Source reduction or process control of fuel-N conversion into reactive NOx precursors (NH3 and HCN) during pyrolysis process was fundamental and essential for clean thermal utilization of straw wastes. In this study, three typical straw wastes (bean, rice and wheat) were employed to probe formation characteristics and regulatory mechanisms of two target NOx precursors via stage pyrolysis method with the help of XPS and chemical absorption-spectrophotometry analytic techniques. Results demonstrated that consistent formation pathways of NOx precursors were elucidated by direct and indirect conversion of similar fuel-N types – amide-N/amine-N/amino-N (N-A) in straw wastes. Specifically, two NOx precursors were hardly linked with primary pyrolysis of N-A types (direct conversion) while dominantly determined by secondary reactions of subsequent nitrogen intermediates in chars and tars (indirect conversion); secondary reactions referring to hydrogenation of heterocyclic-N in chars and dehydrogenation of amine-N in tars were more responsible for NH3-N and HCN-N, respectively, leading to a maximal total yield of 45-50 wt.%. Consequently, compared to single-stage pyrolysis uniformly, two-stage pyrolysis could manipulate intensities of formation pathways at different pyrolysis stages through employing differential intermediate feedstocks for re-pyrolysis, minimizing the ratio of total yield by about 60 % with a greater effect on HCN-N yield (76-83 %) than NH3-N yield (44-50 %), which exhibited an excellent regulatory capacity on NOx precursors formation for straw wastes. These findings were favorable for developing some insights into emission control of N-containing gaseous pollutants during their thermo-chemical conversion processes.
Keywords stage pyrolysis, straw wastes, NOx precursors, secondary reactions, regulatory capacity