2.1 “Full spectrum” CBG utilization
Coal mine gas of different sources has different methane concentrations. Methane accounts for > 90% volume of the gas pre-extracted from ground wells of the original coal mines, 50%–90% volume of the gas extracted from the ground wells in the mining and goaf areas and more than 30% volume of the high-concentration gas captured from underground wells. The above-mentioned CBG with methane concentration > 30% is widely used as a fuel for heating and power generation. Due to technology maturation, their utilization would not result in resources extravagance and environmental pollution. However, for the low-concentration gases including that captured from underground with a methane concentration < 30% and ventilation-exhaust-air, which account for a majority of the coal mine gas, due to imperfective utilization and generalization technologies, most of them are poorly utilized. Especially, those with volumetric methane concentration < 0.75% are directly discharged into the air from coal mines, resulting in massive extravagance of these valuable methane resources. Therefore, in order to achieve the full spectrum CBG utilization, it is necessary to enhance utilization of these wasted low methane concentration gases.
First, CBG extracted from coal mines with 80% methane concentration can be directly used as a fuel for power and heat generation, as well as for chemical industry and the like. This part of gas only accounts for 1% of the total extracted gas amount. Second, CBG with 30%–80% methane concentration can be used for power generation, gas boilers, as well as preparation and purification of liquefied natural gas (LNG) and compressed natural gas (CNG), etc. This part of gas accounts for ~5% of the total amount of gas extracted from coal mines. Third, CBG with 20%–30% methane concentration can be first converted to gas with 90% methane concentration using pressure swing adsorption method and then used to prepare LNG and CNG (Zhen et al. 2010; Yang et al. 2016; Li 2018). In addition, gas with >10% methane concentration can be used for power generation using low-concentration gas power generation technology and gas with <10% methane concentration can be used for power generation using low-concentration gas power generation as well as regenerative oxidation and shaft heating technologies (Zhen et al. 2010; Yang et al. 2016; Li 2018).
The principle of ventilation-exhaust-air thermal-storage oxidation power generation technology is as follows: 1) increase methane concentration to 1.2% by mixing the ventilation air with gas of low methane concentration in the pumping station, 2) instantly and flamelessly oxidize the gas mixture with very low-concentration methane to water and carbon dioxide after injecting it into the reaction chamber of a thermal-storage high-temperature oxidation device. The system could consume more than 95% of total methane and simultaneously release large amount oxidation heat. All the thermal energy except for a small portion used to maintain the reaction temperature will be output to the residual heat boiler for heat exchange with water, which produces over-heated high-pressure water vapor to drive the turbine to generate electricity.
2.2 “Full domain” CBG utilization
The full domain CBG utilization refers to the realization of the entire network CBG supply within the whole coal mine and meet the electricity demand of the whole coal mine area by generating power using the high/low-concentration gas power generation technology and meet the heating demand of the coal mine area by generating heat using CBG as the fuel. In addition, the remaining CBG can also be used for shaft heating, coal slime drying, alumina roasting, and other engineering projects.
2.3 “Full time” CBG utilization
The rational use of CBG resources relies, on the one hand, on expanding its utilization concentration range so as to reduce unnecessary extravagance, while on the other hand, on scientific management means to reduce CBG extravagance in the resource allocation process.
The full time CBG utilization is proposed to realize the reasonable allocation of CBG resources. In detail, that is under the premise of ensuring the normal supply of CBG resources, using data prediction method to real-time analyze the supply and demand relationship of CBG resources in the coal mine area and dynamically predict the variation trend for gas demand so as to take reasonable resource allocation measures. Because the current demand for CBG in the coal mine area is mainly for power and heat generation, which has an obvious time effect. For example, in winter, with the weather gradually turning cold, the demand for electricity and heating grows clearly, thus the demand for CBG also increases greatly. By contrast, in spring and fall, the demand for electricity and heating decreases, so for CBG resources. Thus, one can use the data prediction method to find the gap between the supply and demand of CBG resources, thereby taking rational management measures to balance the demand and supply.
Based on this, a time series algorithm prediction model is used to predict the coalbed methane demand in mining area. The realization of prediction depends on the built-in exponential smoothing model of SPSS (including Winters addition model and Winters multiplication model) and ARIMA model. According to the expert modeler in the software, the appropriate prediction model can be automatically selected, and the periodicity of the time series can be considered. And determine the parameters of the model.
Based on this, a time series algorithm prediction model is used to predict the CBG demand in mining area. The realization of prediction depends on the built-in exponential smoothing model of SPSS (including Winters addition model and Winters multiplication model) and ARIMA model. According to the expert modeler in the software, the appropriate prediction model can be automatically selected, and the periodicity of the time series can be considered and determine the parameters of the model.
The “Full Spectrum-Domain-Time” CBG utilization model is shown in Fig.2.
2.4 CBG utilization technology and its application in Yangquan Coal Group
Overview of Yangquan Coal Group
Shanxi Yangquan Coal Group currently has 40 production and infrastructure coal mines distributing in Taiyuan, Yangquan, Jinzhong, Linfen, Xinzhou and Shuozhou, Shanxi Province, China. Among them are 9 outburst mines, 16 high gas concentration mines, and 15 low gas concentration mines. A total of 54 sets of main ventilators are installed with a total installation power of 102 million kW and the actual ventilation volume of ~640,400 m3/min. A total of 157 extraction and discharge pumps are installed in 34 ground pumping stations with installation power of 130,000 kW and installation capacity of 98,000 m3/min.
Yangquan Coal Group started utilization of CBG in 1958 and established a specialized CBG branch in 1984. Since 1986, it has safely supplied a total of 6.55 billion cubic meters of commercial gas. At present, it has built a complete CBG storage and utilization system and formed a new pattern of large-scale CBG utilization in civilian and industrial power generation, purification and liquefaction, and similar fields.