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.