Estimating carbon dioxide emissions from coal mines
|This article is part of the Global Coal Mine Tracker, a project of Global Energy Monitor.|
Four factors are used to estimate the CO2 emissions from coal mines:
- Coal type conversion factor
- Effective CO2 emissions factor
- Exclusion factor
The CO2 emissions for a coal mine can be calculated with the following formula:
Annual CO2 (in million tonnes) = Production * Coal type conversion factor * Effective CO2 emissions factor/Conversion factor - Exclusion factor
Example for a typical, non-coking bituminous coal mine:
- Production: 5 Million tonnes per annum
- Coal type conversion factor: 25.8 TJ/kt
- Effective CO2 emissions factor: 94600 CO2/TJ
- Conversion factor: 10^6 (tonne of CO2/tonne of coal)
- Exclusion factor: 0.017
Based on these parameters, the annual CO2 for the mine is as follows:
500,000 * (25.8 * 94600) / 10^6 - 0.017 = 12,118,400 tCO2
Coal type conversion factor
Coal type conversion factor is a measure of the energy contents per physical unit of coal subtype, or net calorific value.
In 2006, IPCC Guidelines for National Greenhouse Gas Inventories estimated the TJ/kt content for anthracite, coking coal, other bituminous coal, sub- bituminous coal, and lignite.
|Coal type||Coal type conversion factor (TJ/kt)|
|Bituminous (non coking)||25.8|
These IPCC estimates rely on data from three sources: UNFCCC Annex-1 countries’ national submissions in 2004 on 2002 emissions, International Energy Agency NCV database for all fuels (2004), and the IPCC Emission Factor Database (EFDB), version-1 (2003).
Effective CO2 emissions factor
The variables of Effective CO2 emissions factor include:
- Carbon content (CC): carbon mass per unit energy
- Carbon oxidation factor (COF): the percentage of carbon oxidized when combustion occurs
- Molecular weight ratio of CO2 to C: 44/12
The formula for calculating the Effective CO2 Emissions Factor is CC* COF* 44/12
In 2006, the IPCC Guidelines for National Greenhouse Gas Inventories estimated the CO2 emissions factor for each type of coal in Kg CO2 /TJ.
|Coal type||Effective CO2 emissions factor(kg CO2/TJ)|
When calculating the effective CO2 emissions, carbon content is an influential factor because other factors tend to remain constant. The effective CO2 emissions factor is largely a reflection of the composition of coal, since carbon content varies considerably based on carbon, hydrogen, sulphur, ash, oxygen, and nitrogen.
The default carbon content of different types of coal (kg/GJ):
|Coal type||Coal carbon content (kg/GJ)|
|Bituminous (non coking)||25.8|
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.)
Carbon oxidation factor
The IPCC uses a default carbon oxidation factor of 1, assuming a complete oxidation of the carbon contained in the fuel.
Molecular weight ratio of CO2 to C
Taking into account the weight ratio of CO2 to Carbon is 44/12. According to the US Energy Information Administration, "the addition of two oxygen atoms to each carbon atom forms CO2, which has an atomic weight of 44—roughly 3.6667 times the atomic weight of the carbon, which is 12. For example, subbituminous coal is, on average, 51% carbon, so the carbon in a short ton (2,000 pounds) of subbituminous coal weighs 1,020 pounds. The CO2 emissions from burning a short ton of subbituminous coal weighs approximately 3,740 pounds, or about 3.67 times the weight of the carbon in a short ton of subbituminous coal and 1.87 times the weight of a short ton of subbituminous coal.”
Emission factors for CO2 from the IPCC are in units of kg CO2/TJ on a net calorific value basis (depending on the production units). A conversion factor of 10^6 provides tonnes of CO2 /tonne of coal.
The exclusion factor estimates the fraction of extracted coal that is not ultimately combusted (i.e. lost along the supply chain or used for non-combustion purposes) in a given year.
The Production Gap Report 2019 estimated default exclusion factors for anthracite, bituminous coal, sub-bituminous coal, and lignite to be 0.017 (or 1.7%) of coal that is lost as dust, etc., based on 2016 data of globally averaged annual values of excluded carbon fractions that account for losses.
For coking coal, the Production Gap report estimated 0.645 (64.5%) of the carbon dioxide should be "excluded", meaning these emissions will be counted in the process fuel or industrial category (e.g. metallurgic coke used in iron and steel production) rather than as power sector emissions.
The Global Coal Mine Tracker uses the 0.017 exclusion factor (1.7%) for all coal types, since emissions from coking coal should still be counted in general inventories. When GCMT data is used for a general compilation, it uses 0.017, but when the compilation is relevant to power sector only, it includes 0.645 for coking coal.
Global Coal Mine Tracker
The Global Coal Mine Tracker relies on the following guidelines for CO2 emissions at the mine level.
|Coal type||Coal type conversion factor (TJ/kt)||CO2 Emission factor (kg CO2/TJ)||Conversion Factor (tonne CO2/Tonne of Coal)||Exclusion factor||Adjusted CO2/Tonne of Coal|
|Bituminous (non coking)||25.8||94,600||2.441||0.017||2.399|
Methods for extraction based emissions
While most CO2 emissions frameworks, such as the United Nations Framework Convention on Climate Change (UNFCCC), attribute emissions from the burning of fossil fuels to the country or entity where the fuels are burned, the Production Gap Report (2019) has pushed for a complementary “extraction-based” approach to enable countries to track the “downstream” emissions that will result from the combustion extracted fuels.
The Global Coal Mine Tracker relies on a “bottom up” extraction-based approach to calculate carbon dioxide emissions. This is modified approach (using mine-level rather than national level production) put forth in the Production Gap Report.
Bottom up approach
The Production Gap Report 2019 put together a "bottom up approach" for calculating “extraction-based” emissions:
*Bottom-up: This method involves combining national fossil fuel production statistics with default or country-specific emission factors derived from the energy and carbon contents per physical unit of fuel subtype, and accounting for the fraction of produced fuel that goes toward non-combustion uses. This enables a tiered approach with increasing levels of methodological complexity and data requirements, in a way that is consistent with the IPCC Guidelines for National Greenhouse Gas Inventories (IPCC 2006).
Top down approach
The Production Gap Report 2019 also put together a "top down approach" for calculating “extraction-based” emissions:
*Top-down: National fossil fuel production statistics are combined with globally averaged emission rates per unit of coal, oil, and gas produced. This method has the advantage of aligning with territorial emissions estimates at a global level. However, it makes a simplifying assumption that the emissions factors for each primary fuel type are the same in all countries, which can lead to over- or under- estimates since the energy and carbon contents per physical unit of fuel can vary considerably between countries and between fuel subtypes (e.g. coking coal versus lignite).
The Production Gap Report further noted that “bottom-up” emissions estimates (from extraction-based estimates) exceeded "top-down" estimates by 10-24%, indicating that "excluded carbon fractions derived from the IEA’s World Energy Balances may not sufficiently account for the amount of coal used for non-combustion purposes and/or lost along the supply chain, especially for coking coal."
Other emission factors estimates
The following carbon dioxide emission factors were estimated by the U.S. Department of Energy for coals in the United States, relying on the high heat value approach to estimate pounds CO2 per million Btu.
|Coal type||CO2 emission factors by coal (lbs/MMBtu)|
|Bituminous (non coking)||205.3|
Articles and resources
- Amit Garg, Kainou Kazunari, and Tinus Pulles, Chapter 1: Introduction, IPCC Guidelines for National Greenhouse Gas Inventories (2006)
- "2006 IPCC Guidelines for National Greenhouse Gas Inventories," Table 2.2 "Default Emission Factors for Stationary Combustion in the Energy Industries," page 2.16
- US Energy Information Administration, Why do carbon dioxide emissions weigh more than the original fuel?, FAQs, accessed November 2020
- SEI, IISD, ODI, Climate Analytics, CICERO, and UNEP. The Production Gap: The discrepancy between countries’ panned fossil fuel production and global production levels consistent with limiting warming to 1.5°C or 2°C, 2019
- B.D. Hong and E.R. Slatick, "Carbon Dioxide Emission Factors for Coal," U.S. Energy Information Administration, 1994