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

Abstract

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/m 3 STP ) were largely reduced with temperature, whereas gas production (0.89-1.32 m 3 STP /kg sewage sludge dry and ash free), carbon yield to gas phase (62-90 wt. %), gasification efficiency (39-66 %), and H 2 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/m 3 STP ) and its H 2 /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 favourable for both. The comparison of experimental and theoretical results highlights that equilibrium was not reached during the experimental runs.

Air-steam gasification of char derived from sewage sludge pyrolysis. Comparison with the gasification of sewage sludge

“…%, thus improving the carbon conversion obtained in the direct gasification of sewage sludge. [23], it can be observed that the production of gas has been reduced by half, mainly due to the higher ash content in char. The production of gas during the pyrolysis stage (around 0.06-0.07 m 3 STP N 2 -free/kg SS) is not high enough to offset the difference in the production of gas from the gasification of both materials.…”

“…Furthermore, as a result of the ANOVA analyses, Table 5 presents a summary of the ANOVA results obtained when sewage sludge was the feedstock for the gasification process [23]. This comparative study is based on a single type of sewage sludge and char.…”

“…These variations outweigh the decrease in the content of light hydrocarbons, thus resulting in a positive effect of the temperature on the gas heating value. The composition of the gasification medium also exerts a significant influence on the gas heating value: when [23]. Temperature plays the most important role in the evolution of the gas heating value when char is gasified (Table 4), while the three studied factors exerted similar relative influences on the gas heating value from sewage sludge gasification (Table 5).…”

“…1 shows a diagram of the laboratory installation. A more detailed description of the plant can be found elsewhere [23].…”

“…The three studied factors, together with their respective ranges of study, were chosen based on our previous work on sewage sludge gasification [23] in order to compare the performance of both processes and evaluate how a previous pyrolysis stage affects the subsequent gasification process. The temperature and the ratio between the flow of oxygen or steam and the feed of biomass are among the most studied factors in the airsteam gasification of biomass [22,25].…”