Evolutionary behavior of bed materials in oxygen Carrier–Aided combustion of biomass

Abstract

The application of oxygen carrier–aided combustion (OCAC), referring to partial or complete substitution of conventional inert bed materials with oxygen carriers (OCs), in a circulating fluidized bed (CFB) offers advantages such as uniform distribution of temperature and oxygen (lattice and gaseous), potentially decreasing CO, CH₄, and NO emissions in flue gas. In this study, pine wood chips were employed as the fuel source, while natural hematite was utilized as the OC. The effects of OC proportion (Φ = 25 %, 50 %, 75 %, and 100 %) and air-to-fuel ratio (λ = 1.0 and 1.1) on OCAC performance were evaluated in a 0.5 kW_th fluidized bed reactor operated for 30 h. The effects of lattice oxygen (provided by OCs) and gaseous oxygen (in the air) on CO₂ yield and combustion efficiency were assessed. The results indicated that the inclusion of OCs significantly decreased CO, CH₄, and NO emissions, with CO and CH₄ emissions declining by 55.59 % ± 5.32 % and 55.98 % ± 5.96 % respectively, at λ = 1.0, and NO conversion declining from 1.9 % ± 0.18 % (100 wt% SiO₂) to 0.89 % ± 0.09 % (50 wt% Fe₂O₃). The highest CO₂ gas yield (89.45 % ± 1.05 %) and combustion efficiency (82.98 % ± 1.22 %) were achieved at λ = 1.1 and Φ = 75 %. Notably, OCAC performance gradually diminished with increased operating time of the CFB boiler, which was attributed to the detrimental effect of biomass ash. The used OC was entirely encased within biomass ash, creating an ash shell structure that blocked OC particle pores and negatively affected gas–solid contact.