• Electrical India
  • Oct 5, 2017

Emission Controls for Indian Thermal Power Plants

Thermal power plants are the most common sources for generation of electricity. These plants also pollute the environment mostly. They emit many harmful gases to the atmosphere apart from fly ash, which itself has many harmful health effects. The Ministry of Environment, Forest & Climate Change has notified the revised standards for coal-based Thermal Power Plants in the country, with the primary aim of minimizing pollution.

- Prof Mayadhar Swain


 In the world, installed capacity of thermal power plant is about 65% of the total capacity from all sources. In India, this is 68%. As on February 2017, total installed capacity in India is 315426 MW and that of thermal power plant is 215215 MW (refer Table - 1).

  In spite of emissions of many harmful substances from coal fired plants, we depend on mostly for electricity. It is because these have some strategic advantages over other sources. The main advantage is that its fuel coal is available in plenty in many countries. In our county, the coal will last for another hundred years. India is the world's third largest coal producing country and the fourth largest coal importer. The cost of generation of electricity from coal is cheaper than other sources. Further, its technology is well-established and it takes three to four years for installation from concept to commissioning.

  The most crucial reason is that there is no alternate source to quantitatively substitute thermal power at present. The other conventional sources of power are hydroelectric and nuclear power. In India, the share of hydroelectric power has come down from more than 50% at the time of independence to 14% now due to resettlement and rehabilitation and other problems. Similarly, share of nuclear power is not improving due to several reasons. Although, installation of wind power and solar power is rapidly progressing, it will take some years to fully depend on them.

  Due to these reasons, thermal power has become favorite to planners, although it comes with significant costs to environment and human health.

Thermal Power Plant Emissions

  The water runoff from coal washeries carries pollution loads of heavy metals that contaminate ground water, river and lakes, thus, affecting aquatic flora and fauna. Fly ash residues and pollutants settle on soil contaminating lands and are especially harmful to agriculture.

  Generally, Indian coals have lower calorific value and ash content is more (up to 40%). The typical characteristics of Indian coal used for power generation are given in Table 2.

  Combustion of coal in coal fired power plants releases emissions of sulphur dioxide (SO2), nitrogen oxides (NOx­), particulate matter (PM), carbon monoxide (CO), and volatile organic compounds (VOCs) into the atmosphere through stacks (chimneys). The PM in the flue gas also contains high concentrations of heavy metals such as arsenic, lead, cadmium, mercury, copper and zinc. Chronic and acute exposure to these pollutants has health impacts that include respiratory illnesses, compromised immune systems, cardiovascular conditions, cancer and even premature death. The people living nearby the plants are severely affected. While the impact of the emissions is felt within 200 km of the power plants, under windy conditions the influence can be tracked to distances as far as 400 km from the source region. Another pollutant is carbon dioxide which is a greenhouse gas and is considered as the main source for global warming.

Environmental Regulation

  Some measures have been taken to reduce the effects of pollutants from coal fired power plants. Limits have been fixed for the amount of different emissions. The emission standards for thermal power plants in India are being enforced based on Environment (Protection) Act, 1986 of Government of India and its amendments from time to time. Earlier, only emission standard for particulate matter had been specified and this was also lagging to those implemented in China, Australia, the United States of America and the European Union.

  Ministry of Environment, Forest and Climate Change has announced in December 2015 stringent emission standards for different emissions from different industries (refer Table 3). Thermal power plants are categorized into three categories, namely those:- (i) Installed before 31st December, 2003 (ii) Installed after 2003 upto 31st December, 2016 and (iii) Installed after 1st January, 2017.

  Nearly two-thirds of India's installed coal-based capacity is made up of plants commissioned after 2003 and all of them will now have to be upgraded to the new requirements for NOx, SO2 and PM emissions. The remaining third, primarily older plants commissioned before 2003, will have to at least upgrade their PM control systems.

Emission Control Measures

Flue gas Desulphurization

  Flue Gas Desulphurization (FGD) system is installed to remove SO2 from flue gas. It reduces the emission of SO2 to the atmosphere upto 97%.

Flue Gas Desulphurization Scheme

  However, FGD is not installed for most of the power plants in India as it increases the cost of power generation. Further, it was felt not required for low sulphur Indian coal while considering SO2 emission form individual stack. At the end of 2015, around 24 Indian thermal power plants, mostly, using imported higher sulphur coal had installed FGD system. Now, all new and many older coal-fired plants will have to install FGD system. The most common technologies for this are wet scrubbing using slurry as absorbent, usually lime or limestone and sea water scrubbing.

  For control of SO2 and PM, the height of stack has been specified. The more the height of the stack the more area the emission spreads which dilutes the effect. Stack height for different capacity of power plant is given in Table 4.

Coal Beneficiation

  Coal beneficiation is the process of removing impurities from coal. This is done by washing the coal with water. Coal is first broken down into specified sizes and washing / cleaning methods are then used for various sizes. Coarse coal is handled by dry separation in air jigs or hydraulic jigs, where cyclones and concentrators are used for medium-size coal. Coal fines are separated by floatation or agglomeration. It reduces emissions of ash and sulphur dioxides when the coal is burnt. Coal washing / drying can reduce CO2 emission by as much as 5%.

  Ministry of Environment & Forests, vide notification no. GSR 560(E) & 378(E) dated September 19, 1997 and June 30, 1998 respectively made mandatory use of beneficiated / blended coal containing ash not more than 34 per cent on annual average basis for power plants located beyond 1000 km from pit head and power plants located in critically polluted areas, urban areas and ecologically sensitive areas.

Coal Blending

  Indian coal has higher ash content. So, in many cases, it is blended with imported coals having high calorific value and lower ash content. This method reduces overall cost and improves combustion performance.

Efficiency Improvement at Existing Power Plants

  There is scope for efficiency improvement at existing power plants, where achieving thermal efficiencies upto 40% could reduce CO2 emissions by as much as 22%. This can be done by equipment upgrading and the systematic performance monitoring and diagnostic testing of boilers, turbines, condensers and auxiliary equipment. Effective electrostatic precipitators can remove 99% of the fly ash and PM from the gas. Flue gas desulphurization system should be installed.

Super-critical Boiler

  The emission of carbon dioxide and other substances depends on the efficiency of the power plant. The average efficiency of conventional thermal power plant using sub-critical technology lies between 25% and 35%. The more efficient the plant, the fewer amounts are emissions. Substantial improvements in generation efficiency can be achieved through the use of more efficient super-critical and ultra super-critical technologies. Efficiency of power plant with super-critical technology lies between 37% and 44% and that of ultra super-critical technology lies between 45% and 60%.

(Basic structures of IGCC)

Fluidized Bed Combustion

  Fluidized Bed Combustion (FBC) is well established method of burning low-grade coals, biomass and other waste fuels. It produces less NOx and SO2 than a conventional coal based power plant and is also more efficient.

Integrated Gasification Combined Cycle Power Plant

  Integrated Gasification Combined Cycle (IGCC) power plant consists of a gasifier incorporated into a combined cycle system including coal fired steam power generation and gas turbine based power generation. Typically, the gas turbine produces 65% of the power and the steam turbine produces the remaining 35% power. Coal is partially oxidized in the gasifier producing syn gas (mixture of CO, CO2 & H2). This is called coal gasification. This gas is fed to the combustor as fuel. This is a technology which offers a practical means of utilizing poor quality coal efficiently while at the same time meeting clean environment requirements. In this case, efficiency of the order of 45% to 50% is achieved. This is a very new technology with only a few plants operating in USA and Europe. The technology is also very complex and highly costly.

Carbon Capture & Storage (CCS)

  Carbon capture and storage is an approach to mitigate the contribution of fossil fuel emissions to global warming based on capturing carbon dioxide from large point sources such as thermal power plants. The carbon dioxide can then be permanently stored away from the atmosphere. The carbon dioxide can be separated from the flue gases by chemical absorption processes. CCS applied to a modern large thermal power plant can reduce carbon dioxide emissions to the atmosphere by approximately 80-90% compared to plant without CCS.

Conclusion

  India has little option but to invest in coal, because it is cheap and domestically available. In 2016, coal generated roughly 80% of the country's electricity. Currently, known coal reserves about 250 billion tonnes are expected to last another 100 years. But India has to adopt clean coal technologies to mitigate the effect of climate change and control emissions. At present coal beneficiation is being done in India .Also in all the large power plants of unit size of 660 MW and above, super-critical boilers are being used. It not only reduces emissions, but they also produce power far more economically. Now time has come for India to adopt CCS and IGCC technologies to reduce emissions to the atmosphere. Of course, the technologies are new and their implements may increase the electricity tariff. But for the larger interest of the environment, we may have to bear it.

  In a report, ‘Transitions in Indian Energy Sector - Macro Level Analysis of Demand and Supply Side Options’, The Energy and Resources institute (TERI) indicates that current installed capacity and the capacity under construction would be able to meet demand till about 2026, keeping India power sufficient. The report estimates that no new investments are likely to be made in coal-based power generation in the years prior to that. It further estimates that beyond 2023-24, new power generation capacity could be all renewable.


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