Sewage sludge and digestate gasification in an atmospheric fluidized bed gasifier

The gasification of sewage sludge (SS) and digestate was investigated in a pilot-scale fluidized bed gasifier with an output of 100 kW(t). The treatment of these by-products is an ongoing challenge for sustainable development. SS and digestate are most commonly used as fertilizers. However, regulations restrict their use, mainly because of the content of heavy metals, pathogens and bacteria. Gasification of these by-products instead of application to agricultural land seems to be more efficient, as the syngas can subsequently be used for combined heat and power (CHP) generation. A series of measurements were carried out to get a better understanding of the gasification process of these fuels and to study the effects of gasifying agent on the syngas composition, particulate matter (PM) and tar. The produced syngas and tar were analyzed using a gas chromatograph with mass spectrometry (GC-MS). The results showed that no ash slagging was observed and therefore it is feasible to operate digestate and SS gasification at 750 degrees C. The lower heating value (LHV) of the syngas from digestate and SS with air as the gasifying agent is comparable, 4.06 MJ center dot Nm(-3) for digestate and 4.11 MJ center dot Nm(-3) for SS. The addition of steam had a positive effect on the amount of tar and the tar dew point, which was below 150 degrees C. Tar reduction in digestate was 5037.3 mg center dot Nm(-3) to 3566.3 mg center dot Nm(-3) and in SS 7447.7 mg center dot Nm(-3) to 3390.3 mg center dot Nm(-3). Furthermore, the concentrations of the individual tar compounds were determined and subsequently divided into tar classes.

Automated summary

The gasification process of sewage sludge and digestate was investigated in a pilot-scale fluidized bed gasifier. These by-products are commonly used as fertilizers, but their usage is restricted due to the presence of heavy metals, pathogens, and bacteria. Gasification of these by-products proved to be more efficient as the produced syngas can be used for combined heat and power generation. Analysis of the syngas and tar revealed that the gasification process can be operated at 750 degrees C without ash slagging, and steam addition improved the tar reduction.