Coal-nitrogen release and NOx evolution in the oxidant-staged combustion of coal

Air-staged combustion is one of the most sophisticated and effective technologies for reducing NOx emissions. To better understand NOx formation and destruction in air-staged combustion, measurements of the major gas species and NOx profiles along the axial distance of a furnace were obtained in a 20 kW down-fired pilot-scale combustion facility. The furnace of this facility has a representative configuration of a full-scale boiler with a staged combustion system in terms of residence times and temperature characteristics. The combustion air is divided into primary air, secondary air, and burnout air, which can be heated to 80 °C, 200 °C, and 200 °C, respectively by electric heaters before entering the furnace. Under the staged case, a usual NOx trend was obtained by the measurements: coal nitrogen can be rapidly converted to NOx and then reaches a peak concentration in the primary combustion zone, followed by a significant reduction in NOx in the reduction zone. The greater the staged level is, the earlier the reduction stage starts. For the deepest air-staged case with ratio of burnout air to total air of 0.42, the NOx profile curve does not exhibit the features of the NOx peak and a pronounced reduction, as the NOx values are always at very low levels. There exists an NO reducing saturation phenomenon in an overlong reduction zone. For providing data associated with the oxygen-enriched combustion technology that will be available in the near future, low-level oxygen-enriched combustions of coal (21-30%O2) have been performed to assess their NOx emission characteristics compared to the air-staged configuration. There is no essential difference in the mechanism of NO formation and destruction between the staged combustion with oxygen enrichment and the air-staged combustion. With the increase of the staged degree, the impact of O2 concentration promotion on the rise of NOx emission gradually decreases. The NOx rises again at the point where burnout oxidizer addition is caused by the oxidation of some unknown nitrogen-containing intermediates prevailing in the oxygen-poor reduction zone.