Servizio Comunitario di Informazione in materia di Ricerca e Sviluppo - CORDIS

Basic know-how related to technically successful gasification of sewage sludge and efficient gas cleaning

The fluidised bed test programme demonstrated that circulating fluidised bed gasification of thermally dried sewage sludge with hot gas cleaning is a technically feasible option and no serious problems are expected. The thermally dried sludge was shown to be very reactive, the product gas quality was reasonable and bed sintering and deposit formation could be avoided.

The high nitrogen content of sewage sludge resulted in high levels of NH3 and HCN in the product gas and may require specific attention in product gas cleaning or flue gas cleaning after gas combustion. The gas cleaning requirements can also vary depending on the sludge source, as the presence of harmful trace contaminants in sewage sludge will be dependent on local industry.

Significant formation of heavy hydrocarbons (tars) was avoided, which enabled gas cooling and filtration without tar condensation problems. As the thermal drying process incurs an energy and cost penalty on the process, the option of using composted sludge with an additive (such as wood chips) as a feedstock was also investigated and shown to generate a reasonable quality product gas. Further improvements to gas quality may be possible, if a better quality, composting additive were used.

The combustion performance of the sewage sludge derived product gas generated in the PDU-scale circulating fluidised bed gasifier (CFBG) was good and demonstrated the technical feasibility of co-combusting with higher calorific value fuels, such as natural gas. The results indicated that high levels of NH3 present in the product gas do not necessarily lead to high NOx emissions and, in optimised conditions, can lead to the reduction of NOx compounds to levels which will meet emission limits of the Waste Incineration Directive (WID) for co-combustion.

The sulphur content of the product gas will in part be dependent on the quality/composition of the sewage sludge and may lead to high emissions of SOx. Although a significant proportion of the fuel sulphur is removed during gasification by the addition of limestone to the bed, additional flue gas cleaning systems may be required to meet European emission legislation. Controlling SOx emissions in flue gas, however, is now achievable without extensive investments.

Reported by

Technical Research Centre of Finland
Biologinkuja 5, PO Box 1601
02044 Espoo
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