Fundamental investigation of NOx formation during oxy-fuel combustion of pulverized coal

NOx formation was measured during combustion of pulverized coals and pulverized coal char in N2 and CO2 environments under isothermal and nearly constant oxygen conditions (i.e. using dilute coal loading). Three different oxygen concentrations (12% O2, 24% O2, and 36% O2) and two representative US coals were investigated, at a gas temperature of 1050 °C. To investigate the importance of NO reburn reactions, experiments were also performed with an elevated concentration (550 ppm) of NO in the gases into which the coal was introduced. For low levels of background NO, the fractional fuel-nitrogen conversion to NOx increases dramatically with increasing bath gas oxygen content, for both N2 and CO2 environments, though the fuel conversion is generally lower in CO2 environments. Char N conversion is lower than volatile N conversion, especially for elevated O2 concentrations. These results highlight the importance of the volatile flame and char combustion temperatures on NOx formation. For the high background NOx condition, net NOx production is only observed in the 36% O2 environment. Under these dilute loading conditions, NO reburn is found to be between 20% and 40%, depending on the type of coal, the use of N2 or CO2 diluent, the bulk O2 concentration, and whether or not one considers reburn of volatile-NOx. This dataset provides a unique opportunity to understand and differentiate the different sources and sinks of NOx under oxy-fuel combustion conditions.