Coal characterisation for NOx prediction in air-staged combustion of pulverised coals

A series of world-traded coal samples has been tested using the Imperial College high temperature wire mesh apparatus (HTWM) in order to assess the relationship between high temperature (1600°C) char nitrogen content and NOx formation in Hemweg Power Station (in the Netherlands) using deep furnace air staging. A linear relationship between high temperature char nitrogen and NOx formation has been confirmed. These results suggest that high temperature char N content is the main factor limiting NOx emissions with deep air-staged combustion.Char N and (hence apparently deep air-staged NOx) can be predicted with an accuracy of approximately ±20% for most coals from the coal proximate and ultimate analysis—but this might not be sufficient for stations operating close to their emission limits. Measuring high temperature char N directly reduces the likely uncertainty in deep air-staged NOx emissions for coals (and most blends) to approximately ±10%. Its use should be considered on a routine basis for coal selection on plants employing this technology.