Thermo-economic analysis of syngas production from wet digested sewage sludge by gasification process

Digested sewage sludge (digestate) which is a by-product during treatment of municipal wastewater can be regarded as a reliable biological resource for production of bioenergy as a form of renewable energy. In this study, clean syngas production by gasification process of wet digestate is modeled and analyzed for the effluent of anaerobic digesters at Tehran South Wastewater Treatment Plant. The issues of allowable amounts of inlet feedstock moisture content into different gasifying systems and the criteria for selecting a proper type of gasifier are few in the available literature. Thus, thermal gasification technology (dual fluidized-bed (DFB) steam gasifier coupled with a dryer) and emerging supercritical water (SCW) gasification technology are modeled and investigated here. Results show that the hydrogen mole fraction in the syngas produced by dryer-DFB gasifier is about 0.55 which is almost 11% higher than that for SCW gasifier. However, the SCW gasifier has about 8% higher overall system efficiency (22.6%). Economic assessment shows that SCW gasifier has 19% lower levelized cost of clean syngas production (82.5 $/GJ) than dryer-DFB gasifier (102 $/GJ). Therefore, the SCW gasifier is generally preferred based on thermo-economic analysis of clean syngas production from very wet (75%) digestate.