Air–steam gasification of sewage sludge in a fluidized bed. Influence of some operating conditions

• Experimental work on air–steam gasification of sewage sludge in a fluidized bed.
• Temperature is the most influential factor for most of the variables analyzed.
• Steam presence favors the gas heating value and the H2/CO in the product gas.
• Differences between kinetic and thermodynamic control in a gasification process.

An experimental work was carried out to investigate the viability of energy recovery from the air–steam gasification of sewage sludge. The relative influence of different factors, as well as the effect of their possible interactions, has been determined by means of analysis of variance. Temperature was found to be the most influential factor for most of the variables analyzed. Solid yield (35–41 wt.%) and tar content (11–45 g/mSTP3) were largely reduced with temperature, whereas gas production (0.89-1.32mSTP3/kg sewage sludge dry and ash free), carbon yield to gas phase (62–90 wt.%), gasification efficiency (39–66%), and H2 and CO yields (20–52 and 137–414 g/kg sewage sludge dry and ash free, respectively) were improved at high temperature. Other important parameters for the end-use of the gas such as its heating value (4.12–6.20 MJ/mSTP3) and its H2/CO molar ratio (1.46–3.25) were greatly influenced by the composition of the gasification medium, since the increase in the steam to oxygen ratio was favorable for both. The comparison of experimental and theoretical results highlights that equilibrium was not reached during the experimental runs.