Interactions in NOX chemistry during fluidized bed co-combustion of residual biomass and sewage sludge

This work investigates the interactions in NOX chemistry during biomass co-combustion in a continuous lab-scale bubbling fluidized bed reactor. Co-combustion experiments were performed at air staged and unstaged conditions, and the gas composition in the flue gas and within the reactor was measured. The used biomass fuels were straw, sunflower husk, sewage sludge, and sunflower seed. Based on the NO concentration in the flue gas, strawsunflower husk and straw-sunflower seed co-combustion were additive, while co-combustion of straw and sewage sludge revealed a synergy effect. The main cause was the presence of sewage sludge ash, which could catalyse the formation of NO from NH3 and HNCO, and possibly HCN. The catalytic effect of the ash increased with lower ash preparation temperature and better mixing of the ash with straw. During straw-sewage sludge cocombustion, the NH3 initially released from sewage sludge favoured the reduction of NO, while at later stages, when a significant amount of ash accumulated in the bed, the catalytic oxidation of NH3 to NO was dominant. Compared to air unstaged conditions, the NO emission was reduced and the impact of ash on the nitrogen chemistry was less pronounced at air staged conditions.

Automated summary

This study investigated the interactions in NOX chemistry during biomass co-combustion, finding that the effect varies with different biomass fuels and ash characteristics. The catalytic effect of ash on NO formation is influenced by ash preparation temperature and mixing with fuel. The impact of NH3 on NO depends on the stage of the co-combustion process, and NO emissions are reduced and the influence of ash on nitrogen chemistry is less pronounced under air staged conditions.