DETAILED CHEMICAL KINETICS MODELLING OF NOx REDUCTION IN COMBINED STAGED FUEL AND STAGED AIR COMBUSTION OF BIOMASS

摘要

Fuel and air staging are used to study the potential for reduction of NO_x emissions from biomassrnfuelled reactors. The two well-known methods to reduce NO_x emissions in a combustion process by primaryrnmeasures are air- and fuel staging. Air staged reactors use primary air injection in the fuel bed, and a secondary airrninjection in the combustion chamber. A rich combustion in the first stage (close to stoichiometric condition) gives thernmaximum conversion of fuel-N to N_2 (minimum NO_x emissions after complete combustion). Fuel staging has beenrnintroduced as an alternative emission reduction technique due to the practical demands of air staging (highrncombustion temperature in the primary zone). Fuel-staging is a method of fuel reburning. The present study showsrnthe effect of different variables on NO_x reduction capability. Reactor models consisting of sequences of ideal reactorsrnare employed in combination with a recently available detailed chemical kinetics mechanism, especially suitable forrnbiomass combustion. Nitrogen in the fuel is known to be the largest contributor to NO_x emissions; hence the nitrogenrncontent of the fuel is found to be an important parameter in the reduction of nitrogenous emissions. Furthermore, thernpotential for NO_x reduction is found to be highly affected by the amount of excess air in the primary zone, in advancernof the secondary air inlet, with a clear optimal reduction close to a primary air excess ratio of 1. Increasing reactorrntemperature however has an inverse effect on the reduction potential due to the onset of thermal NO_x production.rnFuel staging is found to have a higher potential for reduction in the majority of the investigated range of primaryrnexcess air ratios. Only in the region close to stoichiometric conditions in the primary zone is a higher NO_x reductionrnobserved for air staging. The achieved NO_x reduction under the optimum conditions was up to 76 percent.