Empirical NOx-Modelling and Experimental Results from Wood Stove Combustion

Abstract

Experiments have been performed in three different wood stoves based on batch combustion with natural draught; a traditional stove, a stove equipped with a catalytic afterburner and a staged air unit with downdraught combustion and good insulation of the secondary combustion chamber. The average wood consumption was varied between 0.8–3.8 kg dry wood/h, the average excess air ratio varied between 1.5–5, and the average CO emission level varied between 5–190 g/kg dry fuel (300–12000 ppm). The average NOx emission level varied between 0.5–1.8 g NO2/kg dry fuel (16–110 ppm), giving a fuel-N to NOx conversion factor between 0.23–0.55 depending on the wood species (spruce, birch or pine), the operating principle of the wood stove and the operating conditions. The NOx emission level is traditionally correlated with excess air ratio, temperature, initial fuel-N content and combustion quality. Using the CO emission level as an indicator of the combustion quality, an empirical model has been developed for the total fuel-N to NOx conversion factor based on average input variables. The excess air ratio and the temperature were found to be the most important input variables, while the CO emission level and the initial fuel-N content only were of minor importance.