Nitrogen sulphur interactions in coal flames

The control of NOx emissions from coal combustion is a major factor in reducing the environmental impact of coal-fired power plants. This paper investigates the effect of sulphur on NO production in pulverised coal flames using an experimental approach. The NO emissions from a 20 kW down-fired combustor were measured with and without SO2 doping for a range of operating conditions and coal types with total sulphur contents between 0.6 and 2.9%. Some tests were repeated using coals that were acid-washed to remove pyritic sulphur. The NO emissions were statistically correlated against 52 parameters describing the physical and chemical properties of the coals. perimental results show that the change in the NO emission due to SO2 addition is dependent upon the stoichiometry of the flame and that sulphur can interact with the production of NO in a heterogeneous coal flame in a manner similar to that reported previously for gaseous and liquid fuel flames. For fuel-rich conditions, sulphur addition enhanced the production of NO from fuel-bound nitrogen by up to around 20%. The change was larger for coals with a lower inherent fuel-S content (<0.5 wt%). The experimental trends also provide strong evidence that the in-flame sulphur nitrogen interactions are controlled by the availability and production of inorganic (pyritic) sulphur from the coal matrix. This is supported by statistical correlation of the data which has identified specific coal properties, such as pyritic sulphur content and the total volatile yield, as being amongst the most influential parameters in predicting the NO emission for the system employed. These parameters are the key indicators for the release of sulphur at low to intermediate temperatures and would imply that the NO enhancement mechanism under fuel-rich conditions appears to be associated with the early release of pyritic-S into the developing flame to subsequently interact with volatile nitrogeneous species.