In-situ nitrogen fixation during sludge pyrolysis integrated with biomass gasification process

Gasification is a promising strategy for sludge valorization; however, the significant emission of nitrogen-containing compounds presents both environmental and operational challenges. To mitigate this, a novel sludge pyrolysis and biomass gasification (SPBG) method has been developed to enable in-situ stabilization of nitrogen compounds. Results show a 57.89 % increase in nitrogen (N) content within the char, with N fixation rates from sludge volatiles (S-volatiles) ranging from 3.10 to 19.91 %. The sludge-biomass (S/B) ratio is the primary determinant of nitrogen fixation, followed by moisture content in volatiles and reaction temperature. A higher S/B ratio enhances the incorporation of nitrogenous S-volatile into biomass, promoting N retention and increasing the prevalence of N-5 structures in char. In contrast, elevated temperatures and moisture levels promote the secondary decomposition of nitrogenous structures. Notably, endogenous nitrogen in biomass primarily derived from intrinsic proteins predominantly forms pyridine (N-6) in char, whereas nitrogen from S-volatiles is preferentially stabilized as pyridine (N-5). Among various biomass feedstocks, corncob exhibits superior nitrogen fixation efficiency. Correlation analysis further indicates that digestible carbohydrates and lipids in biomass play a crucial role in nitrogen fixation by enhancing the reactivity of exogenous nitrogen from S-volatiles, favoring the formation of N-5 structures.