Gasification of waste preserved wood impregnated with toxic inorganic and/or organic chemicals: Gasification tests with impregnated waste wood at the 5 MW Jalasjärvi gasification plant

The overall objective of the project was to reduce or eliminate emissions of toxic compounds by disposal of waste preserved wood by gasification as an alternative method to incineration. One specific objective was to perform experiments with the gasifier in order to develop and test a process for generating combustible gases for production of heat and power from waste preserved wood. This process would ensure that heavy metals were present in the ash without any risk of wash out and that no toxic substances were emitted to the atmosphere. The objective of the gasification experiments in Jalasjärvi was to determine, which compounds emit from gasification of waste preserved wood impregnated with creosote or CCA (copper, chromium, arsenic). A further objective was determine the heavy metals in the ash from the gasifier and determine their leachability. Unfortunately, it was not possible to obtain any ash samples during the tests due to the unloading system for the ash. It is possible to gasify preserved waste wood impregnated with creosote and CCA with low exit concentrations of polychlorinated dibenzo-p-dioxins and dibenzofurans. In all gasification experiments, the concentrations of polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans were well below the limit value for MSW incineration (0.1 I-TEQ ng/m³n, 11% O2). PCDD/F concentrations in the process gas are dependent on HCl concentration in the process gas. Similarly, PCDD/F concentration in the flue gas is dependent on HCl concentration in the flue gas. The congener distribution of PCDD/Fs in the process gas was different from that in the fuels. Some penta and hexa dioxins and furans existed in addition to hepta and octa congeners. The congener distribution of PCDD/Fs in the flue gas differed from that in the process gas but was similar to distribution in the fuels so that octa dibenzo-p-dioxins and dibenzofurans were the main congeners. The concentration of particulate matter is low after the boiler and the condensing heat exchanger. The concentration of arsenic in the process and in the flue gas is originating from arsenic in the fuel. The arsenic concentration in the flue gas of the gasification tests with CCA impregnated wood was too high, compared with the limit value for municipal solid waste (MSW) incineration in Finland, also after the condensing heat exchanger. The limit value for new MSW plants is Sb, As, Pb, Cr, Cu, Mn, Ni, V and Sn in total 0.5 mg/m³n 11% O2 and for old MSW plants Ni and As in total 0.5 mg/m³n 11% O2. The other heavy metal concentrations were within the limits. The hydrogen chloride concentration was highest in test 2 even though the chlorine content of creosote impregnated wood was not higher than in that of the other fuels. HCl concentrations were much smaller in the flue gas before and after the condensing heat exchanger (CHE) than in the process gas. CHE removed almost all HCl from the flue gas. Sulphur dioxide concentrations in the flue gas were higher in the gasification of creosote impregnated wood than the others both before and after the condensing heat exchanger. The CHE removed at least a third of sulphur dioxide from the flue gas.