Research |
Program for Computational Reactive Mechanics (PCRM)Anthropogenic
Emissions from energy activities in India:
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Contents Part I: Emissions from Thermal Power Plants using Coal as Fuel |
Part II: Emissions from Vehicular Transport using Petroleum Fuel Vehicles in India Emissions from Internal Combustion Engines Petroleum Fuels used in India Fuel Stoichiometry Engines for Vehicular Transport Exhaust Emissions Indian Scenario Emissions Factors and Available Inventory Appendix |
Emissions from coal usage
The main emissions from coal combustion at thermal power plants are Carbon dioxide (CO2), Nitrogen oxides (NOx), Sulfur oxides (SOx), Chlorofluorocarbons (CFCs), carbonaceous material (soot), and air-borne inorganic particles such as fly ash, also known as suspended particulate matter (SPM) and other trace gas species. Carbon dioxide, nitrous oxide, and chlorofluorocarbons are greenhouse gases. Evidence accumulated by the Inter-governmental Panel on Climate Change (IPCC) suggests that emissions of these greenhouse gases might be responsible for climate change, a global concern. Possible consequences projected by IPCC include:
- a rise in sea levels
- a more vigorous hydrological cycle that may increase the severity of floods and droughts and may cause more extreme climatic events; and
- ecological change that could threaten agricultural productivity
Oxides of nitrogen and sulfur, also play an important role in atmospheric chemistry and are largely responsible for atmospheric acidity. Particulates and black carbon (soot) are of concern in the radiative10 forcing of the earth. They also have a significant negative impact on human health causing lung tissue irritation and are linked to cancer and other serious diseases.
The pollutants emitted from thermal power plants depend largely upon the fuel burned, the furnace design, the excess air, and any additional devices used to reduce the emissions. At present, the only control device used in thermal power plants in India is electrostatic precipitator to control the emission of fly ash (SPM). CO2, SO2, nitric oxide (NO), soot, and SPM emissions from each of the thermal (coal-fired) power plants in India have been computed using basic principles of combustion. These calculations are based on a theoretical ideal and the input data, such as chemical composition of the coal used in the power plants, coal used per unit of power, excess air used during combustion, and the power generation from each plant. This input data has been collected from the published information. The present method to estimate the emissions is one of the many available methods for emissions inventory process. The other methods used in different countries are based on the guidelines recommended by IPCC11 , and may require large resources. Emissions from combustion of the supplementary fuels such as high-speed diesel (HSD) and furnace oil used in small quantities are not counted in the present calculations.
Emission of carbon dioxide and sulfur dioxide:
Utilities mostly burn coal with approximately 10 -30% excess air. Carbon as obtained from Ultimate analysis is converted to CO2 after the reaction (combustion) is complete. Some carbon is emitted in the form of soot and some carbon remains unburned and mixes with the ash. Different combustion technologies affect the types and concentrations of resultant species e.g. fixed bed combustion results in higher carbon content in the ash.
Carbon in the coal is converted to carbon dioxide (CO2) by the reaction
C |
+ |
O2 |
--> |
CO2 |
Similarly, hydrogen and sulfur are converted to moisture (H2O) and sulfur dioxide (SO2) by the reactions
H2 |
+ |
O2/2 |
--> |
H2O |
S |
+ |
O2 |
--> |
SO2 |
Table 9, as an example, gives the computation of oxygen required for burning one kg of coal used at the Chandrapur thermal power station and the combustion products.
Table 9: Computation of combustion products for the Chandrapur coal
Species |
Mass |
Oxygen required |
Products |
Reaction |
Carbon |
0.3769 |
0.3769 x (32/12) = 1.01 |
CO2: 0.3769 x (44/12) = 1.38 |
C + O2 --> CO2 |
Hydrogen |
0.0266 |
0.0266 x (16/2 ) = 0.21 |
H2O: 0.0266 x (18/2) = 0.24 |
H2 + O2/2 --> H2O |
Sulphur |
0.008 |
0.008 x (32/32) = 0.008 |
SO2: 0.008 x (64/32) = 0.016 |
S + O2 --> SO2 |
Oxygen |
0.0578 |
|||
Nitrogen |
0.0107 |
N2 + O2 --> 2NO |
||
Ash |
0.47 |
Oxygen required to burn 1 Kg of coal = 1.01+0.21+0.01–0.05 = 1.18 Kg
Air required = (Oxygen required)/(Mass fraction of oxygen in the air) = 1.18/0.233 = 5.06 Kg = stoichiometric air
Total air (stoichiometric + 20% excess) = 5.06x1.2 = 6.072 Kg
6.072 air = 1.415(6.072 x 0.233) oxygen + 4.474(6.072 x 0.767) nitrogen
The combustion product with 20% excess air will contain:
0.2348(1.415 – 1.18)Kg O2 and 4.48(4.47 + 0.01)Kg N2 .
Appendix-A gives the general formula for the calculation of required oxygen for burning of one kg of fuel.
Table 10 gives the computation of the flue gas composition from burning one Kg of Chandrapur coal at 20% excess air, as an example.
Table 10: Flu gas composition with 20% excess air
Product |
Mass/Kg coal |
Mol. wt |
Kmoles/Kg coal |
% volume |
CO2 |
1.38 |
44 |
0.03136 |
15.39 |
SO2 |
0.016 |
64 |
0.00025 |
0.12 |
O2 |
0.24 |
32 |
0.0075 |
3.68 |
N2 |
4.61 |
28 |
0.1646 |
80.77 |
Total = |
0.2038 |
Emissions of oxides of nitrogen from coal:
Oxides of nitrogen (NOx) are (i) nitrous oxide (N2O), (ii) nitric oxide (NO), and (iii) nitrogen dioxide (NO2). NO2 is mostly formed by oxidation of the NO, which is discharged in combustion products. About 90% of the NOx is in the form of NO. NO is formed by two mechanisms: (i)oxidation of atmospheric nitrogen, known as 'thermal NO' and (ii) oxidation of nitrogen that is chemically bound within the fuel, known as 'chemical NO'. The amount of NOx varies widely with boiler conditions. NOx emissions are generally functions of flame temperature, excess air, percentage of boiler load, nitrogen content in the coal, and rate of gas cooling. In pulverized coal flames, about 30 - 35% of nitrogen in coal gets converted into NO and remaining nitrogen in the coal gets converted into molecular nitrogen. The actual mechanism, whereby atmospheric nitrogen is oxidized, goes through a complex chain of reactions initiated by oxygen atoms. We can however calculate equilibrium concentrations of NO, using the following reaction:
N2 |
+ |
O2 |
--> |
2NO |
This is a lumped reaction. Generally accepted principal reactions are
O + N2 = NO + N
N + O2 = NO + O
N + OH = NO + H
The concentration of nitric oxide (NO) is given by
Where X is the species concentration and K10.1 is a equilibrium constant and depends upon the temperature of the gas. Appendix-B gives the equations to compute equilibrium constant for NO reaction.
Concentration values X for O2, N2, CO2, SO2, and NO for the Chandrapur coal, as calculated by the above method, are given in Table 11. The value of the equilibrium constant K10.1 computed at 1700 K is 0.007824.
Table- 11: Species concentrations in flue gas for the sample coal
| Air | X(O2) |
X(N2) |
X(CO2) |
X(SO2) |
X(NO) |
Stoichiometric |
0 |
0.831333 |
0.167148 |
0.001512 |
0 |
5% excess |
0.010483 |
0.8293267 |
0.158754 |
0.001436 |
0.00073 |
10% excess |
0.019970 |
0.8274488 |
0.151213 |
0.001368 |
0.001006 |
15% excess |
0.028597 |
0.8257412 |
0.144356 |
0.001306 |
0.001202 |
20% excess |
0.036475 |
0.8241817 |
0.138094 |
0.001249 |
0.001357 |
25% excess |
0.043699 |
0.8227519 |
0.132352 |
0.001197 |
0.001484 |
30% excess |
0.050345 |
0.821436 |
0.127069 |
0.001149 |
0.001591 |
igure 1 gives a comparative behavior of the CO2, SO2, and NO concentrations (in mole fraction) for 5 - 30% excess air. NO concentrations increase, while the concentrations of CO2 and SO2 decrease with the increase in excess air.
Species concentrations (PPM) in the combustion products for the Chandrapur coal are given in Table 12.
Table 12: Species concentrations in parts per million (ppm)
Excess Air % |
CO2 |
SO2 |
NO |
0 |
167148 |
1512 |
0 |
5 |
158754 |
1436 |
730 |
10 |
151213 |
1368 |
1006 |
15 |
144356 |
1306 |
1202 |
20 |
138094 |
1249 |
1357 |
25 |
132352 |
1197 |
1484 |
30 |
127069 |
1149 |
1591 |
Carbonaceous material and black carbon:
Incomplete and/or inefficient combustion processes of fossil fuel generate carbonaceous aerosols. The emitted carbonaceous (soot) aerosols are of two types, namely organic carbon (OC) and black carbon (BC). These two types have different properties in the atmosphere. OC is a reactive species and has scattering properties in the solar spectrum12 while the BC, on the other hand, is non reactive in the atmosphere but has highly absorbing properties in the solar spectrum . In the thermal power plants, most of the soot carbon emitted would be in the form of BC because of the higher temperatures of combustion in the furnaces. When the soot is formed the analysis ranges from C8H to C12H. The importance of BC in the radiative balance of Earth is gradually being understood.
The soot carbon from the combustion of coal in the Indian thermal power plants has been calculated on the basis of prevalent combustion characteristics. It is assumed that approximately, 10% of the coal carbon goes in the bottom ash and about 2% of the carbon forms the soot that may emit with fly ash. Electrostatic Precipitators (ESPs) used in the thermal power plants in India, remove about 99% fly ash from the stack. 1% of the generated fly ash, which contains 2% soot carbon is emitted into the atmosphere. These soot particles are sub-micronic in size. Similar assumption has also been used for calculation of soot carbon from lignite based thermal power plants in India (viz. Kutch and Neyvelli power plants).
Suspended Particulate matter
The fly ash in the form of Suspended Particulate Matter (SPM) is a major pollutant from coal burning power plants in India. SPM has been calculated on the basis of the ash contents of the coal. It is assumed that 85% of the ash in the coal goes out through the stack as fly ash after the combustion. Electrostatic precipitators (ESPs) working on 99% efficiency rate allows only 1% of the formed fly ash to emit as SPM.
10radiative forcing is defined as a change in the average net radiation. It is based on global balance between incoming solar energy and outgoing terrestrial radiative energy that is emitted to space.
11Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories, Edited by JT Houghton et al., IPCC/OECD/IEA, UK Meteorological Office, Bracknell
12Cooke W.F., Liousse C., Cachier H. and Feichter J., `Construction of a 10x10 fossil fuel emission data set for carbonaceous aerosol and implementation and radiative impact in the ECHAM4 model', Journal of Geophysical Research, Vol. 104, No D18, pp 22137-22162, 1999.
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