Experimental and numerical studies on the NO reaction pathways of NH3 cofiring with coal volatile matter

Abstract

Ammonia (NH₃) cofiring provides a promising solution to carbon reduction of coal-fired boilers, but it may lead to increased NO_x emissions. However, many experiments observed that the NO_x emissions exhibited an increase-then-decrease trend with the increase of NH₃ cofiring ratio (R_NH3), indicating that NO_x could be controlled under high R_NH3. To reveal the underlying mechanism, the experiments of NH₃ cofiring with coal volatile were first conducted in a one-dimensional flow reactor to reveal that this trend is primarily attributed to the gaseous reactions of volatile and NH₃. A chemical reactor network model was then constructed to investigate the influences of R_NH3 on the NO reaction pathways of NH₃. The predicted results replicated the increase and then decrease trend of NO_x emissions as R_NH3 increased beyond 25 %. It was found that NO formation is primarily controlled by the reactions between HNO/N and OH/H radicals. Under lower R_NH3, the concentrations of HNO/N increase with the increase of R_NH3 leading to increased NO formation. Under higher R_NH3, however, much of the OH/H radicals are consumed by the dehydrogenation reactions of NH₃ which consequently inhibits the NO formation reactions. Therefore, the root mechanism of the increase-then-decrease trend of NO_x emissions is attributed to the competition for the OH/H radicals between the initial and final steps of the NO formation reaction pathway.