The application of a multistage-bed model for residence-time analysis in chemical-looping combustion of solid fuel

Abstract

The behavior of a 10 kW chemical-looping combustor for solid fuels, normally operated in continuous mode, has been studied by addition of fuel batches. From analysis of gas leaving the air reactor, it was possible to determine the residence time and residence-time distribution of particles in the fuel reactor. Knowing the solids inventory in the fuel reactor, the circulation mass flow could be directly correlated to measured operational data, i.e. pressure drop, temperature and gas flow in air reactor riser. Using results for carbon-capture efficiency and residence-time distribution, a model was developed which could determine a mass-based reaction-rate constant for char conversion. The reaction-rate constant of a Mexican petroleum coke at both 950 and 970 °C was calculated to 8.2 and 28.8 wt%/min, respectively. The reaction-rate constant of a South African coal at 950 °C was calculated to 26.1 wt%/min. This reaction-rate constant could also be determined independently from the conversion rates of char during the batch tests. The results showed a good agreement between the two approaches, indicating that the model well describes the behavior of the unit. From the determination of the circulation mass flow, and comparison with previous testing with different circulation, it was also possible to estimate the limit for which the ilmenite was unable to supply sufficient oxygen to achieve good conversion. This limit was compared to theoretical limits for ilmenite as well as the limit set by a heat balance. It was concluded that the latter will be the one setting the limit.