LITERATURE REVIEW

18

Utilisation of coal for power production is currently

undergoing a rapid expansion. This will continue to increase

as coal is expected to supply one-half to two-thirds of the world's

additional energy requirements during the next 20 years, primarily

through generation of electric power (Warren et aI, 1984).

Therefore, it is an urgent need for every country to consider

the resulting environmental problems •.

Considerable research efforts have been made worldwide to

examine the environmental impact of coal-firing (Heit et aI,

1977; and Shanthanam et aI, 1979). Recently, many countries

(e.g., USA, Japan, Australia, etc) have adopted detailed legis­

lative and regulatory systems or other less formal codes of

practice in an attempt to control the

·resulting from coal combustion (Sharma,

environmental effects

1981) • In Sweden, the

Government decided not to venture on coal firing until a thorough

investigation had shown how the environmental and health problems

resulting from coalfiring could be solved in a. satisfactory way

(Reuss, 1983).

The annual world combustion of coal in electric power stations

is at present about 2450 mtce (million metric tonnes coal

equivalent) ,resulting in the production of 250-300 million tonnes

of fly ash. It has been also predicted that consumption will

increase to about 6500-7000 mtce annually in the year 2000,

resulting in the collection of some 650-800 million tonnes of

19

fly ash (Golden, 1983).

It is generally known that the volume of fly ash produced

in most countries exceeds the volume that can be utilised

productively by a large margin, but country-wise statistics are

not generally available. Very recently a committe has been

appointed to secure and analyse such information on a world-wide

basis (Diamond, 1984) • Table 4 is the committee's compilation

of estimated 1982 fly ash production in a number of countries.

TABLE 4. ESTIMATED 1982 FLY ASH PRODUCTION

COUNTRY PRODUCTION

(Tonnes)

USA 43,600,000

China 31,000,000

UK* 16,000,000

India 10,000,000

South Africa 10,000,000

France** 4,000,000

Canada 2,000,000

Denmark 1,000,000

'. *1980 <:**lQal

20

There are numerous uses of coal ash that have been developed

in several industrialised countries (Adriano et aI, 1980; Murtha

et al 1983; Diamond 1984). But the utilization rates ·vary

considerably from one country to another (e.g., UK: 40%: USA: 17%)

and are very low in many countries because of several practical

limitations (Murtha et al 1983: and Bamber, 1984). Therefore,

the disposal of huge amounts of remainder wastes on land is the

only option in most cases.

One of the main problem in disposing big quantities of fly

ash is the possible washing out of heavy metals. The coal

combustion residues are normally collected in dry or slightly

moist condition or are converted into an aqueous slurry and

transported either by truck or through pipelines to disposal sites.

In the above methods, the ash material comes into contact with

water at one time or another and thereby the aquatic environment

is affected by

1) Soluble and/or suspended materials carried off in the .efflu­ents,

ii) Soluble and/or suspended materials carried off in the surface run off,

iii) Soluble materials carried into ground water percolation through the soil, and

iv) Soluble materials carried off in the s~epages from ash ponds.

During independent studies various investigators (Theis ~

aI, 1978: Chu et aI, 1980; Guthrie et aI, 1983) have reported

21

that coal ash materials when disposed on land surfaces could cause

potential problems in the aquatic environment. A1thoug.h the

various aspects of coal 'fly ash research have been recently

reviewed by different authors (TB;b1e ,I)), there has been no fully

comprehensive review cove,ring all the relevant aspects of fly ash

generated water pollution. In this chapter, a review of 1itera-

ture pertaining to surface water quality problems due to aqueous

coal ash disposal and related laboratory studies is given.

Coal combustion residues

Coal combustion results in huge amounts of solid wastes as fly

ash, bottom (or heavy) ash and furnace slag. Fly ash is a fine

powdery residue that is normally collected from the stack gas by

mechanical collectors and or electrostatic precipitators. Bottom

ash, which is darker than fly ash, is collected in the bottom of

the furnace section. Slag is molten bottom ash which turns black

when quenched with water in the wet-bottom boiler combustion

process.

The amo~nt of ash materials produced, the relative percentage

of bottom and fly ash, and their physical and chemical

characteristics depend on the type and ash content of the' coal

being fired and power station configuration such as the type of

combustion chamber, method of firing, fusion temperature and the ,

efficiency of particulate removal devices (Chu et aI, 1978). Since

· 22

TABLE 5: BIBILIOGRAPHY OF COAL FLY ASH REVIEWS

Page et aI, 1979

Santhanam et aI, 1979

Adriano et aI, 1980

Roy et aI, 1981 --

Tolle et aI, 1982

Murtha et aI, 1983

Ruane ~ aI, 1983 &

Goden, 1983

Diamond, 1984

AREA OF MAJOR CONCERN

Environmental imp~ct and physico-

chemical characterisation of fly ash

Health and environment"al impacts of

coal ash

Impacts of land oriented utilization

and disposal of coal combustion residues

Scientific literature on fly ash and

possible environmental impacts of ash

disposal

Use of fly ash as an amendment for

agricultural

purposes

and land reclamation

Current and proposed methods of utili­

zation and disposal of fly ash

Water pollution problems associated

with coal ash disposal

Utilization of fly ash with special

reference to cement and concrete

23

the total amount of coal ash produced is directly a function of

the ash content of the coal being fired, the higher ash content

of Indian coal ( :::: 35%) results in large amounts of ash materials

compared to western countries u~ing coal of 10% ash content

(Mishra, 19"80).

Characteristics of ash materials

Fly ash from coal combustion is a heterogenous and highly

variable material. It generally occurs as very fine spherical

particles usually ranging in diameter from 0.5 to 100 pm and having

a specific gravity of 2.0 to 2.9 (Dav~sion et aI, 1974; and Fisher

et aI, 1979). Bottom ash and slag occur as angular- and porous­

surface texture particles, ranging in diameter from 0. OS to 50

mm and having a specific gravity of 2.2 to 2.8 (Chu et aI, 1980).

It has been well documented that coal fly ash consists

primarily of a semi-transparent alumino silicate glass with small

amounts of micro-crystalline hematite (Fe203), magnetite (Fe304 ),

<x-quartz (5i02), mullite (3A1

20

3.25i0

2) and gypsum (Ca50

4• 2H

20)

(Na tusch et aI, 1975). All these compounds have low solubility

iIi water.

Morphologically, fly ash consists of i) dark, irregularly

shaped particles, ii) angular, transparent and opaque particles,

and iii) glassy spheres and globules (Roy et aI, 1981). The spheres

24

may be solid, hollow (cenospheres) or filled with smaller particles

and spheres (plerospheres).

Almost all coal-fired power stations produce cenospheres.

Their proportion, however, is dependent upon the nature of minerals

in the coal being burn;-t fusion temperature, type of boiler and

efficiency of fly ash collection (Chu et aI, 1980). Raask (1968)

calculated that the amount of cenospheres can be as much as 20%

by volume of the total fly ash retained in power station

precipitators. These spheres range from 20 to 200 pm in diameter

(Chu et aI, 1980) and predominate in the 20 to 74 pm range (Natusch

3 et aI, 1980) • Their densities are generall y less than 1 gm/ em •

The mechanism of cenosphere and plerosphere formation was

elucidated by Fisher et al (1978) and Natusch et al (1975) and

is still a subject of debate.

Recent studies on coal ashes (Sabbioni et aI, 1980) have

indicated that virtually every mineral constituent accumulated

along coal deposits on the earth's surface can be found in coal

ashes. The enriched presence of major, minor and trace elements

in coal combustion residues have been well estabilished in several

studies (Davision et aI, 1974; Kaakinen. et aI, 1975; and Linton

et aI, 1976). Generally, about 95% to 99% of fly -ash consists

of compounds containing the elements Si, AI, Fe, and Ca. Minor

constituents such as MgJ Na, P, K, Ti, and S make up 0.5 to 3.5%.

Trace elements compose most of the remainder ash (Tolle et al

25

1982). The typical concentration ranges of some elements found

in fly and bottom ashes are shown in Table 6. Due to the nature

of its production, the ash contains no organic matter. However,

the variable carbon content reported in some studies is due to

residual unburnt coal (Bamber, 1984).

TABLE 6: TYPICAL CONCENTRATION RANGES OF SOME ELEMENTS IN COAL

ASH (Dood, 1983)

Concentration Bottom ash Fly ash (g/kg)

10 Si Fe Al Ca K Si Al Fe Ca K

1-10 Na Ti Mg Sr Ba Ti Mg Na Ba Sr

0.1-1 Zn Mn Cr V B Zn Mn V Rb Cr B

0.01-0.1 F Rb Ni Pb Ag F As Cu Pb.Ag Co

As Co Cu Th Ni Th Se

0.001-0.01 Cs D Be Cd Se D Be Cs Br Sb Cd

0.001 Hg Br CI I Hg CI I

Trace elements identified in the literature (Heit, 1977)

as possibly being released into the environment during coal

combustion are Sb, As, Ba, Be, Bi, Cd, Co, Cr, Cu, F, Ga, Pb,

Mn, Hg, Mo, Ni, Se, Ag, Te, TI, Sn, D, V, and Zn. Of these, the

trace elements found to be potentially harmful to man and usually

other biota include: As, Be Cd, Cr, Hg, Ni, Pb, Se, Sn, and Ti.

26

Those elements which are primarily deletrious to organisms other

than man include: Ag, Cu, F, and Zn. Another six elements Ba,

Bi, Ga, Mn, and Te remain questionable as to whether they are

of environmental concern. The mass balance and particulate studies

(Klein et aI, 1975) have shown that many of the above elements

are concentrated and distributed in the ash residues, while a

few (more volatiles) are released along with particulates or gases

into the atmosphere.

The. leaching (Hanson et a1., 1980) and spectroscopic studies

(Linton et al., 1976) have shown that many of the metals present

in coal ash are preferentially concentrated on the surfaces, rather

than evenly distributed throughout the particle. The enhanced

surface concentration is due to condensation or adsorption of

their metal vapours on the available surfaces of fly ash particles

while they are transported in the flue gas to lower temperature

zones (Davision' et aI, 1974) • The extent of enrichment of many

trace metals and fly ash is sometimes large, occassionally reaching

100 times the concentration originally found in the coal (Theis

et al., 1978). The dependence of metal concentration on particle

size of fly ash has been observed by many authors (Davision et

al , 1974; Lee et aI, 1975; and Kaakinen et aI, 1975). In general,

the highest concentration of trace metals are found to be

associated with the smallest particles, due to their large surface

to mass ratio.

27

Solubility

Upto 10% of fly ash can be water soluble, so the potential

exists for release of contaminants through leaching (Shanthanam

et al ,1979). However a number of workers (Shannon et aI, 1974;

Dressen et aI, 1977; James et aI, 1977; and Theis et aI, 1977)

have reported that the bulk solubility of coal fly ash in water

is very low and rarely exceeds 2 -3% by weight.

It has been now well estabilished that most of the soluble

fraction of fly ash is derived from the surface layer of ash

particles and is thus very rich in trace elements (Natusch et

aI, 1975; and Linton et aI, 1976; Goetz, 1983,). The extent of

metal release depends upon the surface concentration of each

element on the fly ash and the amount and type of fly ash in

solution, . the specific chemistry of the metal, and solution pH

which is often influenced by the fly ash eChu et aI, 1980).

Besides, the other phenomena such as adsorption, ion-exchange,

co-precipitation etc. will also influence the trace metal release

in such a complex system (Theis et aI, 1983).

Leaching

As disposal of coal combustion wastes commonly involves

the sluicing of ash with waters in large lagoons, the extent to

which the constituents are leached from the ash in aqueous systems

28

is a matter of interest, particularly in the light of the probable

increased burning of coal in energy production. To evaluate the

potential of contamination of water from the leaching of fly ash,

a variety of extractants and different extraction methods have

been employed to characterise solubility and release of consti-

tuents.

Mineral acid solutions were found to dissolve constituents

from alkaline fly ash much more effectively than water and

subsequently have been used to characterise the reactivity of

such ashes (Theis et aI, 1977; Green et aI, 1978; Hanson et aI,

1980). Hot extractions with concentrated HN03 were used by Hanson

et al (1980) to represent "extreme conditions" or "worst case

conditions" for fly ash dissolution. Cold extractions with dilute

HCl were used to represent biological conditions (Roy et aI, --1981). Although the rationale for using a dilute acid as an

extractant is not well explained, it may be based on the underly-

ing principle that metabolic processess generate acidic con-

ditions (ie., dissolved CO2 in lung fluids). Extractants such

as organic acids, EDTA, chelating agents and bases have been

employed to simulate natural aquatic environments of fly ash

effluents (Dressen et aI, 1977; Eggett et aI, 1978). Ammonium

oxalate and hydroxylamine hydrocholoride were sued to determine

the surface concentrated metals in ash materials (Theis et aI,

1977). Radio-tracer techniques have been employed for the study

29

of trace elements release from coal fly ash by water leaching

(Goetz, 1983). Aqua-regia and HF were employed for complete

dissolution of ash particles and subsequently used to determine

the total metals present in them (Theis et aI, 1977).

Aqueous solutions have been used to determine the solubiliti­

es and leaching behaviours of major, minor and trace e;I.ements

of fl y ash (Shannon et aI, 1974; Theis et aI, 1977; Cox et 81,

1978; Talbot et aI, 1978; Churey et aI, 1979; Elseewi et aI, 1980;

Dudas, 1981; and Kopsick et al 1981; James et aI, 1982). These

studies indicated extreme variability in the chemical composition

of leachates as the variable nature of the ashes and their

extracts are directly related to the operational conditions of

the individual power stations, the composition of the coal being

used and the extraction procedures applied (Roy et aI, 1981).

The composition of aqueous leachates of coal fly ash is

determined by the extent to which equilibrium is achieved between

the solid fly ash and leaching solution. The four important

parameters which commonly influence this equilibrium are the

composition of· the fly ash, the ratio of fly ash to solution

volume, the method by which the fly ash and the solution are

brought into contact, and the chemical characteristics of the

leaching solution (Chu et aI, 1979).

Reed et al (1976) undertook mixing studies using various

30

concentrations of fly ash. From these studies, he concluded

that equilibrium concentrations were established within 20 minutes

of contact. Laboratory tests performed on coal ashes from Tennesse

Valley Authority (TVA) power stations (Chu et aI, 1980) also

indicated that the rate of leaching of inorganic materials,

including trace metals into water was rapid and the concentration

levels reached equilibria in less than 4 hrs of contact time.

Dressen et al (1977) identified the elements in coal fly

ash that are the most extractable in a natural aquatic environment.

He noted increased extraction of trace elements with decreasing

pH.

In a study conducted by Theis et al (l977}, the desorption

of trace metals from fly ash surface in aqueous solution follows

~ predictable pattern of decreasing release 'with increasing

pH. Acconlingly, this study suggests that the greatest

environmental concern arises from those ashes which produce a

low pH in the aquatic environment.

Gangoli et al (1974) reported that fly ash can remove metal

ions from aqueous solutions both by precipitation and adsorption.

The sorption properties of fly ash were investigated by Theis

et al (1977) and his findings indicate that surface coatings of

amorphous iron, manganese or aluminium oxides could provide a

sorptive medium for trace metals. Experimental work at TVA

31

(Chu et aI, 1978) indicates that the fly ash can remove Cu from --. metal cleaning wastes by adsorption at or above pH 8.5. They

reported that the average adsorption capacity of Cu on fly ash

is 4.7 ug of Cu per gm of fly ash.

Ash pond disposal

There are now a number of methods (Shanthanam et aI, 1979)

being employed for the disposal of power station coal ashes.

The most common method of disposal is sluicing the ash particles

with water into settling ponds. After the particles settle down,

the supernatant is narmally either discharged into a receiving

system orrecirculated to the sluicing system. The later is

practised in some power stations (Chu et aI, 1977), primarily,

because of insufficient water supply.

Ash ponds can be generally cl~ssified into three categories

namely i) those that receive only fly ash ii) those that receive

only bottom ash and iii) those that receive both types of ash.

Because of vitrification, bottom ash is quite inert and

contains only small amounts of water soluble materials (Rohraman,

1971) • The direct surface runoff from bottom ash pond is,

therefore, normally suitable for discharge into water bodies

(Murtha et aI, 1983). In the studies (Guthrie et aI, 1976; Cherry

et aI, 1977) conducted at the Savannah river plant, Aiken, South

Carolina, it was shown that the bottom ash pond discharges have

32

relatively minor changes in important water' quality parameters

when compared to the source water used for sluicing.

The coal fly ash settling pond effluents, in general, are

physically, chemically and biologically different from the

receiving natural waters and represents a possible source of heavy

metal contamination when released into natural aquatic drainage

system. Chu et al (1976 & 1978) pointed out that ash pond

chemistry is a function of the i) chemical and physical character

of the ash ii) quantity and chemical composition of the water

used for sluicing and iii) the performance characteristics of

the settling pond. Many factors such as turbulence, hydraulic

distribution and loading, liquid retention time, and the settling

characteristics of the suspended particles affect the efficiency

with which suspended solids are removed by sedimentation in ash

ponds (Roy et a1, 1981) • The effects of these variables are,

however not fully known.

Ash pond discharges

The amounts of ash effluents that pass through the ash pond

or basin is a function of the basin settling efficiency which

depends upon basin volume, length, depth, flow rate and retention

time of sluice water (Cherry et aI, 1984). The composition of

ash effluent discharged to a receiving stream is a function of

the effiCiency of the settling pond, which is determined by pond

design (i.e., primary, secondary or tertiary holding systems)

and the degree to which the pond has filled (Specht et aI, 1984).

33

The pH of ash pond effluents varies from acid to alkaline.

The acidic or alkaline characteristics depend on the content of

sulfide, sulfite and alkaline metal oxides in the ash materials

and on the buffering capacity of the water used for sluicing

(Chu et aI, 1980). Adjustment of acidic ash pond discharges upto

pH 6 or of alkaline ash pond discharges dOIVll to pH 9 is necessary

to minimise its effects on the receiving aquatic systems (Chu

et aI, 1980). However, adjustment of pH between 6 and 9 does

not appreciably reduce total concentrations of As, B, Fe, Mn and

See (Chu et aI, 1976). If their concentration levels are high

in the ash pond effluents, simple pH adjustment may not provide

significant treatment.

Increased concentrations of trace elements have been found

in ash p::md discharges (Cherry et al, 1978). Several trace IJetals

in the TVA ash pond discharges have been reported (Chu et al,

1980) in both dissolved and suspended forms. However, the

environmental effects of these discharges on the receiving waters

have not yet been defined and need further investigation.

High concentrations of suspended solids in the ash pond

discharges have been reported (Chu et a1, 1976 & 1978) and are

believed to be strongly influeTlced by 1m/-density, hollow sphere

ashes called cenospheres. Chu et a1 (1980) found that the

principal constituents of cenospheres are similar to those of

34

fly ash. Since trace metals are also present in the cenospheres,

they can contribute to the toal concentration of trace metals

in the ash pond discharges. The removal of cenospheres from ash

ponds or reducing their concentrations is, therefore, necessary

to reduce trace metals in the discharges.