Key technologies of microbial mining residual coal and CO₂-fly ash co-filling in the impacted geological body of coal mining

The gob of coal mine is an important area for achieving the goal of “dual carbon” in China. The geological body formed by coal mining, which can enrich coalbed methane and provide substrate and space for later microbial activities and mineralization filling is defined as the mining influence body. The proposed technologies include the residual coal extraction through mining influence body microorganisms and the co-mineralization and filling of CO₂-fly ash. The broad prospects of the technology in the secondary development of mining, from the perspectives of necessity and feasibility, are elaborated upon regarding the safe storage of CO₂ and efficient disposal of fly ash solid waste in coal-fired power plants. The overall concept is to utilize mining influence body as a anaerobic fermentation “factory” and microorganisms as “workers” to process the existing raw materials of the “factories”including residual coal, thin coal seams, dispersed organic matter, and injected CO₂. The ultimate goal is to produce methane, thereby achieving the resource utilization of microbial mining for residual coal and CO₂. The combination of CO₂ and alkaline fly ash simultaneously achieves the mineralization storage of CO₂ and the filling of mining influence body. The key scientific issues are involved in this technology encompass the classification of mining influence body and the characteristics of organic matter, elucidating the mechanism of anaerobic fermentation under in-situ conditions specific to mining influence body, investigating the cooperative mineralization mechanism of microbial-CO₂-fly ash, as well as undertaking a demonstration project for constructing the key technology of microbial residual coal mining and filling. The laboratory physical simulation of the in-situ conditions of the mining influence body demonstrates that the residual coal and organic-rich mud shale have the capability to generate biomethane, with methane production further enhanced by a small quantity of fly ash. The dynamic experiment of simulated groundwater recharge demonstrates that the nutrient recharge significantly impacts the anaerobic fermentation system. Specifically, the system with a cycle period of 14 days was consistent with the cycle of methanogens, which can ensure the continuous and efficient operation of the anaerobic fermentation system. After a curing period of 28 days, the test specimen containing high calcium fly ash, CO₂, and mine water exhibited a compressive strength of 12.31 MPa. Additionally, each ton of fly ash had the potential to store approximately 21.99 m³ of CO₂ through mineralization, highlighting the dual benefits of CO₂ emission reduction and goaf solidification achieved by utilizing fly ash. The engineering test target area was optimized based on the purpose of microbial coal residue mining and fly ash filling. Also, the groundwater retention area was identified as the optimal location for CO₂ mineralization and fly ash filling. The natural trap formed by mining activities and the trap formed by artificial filling were one of the more favorable engineering test targets. The proposed technologies of microbial residual coal mining, CO₂ and fly ash co-filling are aimed at providing a novel technical approach for carbon emission reduction and goaf ecological environment management in China.