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Report No.
Thermodynamic Modelling of Energy Recovery Options
from Digestate at Wastewater Treatment Plants
Authors: Karla Dussan and Rory Monaghan
216
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EPA RESEARCH PROGRAMME 2014–2020
Thermodynamic Modelling of Energy Recovery
Options from Digestate at Wastewater Treatment
Plants
(2014-RE-DS-3)
EPA Research Report
Prepared for the Environmental Protection Agency
by
National University of Ireland, Galway
Authors:
Karla Dussan and Rory Monaghan
ENVIRONMENTAL PROTECTION AGENCY
An Ghníomhaireacht um Chaomhnú Comhshaoil
PO Box 3000, Johnstown Castle, Co. Wexford, Ireland
Telephone: +353 53 916 0600 Fax: +353 53 916 0699
Email: info@epa.ie Website: www.epa.ie
ii
EPA RESEARCH PROGRAMME 2014–2020
Published by the Environmental Protection Agency, Ireland
ISBN: 978-1-84095-715-0
Price: Free
June 2017
Online version
© Environmental Protection Agency 2017
ACKNOWLEDGEMENTS
This report is published as part of the EPA Research Programme 2014–2020. The programme is
nanced by the Irish Government. It is administered on behalf of the Department of Communications,
Climate Action and the Environment by the EPA, which has the statutory function of co-ordinating
and promoting environmental research.
The authors acknowledge the guidance and information provided by the members of the steering
committee, namely Fiona Lane (Irish Water), Mick Henry (EPA), Eamonn Merriman (EPA) and
Aisling O’Connor (EPA). Special thanks are due for the technical discussions and comments
provided throughout the project by Dr Xinmin Zhan, Dr Eoghan Clifford, Qingfeng Yang and Edelle
Doherty from the National University of Ireland, Galway.
DISCLAIMER
Although every effort has been made to ensure the accuracy of the material contained in this
publication, complete accuracy cannot be guaranteed. The Environmental Protection Agency, the
authors and the steering committee members do not accept any responsibility whatsoever for loss
or damage occasioned, or claimed to have been occasioned, in part or in full, as a consequence of
any person acting, or refraining from acting, as a result of a matter contained in this publication.
All or part of this publication may be reproduced without further permission, provided the source is
acknowledged.
The EPA Research Programme addresses the need for research in Ireland to inform policymakers
and other stakeholders on a range of questions in relation to environmental protection. These reports
are intended as contributions to the necessary debate on the protection of the environment.
iii
Project Partners
Dr Karla Dussan
Mechanical Engineering Department
National University of Ireland, Galway
University Road
Galway
Ireland
Email: karla.dussan@nuigalway.ie
Dr Rory Monaghan
Mechanical Engineering Department
National University of Ireland, Galway
University Road
Galway
Ireland
Tel.: +353 91 49 4256
Email: rory.monaghan@nuigalway.ie
v
Contents
Acknowledgements ii
Disclaimer ii
Project Partners iii
List of Figures and Tables vii
Executive Summary ix
1 Introduction 1
1.1 Project Components and Research Outcomes 1
1.2 Wastewater and Sludge Treatment in Ireland 2
1.3 Conclusions 4
2 Thermal Conversion Modelling and Outline of Energy Recovery Systems 5
2.1 Pseudo-equilibrium Modelling of Thermal Conversion 5
2.2 Process Outline of the Thermal Conversion of Sludge/Digestate 8
2.2.1 Internal combustion engines 11
2.2.2 Gas turbines 11
2.2.3 Steam turbines 11
2.2.4 Feedstocks: sludge, digestate, wastes and biomass 14
2.2.5 Costs of treatment and costs of electricity for sludge-to-energy systems 14
2.2.6 Carbon emissions due to energy consumption and energy savings in
thermal conversion systems 15
2.3 Conclusions 16
3 Thermal Conversion of Sludge and Integration with AD in WWT Plants 17
3.1 Sludge and Waste Incineration 17
3.2 Gasication of Biomass and Wastes 17
3.3 Thermal Conversion for Waste Management and Energy Recovery 18
3.4 Gasication Performance of Sludge and Digestate 18
3.5 Integration of CHP Technologies for Energy Recovery in WWT Plants 20
3.6 Economic Performance of Thermal Conversion Systems Integrated with AD 22
3.7 Carbon Emissions Due to WWT Plant Operation with Thermal Conversion
Systems Integrated with AD 24
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Modelling of Energy Recovery Options from Wastewater Treatment Plant Digestate
3.8 Advantages and Disadvantages of Thermal Conversion Process Congurations 26
3.9 Optimisation of Energy Recovery Systems Using Gasication and
CHP Modules 27
3.10 Biomass and Waste Co-processing for Improving Energy Efciency and
Reducing Carbon Footprint 29
3.11 Concluding Remarks 31
4 Conclusions and Recommendations 32
4.1 Combustion and Gasication Coupled with Steam Turbines 32
4.2 Gasication Coupled with Gas Turbines 32
4.3 Gasication and AD–Gasication Integrated with Internal Combustion Engines 32
4.4 Recommendations for Future Work 33
4.4.1 Economies of scale 33
4.4.2 Optimisation of anaerobic digestion for biogas production 33
References 35
Abbreviations 39
Appendix 1 Estimation of Costs for Thermal and AD Conversion Systems of
Sludge/Digestate 40
Appendix 2 Economy of Scale: Effect of Sludge Feed Rate on Costs of Treatment
and Levelised Cost of Electricity Generation 41
Appendix 3 Sensitivity Analysis of Gasication Performance 42
Appendix 4 Sensitivity Analysis of Costs of Operation, Levelised Costs of Electricity
and Carbon Emissions of the Gasication and Combustion Engine Process 43
vii
List of Figures and Tables
Figures
Figure 2.1. Pseudo-equilibrium model for thermal conversion of biomass 6
Figure 2.2. Experimental and regressed carbon conversion from air gasication of biomass 6
Figure 2.3. Experimental and regressed carbon yield as char following air gasication
of sawdust in a uidised bed reactor 7
Figure 2.4. Experimental carbon yield as methane following air gasication of different
biomass feedstocks 7
Figure 2.5. Comparison of predictions using a pseudo-equilibrium model with
experimental data in air gasication of different biomass at 923–1123 K 9
Figure 2.6. Process diagram of energy recovery concept from sludge/digestate
conversion through anaerobic digestion and/or thermal conversion 10
Figure 3.1. Performance of gasication of sludge as a function of the equivalence ratio
and moisture content of the sludge 19
Figure 3.2. Total energy recovery efciency (ηel + ηhr) and electrical efciency (ηel) for
Cases TC1–4 and ADTC1–4 21
Figure 3.3. Heat coverage (Chr) and electricity coverage (Cel) for Cases TC1–4 and
Cases ADTC1–4 22
Figure 3.4. Specic capital investment (SCI) and cost of treatment (COT) as functions
of the total energy recovery efciency of the thermal conversion systems 23
Figure 3.5. Specic capital investment (SCI) and costs of electricity as functions of
electrical efciency and net electricity generation for Cases TC3, TC4,
ADTC3 and ADTC4 25
Figure 3.6. Net emissions associated with Irish energy for sludge treatment in Cases
TC1–4 and Cases ADTC1–4 26
Figure 3.7. Sensitivity of efciencies and energy coverage levels using gasication and
internal combustion engines as functions of process parameters 28
Figure 3.8. Process performance as a function of the biomass to dry sludge mass
ratio used in co-processing in a system using gasication and an internal
combustion engine 30
Figure A2.1. Levelised costs of electricity and costs of operation of a sludge gasication
plant as functions of the sewage sludge feeding rate 42
viii
Modelling of Energy Recovery Options from Wastewater Treatment Plant Digestate
Figure A3.1. Sensitivity of the lower heating value of syngas and the cold gas efciency
from gasication as functions of the following process parameters:
equivalence ratio (ER), sludge moisture contents (yM,1 and yM,2), gasication
temperature (TGS), sludge feed rate, and sludge properties (O/C, H/C and ash
content) 43
Figure A4.1. Sensitivity of cost of operation, levelised costs of electricity and carbon
emissions in the system using gasication and combustion engine as
functions of the following process parameters: equivalence ratio (ER),
sludge moisture contents (yM,1 and yM,2), gasication temperature (TGS),
sludge feed rate, and sludge properties (O/C, H/C and ash content) 45
Tables
Table 1.1. Average plant capacity of wastewater treatment facilities in Ireland 2
Table 1.2. Sewage sludge management practices in Ireland in 2014 3
Table 1.3. Anaerobic digestion facilities and current status within wastewater
treatment plants in Ireland 4
Table 2.1. Operational parameters of the gasication–CHP systems 12
Table 2.2. Final analysis of waste and biomass materials used as fuel 14
Table 2.3. Energy and emission factors for Ireland 15
Table 3.1. Process congurations for maximising electricity generation for internal
combustion engines 27
ix
Executive Summary
Stringent emission limits, population growth and
increasing urbanisation continue to drive the
advancement in wastewater treatment (WWT)
technologies and waste management frameworks.
Today, in Ireland, over 96% of the sludge generated
during WWT is spread on agricultural land; however,
restrictions set by agricultural quality assurance
schemes are encouraging the search for new
alternatives. Plants with a capacity greater than
100,000 population equivalent (p.e.) can implement
anaerobic digestion (AD) as a means of sludge
treatment and energy recovery. Pilot and WWT
plant-scale studies have reported biogas yields of
between 4 and 10 GJ t –1 (1–3 kWh kg–1) for dry sludge
through AD of municipal sewage sludge (Qiao et
al., 2011). However, large-scale biogas plants can
consume approximately 40% of their energy yield for
their operation, thus diminishing energy efciency
(Berglund and Börjesson, 2006). Thermal technologies
for the conversion of either sewage sludge or
digestate represent potential routes for both sludge
volume reduction and energy recovery. In particular,
sludge combustion and/or gasication could provide
either thermal or chemical energy for combined heat
and power (CHP) generation and could be readily
integrated into WWT plants.
This project explores the state-of-the-art combustion
(incineration) and gasication technologies used
for biomass and waste conversion. This study
evaluates not only the technical performance of
these technologies, but also the investment and
operational costs, and waste generation, treatment
and valorisation through recovery of materials and
chemicals. Using a pseudo-thermodynamic approach
for modelling thermal conversion, the performance of
combustion and gasication of sludge and digestate
was evaluated under various operational conditions
and for a range of solid material properties (e.g.
moisture and composition). The model evaluated
the technical performance of thermal conversion
processes and the integration of energy carriers
for power generation and heat recovery through
various available technologies, such as steam and
gas turbines, and combustion engines. To support
local and government authorities in the consideration
of these alternatives, different techno-economic
indicators, including energy recovery efciency,
treatment costs, levelised cost of electricity generation
and the carbon footprint of the WWT and sludge
management plants were included and thoroughly
compared to identify potential process alternatives.
Gasication and AD–gasication integrated with
CHP generation was technically feasible and offered
a means to reduce nal waste disposal costs and
improve the energy efciency of the WWT plant. The
most efcient process concept for energy recovery
used internal combustion engines to generate
power from energy carriers that were produced
from gasication and AD–gasication, i.e. biogas
and syngas. The conditions under which electricity
generation was maximised were reached by
undertaking extensive sludge pretreatment, i.e. drying,
which resulted in low heat recovery efciencies. In
contrast, AD integrated with gasication resulted
in greater thermal recovery exibility, leading to
conditions in which net surpluses of both electricity
and heat were achieved. The combination of AD and
gasication offered competitive costs of electricity
generation [20–50 c kWh–1 (euro cent per kWh)], with
a low carbon footprint (< 300 kg CO2 t –1 dry sludge).
Internal combustion engines offer great exibility and
competitive power efciencies at expected scales for
energy recovery in WWT facilities (< 10 MWel).
When gasication was used as the only sludge
conversion treatment, additional energy for heat
generation was required during this process. It was
proposed that this additional heat could be generated
by co-processing with renewable solid fuels and
wastes, i.e. biomass, animal slurries or the organic
fraction of municipal solid waste. Biomass rates of
between 0.8 and 1 times that of the sludge feed rate
were required to meet energy demands with a reduced
carbon footprint.
It is important to emphasise that the scale of the facility
is vital in meeting sustainability criteria, especially in
terms of operational and capital expenditures. The
evaluation presented in this study was applied to the
largest WWT scale in Ireland (1.6 Mp.e.), such as that
of the Ringsend WWT plant, which currently produces
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Modelling of Energy Recovery Options from Wastewater Treatment Plant Digestate
approximately 85 t day–1 (tpd) of dry sludge (digestate).
However, most existing anaerobic digestion facilities
have capacities between 40,000 and 150,000 p.e., with
sludge generation rates of 20–100 dry tpd, depending
on the inuent wastewater. Combustion engines offer
sufcient exibility to operate with the power capacities
expected for these scales (> 100 kWel). However,
installation costs can make the implementation of
gasication challenging at small scales. Raw sludge
generation rates under 130 tpd led to levelised costs
of electricity generation that were slightly above
national costs of fossil-based electricity (24 c kWh–1).
However, gasication and AD–gasication treatment of
sludge was economically competitive, with generation
rates above 25 tpd. This challenge of scale may still
be overcome through the other approaches that
are suggested for future research. On-site thermal
pretreatment can facilitate sludge transport to a
centralised facility, where energy recovery could offset
overall treatment costs in terms of energy and carbon
footprint. A centralised gasication facility would offer
the possibility of implementing biomass and/or waste
co-processing with greater economic and technical
efciencies, while reducing operational challenges.
It is also important to note that sludge transport and
biomass co-processing will have additional energy
penalties, transport costs and carbon footprint effects,
which must be taken into account in the evaluation
of an optimal sludge transport and treatment network
at a county or national level. Use of biomass can
result in direct and indirect carbon emissions linked to
harvesting, use of fertilisers, land use change, import
and transport that were not considered in this study.
These may affect the carbon footprint of large-scale
plants with high biomass-to-sludge co-processing
ratios.
1
1 Introduction
Severe limitations on the emission of pollutants to
water bodies in the future, along with population
growth, increasing urbanisation and changes in
industrial/agricultural practices, mean that wastewater
treatment (WWT) processes are undergoing rapid
development. These processes are required to have
high removal efciencies, while functioning within a
sustainable system that has minimum impact on the
environment and positive economic performance. The
concept of a circular economy has led to searches
for new ways for valorising waste and recovering
resources and energy at all stages of industrial
processes. Water management entities are adapting
conventional processes or shifting to new strategies
in which energy self-sufciency can be guaranteed
at most times (Rygaard et al., 2011). This project
set out to investigate a series of technologies, e.g.
thermal conversion, as new process strategies in
which a circular use of WWT plant waste is employed
for energy recovery via power/heat generation. The
objectives of this work include the following:
●to review and estimate energy requirements
in national and international WWT and sludge
treatment facilities;
●to review the state of the art of thermal conversion
technologies (combustion, pyrolysis and
gasication) for the conversion of biomass and
wastes to energy;
●to create a modelling tool for the implementation
of these technologies in WWT facilities for sludge
treatment, with and without anaerobic digestion
(AD);
●to evaluate the potential generation of power and
heat, coverage of on-site energy demands and
cost of treatment (COT)/power generation using
thermal and AD processes;
●to identify opportunities and challenges in sludge
management practices in Ireland through thermal
conversion and integration with AD.
1.1 Project Components and
Research Outcomes
This work was carried out using a combined empirical
and theoretical approach supported by a literature and
technology survey of WWT and thermal conversion
processes. The project followed four different stages:
1. Sludge properties and scale selection. Sludge
characteristics of interest for thermal conversion
were selected and representative values and
ranges selected based on empirical data collected
from the literature and biomass characterisation
databases.
2. Wastewater and sludge treatment and thermal
conversion technologies. State-of-the-art
processes for WWT and sludge management
in Ireland were reviewed. Technologies for
thermal conversion of sludge/digestate and heat
and power generation were also reviewed, and
heuristics and process conditions were identied
and described.
3. Thermal conversion modelling tool. Modelling
approaches for the prediction of the performance
of thermal conversion processes were reviewed. A
pseudo-equilibrium model was implemented using
MATLAB R2015a and Cantera 2.2.1 software
using empirical data for air gasication of biomass
and wastes.
4. Techno-economic evaluation of integrated
AD, sludge drying and thermal conversion.
Performance and process design factors were
selected, including energy efciency, energy
coverage, specic investments and operational
costs, levelised costs of electricity (LCOE) and
carbon emissions. Different process outlines were
proposed and evaluated to maximise energy
coverage and identify deciencies in the proposed
energy integration systems.
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Modelling of Energy Recovery Options from Wastewater Treatment Plant Digestate
Research and dissemination products from this project
included the following:
● Peer-reviewed scientic publications outlining the
research nding and potential applications of the
proposed technologies. These publications are
“Integrated thermal conversion and anaerobic
digestion for sludge management in wastewater
treatment plants” (Dussan and Monaghan, 2017)
and “Thermodynamic evaluation of anaerobic
digestion and integrated gasication for waste
management and energy production within
wastewater treatment plants” (Dussan et al.,
2016). They also include scientic presentations
at the 26th Irish Environmental Researchers’
Colloquium (ENVIRON 2016) and the 6th
International Conference on Engineering for
Waste and Biomass Valorisation (WasteEng16).
●Poster presentations at the NUIG Energy Night
2016, Galway, Ireland, and at the 24th European
Biomass Conference and Exhibition (EUBCE
2016), Amsterdam, the Netherlands.
●National dissemination through an evening lecture
webinar organised by Engineers Ireland and a
poster presentation at the EPA National Water
Event 2016.
●Project dissemination online via the Therme
research group’s website (http://www.nuigalway.ie/
therme/projects/old/epasludge/).
1.2 Wastewater and Sludge
Treatment in Ireland
Currently, over 500 urban areas in Ireland are
provided with WWT (primary, secondary and/or
nutrient removal treatment) (EPA, 2015). EU Directive
91/271/EEC requires that wastewater discharges from
urban agglomerations greater than 2000 population
equivalent (p.e.) discharging to freshwaters and
estuaries and greater than 10,000 p.e. discharging
to coastal waters must be treated with a minimum
of secondary treatment. According to the EPA, 162
facilities with more than 2000 p.e. include secondary
treatment technologies, corresponding to 94% of the
national wastewater load (EPA, 2015).
Nutrient removal (nitrogen and/or phosphorus) is also
provided at 143 of the 162 WWT plants with more that
than 2000 p.e. Nutrient removal is generally required
under Waste Water Discharge Authorisations issued
by the EPA in settlements with a population greater
than 10,000 p.e. that discharge efuents to designated
sensitive water bodies.
Numbers of WWT plants, average plant capacities
per urban area size and treatment extent are shown
in Table 1.1. In general, small WWT plants serve
small and, in some cases, remote urban areas or
settlements of up to 2000 p.e. A small number of
facilities serving urban areas of more than 2000 p.e.
involve only primary treatment. However, these
facilities are expected to be upgraded to comply with
the Urban Wastewater Directive in the near future.
For urban concentrations of more than 10,000 p.e.,
plants using only secondary treatment have an
average capacity of 157,500 p.e. The design capacity
of treatment works in Ireland does not exceed
160,000 p.e. on average, except in the case of the
Ringsend WWT facility in Dublin City, which has a total
design capacity of 1,640,000 p.e.
Facilities are required to collect and treat sludge
generated from primary and secondary treatment
before nal disposal. These sludge management
methods include mechanical processes (thickening,
dewatering) and biological and thermal treatment.
The main purpose of these operations is to stabilise
Table 1.1. Average plant capacity of wastewater treatment facilities in Ireland (Shannon et al., 2014)
Urban area size Number of plants in each p.e. range and average capacity per plant/p.e.
Primary treatment Secondary treatment Secondary treatment and
nutrient removal
No. of plants Average
p.e.-serving
capacity
No. of plants Average
plant
capacity
No. of plants Average
plant
capacity
Less than 2000 p.e. 233 688 450 827 151 604
Between 2000 and 10,000 p.e. 2 1746 50 4608 120 7176
More than 10,000 p.e. 4 13,500 26 157,545 39 33,066
3
K. Dussan and R. Monaghan (2014-RE-DS-3)
solids to avoid putrefaction and reduce waste
volume, thus decreasing the storage requirement and
transport costs when the sludge is taken to disposal
or treatment sites. As well as stabilising the sludge,
biological processes reduce odour generation and
greenhouse gas (GHG) emissions when sludge is
stored or disposed of. These processes have the
additional advantages of generating either energy and/
or stabilised sludge cake, with 15–20% dry solids that
can be used for agricultural purposes (soil spreading).
Thermal drying represents an optional nal treatment
stage in which the moisture content in the dewatered
or treated sludge/compost is reduced to values around
or below 10% to facilitate storage, packaging and
transport. Properties and production rates of sludge
vary depending on the quality of the wastewater
entering the plant, as well as on the treatment
operation and the efciency with which it has been
separated. In general, primary sedimentation leads
to higher sludge production rates (0.10–0.17 kg m–3)
than from activated sludge or trickling lter (0.06–
0.10 kg m–3) (Metcalf et al., 2014). Further addition
of lime for the chemical removal of phosphorus can
signicantly increase sludge production to between
0.25 and 1.30 kg m–3 (Metcalf et al., 2014). In Ireland,
sludge generation is annually reported by the EPA for
each water service authority (city and county councils).
Average sludge generation in Ireland is in the order
of 0.090 to 0.90 kg m–3 or 10 to 85 kg p.e.–1 year –1
(Shannon et al., 2014).
AD is a vital technology for the improvement of
energy efciency in WWT plants. The Composting &
Anaerobic Digestion Association of Ireland (Cré) has
carried out several surveys within the waste-handling
sector in Ireland to identify the extent of AD use (Cré,
2014). From Cré surveys, it was determined that
303,990 and 331,240 tonnes of organic waste were
processed in 2012 and 2013, respectively, by either
composting or AD. These materials included mainly
brown bin wastes (34%), municipal organic solid
wastes (24%), animal slurries and manures (17%)
and, to a lesser extent, sewage sludge (SS) (16%)
(Cré, 2014).
Irish Water has also surveyed sludge management
methods in Ireland. In their study, the production of
SS and the management methods used by local WWT
authorities in 2014 was investigated (Lane, 2015a).
This information is presented in Table 1.2. A signicant
percentage of the sludge generated in 2014 (50%)
was treated by AD, under either mesophilic (30–40°C)
or thermophilic (> 40°C) conditions. Other pre- and
post-treatments of sludge and digestate, respectively,
are commonly implemented to improve efciency
and reduce waste generation. Thermal processes
(drying, pasteurisation and hydrolysis) are used in
the majority of AD facilities. Lime stabilisation (27.7%)
and composting (11.5%) are other commonly used
methods of sludge management. Composting in itself
can be a nal treatment method, since composted
sludge is widely marketed or distributed to farms for
land application. However, lime stabilisation requires
transport of the treated sludge to the nal disposal site.
Lime stabilisation plants are currently exempt from
waste permits or licences when the stabilised sludge
is used for agricultural purposes (Cré, 2013) and
therefore this is the more common practice.
Table 1.2. Sewage sludge management practices in Ireland in 2014 (Lane, 2015a)
Type of treatment Approximate quantity (tonnes) Percentage by weight (%)
Autothermal thermophilic AD (ATAD) 226 0.4
AD and thermal drying 2124 4.0
AD and lime stabilisation 4529 8.5
AD and pasteurisation 4239 7.9
Composting 6206 11.5
Lime stabilisation 14,815 27.7
Thermal drying 4904 9.2
Thermal hydrolysis, AD and thermal drying 14,220 26.5
Thermal hydrolysis and AD 1543 2.9
No treatment 737 1.4
Total 53,543 100
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Modelling of Energy Recovery Options from Wastewater Treatment Plant Digestate
Cré has promoted the incorporation of legislative
changes to oblige WWT plants of more than 5000 p.e.
[300 kg BOD (biochemical oxygen demand)
day–1] to
implement treatment methods of SS before disposal
or use in landspreading. The necessity to improve
energy efciency in WWT facilities, and pressure
from government and public sectors to implement
sustainable sludge management practices, mean that
the implementation of AD in WWT plants has been
promoted. However, the presence of this technology in
the WWT and organic waste management sectors is
still at an early stage in Ireland.
Irish Water provided an account of Irish AD facilities
and their current operational status (Lane, 2015b).
This information is presented in Table 1.3. WWT plants
with capacities above 20,000 p.e. are usually thought
to be suitable for sludge treatment using AD. Currently,
44% (18 plants out of 41) are using this type of facility
on site (Table 1.3). Among these AD plants, 72% are
currently in operation and 56% have incorporated
energy recovery capabilities from the biogas produced
on site.
1.3 Conclusions
New strategies are being sought to drive the
advancement of the WWT sector in Ireland. These
must be centred on the improvement of efuent
water quality, increased energy efciency during
treatment and the recovery of energy from sludge
and digestates. Although there is a need for further
development of AD facilities at national level, this
technology has great potential as a bridge to integrate
more advanced thermal conversion processes, such
as combustion, pyrolysis and gasication. This study
aimed to investigate the potential and feasibility of
thermal conversion for energy recovery by power/heat
generation in WWT plants with or without AD, as well
as the challenges that the new concepts involve.
Table 1.3. Anaerobic digestion facilities and current status within wastewater treatment plants in Ireland
Area WWT plant License Sludge treatment
capacity (p.e.)
Status Energy recovered from
biogas through CHP
Dublin City Ringsend D0034-01 1,640,000 Active Yes
Cork City Cork City D0033-01 413,000 Active No CHP on site
Kildare Upper Liffey Valley D0002-01 400,000 Active CHP out of service
Waterford City Waterford City D0022-01 190,000 Active No CHP on site
Dún Laoghaire Shanganagh D0038-01 186,000 Active Yes
Louth Dundalk D0053-01 179,000 Active Ye s
Kildare Lower Liffey Valley D0004-01 150,000 Inactive CHP out of service
Louth Drogheda D0041-01 101,000 Active Ye s
Sligo Sligo D0014-01 100,000 Active No CHP on site
Galway City Galway City D0050-01 91,600 Active No CHP on site
South
Tipperary
Clonmel D0035-01 90,000 Active No CHP on site
Donegal Letterkenny D0009-01 80,000 Inactive Ye s
Offaly Tullamore D0039-01 80,000 Active Yes
Fingal Swords D0024-01 60,000 Active CHP out of service
Kerry Tralee D0040-01 50,333. Inactive Yes
Meath Navan D0059-01 50,000 Inactive Ye s
Wicklow Greystones D0010-01 30,000 Inactive Yes
North Tipperary Roscrea D0025-01 26,000 Active Ye s
CHP, combined heat and power
5
2 Thermal Conversion Modelling and Outline of Energy
Recovery Systems
This study evaluated the energetic integration of
combustion and gasication as nal sludge conversion
technologies in WWT facilities. A pseudo-equilibrium
model was implemented using MATLAB software and
thermodynamic properties accessed using Cantera
software. This chapter presents a summary of the
principles applied for the formulation of the thermal
conversion model and process conguration.
2.1 Pseudo-equilibrium Modelling of
Thermal Conversion
Thermal conversion reactions of carbonaceous
materials, such as biomass and wastes, form gas
products, whose composition varies depending
on the reactant gas used and reaction conditions.
With high oxygen concentrations (combustion),
fully oxidised gas products are formed (CO2, H2O),
with minor concentrations of CO and hydrocarbons,
depending on the reactor design and the effectiveness
of the gas/solid contact. When low O2 or mild H2O
concentrations are used instead, the product gas
is richer in mildly oxidised compounds (CO, H2).
This product (synthesis gas or syngas) is obtained
under gasication conditions and is combustible. The
oxidant concentration is commonly evaluated by the
equivalence ratio (ER):
ER =Stoichiometric oxygen concentration
Actual oxyygen concentration
(Equation 2.1)
The stoichiometric amount of oxygen refers to the
minimum amount of O2 required to fully oxidise the fuel
in the thermal conversion process. Under combustion
conditions, ER is ≤ 1, whereas for gasication, ER is
commonly > 2.
However, thermal conversion of carbon materials is not
an ideal or simple process. A fraction of the feedstock
may not be fully converted, leading to residual solid
carbon or char, depending on the equipment design
and operational conditions. In this situation, the
fraction of carbon in the material that is converted to
syngas is dened as carbon conversion, XC:
XC=mols of carbon in syngas product
mols of carboon in biomass
(Equation 2.2)
Hydrocarbons (CH4, C2H6, C2H4, etc.) and high
molecular weight compounds (tars) are also formed
because of limitations in the chemical behaviour
of the process. Thermodynamic approaches, while
quick and easy to use, fail to predict the amount
and properties of the syngas. Jand et al. (2006)
recognised the prediction capability of char and
methane content in the nal products as major
limitations of thermodynamic equilibrium models.
In order to resemble realistic carbon conversions
and nal methane/char yields, these variables were
constrained by employing experimental data, as
indicated in Figure 2.1. By restraining the amount of
carbon converted to syngas and the fraction of carbon
converted to methane (CH4) in the nal gas product
as functions of the ER, the accuracy and applicability
of a thermodynamic equilibrium model was greatly
enhanced.
Figure 2.2 shows the carbon conversion, XC, of
different biomass materials as a function of the
ER. These were reported by several authors for
experimental tests in uidised bed reactors using
different types of biomass (Kersten et al., 2003; Li et
al., 2004; Petersen and Werther, 2005; Jand et al.,
2006; Campoy et al., 2009, 2014; Xue et al., 2014).
This reactor conguration is preferred, since it allows
efcient mixing and high reaction rates, especially with
high-ash content fuels, such as wastes and sludge
(Belgiorno et al., 2003).
Carbon conversion was affected by the ER,
because the gasication temperature (TGS) was
over 1073 K (800°C). The regression presented in
Figure 2.2 was calculated using the assumption
that, when stoichiometric or excess oxygen was
used (combustion), carbon was entirely oxidised and
converted to gas (ue gas).
6
Modelling of Energy Recovery Options from Wastewater Treatment Plant Digestate
Few experimental studies have reported a complete
carbon balance of the biomass gasication tests. Jand
et al. (2006) reported the residual char yield as the
completing balance of the collected tars and syngas
(Figure 2.3).
For this limited dataset at TGS between 1073 and
1103 K, the char yield changed inversely with the ER,
so that, under depleted oxygen conditions, a higher
amount of carbon from the biomass remained as
char. This is also evidence of the kinetic limitation of
char gasication reactions when reactor design is
insufcient to allow effective residence time of the char
or when conditions impede the completion of oxidation/
gasication reactions (Di Blasi, 2009).
Figure 2.4 summarises the carbon yield as methane
reported in the biomass air gasication studies
mentioned above. Variability of the methane yield
in this case was signicant; however, there was no
relationship between ER or TGS and the methane yield.
In most cases, the methane yield was approximately
10%; therefore, it was xed at this value for further
modelling.
Taking into account these ndings, the reactivity of
the sludge/digestate in the equilibrium model was
constrained by dening the carbon conversion as a
function of the ER, so that complete conversion can
be attained when approaching combustion conditions
(ER ≤ 1). Minimum carbon conversion was limited to
85% when ER reaches 4.5. When considering steam
gasication, the TGS will mainly dictate the extent of
carbon conversion, assuming a minimum steam-to-
carbon (SC) molar ratio of 0.6. Assuming an SC ratio
Elemental
distribution
C
H
O
Others
1-XCUnreacted carbon
(Char)
Tars
(Naphthalene)
Methane
Remaining fuel
YC10
YCH4
Chemical
equilibrium
Biomass Final
products
Pseudo-equilibrium model
Figure 2.1. Pseudo-equilibrium model for thermal conversion of biomass.
Figure 2.2. Experimental and regressed carbon conversion from air gasication of biomass.
1357911
0.4
0.6
0.8
1.0
1.2
Carbon conversion, X
C
Equivalence ratio, ER
950
1000
1050
1100
Temperature, K
R
2
= 0.926
XC
ER
7
K. Dussan and R. Monaghan (2014-RE-DS-3)
greater than 1 and TGS greater than 1073 K, carbon
conversion can be considered of the same order as
observed in air gasication systems at ER between 3
and 4. Given the limited experimental data available
for gasication systems using both air/O2 and steam,
it was assumed that oxidation reactions affect the
carbon conversion more strongly than steam reforming
reactions at temperatures between 1023 and 1123 K
during gasication.
Char (as pure carbon) and methane yields were also
constrained as a function of ER and as a xed fraction,
respectively. The excess amount of non-converted
carbon was assumed to be transformed to tars, using
naphthalene (C10H8) as a model compound. In the
estimation protocol, once the non-converted carbon
and corresponding hydrogen was subtracted from the
incoming fuel, a xed amount of carbon was retained
Figure 2.3. Experimental and regressed carbon yield as char following air gasication of sawdust in a
uidised bed reactor (compiled from data in Jand et al., 2006).
3 6 9
0.00
0.05
0.10
0.15
0.20
0.25
0.30
R
2
= 0.963
Carbon yield in char, Y
CHAR
Equivalence ratio, ER
950
1000
1050
1100
Temperature, K
ER
Figure 2.4. Experimental carbon yield as methane following air gasication of different biomass
feedstocks.
1 4 7 10
0.0
0.1
0.2
0.3
Sludge
Carbon yield as CH
4
, Y
C in CH4
Equivalence ratio, ER
950
1000
1050
1100
Temperature, K
Sludge
4
ER
8
Modelling of Energy Recovery Options from Wastewater Treatment Plant Digestate
as methane (Figure 2.1). These constraints, in molar
basis, are represented as follows:
CaHbOcNdSe + y(O2 + N2) + zH2O →
a • YCHARC + a • (1 – XC – YCHAR)C10H8 + (Reacting fuel)
(Equation 2.3)
Reacting fuel = (a • XC)C + (b – 8a • (1 – XC – YCHAR))
H + cO – dN + eS (Equation 2.4)
The gasication reactor was modelled as a
non-stoichiometric equilibrium process using
thermodynamic data accessed using MATLAB and
Cantera software. The reacting solid fuel (sludge or
digestate) and the air/steam/moisture mixture were
allowed to attain chemical equilibrium at a constant
temperature and pressure [TGS, PGS (gasication/
combustion pressure)] in an iterative process to
comply with the process mass and energy balance,
seeking to minimise the total Gibbs free energy of
the syngas product, where i = CO and ns = number of
species:
nG nG RT nf
f
ii
i
ns
ii
i
ns
i
i
ns
i
∑∑ ∑
=+
0
0
ln
(Equation 2.5)
where n corresponds to the moles of gaseous species,
G0 refers to their free energy of formation at the
reference state, and f0 to the gas fugacity (f0 = 1 bar):
fx
P
ii
i
=φ (Equation 2.6)
where xi corresponds to the molar fractions of gaseous
species and P refers to the pressure of the system
(PGS). For ideal gases, activity coefcients (ᶲi) are
approximated to 1.
The approximated carbon conversions, and char and
methane yields were incorporated into the equilibrium
model for the gasication examples in the literature.
Figure 2.5 shows a comparison of the syngas
composition obtained experimentally from various
types of biomass and that predicted using the pseudo-
equilibrium model during air gasication.
The major deciency of the model relates to the
prediction of H2 formation. In all cases, the predicted
H2 concentrations were higher than those observed
experimentally. CO2 and steam reforming of methane
and other minor hydrocarbons (C2H4, C2H6, etc.)
occurs at a low rate at temperatures below 1200 K
and, therefore, these are commonly found in syngas
to a signicant extent (< 8% v/v). Despite this, the
model was capable of estimating the energy content
of the syngas with satisfactory accuracy, because of
the trade-off between H2 and CH4 in contributing to the
syngas heating value. This was deemed appropriate
as an estimation tool for the purposes of the energy
and mass balance analysis in the present study.
2.2 Process Outline of the Thermal
Conversion of Sludge/Digestate
The proposed system for energy recovery from
sludge and digestate residues from WWT facilities is
shown in Figure 2.6. In the rst scenario (solid lines),
dewatered sludge derived from WWT is dried to below
its original moisture content (yM,1 = 75%) to increase
the chemical efciency of the thermal conversion. In
the second scenario, the sludge produced in the WWT
plant was digested anaerobically. The digestate slurry
was dewatered to a moisture content of yM,1 = 75% and
later dried further. This process path is represented in
Figure 2.6 with dotted lines.
Biomass drying is carried out in direct or indirect
dryers that use hot air, ue gas or steam as heat
sources. Theoretical energy consumption associated
with this stage is signicant: 2.3 MJ kg–1 of evaporated
water at atmospheric pressure is required (drying
at 373 K). This corresponds to 10–60% of the total
energy contained in biomass materials and wastes.
However, the rate of energy consumption in common
dryer systems, such as rotary and ash dryers,
can reach over 3 MJ kg–1 evaporated water (Li et
al., 2012). These systems operate at mild to high
temperatures (473–873 K) and with high throughput
capacities. Other dryer congurations, such as belt
conveyors, are preferred, because of their capacity
to operate at lower temperatures (303–473 K), their
lower heat consumption (1.3–2.5 MJ kg–1) and their
easier and safer operation (Li et al., 2012). In this
study, the dewatered sludge or digestate was dried
to a nal moisture content (yM,2) of between 5% and
50% using heat recovered from heat sources in the
system, i.e. syngas from gasication and ue gas from
combustion.
After this, the dried sludge or digestate is fed into
the thermal conversion stage (combustor/gasier),
in which preheated air is used as the gasifying or
combustion agent, depending on the ER. The gasier
or combustor operates at equilibrium temperature (TGS)
or at a minimum gasication temperature of 1073 K.
9
K. Dussan and R. Monaghan (2014-RE-DS-3)
When additional heat is required to attain the minimum
gasication temperature (high ER), an additional
combustor is implemented. In this auxiliary combustor,
the char and a fraction of the formed syngas (xGS) are
used as fuels to provide the additional energy in the
gasier. The air used in the combustor/gasier and the
auxiliary combustor is preheated in heat exchangers
(HE1 and HE2, respectively) using the thermal energy
contained in the syngas and the ue gas from the
corresponding processes.
Subsequently, the syngas is introduced to the rst
heat recovery stage (HR1) in which its temperature
is reduced to 323 K so that the gas is stripped of H2S
and COS. This gas treatment is carried out using an
absorption stage in which aqueous ethanolamine
solutions react through an acid–base mechanism
with the gaseous sulfur species at low temperatures
(Austgen et al., 1991). Energy consumption at this
stage relates to the heat used by the reboiler in the
stripping system used to regenerate the solvent. For
the regeneration of methyldiethanolamine (MDEA)
within an integrated gasier-combined cycle plant,
a heat duty of 3.3 MJ kg–1 H2S has been reported in
stripping columns operating at temperatures between
350 and 373 K (Fiaschi and Lombardi, 2002). An
additional condensation stage is carried out by cooling
the syngas to 283 K to remove any residual moisture.
The total energy removed from the syngas before and
after the gas treatment is considered to be available
thermal energy (QHR1).
After using a fraction of the syngas (xGS) to supply
heat to the gasier when required, the excess syngas
is taken to the combined heat and power (CHP)
component of the system. Three main systems were
considered: (1) reciprocating internal combustion
engine (ICE); (2) gas turbine; and (3) steam turbine
with/without reheating. After the syngas has passed
through the CHP and combustor modules, heat is
recovered in HR2 and HR3 by cooling down the
exhaust ue gas to the stack temperature (323 K).
Heat recovered from the heat sources (QHR1, QHR2,
QHR3) will be employed in the WWT stages of sludge/
digestate drying, AD and other treatment stages, when
required.
Table 2.1 shows a summary of xed and variable
parameters employed in the thermal conversion-
CHP systems considered in this study. The system
performance was evaluated in terms of overall
electrical and heat recovery efciencies (ηel and ηhr)
and energy coverage (Ctot, Cel and Chr), dened as
follows:
ηel
CHP
feed feed
W
mLHV
=⋅×100
(Equation 2.7)
Figure 2.5. Comparison of predictions using a pseudo-equilibrium model with experimental data in air
gasication of different biomass at 923–1123 K.
0 10 20 30 40
0
10
20
30
40
H
2
CO
CO
2
CH
4
LHV
Dry gas concentrations / vol%
LHV dry syngas / MJ Nm
-3
(Pseudo-equilibrium model)
Dry gas concentrations / vol%
LHV dry syngas / MJ Nm
-3
(Experimental)
10
Drying
QDR
Digestate
mD, TD
yM,1≈75% moisture
yM,2≈5-50% Combustor/gasifier
TGS, PGS, QGS
Biomass to
co-process
mB, TB
Combustor
Char + Bed
mC
Bed
HE1
Syngas
mGS, TGS
Syngas
mGS, TGS2
C1
WC1
Air
mGA, Tatm
Air
mGA, TGA
HR1
Syngas
mGS, TGT1
Syngas
mGS, TGT2
Syngas
xGS*mGS
Syngas
(1-xGS)*mGS
AC2
WAC2
HE2
Air
mCA, TCA Air
mCA
Flue-gas
mFC, TFC1
Flue gas
mFC, TFC2
CHP module
WIC
Flue gas
mFG, TFG1
QHR1
Gas treatment
WGT, QGT
Water
H2S COS
HR2
QHR2
Moisture
Anaerobic
digestion module
QAD
Sludge
mS
Biogas
mBG, Tatm
Sludge
mS, TS
yM,1≈75% moisture
HR3
QHR3
Wastewater
Treatement
Plant
1,600,000 p.e.
WWWT
QWWT
Wastewater
Treated water
to the environment
C2
WC2
Flue gas
to stack
Flue gas
treatment
WFGT, QFGT
Figure 2.6. Process diagram of energy recovery concept from sludge/digestate conversion through anaerobic digestion and/or thermal conversion. The
dotted lines represent additional process stages required when additional heat is required for the operation of the combustor/gasier.
11
K. Dussan and R. Monaghan (2014-RE-DS-3)
ηhr
HR HR HR FT
feed feed
QQQQ
mLHV
=
+++
⋅×
123100
(Equation 2.8)
CWQQQ
QQQQQ
tot
CHP
=
+++
++++
,net HR HR FT
WWT AD DR GT GS
12
++
+++∑
WW
WW
i
WWT GT FT Ri
(Equation 2.9)
CW
WW
WW
el
CHP
i
=+++∑
,net
WWT GT FT Ri (Equation 2.10)
CQQQ
QQ
QQQ
hr =
++
++++
HR HR FT
WWT AD DR GT GS
12
(Equation 2.11)
where W and Q correspond to the electricity and heat
demands or generation, and the subscripts CHP, HRi,
WWT, AD, DR, GT, GS, FT and Ri refer to the CHP
unit, heat recovery from heat sources, WWT facility,
AD, drying stage, syngas treatment, gasication
process, ue gas treatment and other auxiliary
demands, respectively, and mfeed and LHVfeed are the
feed rate and the low heating value of dry sludge,
respectively.
2.2.1 Internal combustion engines
Typical electrical efciencies of reciprocating ICEs
vary between 25% and 40% LHV of the fuel gas,
while thermal outputs correspond to 35–55% LHV.
Commonly, engines for large applications have
reported high electrical efciencies and low thermal
energy recoveries (Lantz, 2012). However, this may
vary depending on the quality of the fuel gas, load
level, operation and maintenance. In the rst scenario
of this study, the electrical efciency of the engine was
dened as a function of the capacity of the system
by regressing available data of engine performance
(Lantz, 2012; Darrow et al., 2015).
The exhaust ue gas temperature was dened as a
function of the engine capacity, given that less energy
is generally transformed into work at lower engine
capacities. Engines with a base load electric capacity
of 100 kW have reported exhaust temperatures
of 923 K, while at larger capacities (> 9 MW), the
temperature reported was around 623 K (Darrow et al.,
2015).
2.2.2 Gas turbines
In gas turbines, the available syngas is burnt in a
combustor with excess air to guarantee the turbine
inlet temperature (1373–1773 K). Although current gas
turbine designs for natural gas have reported pressure
ratios well above 20:1 (Taamallah et al., 2015), the
challenges involved in the use of syngas as a fuel
in conventional and t-for-purpose turbines restrict
their performance. Gas turbines designed for low-
BTU (British thermal unit) fuels (syngas) with non- or
partially-premixed ames (170–880 MW) have reported
pressure ratios between 12:1 and 17:1 and efciencies
between 35% and 40% (Taamallah et al., 2015). In the
present study, pressure ratio, turbine inlet pressure
and temperature were varied to reach electrical
efciencies as observed for conventional natural gas
turbines of similar capacities.
2.2.3 Steam turbines
For the steam cycle, a gas boiler is used to burn
the available syngas and generate steam. Typical
industrial and CHP fuel boilers operate with overall
efciencies of 70–85% (Darrow et al., 2015). However,
the electrical efciencies of steam turbines depend on
the steam cycle design, pressure ratio and regime of
the steam turbine.
Since heat is required in the system, a back-pressure
steam turbine conguration was considered in this
study. Superheated steam at 40 to 125 bar was
produced and the pressure ratio in the turbine was
such that low-pressure steam was exhausted from the
turbine at 1.5 to 5 bar and used for heat requirements
in the WWT plant.
The steam was superheated to reach maximum
steam temperature: 723 K for combustion or 923 K
for gasication. Temperature was lower for a sludge
combustion system due to corrosion and damage
of the heat recovery–steam generation (HRSG)
system by chlorine and sulfur in the combustion ue
gas. Taking into account that syngas was scrubbed
prior to the CHP component, the steam temperature
considered in the steam cycle was higher when
analysing the gasication–steam cycle system.
12
Table 2.1. Operational parameters of the gasication–CHP systems
Component Value or range
WWT planta
Installed capacity 1,600,000 p.e.
Sludge generation 130 dry tpd
Digestate generation 85 dry tpd
Energy consumption (WWWT + QWWT)50,400 MWh year–1; 83% electricity, 17% heatb
Biogas production 37,500 m3 day–1 (65% CH4, 35% CO2)
Gasication–combustion
Moisture content in feed (yM,2) 5–50% w.b.
ER Combustion: 0.5–1.0; gasication: 1.5–4.5
Minimum gasier temperature (TGS,min)1073 K
Gasier/combustor pressure (PGS)1.2 bar
Gasier/combustor pressure drop 0.1 bar
ER in combustor 0.9
Air temperature to gasier (TGA)873 K
Air temperature to combustor (TCA)873 K
CHP system 1: ICE
Inlet combustion engine pressure (PIE)1.2 bar
ER 0.9
Electrical efciency in combustion engine (ηEE)0.41–0.16exp(–1.3 × 10–3WCHP)
Exhaust ue gas temperature (TFG1)391.7–4.3 × 10–3WCHP + 305.7exp(1.6 × 10–3WCHP)
CHP system 2: gas turbine
ER in combustor 0.9
Inlet gas turbine pressure (PIGT)20–35 bar
Inlet gas turbine temperature (TIGT)1373–1773 K
Pressure ratio in gas turbine (PRGT) 5–25
Isentropic gas turbine efciency (ηSGT) 85%
13
Component Value or range
CHP system 3: boiler-steam turbine
ER in boiler 0.8
Overall boiler efciency (LHV) (ηB) 80%
Isentropic steam turbine efciency ηSST) 62%
Inlet steam pressure (PIS) 40–60 bar (direct sludge/digestate combustion); 60–125 bar (syngas combustion)
Maximum steam temperature in turbine (Tmax,ST)723 K (450°C) (direct sludge/digestate combustion); 923 K (650°C) (syngas combustion)
Steam pressure for utilities (PS)1.5–5 bar
Other specications
Drying heat (QDR)3.3 MJ kg–1 evaporated H2O
Flue gas temperature to stack 423 K
Flue gas pressure to stack 1.1 bar
Compressor isentropic efciency (ηSC) 75%
Maximum pressure ratio in compressorc (PRC) 2.3–3.0
Centrifugal pump efciency (ηSCP) 75%
Maximum total sulfur in cleaned syngas 1000 ppmv
Sulfur removal energy (HGT)3.3 MJ kg–1 removed H2S
Gas treatment temperature (TGT1)323 K
Temperature drop in gas treatment (∆TGT)8 K
Pressure drop in gas treatment (∆PGT)0.01 bar
Pressure drop in heat exchangers (∆PHE)0.02 bar
Temperature of selective catalytic reduction (TSCR)673 K
Temperature of SO2 scrubbing (TDSO)443 K
Maximum gas temperature in ESP (TESP)423 K
Energy consumption by ESP (WESP)0.450 kWm–3 s–1 ue gas
aEstimated for a WWT plant operating with activated sludge process, tertiary WWT, centrifugal sludge dewatering and mesophilic AD (311 K) with thermal pretreatment (323 K).
bHeating corresponds to that required at the AD stage.
cThis is required for the outlet compressor temperature to be 433 K or higher when another compressor stage follows. Inter-stage cooling temperature is 20 K higher than ambient
temperature (288 K).
ESP, electrostatic precipitator; ppmv, parts per million by volume; w.b., wet basis.
Table 2.1. Continued
14
Modelling of Energy Recovery Options from Wastewater Treatment Plant Digestate
2.2.4 Feedstocks: sludge, digestate, wastes
and biomass
This study focused on the use of SS and digestate
from WWT plants. These materials contain low carbon
[30–40% d.b. (dry basis)] and high ash contents
(> 15% d.b.), which is in contrast to the levels found in
conventional biomass. This directly affects the energy
content of the sludge and digestates when used as
solid fuels and thus the quality of the produced syngas.
In addition, the dewatered moisture content of sludge/
digestate affects the overall energy efciency of the
process. In this study, a dewatered moisture content
of 75% was considered (Werther and Ogada, 1999).
Other biomass and wastes were taken into account for
co-processing. Table 2.2 shows the properties of the
waste and biomass materials considered in this study.
2.2.5 Costs of treatment and costs of
electricity for sludge-to-energy systems
Capital cost data were gathered through a literature
survey and were updated and converted to reect
equivalent costs in euros for 2015. For these
approximations, national consumer price indices
(CPIs) and average international exchange rates
for 2015 were used.1 Equipment costs for the
1 Prices and consumption (Statistics Sweden) (available online: http://www.scb.se/en_/Finding-statistics/Statistics-by-subject-area/
Prices-and-Consumption/); consumer prices indices (Ofce for National Statistics, UK) (available online: https://www.ons.gov.
uk/economy/inationandpriceindices/timeseries/czvl/mm23); CPI data from 1913 to 2016 (US Ination Calculator) (available
online: http://www.usinationcalculator.com/ination/consumer-price-index-and-annual-percent-changes-from-1913-to-2008/);
consumer price index (Statistics Denmark) (available online: http://www.dst.dk/en/Statistik/emner/priser-og-forbrug/forbrugerpriser/
forbrugerprisindeks); and euro foreign exchange reference rates (European Central Bank) (available online: https://www.ecb.
europa.eu/stats/exchange/eurofxref/html/index.en.html).
different modules were taken from the literature
as free-on-board (FOB). Factors were applied to
estimate direct and indirect costs associated with the
modules, covering materials and labour required for
installation, as well as other indirect costs (interest
during commission, contractor fees, contingency,
commissioning), so that:
TCC = FOB × (1 + αTC) (Equation 2.12)
where TCC is the total capital cost of the equipment
considered, FOB refers to the FOB cost of the
equipment or system, and αTC refers to the correction
factor for total direct and indirect costs associated with
the system (αTC = 0.8) (Bridgwater et al., 2002; Yassin
et al., 2009).
Investment costs for combustion plants were based
on facilities processing biomass and wastes using
uidised bed reactors and including the CHP system
(steam cycle) for electricity generation (van den Broek
et al., 1996; Granatstein, 2004; Junginger et al., 2006).
Investment costs for uidised bed gasiers included
costs associated with the feeding mechanism, the
reactor and the syngas cleaning system (Bridgwater et
al., 2002). Installation costs of AD plants (including ICE
modules) were taken from a survey of the literature
and corresponded to agricultural and waste treatment
Table 2.2. Final analysis of waste and biomass materials used as fuela
Content SS SS
digestateb
PM PL PM digestatebPL digestatebMiscanthus Willow
pellets
Moisture (wt%) 75 75 40 40 75 75 25 25
C (wt% d.b.) 36.5 31.0 40.3 36.2 33.8 28.5 47.3 49.0
H (wt% d.b.) 5.2 3.4 5.2 5.2 4.6 3.7 5.4 6.0
O (wt% d.b.) 22.3 21.8 32.0 32.0 20.4 23.7 41.5 42.8
N (wt% d.b.) 5.0 6.1 2.7 4.6 4.7 7.6 0.6 0.5
S (wt% d.b.) 1.5 1.8 0.5 0.6 0.9 1.1 0.1 0.2
Ash (wt% d.b.) 29.5 35.9 20.3 21.3 35.6 35.3 5.1 1.5
LHVc (MJ dry kg–1) 13.6 10.1 12.9 11.7 12.4 9.1 13.9 14.9
aTaken from the ECN biomass database (ECN, 2015).
bEstimated by a mass balance after the biogas production specied in Table 2.1.
cEstimated by the Scheurer’s empirical equation for biomass heating value (Friedl et al., 2005).
d.b., dry basis; PL, poultry litter; PM, pig manure; wt%, percentage by weight.
15
K. Dussan and R. Monaghan (2014-RE-DS-3)
plants (Hjort-Gregersen, 1999; Alakangas and
Flyktman, 2001; Walla et al., 2006). FOB costs of CHP
systems (ICE, gas and steam turbines, and combined
cycles) were also gathered from the literature and
calculated to reect overall price variation as a function
of design capacity or generated electricity (Bridgwater
et al., 2002; ESMAP, 2009; Darrow et al., 2015; NTC,
2015). The data were used to calculate the investment
costs of these systems as a function of either feed
capacity (MWfuel) or electricity generation (MWel).
Operational and maintenance (O&M) costs for all
the process stages were either gathered from the
literature or estimated using the energy and mass
balances of the operations involved at each stage of
the system (van Ree et al., 1995; Bridgwater et al.,
2002; US-NREL, 2006; Tippayawong et al., 2007;
Yassin et al., 2009; Darrow et al., 2015). Appendix
1 summarises the expressions and values used for
estimation of capital and O&M costs. Capital was
amortised for a project period of 20 years with an
annual interest of 5%. COT and LCOE were estimated
after correcting the capital and O&M costs with a xed
annual ination rate of 1% and using the annualised
costs and sludge feed rate and annual net power
generation, respectively:
COT
=
O&M
i
i
∑
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
yearly
m
sludge,yearly
,€ton
–1
⎡
⎣
⎢⎤
⎦
⎥
(Equation 2.13)
COE =
TCCi
i
∑
+O&Mi
i
∑
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥yearly
WCHP –WR
i
i
∑
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥yearly
,c€kWh–1
⎡
⎣
⎢⎤
⎦
⎥
(Equation 2.14)
SCI =
TCC
i
i
∑
W
CHP
–W
R
i
i
∑,k€kW
–1
⎡
⎣
⎢⎤
⎦
⎥
(Equation 2.15)
The costs of disposal of separated ash from
combustion or gasication and solid residues
generated in the removal of sulfur oxide gases (SOX)
gases were also taken into account. Landll gate fees
were assumed to be €80 t –1, in addition to the levy
of €75 t –1 for waste disposed at landlling facilities in
Ireland (EEA, 2013; Government of Ireland, 2013).
2.2.6 Carbon emissions due to energy
consumption and energy savings in
thermal conversion systems
Carbon dioxide equivalent emissions were estimated,
taking into account the energy balance of the process
and the emissions factors associated with Irish energy.
Table 2.3 lists the energy conversion factors used in
this study. Energy demands were converted to the
equivalent amount of primary energy, while energy
generated on site was considered direct energy (1 kWh
renewable energy = 1 kWh primary energy). Net carbon
emissions were estimated in two bases: per m3 of
treated wastewater in the facility or per tonne of dried
sludge:
Carbon emissions per m
El PE R
i
HP
ER
ff Wf
fQ
ii
3=
⋅⋅ +⋅⋅
∑
ii
ww
V
∑
(Equation 2.16)
Carbon emissions per t
El PE R
i
HP
ER
i
ff Wf
fQ
ii
=
⋅⋅ +⋅⋅
∑ ∑∑
mfeed
(Equation 2.17)
Carbon savings per m
El IC
HH
R
i
ww
fW fQ
V
i
3=
⋅+ ⋅
∑
(Equation 2.18)
Table 2.3. Energy and emission factors for Ireland
Energy and emission factors
Electricity
fEl 0.522 kg CO2 kWh–1
fPE 2.37 kJ PE kJ–1
Heat and natural gas
fH0.445 kg CO2 kWh–1
fPH 1.375 kJ PE kJ–1
fEl, factor of equivalent carbon dioxide emission per unit
of output electricity; fH, factor of equivalent carbon dioxide
emission per unit of heat used; fPE, factor of primary energy
consumed per unit of output electricity; fPH, factor of primary
energy consumed per unit of heat used; PE, primary energy.
16
Modelling of Energy Recovery Options from Wastewater Treatment Plant Digestate
Carbon savings per t
El IC
HH
R
i
feed
fW fQ
m
i
=
⋅+ ⋅
∑
(Equation 2.19)
Net emissions = Carbon emissions – Carbon savings
(Equation 2.20)
2.3 Conclusions
A thermodynamic pseudo-equilibrium model was built
to predict the energy value of the syngas generated
from a wide range of solid fuels, biomass and wastes
in uidised bed gasiers. Descriptions were presented
for the empirical and thermodynamic assumptions that
were used in the construction of the computational
energy recovery model. The pseudo-equilibrium model
for gasication was integrated with these modular
modelling tools to describe the performance of the
complete system. User-dened inputs include sludge
properties and operational conditions, such as ambient
temperature, inuent wastewater, sludge generation
rate and biomass co-processing, among others. The
following chapter will present a techno-economic
analysis of the system generated with the use of this
model.
17
3 Thermal Conversion of Sludge and Integration with AD
in WWT Plants
WWT plants consume up to 180 MJ (50 kWh) per
person and produce over 27 Mt of sludge every
year in Europe (Shi, 2011; Eurostat, 2012). This
sector is undergoing continuous change due to
more rigorous environmental regulations on efuent
quality, population growth and increased urbanisation.
Technologies for sludge treatment include thickening
and dewatering, as well as biological processes,
such as AD and composting. A WWT facility can
produce between 10 and 60 kg dry SS per p.e. every
year (Shannon et al., 2014). After thickening and
dewatering, these 60 kg SS can occupy over 240 litres
(25% w/w dry solids), requiring about 50 MJ (14 kWh)
for transport to a disposal site 50 km away (Houillon
and Jolliet, 2005). This is an increase of nearly 30%
in the energy consumed by the WWT plant and in the
indirect carbon footprint of the treatment process.
Plants with a capacity greater than 100,000 p.e. or
sludge hub centres importing sludge from a region
may be suitable for implementing AD to manage
sludge and improve energy efciency. Pilot and
WWT plant scale studies have reported biogas yields
between 4 and 10 GJ t –1 (1–3 kWh kg–1) for dry sludge
through AD of municipal SS (Qiao et al., 2011).
However, large-scale biogas plants can consume up to
40% of this energy for their operation, thus diminishing
energy efciency (Berglund and Börjesson, 2006).
Thermal technologies for the conversion of either SS
or anaerobic digestate represent potential methods for
both sludge volume reduction and energy recovery. In
particular, sludge combustion and gasication could
provide either thermal or chemical energy for CHP
generation, which could be readily integrated to WWT
plants.
3.1 Sludge and Waste Incineration
Incineration has been used as a sludge management
technology following further restrictions on land
spreading of sludge residues in several European
countries. Sludge volume is readily reduced through
auto-thermal oxidation of sludge in multiple hearth
or uidised-bed incinerators when operating at high
temperatures (> 1000 K). All organic content in the
sludge is oxidised and the inorganic components are
obtained in the form of a stabilised bottom or y ash
(10–30% w/w d.b.).
These technologies have been widely implemented
for municipal waste disposal in the last decade,
representing, in some cases, a self-sustained energy
supply for sludge drying and disposal (Werther and
Ogada, 1999). However, emissions arising from the
high nitrogen and heavy metal content in SS and
wastes require additional stages of ue-gas cleaning,
e.g. catalytic and non-catalytic NOX (nitrogen oxide
gases) reduction, tar/polycyclic aromatic hydrocarbons
reduction, staged combustion and adsorption (Sänger
et al., 2001; Yao et al., 2004; Deng et al., 2009).
These technologies have been implemented in
Canada, Germany, the Netherlands and the USA,
where facilities with low or zero additional fuel
consumption requirements operate at scales over
100 dry tpd (Burrowes et al., 2010; Dangtran et al.,
2011). An example of this is the Müllverwertung
Rugenberger (MVR) incineration plant in Hamburg,
Germany. This facility uses a vertical incinerator
equipped with a ue-gas cleaning system (catalytic
NOX reduction, HCl- and SO2-scrubbing, bag lters),
which can process 510 dry tpd of waste to generate
46 MW of steam and 4 MW of electricity (Zwahr, 2003).
This plant has achieved sustainable operation through
extensive recovery of by-products from the wastes
and ue gas treatment, including technical grade
hydrochloric acid, scrap metals, slag and gypsum.
3.2 Gasication of Biomass and
Wastes
Gasication represents an alternative to overcome the
challenges of ue-gas emissions from combustion.
Through this conversion process, the sludge is
volatilised at temperatures between 950 and 1200 K
under oxidant-lean environments to produce syngas
(Huber et al., 2006). Because of the weak oxidative
character of the process, the formation of the toxic
gases NOX and SOX is avoided. In this case, treatment
18
Modelling of Energy Recovery Options from Wastewater Treatment Plant Digestate
of the syngas treatment consisted of H2S removal prior
to any further use as fuel.
In a similar way to combustion, gasication reduces
the sludge volume prior to nal disposal, since it
converts over 80% of the organic fraction to syngas
fuel, while xing any potential harmful metals and
inorganic components, i.e. Cd, Co, As and Hg, in
the char and ash/slag residue (Marrero et al., 2004).
Several technologies are available for gasication
and these have been gradually implemented for
biomass conversion and integration in energy and
fuel production (Kopyscinski et al., 2010), including
xed (downdraft/updraft) reactors, uidised-bed (FB)
gasiers and entrained-ow reactors. FB gasication is
the most attractive technology for biomass and waste
because of its exibility for treating different qualities
of solid fuels, economy of scale and effective process
conguration. FB gasication has been explored in the
technical evaluation of sludge conversion (Dogru et al.,
2002; Petersen, 2004; Petersen and Werther, 2005;
Nilsson et al., 2012; Campoy et al., 2014).
3.3 Thermal Conversion for Waste
Management and Energy
Recovery
One of the main challenges of thermal conversion
processes lies in the poor energy quality of sludge
(10–15 MJ kg–1 dry sludge) and its high moisture
content after dewatering (70–80% w/w). The carbon
content of sludge, and therefore its energy content, is
known to be reduced following AD. Although half the
volatile content of the sludge is converted through AD,
moisture content in the digestate after dewatering is
similar to that of dewatered raw sludge. Sludge drying
not only represents a high energy penalty for thermal
conversion, but is also subject to technical limitations,
principally those related to the poor solid properties of
mildly dried sludge (35–40% w/w moisture) (Werther
and Ogada, 1999). Combustion within WWT plants
that implement AD has been identied as sustainable
for biosolids management, since it improves the
energy balance (up to 83% energy coverage) and
reduces the carbon footprint (~ 4 kg CO2 year–1)
(Burrowes et al., 2010; Stillwell et al., 2010).
Nevertheless, investment costs for both AD and
thermal conversion can be prohibitive when compared
with those of conventional thermal technologies, such
as incineration in grate furnaces (Burrowes et al.,
2010; Shi, 2011).
The present study evaluated the energetic integration
of combustion and gasication as nal sludge
conversion technologies within WWT facilities that
include AD and/or drying as primary management
processes. Different CHP systems were considered for
the transformation of biogas and/or syngas to heat and
electricity.
A parametric optimisation of sludge/digestate
gasication or combustion was carried out using
the electrical and heat recovery efciencies of the
proposed systems for on-site energy demands,
as proposed in Chapter 2. Air gasication using
an indirect heat supply utilising char/syngas
was simulated through a pseudo-equilibrium
thermodynamic model implemented using Cantera and
MATLAB software.
The effects of operational parameters (ER, TGS and
sludge drying extent) and of fuel properties [hydrogen
to carbon molar ratio (H/C), oxygen to hydrogen
molar ratio (O/H) and ash content] on the efciency
of the thermal conversion were evaluated, as well as
the effects of the operational conditions for the CHP
components. An economic analysis was performed to
compare the COT and costs of electricity (COE), when
produced, to determine the most suitable process
congurations for implementation in medium to large
WWT plants.
3.4 Gasication Performance of
Sludge and Digestate
In this study, the reaction temperature of the
gasication stage (TGS) was determined using the
model of an ideal reactor in which heat was provided
by oxidation reactions involving O2. However, low
equilibrium temperatures were attained when low O2
concentrations were used (ER > 1.7–2). The reactivity
or kinetics of char gasication and water–gas shift
reactions are greatly affected by temperature. As
a result, carrying out the process at temperatures
lower than 950 K would be impractical, requiring long
reaction times to reach equilibrium or appropriate
conversions. A minimum TGS of 1073 K was maintained
to guarantee the validity of the pseudo-equilibrium
model. Figure 3.1 shows the predicted gasication
19
K. Dussan and R. Monaghan (2014-RE-DS-3)
performance as a function of ER and moisture content
(yM,2) of the SS in the reactor.
Temperatures between 1100 and 1700 K were attained
at mild gasication conditions (ER = 1.5–2.5). Above
ER = 2–2.5, an external heat source was required to
maintain the minimum TGS.
The energy content of the syngas (LHV) varied
between 2.5 and 7 MJ Nm–3 at the conditions
Figure 3.1. Performance of gasication of sludge as a function of the equivalence ratio and moisture
content of the sludge: (a) gasication temperature (TGS); (b) low heating value of syngas; and (c) fraction
of syngas used in the combustion module to provide heat to gasication.
1700
1500
1300
1100
2.5
3.0
3.5
4.0
4.5
5.0
5.5 6.0
6.5
0.010
0.10
0.20
0.30
0.40
0.50
1.5 2.0 2.5 3.0 3.5 4.0
10
20
30
40
50
Moisture content in sludge yM,2, wt%
Equivalence ratio ER
1100
1300
1500
1700
1900
TGS, K
TGS,min = 1073 K
a)
1.5 2.0 2.5 3.0 3.5 4.0
10
20
30
40
50
Moisture content in sludge yM,2, wt%
Equivalence ratio ER
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
LHV dry syngas,
MJ Nm-3
b)
1.5 2.0 2.5 3.0 3.5 4.0
10
20
30
40
50
c)
Moisture content in sludge yM,2, wt%
Equivalence ratio ER
0.010
0.10
0.20
0.30
0.40
0.50
0.60
Fraction of syngas
used for gasification, xGS
ER
ER
ER
20
Modelling of Energy Recovery Options from Wastewater Treatment Plant Digestate
evaluated. Low moisture contents and high ERs
resulted in improved syngas quality. At low ERs,
oxidation pathways prevailed, leading to the formation
of CO2, H2O and low-LHV syngas. Under these
conditions, the water content of the feed had a minor
inuence on syngas composition. At high ERs, the
negative effect of moisture content was evident.
High concentrations of water or moisture altered the
equilibrium of the water–gas shift reaction, with the
result that CO was consumed and CO2 was produced,
leading to a decrease in the syngas LHV.
The unreacted carbon (char) and a fraction of the
syngas (xGS) were combusted in the combustor
component of the indirect gasier to reach the
minimum gasication temperature at high ERs. The
syngas fraction used for the SS gasier reached
over 50% at ER = 4 and yM,2 = 50%. Because the
syngas quality was higher at low moisture contents, a
smaller amount of syngas was required under these
conditions. For digestates, the heating value of the
produced syngas was reduced to 2–6 MJ Nm–3. The
xGS syngas fraction used to heat the gasier was then
required at lower ERs than those when using SS. At
ER = 4, between 20% and 75% of the syngas was
combusted to maintain the minimum TGS during the
gasication process.
3.5 Integration of CHP Technologies
for Energy Recovery in WWT
Plants
Eight different energy recovery technologies were
evaluated within the case scenarios of the WWT plant
proposed in Figure 2.6. Firstly, only thermal conversion
pathways (Cases TC1–4) were considered, where
sludge was the feed to the system:
●Case TC1: sludge combustion and a steam cycle
were used to recover heat as steam and electricity
through a steam turbine.
●Case TC2: sludge gasication and a syngas-
fuelled boiler (HRSG) were used to recover heat
as steam and electricity through a steam turbine.
● Case TC3: sludge gasication and a syngas-
fuelled reciprocating ICE were used to recover
electricity and heat from the exhaust gases.
● Case TC4: sludge gasication and a syngas-
fuelled gas turbine were used to recover electricity
and heat from the exhaust gases.
In addition, AD was considered a rst stage for the
thermal conversion for the anaerobic digestion coupled
with thermal conversion (ADTC) alternatives:
●Case ADTC1: sludge was digested and the
digestate was dried and combusted. Biogas was
combusted. Heat was recovered to generate
steam used in steam turbines and in process
demands.
●Case ADTC2: sludge was digested and the
digestate was dried and gasied. Syngas and
biogas were used in a boiler to generate steam
and electricity in a steam turbine.
●Case ADTC3: sludge was digested and the
digestate was dried and gasied. Biogas and
syngas were used as fuel in an ICE to generate
electricity. Heat was recovered from the exhaust
gases.
●Case ADTC4: sludge was digested and the
digestate was dried and gasied. Biogas and
syngas were combusted in a gas turbine to
generate electricity. Heat was recovered from the
exhaust gases.
The scenario in which only AD and drying of the
dewatered sludge were performed was used as a
benchmark and will be referred to as Case AD.
The comparison of the energy recovery technologies
was carried out using a range of performance
indicators. For the thermal conversion, the SS and
sludge digestate properties presented in Table 2.2
were used. ER and feed moisture content (yM,2) were
used as the main variable parameters for combustion
and gasication. For the steam cycle, the maximum
steam temperature (TST) and pressure (PST) were also
varied to observe the performance of power generation
and heat recovery. For gas turbines, the pressure
ratio (PRGT), and the inlet gas temperature (TIGT) and
pressure (PIGT) were also varied. Variation in ICE
performance was included in the calculation because
of the dependence of power efciency and heat
recovery on the theoretical installed capacity. Figure
3.2 shows the correlations between the heat recovery
(ηhr), power efciency (ηel) and electrical efciency (ηel)
for Cases TC1–4 and Cases ADTC1–4.
In this analysis, it was observed that:
●There was an inverse proportionality between heat
recovery and electricity generation.
21
K. Dussan and R. Monaghan (2014-RE-DS-3)
● Heat recovery efciencies of between 35% and
75% were achieved, with corresponding power
efciencies of between 2% and 40%.
● When gasication was performed, integration of
AD with thermal conversion increased electricity
generation by 30–70% for a given heat recovery
efciency.
●AD integration with combustion and steam cycles
marginally increased net electricity generation
while improving heat recovery by 15%.
●Poor power generation with high heat recovery
was predicted for Cases TC1 and TC2 and for
Cases ADTC1 and ADTC2, because of the low
power generation capacity of the steam turbines.
●Cases TC3, TC4, ADTC3 and ADTC34 reported
higher combined electricity generation and
heat recovery efciencies due to higher power
efciencies (20–40%) than those of steam turbines
(5–12%) at these scales (3–8 MWel).
Figure 3.3 shows the correlation between heat
coverage (Chr) and electricity coverage (Cel) of the
energy demands of the WWT plant for each case.
Coverage factors below 100% represent a system
conguration in which the energy harvested from the
sludge was not sufcient to cover utility demands (heat
and electricity decits) for the whole plant, whereas
values above 100% indicate an overall energy surplus
that could be used to provide electricity to the grid or
heat for external demands, e.g. district heating.
In relation to the coverage of energy demands, it was
found that:
●The highest power coverage was attained only
when heat coverage was lower than 100%. This
implied that additional fuel would be required to
offset heat demands during operation.
●Cases TC1–4 and ADTC1–4 reported higher
efciencies for power generation and greater heat
integration exibility on site than for Case AD.
●A certain window of conditions for Cases TC3,
ADTC3 and ADTC4 resulted in excess electricity
and heat (top right quadrant of Figure 3.3). Cases
TC3 and TC4 fullled between 30% and 135% of
electricity demands, while providing 80–150% of
the required heat.
●The cases TC1, TC2, ADTC1 and ADTC2 resulted
in scenarios in which up to 45% of the electricity
on site was provided, and heat demands, including
sludge drying and AD heat, were fullled in excess
(100–180%).
●Steam turbine systems led to high excess heat
recovery rates of up to 5 MJ kg–1 or 1400 kWh t –1
dry sludge, in addition to process demands. To put
it in context, a sludge conversion facility (130 tpd)
relying on combustion/gasication and steam
cycle could provide heat to 5000 household units
0 25 50 75 100
0
20
40
60
Case TC1 ADTC1 Case TC2 ADTC2
Case TC3 ADTC3 Case TC4 ADTC4
Case AD
Power generation efficiency
el
, %
Heat recovery efficiency
hr
, %
Figure 3.2. Total energy recovery efciency (ηel + ηhr) and electrical efciency (ηel) for Cases TC1–4 (full
symbols) and ADTC1–4 (empty symbols).
22
Modelling of Energy Recovery Options from Wastewater Treatment Plant Digestate
(13,000 kWh year–1 per dwelling) (Howley et al.,
2015).
●Using steam turbines, combustion led to a more
efcient scenario than gasication, achieving
similar heat to and higher electricity coverage than
Cases TC2 and ADTC2.
● The higher chemical efciency provided by the
formation of methane through AD simultaneously
increased the heat and electricity coverage,
as well as the nominal CHP capacity in Cases
ADTC3 and ADTC4.
●Cases TC4 and ADTC4 reported lower net
electricity generation than Cases TC3 and
ADTC3. Within the range of CHP capacities
explored (3–8 MWel), gas turbines reported lower
efciencies (25–35% LHV) than ICEs (40–42%
LHV) (Darrow et al., 2015).
3.6 Economic Performance of
Thermal Conversion Systems
Integrated with AD
Figure 3.4 shows the specic capital investment (SCI)
and the COT for the different technologies as functions
of their corresponding total energy recovery efciency.
It was found that:
●Lowest SCI costs were reported by Cases TC1
and ADTC1 (€280,000 and €430,000 tpd–1,
respectively). Given the trajectory and
development stage of combustion technologies,
investment costs were 50% lower for Case TC1
than for Case TC2 (€565,000 tpd–1). For a 20 MWth
biomass conversion process, installation costs of
combustion units are up to 2.8 times lower than
those of FB gasiers (Bridgwater et al., 2002).
●The SCI costs for Cases TC3, TC4, ADTC3
and ADTC4, were between €740,000 and
€1,050,000 tpd–1. Investment costs were more
sensitive to process conditions.
●Cases ADTC1–4 had SCI costs that were higher
than in the corresponding Cases TC1–4. In
particular, Case ADTC1 reported SCI costs that
were 60% greater than those of Case TC1. SCI
costs of cases ADTC3 and ADTC4 were 25–30%
higher than in the corresponding Cases TC1–4.
●Similar trends were observed for COT, considering
that annual O&M costs constituted between 3%
Figure 3.3. Heat coverage (Chr) and electricity coverage (Cel) for Cases TC1–4 (full symbols) and Cases
ADTC1–4 (empty symbols).
0 50 100 150 200
0
50
100
150
200
Electricity
+ Heat
surplus
Case TC1 ADTC1 Case TC2 ADTC2
Case TC3 ADTC3 Case TC4 ADTC4
Case AD
Electricity coverage C
el
, %
Heat coverage C
hr
, %
Electricity
+ Heat
deficit
Electricity
surplus
Heat
surplus
23
Figure 3.4. Specic capital investment (SCI) and cost of treatment (COT) as functions of the total energy recovery efciency of the thermal conversion
systems.
20 40 60 80 100 120 140 160
0
400
800
1200
1600
2000
20 40 60 80 100 120 140 160
0
100
200
300
400
500
0 50 100 150 200
1
10
100
1,000
Specific Capital Investment SCI
tpd
,
2015
tpd
-1
sludge
Total energy coverage C
tot
, %
a)
Biomass & wastes
AD-CHP plants
20 kW
el
- 2 MW
el
Cost of treatment COT,
2015
t
-1
sludge
Total energy coverage C
tot
, %
b)
AD-CHP plants
20 kW
el
- 2 MW
el
Biomass & wastes
TC1 ADTC1 TC3 ADTC3 TC3 ADTC3 TC4 ADTC4
24
Modelling of Energy Recovery Options from Wastewater Treatment Plant Digestate
and 20% of the total capital costs of the complete
thermal conversion/ADTC systems.
●Cases TC2, TC3, ADTC2 and ADTC3 reported the
lowest treatment costs (€145–160 t –1 sludge).
●Using syngas for the steam cycle reduced the
COT by up to 25% (€160–200 t –1 sludge).
●Case TC4 reported COTs between €190 and
€300 t –1 sludge, while introducing AD increased
these costs by 40%.
●Conditions at which electricity generation was
maximised in Cases TC3 and TC4 led to an
increase in the COT, while conditions at which
their COT was minimised resulted in low electricity
production.
It is also necessary to analyse the potential of the
sludge management site to operate as an electricity
generation facility. For this analysis, SCI costs were
expressed in terms of potential power generation and
the LCOE was estimated. Figure 3.5 shows SCI per
kW (SCIkW) and COE as functions of the electrical
efciency of these systems. It was found that:
● Congurations using gasication had a direct
correlation between SCIkW and the efciency of the
system, regardless of the CHP technology.
●For electricity coverage above 100%, SCIkW
reached a minimum value of €40,000 kW–1, while
costs as high as €200,000 kW–1 were observed at
the lower end of the efciency scale.
●All SCIkW costs at high energy coverage levels
were well above the range of investment costs
reported for AD-based plants using biomass
and wastes, commonly between €5000 and
€30,000 kW–1 (Hjort-Gregersen, 1999; Walla et al.,
2006).
● The combination of gasication with AD increased
the SCI costs of Cases TC3 and TC4 by less than
20% for any electrical efciency level.
●Similar trends were observed for COE. Cases
TC3 and ADTC3 with high electrical efciencies
reported COEs between 20 and 45 c kWh–1.
● The higher electrical efciency achieved by the
implementation of AD led to lower electricity costs.
●COE for Cases TC3 and ADTC3 were within
the known COE from AD-CHP plants converting
biomass and wastes, commonly between 5
and 52 c kWh–1 (Krich et al., 2005; Walla et al.,
2006; Beddoes et al., 2007; MacDonald, 2010;
Arup, 2011; US EIA, 2015). However, thermal
conversion technologies offer the advantage
of operating with capacities greater than 2 MW,
unlike AD plants.
●CHP technologies for biomass-based electricity
generation have reported potential levelised costs
between 9 and 20 c kWh–1, which are in agreement
with the estimations presented in this report (Arup,
2011; US EIA, 2015).
3.7 Carbon Emissions Due to WWT
Plant Operation with Thermal
Conversion Systems Integrated
with AD
Net carbon emissions were also estimated as kg CO2
equivalent per m3 of wastewater treated in the WWT
facility. Only emissions associated with electricity
and heat consumption/production were estimated,
without taking into account intrinsic emissions from
biological WWT, indirect emissions due to chemicals/
biomass usage or actual CO2 stack emissions from
the CHP module. As a reference, biogenic carbon
emissions associated with biological treatment and
nitrogen removal from wastewater as treated in the
Ringsend plant were estimated to be approximately
350–370 g CO2 m–3 (RTI International, 2010).
Figure 3.6 shows the associated net carbon emissions
as a function of the electrical and heat recovery
efciencies of Cases TC1–4 and Cases ADTC1–4.
Given the operational parameters dened in Table
2.1, the WWT facility considered here would also
emit about 367 g CO2 m–3 (890 kg CO2 t –1) as per the
emission factors dened for the Irish energy mix,
including emissions associated with AD and sludge
drying (20–35 wt% as nal moisture content). The
introduction of a CHP module for energy recovery
in the base case would reduce carbon emissions to
146–215 g CO2 m–3 or 350–520 kg CO2 t –1 dry sludge,
depending on CHP efciency and the extent of drying.
The introduction of thermal conversion as a treatment
had signicant impacts on the overall carbon footprint:
●Cases TC1, TC2 and TC3 and Cases ADTC1,
ADTC2 and ADTC3 showed an overall carbon
footprint that was lower than the maximum carbon
footprint of the base case scenario.
●Carbon footprints above 220 g CO2 m–3 were
reached when the electrical efciency was
maximised in all cases.
25
Figure 3.5. Specic capital investment (SCI) and costs of electricity as functions of electrical efciency and net electricity generation for Cases TC3, TC4,
ADTC3 and ADTC4.
0 50 100 150 200
1
10
100
1,000
0 50 100 150 200
1
10
100
1,000
Biomass & wastes
Specific Capital Investment SCI
kW
,
2015
kW
-1
Electricity coverage C
el
, %
AD-CHP plants
20 kW
el
- 2 MW
el
a)
TC1 ADTC1 TC3 ADTC3 TC3 ADTC3 TC4 ADTC4
Biomass & wastes
Cost of electricity COE,
2015
kWh
-1
Electricity coverage C
el
, %
AD-CHP plants,
20 kW
el
- 2 MW
el
b)
26
Modelling of Energy Recovery Options from Wastewater Treatment Plant Digestate
●The use of gas turbines led to an increase in the
carbon footprint, due to electricity consumption
in auxiliary equipment. For certain conditions,
Cases TC4 and ADTC4 reached carbon emission
rates of between 170 and 340 g CO2 m–3 (400–
800 kg CO2 t –1 sludge), well above the expected
emissions of the WWT–AD facility with energy
recovery.
3.8 Advantages and Disadvantages
of Thermal Conversion Process
Congurations
Some advantages and challenges were identied in
the application of the proposed cases:
●Thermal conversion routes facilitate waste
disposal by reducing the net amount of nal
solid waste (y-ash). For AD, only 40–50% of the
chemical energy contained in the sludge can be
converted to biogas. For a methane productivity
of 210 Nm3 t –1 of volatile solids (VS), about 25% of
the solid is converted to biogas. Given appropriate
process optimisation, combustion and gasication
converts over 80% of the organic content of
biomass and wastes to energy carriers, leaving
only a small fraction of unreacted carbon (char)
and stabilised ash for nal disposal, thus reducing
waste disposal costs.
●In this study, optimal operation conditions were
such that a sufciently rich syngas was produced
with an appropriate drying heat penalty. This is
particularly important for ICEs, in which heating
value and syngas productivity ultimately dictate
efciency.
●For other CHP modules, however, other
parameters also played important roles in
determining the nal efciency. For gas turbines
(Cases TC4 and ADTC4), the inlet pressure of
the syngas to the turbine (PIGT), as well as the
pressure ratio (PRGT), determine the extension
of the energy recovered in the generator.
Nonetheless, nal efciency will be affected by the
performance conditions at which CHP modules
for low energy content syngas are commonly or
potentially built for.
●Special designs are required in gas turbine
combustors to manage low- to medium-BTU gas
fuels (100–500 BTU scf–1, 3.5–20 MJ Nm–3). These
modied systems are currently available only for
large installations (> 100 MWel) (Taamallah et al.,
2015). High H2 concentrations and large variations
in syngas composition can lead to signicant
changes in the transport and thermochemical
properties of the gas. Further development of this
technology is required to make syngas-fuelled
turbines available for smaller scales (1–100 MWel),
with competitive costs and efciencies.
Figure 3.6. Net emissions associated with Irish energy for sludge treatment in Cases TC1–4 and Cases
ADTC1–4.
0 50 100 150 200
0
100
200
300
400
30 90 150 210 270
0
100
200
300
400
TC1 ADTC1 TC2 ADTC2
TC3 ADTC3 TC4 ADTC4
WWT + AD + Drying
Energy recovery
Net carbon emissions,
g CO2m-3
Heat coverage, %
WWT + AD + Drying
No energy recovery
0
200
400
600
800
0
200
400
600
800
kg CO2t-1 dry sludge
27
K. Dussan and R. Monaghan (2014-RE-DS-3)
●ICEs with low to mid-range compression ratios
and direct injection systems can be readily
adapted to low-BTU gases, either in dual fuel
diesel engines (≤ 90% syngas) or syngas-only
spark ignition engines (Hagos et al., 2014).
3.9 Optimisation of Energy Recovery
Systems Using Gasication and
CHP Modules
Thermal conversion following gasication with an ICE
was the scenario selected for further analysis because
of its energy efciency, inexpensive operational costs
and low carbon footprint. Table 3.1 gives a summary of
the process conguration and performance indicators
under which net electricity balance was maximised.
For this system, it was observed that:
●The combined implementation of AD and
gasication doubled electricity generation and
electricity coverage.
● Carbon mitigation was signicant and offered
great potential in diminishing the environmental
effects of WWT.
It is important to highlight that uncertainties concerning
the sludge properties, sludge production and process
control are important in recognising challenges prior
to process design and during process operation. The
sensitivities of the performance indicators (ηel, ηhr, Cel
and Chr) to variations in process parameters within
± 20% of the operational range are presented in Figure
3.7. The base case scenario considered the sludge
properties shown in Table 2.2 and the conditions
shown in Table 3.1. In this analysis, it was observed
that:
Table 3.1. Process congurations for maximising electricity generation for internal combustion engines
Process conguration Gasication AD + gasication Units
Energy and emission indicators
Electricity coverage (Cel) 133.2 164.2 %
Heat coverage (Chr) 83.3 107.4 %
Gross electrical efciency (ηel) 30.6 39.1 %
Gross heat recovery efciency (ηhr) 45.2 46.0 %
Carbon emissions 332 169 kg CO2 t –1
Process gures
Dried sludge feed rate 130 130 tpd
Dry syngas production 3.4 3.2 m3 s–1
LHV of syngas 4.8 5.1 MJ Nm–3
CHP design capacity 6.3 8.0 MW
Auxiliary power 294 352 kW
Gas treatment power 8.5 7.6 kW
WWT power 4.6 4.8 MW
Recovered heat 11.7 12.4 MW
Auxiliary heat 11.3 8.5 MW
Gas treatment heat 2.4 3.1 MW
Economic indicators
Capital costs 137.8 166.5 M€
O&M 7825 7663 k€ year–1
Specic investment costs 21.9 20.8 k€ kW–1
1060 1281 k€ tpd–1
COT 165 161 € t –1
COE 26.7 22.8 c kWh–1
28
Modelling of Energy Recovery Options from Wastewater Treatment Plant Digestate
● The power efciency was strongly affected by ER
following a non-linear correlation. An increase of
ER led not only to an improvement in the chemical
efciency of the conversion, but also to an
increase in the heat demands of the gasier.
● The heat recovery efciency was slightly
decreased with a lower ER (–2.5%). However, an
increase above 5% for this variable did not affect
ηhr. Changes in ER induced similar effects on Chr
as for ηhr.
●Increasing TGS led to an improvement of the
heat recovery by promoting heat exchange
from syngas. When using TGS = 1123 K, the heat
recovery efciency increased by more than 3%.
●Electricity coverage was affected mainly when
reducing ER and increasing TGS. A decrease of
10% in these variables (ER = 2.2, TGS = 1023 K)
decreased the power coverage by 8% and 3%,
respectively.
●Higher ash and oxygen contents in the sludge
reduced the power and heat coverage by 4%
to 9%, which was a result of the effects these
properties have on the sludge energy content.
●The initial sludge moisture content (yM,1) affected
the heat coverage by modifying the heat duty of
the drying stage. When using yM,1 = 77%, the Chr
was signicantly reduced (–10%).
● The nal sludge moisture content (yM,2) had minor
effects on the heat coverage within the evaluated
range (the effects were less than ± 3%).
●Sludge feed rate affected the electrical and heat
recovery efciencies marginally; an increase of
20% increased electrical efciency and reduced
heat recovery efciency by less than 1.5%,
respectively.
Figure 3.7. Sensitivity of efciencies and energy coverage levels using gasication and internal
combustion engines as functions of process parameters: equivalence ratio (ER), sludge moisture
contents (yM,1 and yM,2), gasication temperature (TGS), sludge feed rate, and sludge properties (O/C, H/C
and ash content).
-20 -10 0 10 20
-6
-3
0
3
6
-20 -10 0 10 20
-6
-3
0
3
6
-20 -10 0 10 20
-20
-10
0
10
20
-20 -10 0 10 20
-20
-10
0
10
20
y
M,1
Variation of efficiency
from base case, %
Variable range, %
Base case and evaluation range:
ER = 2.5 [2.2-2.8], y
M,1
= 75% [73-77%],
y
M,2
= 10% [6-14%], O/C = 0.46 [0.43-0.49],
H/C = 1.69 [1.61-1.77], Ash = 30% [24.5-34.5%],
T
GS
= 1073 K [1023-1123K]
Equivalence Moisture, Moisture, O/C H/C
Ash Gasification Feed rate, m
sludge
ratio, ER y
M,2
temperature, T
GS
η
el
Variation of efficiency
from base case, %
Variable range, %
η
hr
Variation of coverage
from base case, %
Variable range, %
Electricity coverage
Variation of coverage
from base case, %
Variable range, %
Heat coverage
29
K. Dussan and R. Monaghan (2014-RE-DS-3)
●An increase in msludge from 130 to 156 tpd improved
Cel by 20%. This would apply to the case in which
the sludge feed rate is increased without altering
the electricity consumption. A further examination
of the sludge feed rate is presented in Appendix 2.
●Sludge properties affected Chr because of their
inuence on the sludge energy content. When the
ash content increased from 29.5 to 34.5%, the
heat coverage decreased by 6%. Increasing the
O/C and H/C molar ratios slightly affected the heat
recovery (< 2%).
The contribution of heat demands to COT, COE
and the carbon footprint was higher than that of
electricity demands under the examined conditions
(intermediate ER, low yM,2). This is particularly
important when considering variations not only in the
inuent wastewater quality, but also in the consequent
sludge properties and process demands required to
meet efuent requirements. Variations can include
higher inorganic and initial moisture contents (yM,1),
which are directly connected to the heat demands.
For further consideration, Appendix 3 and Appendix 4
present additional analyses on the sensitivity of other
performance indicators, such as syngas LHV, cold gas
efciency (CGE), COT, COE and carbon emissions.
3.10 Biomass and Waste Co-processing
for Improving Energy Efciency
and Reducing Carbon Footprint
The use of biomass and waste in co-processing with
the on-site sludge was considered to improve the
process efciency. Two biomass materials, willow
pellets (WIL) and Miscanthus (MIS), as well as poultry
litter (PL) were used (Table 2.2). WIL and MIS were
assumed to be commercially available in Ireland, at
costs of €200 and €75 per tonne, respectively. For PL,
a minimum gate fee of €65 t –1 was considered to be
established by the levy for waste disposal at landlling
facilities in Ireland (EEA, 2013; Government of Ireland,
2013). However, it would be necessary to consider
the costs associated with the acquisition of PL if this
becomes a fuel commodity in the future.
The objective of this analysis was to evaluate the
biomass-to-sludge mass ratio (B:SS) required to offset
the heat demands of the process and reduce the
carbon footprint. Given the associated biomass costs
or fees, the effect of biomass co-processing on COE
was also considered. Figure 3.8 shows Chr, net carbon
emissions and COE as functions of the amount of
co-processed biomass. It was observed that:
●The implementation of biomass or waste
co-processing allowed an increase in the installed
capacity of the CHP system. Feeding equal
quantities of biomass and sludge (B:SS = 1)
increased the capacity over 10 MWel, depending
on the biomass heating value.
●Levels of at least B:SS = 0.18 for WIL or MIS and
B:SS = 0.3 for PL were required to reach complete
heat coverage.
●The addition of biosolids reduced the carbon
footprint, resulting in negative values when using
B:SS > 0.5. These negative values were effectively
equal to zero carbon footprints because of plant
energy demands.
●The type of biomass used during co-processing
was important in dening treatment costs. High-
cost biomass, such as WIL, led to an increase in
the COE of 14%, from 26 to 31 c kWh–1. A cheaper,
high-energy content biomass, such as MIS,
reduced the costs by 5%.
●The associated gate fee for PL decreased the
operational costs and led to a reduction of COE by
25%, to 20 c kWh–1 when 130 tpd PL was used.
The process economics could be further improved
by taking into consideration government aids for
renewable energy generation sites and facilities.
The REFIT scheme (Renewable Energy Feed in
Tariff) in Ireland subsidises electricity sold to the
grid, offering tariffs of 15 c kWh–1 (≤ 500 kWel) or
13 c kWh–1 (> 500 kWel) to plants using AD-CHP and
other biomass-based technologies (Department
of Communications, Energy & Natural Resources,
2015). It is also important to highlight that the indirect
carbon footprint of biomass materials due to land use
and market displacement, biomass plantation and
harvesting, transport and imports are expected to shift
these reductions above the zero carbon footprint limits.
These effects were outside of the scope of the present
study.
30
Modelling of Energy Recovery Options from Wastewater Treatment Plant Digestate
Figure 3.8. Process performance as a function of the biomass to dry sludge mass ratio used in co-
processing in a system using gasication and an internal combustion engine.
0.0 0.2 0.4 0.6 0.8 1.0
50
100
150
200
0.0 0.2 0.4 0.6 0.8 1.0
0
10
20
30
40
50
60
70
0.0 0.2 0.4 0.6 0.8 1.0
-200
0
200
400
600
Heat coverage, %
Biomass to sludge ratio, B:SS
WIL
MIS
PL
Levelised cost of electricity,
2015/kWh
Biomass to sludge ratio, B:SS
Net carbon emissions,
kg CO2MWh-1
Biomass to sludge ratio, B:SS
WIL
MIS
PL
6 8 10 12
Installation capacity, MWe l
Max
11.5-12.0 MW
PL
Gate fee €65t–1
WI, €210t–1
MI, €75t–1
Max
11.5–12.0 MW
31
K. Dussan and R. Monaghan (2014-RE-DS-3)
3.11 Concluding Remarks
●This study undertook a thermodynamic evaluation
of sludge and digestate gasication in WWT plants
as means of sludge volume reduction and energy
recovery.
●The study was extensively supported by empirical
data for relevant pilot and full-scale processes,
including AD, FB gasication, combustion, solids
drying, syngas and ue gas treatment and WWT.
●It was found that integration of energy recovery
from sludge through thermal conversion was
feasible using conventional CHP generation. In
particular, the combination of AD and gasication
could theoretically enable a WWT facility to be
operated with electricity and heat production in
excess of on-site demands.
● ICEs offered sufcient power efciency and
exibility for adapting them in the process
conguration at the plant scale considered in this
study (130 tpd dry sludge, 3–6 MWel).
● Through either gasication or combined AD–
gasication, treatment costs between €55 and
160 t –1 dry sludge were achieved, which are
competitive with European landlling costs and the
operational costs of AD plants.
●The LCOE was within reported costs of electricity
for AD–CHP plants (23–27 c kWh–1) and offer an
opportunity for WWT facilities to implement these
sludge treatments.
●These applications reported heavy capital
expenditures (> €100 million). One of the potential
alternatives to reduce the SCI and improve the
energy balance was co-processing of sludge
with biomass and other wastes. Feeding similar
quantities of biomass with a richer energy content
and sludge neutralised the process carbon
footprint, increased electricity and heat recovery
efciencies, and reduced the COE by up to 35%.
32
4 Conclusions and Recommendations
The WWT sector is in constant need of technological
and economic advancement to deal with the
envisaged increase in the stringency of emission
limits, population growth, urbanisation and changes in
industrial/agricultural practices. WWT technologies are
required to have high pollutant removal efciencies,
to be framed in a sustainable system with minimum
impact on the environment and to be economically
competitive.
Water management entities are adapting technologies
or shifting to new strategies in which self-sufciency
can be guaranteed at all times (Rygaard et al., 2011).
In this context, this project investigated a series
of technologies, i.e. thermal conversion and CHP
generation, as new strategies to employ wastewater
wastes, i.e. sludge and digestate, for energy recovery
on site.
After an evaluation of the current state-of-the-art
combustion and gasication, different process
conguration scenarios were evaluated to nd
conditions under which complete coverage of the
on-site energy demands could be met or even
exceeded.
A modelling tool was built that explored the use of
these technologies in WWT facilities, with and without
AD. Potential energy generation as power and heat,
coverage of on-site demands, COT/power generation
and the carbon footprint were considered performance
indicators of the feasibility and sustainability of the
proposed alternatives.
4.1 Combustion and Gasication
Coupled with Steam Turbines
Although combustion (incineration) is a well-
established and less expensive approach, process
conguration based on this process did not cover
on-site electricity demands for WWT treatment and
sludge processing. Over 15% of the energy contained
in the sludge was required to meet electricity
demands. Despite this, a sludge conversion facility
of this type (1.6 Mp.e. WWT plant) could provide heat
for up to 5000 household units, given proper heat
recovery optimisation. The overall carbon footprint
was within those observed in WWT treatment facilities
undertaking energy recovery using AD (< 500 kg CO2 t –1
dry sludge).
4.2 Gasication Coupled with Gas
Turbines
It was feasible to produce electricity and heat in
excess of on-site demands using AD, gasication
and gas turbines (~ 5–20% surplus energy in sludge).
However, extensive energy use in auxiliary equipment
for fuel gas treatment increased COE generation
(> 50 c kWh–1) and the potential carbon footprint of the
operation of this plant (> 600 kg CO2 t –1 dry sludge).
Likewise, further technological advances are required
in gas turbines to manage low-BTU fuels, such as
syngas and combinations of syngas and biogas, with
lower installed capacities than the current available
units (only > 100 MWel).
The use of gas turbines in waste-to-energy facilities
may become more suitable in the future for sites in
which a high volume of waste is co-processed with
other biomass resources (> 500 tpd).
4.3 Gasication and AD–Gasication
Integrated with Internal
Combustion Engines
It was feasible to produce electricity and heat in
excess of on-site demands using gasication coupled
with ICEs, if all the process congurations were
functioning with the highest possible efciency. An
important feature is that AD integrated with gasication
gave great exibility for thermal recovery, leading to
conditions in which high surpluses of both electricity
and heat were achieved.
These two approaches also offered low operating
costs (€150–170 t –1 dry sludge) and costs of electricity
generation (20–50 c kWh–1), with competitive carbon
footprint levels (< 300 kg CO2 t –1 dry sludge), even
lower than that of WWT facilities using only AD energy
recovery.
33
K. Dussan and R. Monaghan (2014-RE-DS-3)
As an additional advantage, ICEs offer exibility in
terms of scalability for energy recovery at scales
expected in WWT facilities (> 10 MWel) with competitive
power efciencies.
Co-generation also represented a potential alternative
to offset heat demands when gasication alone was
used for sludge conversion, requiring biomass rates
of 0.2 to 0.3 times that of the sludge feed rate to meet
energy demands and give a reduced carbon footprint.
4.4 Recommendations for Future
Work
4.4.1 Economies of scale
This report highlights the importance of facility scale
in meeting sustainability criteria, especially in terms
of operational and capital expenditures. The gures
presented here were applied to the scale of the
largest WWT facility in Ireland (1.6 Mp.e.), which
currently produces about 50 tpd of dry digestate or
an estimated 80–100 tpd dry sludge. However, most
current WWT facilities using secondary treatment
in Ireland have capacities below 10,000 p.e., with
potential sludge production that can vary between 0.2
and 3 tpd dry sludge. In contrast, most Irish AD plants
process between 26,000 and 400,000 p.e., generating
approximately 6–100 tpd dry sludge on site.
Although combustion engines and boilers offer
sufcient exibility to operate with nominal capacities
on these scales (100–700 kWel), installation costs
would make the implementation of combustion or
gasication economically unattractive. Typical sludge
incinerators are designed for processing 30 to 700 tpd,
depending on the reactor type (FB, 30–200 tpd; moving
grate, 120–700 tpd), while gasiers are restricted
to throughputs of between 250 and 500 tpd (EC,
2006). To date, however, incineration and gasication
plants have average capacities of between 160 and
1300 tpd in countries including Denmark, Germany, the
Netherlands, Norway and the UK.
In addition, this study determined that sludge feed
rates of at least 120 tpd raw sludge were required to
generate electricity for at least 24 c kWh–1, which is
within the range of competitive renewable electricity for
Ireland. From the perspective of waste management,
feed rates of at least 25 tpd were required to account
for treatment costs below €250 t –1. However, this would
rely on heavy capital expenditure and fees that would
probably be directed to tax-payers.
This issue may be overcome through other
approaches that are suggested for future
consideration. On-site thermal pretreatment, such as
drying and torrefaction, can facilitate sludge transport
to a centralised facility. Although these treatments
require energy, the electricity and heat recovery
produced by a large-scale centralised facility could
probably offset the treatment costs in terms of overall
energy and carbon footprint. A centralised gasication
facility also offers the possibility of implementing
biomass or waste co-processing with greater economic
and technical feasibilities. Co-processing has the
advantage of reducing operational challenges seen
in decentralised plants, which arise from seasonal
variations in sludge generation and physical/chemical
properties of the waste.
Other advantages offered by co-processing include
the opportunity for utilising non-recyclable waste,
which would facilitate metal and inorganic materials
recovery through thermal conversion, using plasma
gasication and vitrication. These processes allow
the sequestration of toxic heavy metal elements that
are present in the waste incineration ash produced by
high-temperature treatment that allows the formation
of stable and uniform glass products.
It is also important to note that sludge transport and
biomass co-processing will have additional direct
and indirect energy and costs penalties, as well as
increasing the carbon footprint, which must be taken
into account in the evaluation of an optimal sludge
transport and treatment system. Biomass in particular
has direct and indirect carbon emissions linked to
harvesting, use of fertilisers, change in land use,
imports and transport, which were not considered in
the present study but may be signicant at larger plant
scales and greater biomass-to-sludge co-processing
ratios than those evaluated here.
4.4.2 Optimisation of anaerobic digestion for
biogas production
An additional aspect to be considered during the
integration of AD with thermal conversion for sludge
management is the importance of optimising the
efciency of sludge AD. It is possible to increase
methane productivity by co-digestion with other
34
Modelling of Energy Recovery Options from Wastewater Treatment Plant Digestate
readily degradable matter, such as organic fractions of
municipal solid wastes, grease, food waste and animal
wastes (Davidsson et al., 2007; Iacovidou et al., 2012).
Co-digestion reduces investment expenditure by
requiring a greater scale for the digestion stage and
a lower scale for thermal conversion, while improving
the CHP efciency in proportion to the increase in
biogas production from the addition of other biomass.
This was outside of the scope of the present work,
but we highly recommend evaluating which wastes in
Ireland could be potentially accepted at WWT plants
for co-processing with sludge.
35
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39
Abbreviations
αTC Correction factor for total direct and
indirect costs
ηB Overall boiler efciency
ηel Electrical efciency
ηEE Electrical efciency in combustion
engine
ηhr Heat recovery efciency
ηSC Isentropic compressor efciency
ηSCP Centrifugal pump efciency
ηSGT Isentropic gas turbine efciency
ηSST Isentropic steam turbine efciency
AD Anaerobic digestion
ADTC Anaerobic digestion coupled with
thermal conversion
B:SS Biomass to sewage sludge mass
ratio
BTU British thermal unit
Cel Electricity coverage
CGE Cold gas efciency
CHP Combined heat and power
Chr Heat coverage
COE Cost of electricity
COT Cost of treatment
CPI Consumer price indices
Ctot Total energy coverage
d.b. Dry basis
DR Drying stage
ER Equivalence ratio
FB Fluidised bed
fEl Factor of equivalent carbon dioxide
emission per unit of output electricity
fH Factor of equivalent carbon dioxide
emission per unit of heat used
FOB Free-on-board
fPE Factor of primary energy consumed
per unit of output electricity
fPH Factor of primary energy consumed
per unit of heat used
GS Gasication
GT Syngas treatment
H/C Hydrogen to carbon molar ratio
HE Heat exchanger
HR Heat recovery
HRSG Heat recovery–steam generation
ICE Internal combustion engine
LCOE Levelised cost of electricity
LHV Low heating value
MDEA Methyldiethanolamine
MIS Miscanthus
MVR Müllverwertung Rugenberger
incineration plant
NOX Nitrogen oxide gases
O/H Oxygen to hydrogen molar ratio
O&M Operational and maintenance
p.e. Population equivalent
PGS Gasication/combustion pressure
PIE Inlet combustion engine pressure
PIGT Inlet gas turbine pressure
PIS Inlet steam pressure
PL Poultry litter
PRC Maximum pressure ratio in
compressor
PRGT Pressure ratio in gas turbine
Ri Other auxiliary demands
SC Steam-to-carbon
SCI Specic capital investment
SOX Sulfur oxide gases
SS Sewage sludge
TCA Air temperature to combustor
TCC Total capital cost
TDSO Temperature of SO2 scrubbing
TESP Maximum gas temperature in
electrostatic precipitator
TFG Flue gas temperature
TGA Air temperature to gasier
TGS Gasication temperature
TIGT Inlet gas turbine temperature
Tmax,ST Maximum steam temperature in
turbine
tpd Tonnes per day
TSCR Temperature of selective catalytic
reduction
VS Volatile solids
WIL Willow pellets
WWT Wastewater treatment
xGS Fraction of syngas
yM,1 Moisture content of sludge before
drying
yM,2 Moisture content of sludge after
drying
40
Appendix 1 Estimation of Costs for Thermal and AD Conversion Systems of
Sludge/Digestate
System Estimation of costs
Investment, M€ LabouraMaintenance UtilitiesbChemicals
Combustion FOB = 0.464 × Workers per shift = MWfuel × (0.3 – 0.049 ln MWfuel)3% TCC per year Estimated air blower power n/a
Gasication 5% TCC per year Estimated air blower power
and heat required to maintain
TGS; electricity, heat and water
used for ESP and H2S removal;
~ 0.06 m3 water t –1 syngas treated
Dolomite: 0.032 kg kg–1
dry fuel at €75.8 t–1;
costs of chemicals for
H2S removal:
€209 t –1 S removed
Anaerobic
digestion
Hours year–1 kW–1 = –~ 1 MW as heat
~ 185 kW as electricity
–
ICE FOB = 2.0598 × Workers per shift = M€ year –1 = 4% of generated electricity, diesel
consumption: ~ 65 g diesel Nm–3
biogas
–
Gas turbine FOB = 0.7172 × O&M, M€ year –1 = Estimated by power requirement
in compressors
–
Steam turbine FOB = 0.9013 × For MWel < 35 MW:
Workers per shift = MWel × (0.93 – 0.19 ln MWel)
For MWel ≥ 35 MW:
Workers per shift = 1.69 ×MWel
0.446
€0.0055 kWh–1 Power for pressurising water;
water use: ~ 6.5 t MWh–1
–
Combined cycle FOB = 1.821 × Combined from above
Flue gas
treatment
De-SOX: €595 kW–1
De-NOX: €350 kW–1
O&M and chemicals for de-SOX: €2.5 kg–1 removed SO2
O&M de-NOX: €3.5 kW–1 year –1
Estimated for heating and ue
gas blowers to stack
De-NOX chemicals:
€3.1 kW–1 year –1
aThe number of shifts per day was assumed to be three. The average salary for a process engineer in Ireland corresponds to €45,300 or approximately €23.6 per working hour.
bThe Irish business electricity price for band IA is 21 c kWh–1. The Irish business gas price for band I1 is 5.2 c kWh–1. The average national water charge for industry and businesses is
€2.35 m–3.
ESP, electrostatic precipitator.
MWfuel
0.769
()
=×FOBmm0.034 : kg h
fuel fuel
0.698 –1 Workers per shift =0.04 ×mfuel
0.475
()
=×FOBmm3.667 : ton d
fuel fuel
1.053 –1 140.8×kWel
−0.6
MWel
0.836 0.485×MWfuel
0.483 0.14×MWel
0.782
MWel
0.795 0.13×MWel
0.855
MWel
0.719
MWel
0.798
41
Appendix 2 Economy of Scale: Effect of Sludge Feed Rate
on Costs of Treatment and Levelised Cost of Electricity
Generation
These two economic performance factors are
affected exponentially by the capacity of the sludge
management facility in the WWT plant. Currently, in
Ireland, electricity costs vary between 9 and 21 c kWh–1
(Howley and Holland, 2015). Sludge feeding rates
greater than 120 tpd were required to reach a COE
of 24 c kWh–1, near to the electricity prices described
above. In contrast, if the sludge management facility
is considered as a waste treatment site, COT were
maintained below €250 t –1 with sludge feeding rates
above 25 tpd, which gives more exibility to the
implementation of the technology in terms of the waste
management scheme.
0 20 40 60 80 100 120
0
20
40
60
80
100
120
140
Sludge feed rate, tpd
Levelised cost of electricity COE, c kWh
-1
0
100
200
300
400
500
600
700
-1
-1
Maximum: 24c kWh
-1
Figure A2.1. Levelised costs of electricity and costs of operation of a sludge gasication plant as
functions of the sewage sludge feeding rate.
42
Appendix 3 Sensitivity Analysis of Gasication Performance
Figure A3.1. Sensitivity of the lower heating value of syngas and the cold gas efciency from gasication
as functions of the following process parameters: equivalence ratio (ER), sludge moisture contents
(yM,1 and yM,2), gasication temperature (TGS), sludge feed rate, and sludge properties (O/C, H/C and ash
content). The strongest correlation of syngas LHV and CGE was to the ER, since this dened the extent
of the fuel oxidation and, therefore, the energy content of the gas product. Increases of 11% in the dry
syngas LHV and of 6% in the CGE were observed when increasing ER by 10% within the evaluated range.
-20 -10 0 10 20
-10
-5
0
5
10
-20 -10 0 10 20
-10
-5
0
5
10
Base case and evaluation range:
ER = 2.5 [2.2-2.8], y
M,1
= 75% [73-77%],
y
M,2
= 10% [6-14%], O/C = 0.46 [0.43-0.49],
H/C = 1.69 [1.61-1.77], Ash = 30% [24.5-34.5%],
T
GS
= 1073 K [1023-1123K]
Variation LHV
from base case, %
Variable range, %
LHV dry syngas
Equivalence Moisture, Moisture, O/C H/C
Ash Gasification Feed rate, m
sludge
y
M,1
ratio, ER y
M,2
temperature, T
GS
Variation CGE
from base case, %
Variable range, %
Cold gas efficiency
43
Appendix 4 Sensitivity Analysis of Costs of Operation,
Levelised Costs of Electricity and Carbon Emissions of the
Gasication and Combustion Engine Process
●COT was mainly affected by the extent of sludge
drying, represented by the initial moisture content
(yM,1). When yM,1 was increased from 75% to 77%,
COT increased by 5% in relation to the reference
case.
● As with electrical efciency, ER and ash content
had the most signicant inuence on the costs of
electricity generation. Decreasing the ER to 2.2
led to an increase in the COE of 6.9%, while a
10% increase of the ash content raised COE by
7.8%.
●The increase of the associated heat duty for
sludge drying did not affect COE to a signicant
extent (< 2.9%). Similarly, other variations in the
sludge properties, other than ash content, had
minor effects on this economic indicator.
●Implicit to the increase in electricity generation,
a greater sludge feed rate led to reductions in
COE of up to 3% and in COT of up to 2%. This
improvement in the electricity balance illustrates
the effect that the capacity of the thermal
conversion facility can have on the techno-
economic performance of the plant. Greater
scales are linked to higher efciencies and better
economy of scales.
●The process parameters had opposite effects
on the carbon footprint compared with the CHP
coverage in the plant conguration. Decreasing
the amount of oxidising agent (higher ER)
increased net carbon emissions by only 1%;
however, reducing the ER to 2.2 led to an overall
increase in the carbon footprint of 8.4%.
● A higher gasication temperature (TGS = 1123 K)
allowed further heat savings that decreased the
carbon footprint of the process by 4% in relation to
the reference case.
●Both ash and initial moisture content (yM,1) of the
sludge affected the carbon footprint signicantly
because of the effects on electricity and heat
coverage. When the treated sludge had an ash
content of 34.5%, the wastewater and sludge
treatment plant emissions increased by 12%
from the reference case (ash = 29.5%). Similarly,
yM,1 = 77%, only 2% higher than in the original
scenario, led to carbon emissions that were 15%
greater than in the reference case.
●Other sludge properties had minor effects on the
carbon footprint (a change of less than 5%).
●The effect of the sludge feed rate on the electricity
coverage was also reected in carbon emissions:
greenhouse gases associated with operation were
reduced by more than 5%.
44
Modelling of Energy Recovery Options from Wastewater Treatment Plant Digestate
Figure A4.1. Sensitivity of cost of operation, levelised costs of electricity and carbon emissions in the
system using gasication and combustion engine as functions of the following process parameters:
equivalence ratio (ER), sludge moisture contents (yM,1 and yM,2), gasication temperature (TGS), sludge
feed rate, and sludge properties (O/C, H/C and ash content).
-20 -10 0 10 20
-10
-5
0
5
10
-20 -10 0 10 20
-10
-5
0
5
10
-20 -15 -10 -5 0 5 10 15 20
-20
-10
0
10
20
Base case and evaluation range:
ER = 2.5 [2.2-2.8], y
M,1
= 75% [73-77%],
y
M,2
= 10% [6-14%], O/C = 0.46 [0.43-0.49],
H/C = 1.69 [1.61-1.77], Ash = 30% [24.5-34.5%],
T
GS
= 1073 K [1023-1123K]
Equivalence Moisture, Moisture, O/C H/C
Ash Gasification Feed rate, m
sludge
Variation COT
from base case, %
Variable range, %
COT
ratio, ER y
M,1
y
M,2
temperature, T
GS
Variation COE
from base case, %
Variable range, %
COE
Variation emissions
from base case, %
Variable range, %
Carbon
emissions
AN GHNÍOMHAIREACHT UM CHAOMHNÚ COMHSHAOIL
Tá an Ghníomhaireacht um Chaomhnú Comhshaoil (GCC) freagrach as an
gcomhshaol a chaomhnú agus a fheabhsú mar shócmhainn luachmhar do
mhuintir na hÉireann. Táimid tiomanta do dhaoine agus don chomhshaol a
chosaint ó éifeachtaí díobhálacha na radaíochta agus an truaillithe.
Is féidir obair na Gníomhaireachta a
roinnt ina trí phríomhréimse:
Rialú: Déanaimid córais éifeachtacha rialaithe agus comhlíonta
comhshaoil a chur i bhfeidhm chun torthaí maithe comhshaoil a
sholáthar agus chun díriú orthu siúd nach gcloíonn leis na córais sin.
Eolas: Soláthraímid sonraí, faisnéis agus measúnú comhshaoil atá
ar ardchaighdeán, spriocdhírithe agus tráthúil chun bonn eolais a
chur faoin gcinnteoireacht ar gach leibhéal.
Tacaíocht: Bímid ag saothrú i gcomhar le grúpaí eile chun tacú
le comhshaol atá glan, táirgiúil agus cosanta go maith, agus le
hiompar a chuirdh le comhshaol inbhuanaithe.
Ár bhFreagrachtaí
Ceadúnú
Déanaimid na gníomhaíochtaí seo a leanas a rialú ionas nach
ndéanann siad dochar do shláinte an phobail ná don chomhshaol:
• saoráidí dramhaíola (m.sh. láithreáin líonta talún, loisceoirí,
stáisiúin aistrithe dramhaíola);
• gníomhaíochtaí tionsclaíocha ar scála mór (m.sh. déantúsaíocht
cógaisíochta, déantúsaíocht stroighne, stáisiúin chumhachta);
• an diantalmhaíocht (m.sh. muca, éanlaith);
• úsáid shrianta agus scaoileadh rialaithe Orgánach
Géinmhodhnaithe (OGM);
• foinsí radaíochta ianúcháin (m.sh. trealamh x-gha agus
radaiteiripe, foinsí tionsclaíocha);
• áiseanna móra stórála peitril;
• scardadh dramhuisce;
• gníomhaíochtaí dumpála ar farraige.
Forfheidhmiú Náisiúnta i leith Cúrsaí Comhshaoil
• Clár náisiúnta iniúchtaí agus cigireachtaí a dhéanamh gach
bliain ar shaoráidí a bhfuil ceadúnas ón nGníomhaireacht acu.
• Maoirseacht a dhéanamh ar fhreagrachtaí cosanta comhshaoil na
n-údarás áitiúil.
• Caighdeán an uisce óil, arna sholáthar ag soláthraithe uisce
phoiblí, a mhaoirsiú.
• Obair le húdaráis áitiúla agus le gníomhaireachtaí eile chun dul
i ngleic le coireanna comhshaoil trí chomhordú a dhéanamh ar
líonra forfheidhmiúcháin náisiúnta, trí dhíriú ar chiontóirí, agus
trí mhaoirsiú a dhéanamh ar leasúchán.
• Cur i bhfeidhm rialachán ar nós na Rialachán um
Dhramhthrealamh Leictreach agus Leictreonach (DTLL), um
Shrian ar Shubstaintí Guaiseacha agus na Rialachán um rialú ar
shubstaintí a ídíonn an ciseal ózóin.
• An dlí a chur orthu siúd a bhriseann dlí an chomhshaoil agus a
dhéanann dochar don chomhshaol.
Bainistíocht Uisce
• Monatóireacht agus tuairisciú a dhéanamh ar cháilíocht
aibhneacha, lochanna, uiscí idirchriosacha agus cósta na
hÉireann, agus screamhuiscí; leibhéil uisce agus sruthanna
aibhneacha a thomhas.
• Comhordú náisiúnta agus maoirsiú a dhéanamh ar an gCreat-
Treoir Uisce.
• Monatóireacht agus tuairisciú a dhéanamh ar Cháilíocht an
Uisce Snámha.
Monatóireacht, Anailís agus Tuairisciú ar
an gComhshaol
• Monatóireacht a dhéanamh ar cháilíocht an aeir agus Treoir an AE
maidir le hAer Glan don Eoraip (CAFÉ) a chur chun feidhme.
• Tuairisciú neamhspleách le cabhrú le cinnteoireacht an rialtais
náisiúnta agus na n-údarás áitiúil (m.sh. tuairisciú tréimhsiúil ar
staid Chomhshaol na hÉireann agus Tuarascálacha ar Tháscairí).
Rialú Astaíochtaí na nGás Ceaptha Teasa in Éirinn
• Fardail agus réamh-mheastacháin na hÉireann maidir le gáis
cheaptha teasa a ullmhú.
• An Treoir maidir le Trádáil Astaíochtaí a chur chun feidhme i gcomhair
breis agus 100 de na táirgeoirí dé-ocsaíde carbóin is mó in Éirinn.
Taighde agus Forbairt Comhshaoil
• Taighde comhshaoil a chistiú chun brúnna a shainaithint, bonn
eolais a chur faoi bheartais, agus réitigh a sholáthar i réimsí na
haeráide, an uisce agus na hinbhuanaitheachta.
Measúnacht Straitéiseach Timpeallachta
• Measúnacht a dhéanamh ar thionchar pleananna agus clár beartaithe
ar an gcomhshaol in Éirinn (m.sh. mórphleananna forbartha).
Cosaint Raideolaíoch
• Monatóireacht a dhéanamh ar leibhéil radaíochta, measúnacht a
dhéanamh ar nochtadh mhuintir na hÉireann don radaíocht ianúcháin.
• Cabhrú le pleananna náisiúnta a fhorbairt le haghaidh éigeandálaí
ag eascairt as taismí núicléacha.
• Monatóireacht a dhéanamh ar fhorbairtí thar lear a bhaineann le
saoráidí núicléacha agus leis an tsábháilteacht raideolaíochta.
• Sainseirbhísí cosanta ar an radaíocht a sholáthar, nó maoirsiú a
dhéanamh ar sholáthar na seirbhísí sin.
Treoir, Faisnéis Inrochtana agus Oideachas
• Comhairle agus treoir a chur ar fáil d’earnáil na tionsclaíochta
agus don phobal maidir le hábhair a bhaineann le caomhnú an
chomhshaoil agus leis an gcosaint raideolaíoch.
• Faisnéis thráthúil ar an gcomhshaol ar a bhfuil fáil éasca a
chur ar fáil chun rannpháirtíocht an phobail a spreagadh sa
chinnteoireacht i ndáil leis an gcomhshaol (m.sh. Timpeall an Tí,
léarscáileanna radóin).
• Comhairle a chur ar fáil don Rialtas maidir le hábhair a
bhaineann leis an tsábháilteacht raideolaíoch agus le cúrsaí
práinnfhreagartha.
• Plean Náisiúnta Bainistíochta Dramhaíola Guaisí a fhorbairt chun
dramhaíl ghuaiseach a chosc agus a bhainistiú.
Múscailt Feasachta agus Athrú Iompraíochta
• Feasacht chomhshaoil níos fearr a ghiniúint agus dul i bhfeidhm
ar athrú iompraíochta dearfach trí thacú le gnóthais, le pobail
agus le teaghlaigh a bheith níos éifeachtúla ar acmhainní.
• Tástáil le haghaidh radóin a chur chun cinn i dtithe agus in ionaid
oibre, agus gníomhartha leasúcháin a spreagadh nuair is gá.
Bainistíocht agus struchtúr na Gníomhaireachta um
Chaomhnú Comhshaoil
Tá an ghníomhaíocht á bainistiú ag Bord lánaimseartha, ar a bhfuil
Ard-Stiúrthóir agus cúigear Stiúrthóirí. Déantar an obair ar fud cúig
cinn d’Oigí:
• An Oig um Inmharthanacht Comhshaoil
• An Oig Forfheidhmithe i leith cúrsaí Comhshaoil
• An Oig um Fianaise is Measúnú
• Oig um Chosaint Radaíochta agus Monatóireachta Comhshaoil
• An Oig Cumarsáide agus Seirbhísí Corparáideacha
Tá Coiste Comhairleach ag an nGníomhaireacht le cabhrú léi. Tá
dáréag comhaltaí air agus tagann siad le chéile go rialta le plé a
dhéanamh ar ábhair imní agus le comhairle a chur ar an mBord.
www.epa.ie
Inform policy
This techno-economic performance study offers valuable information with respect to
the potential of on-site and centralised thermal conversion of sewage sludge. The
information generated will inform and assist stakeholders and local and government
authorities in their consideration of the establishment of these alternatives in the
future.
The recognised challenges with regard to implementation of gasification and
combustion as waste management techniques will also inform and direct future
focused research and technological development activities in areas of greatest need.
Develop solutions
This study created inexpensive computational tools which will be available for process
modelling and techno-economic evaluation of the thermal conversion of sewage sludge,
and of any characterised organic waste, intended for power and heat generation. These
alternatives offer sustainable means of waste management and renewable energy
production that can significantly reduce the carbon footprint of waste disposal practices
and improve the energy security of Ireland.
EPA Research Report 216
Thermodynamic Modelling of Energy
Recovery Options from Digestate at
Wastewater Treatment Plants
Authors: Karla Dussan and Rory Monaghan
EPA Research: McCumiskey House,
Richview, Clonskeagh, Dublin 14.
Phone: 01 268 0100
Twier: @EPAResearchNews
Email: research@epa.ie
EPA Research Webpages
www.epa.ie/researchandeducaon/research/
Identify Pressures
It is vitally important to evaluate alternative means of sludge disposal in order to avoid
pollution of agricultural land. Thermal conversion technologies can address the problem
of surplus sludge while also providing a means to support the consolidation of a secure
and indigenous energy market in Ireland.
Plant scale and poor fuel properties of sludge were identified as some of the technical
challenges facing the implementation of thermal conversion plants; however, a number of
potential solutions, including centralised plants and waste/biomass co-processing, have
also been identified which may assist in overcoming these challenges.
