Biomass is considered as one of the most promising fuels worldwide, mostly because of its renewability and almost-neutral carbon balance. At the same time, numerous studies have shown that the combustion of biomass fuels results in emissions of multiple gaseous and particle phase pollutants. The aim of this study was to fill the gap in the data of emissions from the combustion of agricultural biomass fuels. Five agricultural residue-derived fuels were tested: sunflower stalk pellets, straw pellets, buckwheat shells, corn stalk pellets, and wheat grain screenings. In addition, wood and sewage sludge pellets were investigated as reference fuels. Experiments were performed in a commercially available domestic 13 kW pellet burner during optimal and stable combustion conditions. The characterization of the emissions of gaseous basic pollutants (CO, CO 2 , SO 2 , NO x ), as well as combustion specific pollutants (size-segregated particulate matter (PM), polycyclic aromatic hydrocarbons (PAHs), as well as BTEX (benzene, toluene, ethylbenzene, xylenes) was conducted. The emissions of PM were mostly represented by PM 1 fraction (PM 1 /TSP > 0.8) in the case of all fuels. Total PM emissions ranged from 0.28 g/kg to 5.23 g/kg. Total emissions of PAHs ranged from 469.4 μg/kg to 7212.2 μg/kg. Size-segregated PAH analysis revealed that the most of PAHs were detected in fine aerosol fraction (0.056−0.18 μm). Sewage sludge pellets were determined as the most polluting fuel, including PAH emissions. Several fuels, including sunflower stalk pellets, buckwheat shells, and sewage sludge pellets, were found to be the least favorable fuels for combustion in a small-scale pellet-type burner, because of increased emissions of CO and PAHs.

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Increasingly stringent limits for NOx and SO2 emission are forcing the investigation of new reduction methods. This study was conducted to determine the combination of primary measures, i.e., flue gas recirculation (FGR) and excess air effects on sulfur, nitrogen and hydrocarbon emissions, in boiler flue gas. Experimental research was performed using an experimental, small-scale (20 kW) model of an industrial biofuel boiler. During combustion of sunflower seed hulls at different FGR ratios and incomplete combustion regimes, the composition of flue gas (NO, NO2, N2O, HCN, NH3, SO2, SO3, H2S, CO, and CxHy) was compared, allowing an explanation of the determinants of emission concentration changes to be provided. Increasing the flue gas recirculation ratio in the primary air had a positive effect on reducing NOX and CO with certain organic compounds. However, an opposite effect on SO2 was observed. NOX and SO2 concentrations were found to be approximately 500 mg/m3 and 150 mg/m3 under a regular combustion regime. When the FGR ratio of 50% NOX concentration decreased by 110 mg/m3, the SO2 concentration increased by 60 mg/m3. The incomplete combustion regime reduced NOX concentration by 70 mg/m3, whereas SO2 concentration increased by 100 mg/m3. The influence of primary measures presented an unclear relationship to hydrocarbon emissions, with concentrations not exceeding 18 mg/m3.

Measurements of pollutant concentrations in the flue gases of a biofuel boiler show that pollutants concentrations fluctuate by 10-100% and more at short time intervals. Large fluctuations in concentrations hinder the effective application of pollution reduction measures. During the research, flue gas composition measurements were performed in 4 MW, 8 MW industrial biofuel boilers and 20 kW experimental model. The results of the research showed that the change of CO, NO, NO2, SO2, H2S, O2 and particulate matter concentrations is related to the periodicity of the boiler grate movement. During the movement of the grate, the surface of the fuel bed is mechanically disrupted. The different composition of the fuel layers is mixed and this affects the changes in the concentration of combustion products. The clarity of the concentration fluctuations depends on the emissions, the type of fuel and the combustion parameters.

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