A study on coal blending for reducing NOx and N2O and levels during fluidized bed combustion

The objective of the study was to investigate the effect of blending on the emissions of NO x and N2O during the fluidized bed combustion of five coals with different levels of nitrogen, volatile matter, and fixed carbon contents, and diverse coal and char petrographic composition. The devolatilization chars were produced in a fluidized bed combustor. Using the petrographic analyses of the coals, sixteen coal blends were made to carry out the combustion tests. The combustion of individual coals and the sixteen blends was carried out in an electrically heated atmospheric fluidized bed combustor. The emissions of NOx reached maximum values at a bed temperature around 1173 K and decreased with lower combustion temperatures. The amount of nitrous oxide formed decreased, for all five coals, as the bed temperature increased from 1053 to 1303 K. These trends are consistent with the observed experimental data obtained by other researchers. It was observed that for ratios of 1.3 to 1.6 of fixed carbon to volatile matter, both NO x and N2O emission levels were minimal and, outside this range, both oxides showed a sharp increase in their formation/emissions. The coal blends were also found to show similar trends with increasing the bed temperatures. The results serve to establish the relative importance of volatile and fixed carbon reactivity and coal and char petrography, which is believed to significantly contribute to devolatilization mechanisms and char reactivity.

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