1、英文原文 Mitigation of Methane Emissions from Coal Mine Ventilation Air Peter Carothers and Milind D.Deo ABSTRACT U S EPA s coalbed methane outreach program ,(CMOP)has prepared a technical assessment of techniques that combust trace amounts of coal mine methane contained in ventilation air. Control of m
2、ethane emissions from mine ventilation systems has been an elusive goal because of the magnitude of a typical airflow and the very low methane concentrations. One established and cost-effective use feeds the air into a prime mover in lieu of ambient combustion air. This method usually consumes just
3、a fraction of the flow available from each ventilation shaft. The authors evaluated the technical and economic feasibility of two emerging systems that may accept up to 100% of the flow from a nearby shaft, oxidize the contained methane, and produce marketable energy. Both systems use regenerative,
4、flow-reversal reactors. One system operates at 1000，and the other uses a catalyst to reduce the combustion temperature by several hundred degrees. Above certain minimum methane concentrations the reactors can exchange high quality heat with a working fluid such as compressed air or pressurized water
5、. This paper discusses two illustrative energy projects where the reactors produce energy revenue and greenhouse gas credits and yield an attractive return on invested capital. KEYWORDS: Coal, Methane, Mining, Ventilation, Combustion, Regenerative, and Greenhouse Gas. INTRODUCTION This paper present
6、s a summary of a draft U.S. Environmental Protection Agency (U S EPA) report. It is a technical assessment of existing and emerging processes capable of removing trace amounts of methane contained in ventilation air streams at gassy underground coal mines. Coaled methane (CBM) is methane that is for
7、med during the coalification process and that resides within the coal seam and adjacent rock strata. Coal mining activity releases methane that has not been captured with drainage systems. The methane then passes into mine workings and on to the atmosphere. Gassy underground mines release significan
8、t quantities of such methane, which is referred to as coal mine methane (CMM). When allowed to accumulate in mine working, CMM presents a substantial danger of fire and explosion. To assure miner safety and maintain continuous production, operators of gassy mines must degasify their mines. The most
9、universally used method of degasification is dilution by ventilation. Ventilation systems consist of inlet and exhaust shafts and powerful fans that move large volumes of air through the mine workings to maintain a safe working environment. Exhausted ventilation air contains a very diluted amount of
10、 methane； typical concentrations range between 0.2 to 0.8% methane, well below the explosion limits. To date (with very few exceptions) ventilation systems release the air-methane mixture to the atmosphere, thus emitting or liberating the methane without attempting to capture and use it. Operators m
11、ay supplement ventilation with another form of degasification, methane drainage technology, which forcibly extracts methane from coal strata in advance of, or after, mining. Some operators to employ a variety of proven methods, capture and use drained CMM but the majority of drained ventilation air.
12、 Methane emissions from ventilation air comprise the largest portion of all CMM liberation worldwide, and they are the most difficult to control. This paper examines the current and future possibilities for destroying and potentially using ventilation air methane. Global Importance of Ventilation Ai
13、r Emissions Methane is a potent greenhouse gas, approximately 21 times more effective per unit of weight than carbon dioxide in terms of causing global warming over a 100-year time frame. Coal mine methane emissions account for approximately 10% of anthropogenic methane emissions worldwide, and they
14、 are the fourth largest source of methane release in the US. By far the largest portion of this methane leaves the mines through the ventilation system. Therefore, the most logical and direct way to reduce CMM emissions would be to find methods to capture, process, and use methane that exits the ven
15、tilation shaft. This paper assesses technologies that can be expected to handle the entire ventilation stream from a single shaft. A typical shaft at a gassy mine in the U.S. will move between 100 to 250 cubic-meters of air per second (m3/s) or approximately 212,000 to 530,000 cubic feet per minute
16、(cfm). Illustrations in this paper assume a unit capacity of 100 m3/s ,a practical modular size that mines could use singly or in multiples. A 100 m3/s ventilation flow containing 0.5%methane will emit 43,200 m of methane per day or about 1.525 mmcfd. Barriers to Current Recovery and Use Ventilation
17、 airflows are very large, and the contained methane is so diluted that conventional combustion processes cannot oxidize it without supplemental fuel. Ventilation air s characteristics make it extremely difficult to handle and process and constitute technical barriers to its recovery and use. Costly
18、Air Handling Systems Typical ventilation airflows are so enormous that a processing system will have to be very large and expensive. Each processing system will have to include a fan to neutralize any pressure drop caused by the reactor and avoid having the mines face costly increases in electric po
19、wer. Low methane concentrations. A methane-in-air mixture is explosive in a concentration range between about 4.5 and 15% .below 4.5% methane will not ignite or sustain combustion unless it can remain in an environment where temperatures exceed 1,000. therefore, any conventional method proposed to u
20、se ventilation air as a fuel, or even to destroy it, would require an endothermic reaction. Variable flows and changing locations Mine operators will face the flow variations typically exhibited by a ventilation system. As mine operations progress underground the working face tends to move away from
21、 the original ventilation shaft. A processing system built to accept a given flow will experience short-term periodic fluctuations and a probable decline over time as other, more distant exhaust shafts take over. IDENTIFICATION OF APPLICABLE TECHNOLOGIES The technologies available to mitigate ventil
22、ation air emissions divide into two basic categories: ancillary uses and principal uses. Ancillary uses The focus of projects in this category is on a primary fuel that is not ventilation air； thus employment of ventilation air is ancillary and restricted to amounts that are convenient for the proje
23、ct. For example, a power plant of other prime mover may use ventilation air(instead of ambient air) as combustion air. Projects of this type normally use only a fraction of the ventilation air. The technique requires a modest air handling and transport system that serves to bring ventilation air from the shaft exit to the prime mover s air intake. The Appin and tower projects owned by BHP steel collieries division in Australia provide an outstanding example of ancillary use. Two facilities totaling 40 and 54MW each produce electric power with a series of one-megawatt caterpillar internal
