Emissions of dioxins and furans during steam gasification of Automotive Shredder residue; experiences from the Chalmers 2-4-MW indirect gasifier

摘要

Emissions of dioxins and furans during the gasification of Automotive Shredder Residue (ASR) were investigated. The experimental work was carried out in a Dual Fluidized Bed (DFB) system, which consists of a 2-4-MW_(th) gasifier that is fluidized with steam, and an interconnected fluidized bed combustor that is fluidized with air. Two different ASR fractions with higher and lower contents of plastic were tested. Measurements were carried out in the flue gas stream exiting the combustion side of the DFB, as well as in the raw gas stream exiting the gasifier side. A calcium (lime) coat was applied to the flue gas filter to ensure compliance with the emissions regulations regarding the retention of HC1 and dioxins. The results showed lower emissions of polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/PCDF) in the flue gas when the raw gas derived from the ASR gasification was combusted, as compared to the direct combustion of ASR. The level of polychlorinated compounds in the flue gas before the lime-coated filter was 0.11 ng/m_(n dry gas) ~3 (I-TEQ) when gasification was used as a pre-step, as compared to 0.27 ng/m_(n dry gas) ~3 (I-TEQ) when the ASR was directly combusted. The raw gas produced by gasification contained very low levels of PCDD/PCDF, whereby the toxicity per kg of ASR was 0.17 ng/kg_(asr), as compared to 3.44 ng/kg_(asr) after passage through the combustion and cooling sections and 0.34 ng/kg_(asr) at the outlet after the lime-coated filter. A higher content of plastic in the ASR led to an increase in the levels of dioxins and furans in the raw gas, with the highest yield seen for highly chlorinated compounds, while higher temperature in the gasifier is shown to be beneficial in reducing dioxin formation.