Characterization of tar content in the syngas produced in a downdraft type fixed bed gasification system from dried sewage sludge

Tar yields in the syngas produced in a pilot-scale downdraft type fixed bed gasification system from dried sewage sludge have been quantified and characterized to identify the effect of equivalence ratio (ER of 0.29-0.36). The increase of ER resulted in higher temperature of oxidation zone because air promoted the combustion reaction. High ER and high temperature also enhanced cracking and combustion of tar. Lower tar mass was observed while increasing ER. The change in tar composition with the change of ER was also observed by using the size exclusion chromatography (SEC). The SEC results showed that heavier molecular tar (in the molecular weight range of 300-500 u) formed whereas lighter molecular tar decreased under the higher ER conditions. Tar removal performances of the gas cleaning system (the venturi scrubbers and the sawdust adsorbers) were also investigated. The tar removal efficiency of the gas cleaning system depended on gasification conditions, tar components and the amount of tar. Tar content in the syngas was reduced to 26-53% and 14-36% (by weight) at the exit of the scrubbers and sawdust adsorbers, respectively. By the action of this gas cleaning system, about 44% of light aromatic hydrocarbon tar was removed while no light PAH tar was detected at the exit of the gas cleaning system.