Production of a producer gas with high heating values and less tar from dried sewage sludge through air gasification using a two-stage gasifier and activated carbon

The production of sewage sludge in South Korea is continually increasing. Until now, much of the sewage sludge in the country has been disposed of by dumping it into the ocean, which the London Convention wants stopped. Therefore, new sewage sludge treatment methods should be sought. Sewage sludge can be energetically utilized through gasification, which produces combustible gases such as H₂, CO, and CH4. During the gasification of a biomass, however, a large amount of tar is also formed. In this paper we report the results of the experiments that were conducted on the air gasification of dried sewage sludge in a newly developed two-stage gasifier. In such experiments, activated carbon was applied to reduce the tar components of the producer gas. The total amount of tar that was reduced was 6-fold when activated carbon was used in the upper reactor of the gasifier. The effects of the reaction conditions, such as the reaction temperature and the equivalence ratio, on the gas composition were also investigated. The producer gas was analyzed using gas chromatographs and a GC-MS system. The producer gases (inclusive of N ₂) that were obtained with the application of activated carbon had high hydrogen contents (24 vol%), and their lower heating values amounted to 11.6 MJ/Nm³ (Nm³ = normal cubic meter). The use of activated carbon led to a strong reduction of tar in the producer gases. In each of the experiments with activated carbon, the tar content of the producer gas was found to be below 10 mg/Nm³. At this tar level, the producer gas seems to be appropriate for use as a fuel gas in internal combustion engines.