Use of sewage sludge combustion ash and gasification ash for high-temperature desulphurization of different gas streams

Abstract

Due to its metal content, sewage sludge ash appears as a potential sorbent material for H2S removal at high temperature. The desulphurization ability of the solid by-products of combustion and gasification of sewage sludge has been evaluated in this work. Ash characterization results revealed that metal fraction in sewage sludge did not remained completely inert during combustion and gasification processes. Iron content was lower in the gasification ash and X-ray patterns showed different crystalline phases in the solids: Fe2O3 in the combustion ash and Fe3O4 in the gasification ash. These differences resulted in a lower sulphur capture capacity of the gasification ash. Desulphurization tests were carried out in a lab-scale fixed bed reactor operating at 600–800°C. Different gases containing 5000ppmv H2S (H2S/N2 mixture and synthetic gasification gas) were used. The H2S breakthrough curves were negatively affected by the reducing atmosphere created by the gasification gas and by the presence of steam in the reaction medium. However, H2S breakthrough curves alone do not provide enough information to evaluate the sulphur capture capacity of the sorbent materials. Ultimate analyses of the spent solid samples showed that the total amount of H2S removed from the gas was only partially captured in the ash. Thermodynamic data pointed to a significant fraction of sulphur forming part of other gases, such as SO2. In the best operating conditions, an outlet gas with less than 100ppmv H2S was obtained during 300min, thus resulting in a sulphur loading of 63mgSgash−1. This experimental sulphur content was 39% lower than the maximum value predicted by equilibrium simulations.