Advances in the chemical utilization of coalbed methane

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Coalbed methane is a mixture of various gases, with methane being the main component; other components include nitrogen, carbon dioxide, acetylene, and oxygen. Coalbed methane is colorless, odorless, flammable, and explosive, with a density lower than that of air. The composition of coalbed methane obtained from surface wells is similar to that of conventional natural gas, with methane being the main component; however, the composition of coalbed methane varies significantly depending on the type of coal, depth of burial, and extraction methods. This article provides an overview of the classification of coalbed methane, its current resource status, and its utilization in the chemical industry. 1 Classification and Resource Status of Coalbed Methane 1.1 Classification Coalbed methane can be divided into three types based on its storage state, namely free coalbed methane, adsorbed coalbed methane, and water-soluble coalbed methane. Depending on the method of extraction, it can also be divided into 3 types: surface extraction of coalbed methane, extraction of coalbed methane from underground coal mines, and coalbed methane from abandoned mines. The exploitation of coalbed methane at the surface primarily involves the use of surface wells, and it is a extraction technique developed on the basis of conventional natural gas extraction methods; it is mainly applied to undeveloped coalfields. This type of coalbed methane is characterized by a high methane content, with a volume fraction of generally ≥90%. It can be extracted on a large scale with stable yields, and the extracted coalbed methane can be directly fed into natural gas pipelines after slight processing for use in distant areas. Coalbed methane is extracted from underground coal mines primarily to ensure safe coal mining operations; it is obtained from the coal seam and surrounding rock before, during, and after coal extraction. This type of coalbed methane is diluted due to the presence of large amounts of air, resulting in a low methane content; its volume fraction is generally between 20% and 60%, with nitrogen and carbon dioxide being the other main components. Coalbed methane from abandoned mines exists in coal mines that have been shut down for various reasons; it is the coalbed methane released by the remaining coal in those mines. Coalbed methane extracted from underground coal mines and that from abandoned mines is generally referred to together as coalbed methane in coal mining areas. In our country, coalbed methane is primarily extracted from coal mines underground, and this method has the following issues: ① The difficulty of extracting gas from coal mines increases. ②There is a lack of basic theoretical research and technological innovation. The overall productivity level of coal mines in our country is low, and the technical capabilities for extracting and utilizing coal mine gas are insufficient; there are many key challenges both in terms of theory and technology. ③Lack of strong regulatory measures ； At present, the extraction and utilization of coalbed methane lack safety management guidelines, industry standards, and regulatory regulations, which hinders the healthy and orderly development of the coalbed methane industry. In some areas, the extraction of coalbed methane is not coordinated with coal mining; some coalbed methane development companies expand their operations without conducting proper exploration, while other coal mining enterprises focus on extraction rather than utilization. This not only hinders the motivation of all parties involved in the development and utilization of coalbed methane but also affects the sustainable development of the coal industry. ④The utilization of coalbed methane is constrained. In the coalbed methane development areas, there are no corresponding long-distance pipelines, resulting in a disconnect between development and the market. There are still no standards or guidelines for the transportation and utilization of low-concentration gas; large amounts of such gas can only be diluted before being discharged. The low grid price for electricity generated from coal mine gas means that power generation companies cannot make a profit, which in turn limits the utilization of gas from mines. In 2005, the utilization of coal mine gas nationwide accounted for 43% of the amount extracted. 1.2 Current status of coalbed methane resources Currently, the world holds approximately 240 trillion cubic meters of coalbed methane resources. The top 7 countries in terms of these resources are as follows: the Commonwealth of Independent States with 17 trillion to 113 trillion cubic meters, Canada with 5.66 trillion to 76.40 trillion cubic meters, China with 30 trillion to 35 trillion cubic meters, the United States with 11.35 trillion cubic meters, Australia with 8.5 trillion to 14.16 trillion cubic meters, and both Germany and Poland with 2.8 trillion cubic meters each. China is rich in coalbed methane resources, which are widely distributed. Preliminary estimates indicate that the coalbed methane resources in China at depths of 300–1,500 meters amount to 27.39 trillion cubic meters, accounting for approximately 10% of the world’s total coalbed methane resources. In China, the coalbed methane resources at depths of 300–1500 meters are primarily concentrated in the coal-rich regions of North China and Northwest China, accounting for over 90% of the country’s total coalbed methane resources. Shanxi Province has the richest reserves; it is estimated that the coalbed methane resources at depths of over 2000 meters there amount to 10 trillion cubic meters, which constitutes about one-third of the country’s total coalbed methane resources. The South China coal-rich region also has significant amounts of coalbed methane. In addition, there are certain amounts of coalbed methane reserves in the Northeast and Yunnan-Tibet coal-rich regions as well. 2 Comprehensive utilization methods of coalbed methane The utilization methods of coalbed methane are shown in Figure 1. http://pub2.hi2000.com/upload1/0709101000474963.jpg Overall, the utilization of coal mine gas extraction in China has entered a new stage of development; the technology for extracting gas underground has been systematized and is now widely applied in mines with high gas levels. In 2005, 2.3 billion m3 of gas was extracted from coal mines across the country, with over 1 billion m3 of that gas being utilized. Seven mining areas, including Yangquan, Huainan, Shuicheng, Panjiang, Songzao, Jincheng, and Fushun, extract over 100 million cubic meters of gas per year. The development of coalbed methane on land has entered the commercial development phase following the completion of demonstration projects. In 2005, 328 coalbed methane wells were drilled nationwide, exceeding the total number of wells drilled over the previous history (287 coalbed methane wells drilled as of 2004). At the end of December 2005, the technology for transporting low-concentration gas from Huainan coal mines and generating electricity safely passed expert evaluation. 3 Chemical applications of coalbed methane: The utilization methods for coalbed methane are similar to those for natural gas, with it being mostly used for domestic cooking gas and gas-powered power generation. The chemical applications of coalbed methane also resemble those of natural gas, as it is used as a raw material in the production of various chemical products derived from natural gas, such as synthetic ammonia and its related products, synthetic oils and organic chemicals, synthetic methanol and its downstream products (ethylene, propylene, dimethyl ether MTBE, dimethyl carbonate, acetic acid, etc.). 3.1 Hydrogen production from coalbed methane: At present, a large amount of coalbed methane in China is extracted underground, primarily to ensure the safe operation of coal mines. Due to the backwardness of in-situ extraction technologies, the methane volume fraction in the coalbed methane obtained is not high, ranging only from 30% to 60%, and it fluctuates depending on the mining conditions. Coalbed methane with a medium methane volume fraction (30%–60%) can generally be used as a fuel for domestic use, for utility purposes, and for power generation. Due to limited domestic demand and low power generation efficiency, most of the extracted coalbed methane is released into the atmosphere, resulting in both waste of resources and environmental pollution. To expand other uses of such coalbed methane, the methane concentration is often increased using pressure swing adsorption or cryogenic methods. Since coalbed methane contains oxygen, it must be removed before methane concentration to eliminate the risk of explosion; simultaneously, desulfurization and decarburization are also necessary. Regardless of the separation method used, the investment is high and the operating costs are substantial. Since methane in coalbed methane is an excellent raw material for hydrogen production, coalbed methane can be converted into hydrogen products. This is achieved by utilizing the heat of combustion between the oxygen in coalbed methane and methane to carry out a conversion reaction between methane and water vapor, resulting in a mixture containing hydrogen, CO, CO2, and nitrogen (present in coalbed methane). All impurities are then removed in one step using pressure swing adsorption to obtain pure hydrogen. In coalbed methane, the methane concentration is inversely related to the oxygen concentration; that is, the lower the methane concentration, the higher the oxygen concentration, and vice versa. For coalbed methane with low methane concentrations, the relatively high oxygen concentration enables the heat of combustion to achieve a methane conversion rate of over 90%. Conversely, when the methane concentration is high and the oxygen concentration is low, less heat of combustion is available, and the conversion rate of methane is only around 30%; in such cases, external heat is required to facilitate the methane steam reforming reaction and achieve a methane conversion rate of over 95%. For coalbed methane with a low methane concentration, when the volume fraction of methane in it is around 30%, the possible process combinations are: autothermal reforming + pressure swing adsorption to produce pure hydrogen (Process 1), or autothermal reforming + CO conversion + pressure swing adsorption to produce pure hydrogen (Process 2). For coalbed methane with a methane volume fraction of around 30%, conversion through an autothermal reaction can reduce the methane volume fraction to less than 2%, while increasing the hydrogen volume fraction to over 45%; further conversion can increase the hydrogen volume fraction by 3–5 percentage points. For coalbed methane with a high methane concentration, when the volume fraction of methane in it is above 40%, the possible process combinations are: autothermal pre-conversion + methane steam reforming + pressure swing adsorption to produce pure hydrogen (Process 3), or autothermal pre-conversion + methane steam reforming + CO conversion + pressure swing adsorption to produce pure hydrogen (Process 4). All the processes mentioned above are mature technologies, and Table 1 shows a comparison of the hydrogen production rates for these four combined processes. http://pub2.hi2000.com/upload1/0709101001267063.jpg 3.2 Production of methanol from coalbed methane: As a new area of research, the use of coalbed methane as a raw material for methanol production is attracting the interest of scientists around the world. Methanol synthesis generally consists of four main processes: gas generation, purification, synthesis, and distillation. To overcome the limitations of traditional processes, such as low one-way conversion rate of CO, high recycle ratio, and high energy consumption, some large companies at home and abroad have been working on the development of new processes in recent years. The main methanol production technologies in the world are the medium-pressure method and the low-pressure method, with the low-pressure method being the most common. The low-pressure gas-phase methanol synthesis processes mainly include those developed by companies such as ICI in the UK, Lurgi in Germany, Topsφe in Denmark, and Mitsubishi in Japan, with the first two processes being the most common. The specific process flow of the Lurgi low-pressure gas phase method is as follows: First, coalbed methane is compressed and subjected to pressure swing adsorption treatment; after further compression, methane conversion takes place. Subsequently, it is mixed with water gas, and then undergoes decarburization, desulfurization, and compression, before methanol is synthesized. Currently, there are mainly two types of raw materials for methanol production: one is methanol produced from gasification syngas, which has lower raw material costs but requires high investment, and scale is necessary to achieve good economic benefits ； Second is the production of methanol using natural gas as a raw material; the total investment is low, but its production costs are greatly affected by natural gas prices. The gasification process is required to produce syngas through pulverized coal gasification in order to manufacture methanol. However, when using coalbed methane as a raw material for methanol production, a large-scale gasification process is not necessary; a small gasifier is sufficient, eliminating the need for the substantial investment associated with coal gasification processes. Moreover, the price of coalbed methane is lower than that of natural gas, and coupled with the various preferential policies in place for its development, the production cost of methanol using coalbed methane as a raw material is lower than that using natural gas. Methanol synthesis requires a H2/CO ratio (on a volume basis) of 2.0–2.1 in the feed syngas, whereas the H2/CO ratio in the gas obtained from coalbed methane conversion is 2.66; thus, the gas is rich in H2. To bring the H2/CO ratio in the blended feed gas to the level required for methanol synthesis, gas blending is necessary. Producing methanol from coalbed methane offers the advantages of lower investment and higher profits compared to using natural gas as a raw material, while also possessing the benefits of low cost and large-scale production associated with using coal as a raw material. Table 2 shows a cost comparison for producing methanol from four raw materials: natural gas, naphtha, residue oil, and coal. Table 3 shows the technical and economic indicators of the coalbed methane to methanol process. However, the aforementioned process suffers from a low hydrogen-to-carbon ratio, which requires the addition of CO2 for adjustment. http://pub2.hi2000.com/upload1/0709101002148963.jpg 3.3 Power generation using fuel cells powered by coalbed methane. As is well known, in traditional coal-fired power generation, only a portion of the energy generated by burning fuel (less than 40%) is converted into electrical energy; the remaining energy is lost inevitably. Fuel cells, lacking mechanical and thermal intermediaries, feature high efficiency, low pollution, and low operational noise ； Depending on the application, its utilization rate can exceed 90%, with NOx emissions of less than 4 mg/m3. The structure of a fuel cell basically consists of 2 electrodes and an electrolyte; the fuel and oxidant undergo electrochemical reactions at these two electrodes, while the electrolyte forms the internal circuit of the cell. Fuel cells can be broadly divided into the following 5 categories: alkaline fuel cells (AFC), phosphate fuel cells (PAFC), molten carbonate fuel cells (MCFC), solid oxide fuel cells (SOFC), and polymer electrolyte fuel cells (PEFC). Among these, PAFC and SOFC are particularly suitable for using coalbed methane as fuel. The key issue in using coalbed methane to develop fuel cells is the pretreatment of this gas, aimed at removing the small amounts of pollutants present in it, primarily sulfur and other halogen elements, so that their volume fraction is less than 0.0003% before it enters the fuel cells. A simplified fuel cell pretreatment process is shown in Figure 2; it includes several steps such as H2S removal, cooling, condensation, drying, re-cooling, hydrocarbon separation, and filtration. The process involves first removing and separating H2S and water vapor, so that activated carbon can eliminate other pollutants with higher concentrations at lower temperatures and humidity levels. Pre-treatment includes 3 sub-processes, namely the gas purification process, the regeneration process, and the cooling process. http://pub2.hi2000.com/upload1/0709101002559957.jpg Essentially, a fuel cell is a solid-state generator that converts the chemical energy of processed fuel (coalbed methane) into electrical energy and heat energy. It mainly consists of three units: a fuel processing unit, a power generation unit, and a current conversion unit. The fuel processing unit is primarily used to convert coalbed methane containing high concentrations of methane into a hydrogen-rich fuel, which then undergoes an electrochemical reaction with oxygen from the air in the power generation unit, under the action of a catalyst, to produce direct current electricity ； The unreacted fuel, along with the water vapor from the regeneration process, is recycled back to the fuel processing unit. The current conversion unit converts the direct current generated by the fuel cell into alternating current, which is then supplied to the user along with the heat generated in the power generation unit. However, the main problems facing fuel cells are their short service life and high cost. It is currently believed that fuel cells can be brought to industrial application as long as their lifespan reaches 40,000 hours and their cost is between $1,500 and $2,000 per kW (for capacities of 500 k–2 MW), which is about 5 times higher than the cost of conventional power generation. Before achieving large-scale commercialization, some technical obstacles must still be overcome. In this process, only by continuously deepening fundamental research can the obstacles encountered in the industrialization of fuel cells be overcome. 4 Conclusions and Recommendations (1) Coalbed methane is a technology-intensive energy industry whose commercial application has only just begun. There are significant fundamental issues unresolved regarding the estimation of its resources. Moreover, since production levels are low during the first 1–4 years of extraction, the payback period for investments is long. In addition, China provides insufficient policy support for the development and utilization of coalbed methane, its infrastructure for transporting gas is weak, and its market mechanisms are imperfect; as a result, the industrialization of coalbed methane in China lags far behind that in other countries ； (2) Using coalbed methane as fuel offers stable economic benefits, but the issue of transportation pipelines needs to be addressed ； (3) Using coalbed methane for power generation offers the advantages of on-site conversion and convenient transportation ； (4) Using coalbed methane to produce carbon black with methanol as a by-product offers significant economic benefits ； (5) The utilization of coalbed methane is still in its infancy, and many aspects related to project integration and economic-technical evaluation need to be improved.