Abstract
This work investigated the effects of different ash co-products on the combustion of solid fuels, in particular the fuel-nitrogen behaviour: The fuel-ash additive combinations investigated were: Firstly, biomass ashes added to bituminous coals, representative of those used in power stations; Secondly, a low reactivity coal; Thirdly, a high-N biomass (olive cake) was chosen as a high reactivity fuel and studied with a power-station pulverised coal fly ash as an additive. These five solid fuels have a wide fuel ratio, FR (i.e. the ratio of fixed carbon to volatile matter content). The ash additives were a pulverised fly ash (PFA) and a furnace bottom ash (FBA) from wood pellet combustion in a UK power station. Fuels (with and without additives) were studied for nitrogen partitioning during (i) devolatilisation and for (ii) NOx formation during combustion, using two different electrically heated drop tube furnaces (DTF) operating at 1373 K. Devolatilisation was also studied via ballistic-heated thermogravimetric analysis (TGA). The extent of impact of additives on volatile yield under devolatilisation conditions was dependent on fuel ratio, high FR has the greatest increase in volatile release when co-feeding the additive. Under devolatilisation conditions, there is a correlation between volatile nitrogen and carbon conversion for all the fuels tested. Thus, additives liberate more volatile-nitrogen from the coals and also deliver enhanced carbon conversion. A mechanism is proposed whereby ultra-fine particles and vapours of reactive compounds from the additives interact with the reacting fuel/char particle and influence N-release during both devolatilisation and char burn-out. The enhanced conversion of fuel-nitrogen to volatile-nitrogen and the reduction of char-nitrogen can lead to reductions of NOx emissions in emissions-controlled furnaces. This approach could assist fuel-flexible power stations in achieving their NOx emission targets.
Highlights
In 2016, 3.8 Gtoe of coal was mined and 28.1% of global energy consumption came from coal [1]
The general trend is for the carbon conversion and the volatile nitrogen release from the coals to move to the right on the graph but still follow the trend line
During pulverised fuel coal combustion, low-NOx burners promote the release of fuel-N in low oxygen environments which enhances the formation of N2 over NOx. Results from both thermogravimetric analysis (TGA) and DTF1 show that the additives enhance the conversion of the fuel, which is clearly a desirable attribute to help lower NOx emissions
Summary
In 2016, 3.8 Gtoe of coal was mined and 28.1% of global energy consumption came from coal [1]. The combined consequence of the Large Combustion Plant Directive, followed by the IED, together with incentives to decarbonise power generation, has seen the development of fuel-flexible power stations, which are managing coal fuel inventories, and biomass fuel inventories, together with resultant ash co-products which can be fossil derived, biomass derived, or a mix of the two. This raises the opportunities for improving combustion performance through the addition of ash co-products in the boiler. Experimental Data are provided in full in the results section of this paper
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