Assisting denitrification and strengthening combustion by using ammonia/coal binary fuel gasification-combustion: Effects of injecting position and air distribution

Abstract

The direct co-firing of ammonia (NH₃) with coal is an effective approach for reducing carbon emissions from coal-fired plants. However, the limitations of NH₃ combustion, such as its high ignition energy requirement and high NO_x emissions, restrict its large-scale co-combustion in coal-fired units. In this study, a self-sustaining gasification–combustion experimental system was employed, and its denitrification performance and combustion strengthening capability for NH₃/coal binary fuel under different NH₃ injection positions and air distribution methods were evaluated. The results of co-firing 20 % NH₃ in the gasifier revealed that its operating temperature can be flexibly controlled within the optimal reaction temperature window of SNCR by adjusting the primary air ratio (λ₁). Increasing λ₁ was beneficial for NH₃ and coal conversion and denitrification in the gasifier; at λ₁ = 0.53, the conversion rate of NH₃ and N₂ reached 86.37 % and 83.59 %, respectively, with no NO_x being detected at the gasifier outlet. Furthermore, increasing λ₁ promoted the development of gasified char pore structure, thereby improving reactivity. After entering the down-fired combustor (DFC), increasing λ₁ promoted NH₃ and coal burnout and effectively controlled NO_x emissions, reaching a level similar with that of pure coal under λ₁ = 0.53. Co-firing 20 % NH₃ in the DFC also demonstrated that the introduction of inner secondary air was conducive to char burnout, however, increased NH₃ slip. The outer secondary air promoted NH₃ combustion, however, exacerbated NO_x emissions and inhibited gasified char combustion. The uniform air distribution method balanced the combustion of NH₃ and gasified char, and effectively controlled NO_x emissions. When the same air distribution method was used, co-firing NH₃ in the gasifier was more favourable for NO_x control, whereas co-firing NH₃ in the DFC benefited coal burnout. This study provides innovative ideas and serves as a reference for developing enhanced combustion and low NO_x emission technologies for large-scale NH₃ co-firing in coal-fired units.