Estimating carbon dioxide emissions from coal plants

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Four factors are used to estimate the CO2 emissions from coal plants:

  • Plant capacity
  • Plant capacity factor
  • Emissions factor
  • Combustion technology

Formula

The CO2 emissions from a proposed coal plant can be calculated with the following formula:

annual CO2 (in million tonnes) = capacity * capacity factor * heat rate * emission factor * 3.97347 x 10^-9

Example for a typical coal plant

  • Size: 1,000 MW
  • Capacity factor: 80%
  • Supercritical combustion heat rate: 8863 Btu/kWh
  • Sub-bituminous coal emission factor: 211.9 pounds of carbon dioxide per million Btu

Based on these parameters, the annual CO2 for the plant is as follows: Annual CO2 (million tonnes) = 1,000 * .8 * 8863 * 211.9 * 3.97347 * 10^-9 = 5.97 million tonnes

Capacity factor

"Capacity factor" or "load factor," is a measure of the amount of power that a plant produces compared with the amount it would produce if operated at its rated capacity nonstop. 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.

Worldwide

As shown in table below, the average capacity factor for coal plants worldwide, based on data for 2014 was 58.9%, down from 59.3% in 2013. Individual countries vary from 36.8% (Russia) to 79.7% (Japan).

Table 1: Capacity factor for leading coal countries and regions worldwide[1]

Country or region Coal-fired
electricity generation
in 2014 (TWh)
Coal-fired
generating capacity
in 2014 (GW)
Capacity factor
China 4146 855 55.4%
US 1713 302 64.7%
India 967 176 62.7%
EU 841 177 54.2%
Japan 349 50 79.7%
South Africa 232 39 67.9%
Russia 158 49 36.8%
OECD 3478 614 64.7%
World 9707 1882 58.9%

Other Estimates

China: According to the China Electricity Council, the "national average utilization hours for thermal generation" during the first six months of 2013 was 2,412 hours. Compared to the actual number of hours in those months (4380), this suggests a capacity factor of 55.07 percent.[2]

India: For the period April 2011 - December 2011 India's Central Electricity Authority reported an all-India load factor of 72.10 percent.[3] A slightly lower figure of 69.63 percent (still high compared to the worldwide average) was reported by the Central Electricity Authority for the period April 2012 - December 2012.[4]

United States: Average capacity factor for coal plants dropped in the United States from 73 percent in 2008 to 60% in 2013.[5]

Heat rate

A coal plant's heat rate is a measurement of how well a plant performs the task of converting one form of energy (coal) to another form of energy (electricity). Note that in order for various measures of heat rate to be useful for comparative purposes, definitions must be consistent. For this reason the standard way of measuring heat rate is to compare the quantity of energy contained in coal as it enters the plant site to the quantity of energy contained in the electricity that exits the plant site into the grid. (Energy consumed to mine and transport coal is not included, nor is energy lost in moving electricity through the grid.) There are two common ways of expressing heat rate, one using Btu/kWh and the other using kcal/kWh. A plant that was 100 percent efficient at converting all its coal energy into electrical energy would have a heat rate of 860 kcal/kWh or 3412 Btu/kWh; of course, it is not possible for such a plant to exist. The higher the heat rate, the lower a plant's efficiency. Heat rate is determined by the type of combustion technology, the type of coal, and the size of the plant. Additional factors include continuity of operations (starting and stopping a plant degrades efficiency) and ambient temperature (the warmer temperature of the available cooling water will make a plant running in a hot climate less efficient than a plant running in a cold climate).

MIT's Future of Coal study (2007)

In 2007 MIT's Future of Coal study examined the coal combustion technologies used by nearly all coal plants.[6] According to the study, improved technologies (supercritical, ultra-supercritical) deliver significantly better efficiencies. Note that the Sargent & Lundy study (see below) showed more modest differentials among the various technologies.

Table 2: Heat rate for coal plants (Future of Coal, 2007)[6]

Technology Type of Coal Plant Capacity
(MW)
Net Heat Rate
(kcal/kWh)
Net Heat Rate
(Btu/kWh)
Net Plant
Efficiency
Subcritical Illinois #6 (bituminous) 500 2,509 9,950 34.29%
Supercritical Illinois #6 (bituminous) 500 2,237 8,870 38.47%
Ultra-supercritical Illinois #6 (bituminous) 500 1,987 7,880 43.30%
Subcritical CFB Lignite 500 2,474 9,810 34.78%

Sargent & Lundy study (2009)

In 2009 the U.S. Environmental Protection Agency commissioned the consulting firm of Sargent & Lundy to make a study of heat rates at various types of coal plant. The study looked at the performance of currently available plant types (subcritical, supercritical, and ultra-supercritical) as well as technology not yet commercially available (advanced ultra-supercritical) or available but not widely deployed (IGCC). Coal types included lignite (Texas), subbituminous (Powder River Basin), and bituminous (Illinois #6). Plant size is measured in terms of gross capacity, i.e. prior to subtracting capacity devoted to internal plant functions. Heat rates were calculated on a net basis, i.e. based on a plant's actual delivery of electricity to the grid. Pollution controls included selective catalytic reduction (SCR), flu gas desulfurization (FGD), activated carbon injection (ACI), and baghouse.[7] As shown in the table, larger plants and higher grade coals result in better plant efficiency. (The sole exception to that principle is IGCC plants, which are expected to perform more efficiently with subbituminous coal than with bituminous coal.) The efficiency figures and heat rates provided by the Sargent & Lundy study were computed using the higher heating value (HHV) method, which is standard for coal plant ratings in the United States, rather than the lower heating value (LHV) method, which is standard outside the United States. The LHV method reflects actual field conditions, since it assumes that the condensate energy in plant steam is lost into the environment. The LHV method is used by the International Energy Agency. To convert HHV efficiencies to LHV efficiencies, two percentage points are added for bituminous coals, three percentage points are added for subbituminous coals, and four percentage points are added for lignite coals, as shown in Table 2.[8]

Table 2: Heat rate for coal plants, based on Sargent & Lundy, 2009[7]

Technology Type of Coal Plant Capacity
(MW)
Heat Rate Based on HHV Method
(kcal/kWh)
Heat Rate Based on HHV Method
(Btu/kWh)
Plant Efficiency Based on HHV Method
HHV to LHV Conversion Factor Plant Efficiency Based on LHV Method Heat Rate Based on LHV Method
(Btu/kWh)
Subcritical Bituminous 400 2,357 9,349 36.50% 2% 38.50% 8,863
Subcritical Bituminous 600 2,346 9,302 36.68% 2% 38.68% 8,821
Subcritical Bituminous 900 2,343 9,291 36.72% 2% 38.72% 8,811
Subcritical Subbituminous 400 2,376 9,423 36.21% 3% 39.21% 8,702
Subcritical Subbituminous 600 2,363 9,369 36.42% 3% 39.42% 8,656
Subcritical Subbituminous 900 2,360 9,360 36.45% 3% 39.45% 8,648
Subcritical Lignite 400 2,512 9,963 34.25% 4% 38.25% 8,921
Subcritical Lignite 600 2,499 9,912 34.42% 4% 38.42% 8,880
Subcritical Lignite 900 2,497 9,901 34.46% 4% 38.46% 8,871
Supercritical Bituminous 400 2,284 9,058 37.67% 2% 39.67% 8,601
Supercritical Bituminous 600 2,274 9,017 37.84% 2% 39.84% 8,564
Supercritical Bituminous 900 2,267 8,990 37.95% 2% 39.95% 8,540
Supercritical Subbituminous 400 2,302 9,128 37.38% 3% 40.38% 8,450
Supercritical Subbituminous 600 2,290 9,080 37.58% 3% 40.58% 8,409
Supercritical Subbituminous 900 2,284 9,057 37.67% 3% 40.67% 8,389
Supercritical Lignite 400 2,433 9,647 35.37% 4% 39.37% 8,667
Supercritical Lignite 600 2,422 9,603 35.53% 4% 39.53% 8,631
Supercritical Lignite 900 2,415 9,576 35.63% 4% 39.63% 8,609
Ultra-Supercritical Bituminous 400 2,250 8,924 38.23% 2% 40.23% 8,480
Ultra-Supercritical Bituminous 600 2,238 8,874 38.45% 2% 40.45% 8,435
Ultra-Supercritical Bituminous 900 2,233 8,855 38.53% 2% 40.53% 8,418
Ultra-Supercritical Subbituminous 400 2,268 8,993 37.94% 3% 40.94% 8,334
Ultra-Supercritical Subbituminous 600 2,254 8,937 38.18% 3% 41.18% 8,286
Ultra-Supercritical Subbituminous 900 2,250 8,921 38.25% 3% 41.25% 8,272
Ultra-Supercritical Lignite 400 2,396 9,502 35.91% 4% 39.91% 8,550
Ultra-Supercritical Lignite 600 2,383 9,449 36.11% 4% 40.11% 8,507
Ultra-Supercritical Lignite 900 2,378 9,430 36.18% 4% 40.18% 8,491
Advanced Ultra-Supercritical Bituminous 400 2,105 8,349 40.87% 2% 42.87% 7,959
Advanced Ultra-Supercritical Bituminous 600 2,094 8,305 41.08% 2% 43.08% 7,919
Advanced Ultra-Supercritical Bituminous 900 2,088 8,279 41.21% 2% 43.21% 7,896
Advanced Ultra-Supercritical Subbituminous 400 2,122 8,414 40.55% 3% 43.55% 7,834
Advanced Ultra-Supercritical Subbituminous 600 2,109 8,363 40.80% 3% 43.80% 7,790
Advanced Ultra-Supercritical Subbituminous 900 2,103 8,341 40.91% 3% 43.91% 7,771
Advanced Ultra-Supercritical Lignite 400 2,240 8,882 38.41% 4% 42.41% 8,044
Advanced Ultra-Supercritical Lignite 600 2,228 8,834 38.62% 4% 42.62% 8,005
Advanced Ultra-Supercritical Lignite 900 2,221 8,808 38.74% 4% 42.74% 7,984
IGCC Bituminous 600 2,124 8,425 40.50% 2% 42.50% 8,029
IGCC Subbituminous 600 2,033 8,062 42.32% 3% 45.32% 7,528
IGCC Lignite 600 2,147 8,515 40.07% 4% 44.07% 7,742

Government of India CO2 Baseline Database (2013)

As part of its participation in the Clean Development Mechanism (CDM) under the Kyoto Protocol to the United Nations Framework Convention on Climate Change (UNFCCC), the government of India Central Electricity Authority publishes an annual report on the carbon dioxide emissions of the country's existing coal plants larger than 25 MW in size. Version 8 of this report, the results of which are shown in Table 3, is based on plants commissioned through 2012 and on 120 samples of coal from different Indian coal fields. While the characteristics of that coal are not included in the study, "Indian coal" typically refers to subbituminous coal.[9]

Table 3: Heat rate for existing coal plants in India[9]

Technology Type of Coal Plant Capacity
(MW)
Net Heat Rate
(kcal/kWh)
Net Heat Rate
(Btu/kWh)
Net Plant
Efficiency
Unspecified Indian coal 67.5 3,125 12,393 27.53%
Unspecified Indian coal 120 2,747 10,894 31.32%
Unspecified Indian coal 200-250 2,747 10,894 31.32%
Unspecified Indian coal 300 2,582 10,239 33.32%
"Type 1" Indian coal 500 2,622 10,398 32.81%
"Type 2" Indian coal 500 2,545 10,093 33.81%
Unspecified Indian coal 600 2,545 10,093 33.81%
"Type 1" Indian coal 660 2,329 9,236 36.94%
"Type 2" Indian coal 660 2,274 9,018 37.84%
Unspecified Indian lignite 75 3,125 12,393 27.53%
Unspecified Indian lignite 125 2,909 11,536 29.58%
Unspecified Indian lignite 210-250 3,014 11,953 28.55%

Emission factor

The following carbon dioxide emission factors were estimated by the U.S. Department of Energy for coals in the United States.[10]

  • 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, higher grade coals do produce more carbon dioxide than lower grade coals.)

Coefficient

3.97347 x 10^-9

This is basically 8760 hours per year divided by 2,202.31 lb/tonne

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


Articles and resources

References

  1. World Energy Outlook 2016, International Energy Agency, Annex A, November 2016 (fee required)
  2. "2013年1-6月份电力工业运行简况" China Electricity Council, 2014
  3. "All India Plant Load Factor," Central Electricity Authority, accessed July 2014
  4. "All India Plant Load Factor Thermal," Central Electricity Authority, accessed June 2014
  5. "Benchmarking Emissions," Natural Resources Defense Council, 2014, page 2.
  6. 6.0 6.1 "The Future of Coal," Massachusetts Institute of Technogy, 2008, Table 3.1: Representative Performance and Economics for Air-Blown PC Generating Units," p. 19.
  7. 7.0 7.1 "NEW COAL-FIRED POWER PLANT PERFORMANCE AND COST ESTIMATES," Sargent & Lundy, 2009
  8. "High Efficiency Electric Power Generation; The Environmental Role," János Beér, Massachusetts Institute of Technology, undated
  9. 9.0 9.1 "CO2 Baseline Database for the Indian Power Sector: User Guide Version 8.0," Central Electricity Authority, January 2013, Appendix B
  10. B.D. Hong and E.R. Slatick, "Carbon Dioxide Emission Factors for Coal," U.S. Energy Information Administration, 1994

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