Fate of heavy metals during fixed bed downdraft gasification of willow wood harvested from contaminated sites

The combination of energy production from biomass with phytoremediation opens perspectives for bringing land with moderate metal contamination into safe and beneficial use. Small scale fixed bed downdraft gasifier installations (150 kWe–1 MWe) can be employed for on-site conversion of biomass to electricity and heat. To assess the fate of heavy metals, batch gasification operations were run in a small scale fixed bed downdraft gasifier (100 kW thermal power). The gasification of 1 kg of wood resulted in the production of 40 g of ashes, 1.2 kWh of electricity and 9 MJ of heat. Per kg dry mass of willow wood, 31 g bottom ashes, 7 g cyclone ashes, 2 g filter ashes, and 18 g DM of gasifier bed ashes were produced. Cd, Zn and Pb were enriched by a factor 7–100 in the fly ashes compared to the bottom ashes. Low mass recoveries (30–40%) were found for Cd, Cr, Ni and Pb. This was mainly attributed to measurement uncertainties. 60% and more of recovered Cd, Pb and Zn were present in the finer filter and cyclone ashes, while Cr, Cu and Ni were mainly found in the bottom ashes and to a lesser extent the bed ashes. The scrubber placed after the hot cyclone has an essential role in intercepting the more volatile heavy metals before combustion of the gas. The bottom ashes constituted the largest fraction and ideally should be recycled if limits for metal contents are met. Only Cd and Zn exceeded Flemish threshold values for use of this fraction as a fertilizer, albeit by a small margin. This leaves perspectives for future research to optimise process parameters towards the production of bottom ashes that can be recycled.