Influence of temperature on the formation of NO x during pulverized coal combustion

Abstract

An electrically heated, quasi one-dimensional pulverized coal combustion furnace was used to examine the processes of NO formation and reduction for one-stage and two-stage combustion covering a large range of stoichiometric ratio and of temperature under nearly isothermal conditions. This externally heated, temperature-controlled combustor permits studying the influence of temperature on NO formation unbiased by fuel loading and thermal furnace characteristics. Non-staged NO emissions increased strongly with stoichiometric ratio between 0.8 and 2.0 and with temperature which was varied from 1000 to approximately 1500°C. Air preheat and wall temperatures are responsible for coal devolatilization, flame ignition and early combustion. Highest fuel NO formation was ascertained in coals which evolve the most reactive nitrogen, and lowest when the nitrogen is retained in the solid phase. Minimum staged NO emissions ranged from 180 vppm to 340 vppm based on zero percent excess O2. NOx reduction in the first stage was influenced by stoichiometric ratio, residence time, temperature, and coal rank. In the case of decreasing the first stage stoichiometry, inflame measurements indicated an increase in gaseous nitrogen species and solid phase nitrogen retention. The optimum first stage stoichiometric ratio dropped to lower values resulting in higher temperatures in the primary zone. Highly volatile coals lost most of the fuel nitrogen in the first stage by combustion and gasification. In the rich stage these nitrogen species can be reduced to molecular nitrogen. Exhaust NO results from oxidizable gaseous and nitrogen components in the second stage. The conversion rate of the volatile fraction is higher than of the fixed nitrogen, but in both cases it is dependent on the second stage temperature. Compared to the nonstaged combustion NO emissions could be reduced by up to 70 or 80 percent. The potential of reduction is greater for highly volatile and highly reactive coals. The findings support most of the available evidence and give additional information on the influence of temperature under controlled conditions.