Fly ash – waste management and overview : A Review

Abstract

Fly ash (FA)-a coal combustion residue of thermal power plants has been regarded as a problematic solid waste all over the world. India has some of the largest reserves of coal in the world. Indian coal has high ash content and low calorific value. Nearly 73% of the country's total installed power generation capacity is thermal of which coal-based generation is 90%. Some 85 thermal power stations, besides several captive power plants use bituminous and sub-bituminous coal and produce large quantities of fly ash. High ash content (30%-50%) coal contributes to these large volumes of fly ash. Current annual production of Fly ash, a by-product from coal based thermal power plant (TPPs), is about 112 million tones (MT). Some of the problems associated with Fly ash are large area of land required for disposal and toxicity associated with heavy metal leached to groundwater. Fly ash, being treated as waste and a source of air and water pollution till recent past, is in fact a resource material and has also proven its worth over a period of time. The present paper reviews the potential applications for coal fly ash as a raw material: as a soil amelioration agent in agriculture, use, in highway embankments, in construction of bricks, as an aggregate material in Portland cement, filling of low lying areas etc in the manufacture of glass and ceramics, in the production of zeolites, in the formation of mesoporous materials, in the synthesis of geopolymers, for use as catalysts and catalyst supports, as an adsorbent for gases and waste water processes, and for the extraction of metals. Thus fly ash management is a cause of concern for the future. This article attempts to highlight the management of fly ash to make use of this solid waste, in order to save our environment.

Full text

Recent Research in Science and Technology 2014, 6(1): 30-35
ISSN: 2076-5061
Available Online: http://recent-science.com/
Fly ash – waste management and overview : A Review
Aakash Dwivedi* and Manish Kumar Jain**
Department of Environmental Science and Engineering, Indian School of Mines, Dhanbad-814004, Jharkhand, India
Abstract
Fly ash (FA)-a coal combustion residue of thermal power plants has been regarded as a problematic solid waste all over the
world. India has some of the largest reserves of coal in the world. Indian coal has high ash content and low calorific value.
Nearly 73% of the country’s total installed power generation capacity is thermal of which coal-based generation is 90%. Some
85 thermal power stations, besides several captive power plants use bituminous and sub-bituminous coal and produce large
quantities of fly ash. High ash content (30% - 50%) coal contributes to these large volumes of fly ash. Current annual
production of Fly ash, a by-product from coal based thermal power plant (TPPs), is about 112 million tones (MT). Some of the
problems associated with Fly ash are large area of land required for disposal and toxicity associated with heavy metal
leached to groundwater. Fly ash, being treated as waste and a source of air and water pollution till recent past, is in fact a
resource material and has also proven its worth over a period of time. The present paper reviews the potential applications for
coal fly ash as a raw material: as a soil amelioration agent in agriculture, use, in highway embankments, in construction of
bricks, as an aggregate material in Portland cement, filling of low lying areas etc in the manufacture of glass and ceramics, in
the production of zeolites, in the formation of mesoporous materials, in the synthesis of geopolymers, for use as catalysts and
catalyst supports, as an adsorbent for gases and waste water processes, and for the extraction of metals. Thus fly ash
management is a cause of concern for the future. This article attempts to highlight the management of fly ash to make use of
this solid waste, in order to save our environment.
Keywords: Fly ash, particulate matter, thermal power plants, waste management, water pollution.
INTRODUCTION
India is the third largest producer of coal and coal based
thermal power plant installations in India contribute about 70% of the
total installed capacity for power generation [1]. However, the
bituminous and sub-bituminous coals used contain over 40% ash
content. At present, 120-150 million tons of coal fly ash is generated
from 120 existing coal based thermal power plants in India [2]. Coal
fly ash is an industrial waste generated from coal combustion
process in thermal power plants. It is a fly ash, a coal combustion
residue having a complex heterogeneous mixture of amorphous and
crystalline phases and is generally fine powdered
ferroaluminosilicate material with Al, Ca, Mg, Fe, Na and Si as the
predominant elements. The coal fly ash also contains significant
amounts of toxic metals such as As, Ba, Hg, Cr, Ni, V, Pb, Zn and Se
characteristically enriched in coal fly ash particles [3-5]. The coal fly
ashes occupy more space in the premises of industrial plants and
are mixed with water to discharge into fly ash settling ponds or land
fills. Large quantities of coal fly ashes are stored in the form of waste
heaps or deposits, whose contamination poses a serious threat to
the environment as a major source of inorganic pollution. The
behavior of many metal pollutants and the release of such metals
during storage can have deleterious effects on the environment as
well as on human health [6]. Metals present in the ashes are
originated from the composition of the coal used in combustion,
combustion conditions, removal efficiency of air pollution control
device and method of coal fly ash disposal [7].
Fig 1. Fly ash production (million tonnes/year) in different countries
(source: http://www.tifac.org.in)
Metals present in the ashes are originated from the compo-
Large number of innovative alternate building materials and low cost
construction techniques developed through intensive research efforts
during last three to four decades satisfies functional as well as
specification requirements of conventional materials/techniques and
provide an avenue for bringing down the construction cost. Fly Ash,
an industrial by-product from Thermal Power Plants (TPPs), with
*Corresponding Author
Aakash Dwivedi
Department of Environmental Science and Engineering, Indian School of Mines,
Dhanbad-814004, Jharkhand, India
Email: aakashdwivedi4@gmail.com

Recent Research in Science and Technology 2014, 6(1): 30-35
31
current annual generation of approximately 112 million tones and its
proven suitability for variety of applications as admixture in
cement/concrete/mortar, lime pozzolana mixture (bricks/blocks) etc.
Cement and Concrete Industry accounts for 50% Fly Ash utilization,
the total utilization of which at present stands at 30MT (28%). The
other areas of application are Low lying area fill (17%), Roads &
Embankments (15%), Dyke Raising (4%), Brick manufacturing (2%)
and other new areas for safe disposal of fly ash is in paint industry,
agriculture etc [8].
Fig 2. Utilization (%) of total produced fly ash in different countries
(Source: http://www.tifac.org.in)
Effluent and disposal
Disposal and management of fly ash is a major problem in
coal-fired thermal power plants. Fly ash emissions from a variety of
coal combustion units show a wide range of composition. All
elements below atomic number 92 are present in coal ash. A 500
MW thermal power plant releases 200 mt SO2, 70 t NO2 and 500 t
fly ash approximately every day. Particulate matter (PM) considered
as a source of air pollution constitutes fly ash. The fine particles of fly
ash reach the pulmonary region of the lungs and remain there for
long periods of time; they behave like cumulative poisons. The
submicron particles enter deeper into the lungs and are deposited on
the alveolar walls where the metals could be transferred to the blood
plasma across the cell membrane (fig. 1). The residual particles
being silica (40–73%) cause silicosis. All the heavy metals (Ni, Cd,
Sb, As, Cr, Pb, etc.) generally found in fly ash are toxic in nature [9].
Fly ash can be disposed-off in a dry or wet state. Studies show that
wet disposal of this waste does not protect the environment from
migration of metal into the soil. Heavy metals cannot be degraded
biologically into harmless products like other organic waste. Studies
also show that coal ash satisfies the criteria for landfill disposal,
according to the Environmental Agency of Japan. According to the
hazardous waste management and handling rule of 1989, fly ash is
considered as non-hazardous. With the present practice of fly-ash
disposal in ash ponds (generally in the form of slurry), the total land
required for ash disposal would be about 82,200 ha by the year 2020
at an estimated 0.6 ha per MW. Fly ash can be treated as a by-
product rather than waste [10].
Laws and Legislation of Disposal of Flyash
Historically, wastes have always created a disposal problem.
The problem of flyash disposal has assumed such an enormous
scale in the country that the Ministry of Environment and Forests
(MoEF) issued a regulation on 14 September 1999 specifying
normative levels for progressive utilization of flyash. According to the
regulation, it is mandatory for the existing (old) and new coal based
thermal power plants to utilize 100% of the flyash produced in a
stipulated time horizon. The new coal thermal power plants are
required to use 100% of the flyash produced within nine years of
commencing operation. The old power plants, however, are required
to achieve 100% flyash utilization goal with in 15 years from the date
of issue of the regulation [11].
Table 1. Thermal power generation, coal consumption and ash generation in India
(Source: Current Sc 1792 IENCE Vol. 100,No. 12, 25 June 2011)
1995
54,000
200
75
2000
70,000
250
90
2010
98,000
300
110
2020
137,000
350
140
Fig 3. Penetration of tiny particles into the lungs.( Source:- Current Sc 1792 IENCE Vol. 100,No. 12, 25 June 2011)

Dwivedi and Jain
32
Classification of fly ash
Fly ash particles are generally spherical in shape and range in
size from 0.5 µm to 100 µm. They consist mostly of silicon dioxide
(SiO
2
), which is present in two forms: amorphous, which is rounded
and smooth, and crystalline, which is sharp, pointed and hazardous;
aluminum oxide (Al
2
O
3
) and iron oxide (Fe
2
O
3
). Fly ashes are
generally highly heterogeneous, consisting of a mixture of glassy
particles with various identifiable crystalline phases such as quartz,
mullite, and various iron oxides.
Two classes of fly ash are defined by ASTM C618: Class F fly
ash and Class C fly ash. The chief difference between these classes
is the amount of calcium, silica, alumina, and iron content in the ash.
The chemical properties of the fly ash are largely influenced by the
chemical content of the coal burned (i.e., anthracite, bituminous, and
lignite) [13].
Class C fly ash
Fly ash produced from the burning of younger lignite or sub
bituminous coal, in addition to having pozzolanic properties, also has
some self-cementing properties. In the presence of water, Class C fly
ash will harden and gain strength over time. Class C fly ash
generally contains more than 20% lime (CaO). Unlike Class F, self-
cementing Class C fly ash does not require an activator. Alkali and
sulfate (SO
4
) contents are generally higher in Class C fly ashes [12].
Class F fly ash
The burning of harder, older anthracite and bituminous coal
typically produces Class F fly ash. This fly ash is pozzolanic in nature,
and contains less than 10% lime (CaO). Possessing pozzolanic
properties, the glassy silica and alumina of Class F fly ash requires a
cementing agent, such as Portland cement, quicklime, or hydrated
lime, with the presence of water in order to react and produce
cementitious compounds. Alternatively, the addition of a chemical
activator such as sodium silicate (water glass) to a Class F ash can
lead to the formation of a geopolymer [12].
Fig 4. Typical ash colors (Class „F‟ & „C‟ Fly ash)
(Source:- International journal of emerging trands in Engineering and Development Issue1, Vol 1August2011)
Fly ash utilization
During the last 30 years, extensive research has been carried
out to utilize the fly ash in various sectors, as this is not considered
as hazardous waste. Broadly, fly ash utilization programmes can be
viewed from two angles, i.e. mitigating environmental effects and
addressing disposal problems (low value–high volume utilization) [9].
Following are some of the potential areas of use of fly ash:-
Development of Fly Ash Based Polymer Composites as Wood
Substitute
Fly ash based composites have been developed using fly ash
as filler and jute cloth as reinforcement. The technology on fly ash
Polymer Composite using Jute cloth as reinforcement for wood
substitute material can be applied in many applications like door
shutters, partition panels, flooring tiles, wall panelling, ceiling, etc.
This technology has been developed by Regional Research
Laboratory, Bhopal in collaboration with Building Materials &
Technology Promotion Council (BMTPC) and TIFAC. One
commercial plant has also been set up based on this technology
near Chennai [13].
Fly Ash Based Cement
As per the specifications of Bureau of Indian Standards fly
ash upto 35% can be used in manufacture of PPC, while worldwide
there are examples of countries that permit upto 55% utilisation of fly
ash in PPC production. Setting aside 25% of cement production for
OPC for such applications, the balance 75% can be PPC with an
average fly ash content of 30% [14]. It would consume around 25 MT
fly ash, replacing same amount of cement clinker and resulting in net
saving Rs. 2500 crore [15].
Role of bio-amelioration of FA on soil
Recent investigations suggest that FA can find better
application if combined with organic amendments such as cow
manure,press mud, paper factory sludge, farmyard manure, sewage
sludge,crop residues and organic compost for improvement of
degraded/marginal soil [16]. Few beneficial combined effects of FA
and organic matter on soil have been found such as reduced heavy-
metal availability and killing pathogens in the sludge [17]; improved
soils through higher nutrient concentrations,better texture,lower bulk

Recent Research in Science and Technology 2014, 6(1): 30-35
33
density, higher porosity and mass moisture content and higher
content of fine-grained minerals [18]; enhanced the biological activity
in the soil [19]; reduced the leaching of major nutrients [20]; and
beneficial for vegetation [21]; .Use of swine manure with FA
increased the availability of Ca and Mg balancing the ratio between
monovalent and bivalent cations (Na
++
K
+
/Ca
2+
Mg
2+
), which
otherwise proves detrimental to the soil [22]; Co-utilization of ‘slash’
a mixture of FA, sewage sludge and lime in the ratio of 60:30:10 had
beneficial soil ameliorating effect. ‘Slash’ incorporation in soil showed
positive effects on soil pH and Ca, Mg and P content and reduction
in the translocation of Ni and Cd [23] and enhanced growth and yield
of corn, potatoes and beans in pot trials. So, amendment with FA will
enhance agricultural sector for crop production. Further, organic
amendment application will provided anchorage and growth of the
plant on a FA dumping site [24].
Fly ash bricks
The Central Fuel Research Institute, Dhanbad has developed
a technology for the utilization of fly ash for the manufacture of
building bricks [9]. Fly Ash can be used in the range of 40-70%. Our
current clay brick production exceeds 100 billion bricks a year. In
such circumstances and when fly ash brick is technically acceptable,
economically viable and environment friendly, it may not be wrong to
target to produce at least 2 billion fly ash bricks per year. It would
consume about 5 million tonne of flyash/year, yielding a net saving of
around Rs. 20 crores per annum. Fly ash bricks have a number of
advantages over the conventional burnt clay bricks. Unglazed tiles
for use on footpaths can also be made from it. Awareness among the
public is required and the Government has to provide special
incentives for this purpose [21].
Fly ash in distemper
Distemper manufactured with fly ash as a replacement for
white cement has been used in several buildings in Neyveli, Tamil
Nadu, in the interior surfaces and the performance is satisfactory.
The cost of production will only be 50% that of commercial distemper
[9].
Fly ash-based ceramics
The National Metallurgical Laboratory, Jamshedpur has
developed a process to produce ceramics from fly ash having
superior resistance to abrasion [9].
Ready mixed Fly ash concrete
Though Ready Mix concrete is quite popular in developed
countries but in India it consumes less than 5 percent of total cement
consumption. Only recently its application has started growing at a
fast rate. On an average 20% Fly ash (of cementitious material) in
the country is being used which can easily go very high. In ready mix
concrete various ingredients and quality parameters are strictly
maintained/controlled which is not possible in the concrete produced
at site and hence it can accommodate still higher quantity of fly ash
[25].
Minefills
Nearly one third of our thermal power stations are at or near
to pit heads. Most of these mines cart sand for backfilling from river
beds, which are normally 50-80 kms away. Apart from the royalty,
huge amount of expenditure is incurred on transportation of sand. It
is estimated that about 15-20 million tonne of ash per annum can be
safely consumed in minefills yielding a saving of about Rs. 150 crore
a year [14].
Fly Ash in Road Construction
Fly ash can be used for construction of road and embankment.
Saves top soil which otherwise is conventionally used, avoids
creation of low lying areas (by excavation of soil to be used for
construction of embankments) [8]. Fly Ash may be used in road
construction for: Stabilizing and constructing sub-base or base;
upper layers of pavements; filling purposes. Concrete with Fly Ash
(10-20% by wt) is cost effective and improves performance of rigid
pavement; Soil mixed with Fly Ash and lime increases California
Bearing Ratio (CBR), increased (84.6%) on addition of only Fly Ash
to soil. National Highway Authority of India (NHAI) is currently using
60 lakh m
3
of Fly Ash and pr oposed to use another 67 lakh m
3
in
future projects.
Embankment
Fly ash properties are somewhat unique as an engineering
material. Unlike typical soils used for embankment construction, fly
ash has a large uniformity coefficient consisting of clay-sized
particles. Engineering properties that will affect fly ash use in
embankments include grain size distribution, compaction
characteristics, shear strength, compressibility, permeability, and
frost susceptibility. Nearly all fly ash used in embankments are Class
F fly ashes [9]
In view of the growing need for development of road
infrastructure in the country, conservative estimates show that about
15-20 MT ash can be used in construction of road and flyover
embankments per annum in the vicinity of TPPs. This would yield a
saving of around Rs. 100 crore per year [16].
Roller compacted concrete
Another application of using fly ash is in roller compacted
concrete dams. Many dams in the US have been constructed with
high fly ash contents. Fly ash lowers the heat of hydration allowing
thicker placements to occur. Data for these can be found at the US
Bureau of Reclamation. This has also been demonstrated in the
Ghatghar Dam Project in India [14].
Asphalt concrete
Asphalt concrete is a composite material consisting of an
asphalt binder and mineral aggregate. Both Class F and Class C fly
ash can typically be used as a mineral filler to fill the voids and
provide contact points between larger aggregate particles in asphalt
concrete mixes. This application is used in conjunction or as a
replacement for, other binders (such as Portland cement or hydrated
lime) [14]. For use in asphalt pavement, the fly ash must meet
mineral filler specifications outlined in ASTM D242. The hydrophobic
nature of fly ash gives pavements better resistance to stripping. Fly
ash has also been shown to increase the stiffness of the asphalt

Dwivedi and Jain
34
matrix, improving rutting resistance and increasing mix durability [8].
Use of Fly Ash in Agriculture
Agriculture and waste land management have emerged as
prime bulk utilization areas for fly ash in the country. It improves
permeability status of soil; improves fertility status of soil (soil health)/
crop yield; improves soil texture; reduces bulk density of soil;
improves water holding capacity/porosity; optimizes pH value;
improves soil aeration; reduces crust formation provides micro
nutrients like Fe, Zn, Cu, Mo, B, Mn; provides macro nutrients like K,
P, Ca, Mg, S etc; works as a part substitute of gypsum for
reclamation of saline alkali soil and lime For reclamation of acidic
soils; ash ponds provides suitable conditions and essential nutrients
for plant growth, helps improve the economic condition of local
inhabitants; crops grown on fly ash amended soil are safe for human
consumption & groundwater quality is not affected [8].
Use of fly ash in agriculture has also proved to be
economically rewarding. The improvement in yield has been
recorded with fly ash doses varying from 20 tonne/hectare to 100
tonne/hectare. On an average 20-30% yield increase has been
observed Out of 150 million hectare of land under cultivation, 10
million hectares of land can safely be taken up for application of fly
ash per year. Taking a moderate fly ash dose of 20 mt per hectare it
would consume 200 million tonne flyash per year. This is more than
the annual availability of fly ash, therefore the shortfalls would be met
from accumulated 1500 million tonne stock of fly ash (available in
ash ponds). The fly ash treated fields would give additional yield of 5
million tonne foodgrains per year valued at about Rs. 3000 crore [15].
Table 2. Economic benefits of fly ash management
S.No
.
Utilisation
Fly Ash Consumption (Million
tonnes/year)
Savings per ye
ar (rupees in
crore)
1
Cements
25
2500
2
Roads and Embankments
15
-
20
100
3
Minefills
15
-
20
150
4
Bricks
5
20
5
Agriculture
200
3000
Total
5770 around 1.2billion US$
CONCLUSION
It has been recognized worldwide that the utilization of an
enormous amount of fossil fuels has created various adverse effects
on the environment, including acid rain and global warming. An
increase in the average global temperature of approximately 0.56 K
has been measured over the past century (global warming). Gases
with three or more atoms that have higher heat capacities than those
of O
2
and N
2
cause the greenhouse effect. Carbon dioxide (CO
2
) is a
main greenhouse gas associated with global climate change. The
disposal, management and proper utilization of waste products has
become a concern for the scientists and environmentalists. Proper
management of solid-waste fly ash from thermal power plants is
necessary to safeguard our environment. Because of high cost
involved in road transportation for the dumping of fly ash, it is
advisable to explore all its possible applications. Fly ash is a
potential source of pollution not only for the atmosphere but also for
the other components of the environment. Deposition in storage
places can have negative influences on water and soil because of
their granulometric and mineral composition as well morphology and
filtration properties. This waste has found application in domestic and
wastewater treatment, purification, paint and enamel manufacturing.
In future, large-scale application of this waste product may be
possible for recovery of heavy metals, reclamation of wasteland, and
floriculture. The detailed investigations carried out on fly ash
elsewhere as well as at the Indian Institute of Science show that fly
ash has good potential for use in highway applications. Its low
specific gravity, freely draining nature, ease of compaction,
insensitiveness to changes in moisture content, good frictional
properties, etc. can be gainfully exploited in the construction of
embankments, roads, reclamation of low-lying areas, fill behind
retaining structures, etc.
On the other hand it can safely be concluded that fly ash,
which till recent years has been treated as a waste product of
thermal power stations, is in fact a valuable resource material.
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