Abstract
An experimental investigation was carried out to study the NOx 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 NOx 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, CO2, CxHy, O2, NO, NO2, N2O, 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 NOx reduction of 50–80% was achieved, where the highest reduction represents the case with the highest fuel-N content. The NOx 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 NOx 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 NOx 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 NOx reduction and low fuel-N conversion to NOx. Tops and branches did not show desirable NOx reduction and made the combustion also more unstable. N2O emissions were very low, typically below 5 ppm at 11% O2 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 problematic fuels, also considering ash content and corrosive compounds which have been discussed elsewhere.
Highlights
According to the analysis carried out by the European Renewable Energy Council, the EU aims for a 100% renewable energy future by 2050, where biomass will potentially supply about 36% of the totalEuropean primary energy consumption, while the potential for many developing countries is higher since the resources are larger in such areas [1]
Biomass can be converted to synthetic natural gas or liquid fuels like methanol in order to be used in the transportation sector [2,3]
The deviation between the measured and calculated values is in the range of ±2%, so the experimental measurements are reliable with low uncertainty
Summary
According to the analysis carried out by the European Renewable Energy Council, the EU aims for a 100% renewable energy future by 2050, where biomass will potentially supply about 36% of the total. While the potential for many developing countries is higher since the resources are larger in such areas [1]. With large resources around the World, which contains carbon in its structure, is almost a CO2-neutral fuel. Combustion is the conventional way of extracting energy from biomass to produce heat, electricity or combined heat and power (CHP). As a carbon- and nitrogen-containing fuel, combustion of biomass is associated with emissions of harmful pollutants. Emissions from a biomass combustion process are normally divided into three groups [4,5]:
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