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ReuseOptionsforCoalFiredPowerPlantBottomAshandFlyAsh

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Reuse options for coal fired power plant bottomash and fly ash

Madawala Liyanage Duminda Jayaranjan •

Eric D. van Hullebusch • Ajit P. Annachhatre

� Springer Science+Business Media Dordrecht 2014

Abstract Reuse options for coal fly ash and coal

bottom ash are reviewed in this paper. Although,

significant quantities of coal fly ash and coal bottom

ash are produced worldwide every year, less than

30 % of coal ash produced is reused. Coal ash is

mainly reused in civil engineering applications such as

road construction, embankments, construction mate-

rials, geo-polymer applications and in cement pro-

duction. Other potential reuse options for coal ash

include applications such as glass ceramics, water and

wastewater treatment, agriculture as well as for

making high value products (e.g. telescope mirrors,

break-liners, fire proof products etc.). Considering that

only a small fraction of coal ash is reused, other reuse

options for commercial applications need to be

explored.

Keywords Power plant � Bottom ash � Fly ash �Reuse � Materials recovery

Abbreviations

ASTM American Society of Testing of Materials

CBA Coal bottom ash

CFA Coal fly ash

DNA Deoxyribonucleic acid

EU European Union

FGD Flue gas desulfurization

GBA Ground bottom ash

HeCB Heptachloro biphenyl

LOI Loss on ignition

OPC Ordinary Portland cement

PAH Polycyclic aromatic hydrocarbon

PCB Polychlorinated biphenyl

TCB Tri chloro biphenyl

TOC Total organic carbon

TPPs Thermal power plants

TW Tinacal ore waste

ZFA Zeolited fly ash

USA United States of America

1 Introduction

Economic growth is correlated with the use of energy.

However, clean energy with minimum environmental

impacts is vital for socioeconomic development,

particularly in developing countries. Today, fossil

fuels are the major source of energy generation.

Demand for fossil fuels is increasing at a rapid rate for

M. L. D. Jayaranjan � A. P. Annachhatre (&)

School of Environment, Resources and Development,

Asian Institute of Technology, P.O. Box 4, KhlongLuang,

Pathumthani 12120, Thailand

e-mail: ajit@ait.ac.th

M. L. D. Jayaranjan

e-mail: duminda@gmx.net

E. D. van Hullebusch

Laboratoire Geomateriaux et Environnement, Universite

Paris-Est, EA 4508, 5 bd Descartes, 77454 Marne la

Vallee Cedex 2, France

123

Rev Environ Sci Biotechnol

DOI 10.1007/s11157-014-9336-4

the last two decades due to increasing consumption of

fossil fuels by USA, India, China and EU (Chinda-

prasirt et al. 2009; IEA 2012). It is anticipated that

energy generation from nuclear sources in the EU

would decline (Kurama and Kaya 2008) due to

possibilities of nuclear accidents such as Fukushima

incident occurred in the year 2011 in Japan (Davies

2011; Vivoda 2012; Vujic et al. 2012).

Today coal still remains as a major source for

energy generation worldwide. Use of coal in power

plants generates fly ash as well as bottom ash in high

quantities. Typically burning of 15–18.75 tons of coal

generates 1 megawatt of electricity as well as

4.3–11 tons of bottom ash and fly ash (Asokan et al.

2005; EGAT 2010). Total ash generation from burning

of coal can be 25–60 % (Asokan et al. 2005; Kizgut

et al. 2010). Approximate total annual coal ash

generation in the world is in the range of 600–800 mil-

lion tons (Wang et al. 2005; Hui et al. 2009). Fly ash

and bottom ash contribution from the total ash varies

in ranges of 65–95 and 5–35 % respectively (Levan-

dowski and Kalkreuth 2009; Wang et al. 2005). Total

country wise annual ash generation is as follows:

USA: 125 million tons, Europe: 100 million tons

(Kizgut et al. 2010), China: 150 million tons (Cao

et al. 2008), India: 105 million tons (Asokan et al.

2005) and Thailand: 4 million tons (Chindaprasirt

et al. 2009).

Coal ash is disposed of in ash dumps either as wet or

in dry form (Lopez-Anton et al. 2007). Bottom ash and

fly ash are commonly disposed of as slurry after

mixing them with water (Chakraborty and Mukherjee

2009). The slurry is often discharged into ponds or

lagoons (Asokan et al. 2005; Prezzi and Kim 2008) or

sometimes into ocean (Horiuchi et al. 2000). In dry

disposal system, coal ash may be disposed of into

landfills often mixed with waste gypsum produced

from flue gas desulfurization (FGD) unit from the

power plant (Sathonsaowaphak et al. 2009).

Coal ashes contain heavy metals and metalloids, i.e.

Arsenic (As), Lead (Pb), Zinc (Zn), Nickel (Ni), Copper

(Cu), Manganese (Mn), Cadmium (Cd), Chromium

(Cr) and Selenium (Se) at trace levels (Dincer et al.

2007; Pandey et al. 2011; Su and Wang 2011;

Jayaranjan and Annachhatre 2012). These elements

can be leached out under acidic conditions and can

contaminate the surrounding soils, surface water and

ground water sources (Bashkin and Wongyai 2002).

Furthermore coal fly ash also may contain polycyclic

aromatic hydrocarbons (PAHs). In some cases, radio-

active contamination can also be present (Popovic et al.

2001). Such contamination of heavy metals can enter

the food chain leading to genotoxic effects to DNA

(Chakraborty and Mukherjee 2009). Coal ash often is

considered as ‘‘non-hazardous’’. However, the effects

may be many-folds since high volumes of waste remain

in the dumps for a long time (Twardowska and

Szczepanska 2002). At present, coal ash disposal is a

major issue worldwide due to its possible adverse

environmental impacts as well as due to its high volume

of generation which requires large land area for

disposal. It is observed that wet ash disposal methods

lead to greater adverse environmental impact than dry

ash disposal methods (Hansen et al. 2002).

Coal ash and bottom ash are reused in sectors such

as, cement, concrete, structural fill, soil stabilization

and agriculture (Rifa et al. 2009; Chindaprasirt et al.

2009). It is estimated that about 2.2 billion tons of

cement is annually produced worldwide (Yang et al.

2007). Considering that fly ash and cement can be

mixed in proportion of 1:1 in concrete, up to 2.2 billion

tons of fly ash can be utilized for concrete production

(Mehta 1998). Therefore, reuse of fly ash and bottom

ash is a great challenge as only 30 % total ash

produced in the world is currently reused. Accord-

ingly, this paper reviews existing utilization and reuse

options for both fly ash and bottom ash. Use of fly ash

and bottom ash in sectors like wastewater treatment,

zeolites, adsorbents and material recovery options etc.,

are reviewed.

2 Properties of power station coal fly ash

Physical and chemical properties of coal fly ash are

influenced by the raw coal source, size, type of coal

burner and the operating conditions (Sharma et al.

1989). During the coal combustion process in the

power plant, the minerals in coal may undergo through

sequential conversion and eventually may be present

in the fly ash.

2.1 Physical properties of coal fly ash

The coal fly ash particles are generally grey in colour.

They can be light grey or dark grey in color depending

upon unburned carbon content and combustion tech-

nology applied (Collot 2006; Kimura et al. 1995). Other

Rev Environ Sci Biotechnol

123

main properties of fly ash such as bulk density, porosity,

particle size distribution, etc. are important for its use in

civil construction engineering or in agriculture (Sato

and Nishimoto 2001; Horiuchi et al. 2000). Typical

values of properties are: specific gravity:

2.10–2.81(Prezzi and Kim 2008; Sharma et al. 1989),

particle size distribution: 0.001–0.075 mm (Prezzi and

Kim 2008), moisture content: 7.75 wt%, bulk density:

1.12–1.28 g cm-3 and specific surface area:

1.0–9.44 m2 g-1 (Theis and Gardner 1990).

2.2 Structure and chemical composition of coal fly

ash

Coal fly ash is divided into two main types, Class F and

Class C. Fly ash consists of mainly oxides such as

SiO2, Al2O3, Fe2O3, TiO2 and CaO (Iyer 2002). High

CaO ([8 %) fly ash is classified as Class C and Low

CaO (\8 %) fly ash defined as Class F (Manz 1999).

Class C fly ash is a product from the burning of lignite

or sub-bituminous coal, while Class F is a product

from combusting of bituminous or anthracite coal

(Furlong and Hearne 1994). ASTM standard for fly

ash classes are mentioned in Table 1.

Chemically all naturally occurring elements can be

found in coal ash; Table 2 shows the major elements

present in coal ash. Also, an empirical formula has

been developed for coal fly ash (Fisher et al. 1976) as:

Si1:00Al0:45Ca0:051Na0:047Fe0:039Mg0:020K0:017Ti0:011

The mineral composition of coal fly ash from

burning of lignite, sub-bituminous, bituminous or

anthracite coal can be in the range of 0–60 % by

weight. Chemical composition of fly ash can be

typically as follows (in wt%): SiO2, 15–60; Al2O3,

3.68–35; Fe2O3, 4–40; CaO, 1–40; MgO, 0–10; LOI,

0–18.77 (Ahmaruzzaman 2010; Rifa et al. 2009; Silva

et al. 2010) (Table 2). Furthermore, heavy metals and

metalloids may also be present in coal fly ash in

concentrations as presented in Table 3.

These heavy metals may be subjected to leach out

to the environment under various pH values. Some

cations tend to leach extensively under acidic condi-

tions while some oxyanionic elements such as As, B,

Table 2 Major elemental composition of coal fly ash

Composition Lignitea Sub-bituminousb Bituminousc Anthraciteb

SiO2 (wt%) 14.80–50.00 52.2–55.2 56.7 43.5–47.3

Al2O3 (wt%) 3.40–25.70 19.9–23.1 38.4 25.1–29.2

Fe2O3 (wt%) 0.86–11.80 6.1–9.7 2.5 3.8–4.7

MgO (wt%) 0.50–9.10 1.0–1.2 0.2 0.7–0.9

CaO (wt%) 13.00–54.10 3.7–3.8 1.1 0.5–0.9

Na2O (wt%) 0.18–1.23 0.3 0.04 0.2–0.3

K2O (wt%) 0.20–4.89 1.0 0.6 3.3–3.9

TiO2 (wt%) 0.23–1.68 1.1–1.2 0.5 1.5–1.6

P2O5 (wt%) – 0.3–0.5 0.02 0.2

MnO (wt%) 0.04–0.21 0.1 0.02 0.1

SO3 (wt%) 3.00–22.10 – 0.2 –

S (wt%) – 0.1–0.7e 0.08–0.67f 0.1

LOI – 1.8–2.7e – 8.2d

a Baba and Kaya (2004), b Choi et al. (2002), c Pires and Querol (2004), d Haibin and Zhenling (2010), e Vassilev et al. (2005), f Lee

et al. (2011)

Table 1 Composition of fly ash classes as per ASTM

standards

Chemical differences Class F Class C

SiO2 ? Al2O3 ? Fe2O3, minimum % 70.00 50.00

SO3, maximum % 5.00 5.00

Moisture content, maximum % 3.00 3.00

LOI, maximum % 6.00 6.00

Available alkalis (as Na2O), maximum % 1.50 1.50

Source: ASTM standard C 618 - 95; composition requirement

for fly ash classes

Rev Environ Sci Biotechnol

123

Cr, Mo, V and W tend to leach out under alkaline

conditions (Izquierdo and Querol 2012). Many studies

have been conducted highlighting metal mobility

behavior of coal fly ash (Choi et al. 2002; Dutta

et al. 2009; Janos et al. 2002; Kim and Hesbach 2009;

Nathan et al. 1999; Popovic and Djordjevic 2009;

Izquierdo and Querol 2012).

3 Coal fly ash utilization options

Several options are available for reuse of coal

combustion waste (CCB) which gives additional

economic benefits. These include use in civil con-

struction, in wastewater treatment, for recovery of

metals and for production of materials like zeolites.

High proportion of coal fly ash is use in cement for

civil construction (Kizgut et al. 2010) and in highway

construction applications (Prezzi and Kim 2008). As

brought out earlier, worldwide utilization of ash is

15 % of its total production (Hui et al. 2009). This

proportion is still considerably low compared to the

current rate of generation of fly ash (1.65–2.20 million

tons day-1). Hence, it is important to search for fly ash

utilization options in various sectors.

3.1 Civil engineering applications, road

construction and in embankments

Coal fly ash is available at a cheaper price than soil

(Dewangan et al. 2010), hence civil contractors are

automatically encouraged to replace soil by fly ash.

Fly ash can be used as a land filler which improves soil

properties and also acts as a stabilizer (Sato and

Nishimoto 2001). Untreated coal fly ash mixed with

coal bottom ash was used to support the foundation of

an electrostatic precipitator in a coal fired power plant

(Leonards and Bailey 1982). Fly ash also has been

used as a structural filling material (Dewangan et al.

2010). Coal fly ash slurry could be utilized as an

effective filling material in civil engineering applica-

tions like underwater fills, light weight back fills and

for light weight structural filling applications (Horiu-

chi et al. 2000; Moulton et al. 1973). Use of fly ash as

filling material is considered as sustainable because of

the higher price of soil (Dewangan et al. 2010).

Table 3 Trace elements concentrations in coal fly ash

Heavy metal Heavy metal composition of coal fly ash (mg kg-1 dry basis)

Lignitea Sub-bituminousc Bituminousb Anthracited

As 13.5–172 110–141

B 386–400

Ba

Cd 1–312 18–35

Co 16–57 18–53 21–25 8–12

Cr 31–160 69–95 98–128 80–498

Cu 24–71.8 63–66 64–64 77–109

Hg 0.01–8.8

Li 113–119

Mn 182–566 460–588

Mo 162–386

Ni 36–242 74–174 49–61 41–72

Pb 9–847 29–32 20–1,192 36–103

Sn 101–109

Zn 59.6–249 93 3,500–5,800 43–167

V 147–210 10–30 130–175

U 3.8–4.6

Ti 28–64

a Brigden et al. (2002), Baba et al. (2008), Suwanvitaya and Wattanachai (2006), Sharma et al. (1989), b Levandowski and Kalkreuth

(2009), c,d Choi et al. (2002)

Rev Environ Sci Biotechnol

123

Mixture of fly ash and bottom ash can be useful

additives in soft soils to improve its engineering

properties such as strength, bearing capacity and for

decreasing displacement (Rifa et al. 2009). As a result

its use in highway construction has long been

practiced (Moulton et al. 1973). Extensive use of coal

fly ash with low proportion of bottom ash has also been

widely practiced as a construction material for high-

way embankments (Kim et al. 2005; Kumar and Patil

2006). Class F fly ash mixed with cement (2–10 wt%)

can be used very effectively as a base material in road

construction (Lav et al. 2006).

For above usage, that lignite or hard coal fly can be

used. Basically, the compaction ratio is important for

filling. Particle size distribution, geotechnical proper-

ties like, specific gravity, permeability, angular fric-

tion and consolidation characteristics are in concern

(Ahmaruzzaman 2010).

3.2 Binder material, OPC cement, geo-polymer

and sand replacement material

Ordinary Portland Cement (OPC) is the most widely

used binder material in the construction industry.

Researchers have found that OPC can be partly

replaced by fly ash and bottom ash (McCarthy and

Dhir 1999; Kula et al. 2002). OPC mixed with fly ash

when used in construction, exhibited improved product

qualities such as reduced crack-width and free drying

shrinkage index (Yang et al. 2007). Coal fly ash can be

used as a raw material to produce geo-polymer mortars

(Chindaprasirt et al. 2009; Izquierdo et al. 2009).

Unprocessed low lime (CaO) coal fly ash can also be

successfully used in concrete as a sand substitute

material (Jones and McCarthy 2005). Basically, in

order to use fly ash in cement, SiO2 content should be

more than 25 % and SiO2 ? Al2O3 ? Fe2O3 must be

greater than 70 %. Also, alkali (Na2O), SO3 and MgO

should be lower than 5, 3 and 4 wt%, respectively

(Solis-Guzman et al. 2011). Properties of light weight

aggregate such as density, water adsorption and

strength of aggregate are vital in these usages. Pozzo-

lanic reactivity of fly ashes is too important for the

usage (Geetha and Ramamurthy 2010). For above

groups of uses, it was found that with higher LOI,

coarse and conditioned fly ash is more suitable

(McCarthy and Dhir 1999). However, as per BS 3892

standards, maximum LOI which is allowable for the

usage is 7 % while recommended Chloride is 0.1 %.

3.3 Construction materials and engineering

products

3.3.1 Bricks, tiles and cement composites

Fly ash itself or mixed with bottom ash, sand and lime

have been used by researchers to develop solid bricks,

ceramic tiles and concrete blocks. Solid bricks were

made by compressing coal fly ash, bottom ash,

gypsum, calcium carbonate and lime (Furlong and

Hearne 1994). Other researchers used Class F fly ash,

coal bottom ash and waste foundry sand, mixed with

OPC to make bricks (Chaulia et al. 2009). Bricks made

from fly ash, mixed with clay in various proportions

possessed higher crushing strength compared to

normal clay bricks (Asokan et al. 2005). Furthermore,

up to 30 % cost savings where obtainable from fly ash

based bricks as compared to conventional bricks

(Chaulia et al. 2009). These researchers also con-

firmed that fly ash based bricks showed higher

compressive strength where lighter in weight and

showed lower water absorption as compared to

common clay bricks. Bricks made from 100 % lignite

fly ash mixed with water showed high compressive

strength as compared to red-clay bricks (Pimraksa

et al. 2000). Concrete bricks (solid blocks), hollow

blocks and paving stones can be made from Class F fly

ash mixed with OPC, bottom ash and foundry sand

(Kraus et al. 2003; Naik et al. 2005). Researchers also

have found that coal fly ash can replace OPC in

traditional recipe (Naik et al. 2005; Yang et al. 2007).

Pressed ceramic tiles have been made with coal fly

ash and stoneware clay with a composition of 70 and

30 wt% respectively (Sokolar and Smetanova 2010).

Ceramic wall tiles with improved strength could be

developed by using 6 % Tincal ore waste (borax solid

waste) and fly ash mixture as compared to standard

wall tiles (Olgun et al. 2005). Terracotta tiles made

from coal fly ash mixed with traditional raw material

showed improved physical and mechanical properties

as compared to usual clay and terracotta tiles (Kara

et al. 2004). Door shutters and window frames using

fly ash mixed with polymer based organic fiber have

also been developed (Saxena and Prabakhar 2000).

3.3.2 Glass–ceramics

Glass–ceramics have a number of uses in various

sectors ranging from construction industry to

Rev Environ Sci Biotechnol

123

manufacture of specialized products such as telescope

mirrors. Coal ash from bituminous coal was used to

synthesize glass and glass–ceramics which had Anor-

thite (CaAl2Si2O8) as the main glass–ceramic phase

(Cumpston et al. 1992). Vitrified coal fly ash with

Na2O ? CaO and BaO ? CaO as additives was used

to obtain nano-crystal glass ceramics with average

crystal size below 300 nm: the main phase of this

glass–ceramics was identified as Wollastonite (Ca-

SiO3) (Peng et al. 2004). Amorphous glass–ceramics

were produced using coal ash (40–50 % by weight)

mixed with other wastes like glass cullet, float

dolomites (Barbieri et al. 1999; Francis et al. 2004).

Glass–ceramics possess very unique mechanical

properties, such as abrasion resistance, high mechan-

ical strength and stability for a wide range of

utilization options. Fly ash based glass ceramics were

tested for physical and mechanical properties, thermal

expansion coefficient, density, hardness and bending

strength and very positive results were obtained

ensuring a wide range of applications in the construc-

tion industry (Peng et al. 2004; Cumpston et al. 1992).

Dilithium Dialuminium Trisilicates phases of

glass–ceramics have been used as materials for

making products like telescope mirrors, kitchen stove

hot plates etc.,. Coal fly ash and Lithium Hydroxide

Monohydrate (LiOH�H2O) were used to prepare

DilithiumDialuminiumTrisilicates (Li2Al2Si3O10)

based glass–ceramics (Yao et al. 2011).

Quartz in ash helps to obtain a relatively high

homogeneity of the glass–ceramics obtained (Cump-

ston et al. 1992).

3.3.3 Fire proof products

Fire-proof products can be developed using coal fly

ash by mixing with other waste types. Fly ash was

mixed as a main component with residues of waste

paper and other industrial wastes (Vilches et al. 2003),

RTi waste (a waste after the first attack on ilmenite in

the production of titanium dioxide) and Vermiculite

(up to 10 %) (Vilches et al. 2002) to make plates

which have higher insulating properties. These panels

and plates can be used to produce fireproof doors,

windows and for other fire-resistant products (Vilches

et al. 2002, 2003). It was found that both fly and

bottom ash increases the fire resistance of products

made with fly ash, due to the wide evaporation plateau

in ashes (Solis-Guzman et al. 2011). Then, it would be

made high fire resistance with fine particles, as high

surface area possess.

3.3.4 Sag-resistant gypsum boards

Gypsum boards are made by gypsum slurry, a

monolithic cellular core of set gypsum, which has

specific properties such as sag resistance for use in

various applications. In the common production

method, gypsum boards are produced by feeding

stucco, water, foam and additives between two paper

layers made of fibrous material. Gypsum boards made

of coal fly ash up to 20 wt% of stucco (calcium sulfate

hemihydrates) in gypsum slurry exhibited improved

sag resistance (Bruce and Kuntze 1983). The sag

resistance properties of fly ash are due to oxides of Ca,

Fe, Al and Si (Bruce and Kuntze 1983). These oxides

are present in considerable amounts in all fly ash types.

However, the exact mechanism for sag resistance

property of fly ash still has not been identified (Bruce

and Kuntze 1983).

3.4 Water and wastewater treatment applications

This chapter describes how the fly ash has been used in

pollution control. A summary for coal fly ash in

pollution control has been presented in tabulated form

(Table 4) for a better clarification to the areas of use as

below.

3.4.1 Low cost adsorbents for wastewater treatment

Coal fly ash (either treated or raw form) has a wider

range of uses as low cost adsorbents to remove toxic

compounds from contaminated water. Lignite fly ash

can be converted to zeolite, a potential adsorbent used

to remove synthetic dyes from waste waters by

sorption (Janos et al. 2003; Mohan et al. 2002).

Untreated coal fly ash itself was also used effectively

to remove textile anionic reactive dyes such as Acid

Red (AR1), Remazol Brillant Blue (RB), Remazol Red

133 (RR) and Rifacion Yellow HED (RY) from textile

dye processes (Dizge et al. 2008). Typical basic dyes

like Methylene Blue can be effectively removed from

aqueous solutions using heat and acid (like HNO3)

treated fly ash-red mud mixture (Wang et al. 2005).

NaOH treated coal fly ash was also effectively used to

remove cationic dyes such as Methylene Blue (Woo-

lard et al. 2002).

Rev Environ Sci Biotechnol

123

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Rev Environ Sci Biotechnol

123

Coal fly ash can be widely used to remove heavy

metals and metalloids from aqueous solutions. Fly ash

was used to remove Cadmium (Cd) and Copper (Cu)

from wastewaters effectively (Ayala et al. 1998;

Papandreou et al. 2007). Another researcher used

modified coal fly ash (by 3 M NaOH) to remove Lead

(Pb) at pH 5.0 (Woolard et al. 2000). Coal fly ash based

adsorbents have been used to remove Copper (Cu) and

Lead (Pb) from municipal wastewaters to reduce its

toxicity (Gupta and Torres 1998). Removal of Arsenic

(As) was achieved at an optimum pH of 4.0 (Diamad-

opoulos et al. 1993) while Mercury II (Hg2?) was

removed at pH 3.5–4.0 (Sen and De 1987). Heavy metal

removal efficiency by fly ash as adsorbent was found to

be pH dependent and heavy metals removal was found

satisfactory, if the pH of the solution was not strongly

acidic (Cho et al. 2005). On the other hand adsorption of

Arsenic (As) by coal fly ash was found to follow

Freundlich’s isotherm (Diamadopoulos et al. 1993).

Researchers have found that coal fly ash can also be

used to remove phosphate from aqueous solutions

(Agyei et al. 2002; Ugurlu and Salman 1998). However,

the adsorption was pH and temperature dependent.

Furthermore, phosphate removal up to 99.8 % at 40 �C

was obtained which could be attributed to the high

amount of calcite (CaO) present in coal fly ash (Ugurlu

and Salman 1998). A mixture of lignite coal and coal fly

ash was used as a low cost adsorbents to remove Boron

(B) from seawater very effectively, showing removal

efficiency as high as 95 % at pH 9.00 (Polat et al. 2004).

Polychlorinated biphenyl (PCB) is one of the persistent

organic pollutants which have adverse environmental

and toxicological impacts when discharged into the

environment. Several congeners of PCB from waste-

water could be removed by adsorption on to fly ash with

removal efficiency up to 97 % at 25 �C and pH 7.00

(Nollet et al. 2003). The property of fly ash for

adsorption characteristics was due to high surface area,

porosity and CaO percentage. Calcite makes hydrate

bonds when contacts with water and it lead to high

porosity, hence Class C fly ash having high CaO is best

suitable as an adsorbent (Papandreou et al. 2011).

3.4.2 Mine water treatment and control

Pumping of water from underground mines is a

common practice. Water pumped-out from under-

ground mines often has a high total hardness which

makes it difficult for domestic use. Researchers haveTa

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4co

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(Sec

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Rev Environ Sci Biotechnol

123

used coal fly ash treated with NaOH to remove

hardness from mine waters. Hardness removal up to

72 % could be obtained with fly ash dosage of

40 g L-1 (Prasad et al. 2011).

Mine tailings when exposed to rainwater and

oxygen from the atmosphere can generate acid mine

drainage (AMD). AMD can be highly acidic with

several heavy metals in dissolved form which can lead

to adverse environmental impacts. Fly ash can be

mixed with sulfidic mine tailings to control acidity.

Researchers found that lignite fly ash could increase

pH up to 10 by mixing it with 10–63 % of sulfidicmine

tailings, resulting in reduced Manganese (Mn) and

Zinc (Zn) in drainage waters (Xenidis et al. 2002).

Other researchers also demonstrated, utilization of fly

ash for reclamation of mine spoils (Ram and Masto

2010), and for controlling heavy metal mobility from

abandoned mines (Prasad and Mondal 2008; Gitari

et al. 2010). Neutralizing capacity of fly ash is the key

property for the usage.

3.5 Hazardous waste management

Stabilization and solidification (SS) applications are

widely used in hazardous waste management. Several

studies have been conducted using pozzolanic-based

coal fly ash as a binder and as the main raw material for

stabilization and solidification of heavy metals [Lead

(Pb), Cadmium (Cd), Chromium (Cr) and Zinc (Zn)]

present in waste arc furnace dust (Pereira et al. 2001).

Other researchers investigated, coalfly ash -quicklime

(CaO) mixture to stabilize Lead (Pb) and trivalent

Chromium (Cr3?) as well as hexavalent Chromium

(Cr6?) present in contaminated soils (Dermatas and

Meng 2003). Researchers also used fly ash as a partial

substitute for cement in hazardous waste stabilization

and solidification process (Conner and Hoeffner

1998). It was found, that fly ash can be mixed with

natural organic matter like peat to stabilize Copper

(Cu) and Lead (Pb) in soil with efficiencies of 98.2 and

99.9 % respectively (Kumpiene et al. 2007).

Liner is an important impermeable barrier of

hazardous waste disposal sites, which stops leaching

out of possible hazardous leachates into the soil. Type

C fly ash was employed together with rubber and

bentonite to develop low hydraulic conductivity liner

materials. Leachate analysis and other performance

tests on such fly ash based liner material had shown

satisfactory results (Cokca and Yilmaz 2004).

3.6 Agriculture

Many studies have been conducted on fly ash usage as

a soil amendment material in agriculture. Physical

properties like texture, water holding capacity, bulk

density and pH of the fly ash and enrichment with all

the essential plant nutrients may have the potential for

fly ash to be utilized as a valuable resource material in

agriculture (Singh et al. 2010). Ram et al. (2007) used

lignite fly ash as a fertilizer by adding to crops such as

groundnut, sun hemp and maize with a dose of 200

tons of coal fly ash per acre resulting in higher crop

yields. It was revealed, fresh coal fly ash can improve

soil qualities than weathered or pond ash (Ram et al.

2007). Mixing of coal fly ash with agricultural soils at

proportions\2 wt% (or\12 Mg ha-1), can improve

physical and hydrological characteristics like aggre-

gation and plant water availability of the soil (Yunusa

et al. 2011).

3.7 Artificial habitats

Artificial reefs are constructed in the vicinity of

seashore areas with a view to stop trawling, to provide

protection for eggs and juveniles and also to enhance

the restocking of fish populations. Studies were

conducted to construct such artificial reef blocks using

coal fly ash which was found compatible with living

organisms in the sea (Sampaolo and Relini 1994). Reef

blocks made of coal fly ash yielded higher compati-

bility with living organisms as compared to blocks

made from concrete. Negligible leaching of elements

was observed with long term leaching tests, ensuring

utility of coal fly ash for constructing artificial

habitats.

3.8 Value added materials

3.8.1 Zeolite materials

Zeolites have a wide range of applications in industry.

Zeolites are used as adsorbents, for ion exchange and

for controlling of mobilization of heavy metals (Shih

and Chang 1996; Juan et al. 2007; Querol et al. 2002;

Koukouzas et al. 2010). Zeolites could be synthesized

from the class F fly ash by treating it with NaOH (Shih

and Chang 1996). These zeolites have 70–80 % of

adsorption capacity as compared to commercial grade

zeolites (Mondragon et al. 1990). Researchers also

Rev Environ Sci Biotechnol

123

found that zeolites synthesized from fly ash (ZFAs)

could be used effectively to remove trivalent Chro-

mium (Cr3?) from wastewaters (Wu et al. 2008).

Zeolites produced from fly ash are environmentally

friendly and do not release heavy metals into the

environment compared to the raw fly ash from dump

sites (Steenbruggen and Hollman 1998).

Researchers have produced nano sized porous

zeolite materials (Zeolite A and MCM-41) from coal

fly ash, compatible with commercial grade types (Hui

et al. 2009). These zeolites have wide scientific and

industrial applications such as air and water purification

and gas adsorption. Hui and Chao (2006) succesfully

produced zeolites from fly ash which could be used as

detergent builder. This Zeolite 4 A product areequally

effective in removing calcium ions from water during

washing compared to the commercial detergent grade

zeolite 4A (Hui and Chao 2006; Hui et al. 2005).

3.8.2 Extraction of alumina and mullite

As brought out in the earlier section, approximately

3–35 % of Al2O3 is present in coal fly ash. Acid or base

soluble calcium aluminates can be prepared by calcining

coal fly ash and CaO mixture at 1,000–1,200 �C. Such

calcined ash sintered pellets could be used to extract

alumina materials using sulfuric acid as the solvent at an

efficiency of 85 % at 80 �C (Matjie et al. 2005).

Researchers have carried out studies to produce

mullite materials from coal fly ash. Gamma alumina

(alumina powder mixture) and pretreated coal fly ash

was used to produce mullite at a temperature of

1,400 �C (Ohtake et al. 1991). Amullite production

efficiency of 80 % was reported.

3.8.3 Extraction of cenospheres and rare earth

materials

Cenospheres consist of alumina and quartz composi-

tion, thermally stable up to 280 �C and having a

density of 0.3–0.6 g cm-3 (Kolay and Singh 2001).

Cenospheres are lightweight, low density, inert and

hollowed spherical material filled with air or gas.

These characteristics, make Cenospheres useful in

making value added products like light weight

concrete, structural materials and as well as to produce

ultra-light composite materials.

A study was conducted to extract Gallium (Ga)

from coal fly ash using foam extraction technique

(Fang and Gesser 1996). The efficiency of this

hydrometallurgical process of extraction was depen-

dent on acid strength, time and temperature.

3.9 Automotive brake-lining

Fly ash has thermally resilient properties, which can

withstand breaking temperatures. Automotive brake

linings are usually made of friction composites consist-

ing of phenolic resin, aramid pulp, glass fiber, potassium

titanate, graphite aluminium powder and copper powder.

Researches could produce friction composites incorpo-

rating coal fly ash more the 50 % by weight along with

standard materials as reported above (Mohanty and

Chugh 2007). Brake liners thus produced had wear rates

lower than 12 wt% with consistent friction coefficients in

the range of 0.35–0.40, thus confirming the utility of fly

ash as an ingredient for friction composites.

3.10 Sequester carbon dioxide

Coal fly ash consists of calcites (CaO) which can be

used to sequester CO2 by first carrying out reactions like

hydration of lime [CaO ? H2O ? Ca(OH)2] followed

by spontaneous carbonation of calcium hydroxide

[Ca(OH)2 ? CO2 ? CaCO3]. Coal fly ash was used

with 4.1 wt% of lime for CO2 sequestration by

performing CaO–CaCO3 chemical transformation

through aqueous carbonation at 82 % of carbonate

conversion efficiency (Montes-Hernandez et al. 2009).

Also, it was shown that 26 kg of CO2 can be captured

by 1 ton of fly ash. The CO2 sequester effectiveness by

fly ash was reported in literature as about 5 wt% at

75 �C (Arenillas et al. 2005). Also, it was reported that

CaO rich fly ashes and brine solution together were

found to be effective as a CO2 sequester (Soong et al.

2006). Further, CO2 adsorbents made from coal fly ash

are now commercially available. Therefore, fly ash—

CO2 sequestration adsorbents can be applied to mitigate

carbon dioxide (Palumbo et al. 2004) emissions from

power plants and industrial boilers.

4 Properties of coal bottom ash

4.1 Physical properties of coal bottom ash

Typically bottom ash production from coal power plant

ranges between 0.6 and 2.10 tons per megawatt of

Rev Environ Sci Biotechnol

123

energy produces per day (EGAT 2010; Asokan et al.

2005). Coal bottom ash has particle size in the range of

0.1–10 mm, with apparent dark grey colour. Other

characteristics of coal bottom ash are as follows. TOC:

11.74–52.24 wt% (Levandowski and Kalkreuth 2009),

specific gravity: 2.30–3.00, bulk density:

1.15–1.76 g cm-3 and specific surface area:

0.17–1.0 m2 g-1 (Theis and Gardner 1990; Ahmaruzz-

aman 2010). As compared to fly ash CBA is a bristly

(coarse) and granular materials (Mukhtar et al. 2003).

4.2 Structure and chemical composition of coal

bottom ash

CBA mainly consists of silicate, carbonate, aluminate,

ferrous materials and several of heavy metals and

metalloids. The exact composition of the CBA may

depend upon the raw coal source, size, type of coal

burner and the operating conditions of the burner.

Typical composition of coal bottom ash as reported by

researchers is presented in Table 5. Likewise heavy

metal and metalloid composition reported by research-

ers is presented in Table 6.

5 Coal bottom ash utilization options

5.1 Civil engineering applications

Natural aggregates like sand and coarse materials are

widely used in construction applications. Early

researchers suggested the use of coal bottom ash as a

replacement material of natural aggregates in con-

struction applications (Moulton 1973). Coal bottom

ash has similar geological characteristics as compared

to natural sand materials (Kumar and Stewart 2003b).

A study was conducted for utilizing bottom ash as a

partial or full replacement for natural sand (Suwanvi-

taya and Wattanachai 2006). However, it was con-

cluded that the compressive strength reduced with an

increase of CBA replacement percentage.

Several other studies have been conducted to find

the effectiveness of untreated pulverized coal bottom

ash as a structural fill material. Field density and plate

load tests were performed during fill construction to

check the consistency of the stabilized soil containing

CBA (Sato and Nishimoto 2001; Leonards and Bailey

1982). Coal bottom ash together with powdered

bentonite was used as an efficient construction fill

material (Kayabal and Bulus 2000). It has been found,

bottom ash modified with 15 % bentonite can be used

to construct landfill barriers having a suitable hydrau-

lic conductivity (Kumar and Stewart 2003b).

Researchers found that addition of bottom ash and

lime on soft soils improved engineering properties

such as strength, bearing capacity while minimizing

displacement properties (Rifa et al. 2009). Bottom ash

was also used effectively in highway construction for

base course applications (Moulton et al. 1973). Also

CBA together with Class F fly ash mixture was found

suitable for highway embankment construction. How-

ever, in this case, low bottom ash to high fly ash ratio

(ranging from 50 to 100 % of fly ash content) was

recommended for better performance and stability

(Kim 2003; Kim et al. 2005).

Low specific gravity bottom ashes lead to low

durability, hence not suitable for filing applications.

Therefore, relatively high specific gravity BA

(1.9–3.0) must be used for engineering applications

(Huang 1990). As stated above, these properties

depend upon the furnace type. The dry bottom ash is

considered as well graded and suitable for road bases,

however it was reported grading from no. 200 sieve

must not exceed 8 % (Huang 1990).

5.2 Portland cement and geo-polymer

Ordinary Portland cement (OPC) is used in mortar

preparation and cement mixtures. CBA could be used

as a pozzolana either in ground form by grinding in a

ball mill to small size, as raw bottom ash (Cheriaf et al.

1999) or as a Portland cement replacement material

(Jaturapitakkul and Cheerarot 2003). Ground bottom

ash (GBA) showed improved pozzolanic properties

(Jaturapitakkul and Cheerarot 2003) as well as with,

increased 28 days strength activity index by 27 %

(Cheriaf et al. 1999). Furthermore 25 % of ash mixed

with cement yielded 48.86 N mm-2 90 days curing

strength (Kizgut et al. 2010). In another study, Tinacal

ore waste together with CBA was utilized as a partial

replacement to OPC (Kula et al. 2002). Also, it was

found, cement mortars with 10 % replacement of the

OPC with bottom ash could yield compressive

strength up to 45.1 N mm-2 which is well suited for

construction applications (Kurama and Kaya 2008).

Geo-polymer mortars are also used in geotechni-

cal engineering applications. Preparation of geo-

polymer mortar was possible with coal bottom ash

Rev Environ Sci Biotechnol

123

as a raw material. Relatively high strength geo-

polymer mortar was developed with GBA mixing

with liquid Sodium silicate and NaOH (Sathonsaow-

aphak et al. 2009). Such geo-polymer mortar with

3 % GBA of 15.7 lm particle size could achieve a

mortar strength of 24–58 MPa. Strength of geo-

polymer was improved with the fineness of GBA.

Other researchers used dry bottom ash, amended

with bentonite for geotechnical engineering applica-

tions (Kumar and Stewart 2003a). However, unpro-

cessed coal bottom ash is less reactive than fly ash

in geo-polymerization (Chindaprasirt et al. 2009).

As a result use of ground bottom ash in geo-

polymerization applications are recommended.

Table 5 Major elemental composition of coal bottom ash

Composition as a percentage (%) otherwise stated

Composition Lignitea,h Sub-bituminousc Bituminousb,e Anthracited

SiO2 10.80–48.30 45.3 48.81–58.9 53.5

Al2O3 2.50–24.90 24.0 10.12–36.0 27.6

Fe2O3 0.50–8.20 18.0 2.4–6.10 6.0

MgO 0.40–4.60 0.58 0.2–5.61 2.1

CaO 8.60–45.10 1.4 1.3–11.81 3.4

Na2O 0.15–1.15 0.45 0.04–0.92 1.0

K2O 0.02–3.60 0.53 0.6–2.31 4.9

TiO2 0.18–1.32 1.5 0.39–0.60 1.0

P2O5 – 2.2 0.02–0.79 0.5

MnO 0.03–0.21 0.05 0.02–0.08 0.0

SO3 5.10–20.20 2.2 \0.1–4.06 –

S 0.1g 0.2–0.3f 0.01 0.54i

LOI 4.6g 9–17.8f 9.75

a Baba and Kaya (2004), b Ucurum et al. (2011), c Wee et al. (2005), d Russell et al. (2002), e Pires and Querol (2004), f Vassilev

et al. (2005), g Cheriaf et al. (1999), h Rifa et al. (2009), Cheriaf et al. (1999), Chindaprasirt et al. (2009), Dincer et al. (2007)

Table 6 Trace elements concentrations in coal bottom ash

Trace elements Trace element composition of bottom ash (mg kg-1 dry basis)

Lignitea Sub-bituminousc Bituminousb Anthracited

As – 25–30 1.8 \5

B – 321–467 15.30 –

Ba 62–109 428–523 – –

Cd \5 0.5–0.6 0.3 \2

Co 3–7 10–13 17.5

Cr 47–194 65–99 47 21–30

Cu 18–121 33–49 32 42–80

Hg 0.4–1.6e – – \0.5

Li 4–30e 93–147 28 –

Mn 97–328e 295–402 991 –

Ni 30–293 34–53 30 –

Pb 5–33 16–29 2.6 62–80

Zn 33–226 59–99 47 1,250–2,000

a Baba and Kaya (2004), b Pires and Querol (2004), c Vassilev et al. (2005), d Taeyoon (2011), e Bhangare et al. (2011)

Rev Environ Sci Biotechnol

123

5.3 Bricks and solid concrete products

Researchers produced solid bricks by compressing the

mixture of coal bottom ash (40–45 % by weight) with

gypsum, fly ash, calcium carbonate and limeup to

2,350 psi (Furlong and Hearne 1994). The product

retained sufficient structural strength for construction

applications. Researchers also used CBA, foundry

sand and Class F fly ash together with Portland cement

in various ratios to make cast concrete products like,

bricks, blocks and paving stones (Kraus et al. 2003).

5.4 Glass–ceramic products

CBA generally has low quartz (SiO2) and mullite

(3Al2O3�2SiO2) phases. Coal bottom ash could be

treated with Li2CO3, TiO2 and Al2O3to make

DilithiumDialuminiumTrisilicates (Li2Al2Si3O10)

glass–ceramics (Kniess et al. 2007). TiO2 could be

used to improve the crystallinity of the glass–ceramics

(Cumpston et al. 1992). The results showed high

crystallinity, however thermal expansion coefficient

of this glass ceramics was approximately less than

18 % compared to the commercial grade lithium

glass–ceramics.

5.5 Water and wastewater treatment

Studies have been carried out using CBA to remediate

domestic and industrial wastewaters. Use of CBA as a

potential adsorbent for removal of toxic dyes has been

extensively researched. It has been found that bottom

ash and de-oiled soya mixture can be used effectively

to remove water soluble azo dyes from textile

wastewaters (Mittal et al. 2005). A mixture of CBA

and de-oiled soya was also used to recover and remove

hazardous Tryphenylmethane dye and Brilliant Blue

FCP; a colorant in textile and leather industry (Gupta

et al. 2006). Other common dye types such as Vertigo

Blue 49 (CI Blue 49), Orange DNA13 (CI Orange 13)

and Malachite green from textile wastewaters were

also effectively treated using CBA (Dincer et al. 2007;

Gupta et al. 2004). Removal of Chemical Oxygen

Demand (COD) from coking and papermaking waste-

waters by the CBA was investigated. It was found,

10 g per 100 mL dosage of \0.074 mm particle size

CBA could remove organic pollutants by reducing

COD levels more than 40 % (Sun et al. 2008). Coal

bottom ash could also be utilized effectively as an

alternative medium in on-site sewage treatment

methods like mound type soil adsorption systems

(Viraraghavan and Dronamraju 1992). Trace elements

such as Silicon and aluminium in coal bottom ash

make it to have good adsorption capacity (Gorme et al.

2010). As, Si and Al present in all coal types, that

bottom ashes from lignite as well as hard coal can be

used as an adsorbent as stated above, either with

modifications or with mixing with other materials. A

summary of CBA in pollution control is given in

Table 7.

5.6 Soil reclamation and in agriculture

Studies have been conducted using CBA in agriculture

and for soil reclamation. Soil properties can be altered

in favor of the environment. It was revealed that clay

soils mixed with CBA can enhance tilth, reducing the

crust formation and higher friability of the clay soil

(Sell et al. 1989). Soils mixed with CBA were showed

high water holding capacities as well as air content,

also provides some mineral ingredients which are

favorable for plant growth. It was shown high yields of

peanut when CBA added at 15 kg m-2 proportion of

soil (Wearing et al. 2004). Some soils need a lime

addition to maintain its pH value. CBA can be used as

an alternative source for such lime requirement to

amend the soil (Korcak 1998). It may improve soil

structure, water infiltration and increase pH of the soil.

Such soils with enhanced texture are highly suitable

for cultivating of crops (Wearing et al. 2004; Sell et al.

1989). Another researcher used CBA with composted

dairy manure as a soil amendment. However, as CBA

contains heavy metals and it may leach out heavy

metals under some environmental conditions making a

threat to water quality (Mukhtar et al. 2003).

6 Risk assessment of coal ash reuse options

As an overview, use of ash in various sectors is

presented in the Fig. 1. As brought out in this figure,

majority of reuse options are cement based products

(ACAA 2010; ECOBA 2008; Dewangan et al. 2010;

Cao et al. 2008).

Potential hazards from coal ash and its products

mainly come due to its heavy metal contents. As

Rev Environ Sci Biotechnol

123

brought out in this report, coal ash contains several

heavy metals such as As, Pb, Zn, Ni, Cu, Mn, Cd, Cr

and Se. Although coal contains Mercury (Hg), a large

proportion of Hg is vaporized during combustion

process and remaining Hg usually is tightly bound to

the coal ash particles and therefore is not released into

the environment (Fairbrother et al. 2010). Other

metals present in coal such as Cd, Pb and As can pose

potential hazards to the environment, particularly

under acidic conditions when they can be leached out.

These metals can be taken up by the plants and

incorporated into the food chain due to their bioaccu-

mulation. Furthermore, other elements present in coal

ash such as As, Se and B are highly mobile in soil and

sediments and can be readily leached out. Out of these

elements, inorganic form of arsenic through drinking

water is poses significant health risk. However, since

environmental risk is a function of toxicity and

exposure, risk from coal ash can be reduced by

reducing its exposure.

On the other hand, researchers have reported that

leaching of elements from products containing coal

ash such as autoclaved aerated concrete (ACC) poses

no environmental threats (EPRI 1996). This is mainly

due to the fact that during concrete applications, the

trace elements in fly ash are stabilized in the hardened

concrete matrix, reducing their ability to leach out.

Solidification and stabilization (S/S) is a well-estab-

lished process which involves concrete to solidify and

stabilize heavy metals from coal ash. Also cement is

considered as the most adaptable binder for immobi-

lization of heavy metals (Giergiczny and Krol 2008;

Chen et al. 2009), thus, reducing the leachability of

heavy metals (USEPA 2011).

Following the accidental release of over one billion

gallons of ash from Tennessee Valley Authority

(TVA) Kingston Power Plant in December 2008, the

USEPA has proposed two regulatory options for coal

combustion products disposed of in landfills or surface

impoundments. Under the first option, Coal ash

residues disposed of in landfills or surface impound-

ments would be regulated as hazardous wastes under

subtitle ‘‘C’’ of Resources Conservation and Recovery

Act (RCRA). Under the second option, USEPA has

proposed to regulate coal as residues as non-hazardous

waste under RCRA subtitle ‘‘D’’. Beneficially used

residues such as CCP would be excluded from

definition of hazardous waste and hence would not

be regulated (USEPA 2013).Ta

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Rev Environ Sci Biotechnol

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7 Conclusions

Reuse options for coal fly ash and coal bottom ash are

reviewed in this paper. Significant quantities of coal

fly ash and coal bottom ash are produced worldwide

every year. Coal ash poses potential environmental

hazards due to its heavy metal contents which may be

leached out into environment. Only up to 30 % of fly

ash produced worldwide is reused. Coal fly ash is

mainly reused in civil engineering applications. These

applications include road construction, embankments,

construction, geo-polymer applications, cement etc.

However coal fly ash also can be reused in other

applications such as making glass–ceramics, water

and wastewater treatment, agriculture as well as for

making high value products. Considering that, only a

small fraction of coal ash (bottom ash and fly ash)

produced is reused, other reuse options need to be

explored for commercial applications.

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