Coal combustion residues—environmental implications and recycling potentials

Resources, Conservation and Recycling 43 (2005) 239–262

Coal combustion residues—environmental implications and recycling potentials P. Asokana,∗ , Mohini Saxenaa , Shyam R. Asolekarb a

Scientists, Regional Research Laboratory (CSIR), Habib Ganj Naka, Bhopal 462026, India b Professor, Indian Institute of Technology, Bombay, Mumbai 400076, India Received 10 September 2002; accepted 10 June 2004

Abstract To meet the electric power requirement, the world population is greatly dependent on fossil fuel. Presently in India, about 75% of the total electrical energy (i.e. ∼100,000 MW) is generated from fossil fuel and about 105 million tons of coal combustion residues (CCRs) as solid waste/by-product is being released annually during combustion of pulverised bituminous, sub bituminous, and lignite coal. Indian coal typically has ash content of 30–60%, which results in low calorific value however low in sulphur, radioactive elements and heavy metals content. Mostly, the CCRs is being disposed to the ash pond as thin slurry, and more than 65,000 acres of land is occupied in India for storage of this huge quantity of ash which leads ecological and environmental problems. Presently about 27% of the total CCRs produced in India is being recycled and used in various applications. The major utilisation is in cement, concrete, bricks, wood substitute products, soil stabilisation, road base/embankment, and consolidation of ground, land reclamation and for agriculture. In this paper, an attempt has been made to assess the global generation of CCRs, present utilisation and acceptability in Indian context, implications and future potentials to achieve environmental sound management. © 2004 Elsevier B.V. All rights reserved. Keywords: Coal combustion residues; Characterisation; Resources; Building materials; Conservation; Disposal; Recycling; Engineering applications; Utilisation and safe-management

∗

Corresponding author. Tel.: +91 755 2589827/2488767; fax: +91 755 2587042/2488985. E-mail address: [email protected] (P. Asokan).

0921-3449/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.resconrec.2004.06.003

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1. Introduction Environmental pollution by the coal based thermal power plants all over the world is cited to be one of the major sources of pollution affecting the general aesthetics of environment in terms of land use, health hazards and air, soil and water in particular and thus leads to environmental dangers. Coal combustion residues (CCRs) is a collective term referring to the residues produced during the combustion of coal regardless of ultimate utilisation or disposal. It includes fly ash, bottom ash, boiler slag, and fluidised bed combustion ash and other solid fine particles (Asokan, 2003; Keefer, 1993). As per the ASTM standards, in India bituminous and sub-bituminous coal results in class ‘F’ash and lignite coal produces class ‘C’ ash having high degree of self-hardening capacity. Physical, chemical and mineralogical, morphological and radioactive properties of CCRs in general vary as they are influenced by coal source/quality, combustion process, degree of weathering, particle size and age of the ash (Adriano et al., 1980; Asokan, 2000; McCarthy and Dhar, 1999). In India, presently coal based thermal power plants are releasing 105 MT of CCRs which possess major environmental problems (Kumar and Mathur, 2004; Sharma et al., 2003)). Presently from all these thermal power plants, dry fly ash has been collected through Electro Static Precipitator (ESP) in dry condition as well as pond ash from ash ponds in semi-wet condition. In India most of the thermal power plants do not have the facility for automatic dry ash collection system. Commonly both fly ash and bottom ash together are discharged as slurry to the ash pond/lagoon. Year wise CCRs generation for the past more than one decade in India is shown in Fig. 1. In 1995 CCRs generation in India was only 40 MT. Although the rate of CCRs generation is not uniform in all the years from 1992–2004, as an average of 7.4% of annual increase in CCRs could be seen. It is obvious that the CCRs generation increased when the power generating capacity increased from the last five decades from 1350 MW in 1947 to ∼100,000 MW in 2004 to cater the need of the Nation. In the year 1994, 1999 and 2000 there was not much increase in power generation while comparing the previous years due to the failure in the boilers in some of the power plants. Out of the present installed capacity, about 75,000 MW of electricity is from the coal-based thermal power stations, ∼20% is from hydro-electric plant and the rest is from nuclear and non-conventional energy sources (Kumar and Mathur, 2004; Mishra, 2004; Roongta, 2000). India has about 211 billion tons of coal reserves, which is known to be the largest resource of energy and presently ∼240 MT of coal is being used annually to meet the Nation’s electricity demand. In terms of energy, India stands at world sixth position accounting ∼3.5% of the world commercial energy demand in 2001, but the electricity generation yet not completely fulfilled the present requirement. Though nuclear power programme envisaged for generation of 20,000 MW of nuclear energy by the year 2020, India do not have option in the foreseeable future, except the fossil fuel mainly based on coal sources. The rate of annual increase in power generation in India is ∼5%. And at this rate the annual power generation by the year 2020 is expected to be 180,000 MW, which may release about 190 MT of CCRs per annum. However, to achieve sustainable development the Nation may have to generate at least 260,000 MW of power (i.e. 10% increase in rate of annual electricity generation) by the year 2020 and as consequence ∼273 MT of CCRs is expected to be released. Keeping in view of the formidable future problems due to these huge quantity of CCRs to achieve Environmental

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Fig. 1. Indian CCRs generation scenario (Rengaswami et al., 2000).

Sound Management, it is very crucial time for confidence building on CCRs utilisation and increase in acceptability of CCRs based products among the end users. The growing consensus on energy requirement and demand world wide, the electricity generation will increase rapidly in the immediate future. Though world nuclear energy share has increased, major source of energy is contributed from fossil fuels. To meet the electric power requirement the world population is greatly depend on combustion of coal (World Coal Institute home page, October 2003; Mineral home page, November 2003)). As a consequence the world CCRs generation from thermal power plants is tends to be in exponential growth and is expected to reach 2000 MT per annum by the year 2020 except the Republic of Korea. The present status on the world scenario of CCRs generation is shown in Fig. 2. All these data were compiled between the end of the year 2001 to April 2004. The major quantity of CCRs producing countries are China, USA and India. However, the broken away all Soviet Union together annually releases about 125 MT. Irrespective of the US slower population growth over the world growth, per capita electricity consumption in US is among the world highest (World Coal Institute home page, October 2003; Oak Ridge National Laboratory home page, October 2001). More than one-half of the US electricity is generated from burning coal from ∼600 coal fired power plants and at the end of the year 2002, US produced 128.7 MT of CCRs and utilised 35.4% (American Coal Ash Association home page, April 2004). Among the 13 European Union Countries under the European Coal Combustion Products Association (ECOBA) member countries, Germany,

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Fig. 2. Major CCRs generating countries – World current scenario.

United Kingdom and Poland produced large quantity of CCRs. By the end of 2003 Australia produced 13 MT of CCRs and they are using it in various value added products. The utilisation of CCRs in Australia during 2002 was 4.1 MT (Ash Development Association, Australia home page, March 2004). In Japan, 8.8 MT of CCRs was produced in total during 2003, out of which 82% was effectively utilised. The ratio of CCRs production from coal consumption in 2000 was 12.1% and further it was reduced in 2002–2003 (Centre for Coal Ash Utilisation Japan home page, March 2004). Developed countries have well defined quality of segregated CCRs at different stages and they utilise an average of more than 33% of the CCRs generated in their country (Asokan, 2003; Mineral web site, 2001; Manz, 1999). Literature review indicates that CCRs has many potential uses in back fill, developing bricks, cement, concrete, adhesives, wall board, and agriculture/soil amelioration, wood substitute, paint, etc. (Ferraiolo et al., 1990; Iyer and Scott, 2001; Kazuo, 2000; Martin et al., 1990; Murarka et al., 1993; Saxena and Asokan, 2003; Sharma and Jain, 1993; Sumio et al., 2000). Use of CCRs increases various environmental benefits besides, economy. Further during the production of each ton of cement, ∼1 t of CO2 is released along with NOx and CH4 , and CCRs incorporation in cement manufacturing contributes to such emission reduction. The other studies indicate that use of 1 t of fly ash in concrete will avoid 2 t of CO2 emitted from cement production and reduces green-house effect and global

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warming (Krishnamoorthy, 2000; Naik and Tyson, 2000). World wide the electricity generation process in all thermal power plants is almost identical but, the quantity and quality of CCRs varies distinctly due to the quality of coal, temperature maintained in the boiler, efficiency of electrostatic precipitator, etc., CCRs is the world’s largest mineral resource, it’s processing, handling ultimate utilisation and safe management are the major concern for the environmental sound management and sustainable development.

2. Significance of CCRs characteristics 2.1. Physical properties The CCRs particles are generally grey in colour and some of the pond ashes are blackish grey, which are devoid of unburnt carbon. But when CCRs is mixed with bottom ash, overall carbon contents in the CCRs conglomerate and vary between ranges 3 and 5%. Table 1 shows the physico-chemical properties of India CCRs. pH of CCRs varies from acidic to alkaline with electrical conductivity as high as 1 dS/m. Physical properties like bulk density, texture, porosity and water holding capacity, etc., play an important role as far as the utilisation is concerned whether in engineering applications or in agriculture purpose (Adriano et al., 1980; Asokan, 2003; Ferraiolo et al., 1990; Schure et al., 1985; Sridharan et al., 1996). CCRs constitute an assemblage of particles of wide variety of shapes and sizes, ranging from coarse sand to clays. As per the United States Department of Agriculture standards, ∼55% of Indian dry fly ash collected from ESP falls within the silt and clay sized particles and the rest is sand sized particles (Asokan et al., 1999). Fig. 3 shows the particle size distribution and textural classification of CCRs collected from Satpura Thermal Power Station, Central Indian, ESP hopper No.4 and from the Ash Pond at a distance of 150 m from the ash slurry discharge zone. The study carried out by Wigley and Williamson (1998), indicates that medium size of fly ash particle diameter is 20 ␮m and the maximum fly ash particles are usually in the range of 150–200 ␮m. The particles size distribution and texture of the CCRs varied distinctly based upon the source, topography of disposal site Table 1 Physico-chemical properties of Indian CCRs Sl. no.

Parameters

Range

1 2 3 4 5 6 7 8 9 10

Colour Bulk density (kg/m3 ) Porosity (%) Water holding capacity (%) Sand (%) Silt (%) Clay (%) Specific surface area (m2 /g) pH Electrical conductivity (dS/m)

Grayish 960–1500 30–55 35–55 60–80 10–35 0.5–15 0.1038–2.4076 3.5–12.5 0.075–1.0

Source: Asokan (2003), Asokan (2000), Dubey et al. (2000), Pandian et al. (1988) and Sridharan et al. (1996).

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Fig. 3. Particle size distribution and textural classification of CCRs.

and location from where the ash is collected and which was confirmed by several authors (Asokan, 2000; Rajasekhar, 1995; Sivapullaiah et al., 1998; Skarzynska et al., 1989). The size distribution and surface area of a particular ash are important because they tend to influence the texture, sorption capacity, physico-chemical and engineering properties for different applications. 2.2. Chemical properties The major constituents of CCRs consist of silica, alumina and iron oxides (∼87%). The chemical constituents present in Indian CCRs are shown in Table 2. One of the major concerns with CCRs disposal is the leaching of heavy metals to surface and underground water source, which may contaminate the ground water quality nearby the ash disposal area (Anderson et al., 1993; Sandhu et al., 1992). The trace elements in CCRs, such as Zn, Cd, Pb, Mo, Ni, As, Se and B are important concern for land disposal due to their environmental significance (Keefer, 1993; Spears, 2000). But the ultimate impact of each trace element will depend upon its state in CCRs and toxicity, mobility and availability in the ecosystem. Most of the heavy metals/trace elements in Indian CCRs found in lower concentration than that of abroad, which is one of the advantages for utilisation in agriculture and in embankment

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Table 2 Chemical properties of typical Indian CCRs Sl. no.

Parameters

Indian CCRs

Range (%) 1 2 3 4 5 6 7 8 9 10

Aluminium (Al) Calcium (Ca) Iron (Fe) Manganese (Mn) Magnesium (Mg) Phosphorous (P) Potassium (K) Silicon (Si) Sodium (Na) Sulphur (S)

15.167–20.45 0.37–0.76 4.447–6.562 0.002–0.84 0.02–0.9 0.06–0.3 0.14–1.8 27.413–29.554 0.07–0.71 0.03–0.055

Range (ppm) 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Arsenic (As) Barium (Ba) Boron (B) Cadmium (Cd) Chromium (Cr) Copper (Cu) Cobalt (Co) Lead (Pb) Nickel (Ni) Mercury (Hg) Molybdenum (Mo) Scandium (Sc) Selenium (Se) Vanadium (V) Zinc (Zn)

5–68 26–1275 100–1000 1–26 10–353 39–1000 7–128 10–144 29–265 0–0.005 8–100 0.5–106 1–10 40–190 10–250

Source: Asokan (2003), Saxena and Asokan (2001), Asokan (2000), Dubey et al. (2000) and Pandian et al. (1988).

where leaching of toxic metals found to be below the USEPA standard (Asokan, 2000; Dube, 1994; Dubey et al., 2000; Furr et al., 1978). 2.3. Morphology Several morphological classes of CCRs particles observed by scanning electron microscopy showed that most of the CCRs particles were regular in shape and size (Fig. 4). And some of them were spherical, hollow shaped and cenospheres in nature (Asokan, 2003; Fisher et al., 1978; Kolay and Singh, 2001; Murarka et al., 1993; Norton et al., 1986). Microstructure of CCRs may influence the binding properties, sorption characteristics of the final products or process. The morphology, physico-chemical characteristic of CCRs significantly contribute even to improve the long-term properties of high performance concrete with 25–50% CCRs application (Peter and Gopalakrishnan, 2004). It may also be seen in the micrograph where some of the cenospheres smaller size particles were adhered on bigger size particles. Further the cenospheres present in CCRs helps as aggregate in developing lightweight concrete and other lightweight sound absorbing structural materials (Blanco et al., 2000; Tiwari et al., 2004).

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Fig. 4. Microstructure of CCRs showing spherical, hollow shaped and cenospheres in nature.

2.4. Mineralogy Mineralogical analysis revealed that CCRs could be separated into three major matrices: glass such as mullite, quartz and magnetic spinal (Twardowska and Szczepanska, 2002). In CCRs, quartz (SiO2 ), Alumino silicate (gehlenite, Ca2 Al2 SiO7 ) and hematite (Fe2 O3 ) are the predominant phase constituents, which influenced the concentration of alumina, silica and iron oxide (Asokan, 2003; Janos et al., 2002). The studies carried out by Kolay and Singh (2001) and Saxena et al. (1998) revealed that there were other mineral phases such as Albite (KalSi3 O8 ), Mullite (Al6 Si2 Ol3 ), Esperite (CaPb)ZnSiO4 , Nepoutite (NiMg)3 Si2 O15 (OH)4 and Tenorite (CuO) also present in CCRs. However, the mullite crystals are largely attributed to kaolinite and illite contributes towards the glass and cenospheres (Spears, 2000). 2.5. Engineering and geo-technical properties The particle size and density relation is an important aspect to predict the Fe2 O3 distribution and cenosphere content in CCRs (Ghosal and Self, 1995). This work carried out by Matsunaga et al. (2002) showed that the crystalline to amorphous ratio in CCRs varies with the particle size, where as the crystallinity of cenospheres decreased as the particle

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Table 3 Average concentration of natural radio nuclides in some of Indian CCRs Name of thermal power plant (TPP) CCRs

Average concentration range (bq/kg) 226 Ra

Badarpur TPP (New Delhi, Northern India) Rihand TPP (Uttar Pradesh, Northern India) Talcher TPP (Orissa, South Eastern India) Farakka TPP (West Bengal, Eastern India) NLC (Tamil Nadu, Southern India) Raichur TPP (Karnataka, Southwest India) Satpura TPP (Sarni, Central India)

(Radium)

70.7–71.9 80.8 70.0–97.6 86.7–91.2 43.6–48.8 99.7–103.7 54.13–67.20

228 Ac

(Actinium)

97.3–108.4 97.4 82.0–102.7 99.2–103.2 55.0–69.2 108.2–110.3 74.03–77.27

40 K

(Potassium)

290.9–298.7 284.5 292.2–374.4 305.5–313.8 280.7–291.9 359.3–368.7 281.8–314.0

Source: Asokan (2000) and Vijayan and Behera (1999).

size increased. The young’s modules of cenosphere particles found to be 13–17 GPa as against the 126 GPa of CCRs and coarser size CCRs (120 ␮m) exhibits wider range of hardness (160–400 kg mm−1 ) while fine size (20 ␮m) exhibits narrow range of hardness (250–270 kg mm−1 ). The specific gravity of most of the CCRs is considerably less than that of soil due to variation in particle size, shape, chemical composition and mineralogy, etc. (Asokan, 2000; Rao, 1999). Specific gravity of CCRs plays an important role in geotechnical application and it varies from 1.66 to 2.86 with high shear strength (Pandian et al., 1988). The co-efficient of permeability of CCRs varies from 10−4 mm/s to 10−3 mm/s (Rajasekhar, 1995). Lime reactivity of fly ash is much greater than that of bottom ash and pond ash. The lime reactivity depends on the proportion of silica content. The study of Sivapullaiah et al. (1998) and Prasad and Bai, 1999 showed that a high percentage of free lime content in fly ash contributed to increase its compressive strength with the curing time due to the pozzolanic reactivity. The California bearing ratio (CBR) value of soaked fly ash varied from 6.8 to 13.5% and that of unsoaked fly ash from 10.8 to 15.4%. CBR value of bottom ash (15.3–36.5%) was found higher than that of fly ash. But the pond ash has 20% CBR value (Toth et al., 1987). Compacted fly ash has the requisite properties for use in load-bearing fills or highway sub-bases (Sridharan et al., 1996). Low unit weight of CCRs is quite suitable for structural fills over soils with low pre-consolidation pressures. Though pond ash has very lower compacted dry density it exhibits higher shear strength. When mixed with mooram, it develops a fairly high CBR value (Srinivas et al., 1999; Sridharan et al., 1996). However, the complete micro information on density variation, crystallinity, micro chemistry, mechanical and other engineering properties of CCRs as a function of size distribution for a well defined source are not yet available and these caps are essentially to be filled through micro studies to explore further potentials on use of CCRs with clear understanding on its effect in different applications. 2.6. Radioactivity The radioactivity level of Indian CCRs and pond ash is almost similar to that of normal soil (Saxena and Asokan, 2001). But the radioactive level in lignite CCRs is found less

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than that of bituminous and sub-bituminous coal ash (Vijayan and Behera, 1999). Table 3 shows the normal range of radionuclides present in some of Indian CCRs. The measured level of these radionuclides in Indian CCRs is below the limits specified for environmental point of view. The upper limit for naturally occurring radionuclides such as 232 Th (parent radionuclide of 228 Ac), 226 Ra and 40 K are 259 bq/kg, 370 bq/kg and 925 bq/kg, respectively (Moghissis et al., 1978). Also the radioactivity concentration of Indian CCRs is similar to the concentration reported for the CCRs of United States and England and less than the corresponding level in the CCRs of Poland, Denmark and Australia (UNSCEAR, 1982; Vijayan and Behera, 1999). The study carried out by Ghosal and Self (1995) indicate that the radioactive properties in cenospheres content found to be lesser than the CCRs due to low porosity and presence of the bubble(s).

3. CCRs utilization scenario in Indian context Looking into the physico-chemical, engineering, mineralogical and morphological properties of ash, the Bureau of Indian Standard has released IS 10153:1982 indicating various applications of CCRs. Presently in India, CCRs is being used as a raw material in cement, cellular concrete, fly ash lime bricks, fly ash lime gypsum block, building tiles; as admixture in cement concrete, timber substitute products; as aggregate in concrete, road and building block; as pozzolana in lime pozzolana mortars/plasters, portland pozzolana cement; as stabiliser in soil stabilisation, road construction; as filler in consolidation of ground, land and mine-filling. The other applications of CCRs are metals extraction, cenosphere, soil amendment/soil modifier, fertiliser and wastewater treatment (Asokan et al., 1999; Chandrasekar, 1997; Iyer and Scott, 2001; Kolay and Singh, 2001). In India several laboratories of Council of Scientific and Industrial Research, Agricultural Universities, Indian Institutes of Technology, Tata Energy Research Institute, National Thermal Power Corporation, various Governmental and Non Governmental organisations are actively involved in conducting various in-depth experiments and demonstration trials in recycling and use of CCRs effectively. As a result, in India the CCRs utilisation rate has considerably increased and 27% of it was used in various applications by the year April, 2004 (Kumar and Mathur, 2004). Fig. 5 shows Indian scenario, from 1992 to 2004, representing quantity and rate of CCRs utilisation. As it can be seen that during 1993 utilisation of CCRs in India was only 2.3% out of the annual generation of 35 MT. The Ministry of Environment and Forest (MOEF, 2003), Government of India regulatory framework, due to various environmental concern, has stressed the importance in increasing use of CCRs. However, lack of awareness among the users on the beneficial aspect of CCRs based products, the utilisation rate greatly influenced. But in India availability of quality CCRs confirming to IS 3812:1981 from modern Thermal Power Station (TPS) and various proven research work through demonstration trials on use of CCRs both in civil engineering applications such as developing building materials (cement, bricks, concrete, aggregate, wood substitute, etc.); road embankment; wasteland development and agriculture have generated confidence and thus the rate of utilisation has substantially increased. Further keeping in view of the present growth, demand and necessity, it is expected that by the year 2020, CCRs utilisation rate may reach upto 60% in India. However, micro information on density

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Fig. 5. Quantity and rate of CCRs utilisation – Indian scenario.

variation, crystallinity, mechanical properties, microchemistry of CCRs as a function of CCRs size for a well-defined source are not yet available. And still more micro studies are required to be carried out to explore further potentials on use of CCRs with clear understanding on the effect of CCRs on different applications as a function of all above properties. The details on utilisation of CCRs in different applications is summarised in the following section. 3.1. Utilisation of CCRs in cement and asbestos Experimental investigations indicate that in cement Industry upto 22.5% of dry fly ash is being used as major raw material for the production of CCRs blended cement. Studies reveal that fly ash based blended cement is much superior to ordinary portland cement on account of its higher resistance to lime leaching, alkali aggregate reactions, higher resistance to carbonation, smoother surface, lower water permeability and penetration by chloride and sulphate ions (Kishore, 2000). Silica, Alumina and iron oxide are the major chemical constituents, which contribute to achieve superior quality of blended cement as per the IS 3812-1981. The presence of SiO2 + Al2 O3 (70%), SiO2 (35%), MgO (5%), SiO3 (2.75%), and Na2 O (1.5%) in CCRs are the requisite properties for making pozzolana cement (Roongta, 2000). Production of CCRs based cement also increases overall availability of cement production and is cost effective. In manufacturing of cement and asbestos, about 12 lakh tons of ash is being used every year in India (Roongta, 2000).

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3.2. CCRs based bricks The CCRs bricks can be broadly categorised in to two types, namely, clay CCRs bricks (sintered bricks) and CCRs sand lime bricks (calcium silicate bricks). The CCRs sintered brick contributes to replace the topsoil and thus silica and oxides of iron and aluminium play an important role. The presence of unburnt carbon in the CCRs becomes an advantage as it saves fuel consumption. On the other hand, in case of air and water cured calcium silicate bricks, CCRs plays an important role as a pozzolonic material and presence of CaO, soluble silica, Al2 O3 and higher surface area, help in improving the brick/block quality (Kumar et al., 1999). But the presence of organic carbon adversely affects the quality of bricks. The compressive strength of clay CCRs brick is as high as 120 kg cm−1 , water absorption is less than 18% and shrinkage is less than 10% (Karade et al., 1995). Another fly ash based pozzolona product is FaL-G brick, in which 60–75% of fly ash is being used. The compressive strength of FaL-G brick varies from 80–160 kg cm−2 (Bhanumathidas and Kalidas, 1997). There are various methods of manufacturing ash bricks with application of ash content varying from 30% for ash clay bricks to 80% for fly ash-lime-gypsum bricks without compromising the quality. For brick manufacturing Tamil Nadu, Maharahstra and Punjab together use about 57,000 m3 of ash annually (Roy, 2000). Other states in India like Andhra Pradesh, Uttar Pradesh also use significant amounts of CCRs in manufacturing different types of bricks. It is reported that fly ash-lime bricks have shown better crushing strength than clay bricks. These ash bricks are more resistant to salinity and water. In India, the Ministry of Environment and Forest, Government of India, has issued notification, in April 2003, that the use of CCRs is mandatory in making bricks and use for all types of constructions around 100 km radius of each thermal power station (MOEF web site, 2003). 3.3. CCRs based binder in concrete It is reported that conditioned fly ash can be used as binder and that will find large-scale utilisation in replacement/substitution in Portland cement and admixture (McCarthy and Dhar, 1999; Man and Yeung, 2000). In construction work, where cementatious binders are required, CCRs can be used and will give equal or improved properties as compared to Portland cement binder. Earlier studies indicate that fly ash-lime-phosphogypsum based binder decreases the strength and loss in weight of binder with increase in temperature of 27–60 ◦ C (Garg et al., 1996). Concrete blocks were developed from stone dust waste with 50% CCRs. The compressive strength of block was 80–130 kg cm−2 ; water absorption was 5–10% (Karade et al., 1995). The combined effect of particle shape, grading and particle density of CCRs caused a substantial reduction in the water demand of concrete mix. Such concrete gives much higher long-term strength, lower permeability and increased resistance to chemical attack (Throne and Watt, 1965). Fly ash being used as a major filler and binder material due to the pozzolanic properties meets the IS 456:2000 specification. However, studies carried out by National Council for Cement and Building Material, India showed that use of CCRs from 15 to 25% in normal concrete and even more for mass concrete works resulted in impairing the total compressive strength.

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3.4. Timber substitute products To reduce deforestation and growing environmental hazards, the use of timber is becoming restricted in India. Timber substitute products such as door shutters, window frames, false ceilings and partition walls, etc., have been developed using organic fibre as a reinforcement and fly ash as filler in polymer matrix composites. The tensile strength, compressive strength and flexural strength of CCRs based timber were showed comparatively more than that of teak wood (Saxena and Prabakar, 2000). This is an environmentalfriendly technology, quality is better than timber, and products are corrosion resistant and safe from the menace of termites, fungi, rot and rodents. It is almost timber free product in which upto 50% CCRs can be utilised (Saxena and Asokan, 2002). The study carried out by Matsunaga et al. (2002) showed that there are further potentials for use of CCRs in metal matrix composites (MMC) and advanced polymer-matrix composites (PMC). 3.5. CCRs as base course in embankments and roads construction Pond ash and bottom ash which range in particle size from fine to coarse sand have been used as a granular sub-base material for construction of embankment and road (Sikdar et al., 2000). A mixture of local soil and CCRs stabilised with 3–5% lime provides good subbase course. Utilisation in structural fill, back fills for reclamation of undulated land and abandoned mines were to be found most effective for bulk utilisation (Saxena and Asokan, 2000). Under water placement of the CCRs slurry was found suitable for stable ground construction such as harbour and airport construction (Sumio et al., 2000). Compacted pond ash and bottom ash possess good bearing strength and also meet gradation requirements for use as a sub-base material (Martin et al., 1990). CCRs added to cement concrete mix permits easier placement and finishing in which upto 50% of sand can be replaced with CCRs in road construction. In lime CCRs bound macadam, lime CCRs mix was used as filler in the Water Bound Macadam (WBM) construction to provide additional stability. The pond ash has been found a very useful material for the replacement of soil for making of embankments and for raising of outer bunds of ash dump areas. During 1999–2000, about 10 lakh tons of CCRs was used for raising of the ash pond at NTPC, Rihand, Korba and Badarpur Captive Power Plant (CPP), India. CPP, National Aluminium Company Limited Orissa utilised about 2 lakh tons of CCRs both in ash pond raising and land reclamation. For widening the embankments of Nizammuddin Bridge, New Delhi, about 1.5 lakh tons of ash was used along with soil cover of about 1 m thickness. In addition CCRs has also been used for embankments of various fly-over bridges in Delhi (Sikdar et al., 2000). The other study showed that bottom ash was used successfully as sub-base course for roads of NTPC’s Dadri in association with Central Roads Research Institute (CRRI), New Delhi. About 20,000 t of ash was used by NTPC for road works at Talcher-Kaniha, Orissa during 1999–2000 (Mathur, 2000). Apart from this, more than 7 lakh tons of CCRs is being utilised for construction of roads and ash ponds by most of the NTPC units located in different parts of India.

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Fig. 6. Increase in edible yield of cabbage due to CCRs application over control.

3.6. CCRs for reclamation of wasteland and agriculture The advantages and disadvantages of CCRs applications to improve the soil fertility have been published by a large number of researchers in India and abroad (Menon et al., 1993; TIFAC, 2003 home page, October 2002; Schwab, 1993; Saxena et al., 1998). Long-term studies, on the effect of fly ash on soil fertility and crop yields, carried out by Saxena and Asokan (2001) revealed that CCRs can be used as a enriching medium for improving the productivity of wasteland soils and increase the yield of most of the crops, vegetables and cereals without affecting the food quality and soil fertility. As a case study, conducted in pilot scale at NTPC, Rihand Nagar, Northern India, the effect of CCRs on edible yield of cabbage (Brassica oleracea) over a period of 6 years winder cropping (1993–1994 to 1998–1999) on sandy soil is shown in Fig. 6 (Saxena and Asokan, 2000). Result revealed that there is a significant increase in edible yield of cabbage over control due to CCRs treatment. However, CCRs treatment at 1170 t/ha. (i.e. 650 t of CCRs per hectare of land in the first year + 130 t/ha. in each subsequent years) resulted maximum (average 42.5%) yield over control where no CCRs was applied. In sandy soil CCRs application decreased the particle size distribution, porosity and thus increased the water holding capacity. Further the presence of plant nutrients such as P, K, Cu, Zn, Fe, Mn, S, etc., in CCRs found higher than the soil and due to presence of moisture these nutrients get mobilised/released and thus contributed to improve the fertility and productivity of soil. These effects are usually observed when CCRs overcomes nutrients deficiency in the soil to which it has been introduced. Apart from the increased bio mass generated in the plot due to CCRs treatment during the first year further contributed to enrich the fertility status of soil in the subsequent years. The edible

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yield of cabbage grown on CCRs treatments were tested and confirmed that the nutritional and heavy metals contents are comparable to the control and meets the quality standard as well as consumer acceptability. The decrease in yield of cabbage during II and V year cropping was due to decreased temperature below 6 ◦ C followed by the unusual rainfall in which the fungus attack greatly influenced the growth and resulting less yield as compared to the remaining years (Saxena and Asokan, 2001). But the yield was always higher with CCRs treatment as compared to the without CCRs. This was further supported by Keefer (1993) that CCRs are known to improve crop growth by neutralising soil acidity. On the contrary few investigations involving use of CCRs in agriculture show that CCRs produced undesirable effects on crop yield and on development of plants. The most frequently cited cause of these effects is heavy metals and boron toxicity (Ferraiolo et al., 1990).). In some cases CCRs is shown to induce P deficiency, salt injury, pozzolonic effects and heavy metal toxicity to crops (Shukla and Mishra, 1986). But the effect of CCRs in agriculture may depend on soil and CCRs texture, structure, pH, moisture content, reactivity of ash and soil, ion exchange capacity, method of application and percentage addition of ash. One of the study carried out by Saxena and Asokan (2000) showed that for reclamation of land ∼40,000 t/ha. of ash was used at NTPC, Rihand Nagar, Northern India (TIFAC Home Page, October 2002). Also it is reported that about 3.0 lakh tons of ash was used as filler material for reclamation of low-lying area at NTPC; Badarpur Delhi (Mathur, 2000). Apart from this, during 1995–2000, NTPC, located at different places of the country, used more than 15 lakh tons of ash for various land reclamation and agricultural applications. This study indicates that there is a great potential not only for CCRs safe management but also to convert the wasteland and increase the fertility of soil.

3.7. CCRs for reclamation of abandoned mine Bulk quantities of CCRs have been used to replace the conventionally used sand for reclaiming underground mines. During 1999–2000, NTPC used about 60,000 t of ash for backfilling underground mines of Singareni Colliery Company Limited, Southern India, in collaboration with Central Mining Research Institute, India (Mathur, 2000). The potential application of CCRs in reclaiming abandoned coal mine is of great practical significance. Research and Development are still on for commercial use of such huge quantum of CCRs as mine-filling material. Since about 80% coal is produced from open cast mines, Coal India Ltd., is in crucial stage of being able to handle this excessive overburden and planning for CCRs back filling in the abandoned mines for eco-engineering development with viable plant life. On the contrary Coal India Ltd., itself, is facing problem of disposal of the abundant overburden wastes (∼6000 million m3 ) as against their in situ volume of the available open cast mine pits (4000 million m3 ) and regaining the configuration of the landscape (Pan, 2000). Since there are various limitations and threats to environmental degradation, effective scientific work is to be done before a firm decision is taken for bulk use of CCRs in reclaiming abandoned mines.

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3.8. CCRs in solidification/stabilisation of hazardous wastes Solidification/stabilisation (s/s) is a technique widely used world wide for remediation of hazardous wastes/priority pollutants. This process reduce the leachability/movements of toxic metals as well as offers improvement in the physical characteristics that help easy accessibility and transport of hazardous wastes. Studies indicates that for safe disposal of hazardous wastes by immobilizing toxic species, to protect the environment, CCRs can be used as a binder/raw materials or partial substitute for cement in s/s process (Couner and Hoeffner, 1998). During this process the fixation/stability of heavy metals Pb, Cd, Cr, Zn and Cu, etc., present in the hazardous wastes greatly impart through physico-chemical means (Constantino et al., 2001; Jang and Kim, 2000; Yousuf et al., 1995). The mechanism of reaction in s/s products mainly depends on the pH, hydration rate, microstructure, reactivity of wastes in relation with the binder (Li et al., 2001; Pereira et al., 2001; Wang and Vipulanandan, 1996). The studies carried out on s/s of toxic metal wastes using coke and coal combustion by-products revealed that alkaline wastes can retain low concentration of toxic metal ions and solidification and sorption of metals were significant due to the presence of CaO and CaSO4 in CCRs (Vempati et al., 1995). Another studies showed that CCRs can be used to neutralise and stabilise the sulphuric acid containing waste of titanium dioxide production as well as Pb, Zn, and sulphur containing waste from Zinc metal production, respectively (Asokan, 2003; Vondruska et al., 2001). Also CCRs substitution as aggregate resulted higher strength, water binding and lower porosity and which is proportional to the presence of calcium and active silica in the mortar. But when CCRs substituted as Portland cement, strength maintained constant and it increased when active silica is higher than that of cement (Papadakis, 2000).

3.9. Extraction of cenospheres, metals, developing paint and other environmental applications In CCRs a small quantity (∼2%) of particles consist of hollow and solid spheres known as cenospheres and plerospheres. The Greek words Kenos (hollow) and Sphaira (sphere) later termed as cenosphere (Kolay and Singh, 2001). The cenospheres content in CCRs is depends on its carbon and iron content. The other study showed that the solid spherical particle of CCRs are known as precipitator CCRs and hollow particles (density less than 1 g cm−3 ) are called cenospheres (Matsunaga et al., 2002). The studies carried out by Matsunaga et al. (2002) and Kolay and Singh (2001) showed that cenospheres ash are low in density (0.3–0.7 g cm−3 ), high strength, high temperature and chemical resistance as well as thermally stable (upto 280 ◦ C). The particle size ranges from 20–200 ␮m with dominant in alumina and quartz phase with uniform shape (Blanco et al., 2000). Cenospheres ashes are lightweight and can be used in various high value added applications such as lightweight concrete, structural materials, synthesis of ultra-light composite materials, MMC and PMC, etc. (Matsunaga et al., 2002; Spears, 2000; Tiwari et al., 2004). Also, CCRs is being used in synthesis of mullite, zeolite, granite, alumina and germanium, ceramics products (Chandrasekar, 1997; Mondragon et al., 1990; Roongta, 2000; Shrivastava, 2000).

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Tiwari and Saxena (1999) reported that paints can be developed using CCRs for protection of metallic and non-metallic structures and generally for the protection of corrosive environment. CCRs were used (30%) as extender due to its chemical inertness, abrasion resistance, less oil absorption and low specific gravity. The CCRs paints showed resistance to water, acid, alkalis, organic solvents and have improved abrasion resistance. Some of the studies showed that a series of aromatic substrates were adsorbed on the surface of ash and found as source for the extraction of chlorinated compounds (Robert and Kenneth, 1990). Addition of ash in soils contributes to detoxification of lindane residues (Albanis et al., 1988). Application of CCRs was also found useful in environmental field for reducing the polluting content of waste water, for de-watering biological sludge and for cleaning oil-polluted seawaters (Ferraiolo et al., 1990). In chemical and paper industries the COD and absorbency reduction were induced addition of CCRs. Fine particles of CCRs were used for removal of dye under acidic condition at low temperature. Even CCRs was used in liner materials for waste disposal to augment the de-contamination of soil (Iyer and Scott, 2001). The presence of unburned carbon in the CCRs could be activated to further improve the sorption capacity.

4. Constrains and barrier for CCRs utilisation Presently, India is producing ∼105 MT of CCRs which is ∼7-fold than that of the year 1990. Utilisation of CCRs also increased similarly from 4.5% in 1990 to 27% in 2004. Indian CCRs has lower lime reactivity (puzzolanicity), higher unburned carbon and crystalline phase in comparison to European and North American CCRs. In India, the characteristics of CCRs are not uniform in nature and quality control parameters such as quality of coal, combustion process, ash handing system, uniform design of ash pond, etc., have to be maintained efficiently in each TPS. In India ∼19% of the total cement production is Fly Ash Portland Pozzolana cement (FAPPc). Inspite of the PPC demand in southern India, the availability is much less. On the contrary in the northern parts of the country there is no demand for FAPPC and the manufacturers are finding difficulties in marketing FAPPC. Such gap should be filled to increase the CCRs utilisation in cement application. If about 50% of the total cement production in India is FAPPC with 35% ash substitution, about 10 million tons of CCRs can be utilised annually (Krishnamoorthy, 2000). Indian standard (IS 1480–1996) for making PPC and reinforcement concrete (IS 456:2000) are more specification-oriented (physicochemical properties) rather than performance-oriented and ignore the long-term durability and hence customary standards are required to meet the compelling need of the large scale utilisation of CCRs. Settings up of CCRs based building products units are capital intensive. For setting up of brick making plant, capacity of 5.4 million brick per day, to utilise 10,000 t of ash an investment of approximately Rs. 600 crore is required. Also for utilising same quantity of CCRs for manufacturing sintered lightweight aggregates or aerated cellular concrete block making plant an investment of Rs. 350 crore and Rs. 2000 crore, respectively is required (Krishnamoorthy, 2000). Entrepreneurs/manufacturers are not showing much interest in investing such huge amount. For developing good quality of building products,

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good quality of CCRs is required. In order to achieve this the existing CCRs disposal system in the power plants India should be modified and suitable technique to be developed for size-wise segregation of ash and make available finer dry ash in huge quantity through automatic collection system for cement production and coarse ash for building, road and mine reclamation. CCRs transportation cost for reclamation of abundant mine and road construction is a major constraint. Restriction of excavation of earth for filling low-lying areas and construction of embankment within 200 km radius of thermal power plant is essential. Further, there should be a mandatory condition in the policy legislation to use CCRs in place of soil for such applications. Lack of awareness on the advantages of CCRs based products among end-users is limiting new initiatives and market potential. There should be an integrated approach by the coordination of technologists, architects and manufacturers for the production of superior quality of CCRs based products to meet the consumer acceptability and increased marketability. In addition, in association with scientists, policy makers and CCRs generators, awareness of the quality parameters and beneficial effects of CCRs based building materials and its utility should be made clear to the general public for mass consumption and effective utilisation of CCRs.

5. Future potentials Coal is the major natural resource available in abundance in India and hence the coal based electricity generation obviously will increase in the present and future to meet the demand and per capita consumption due to the growing industrialisation and advancement in newer technologies. In India about 75% of the power generated are coal based and similar trend is expected for another couple of years. Further the lignite reserves in India is estimated as ∼35 billion tons. The lignite reserves are mostly in southern and western part of the country. Even though lignite use is further proposed to expand 24.3 MT in 2001–2002 to 44.96 MT in 2006–2007 (Poothia and Basu, 2004). As a consequence, there is a production of huge quantity of CCRs followed by emission of green house gases and intrude significantly for global warming. Also similar situations exist in may developed and developing countries. Hence, to comply with environmental requirement, serious efforts are to be made to tackle this alarming situation of fly ash management to reduce the adverse effect on environment and ecology and future hypothesis by finding remedial measures for the social development. In view of the complexity of CCRs disposal, Fly Ash Utilisation Programme, Department of Science and Technology, Government of India has taken several effort in “Confidence building” in CCRs based products and technology (TIFAC Home Page, October, 2002). Ministry of Environment and Forest, Building Materials Technology and Promotion Council and Housing and Urban Development Corporation, Government of India have taken various initiatives and contributed not only for the financial and technical support to carryout R&D work but also for promoting of CCRs based building materials for large-scale production and utilisation. As a result the Bureau of Indian standards, 3812:1981 and 456:2000, has been revised to use CCRs as pozzolana and admixture. By substituting ash the quality parameters has been further tightened.

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Soil is one of the valuable resources. It takes several hundred years to produce a layer of ∼2.5 cm thick natural soil by weathering. In India ∼53% of the total land (∼175 million ha.) are non-cultivable due to water logging, sandy, rocky nature, undulation, salinity, alkalinity and acidic nature. The technology demonstrated by several research organisations at different parts of the country indicates that CCRs has wider potentials to increase the agricultural productivity and convert the wasteland in to agricultural land (Saxena et al., 1998; Shukla and Mishra, 1986). The usage of soil for road embankments, mine filling and uses as aggregate in various civil construction applications like bricks, concrete filler materials in foundations, etc., destructs the soil system and contributes to the ecological imbalance. In agriculture, depending up on the characteristics of soil 50–650 t of fly ash per hectare of land can be utilized to improve the agricultural productivity. Building materials like bricks, blocks, paints, timber substitute products, etc., is also another important area in which 35–50% of CCRs can be utilized without compromising the quality. Cost benefit analysis of CCRs versus conventional building materials are needed to be thoroughly evaluated for the concrete recommendation for maximising the use of CCRs. For the effective and efficient utilization of CCRs, there is an urgent need for extensive R&D work towards exploring newer applications and maximizing of use of existing technology. Large-scale CCRs utilisation as replacement of soil can be a worth full proposition. Apart from fiscal incentives to encourage the entrepreneurs, presently Government of India has made legislation for restricting the use of topsoil for construction activities and instead to use 25% of CCRs in bricks, blocks, tiles within a radius of 100 km of each thermal power plants without excise duty (MOEF home page, April 2003). All these efforts were significantly contributed to maximise the CCRs utilisation in India. Keeping in view of the

Fig. 7. Potentials of CCRs utilisation.

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growth and the current rate of ash utilisation in different applications, as cited above, it is expected that by the year 2020, in India the CCRs utilisation will reach upto 60% and the potentials areas of CCRs utilisations, for zero waste discharge, are shown in Fig. 7. Now the liability of CCRs safe disposal involves not only the CCRs generator but also the scientists, technocrats and the society as a whole.

6. Conclusion In India major source of power is generated through coal based thermal power plants resulting 105 MT of CCRs and hence its safe disposal is an important concern to safeguard the cleaner environment. This huge quantity of wastes, indeed resource, has to be recycled and used in an effective manner with Life Cycle Assessment Studies. Presently 33% of the CCRs produced worldwide, finds market acceptability. In India, due to the incessant effort of R&D, today CCRs is being used ∼27% of total generation in building materials, road and embankment, land development and agriculture, extraction of metal and cenospheric ash, paints, waste treatment and hazardous waste management. Attempts are being also made to recycle and use huge quantity of CCRs for reclamation of abundant coalmines for socio-techno economic development. Long term perspectives of CCRs management is deemed imperative and now it is necessary for the CCRs generators, scientists, technocrats, entrepreneurs, consumers alongwith decision/policy-makers to jointly put an effort for evolving a suitable paradigm for effective management of Coal Combustion Residues. This will open a new avenue for large scale utilization of CCRs followed by providing a multidisciplinary solution to cater the need and safe guard the environment.

Acknowledgements The authors are thankful to Dr. Aparna Asokan for correcting the manuscript. Authors are also wish to thanks to Dr. N. Ramakrishnan, Director, Regional Research Laboratory, Bhopal, India for the support and permission to publish this paper.

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