Estimates of Emissions from Coal Fired Thermal Power Plants in India

Coal is the primary fuel for electricity generation in India and its usage is continuously increasing to meet the energy demands of the country. This paper presents emissions of carbon dioxide (CO2), sulfur dioxide (SO2), and nitric oxide (NO) from thermal power plants in India for a period of nine years from 2001-02 to 2009-10. The emission estimates are based on a model in which the mass emission factors are theoretically calculated using the basic principles of combustion and operating conditions. Future emission scenarios for the period up to 2020-21 are generated based on the estimates of the nine years from 2001-02 to 2009-10. Power plants in India use different qualities of coal, different combustion technologies and operating conditions. As a result, these plants have differences in achieved efficiencies (coal usage per unit of electricity). The estimates show region wise differences in total emissions as well as differences in emissions per unit of electricity. Computed estimates show the total CO2 emissions from thermal power plants have increased from 323474.85 Gg for the year 2001-02 to 498655.78 Gg in 2009-10. SO2 emissions increased from 2519.93 Gg in 2001-02 to 3840.44 Gg in 2009-10, while NO emissions increased from 1502.07 Gg to 2314.95 Gg during this period. The emissions per unit of electricity are estimated to be in the range of 0.91 to 0.95 kg/kWh for CO2, 6.94 to 7.20 g/kWh for SO2, and 4.22 to 4.38 g/kWh for NO during the period 2001-02 to 2009-10. The future emission scenario, based on the projected coal consumption in Indian thermal power plants by Planning Commission of India under ‘Business-as Usual (BAU)’ and “Best case Scenario (BCS)’ show the emission in the range of 714976 to 914680 Gg CO2, 4734 to 6051 Gg SO2 and 366 to 469 Gg NO in the year 2020-21. Increase in coal use efficiencies in electricity generation by thermal power plants can significantly reduce the emissions of greenhouse and polluting gases. This methodology provides a useful tool for inventory preparation in a sector where measured values for emissions factors are very sparse. INTRODUCTION Emissions of greenhouse gases and other pollutants are increasing in India with the increasing demand for electricity. The aspiration for rapid economic growth leading to rapid industrialization coupled with accelerated urbanization and mechanization of agriculture has been responsible for this increasing demand of electricity ever since the independence. The electricity consumption grew from 375.39 (billion kWh) in 2000 to 600.65 (billion kWh) in 2008 at an annual growth rate of 6.67% , while the electric power generation grew from 529.12 billion kWh in 2000 to 835.27 billion kWh at an annual growth rate of 5.78% (www.eia.doe.gov, 2010). Fig. 1 shows the growth of electricity generation and usage in India and China during the period 2000 to 2008 based on the EIA (www.eia.doe.gov, 2010) 2 data. Large difference between electric power generation and consumption are due to transmission and distribution losses. In India, the losses are extremely high and vary between 30 to 45%. To ensure ‘Power on Demand’, India has envisioned an additional generating capacity of 100,000 MW by the year 2012. It is estimated that electricity demand outstrips supply by 7-11%. With India’s population of more than a billion that is growing at an annual rate of about 2% (World Bank, 2000), the gap between demand and supply of the electricity may rise further. India ’s current (www.powermin.nic.in – Annual Report 2008-09) electric power availability is approximately 11.1 % short of demand with peak load shortages of 11.9 %, whereas in 2000-01, power capacity was 7.8 % short of demand with peak load shortage of 13%. Percentage of the electric energy shortage and peak shortage from the period 2000-01 to 2008-09 are shown in Fig. 2. Coal is the favorite fuel for the electricity generation in countries like India and China. Abundant supply of coal locally and sustained high prices for imported natural gas and oil make coal-fired generation of electricity more attractive economically. Coal is approximately 90% of the total fuel mix for electricity generation. Fig. 3 shows the percentage of generating capacity of all categories that includes natural gas, diesel, nuclear, hydro, and renewal energy sources like biomass power, urban & industrial waste power, and wind energy. Coal and lignite based power plants have approximately 54.42% of the total electric power generation capacity in India. However, relatively lower calorific value, coupled with high ash content and inefficient combustion technologies aggravates emission of greenhouse gases and other pollutants from India’s coal and lignite based thermal power plants. Main emissions from coal fired and lignite based thermal power plants are CO2, NOx, SOx, and air-borne inorganic particles such as fly ash, carbonaceous material (soot), suspended particulate matter (SPM), and other trace gas species. Thermal power plants, using about 70% of total coal in India (Garg et. al., 2002), are among the Large Point Sources (LPS) having significant contribution (47% each for CO2 and SO2) in the total LPS emissions in India. Only limited efforts (Chakraborty et al., 2008) have been made so far for measuring the plant specific emissions of different gases and particulate matter in India to generate plant specific emission factors. These measurements were taken at few small plants of less than 250 MW capacities. For preparing the national green house gas (GHG) emission inventories from electricity generation in Indian thermal power plants, default emission factors prescribed by the Intergovernmental Panel on Climate Change (IPCC) [IPCC, 1996] have so far been used along with the country specific net calorific values (NCV) of Indian coal types (INCCA, 2010). Earlier electric power generation from thermal power plants was estimated to have contributed about 96% of total carbon dioxide (CO2) emissions during 1990 (ALGAS ,1998) while in 1994, it was estimated about 62% (NATCOM, 2004) and in 2007, it has been estimated about 69% of the total CO2-equivalent emissions from energy sector (INCCA, 2010). The differences between 1990 estimates and 1994 and 2007 estimates also capture the different methodological approaches followed in these sets of estimates. The IPCC default emission factors, used in Indian inventory estimation, represents the average of available emission values using the similar fuel and technical processes under similar national circumstances and do not account for Indian coal characteristics or the operating conditions at the various thermal power plants in India. A time series of emission trends of CO2, NOx, and SOx from the Indian coal fired and lignite based thermal power plants over a decade (2001-02 to 2009-10) is presented here. Eighty six power plants with total installed capacity of 77682 MW are considered in this analysis for which required input 3 data was available from Central Electricity Authority of India (CEA). These plants represent about 76% of the total installed capacity of thermal power plants in India. As of March 2010, there are 105 thermal power plants in India of more than 100 MW capacity each, with total installed generation capacity of 93772 MW. As per the CEA (www.cea.nic.in, 2011), public sector thermal power plants have total installed capacity of about 87592 MW while the private sector has a total installed capacity of about 14962 MW which include captive thermal power plants of less than 100 MW installed capacity. The combustion technology in these 86 plants is based on pulverized coal burning but the type of furnace technology, design of the boiler, forced draught fans etc. differ with plants. Based on the CEA data (www.cea.nic.in 2010), specific coal usage at three plants is less than 0.6 kg/kWh, at 19 plants, usage is between 0.6-07 kg/kWh, thirty nine plants use coal between 0.7-0.8 kg/kWh, fifteen plants use between 0.8-0.9 kg/kWh, seven plants use between 0.9-1.0 kg/kWh, and at three plants coal usage exceeds 1.0 kg/kWh. Hence the efficiency of the plants and the coal usage per unit of electricity generation also differ at each plant. There is a need to modernize India’s thermal power plants and reduce the coal usage per unit of electricity generation (kg/kWh). Modernization with reduction in coal usage (kg/kWh) will help in reducing the national emissions. Quality of Indian coal will remain same but with the improvement in combustion technologies, emissions can be reduced. It is estimated that 1% to 2% increase in heat rate improvements leading to efficiency improvement will result in 1% to 2% decrease in emissions per unit of electricity (USAID/TVA/NTPC 2000). Here, the emission factors for each of the 86 plants are computed based on the basic principles of coal combustion, characteristics of the coal used in different thermal power plants, and the operating conditions in these plants. This methodology provides a `bottom-up’ approach for the development of emission inventory. This is the first study where emission inventory of CO2, SO2, and NO are developed specifically for coal fired thermal power plants in India based on the characteristic of coal and operating conditions prevalent at the plant. METHODOLOGY The combustion process of the pulverized coal in the boiler is a complicated non-linear phenomenon. The pollutants emitted from thermal power plants depend largely upon the characteristics of the fuel burned, temperature of the furnace, actual air used, and any additional devices to control the emissions. At present, the control devices used in thermal power plants in India is electrostatic precipitator (ESP) to control the emission of fly ash (SPM). Some new plants use low NOx burners for high temperature (> 1500 K) combustion technologies and dry/wet SO2 scrubber, if chimney height is less than 275 meters. Mass emission factors for CO2, SO2, and nitric oxide (NO) are computed based on the input data, such as chemical composition of the coal used at the power plants and the actual air used during combustion. These calculations are based on theoretical ideals and do not take account for the control devices. Indian coal generally has low sulfur contents. The operative combustion temperature is assumed to be 1200 K. Carbon Dioxide and Sulfur Dioxide From the elemental analysis of the coal, the percentage of carbon, hydrogen, nitrogen, oxygen, ash, and moisture in the coal is known. Let C be the mass of the carbon, S of the sulfur, H of the hydrogen, O2 of the oxygen, and N2 of the nitrogen, then Oxygen (Or) required to burn one kilogram (kg) of coal = 4 Or = C*(32/12) + H*(16/2) +S*(32/32) –O2 (1) Air mass required for Or kg of oxygen = ( Or / mass fraction of O2 in the air ) = Or / 0.233 (2) If E is the percentage of excess air used in the furnace to burn the coal, the air mass used = Air (used) = (1 + E)* Or / 0.233 (3) Knowing the air mass used to burn one kg of coal, mass of O2 and N2 are calculated as O2 in the air used = (1+ E)* Or (4) N2 in the air used = 0.767* (1+ E)* Or / 0.233 (5) Mass of CO2, SO2, NO, and H2O are calculated by mass balance as CO2 = C*44/12 (6) SO2 = S*64/32 (7) H2O = H*18/2 (8) Oxidation of Nitrogen Oxides of nitrogen (NOx) are nitrous oxide (N2O), nitric oxide (NO), and nitrogen dioxide (NO2). The formation of NOx during coal combustion is a complex process involving both homogeneous and heterogeneous reactions. Most (about 90% or higher) of the NOx emitted during combustion process is in the form of NO. NO is formed by oxidation of (i) atmospheric nitrogen, known as ‘thermal NO’ and (ii) chemically bound nitrogen within the fuel matrix, known as ‘chemical NO’. Formation of ‘thermal NO’ is temperature sensitive whereas formation of ‘chemical NO’ is insensitive to temperature and occurs on a time scale comparable to that of combustion reactions. A kinetic model is needed to describe the detailed mechanism of the formation of NO in flames and the prediction of NO concentration in combustion products. Emission of NO varies widely with boiler conditions and is generally functions of flame temperature, excess air or concentration of oxygen in the system, percentage of boiler load, nitrogen content in the coal, and rate of gas cooling. The actual mechanism, whereby atmospheric nitrogen is oxidized, goes through a complex chain of reactions initiated by oxygen atoms. Generally accepted principal reactions (Zeldovich, 1946) for ‘thermal NO’ formation are O + N2 = NO + N (09) N + O2 = NO + O (10) N + OH = NO +H (11) A kinetic model is beyond the scope of present analysis. Present estimates give the equilibrium concentrations of NO assuming long residence time as found in large boilers. The oxidation of nitrogen is represented by the overall balance (Hanby, 1994). N2 + O2 → 2NO (12) A simple stoichiometric calculation gives the equilibrium NO concentration as 0.5 0.5 10.1 N O 2 2 K ( ) ( ) NO χ = χ χ , (13) where χ is the species concentration and K10.1 is is an equilibrium constant that depends upon the temperature of the gas. At 1200 K, K10.1 = 0.00526, (Hanby, 1994). Characteristics of the Indian Coal