Coal mine methane (CMM) released during coal mining attributes to unsafe working conditions and environmental impact. China, the largest coal producer in the world, is facing problems associated with CMM such as fatal gas accidents and intense greenhouse gas emission along the path to deep mining. Complicated geological conditions featured with low permeability, high gas pressure and gas content of Chinese coal seams have been hindering the coal extraction. To solve these problems, a model of coal–methane co-exploitation is proposed. This model realizes the extraction of two resources with safety ensured and has been successfully applied in Huainan coalfield, China. The current situation of drainage and utilization of CMM in China are diagnosed. Connections between the coal production, methane emissions, drainage and utilization are analyzed. Estimations of future coal production, methane emissions, drainage and utilization are made in a co-exploitation based scenario. The emitted, drained and utilized CMM are projected to reach 26.6, 13.3 and 9.3 billion m3, respectively by adapting the assumption of 3800 million metric tons of coal production by 2020.
Powered by robust economic development, China has become the world's second largest energy producer and consumer (NBSC, 2009). Coal, as a primary energy source, accounts for 70% of the nation’s total energy supply (BP, 2010). The Chinese coal production has increased significantly from 1299 million metric tons (Mt) in 2000 to 3050 Mt in 2009, an annual growth rate of 10% (BP, 2010). It is foreseen that coal will still play a leading role in the energy structure of China for a long time.
While the contributions from coal mining industry to Chinese energy supply are irreplaceable, the conditions for coal exploitation are deteriorating. With the continued dependence on coal production, coal extraction is expected to become increasingly challenging as shallow reserves are exhausted and deeper and more gassy seams are mined (U.N. ECE and M2M, 2010). The deeper mining levels are increasing the emissions of coal mine methane (CMM), which could lead to serious mining accidents like outbursts.
The CMM involved accidents are the dominant cause that is threatening mining safety. They account for 45.8% of severe coal mine accidents where there are more than ten fatalities, according to the 2010 China's coal mine accidents report (SACMS, 2011). Typical CMM accidents in China include gas explosion, gas outburst, gas ignition and suffocation due to a high gas concentration.
Although it poses a major threat to coal mine safety, methane is also a clean and high-efficiency fuel (Flores, 1998). The energy released in the combustion of 1 m3 of methane is 35.9 million Joules, equivalent to the combustion of 1.2 kg of standard coal. In the mean time, methane is also an intense greenhouse gas (GHG) with a Global arming Potential (GWP) of 25, i.e. 25 times of the environmental impact over carbon dioxide, in a 100-year span (IPCC, 2007).
Therefore, measures to control CMM in China bear the multiple purposes of promoting mining safety, recovering the methane resource and abating the emission of GHG. Chinese government now plans to use the recovery and utilization of CMM as the core stimulus of CMM control. By spurring methane drainage through increasing the utilization of CMM, the administration expects to promote mining safety and reduce greenhouse methane emissions (Cheng et al., 2011).
This paper gives an analysis of the drainage and utilization of CMM in China by analyzing the methane-involved problems with coal mining and raises a solution of coal–methane co-exploitation model. Projections of future drainage and utilization based on this model are made while China climbs to its coal peak.
As a by-product of the robust Chinese coal mining industry, the CMM plays multiple roles as a threat to mining safety, an intense GHG and a high-efficiency clean fuel. Traditional methodology on dealing with the CMM is no longer applicable when most of the Chinese coal mines are digging deep and encountering the problems of high gas pressure, high methane emissions and low permeability. A model of coal–methane co-exploitation offers a solution by placing equal emphasis on methane drainage and utilization as with coal extraction. Through different techniques aiming at different coal seam conditions, this co-exploitation model allows the recovery of both resources without compromising safety.
Coal production is expected to increase rapidly before it reaches the peak in 10–20 years. CMM emissions will keep increasing with the coal production. The coal–methane co-exploitation model will substantially promote methane drainage and utilization. Besides the rising coal production, the technology advancement and economic stimulus will also contribute to the increasing amount of methane drainage and utilization. Based on assumption of 3800 Mt of coal production by 2020, the emitted, drained and utilized CMM are projected to reach 26.6, 13.3 and 9.3 Bm3 respectively. The corresponding drainage rate and utilization rate will reach 50% and 70%, respectively.
