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American Journal of Environmental Sciences 4 (5): 482-490, 2008
ISSN 1553-345X
© 2008 Science Publications
Corresponding Author: Ali Elkamel, Department of Chemical Engineering, University of Waterloo, 200 University Avenue
West, Waterloo, ON N2T 2R8, Canada
482
Analysis and Optimization of Carbon Dioxide Emission
Mitigation Options in the Cement Industry
1Mohammed Ba-Shammakh, 2Hernane Caruso, 2Ali Elkamel, 2Eric Croiset, and 2Peter L. Douglas
1Department of Chemical Engineering, King Fahd University of Petroleum & Minerals,
Dhahran, Saudi Arabia
2Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario Canada N2L 3G1
Abstract: The cement industry is responsible for approximately 5% of global anthropogenic carbon
dioxide emissions emitting nearly 900 kg of CO2 for every 1000 kg of cement produced. Effective
control strategies to mitigate these emissions are discussed and a mathematical programming model
able to suggest the best cost effective strategy is outlined. Control costs consisting of operating and
investment costs along with the efficiency of control options are taken into account in the model. A
representative case study from the cement industry was considered in order to illustrate the use of the
model in giving optimal control strategies. Efficiency improvement measures were found to be
effective options for reduction targets up to 10 %. The model suggested that fuel switching and carbon
capture must be considered at reduction targets higher than 10%. The cost of cement production was
shown to increase dramatically with an increase in reduction target.
Key words: Cement industry, CO2 emissions, CO2 capture, process optimization
INTRODUCTION
The rapid deterioration of the global environment
forces governments around the globe to increasingly
take into consideration environmental issues. One of the
most important and debated issues is the enhanced
greenhouse effect due to greenhouse gases (GHG)
emissions. The burning of fossil fuels releases more
than six billion tonnes of carbon dioxide (CO2) into the
atmosphere each year [1]. The cement industry is among
the industries that release the most CO2 to the
atmosphere. Concrete is the world’s most important
construction material, and for each tonne of Portland
cement (an essential component of concrete) produced,
approximately one tonne of CO2 is emitted to the
atmosphere [2]. The cement industry generates
approximately 5% of the global anthropogenic CO2
emissions [3]. This is mostly due to combustion at the
manufacturing operations, transportation, and
combustion of fossil fuel required to produce the
electricity consumed by the cement industry.
Environmental policies related to CO2 emissions can
potentially affect greatly the cement industry. Today,
there are some economically acceptable alternatives for
manufacturing an environmentally-friendly Portland
cement, e.g. substitute materials and alternative fuels.
Whatever alternatives are implemented, they must be
pragmatic. The possibility of making a profit with CO2
emissions is also a parameter that may impact the
competitiveness of the cement industry.
CEMENT FABRICATION PROCESS
Portland cement manufacturing requires a precise
mix of raw materials. This mix is commonly called the
raw mix and mainly consists of two main natural raw
materials: limestone (calcium carbonate-CaCO3) and
argillaceous materials. The cement industry must
therefore start by quarrying limestone and clay. The
main objective of raw material control is to produce a
Kiln feed that will allow the production of a quality
cement clinker, while conserving as much energy as
possible. The cement clinker (clinker) requires a
defined proportion of the elements calcium, silicon,
aluminum, and iron; all these raw materials together
with the fuel ash must combine and form the typical
clinker composition. Figure 1 shows the process flow
diagram of a typical cement plant [1].
The raw material mix is grounded up before being
sent to the process stage. The grinding process can be
performed using either ball mills or vertical roller mills.
During this stage, part of the excess heat from the Kiln
is used to dry the raw mix. In order to reduce the
natural chemical variation in the various raw material
Am. J. Environ. Sci., 4 (5): 482-490, 2008
483
sources it is necessary to blend and homogenize the raw
material efficiently. The main objective of this step is to
minimize impacts on the efficiency of the Kiln.
Fig.1: Process diagram of a typical cement plant
The raw material burning or clinkering step takes
place in the Preheater Tower and in the Kiln. The
Preheater Tower is composed of a series of
countercurrent flow cyclones that transfer heat from the
Kiln to the raw materials. Some of the newest Preheater
Towers have a section which contains a fuel
combustion chamber, called precalciner. In this stage,
calcination of the raw materials will start and CO2 will
be formed. The Kiln is the main piece of equipment in
the cement plant and “are the world’s largest piece of
moving industrial process equipment and one of the
hottest” [4]. The Kiln is a long, horizontal, rotating,
cylindrical pipe that is at least 60 m long and can be up
to 200 m long and with diameters ranging from 3 to 9
m. Its internal surface is covered with refractory bricks.
The blended raw materials are fed in to the upper
end of the Preheater Tower going all the way through
the end of the rotary Kiln. The Kiln slowly rotates,
approximately one to four revolutions per minute, and
the raw material tumbles through increasingly hotter
zones. The flame at one end of the Kiln can be fuelled
by powdered materials such as coal, petroleum coke,
or by natural gas, oil, and recycled materials. The heat
will start a series of chemical reactions and the raw
material becomes molten, and fuses together into
modules, called clinker, which are the final product
from the Kiln. The clinker is discharged red-hot from
the end of the Kiln and conducted through different
types of coolers to partially recover the thermal energy
and lower the clinker handling temperature. The
clinker coming out of the Kiln is approximately at
1500ºC. It is cooled in an air-cooled cooler. Ambient
air is blown into the cooler to exchange heat between
the hot clinker and the ambient air. After cooling the
clinker temperature drops to approximately 170ºC. The
final step to produce cement is the cement grinding,
where the clinker is ground together with additives in a
cement mill. The cement mill is a horizontal metallic
cylinder containing metallic balls. As it rotates the
crushing action of the balls grinds and mixes the
clinker and additives, forming the final product.
The above description illustrates clearly the energy
intensity of the different steps involved in the cement
industry and the corresponding CO2 implications.
Several studies have been conducted to reduce CO2
emissions from the cement industry. Sheinbaum et al.
[5] considered CO2 abatement costs through efficiency
improvement in Mexico. Hendriks et al. [6] shows that a
wide range of options exists to reduce CO2 emissions
from a cement plant and provided cost data. Das and
Kandpal [7] made an attempt to estimate CO2 emissions
from the cement industry in India. The impact of
variations in product mixes and technology on CO2
emissions were also analyzed. Martin et al. [8] presented
energy efficiency and CO2 emission reduction
opportunities in the U.S. cement industry. Carbon
capture and storage (CCS) can also represent a
promising option to reduce significantly CO2 emissions
[1].
In this article, after identifying the various sources
of CO2 in a cement plant and after introducing some
possible mitigation measures, a mathematical
programming model that will determines the optimal
CO2 mitigation strategy with the least cost is presented.
The objective of the model is to minimize the total
control cost consisting of operating costs and
investment costs. The model takes into account the
sources of CO2 emissions, cost, and efficiency of
control options. The model is then illustrated on a case
study.
CO2 EMISSIONS IN CEMENT INDUSTRY
The main sources of carbon dioxide in cement
manufacturing are combustion of fossil fuel and
limestone calcinations. The most common fuels are
coal, petroleum coke, fuel oil and natural gas.
Currently, the cement industry in North America and
Europe base its fuel choice on three basic factors: cost,
product quality, and environmental impact. The fuel
that best fills these three basic requirements will be the
preferred choice. It is important to note that factors such
as the cost of a new firing system, the amount of
storage, and local fuel availability also play a key role
in the decision process.
During the clinker process, limestone will suffer
calcination and CO2 will be formed. The limestone
chemical reaction can be expressed by the equation
below:
CaCO3 CaO + CO2 (1)
Prehe
Rot
ary
l
Raw
Materia
Stock
Raw Mill
Dedusting
Plant
Blending
Silos
Air
q
uenching
Cooler
Clinke
r
Silos
Additiv
es
Cement
Mill
Cement
Cooler
Cement
Silos
Packing
Plant
Am. J. Environ. Sci., 4 (5): 482-490, 2008
484
The percentage of calcium oxide (CaO) in clinker
is usually between 64 and 67 per cent. The balance is
comprised of iron oxides, silicon oxides and aluminum
oxides. The amount of CO2 generated by the process
varies based on the specific loss of the raw materials
(limestone) on ignition. An example of mass balance
for for production of one tonne of cement is shown in
Figure 2 [9].
There are other sources for CO2 emissions in a
cement plant, such as electricity and mobile
equipments. These represent, however, a small
contribution to the total CO2 generated by the cement
manufacturing. Approximately, half of the CO2 emitted
by the cement industry originates from the fuel and half
from the calcinations (chemical reactions) that will
convert raw materials into clinker [1].
Fig. 2: Typical cement plant mass balance
CO2 MITIGATION OPTIONS
GHG mitigation has now become an important
factor in creating a sustainable cement industry.
Although regulated locally by different countries, the
top 10 cement producers have their plants spread
around the globe, and as part of a sustainability
strategy, the cement industry is forced to reduce
emissions. GHG mitigation has to overcome
commercial and economical barriers. During the last 20
years environmental matters have had more influence in
different global agreements; however, since solutions
could result in a reduction in the profit margin of
certain multinational corporations or adversely impact
the economy of industrialized countries, the only
possible solution is one that will offer environmental
gains and strong business opportunities. In order to
achieve CO2 mitigation targets while promoting the
sustainability of the cement industry, below is a list of
options that can impact on CO2 emissions.
Maintenance: One of the most important parts of a
maintenance system is preventive maintenance.
Preventive maintenance can increase plant efficiency
and reduce the cost of corrective maintenance. One
example of results delivered by a successful
maintenance system is energy savings. Actions such as
false air survey and control of the leaking point can
significantly increase the kiln thermo efficiency. It is
estimated that a simple air leak at the kiln hood can
contribute to a 46 kJ/kg of clinker increase on the kiln
thermal consumption [9].
Other strategies to reduce energy consumption
include the gradual substitution of old motors by high-
efficiency motors and the implementation of an
integrated management system where the daily process
routine contributes directly to increased maintenance
effectiveness. The feeders and scales performance are
examples of equipment that have direct influence on the
kiln feed quality. A developed maintenance plan will
support the kiln feed quality reducing the deviation on
the material proportions which directly affect the fuel
consumption. In general, a good maintenance program
will contribute to an increase in the plant utilization
ratio reducing the numbers of start-up and kiln preheats
during the year [10]. Although not easily quantified, it is
clear that a well structured maintenance program can
highly contribute to emission reduction and plant
performance improvement.
Plant Optimization: Plant optimization has been
largely implemented in the cement industry not only as
an action to reduce emissions, but also to promote
higher kiln productivity and runtime. It is common
knowledge in cement plants that many minor problems
such as kiln seal leaks, cooler inefficiency, fuel
atomization or fineness can compromise and impact
plant performance. These problems alone can lead to
thermal waste of up to six per cent. An optimization
strategy should minimize fuel consumption and
maximize clinker production correcting the clinker
quality as required. The main idea is to make the
process more consistent and reliable. For example, the
operator might increase fan speed or reduce fuel
injection based on the tower oxygen levels. It is
estimated that such strategy can reduce heat
consumption by three to five per cent and improve
refractory life by 30 to 50 per cent [11].
Alternative Fuel and Pyroprocessing Improvements:
The main opportunities for improvements and reduction
1150 kg
raw
material
0.94 kg
air
600 kg
CO2
1566 kg
N2
262 kg
O2
63 kg
fuel
oil
1050
kg air
Am. J. Environ. Sci., 4 (5): 482-490, 2008
485
of emissions associated with the cement industry are in
the pyroprocess. As discussed previously, a large part
of energy consumption, and consequently emission
generation, takes place during the burning process. It is
estimated that the average pyroprocess efficiency in the
U.S. is about 34 per cent. Opportunities for
improvement can be found mainly in process upgrades
such as replacing wet systems and upgrading preheaters
and precalciners. It is important to recognize that new
burner designs and fuel systems can also play a
considerable part in reducing emissions. New burners
and fuel systems can contribute to reduced emissions by
improving a cement plants’ flexibility to burn
alternative fuels, and replacing high fossil carbon fuels
with low fossil carbon fuels. An example of fuel
substitution is the use of natural gas instead of coal.
Some other types of alternative fuels include:
• Gaseous: refinery gases and landfill gas;
• Liquid: mineral oils, distillation residues, hydraulic
oil; and,
• Solid: sewage sludge, plastic, tires, petroleum coke
and tar.
Other impacts of alternative fuel on the plant
operation are the refractory utilization rate and
preheater tower pressure loss [12]. The organic portion
will burn and generate energy required for the process.
The mineral part will be integrated into the process and
will contribute as raw material. Fly-ash is a typical
example of alternative raw material that will contribute
not only as a raw material but also as an energy source.
Replacement of Fossil Fuel by Waste-Derived Fuel
(WDF): It is estimated that the use of waste-derived
fuel (WDF) will increase by one per cent worldwide
per year. The alternative implemented by some cement
plants is to use approximately one per cent of WDF to
replace fossil fuel [13]. It is important to note that this
mitigation is indirect, because if these waste products
had not been burned in cement kilns, they would have
been incinerated or sent to a landfill, generating further
CO2 emissions together with the CO2 generated by the
fossil fuel that was not replaced. This alternative has a
potential to add great environmental value by solving
the serious problem of waste disposal. Unfortunately,
fossil fuel substitution by WDF is not an alternative
supported by the general public. The public perception
is that it would convert the cement kiln into a simple
incinerator. This perception from the public pressures
the local authorities to not consider this as a reasonable
alternative to reduce fossil fuel consumption.
Raw Meal Burnability: The contribution of the raw
materials burnability is difficult to measure. In general
cement plants have targets for production
improvement and profit margin when this alternative is
considered. Raw materials fineness, composition and
chemical module are the main improvements that must
be made to achieve a constant raw material
burnability. Such improvements could directly impact
the amount of fuel used daily by the kiln. These
improvements would also extend the refractory life
cycle and reduce power consumption [14].
Use of By-products: This alternative can provide a
practical solution to the usage of huge amounts of by-
products generated, such as fly ash from power
plants. In some cases like fly-ash, the by-product can
contribute to improve concrete durability. This
alternative needs to be studied locally to determine
the availability and cost. European countries have
been using by-products in high amounts. In general, it
is important to note that cement standards need to be
reviewed to accommodate the use of by-products as
alternatives in the process of reducing GHG
emissions [15].
Replacing Raw Material Limestone by Slag: Blast
furnace slag is a non-metallic by-product from the iron
production process. It is comprised of silicates,
aluminosilicates, and calcium-alumina-silicates. By
replacing raw material limestone with slag it is
possible not only to prevent CO2 emissions due to
limestone decomposition, but also to improve raw
material burnability. Blast furnace slag is not a new
supplementary cementitious material; it has been used
by the cement industry as a component blended in
cement or as aggregate material in the concrete
mixture for the past ten years. Blast furnace slag
incorporation in Portland cement is specified by
AASHTO M302 [16]. Although blast slag has
great use in the cement industry, its use cannot be
generalized worldwide, since factors such as the cost
of slag and transportation are prohibitive. It is
important to observe that only 25 per cent of the
energy used to manufacture Portland cement is
required. The use of slag has important ecological and
economical benefits. For example, the use of slag in
Europe has contributed significantly to the efforts to
meet the Kyoto targets, and has reduced the energy
and raw materials necessary in the cement process [17].
Electrical Energy Savings: Electrical energy is used
in the cement plant to drive fans, rotate the kiln and to
Am. J. Environ. Sci., 4 (5): 482-490, 2008
486
move materials. In general, the power used in the kiln
corresponds to 40 to 50 kWh/tonne clinker. Power
savings from the use of high efficiency motors will
vary from plant to plant and from case to case. Most of
the motor substitution is done during the replacement
period when the motor life is nearly done. Another
energy consumption point in the cement process is the
adjustable speed drivers. Drivers are, in most cases,
the largest power consumers in the cement process.
Adjustable drivers can produce savings from 7 to 60
per cent [4]. These savings will be based on the
application and the load applied to the motor and the
application in the process.
New Preheater Tower: The preheater tower is a vital
part of the process. A new preheater tower with low
pressure drop cyclones will reduce the power
consumption of the kiln fan system. It is possible to
achieve a reduction of 0.6 to 1.1 kWh/t depending on
the fan efficiency. A new installation can be
expensive. In addition, installation and modification
are site-specific, which makes it difficult to point out a
general return on the investment. A new cyclone
system can increase the overall dust transport cost [18].
This indicates that this solution is recommended for
dry preheater and precalciner kilns older than 15 years
of age.
Kiln Burner: Burner technology has improved
quickly. A number of different burners have improved
flame control and optimized fuel usage. One of the
main objectives of the new burner technology is to
create a more stable flame independent of the fuel
type. Flame stability is one of the most important
factors in maintaining a stable kiln operation and
maximizing combustion effectiveness.
CO2 Capture and Disposal: Different methods for the
capture and disposal of CO2 at the point of combustion
have been researched and developed [1]. Examples of
possibilities are: chemical stripping, membrane system,
cryogenic separation and physical absorption. The
implementation cost of each one of these possibilities is
highly uncertain; costs are directly related to technical
performance, economic growth and fuel type.
Moreover, the disposal solutions available today
present a great level of doubt regarding the technical
feasibility for a full-scale implementation.
The CO2 concentration in a cement plant is higher
than in a power generation process. Studies have shown
that the cement production process has a high quantity
of low quality heat. This extra heat could be used in the
CO2 capture process [19]. In general, the average cost to
capture one tonne of CO2 is estimated to be around
USD 50 [1].
Chemical scrubbing is considered to be the most
mature process to capture CO2. The chemical stripping
method is based on Henry’s Law where the absorption
depends on the temperature and pressure of the system.
Chemical absorption is mainly applicable for a system
where the exhaust gases present low concentration of
CO2 and the system pressure is close to atmospheric
pressure. The main steps of the stripping method are
absorption of CO2 by chemical solvents and recovery of
CO2 from chemical solvents by using low-grade heat
(usually extracted from power plants). One of the
available technologies for removing CO2 from the gas
stream is chemisorption using monoethalnolamine. The
design and costing of CO2 capture from cement plant
flue gas is similar to the design and costing of capturing
CO2 from power plant using monoethalnolamine [1]. At
a typical cement plant the cost for this method was
estimated to be approximately $49-$54 per tonne of
CO2 captured [1].
Physical absorption is another option for capturing
CO2 and has its main application with low
concentration gases and vapors that are retained in a
surface of porous solid materials. CO2 is held on the
surface of the porous material by (non-chemical)
surface forces. The solid adsorbent material is
regenerated using heat and the CO2 capture is complete
[20]. Membrane systems are based on different physical
and chemical interactions between the gas stream and
the membrane material. Carbon dioxide capture by a
membrane system is currently not considered as a
common approach. The main obstacle for this
technology is the necessity of multiple stages or cycles,
which directly increases energy consumption and
consequently, cost.
the cryogenic fraction method is based on the
compression of the gas stream and subsequently, the
gas temperature is reduced where the separation is
possible by distillation. This method is mainly
recommended in cases of high CO2 concentration (more
than 90 per cent) and therefore is not suitable for the
cement industry.
As for CO2 disposal, the options are: discharge
into natural gas reservoirs or aquifers, discharge deep
into the ocean or reuse the CO2 in useful organic
compounds. Reviewing all the solutions available
today, the ocean scenario has the highest capacity to
store CO2, and absorbs the CO2 quantities generated by
the actual necessity of reduction [21]. It is expected that
in the next few years, CO2 underground storage will be
Am. J. Environ. Sci., 4 (5): 482-490, 2008
487
a technical and economical option for CO2 disposal,
especially in the case of enhanced oil recovery and
eventually coal bed methane recovery.
MATHEMATICAL MODEL
The above section illustrates clearly that there is a
wide range of options that can be used to reduce CO2
emissions in the cement industry. These include:
• Improvement of the energy efficiency of the
process
• Shifting to a more energy efficient process (e.g.
from (semi) wet to dry process)
• Replacing high carbon fuels by low carbon fuels
• Removal of CO2 from the flue gases such as by
absorption (MEA process)
In order to help decision makers to adapt an
appropriate CO2 reduction strategy, an optimization
model for the cement industry is presented in this
section. The mathematical model consists of an
objective function to be minimized and equality and
inequality constraints. The objective of the model is to
find the best strategy or mix of strategies to reduce CO2
up to a certain target with minimum overall cost for
cement production while meeting the demand.
The objective function to be minimized can be written
as:
($/ ) r r if if
r i f
if if ie ie ic ic
i f i e i c
Z yr C R C P
R X C Y C Z
= + +
+ +
(2)
Where:
Z : annualized capital and operating cost of the cement
plant ($/yr)
Cr : cost of purchasing raw material r ($/tonne)
Rr : purchased amount of raw material r (tonne/yr)
Cif : operating cost for a unit i with fuel f ($/tonne)
Pif : amount produced from unit i using fuel f (tonne/yr)
Rif : annualized retrofit cost for switching unit i to run
with another fuel f ($/yr)
Xif : binary variable representing switching or not.
Cie : annualized cost of applying efficiency
improvement technology (e) on unit i ($/yr)
Yie: binary variable representing applying efficiency
improvement technology (e) or not.
Cic: annualized cost of applying CO2 capture
technology (c) on unit i ($/yr)
Zic: binary variable representing applying CO2 capture
technology (c) or not.
The first term in the objective function represents
the cost associated with purchasing the raw material.
The second term takes into account the operating cost
for different units. The cost of switching to less carbon
content fuel is shown in the third term. The fourth term
represents the cost associated with applying efficiency
improvement technologies. The remaining term adds
the cost that results from applying CO2 capture
technology. A binary variable is defined for each CO2
mitigation option under study.
Constraints: The constraints for demand satisfaction,
fuel selection and CO2 emissions reduction are
described as follows:
a) Demand satisfaction
This constraint simply says that the total cement
produced should be greater than or equal to the demand.
if
i f
P demand
≥
(3)
b) Fuel selection
Each unit i has to run with only one fuel f. For this
reason, a binary variable is introduced to represent the
type of fuel used in a given unit.
∀=
f
if iX 1 (4)
c) Emission constraint
The CO2 emitted from all units must satisfy a CO2
reduction target. Different technologies, e, to improve
the efficiency are implemented in the mathematical
model. It is assumed that the effect of these
technologies is additive. The emission is also affected
by applying CO2 capture technology.
( )
2
2 2
1 1
1 %
if ie ie ic ic if
i f e c
CO e Y Z P
CO CO
ε
− −
≤ −
(5)
Where:
CO2if : CO2 emissions from unit i using fuel f (tonne per
tonne cement produced)
eie : percent gain in efficiency associated with
applying technology e on unit i
Yie: binary variable for applying efficiency
improvement technology e or not
εic : percent CO2 capture
Zic: binary variable for applying CO2 capture
technology c or not
Am. J. Environ. Sci., 4 (5): 482-490, 2008
488
% CO2: reduction target
CO2 : Current CO2 emissions (tonne/yr)
d) Selection of CO2 capture process to be installed
This constraint let the model select only one capture
process for each unit i
iY
c
ic ∀≤
1 (6)
e) Non-negativity constraints
The amount produced must be greater than zero
iPif ∀≥ 0 (7)
The developed model is illustrated in the following case
study.
Case Study
The problem of reducing CO2 emissions from
combustion sources within a cement plant is considered
with three different mitigation options. The first option
is the application of efficiency improvement
technologies to reduce CO2 emissions. Table 1 shows
the different technologies considered in this study,
along with their corresponding reduction in CO2
emissions. The second option for reducing CO2
emissions is the switching of fuel to a less carbon
content fuel (e.g. switch from coal to natural gas). The
third option is the application of CO2 capture
technologies.
An existing cement plant [22] with the following
data was studied and the aim is to minimize the cost of
cement production while reducing CO2 emissions by a
fixed target.
Cement production: 712,600 tonne/yr
Current total CO2 emissions: 553,800 tonne CO2/yr
Current total annualized cost: 25 x 106 $/yr
Three CO2 mitigation options will be considered and
these are:
• Applying efficiency improvement technologies to
reduce CO2 emissions shown in Table 1.
• Switching to less carbon content fuel such as from
coal to natural gas
• Applying “end of pipe” CO2 capture technologies.
The chemical absorption (MEA) process is the only
considered option in this study with a cost of 50
$/tonne CO2 captured.
The model is first formulated as a mixed integer
nonlinear model (MINLP). This model was then
linearized using an exact linearization scheme similar to
what we have employed in our previous work on
carbon dioxide mitigation in the power industry [23]. The
resulting linearized model (mixed integer linear
program or MILP) was coded in GAMS (General
Algebraic Modeling System).
Table 1: Technologies for efficiency improvements
Technology % CO2
Reduction
High efficiency motors and drives
Adjustable Speed Drives
High efficiency classifiers
Efficient grinding technologies
Conversion from wet to dry process
4
5.5
8.1
10.5
50.0
DISCUSSION
The CO2 mitigation options discussed earlier were
incorporated into the model to select the least cost
option to reduce CO2 emissions to a specified target.
Different CO2 reduction targets were specified. Table 2
shows the results for different CO2 reduction targets.
For 1% reduction target, for example, the optimization
process chose to apply the technology of high efficient
motors and drives. The cost of production increased by
about 2%. A second improvement technology was
selected by the model at a reduction target of 5%. No
fuel switching was applied up to 10 % where efficiency
improvement technologies can be applied with an
increase of about 7% in the cost. For a 20 reduction
target, fuel switching, from coal to natural gas, was
selected with only one technology for efficiency
improvement (installation of high efficient motors and
drives). The cost increased by about 17%. Carbon
capture technology was selected at reduction targets
higher than 30%. For a 50% reduction target, the
optimization process still chose to apply capture
technology.
Table 2: Summary of results for different CO2 reduction target
% CO2
reduction
Cost
(million $/yr)/
($/tonne cement)
% Increase in cost
0
1
5
10
20
30
50
25.0 / 35.1
25.6 / 35.9
25.7 / 36.1
26.8 / 37.6
29.4 / 41.2
33.3 / 46.7
38.9 / 54.5
0
2.4
2.9
7.3
17.4
33.2
55.4
Am. J. Environ. Sci., 4 (5): 482-490, 2008
489
Figure 3 shows the increase in the production cost
for each CO2 reduction target. The line starts to sharply
increase at reduction targets ranging from 20 to 50%.
This is expected since this is where optimization lead to
decisions that employ CO2 capture options which have
in general a much higher cost than other mitigation
options.
0
10
20
30
40
50
60
0 5 10 15 20 25 30 35 40 45 50
% CO2 reduction
% Incr ease in c ost
Fig. 3: Percent increase in cost for different CO2
reduction targets
CONCLUSIONS
An optimization model was developed in order to
meet demand at a given CO2 reduction target. Three
mitigation options were considered. The model chose
the best strategy or mix of strategies in order to meet a
certain CO2 reduction target with the least cost
providing that the demand and other requirements were
met. The model was formulated as a MINLP, exactly
linearized, and then coded in GAMS.
It was found that the application of different
efficiency improvement technologies represent good
options especially at reduction targets up to 10%.
Beyond this reduction target, fuel switching should be
applied to achieve a reduction target such as 20%. At
reduction targets higher than 20%, carbon capture
technologies should be applied and efficiency
improvement technologies are no more a good
mitigation option. The cost of production increased
dramatically when the reduction target is beyond 20 %.
This is because carbon capture technologies were
selected at these ranges and since these were the most
expensive mitigation options. The cost per tonne
Portland cement produced increased from 35.1 $/tonne
to about 55 $/tonne which is about 20 $ increase for
each tonne produced.
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