Experimental study of effect of a novel ammonia/coal co-firing mode on NOx emission under high temperature conditions

Coal co-firing with green ammonia synthesized by renewable energy is a promising method to reduce CO2 emissions from coal-fired power plants. Enormous NO emission is the principal barrier to realized industrial application of ammonia/coal co-firing in coal-fired boilers with high combustion environmental temperatures. This paper proposed a new ammonia/coal co-firing mode, named lagging co-firing. This mode places the ammonia channel separately in the center of the coal channel, with the ammonia outlet following after the coal outlet. The objective of this design is to allow the pulverized coal and air to enter the furnace first, forming a high temperature flame zone, enabling ammonia decomposed in a high temperature and oxygen-deficient environment, potentially lowering NO emissions. Experiments were carried out in a 45 kW wall-temperature-controlled staged combustion downstream furnace at 1100/1400 °C to study NO emission under air-staged and non-staged condition. It was discovered that lagging co-firing mode successfully reduced NO emissions from ammonia/coal co-firing, especially at high combustion environmental temperatures. In 1400 °C, NO emission concentrations under air staged combustion were 1007 mg/m3 with premixed co-firing mode whereas only 358 mg/m3 with lagging co-firing mode. Using lagging co-firing mode, increasing the ammonia co-firing ratio results in just a minor increase in NO emissions under 1400 °C. Lagging co-firing mode disrupts the NH3→NO reaction pathway and transforms ammonia combustion to hydrogen combustion. Consequently, under high combustion environmental temperatures, lagging co-firing mode successfully tackles the issue of excessive NO emissions in ammonia/coal co-firing, encouraging the industrial development of ammonia/coal co-firing.