Influence of multiple air staging and reburning on NOx emissions during co-firing of low rank brown coal with woody biomass and natural gas

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The paper presents an experimental research on NOx emissions during co-firing of coal with woody biomass and natural gas. The aim of the research was investigation of synergetic influence of advanced and simultaneously applied different primary reduction measures on NOx emissions in a multifuel combustion system. During the tests run on a 20kW PC lab-scale furnace equipped with a swirl burner and over fire air system (OFAS), the process conditions such as fuel composition, combustion temperature, overall air amount and air distribution (air staging and OFA) and fuel distribution (reburning) have been varied. Three single low rank brown coals from Middle Bosnia coal basin and their blends have been used in combination with sawdust and natural gas. Woody biomass fraction was varied between 5 and 20%w in the fuel blend, while the fraction of natural gas used for reburning was varied between 5 and 10%th. Various process temperatures were applied to investigate their influence on the NOx emissions. Besides multiple air staging, effects of the fuel staging in respect to the NOx emissions have been investigated, first by staging the basic fuels, that is coal and woody biomass, and then by natural gas reburning.In a multi-fuel case investigated, it has been demonstrated that over 50% of NOx reduction could be achieved at 0.1 cofiring at the same process temperature, and 67% if the process temperature is reduced. It has been shown that the coal blends at process temperatures above 1400°C, with no fuel staging and OFA switched off, yield highest NOx emissions (above 1100mg/mn3 at 6% O2 dry). On the other side, NOx emissions reduced significantly with the increase of the staging air portion and also with an increase in the distance between the burner and OFA nozzle. Furthermore, if the reburning is performed by natural gas with OFA switched on, NOx emissions reduce below 500mg/mn3 at 6% O2 dry. If the temperature is reduced below 1100°C, NOx emissions decreased to 400mg/mn3 at 6% O2 dry. The results confirmed that the design and operation of multi-fuel system using coal, biomass and gas could be optimized, such benefiting further reduction of NOx emissions.