Influence of alkali metals on combustion chemistry - The effect of sulfur additions and alkali metals interaction with the radical pool on CO oxidation in biomass combustion

This project investigates how alkali metals, chlorine and sulfur interact and affect the oxidation of CO and H2 under post flame conditions. The investigation includes a test with sulfur additions in a 10 MW circulating fluidized bed(CFB) boiler and detailed reaction modelling of the combustion chemistry during conditions representative of the cyclone in the CFB boiler used in the experiments. The experiments were performed by continuous and batch feeding of sulfur into the CFB boiler during operation with biomass. Temperatures and gas compositions in the boiler was measured. The other main method in this project was to model the combustion chemistry in the CFB boiler and its influence of K-S-Cl interactions. The detailed reaction modeling focused on how the combustion chemistry was affected by different K/S/Cl ratios by investigating reactions connecting the influence of K-S-Cl to oxidation of CO. The results shows that the S/K ratio is important to the oxidation of CO. When sulfur was added in the experiments, CO concentration decreased drastically. Equilibrium calculations showed that K2SO4 is likely to form under present conditions. The modelling shows that sulfur additions decreases KOH content and forms K2SO4. KOH in excess of sulfur inhibits CO oxidation as it interacts with radicals important to CO oxidation. Excess KOH forms KO, which favours CO oxidation. Chlorine is most likely present as KCl and does not affect CO oxidation if KOH is in excess. The modelling does, however, not predict the significant decreases in CO oxidation during sulfur additions seen in the experiments. Continued work should focus on experimentally investigate the effect of the K/S ratios on CO oxidation under more controlled conditions in order to validate the models and develop the understanding of the mechanisms affecting CO oxidation