NO Emission Characteristics of Pulverized Coal Combustion in O2/N2 and O2/H2O Atmospheres in a Drop-Tube Furnace

Oxy-steam combustion is a new oxy-fuel combustion technology. This paper focuses on the NO emission characteristics during the combustion of SF (Shen Fu) coal in O₂/N₂ and O₂/H₂O mixtures. Experiments were performed in a drop-tube furnace. Combustion tests were carried out in O₂/N₂ and O₂/H₂O atmospheres for various O₂ concentrations (21%, 30%, 40%, and 60%) at different temperatures (1173 K, 1273 K, and 1373 K). In addition, combustion experiments at different excess oxygen ratios (λ) were conducted in O₂/N₂ and O₂/H₂O atmospheres. The influences of the atmosphere, oxygen concentration, temperature, and excess oxygen ratio on NO emissions were analyzed. The results show that the NO concentrations of SF coal combustion in the 21% O₂/79% H₂O atmosphere were much lower than those in the 21% O₂/79% N₂ atmosphere at the three temperatures considered. This was because a large amount of NO was decomposed during the SF coal combustion in the O₂/H₂O atmospheres. The reasons for the decomposition of NO include the selective non-catalytic reaction (SNCR) mechanism and char's important role as a catalyst for the destruction of NO, either directly or by reacting with CO or H₂. In oxy-steam combustion, the NO concentrations significantly increased with the increase in the oxygen concentration from 21 vol.% to 60 vol.% and the temperature from 1173 K to 1373 K. The excess oxygen ratio (λ) slightly impacted the NO emissions in the O₂/H₂O atmosphere.