An experimental investigation was carried out to study the NO x formation and reduction by primary measures for five types of biomass (straw, peat, sewage sludge, forest residues/Grot, and wood pellets) and their mixtures. To minimize the NO x level in biomass-fired boilers, combustion experiments were performed in a laboratory scale multifuel fixed grate reactor using staged air combustion. Flue gas was extracted to measure final levels of CO, CO 2 , C x H y , O 2 , NO, NO 2 , N 2 O, and other species. The fuel gas compositions between the first and second stage were also monitored. The experiments showed good combustion quality with very low concentrations of unburnt species in the flue gas. Under optimum conditions, a NO x reduction of 50-80% was achieved, where the highest reduction represents the case with the highest fuel-N content. The NO x emission levels were very sensitive to the primary excess air ratio and an optimum value for primary excess air ratio was seen at about 0.9. Conversion of fuel nitrogen to NO x showed great dependency on the initial fuel-N content, where the blend with the highest nitrogen content had lowest conversion rate. Between 1-25% of the fuel-N content is converted to NO x depending on the fuel blend and excess air ratio. Sewage sludge is suggested as a favorable fuel to be blended with straw. It resulted in a higher NO x reduction and low fuel-N conversion to NO x . Tops and branches did not show desirable NO x reduction and made the combustion also more unstable. N 2 O emissions were very low, typically below 5 ppm at 11% O 2 in the dry flue gas, except for mixtures with high nitrogen content, where values up to 20 ppm were observed. The presented results are part of a larger study on OPEN ACCESSEnergies 2012, 5 271 problematic fuels, also considering ash content and corrosive compounds which have been discussed elsewhere.

The combustion of biomass, in this case demolition wood, has been investigated in a grate combustion multifuel reactor. In this work a temperature range of 850À1000°C is applied both for staged air combustion and nonstaged combustion of biomass to investigate the effects of these parameters on the emission levels of NOx, N 2 O, CO, hydrocarbons (C x H y ) and different other components. The composition of the flue gas is measured by four advanced continuous gas analyzers including gas chromatograph (GC), two Fourier transform infrared (FTIR) analyzers, and a conventional multispecies gas analyzer with fast response time. The experiments show the effects of staged air combustion, compared to nonstaged combustion, on the emission levels clearly. A NOx reduction of up to 85% is reached with staged air combustion. An optimum primary excess air ratio of 0.8À0.95 is found as a minimizing parameter for the NOx emissions for staged air combustion. Air staging has, however, a negative effect on N 2 O emissions. Even though the trends show a very small reduction in the NOx level as temperature increases in nonstaged combustion, the effect of temperature is not significant for NOx and C x H y , neither in staged air combustion or nonstaged combustion, while it has a great influence on the N 2 O and CO emissions, with decreasing levels with increasing temperature.

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Flue gas recirculation (FGR) is a conventional means of reducing NO x emissions that involves lowering the peak flame temperature and reducing the oxygen concentration in the combustion region. Staged air combustion is also an effective means of NO x reduction, especially in biomass combustion. This article reports results on NO x emissions in a set of experiments combining FGR and staged air combustion in a grate-fired laboratory-scale reactor. Two different compositions of the recirculated flue gas were used: CO 2 and CO 2 + NO. The CO 2 concentration varied between 0−8 vol % of the total inlet flow rate and the NO concentration varied between 0 and 64 ppm. Two different FGR locations were also tested: above and below the grate. The results are compared with a reference experiment performed without FGR. The NO x reduction level from staged air combustion at the optimal primary excess air ratio is ∼70%, while employing FGR can reduce the NO x emissions by an additional 5%−10%. The optimal primary excess air ratio range is 0.9−1. However, FGR more effectively reduces NO x when employed outside of the optimum primary excess air ratio range, i.e., excess air ratios higher than 1 and less than 0.9. The experiments with FGR located above the grate exhibit higher reduction potential, while FGR located below the grate produces decreased reduction. The recycled-NO conversion factor, which gives a measure of maximal FGR efficiency, at the maximum point, is nearly 100% when FGR is applied below the grate and is 85%−100% in the case of recirculation above the grate.

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