Formation and regulatory mechanisms of N-containing gaseous pollutants during stage-pyrolysis of agricultural biowastes

Abstract

Abstract Process control of fuel-bound nitrogen conversion into N-containing gaseous pollutants was essential for thermal utilization of agricultural biowastes. In this study, decisive formation pathways and regulatory mechanisms of two N-containing gaseous pollutants (NH3 and HCN) were probed via stage pyrolysis of three typical ones involving bean straw, rice straw and wheat straw. Formation characteristics of N-containing gaseous pollutants for single-stage and two-stage pyrolysis were quantitatively compared. Results indicated that consistent formation pathways of N-containing gaseous pollutants were elucidated by direct and indirect conversion of similar types of fuel-bound nitrogen - amide-N/amine-N/amino-N in agricultural biowastes. Specifically, two N-containing gaseous pollutants were hardly associated with primary pyrolysis of amide-N/amine-N/amino-N 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 under same conditions, 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 57–60% with a greater effect on HCN–N yield (76–83%) than NH3–N yield (45–50%), which exhibited an excellent regulatory capacity on the formation of N-containing gaseous pollutants for agricultural biowastes. These findings were favorable for developing some insights into emission control of N-containing gaseous pollutants (intermediate and final) during their thermal utilization.