Thermodynamic performance comparison of calcium looping processes for post-combustion capture: Influence of CO2 enrichment routes among three heat supply methods

Calcium looping process is recognized as a promising option for low-energy consumption post-combustion CO2 capture. This paper introduced three calcium looping processes with different fossil-fuel-based heat supply methods including air combustion (CaL-AC), oxy-fuel combustion (CaL-Oxy), and chemical looping combustion (CaL-CLC). The sensitivities of key parameters on system performance are investigated, and the detailed energy analysis is conducted to reveal the thermodynamic performance difference. Results show that the changes of the average CaO conversion ratio and the solids make-up ratio bring about drastic variation of energy distributions and the specific primary energy consumption for CO2 avoidance (SPECCA) in the CaL-AC process. While temperature difference of supplying heat for calcination has a significant influence on the system performance of the CaL-CLC process. Besides, eliminating air preheating leads to the increase of the SPECCA from 3.12 MJ/kg CO2 to 4.53 MJ/kg CO2 in the CaL-AC process, which is inferior than that in the CaL-Oxy process. Furthermore, the minimum CO2 enrichment work of different pathways is examined. The unit minimum enrichment work in the CaL-CLC process is 9.92 kJ/mol CO2, lower than those in the other two processes due to the avoidance of minimum enrichment work for fuel decarbonization. Through reaction coupling, the Gibbs free energy of the combustion reaction offsets the minimum work required for O2 release, thereby avoiding the minimum separation work. In post-combustion CO2 capture processes, avoiding the CO2 enrichment work requirement during fossil fuel conversion will offer another way to reduce energy consumption.