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

•Increased content of fixed carbon in the solid after sewage sludge pyrolysis.•Higher gas yield from dried and ash-free (daf) char than from sewage sludge (daf).•Average tar yield decreased by 45% when gasifying char instead of sewage sludge.•Average CO yield was 70% higher when gasifying char (daf...

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Bibliographic Details
Published in:Fuel (Guildford) 2014-08, Vol.129, p.147-155
Main Authors: Gil-Lalaguna, N., Sánchez, J.L., Murillo, M.B., Ruiz, V., Gea, G.
Format: Article
Language:English
Subjects:
Air-steam gasification
Applied sciences
Char
Energy
Energy. Thermal use of fuels
Exact sciences and technology
Fast pyrolysis
Fluidized bed
Fuels
Sewage sludge
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Summary:•Increased content of fixed carbon in the solid after sewage sludge pyrolysis.•Higher gas yield from dried and ash-free (daf) char than from sewage sludge (daf).•Average tar yield decreased by 45% when gasifying char instead of sewage sludge.•Average CO yield was 70% higher when gasifying char (daf basis for solids).•Temperature was the most influential factor for most of the studied variables. Air-steam gasification of char derived from fast pyrolysis of sewage sludge has been experimentally evaluated in a fluidized bed as a route towards a full recovery of energy from sewage sludge. The results have been compared with those obtained from the direct gasification of sewage sludge in order to evaluate how the previous pyrolysis stage affects the subsequent gasification process. The fixed carbon content in the solid increased after the pyrolysis stage so that heterogeneous reactions of carbon with steam or CO2 assumed greater importance during char gasification than during sewage sludge gasification. Furthermore, char gasification led to an improvement in the gas yield -calculated on a dry and ash-free basis (daf)- due to the increased concentration of carbon in the organic fraction of the solid after the pyrolysis step, with an increase in the average CO yield of about 70% -in terms of g/kg solid daf-. The reduction in the fraction of carbon which forms tar is another advantage of char gasification over the direct gasification of sewage sludge, with an average decrease of about 45%. Regarding the influence of the operating conditions, the response variables were mainly controlled by the same factors in both processes.
ISSN:0016-2361
1873-7153
DOI:10.1016/j.fuel.2014.03.059