Emission characteristics and formation mechanisms of ammonia and amines during residential solid fuel combustion at varying temperatures

Abstract

Residential solid fuel combustion (RSFC) is a significant source of ammonia (NH₃) and amines; however, their emission characteristics and influencing factors remain poorly understood. This study investigated the emission behavior of NH₃ and C1∼C6 alkylamines from 12 fuel types categorized into three major groups (straw, firewood, and coal) via field and laboratory experiments at different temperatures. The results revealed that NH₃ emissions (159.5 ± 215.6 mg/kg) were an order of magnitude higher than amine emissions (20.1 ± 23.1 mg/kg). Additionally, NH₃ and amine emissions exhibited a strong synergistic relationship, with a Pearson correlation coefficient (R) of 0.5–0.8, indicating a moderate to strong positive linear correlation. Temperature and fuel types were identified as key factors influencing the emissions of NH₃ and amines. Increasing the temperature from 500 to 800 °C reduced NH₃ and amine emissions from biomass burning (BB) by 2–5 times but increased the emissions from low-volatility coal combustion (CC), reflecting different formation mechanisms. At low temperatures, NH₃ and amines in BB are released primarily via amino acid deamination, whereas at high temperatures, they are consumed. In contrast, CC emissions are mainly driven by the high-temperature ring-opening oxidation of tightly bonded nitrogen in pyrroles and pyridines. Random forest models revealed that BB emissions are more temperature sensitive, while CC emissions are more fuel-type sensitive. High temperatures promote large-molecule amine formation in RSFC, with the proportion of diethylamine and triethylamine rising from 8.2%–11.8% at 500 °C to 18.2%–31.8% at 800 °C, and small-molecule monomethylamine rapidly decreasing to below 10%. This study indicates that synergistic control strategies based on fuel type and combustion temperature should be implemented to reduce NH₃ and amine emissions.