Fuel upgrade and soil contamination effects on wood chip quality and combustion behaviour

Contamination of wood chip fuels with soil material, e. g. during logging operations, might result in a deterioration of fuel quality and in elevated gaseous and particulate emissions, corrosion, or slag formation during combustion in biomass boilers. In contrast, fuel upgrading by washing/screening might improve combustion behaviour. To investigate these effects, wood chips from stemwood and forest residues originating from three different forest growing regions (A to C) in Bavaria, Germany, were deliberately contaminated with mineral soils at two mixing ratios (5 %, 10 %) or were mechanically upgraded by screening or washing (only fuels from region A). Contamination was performed using three typical forest soils from Bavaria that corresponded to the selected fuel growing areas (soil A, B and C). Wood chips were tested for fuel quality according to ISO standards for solid biofuels and combusted in a 30 kW biomass boiler. Throughout the trials, fuel upgrading usually improved combustion behaviour. Total particulate matter (TPM) emissions increased for wood chips from stemwood when contaminated with soil A and C from 50 mg/m3 up to 236 mg/m3 (STC), compared to the reference fuel, while emissions for wood chips contaminated with soil B remained constant at approx. 50 mg/m3 (STC). Carbon monoxide (CO) emissions decreased for contaminated wood chips by up to 89 %. For NOX emissions, no clear effect was observed. In addition, contamination mostly increased slagging of fuels when contaminated with soil. Overall, the contamination of woody biomass had a noticeable effect on pollutant emissions and slag formation and should be avoided.