Fly ash from thermal power plants – waste management


Energy requirements for the developing countries in particular are met from coal-based thermal power plants. The disposal of the increasing amounts of solid waste from coal-fired thermal power plants is becoming a serious concern to the environmentalists. Coal ash, 80% of which is very fine in nature and is thus known as fly ash is collected by electrostatic precipitators in stacks. In India, nearly 90 mt of fly ash is generated per annum at present and is largely responsible for environmental pollution. In developed countries like Germany, 80% of the fly ash generated is being utilized, whereas in India only 3% is being consumed. This article attempts to highlight the management of fly ash to make use of this solid waste, in order to save our environment.



COAL-based thermal power plants have been a major source of power generation in India, where 75% of the total power obtained is from coal-based thermal power plants. The coal reserve of India is about 200 billion tonnes (bt) and its annual production reaches 250 million tonnes (mt) approximately. About 70% of this is used in the power sector. In India, unlike in most of the developed countries, ash content in the coal used for power generation is 30–40%. High ash coal means more wear and tear of the plant and machinery, low thermal efficiency of the boiler, slogging, choking and scaling of the furnace and most serious of them all, generation of a large amount of fly ash. India ranks fourth in the world in the production of coal ash as by-product waste after
USSR, USA and China, in that order. Fly ash is defined in Cement and Concrete Terminology (ACI Committee
116) as the ‘finely divided residue resulting from the combustion of ground or powdered coal, which is transported from the fire box through the boiler by flue gases’. Fly ash is fine glass powder, the particles of which are generally spherical in shape and range in size from 0.5 to 100 μm. Fly ash is classified into two types according to the type of coal used. Anthracite and bituminous coal produces fly ash classified as class F. Class C fly ash is produced by burning lignite or sub-bituminous coal. Class C fly ash has self-cementing properties.

Effluent and disposal

Disposal and management of fly ash is a major problem in coal-fired thermal power plants. Fly ash emissions from a variety of coal combustion units show a wide range of composition. All elements below atomic number 92 are present in coal ash. A 500 MW thermal power plant releases 200 mt SO2, 70 t NO2 and 500 t fly ash approximately every day. Particulate matter (PM) considered as a source of air pollution constitutes fly ash. The fine particles of fly ash reach the pulmonary region of the lungs and remain there for long periods of time; they behave like cumulative poisons. The submicron particles enter deeper into the lungs and are deposited on the alveolar walls where the metals could be transferred to the blood plasma across the cell membrane (Figure 1). The residual particles being silica (40–73%) cause silicosis. All the heavy metals (Ni, Cd, Sb, As, Cr, Pb, etc.)
generally found in fly ash are toxic in nature .
                                                                     
Fly ash can be disposed-off in a dry or wet state. Studies show that wet disposal of this waste does not protect the environment from migration of metal into the soil.
Heavy metals cannot be degraded biologically into harmless products like other organic waste. Studies also show that coal ash satisfies the criteria for landfill disposal, according to the Environmental Agency of Japan2. According to the hazardous waste management and handling rule of 1989, fly ash is considered as non-hazardous. With the present practice of fly-ash disposal in ash ponds (generally in the form of slurry), the total land required for ash disposal would be about 82,200 ha by the year 2020 at an estimated 0.6 ha per MW. Fly ash can be treated as a by-product rather than waste.

Fly ash utilization

The ash generated from volcanoes was used extensively in the construction of Roman structures. The colosseum (constructed in AD 100) is a classic example of durability achieved using volcanic ash . Fly ash is generated in artificial volcanoes (coal-fired). Volcanic ash acts just like fly ash obtained from coal-fired thermal powerplants. During the last 30 years, extensive research has been carried out to utilize the fly ash in various sectors, as this is not considered as hazardous waste4,5. Broadly, fly ash utilization programmes can be viewed from two angles, i.e. mitigating environmental effects and addressing disposal problems (low value–high volume utilization).

Following are some of the potential areas of use of fly ash.

Fly ash bricks

The Central Fuel Research Institute, Dhanbad has developed a technology for the utilization of fly ash for the manufacture of building bricks. Fly ash bricks have a number of advantages over the conventional burnt clay bricks. Unglazed tiles for use on footpaths can also be made from it. Awareness among the public is required and the Government has to provide special incentives for this purpose.

Fly ash in manufacture of cement

Fly ash is suitable for use as pozzolana. In the presence of moisture, it reacts chemically with calcium hydroxide at room temperature to form compounds possessing cementitious properties. Fly ash has a high amount of silica and alumina in reactive form. These reactive elements complement the hydration chemistry of cement. On hydration, cement produces C–S–H gel and free lime, i.e. Ca(OH)2. The C–S–H gel binds the aggregates together and strengthens the concrete. Water, sulphates and CO2 present in the environment attack the free lime causing deterioration of the concrete. A cement technologists observed that the reactive elements present in fly ash convert the problematic free lime into durable concrete. The difference between fly ash and portland cement becomes apparent under a microscope. Fly ash particles are almost totally spherical in shape, allowing them to flow and blend freely in mixtures. This property make fly ash a desirable admixture for concrete.

Fly ash in distemper

Distemper manufactured with fly ash as a replacement for white cement has been used in several buildings in Neyveli, Tamil Nadu, in the interior surfaces and the performance is satisfactory. The cost of production will only be 50% that of commercial distemper.

Fly ash-based ceramics

The National Metallurgical Laboratory, Jamshedpur has developed a process to produce ceramics from fly ash having superior resistance to abrasion.

Fly ash as fertilizer

Fly ash provides the uptake of vital nutrients/minerals (Ca, Mg, Fe, Zn, Mo, S and Se) by crops and vegetation, and can be considered as a potential growth improver. It serves as a good fertilizer.

Fly ash in road construction

The use of fly ash in large quantities making the road base and surfacing can result in low value–high volume utilization.

Conclusion

It has been recognized worldwide that the utilization of an enormous amount of fossil fuels has created various adverse effects on the environment, including acid rain and global warming. An increase in the average global temperature of approximately 0.56 K has been measured over the past century (global warming). Gases with three or more atoms that have higher heat capacities than those of O2 and N2 cause the greenhouse effect. Carbon dioxide (CO2) is a main greenhouse gas associated with global climate change. The disposal, management and proper utilization of waste products has become a concern for the scientists and environmentalists. Proper management of solid-waste fly ash from thermal power plants is necessary to safeguard our environment. Because of high cost involved in road transportation for the dumping of fly ash, it is advisable to explore all its possible applications. The Pradhan Mantri Gramya Sadak Yojana would be a successful and economically viable project with the utilization of fly ash in road construction in remote and rural areas. Every village in India will have concrete roads and large amounts of fly ash can be consumed in this process. Concentrated efforts are needed to utilize fly ash in the manufacture of building bricks, cement and ceramics, and mitigate the unemployment problem as well.