Co-combustion behaviors and NO formation characteristics of semi-coke and antibiotic filter residue under oxy-fuel condition

•The oxy-fuel co-combustion of antibiotic filter residue and semi-coke was examined.•The fuel-nitrogen transformation mechanisms in co-firing process were elucidated.•High volatile matter of antibiotic filter residue results in its large mass loss.•The ratio of nitrogen converted to NOx of blend rises with the semi-coke fraction.•Effects of oxygen content on semi-coke are great due to its low volatile content. The oxy-fuel combustion technology is helpful to realize the carbon neutrality and reduce the CO2 emission. Antibiotic filter residue (AFR) is a kind of biomass solid waste from medical treatment with a high-level of volatile matter but a low content of fixed carbon. Semi-coke (SC) is the by-product from coal-chemical industry with poor combustion feature, and the SC powder is usually regarded as a carbon-based solid waste. The co-combustion of SC and AFR in oxy-fuel atmosphere could take advantages of both physical–chemical features of two solid wastes. However, few scholars have focused on the oxy-fuel co-firing features of AFR and SC blend, especially the NO formation characteristics and the effects of interactions between two samples on co-combustion behavior. Here, a drop-tube furnace co-firing experimental system is mainly employed to elucidate the co-combustion features of two solid wastes under oxy-fuel condition, together with the physical–chemical properties and reactivity of solid samples. The physical–chemical features of AFR and SC are quite different due to the additives during fermentation and extraction process of antibiotic. The high volatile matter of AFR leads to a much larger mass loss than SC. The conversion ratio of fuel-N to NO (XNO) of blend rises with the proportion of SC, and the increase of XNO reaches by 17.1% with the SC blending ratio raised from 80% to 100%. The XNO of blend in oxy-fuel atmosphere is lower than that in O2/N2 and O2/Ar atmospheres. The XNO and combustion efficiency both increase with the oxygen content in O2/CO2 atmosphere, and the effects of oxygen concentration elevated on co-firing characteristics are raised with the volatile content of blend reduced. The effects of increasing temperature could enhance the combustion reactivity of blend, which are more significant on blend with low-volatile matter. The present studies could provide guidance for effective and clean utilization of solid wastes, and be beneficial for the carbon neutrality and large-scale CO2 reduction.