Formation characteristics of NO_x precursors during gasification of N-rich biomass

Based on gasification of 4 N-rich biomass (SBS, RS, CS and MDF) in a horizontal tubular quartz reactor, formation characteristics of NO_x precursors were investigated with the help of chemical absorption-spectrophotometry and XPS methods. Effects of fuel's properties (nitrogen functionality and nitrogen content) and gasification conditions (temperature and gasifying agent) were discussed and compared. The results indicate that NH₃-N is the predominant NO_x precursor species mainly produced during devolatilization stage. Each operational factor would alter the yield of each NO_x precursor by affecting their formation pathways. On one hand, thermal stability of nitrogen functionality in fuels (N-A) is a much more important factor than nitrogen content among fuel's properties. Subsequently, due to the primary cracking of unstable N-A (polyamide), total yield of NO_x precursors for MDF reaches up to 74.7% which is higher than that for other straw biomass by 15%. On the other hand, gasification conditions would influence the reaction routes relevant to NO_x precursors during secondary reactions, especially for the hydrogenation reaction. As a consequence, during SBS gasification, when temperature rises from 800 to 1000℃, NH₃-N yield keeps a constant increase from 38.9% to 47.7% while HCN-N increases first and then decreases with a peak value of 18.3%, which may depend on the balance between reaction routes affected by temperature. As for gasifying agent, the presence of CO₂ would partly inhibit HCN-N yield while the introduction of H₂O would moderately promote NH₃-N yield, which is attributed that the hydrogenation reaction would be strongly affected by gasifying agent. Hence, it is concluded that the selectivity and partitioning of NO_x precursors could be changed by employing different ratio of gasifying agents.