Reviewing the Operational Experience on Chemical Looping Combustion of Biomass at CSIC: <i>i</i>G-CLC vs CLOU

The unmixed combustion of biomass by chemical looping (Bio-CLC) has outstanding properties to produce energy by thermochemical conversion with CO₂ capture at low economic and energetic costs as well as minimizing NOx emissions to the atmosphere. A detailed analysis of the experimental results during the biomass combustion in chemical looping combustion (CLC) units is useful to evaluate the Bio-CLC potential for future industrial up-scaling. This work compiles the results obtained during more than 500 h of combustion in the previous years at the Instituto de Carboquímica (ICB-CSIC). Different biomasses were burnt with several oxygen carriers in two singular CLC units. Critical key performance indicators, such as the CO₂ capture rate and combustion efficiency, have been evaluated considering the combustion mode and the fluidization regime in the fuel reactor. Regarding the combustion mode, results by in situ gasification (<i>i</i>G-CLC) vs chemical looping with oxygen uncoupling (CLOU) are compared. As for the fluidization regime, it was bubbling in a 0.5 kW_th CLC unit and circulating in a 50 kW_th CLC unit. A methodical comparison of the results allows us to understand the fundamentals of the processes and to evaluate the different behaviors observed in each case. The operating conditions having major effects on the Bio-CLC performance were identified, as well as the operating conditions necessary to optimize the CO₂ capture and the combustion efficiency. In addition, other issues are evaluated, such as the fate of fuel-N in a CLC process, the presence of tar compounds in the CO₂ stream, or the interaction of ash compounds with the oxygen carrier particles. Finally, possible future actions are discussed to improve the performance of the Bio-CLC process as well as the possible development paths of new oxygen carriers.