Additives for the reduction of NOx emissions in large scale solid fuel combustion

Oxides of nitrogen (NOx) are gases that can cause harm to flora, fauna and human health. NOx emissions can result in acid rain, act as a nucleus to form particulate matter (PM) and have been known to cause respiratory disorders in humans. Recent regulations to curtail the emissions of NOx (and other emissions) have been implemented globally. The current legislation, the Industrial Emissions Directive (IED) has set ever more stringent limits for NOx from existing power stations across the European Union (EU). To meet these targets, power stations operating with pulverised fuels will be required to use multiple reduction technologies.
This research is focused on using by-products of biomass combustion, pulverised fly ash (PFA) and furnace bottom ash (FBA) in co-combustion with coal to reduce NOx in fuel flexible power stations. The increase in K, Ca, Na and Mg in the fuel blends may provide catalysts to promote a reduction mechanism for NOx to diatomic nitrogen during combustion. Three bituminous coals, representative of those used for power production were selected for co-combustion with the two ash additives. To compliment the research, a low reactivity coal was also examined under the same conditions. A high reactivity biomass was also examined using coal PFA as an additive, with relatively high Fe. The experimental work investigated the nitrogen partitioning and carbon conversion of the fuel blends through a variety of conditions; Low heating rate (LHR) and high heating rate (HHR) analysis was carried out. The range of experimental work revealed, through the addition of the FBA and PFA to all of the coals, volatile nitrogen was increased and char nitrogen reduced. Carbon conversion was also increased across all of the coals. The addition of the coal PFA to the biomass also realised the same trends in volatile nitrogen and carbon conversion. These are favourable conditions in a large-scale furnace fitted with Low-NOx technology.
A slice model was used to represent of a well stirred pulverised fuel furnace. By dividing the furnace into several 3-dimentional slices the model can calculate the overall performance of a furnace. The slice model can be used for the prediction of NOx emissions from a furnace based on the fuel properties, and furnace operating conditions. When the adapted model was applied to the bituminous coals with and without the additives, accuracies within 10% of experimental work were achieved.
The use of PFA and FBA may present a cost-effective NOx reduction technique when combined with other low NOx technologies in fuel flexible power stations.