Abstract
The gasification and torrefaction of sewage sludge have the potential to make the thermal utilization of sewage sludge fully sustainable, thus limiting the use of expensive fossil fuels in the process. This includes sustainability in terms of electricity consumption. Although a great deal of work has been performed so far regarding the gasification of sewage sludge and some investigations have been performed in the area of its torrefaction, there is still a gap in terms of the influence of the torrefaction of the sewage sludge on its subsequent gasification. This study presents the results from the torrefaction tests, performed on a pilot scale reactor, as well as two consecutive steam gasification tests, performed in an allothermal fixed bed gasifier, in order to determine if torrefaction can be deemed as a primary method of the reduction of tar content for the producer gas, from the aforementioned gasification process. A comparative analysis is performed based on the results obtained during both tests, with special emphasis on the concentrations of condensable compounds (tars). The obtained results show that the torrefaction of sewage sludge, performed prior to gasification, can indeed have a positive influence on the gas quality. This is beneficial especially in terms of the content of heavy tars with melting points above 40 °C.
Highlights
Torrefaction led to the improvement of the calorific value of the producer gas and its quality
The severity of the torrefaction was high, which resulted in a good quality of the torgas and a relatively high amount of the chemical energy going to torgas
Even though it could potentially lead to the thermal runaway of the installation, this solution seems to be beneficial, as torgas could potentially be mixed with the producer gas in the gasifier
Summary
Sewage sludge is a residue of wastewater processing, is biologically active and consists of water, organic matter, including dead and alive pathogens, as well as organic and inorganic contaminants such as polycyclic aromatic hydrocarbons (PAHs) and heavy metals [1,2,3]. Utilization methods leading to stabilization and safe recycling are gradually replacing storage, landfilling and land-spreading. In the EU countries, novel methods are becoming increasingly popular [4], due to both environmental and economic reasons, as landfilling is deemed to be the most costly way to dispose of the sewage sludge [5]. Land-spreading is typically the most economical way to dispose of sewage sludge [5]. The cost of this can be subject to significant changes, depending on the distance between the sewage
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