United Nations Conference on the Environment. Such reductions are currently voluntary, although the Clinton administration is aggressively and successfully pursuing utility participation. The EPACT also involves utilities in programs to establish baseline CO2 emissions.

The most cost-effective method of reducing CO2 emissions from power generation and other coal-based systems is to improve the systems' overall efficiency. DOE's strategic objectives for its Advanced Power Systems Program are consistent with this approach (see Chapter 2). Technology exists to remove CO2 from combustion gases and other coal-based gas streams, but the costs of doing so are high (MIT, 1993), and no proven methods yet exist for disposing of the collected CO2. Beyond the 2040 planning horizon considered in the present study, very high-temperature nuclear reactors might be used as an energy source in fossil fuel conversion processes, such as steam gasification of coal, to reduce their greenhouse gas emissions (NRC, 1990).

Methane from coal mining is also of concern as a greenhouse gas. It has been estimated that in the United States approximately 3.6 million metric tons of coalbed methane is released each year in this process. A large percentage of this total is from underground mining. 12 About 30 percent of concentrated methane from wells in the coal seam is now collected and used. The ventilation air exhaust, which typically contains less than I percent methane, is not generally collected and makes up over 70 percent of the total methane released to the atmosphere from coal mining (CIAB, 1992). Estimates indicate that the greenhouse effect of the methane released from underground coal mining represents up to 8 or 9 percent of the greenhouse effect of the CO2 released in burning the mined coal.13 For a 40 percent thermal efficiency power plant, the additional greenhouse effect of methane released from coal mining is equivalent to decreasing the plant's efficiency by up to about 2 percent. Control of coal mine methane emissions, therefore, has less potential for reducing greenhouse gases than achieving higher plant efficiency through the use of advanced technology. However, methane emissions from coal mining are independent of coal use in combustion equipment; current understanding of global warming issues suggests that they are

12  

Methane from U.S. underground mining comes from mine ventilation air (2.29 million metric tons/year), coal seam degasification (1.00 million metric tons/year), and postmining emissions (0.24 million metric tons/year).

13  

In 1992, 384 million metric tons of coal were produced by underground mining in the United States, with a net release of 3.22 million metric tons of methane. Combustion of the same coal liberated approximately 800 million metric tons of CO2. On a weight basis, the direct and indirect effects of methane have been estimated to be 21 times more powerful than CO2 as a greenhouse gas (NRC, 1992b). The greenhouse effect of the methane released from mining compared to the effect of CO2 from combustion is therefore 21 x 3.22 x 100/800 = 8.5%.

More recent studies by the Intergovernmental Panel on Climate Change no longer quantify the indirect effects of methane; rather, only the direct effects are included in the Global Warming Potential. This gives an index of 11 rather than 21 for a 100-year averaging time.



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