The rise of coalbed methane bioengineering enables the conversion and utilization of carbon dioxide through microbial action and the carbon cycle. The environment of underground coal reservoirs is the result of a comprehensive effort by microorganisms. Some studies on reservoir microorganisms have progressed in laboratory conditions. However, it does not replicate the interaction between microorganisms and the environment on site. Hydraulic fracturing is an engineering technology to improve the natural permeability of tight reservoirs and is also a prerequisite for increasing biomethane production. In addition to expanding the pore and fracture systems of coal reservoirs, hydraulic fracturing also improves the living conditions of microbial communities in underground space. The characteristics of microbial communities in the reservoir after hydraulic fracturing are unclear. To this end, we applied the 16S rRNA sequencing technique to coalbed methane production water after hydraulic fracturing south of the Qinshui Basin to analyze the microbial response of the hydraulic fracturing process in the coal reservoir. The diversity of microbial communities associated with organic degradation was improved after hydraulic fracturing in the coal reservoir. The proportion of Actinobacteria in the reservoir water of the study area increased significantly, and the abundance of Aminicenantes and Planctomycetes increased, which do not exist in non-fracturing coalbed methane wells or exist at very low abundance. There are different types of methanogens in the study area, especially in fracturing wells. Ecological factors also determine the metabolic pathway of methanogens in coal seams. After hydraulic fracturing, the impact on the reservoir’s microbial communities remains within months. Hydraulic fracturing can strengthen the carbon circulation process, thereby enhancing the block’s methane and carbon dioxide circulation. The study provides a unique theoretical basis for microbially enhanced coalbed methane.

Coal-fired thermal power plants that meet the huge energy demand of China largely contribute to carbon emissions, environmental pollution, and human health issues. To investigate the impact of coal-fired power plants on the environment in the most developed region of eastern China, samples of feed coal, limestone, slag, fly ash, and flue gas desulfurization (FGD) gypsum were collected from two coal-fired power plants in Hefei City, Anhui Province, China. The concentrations of trace elements in these samples were determined using inductively coupled plasma mass spectrometry (ICP-MS). The results show that the feed coal was slightly rich in V, Cr, Se, In, Sb, Pb, and Ga compared to common Chinese coal. Most trace elements were concentrated in fly ash, especially in the fine particles. Only Sc, Cr, Rb, and Ba were enriched in slag. The smaller the particle size of the fly ash, the higher the fraction of volatile trace elements. The elemental concentrations of old fly ash, which was precipitated in the well of the flue gas pipeline, did not show noticeable differences from fresh fly ash. In addition, the short-time storage of fly ash did not influence the trace elements. To understand the possibility of recovering rare earth elements from coal combustion by-products, the outlook coefficients (C_outl) of coal combustion by-products for the Wanneng (WN) and Tianyuan (TY) power plants were calculated, and they were found to be promising for rare earth elements plus yttrium (REY) recovery. However, as per the evaluation diagram of cut-off grade and C_outl values, all feed coal and its by-products from these two coal-fired power plants are projected to be ‘unpromising’. Therefore, the coal combustion by-products from the WN and TY coal-fired power plants were not candidates for REY recovery.

As an essential unconventional natural gas resource, China’s coalbed methane resources are only commercially exploited in a few areas, such as the Qinshui Basin and the Ordos. The rise of coalbed methane bioengineering makes it possible to realize the conversion and utilization of carbon dioxide through microbial action and the carbon cycle. According to the metabolic behavior of the underground microbial community, if the coal reservoir is modified, it may stimulate the microorganism to continuously produce biomethane to prolong the production life of depleted coalbed methane wells. This paper systematically discusses the microbial response to promoting microbial metabolism by nutrients (microbial stimulation), introducing exogenous microorganisms or domestication of in situ microorganisms (microbial enhancement), pretreating coal to change its physical or chemical properties to improve bioavailability, and improving environmental conditions. However, many problems must be solved before commercialization. The whole coal reservoir is regarded as a giant anaerobic fermentation system. Some issues still need to be solved during the implementation of coalbed methane bioengineering. Firstly, the metabolic mechanism of methanogenic microorganisms should be clarified. Secondly, it is urgent to study the optimization of high-efficiency hydrolysis bacteria and nutrient solutions in coal seams. Finally, the research on the underground microbial community ecosystem and biogeochemical cycle mechanism must be improved. The study provides a unique theory for the sustainable development of unconventional natural gas resources. Furthermore, it provides a scientific basis for realizing the carbon dioxide reuse and carbon element cycle in coalbed methane reservoirs.