Biomass as a reburning fuel: a specialized cofiring application

Abstract

Reaction Engineering International has performed a series of combustion tests to evaluate the potential for utilizing wood biomass as a reburn fuel for nitrogen oxides (NO x ) control. Reburning is an effective NO x reduction technology that utilizes fuel injection above the main burner zone. Studies with other hydrocarbon fuels such as coal and natural gas as reburn fuels have shown that NO x emissions can be reduced by more than 50–60% with about 15% of the heat input coming from the reburn fuel. Two different biomasses, a hardwood and softwood, were evaluated as reburning fuels and compared to coal and natural gas. The use of wood to reduce NO x is attractive for several reasons. First, wood contains little nitrogen, as compared with coal which is also used as a reburning fuel. This results in lower NO x production from fuel nitrogen species for wood. In addition, wood contains virtually no sulfur, so sulfur dioxide (SO 2) emissions are reduced in direct proportion to the coal replacement. Wood is a regenerable biofuel; when a fossil fuel is replaced by a biofuel, there is a net reduction in carbon dioxide (CO 2) emissions. Finally, since the reburning fuel is normally 10–20% of the total heat input, large quantities of wood are not necessary. Experimental results showed NO x reductions of as high as 70% were obtained with approximately 10–15% wood heat input. The stoichiometric ratio in the reburn zone was the single most important variable affecting NO x reduction. The highest reductions were found at a reburn stoichiometric ratio of 0.85. NO x reduction fell to about 40–50% at slightly higher stoichiometric ratios (0.9<SR<0.95) and to 30% at stoichiometric ratios of approximately 1.0. The NO x reduction was strongly dependent on initial NO x concentration and only slightly dependent upon temperature, where increased temperature increased NO x reduction. Finally, the experimental results suggest that wood is as effective as natural gas or coal as a reburning fuel. In addition, REI has completed computer simulations of a full-scale boiler to evaluate the conditions that maximize the NO x reduction efficiency using biomass as the reburn fuel. Computer modeling of the TVA Allen Station Unit 3, a 265 MW e cyclone-fired boiler, showed that NO x reductions as high as 50–60% could be achieved within the constraints set by the boiler and operations. The most important parameters affecting final NO x emissions are the cyclone barrel stoichiometry, residence time in the reburn zone, and mixing in both the reburn and overfire air zones. The combination of computer simulations and experimental programs has provided the engineers with the tools needed to optimize biomass as a reburn fuel to maximize NO x reduction.