A Ca-Cu chemical loop process for CO₂ capture in steel mills: system performance analysis

The conceptual design and modelling of the Calcium Assisted Steel-mill Off-gas Hydrogen (CASOH) process for the conversion of blast furnace gas (BFG) into H2-rich stream and CO2-rich stream at a large scale is discussed in this work. High temperature reactors packed with CaO- and Cu-based materials are used to remove CO2 from the gaseous phase and simultaneously shifting the WGS equilibrium towards H2-rich products. The incorporation of a Cu/CuO chemical loop to such sorption process allows an efficient regeneration of the CO2 sorbent. In this case, the heat needed for the calcination of the CaCO3 is supplied in situ by the exothermic reduction of CuO to Cu with a gaseous fuel (e.g. CH4, CO or H2). The Cu-based solid is firstly converted to CuO(s) during the oxidation step with air and later reduced during the regeneration step, which involves the combination of the endothermic reaction of CaCO3(s) calcination and the exothermic gas-solid reduction of CuO(s) to Cu(s) using a gaseous fuel (typically BFG or natural gas producing a highly concentrated stream of CO2 and H2O(v)). In this paper, the three reaction stages of the CASOH process are modelled with a new 1-D reactor model that integrates state of the art kinetic information on the gas solid reactions, predicting the molar composition of the product gases (dry basis) at the outlet of the packed-bed reactor and maximum temperature achieved in each stage. The 1-D reactor modelling results confirm that process allows the conversion of up to 99% of the inlet CO to H2 at intermediate temperatures (about 650 °C), because of the efficient and continuous removal of CO2 from the gas phase. The high pressure (10 bar) during the Cu-oxidation step causes a very low leakage of CO2 (1.1 vol. %) due to the partial calcination of CaCO3, i.e. only 8 wt. % of the CaCO3 is calcined in this stage. Finally, the feed of BFG as reducing gas during the regeneration stage leads to a maximum temperature of 850 °C in the bed, which allows the complete calcination of the sorbent and gives as a result a CO2-rich stream ready for purification and subsequent use or storage.