Energy-carbon comprehensive efficiency evaluation of a hydrogen metallurgy system with low-temperature waste heat recovery

Hydrogen-based direct reduction technology holds both energy conservation and carbon emissions reduction potential, emerging as a promising paradigm for low-carbon direct reduction iron (DRI) production. Nevertheless, current hydrogen metallurgy processes suffer from poor utilization of low-temperature waste heat, limited adoption of electrified equipment, and a decoupling between energy output and carbon emissions assessment, which obscures potential energy savings and impedes the steelmaking sector’s climate-change mitigation goals. To address these gaps, this paper proposes a novel zero-carbon hydrogen metallurgy system with fully electrified equipment that integrates the utilization of low- and high-temperature waste heat, internal energy, and cold energy during hydrogen production, storage, reaction, and circulation. Mathematically, detailed models are developed to quantify the energy and exergy inflows and outflows of every operating unit in the proposed zero-carbon hydrogen metallurgy system. Energy, exergy, and energy-carbon efficiency indices are proposed. Each metric is formulated across the complete process-chain energy flow, thereby eliminating the counting of the chemical energy in the circulating furnace top gas and ensuring accuracy of evaluation. Energy, exergy, and energy-carbon efficiencies of the proposed zero-carbon hydrogen metallurgy system are compared with those of an existing actual DRI production system with HX2/CO ratios of 6:4 and 8:2. The comparative results demonstrate the superiority of the proposed zero-carbon hydrogen metallurgy system and the effectiveness of low-temperature waste heat recovery.