Prediction of unburned carbon and NO formation from low-rank coal during pulverized coal combustion: Experiments and numerical simulation

Coal in pulverized form has been utilized to generate heat for boiler operation, most of which is used to run power generation plants. Owing to the issues of the pollutant gas production within the pulverized-coal-fired boiler, the CO, NO, and unburned carbon (UBC)concentrations need to be reduced despite the formation of SO, which is also a potential pollutant. In order to study the pollutant generation mechanism of pulverized coal and predict its contents, the drop-tube furnace (DTF) experiment has been established and used extensively by many researchers worldwide. The DTF experiment itself, including the sample preparation processes, takes a long time to be performed. Therefore, in the present study, DTF modeling using one-dimensional numerical simulation is established for the rapid, good-accuracy prediction of the output gas and UBC concentrations. The DTF model is successfully established and found to provide results with trends similar to those of experimental results. Coal with high reaction rates is observed to have higher NO emissions than that with low reaction rates; this is attributed to a higher peak temperature in the combustion cloud around the coal particles, which leads to thermal NO formation. Suppression in the NO formation rate is observed in low-stoichiometric-ratio combustion, even under high-temperature conditions. The existence of a high CO gas concentration would assist in suppressing the NO formation during the combustion of coal char particles.