Estimating carbon dioxide emissions for proposed coal plants in India

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With over 500 coal plants proposed or under construction in India, estimating carbon dioxide emissions from those plants is important for assessing potential climate impacts.

Table 1: Summary statistics for proposed coal plants in India

Status # of Plants Capacity (MW) Annual tons of CO2
Proposed 154 182,199 1,077,467,415
Early development 103 138,122 816,812,182
Advanced development 53 57,115 337,761,021
Construction 123 101,294 599,022,409
Newly operating (since 1/1/2010) 53 30,478 180,237,773
Deferred 21 31,510 186,340,712
Cancelled 15 18,150 107,333,670
Unconfirmed 21 25,735 152,189,090
Uncertain 9 19,600 115,908,536
Total 552 604,203 3,573,072,808


Factors used in calculating the CO2 estimate

  • Capacity factor: 70%
  • Coal mix: 2/3 domestic sub-bituminous; 1/3 imported bituminous
  • Weighted carbon dioxide emission factor for coal: 209.7 pounds of CO2 per million Btu
  • Generating units: 2/3 supercritical; 1/3 subcritical
  • Weighted plant efficiency: 37.1%
  • Average power plant output rate: 964.4 tons CO2/GWh, or 1.93 pounds CO2/kWh

Formula

Annual electricity output (GWh) x Emissions rate (tons of CO2 per GWh) = Total emissions of CO2 per year

Notes on annual electricity output

In order to estimate how much electricity India's proposed coal plants will generate, we need first to make a projection about how heavily the plants will actually be run. This figure is called "capacity factor" or "load factor." A capacity factor of 100% means that a plant is run at its maximum capacity for every hour of the year. Actual capacity factors are lower than 100% because of routine maintenance, fuel availability problems, and variations in demand. In 2010, India's coal plants totalled 159,398 MW of capacity and generated 771,173 GWh of electricity, implying a systemwide capacity factor of 55%.[1] A higher figure (69.63 percent load factor) was reported by the Central Electricity Authority for the period April 2012 - December 2012.[2]

This compares to average capacity factors in the United States of 66% to 75% in recent years, as shown in Coal-fired power plant capacity and generation, Table 1. Since new coal plants tend to be run more heavily than older, less efficient plants, it is reasonable to assume that India's proposed new coal plants will feature capacity factors higher than the current national average, perhaps in the range of 65% - 75%. For the purposes of this estimate of carbon dioxide generation from proposed coal plants in India, it is assumed that the average capacity factor will be 70%.

Notes on output rate (964.4 tons of CO2/GWh, or 1.93 pounds of CO2/kWh)

Estimating carbon dioxide output rate depends on the following factors:

  • carbon dioxide emission per million Btu from various types of coal
  • plant efficiency (i.e. heat rate) expressed in kWh/Btu

Carbon dioxide emission factor from various types of coal

The following carbon dioxide emission factor were estimated by the U.S. Department of Energy for coals in the United States.[3]

  • Lignite (i.e. brown coal): 216.3 pounds of carbon dioxide per million Btu
  • Subbituminous coal: 211.9 pounds of carbon dioxide per million Btu
  • Bituminous coal: 205.3 pounds of carbon dioxide per million Btu
  • Anthracite: 227.4 pounds of carbon dioxide per million Btu

Note that, perhaps counterintuitively, carbon dioxide emission factors are not necessarily lower for higher quality coals. For example, anthracite coal, which is the highest quality coal, produces more carbon dioxide per Btu than low-quality lignite. This is because anthracite lacks hydrogen, which is a small portion of the content of lower grade coals. When burned, hydrogen is transformed into water vapor (H2O) rather than carbon dioxide (CO2). Therefore, nearly all the energy in anthracite comes from the combustion of carbon, resulting in higher carbon dioxide emission rates per unit of energy than when lower grade coals containing some hydrogen are burned. (Of course, on a tonnage basis, lower grade coals do produce more carbon dioxide than higher grade coals.)

Domestic coal in India is typically sub-bituminous (heat rate 4,200 kcal/kg or 7,560 Btu/lb), while imported coal is typically bituminous (6,665 kcal/kg or 11,997 Btu/lb). For purposes of estimating carbon dioxide emissions for proposed coal plants in India, it is assumed that these plants will use 2/3 domestic coal and 1/3 imported coal, for a carbon dioxide emission rate of 209.3 pounds of carbon dioxide per million Btu.[4]

Plant efficiency

MIT's "Future of Coal" study provides the following estimates of coal plant efficiency (i.e., percent of energy content of fuel converted into electricity) for various coal plant technologies, assuming no carbon capture:[5]

  • Subcritical - 34.3%
  • Supercritical - 38.50%
  • Ultra-supercritical - 43.3%

For purposes of estimating carbon dioxide emissions for proposed coal plants in India, it is assumed that 2/3 of the plants will use supercritical technology and 1/3 will use subcritical technology, for a weighted plant efficiency of 37.1%.

Formula

CO2 emissions per unit of energy in coal (expressed in Btu) x Btu/MWh x Plant efficiency = Emissions per MWh

Calculation

209.7 lbs CO2 per million Btu x 3412.3 x 103 Btu/MWh x 37.1% = 1.93 lbs CO2/kWh = 964.4 tons CO2/GWh

Comparing carbon dioxide output rate from existing coal plants in the United States to projected carbon dioxide output rate from proposed coal plants in India

The output rate of 1.93 pounds CO2/kWh calculated in this analysis is slightly lower than published estimates for existing U.S. coal plants. The Energy Information estimated an output rate of 2.117 in 1998 for U.S. electricity generation, and an output rate in 1999 of 2.095. [6] The probable reason for the higher U.S. rate is the lower efficiency of the U.S. legacy coal fleet compared with the projected efficiency of new coal plants in India.

Articles and resources

References

  1. S.K. Behera, J.A. Farooquie, and A.P. Dash, "Productivity changes of coal-fired thermal power plants in India: a Malmquist index approach," IMA Journal of Management Mathematics (2011) 22, 387-400, Table 1. (Subscription only)
  2. "All India Plant Load Factor Thermal," Central Electricity Authority, accessed June 2014
  3. B.D. Hong and E.R. Slatick, "Carbon Dioxide Emission Factors for Coal," U.S. Energy Information Administration, 1994
  4. For estimates of heat rate from Indian domestic and imported coal, see "Alternate fuel stock," BAIL (India) Limited, accessed December 2011
  5. "The Future of Coal", MIT, 2007, Table 3.1, page 19
  6. "Carbon Dioxide Emissions from the Generation of Electric Power in the United States," July 2000, U.S. Department of Energy

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