EDUCATION: GLOBAL METHANE INVENTORY

Methane Emissions from Soft Coal

By L. Dolan

Note: The following is a student paper which has not been published or peer-reviewed. It is made available for archival purposes only. All results are preliminary. Please do not reference the paper for results or statistics.

Introduction

Methane is one of the most important trace gases in the atmosphere because it is a greenhouse gas and is involved in the destruction of the ozone. As a greenhouse gas, methane absorbs the infrared radiation reradiated from the Earth's surface, which causes warming. The small concentrations of methane play an important role in global warming because it absorbs about 25 times more energy than carbon dioxide, which is considered the most important greenhouse gas. The concentration of methane in the last 300 years has doubled which is thought to be related to human actions.

Atmospheric concentrations of methane reflect the emitted methane and the methane sinks. Methane is emitted from natural and anthropogenic sources. The natural sources include wetlands, oceans, and termites. Landfills, fossil fuels, rice cultivation, animal wastes, and biomass burning are anthropogenic sources than emit more that double the methane of the natural sources. Atmospheric sinks for methane occur when methane reacts with the OH (hydroxyl) radical. The reaction of methane with OH causes methane to break down into other molecules. It usually takes about ten years for a molecule of methane to react with OH; therefore it is difficult to observe a substantial decrease or increase without waiting ten years. A decrease or increase in atmospheric concentrations of methane could be caused of either a change in sink and/or source.

In the past, ICP students have calculated emissions for sources including ruminant animals, rice cultivation, and landfills. Collecting the yearly data for methane sources can help us determine if and which source or sources dictate the change in atmospheric concentration.

Our project is focused on calculating methane emissions from fossil fuels, which is the main anthropogenic source of methane. Coal, crude oil, and natural gas produce methane while being extracted, processed, and transported. We are focusing on fossil fuels because historical methane emissions from all fossil fuels have never been calculated. It is important to know the emissions from fossil fuels because they produce about 20% of global methane emissions. The purpose of my project is to calculate the emissions of soft coal so that we can see if the coal emissions have contributed the decreased growth rate of atmospheric methane

Background

Methane and coal bond together during coalification, which is when swamp vegetation is transformed by geological and biological forces into coal (US EPA, 1993). Coal is separated into two main categories: hard and brown coal (lignite). Hard coal mining releases methane from the ventilation shafts, gas drainage systems, and coal crushing. In both types of coal, methane is released from the seams of the coal disturbed during mining. This gas released is kept away from the miners by filling the mines with large quantities of ventilation air, which releases methane into the atmosphere. Gas drainage systems are used mostly in hard coal mines. This type of drainage is accomplished by drilling wells into the coal mines before mining to drain methane. In Europe, methane recovered from drainage systems is used as an energy source for the mine. After coal is mined, the coal goes through several processes which can include crushing, separating impurities, size classification, transportation, and storage. During these types of processes hard coal produces more methane than does lignite. (Kirshgessner, 2000)

Different types of coal release different amounts of methane because of physical factors including pressure (depth), coal rank, and moisture. The higher the pressure, the more methane is kept in the coal seams. A high coal rank, which is determined by high carbon content, indicates higher methane content. Joubert at al. (1974) showed that moisture in coal decreases methane concentration up to 1 to 3 percent moisture at which no significant change in methane content is observed.

Hard coal (mostly from underground mines) produces more methane than the same amount of brown coal (surface mines) because it is deeper and is a higher coal rank. Therefore emissions from underground mines are measured more frequently and are relatively well estimated while surface mine emission rates are not as well known.

Methods

To calculate the methane emissions from coal we used the Intergovernmental Panel on Climate Change (IPCC) methodology and the data obtained from International Energy Agency (IEA). IEA contained data for 140 countries and annual statistics for all countries were obtained for 1980-1998. The data taken from IEA had information contained information on the production, exports and imports.

For hard and brown coal we used the production numbers to calculate the methane emissions from mining. IEA gave the numbers for production in 1000 tons, which was then converted to Mt by multiplying by 10^-3.

Emissions (Gg CH₄ ) = Emission Factor (m³ CH₄ / ton of coal)
       × ton of coal produced
       × Conversion Factor (Gg/10⁶ m³ )

Once the coal production was in Mt we multiplied those numbers by an emission factor and a conversion factor. The conversion factor converted the volume of methane to a weight measurement and is 0.67 Gg CH₄/10⁶m³CH₄, which is the density of methane at 20°C and 1 atmosphere (IPCC, 1996). IPCC gave default emission factors for some of the main coal producing countries. Then we searched many books and country reports to find more emission factors. Table 1a lists the final emission factors used in our study. We were able to calculate and find a few more emission factors. For those countries for which we were not able to find an emission factor, we used default values of 17 m³CH₄/ton for hard coal which is the mean number from the range given by IPCC of 10-25 m³CH₄/ton (IPCC, 1996). The 1996 values in IPCC traced back to publications from as early as 1990.

Table 1a - Methane Emissions Rates from Hard Coal
Country	Methane Emission (m3CH4/ton)	Source of Methane Emission
Former USSR	20.0	1996 IPCC
US	13.15	1996 IPCC
Germany	22.4	1996 IPCC
Poland	9.4	1996 IPCC
United Kingdom	15.3	1996 IPCC
Czechoslovakia	23.9	1996 IPCC
Australia	15.6	1996 IPCC
China	19.04	Calculated from EPA 1993: Options for reducing methane emissions Internationally Volume II

Lignite methane emissions are calculated in the same manor as hard coal emissions except the emission factors are different. Since lignite comes from surface mines, the methane emission was not as well measured as hard coal. IPCC provided a global range of 0.3 Ò 2.0 m³CH₄/tonne for lignite. Therefore we used the mean number of 1.15 m³CH₄/ton for all countries. Since there was little information given it was difficult to find any specific values.

The emission factors differ by region because each region handles coal in ways that produce more or less methane. The management of the methane coal can significantly change the amount of methane produced. Some developed countries have started to use methane to power parts of factories, which reduces the methane emissions. Also coal rank varies among countries and this causes disparity in emission factors. Therefore using a global emission factor is much less precise.

Results

The world lignite production (Figure 1a) increases from 1980 until 1989 about 20% and then steadily decreases until 1998. In 198 the methane emission level was below that of 1980. The peak of production is about 1.25 (Gt), which is about one third the hard coal peak production of 4.13 (Gt)). This is significant since the peak methane emissions from lignite are about 1/45 of the peak hard coal methane emission.

The results of methane emissions from lignite (Figure 1b) show a steady increase, after which the emissions decrease significantly. Starting in 1994, the methane emissions level off at around 700 Gg. Throughout the 20 year period, the main methane emitters were Germany and the former USSR. In 1989, Germany produced 317 Gg of CH4 (33% of the total) and the former USSR produced 126 Gg of CH4 (13% of the total). By 1998 both Germany and the former USSR emissions had decreased by almost 50%.

Regionally Europe dominates lignite methane emissions (Figure 2a) therefore the world methane emissions (Figure 1b) mimics the Europe methane emissions (Figure 2a) accounting for about 90% of the global total. Even though the North American, Asian, and Australian emissions (Figures Figure 2b, Figure 2c, Figure 2d) increased steadily from 1980, world methane emissions from lignite gradually decline starting in 1990 because of the European emissions. Africa, the Pacific, and the Caribbean emit extremely small amounts of methane from lignite.

Methane emissions from lignite in the last 20 years peaked at 0.964 Tg/yr while the peak emissions from hard coal was 46.4 Tg (see Adams, 2001). Compared to the hard coal, lignite emissions are insignificant even though lignite is produced in large amounts. Also very little research has been done about the emission rates from lignite. Therefore the emission rates used in our datasets increase the amount of uncertainty in our results.

The post-mining emissions (storage and processing) for lignite was not shown because it was insignificant. The peak in 1989 was only 84.9Gg (0.0849Tg).

In the regional Methane Emission figures the scale used for each region is different and all the emissions are in Gigagrams.

Discussion

Since soft coal emits methane in such small amounts, it cannot be a major contributor to the decline in the growth rate of atmospheric methane. Lignite methane production emissions started to decrease around 1990, which is the same time the atmospheric growth rate started to decrease. However, as seen in Amelia AdamsÌ paper, the hard coal production emissions do not decline substantially until the very end of the 1990's. The two major features in the atmospheric methane data are the low growth in 1992, which is now attributed to wetlands, and the large growth rate in 1998, which is not seen in either the soft or hard coal data but is also accounted for by wetlands (Dlugokencky et al, 2001).

The fact that the world methane emissions from lignite follow the same trends as the European methane emissions (Figure 2a) confirms that Europe dominates the lignite methane emissions. The upward trend and small numbers of the Asian, North American, and Australian emissions has little effect on the world methane emissions because they produce little lignite. The global trend (Figure 1b) of gradual decrease implies that it will continue to decrease in the future.

EIA estimated methane emissions for lignite for the US and published them on the internet. Their data was significantly lower than ours (~30%, which could be the result of our use of a global emission factor. Their dataset followed the same trends of ours. The validity of their results are questionable since they seemed to use an emission factor lower than the range given by IPCC 1996. Our use of a global emission factor could have caused a problem since it is known that each country has different coal rank. But few people measure the emissions from surface mines (lignite) since the methane released from the mines is not a threat to the miners since it is released directly into the atmosphere unlike underground mining.

Future research could be conducted on the use of lignite compared to hard coal. If lignite were an efficient energy source, a way of lowering methane emissions would be to use more lignite. The heat content of lignite is much lower than for hard coal therefore one would have to burn much more of it to get the same amount of heat. Other impacts that would have to be researched would be the amount of carbon dioxide, sulfur, and particulates (soot) released from burning coal. Using the data from David Sarma and Amelia Adams on hard coal, oil, and natural gas, we could research the efficiencies of the fossil fuels compared to the amount of methane produced. This type of research could indicate the fossil fuel that was the most efficient for the amount of methane produced.

The results of our study on lignite indicate that lignite is not an important methane emitter. Even though we only had a single global methane emission factor, the methane emissions are so small that finding more accurate emission factors are unnecessary.

Bibliography

• Augenbraun, H., D. Sarma, and E. Matthews, The Greenhouse Effect, Greenhouse Gases, and Global Warming, Available: http://icp.giss.nasa.gov/research/methane/greenhouse.html, 1 July 1999.
• Augenbraun, H., D. Sarma, and E. Matthews, The Global Methane Cycle, Available: http://icp.giss.nasa.gov/research/methane/gmc.html, 1 July 1999.
• Dlugokencky, E.J., B.P. Walter, K.A. Masarie, P.M. Lang, and E.S. Kasischke, Geophys. Res. Lett. 28, 499-502, 2001.
• IPCC, OECD, IEA. Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories, J.T.Houghton, L.G. Meira Filho, B. Lim, K. Treanton, I. Mamaty, Y. Bonduki, D.J. Griggs, and B.A.Callender (eds.), Available: http://wwww,ipcc-nggip.iges.or.jp/public/gl/invs1.htm. UK Meteorological Office: Bracknell, 1996
• Kirshgessner from Khalil, M.A.K., Atmospheric Methane: Its Role in the Global Environment, Springer-Verlag, Berlin, 2000
• U. S. Department of Agriculture, E.R.S., Options f.or Reducing Methane Emissions Internationally Volume II: International Opportunities for Reducing Methane Emissions, U.S. Government Printing Office, Washington, D. C., 1993.

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