The Ecology of Thermal Power

Published on 10/05/2011

First Update 10/03/2017

Earth has evolved out of millions of years of intense Biogeochemical activity. Early earth did not have free oxygen, the environment was anoxic, the type you have besides polluted rivers and lakes, composed of marsh gases like Hydrogen Sulphide, Ammonia and Methane etc. It was within this environment and the large water masses that the precursors to early life developed. The system was chemoautotrophic and fermentative. The end products of metabolism were anaerobic gases, the type I have mentioned above. Out of a total of 5 Billion years of existence it was only about 2.8 Billion years ago that the most remarkable synthesis occurred on earth.

The synthesis of chlorophyll triggered a process in which the Carbon Dioxide present in the atmosphere (probably about 10000 times more than of now) entered a reaction with water in the presence of sunlight. This was the beginning of a transition from the chemoautotrophic, fermentative and anaerobic mode of metabolism to the autotrophic, photosynthetic and aerobic form of metabolism.

The process also heralded the way for the evolution of plants – for the fixation of carbon and nitrogen- the synthesis of biomass and all primary food production. Oxygen evolved as a by-product of the reaction but it took another about 0.8 billion years for earth to get free Oxygen. The all-important ozone layer was formed. As plant activity achieved dominance plant material became embedded in the lower crusts of the earth to finally form fossils. Coal is one such fossil, which has been thus formed.

Man in his quest for excellence, saw this coal and recognized its potential as a slave. He had discovered fire already. This was the beginning of the Industrial revolution- of smoking chimneys serving as signatures of prosperity and development. Little did man know that the process could recoil as a danger to mans very existence on earth. That man would eventually cut the very branch on which he is sitting. Electrical energy is a very important part of all development activity. 1995 estimates for the per capita consumption of electrical energy in India were 360 units which compares very badly with 6000 to 10000 units from the industrially developed countries. Estimates for the demand for power during1992-1997 reveals a demand of 1783989 million kWh, a supply of 1626544 kWh and a deficit of 8.8% on the demand. Indias coal reserves are estimated to be just about 1% of the worlds reserves while its population is 16% of the global population. India has a coal reserve of 200 billion tons and a current annual production of 295.93(1997-1998) million tons. Seventy percent of the total coal production and virtually the entire lignite production goes into power generation. In the early years of this century India will be producing 400 million tons of coal and lignite of which about 330 million tons would be needed for power generation.

Interestingly the production of coking and non-coking coals for 1962-1963 was just 55.23 million tons. About 75% of electrical energy is produced by the thermal power plants in India. Emissions from the combustion of coal are one of the basic environmental problems associated with the thermal power plants. The World Health Organisation has prescribed the following emission factors for thermal power plants: Particulates- 3.5(A) Kg. per ton of Lignite burnt; 8(A) Kg. Per ton of Bituminous coal burnt; 8.5 (A) Kg per ton of Anthracite burnt. Here A is the ash content of combustible coal by weight. Sulfur Dioxide- 15 (S) Kg. Per ton Lignite burnt; 19 (S) Kg. Of coal and Anthracite burnt. Here S is the percentage combustible sulfur by weight. Nitrogen Oxides- 7 Kg. Per ton of lignite burnt, 9 Kg. Per ton of Anthracite and 9Kg. Per ton of Bituminous coal. Hydrocarbons- 0.5 Kg per ton of lignite, 0.015 Kg. Per ton of Anthracite burnt and 0.15 Kg. Per ton of bituminous coal burnt. Carbon monoxide- The emission of carbon monoxide from all sources is prescribed as 0.15 Kg. Per ton.

 Indian coal has a high Ash content sometimes exceeding 40%and a Sulfur content ranging from 0.2 to 8 % with an average of 2%. With low conversion efficiency, thermal power plants release almost about 1.5 to 2 MW of thermal energy per MW of power produced in the environment. About 15 % of this is released along with the flue gases and the rest is discharged along with cooling water. It is estimated that a 500MW coal fired power plant having no pollution control equipment would emit nearly 100 tons of Sulfur Dioxide, 20 tons of Nitrogen Oxides and 6 tons of Ash daily. Existing power plants produce about 50 million tons of fly ash per annum needing 40000 acres of precious land for disposal of fly ash during their span of 30 years. Combustion products from thermal power plants have great environmental significance. Acidic gases have a tendency to form acid rain. While the problem of acid rain is not yet severe in India yet the increasing use of coal is likely to increase the possibility. Soot from chimneys has a low particle size and may tend to deposit in the tissues of lungs where it embeds itself, may stay for two to six weeks and in the process because of adsorbent capacities, adsorb acidic gases, heavy metals and other particulate air pollutants which are inhaled.

Heavy metals are an important constituent of fossils and combustion of coal releases in the atmosphere a number of heavy metals. These either come out as slag from boilers or are impregnated on the soot particles and with the slightest of acidic conditions may resolubilise in the environment causing metal pollution of air and water. This class of pollutants, because of bioaccumulative properties needs special mention. Bioaccumulation leads to magnification and long term exposure to very small concentrations may cause severe problems. Episodal pollution of this kind is best exemplified by the Minamata Bay incident where Methyl Mercury created problems of fish and human mortality.

In a 200 MW power plant in India burning about 9000 tons of coal per day leaching of a mere 15% of heavy metals from the surface of Ash will cause a nearby river to receive daily 208 Kg of Iron, 56 Kg of Zinc, 45 Kg of Copper, 5 Kg.of Cadmium, 56 Kg of Nickel, 4.6 Kg. of Uranium, 16.5 Kg of Thorium, 60.6 Kg of Chromium and 11.2 Kg of Cobalt daily. The transformation, which these metals undergo in the polluted anaerobic waters, the effects of bioaccumulation on the flora and fauna and the impact of biomagnification need special attention. Chlorine and Fluorine are also constituents of coal and it is expected that photochemical reactions would trigger out a process of synthesis of Chlorofluorocarbons which may have far reaching consequences.

We all know by now that Chlorofluorocarbons are causative in the catalytic breakdown of Ozone and the consequent depletion of the ozone layer. The life of these molecules (Chlorine) is almost 100 years and it is for this extent of time that it would go on damaging the ozone layer. We must be cautious. Most of the gases released from fossil fuel burning contribute to the green house effect. Global warming is a consequence. It is also significant that every three tones of carbon burnt consume 8 tons of oxygen and that we are drawing excessively on the oxygen resource of earth. Fly Ash disposal is a major concern for the thermal power plants. Generally, for every MW of installed capacity approximately one acre of land is required for the ash generated, the material accumulating to a height of 8 – 10 metres.

Fly ash is a harmful environmental pollutant. Being light it gets air-borne very fast. Long inhalation causes silicosis, fibrosis of lungs, bronchitis and pneumonites etc. It corrodes structural surfaces and deposition effects horticulture. Slurry disposal lagoons/ settling tanks become sources of mosquitoes and bacteria. It holds the potential to contaminate the underground resources with traces of toxic metals present in it. The ash handling system may account for 5% of the total cost of a power project.