Theoretical basis and technical conception of backfill carbon fixation in coal mine

Under the national “dual carbon” goal, how to reduce the carbon emissions of the coal industry and achieve carbon storage has become an urgent problem to be solved. The coal industry is the producer of high carbon fossil energy and the main carbon emission source provider. In the process of production and consumption, the accumulation of bulk solid waste, the formation of large goaf and a large amount of CO₂ emissions are the bottlenecks that restrict the sustainable development and utilization and the green and healthy development of the coal industry. In order to solve the problem of carbon dioxide storage and mine waste consumption, the bulk solid waste disposal, high-value solid waste utilization, CO₂ storage and goaf utilization were organically combined, the concept of carbon dioxide backfill was put forward, and three types of dioxide backfill were defined from the perspective of carbon sink capacity assessment. ① The basic theories involved in the transportation process of CO₂ filling slurry and mineralization reaction process are analyzed. The mathematical equations for each process and calculation formulas for carbon sequestration amount are provided. The influence of factors such as temperature and humidity on the mineralization reaction mechanism, carbon sequestration amount, and strength of the backfill body are pointed out. ② The carbonation technological approaches, CO₂ sequestration capacity of major alkaline industrial solid wastes and enhancing measures of CO₂ mineralization are summarized. On this premise, two types of CO₂ backfill materials preparation technique based on direct carbonationand or indirect carbonationand are presented, which can fulfill the criterion of mine backfill in fluidity, solidification characteristics and strength. ③ To solve the problem of decomposed CO₂ escaping during CO₂ backfill, two technical paths of strip roadway paste backfilling and intermittent backfilling behind packed hydraulic support have been proposed. The former sequesters decomposed CO₂ by constructing backfilling bodies with multiple through holes in the weak backfilling strip, while the latter uses packed hydraulic support and chain self-filling baffling to construct backfilling strips in longwall goaf to control roof caving and form a CO₂ physicochemical storage space. ④ In order to evaluate the carbon balance effect of CO₂ backfill, the calculation boundary of carbon footprint and carbon sequestration in CO₂ backfill was defined according to the life cycle method, including the stages of raw material mining, transportation, processing, injection, solidification, etc. Then, the carbon footprint and carbon sequestration in the CO₂ backfill process were sorted out, and factors such as the source, dosage, loss, and conversion of CO₂ were considered. Next, the calculation methods of carbon footprint and carbon sequestration in the processes of raw material transportation, filling slurry preparation, underground injection and filling were given. The research results are expected to reduce the energy consumption and cost of CO₂ storage, and have far-reaching significance for green coal mining and sustainable development and utilization.