Experimental Parameter Study on Synthesis Gas Production by Steam-Oxygen Fluidized Bed Gasification of Sewage Sludge

Max Schmid,Selina Hafner,Günter Scheffknecht

doi:10.3390/app11020579

Abstract

The conversion of biogenic residues to fuels and chemicals via gasification and synthesis processes is a promising pathway to replace fossil carbon. In this study, the focus is set on sewage sludge gasification for syngas production. Experiments were carried out in a 20 kW fuel input bubbling fluidized bed facility with steam and oxygen as gasification agent. In-situ produced sewage sludge ash was used as bed material. The sensitivity of the key operation parameters gasifier temperature, oxygen ratio, steam to carbon ratio, and the space velocity on the syngas composition (H2, CO, CO2, CH4, CxHy, H2S, COS, NH3, and tars) was determined. The results show that the produced syngas has high H2 and CO concentrations of up to 0.37 m3 m−3 and 0.18 m3 m−3, respectively, and is thus suitable for synthesis of fuels and chemicals. By adjusting the steam to carbon ratio, the syngas’ H2 to CO ratio can be purposely tailored by the water gas shift reaction for various synthesis products, e.g., synthetic natural gas (H2/CO = 3) or Fischer–Tropsch products (H2/CO = 2). Also, the composition and yields of fly ash and bed ash are presented. Through the gasification process, the cadmium and mercury contents of the bed ash were drastically reduced. The ash is suitable as secondary raw material for phosphorous or phosphate fertilizer production. Overall, a broad database was generated that can be used for process simulation and process design.

Highlights

The mitigation of climate change requires the substitution of fossil carbon in carbonaceous fuels and goods by renewable carbon sources
In this paper experimental runs investigating the influence of key operation parameters on sewage sludge gasification are presented
The experimental results can be utilized for process design and to set-up and validate a gasifier model, since product gas, char, tar and impurity yields as well as their dependencies on the operation conditions are presented

Summary

Introduction

The mitigation of climate change requires the substitution of fossil carbon in carbonaceous fuels and goods by renewable carbon sources. For instance, biomasses, such as wood or energy crops. Besides the utilization of the energy and carbon contents of sewage sludge, the recovery of phosphorous (e.g., from sewage sludge ash) to close the nutrient cycle and the safe recycling or disposal of hazardous components such as heavy metals are of great importance [5]. A change from the currently mostly practiced sewage sludge disposal methods, e.g., land-“use”, landfill, and co-incineration in coal-fired power plants or cement plants, to mono-treatment recycling methods is required in order to fulfill the goals of a circular economy and climate change mitigation. The utilization of sewage sludge as fuel is economically favorable as high disposal fees are attainable

Methods

Results

Conclusion