Contamination of Wood Pellets with Selected Mineral Soils - Fuel Quality and Combustion Behaviour

Due to careless operation during fuel production, considerable shares of mineral soil might be added to wood chips and wood pellets leading to contamination of the biofuels. This can result in high gaseous and particle emissions, corrosion or slag formation during combustion. To investigate this effect, four pellet fuels were produced at TFZ using coniferous wood of Norwegian spruce, i. e. a pure sample as reference and three pellets that were contaminated with different mineral soils (2 w-%). The contaminated pellets had significantly higher ash contents (2.47 to 2.63 w-%, d.b.) compared to the reference fuel (0.83 w-%, d.b.). The reference sample fulfilled the specifications for A2 according to DIN EN ISO 17225-2, while the contaminated pellets only met the I3 quality criteria. A visual differentiation between contaminated and uncontaminated fuels was not possible. The pellets were combusted in a pellet boiler with a nominal heat output of 15 kW. During combustion of the contaminated pellets, the boiler shut down because of to severe slag and ash formation after 118, 120 and 278 minutes of full load operation. Particle emissions and NOX decreased for the soil contaminated pellets compared to the reference fuel while SOX increased. Carbon monoxide was on a very low level for all fuel assortments. A final interpretation of the correlation between soil contamination of wood pellets and their respective emission behaviour is not possible yet as analysis of chemical element concentration is still ongoing. Overall, the contamination of woody biomass had a noticeable effect on emissions and on the continuous, undisturbed operation of the pellet boiler and should be avoided.
Proceedings of the 27th European Biomass Conference and Exhibition, 27-30 May 2019, Lisbon, Portugal, pp. 673-680