Abstract
In this work the effects of fuel parameters and under grate flue gas recirculation (FGR) are investigated when applying staged air combustion of solid fuel for the purpose of NOx reduction. In addition to temperatures, residence times and primary and overall excess air ratio, the fuel-N content and speciation, the relative fuel-C, fuel-H and fuel-O contents, the fuel moisture content, the fuel gas composition from the dry fuel and the use of FGR (in the primary and/or secondary combustion zone) influence the NOx reduction potential to a larger or smaller extent. To assess the influence of different parameters, a simulation matrix using one ideal plug flow reactor representing only the primary zone and a recent detailed chemical kinetics mechanism was carried out. The results show a large influence of the fuel-N content and speciation on the NOx reduction potential in the primary zone, and a significant influence of the fuel moisture content due to its fuel gas dilution effect. At the close to optimum NOx reduction conditions studied in this work (973 K, primary excess air ratio of 0.9), the influence of the fuel gas composition from the dry fuel on the achievable total fixed nitrogen reduction in the primary zone is relatively small, as well as its influence on the radical pool available for the fuel-N chemistry. The influence of the relative content of C, H and O in the fuel at a constant primary excess air ratio is also relatively small. From a kinetics point of view, there is a positive effect of applying under grate flue gas recirculation if the residence time is sufficiently long, however, the effect is relatively low compared to the other main influencing parameters. At higher temperatures the importance of the radical pool availability as well as its speciation increases, and a more significant part of the NH3 and especially HCN reduction in the primary zone results in increasing NO formation with increasing temperature.
