Closed-loop synergistic strategy for low-carbon calcium carbide production: Integrating biochar-gas co-supply, multi-stage waste heat valorization, and solid waste calcium-looping

The typical energy-intensive calcium carbide industry generates substantial amounts of high-grade waste energy and solid waste resources. Its potential in future green and low-carbon industrial scenarios deserves in-depth exploration. In this regard, this study proposes a low-carbon synergistic calcium carbide system to form a novel closed-loop strategy combining biomass charcoal-gas cogeneration, steam Rankine cycle thermoelectric conversion, solid waste recycling and multi-stage heat utilization of residual energy. The thermodynamic, environmental and economic performance of this innovative system is assessed, with particular attention to critical parameter impacts. Findings reveal that the cogeneration of biochar and gas, alongside calcium recycling of solid waste carbide slag, has substantially unlocked new low-carbon sources of carbon and calcium for the calcium carbide industry. The CO2 emission of the proposed system is 0.234 t/GJ, notably superior to the coal industry's 0.328 t/GJ. Steam Rankine cycle thermoelectric conversion and multi-stage heat utilization effectively recover approximately 3000 kW of system waste energy, with the working range of steam Rankine cycle module featuring turbine steam heat regeneration being expanded, resulting in a further 6.5 % enhancement in steam Rankine cycle efficiency, enabling the calcium carbide system to co-generate around 1300 kW of electrical energy. Economic analysis identifies the critical market price 1.83 USD/kg as the key criterion for evaluating the system's profitability feasibility. Furthermore, the analysis of critical parameters indicates that the optimal reaction temperature for calcium carbide production and biomass gasification pressure are 1700 °C and 45 bar, respectively, which significantly enhance products output, thermodynamic efficiency and emissions reduction. The proposed system holds significant practical value for addressing the challenges of high energy consumption and high emissions in traditional industries and propelling the green and low-carbon transformation of the chemical industry.