NOx Emission of Fine- and Superfine- Pulverized Coal Combustion in O2/CO2 Atmosphere

The effects of particle size, stoichiometric ratio (lambda), atmosphere, temperature, and recycled NO on the emissions of three fractional nitrogens, [N](N2O), [N](NO) and [N](NO2), during the combustion of superfine pulverized coal in O-2/CO2 atmosphere were investigated in the present study. NO2 has shown little contribution to NOx compared with N2O and NO in all the cases. As the stoichiometric ratio increases, the trend is much similar in CO2/O-2 and N-2/O-2. However, the conversion ratio from fuel-N to NOx in CO2/O-2 atmosphere is less than that in N-2/O-2 atmosphere especially at lambda > 1.2. [N](NO) makes up the largest portion of [N](NOx) at lambda > 1, and [N](N2O) dominates at lambda < 1 resulting from the presence of hydrocarbons and CO at low stoichiometric ratio. For the three coals, [N](N2O) increases as the mean particle size increases while [N](NO) shows the opposite trend because the evolution of volatile nitrogen was delayed and the reburning-like scheme happened. There exists a minimum for the conversion ratio from fuel-N to NOx at the particle size range of 15-25 mu m under the combined effects of [N](N2O) and [N](NO). As temperature goes up, [N](NO) and the conversion ratio from fuel-N to NOx increase while the [N](N2O) decreases obviously. The conversion ratio from fuel-N to NOx decreases while the reduction rate increases as recycled NO increases. Recycled NO is destroyed in the flame through its reactions with hydrocarbon radicals in the form of CHi, and reduction reactions occur between recycled NOx and fuel-N. The increase in NO concentration accelerates the formation reactions of N2O and also promotes the conversion of char-N to N2O.