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Cogasification of sewage sludge mixed with coal showed that the amount of sewage sludge supplied to the gasifier, depending on its availability, could vary without affecting the gasifier performance; however, it had an influence on the syngas composition. The use of sewage sludge during coal gasification gave rise to an increasing gas yield and energy conversion, mainly because the gas produced had a greater hydrocarbon content. H2S, HCl, and especially NH3 were also found to increase, due to higher contents of nitrogen in the sewage sludge compared with coal. The rise of both the temperature and the air flow rate resulted in the production of more gas and a lowering of hydrocarbon, char, and tar contents. A decrease in NH3 content was also observed, as the increase of these parameters promoted the destruction of this compound. The rise in the equivalent ratio also led to lower contents of H2S and HCl, probably due to the partial oxidation of these compounds; however, the total amount of these elements released to the gas phase was not considerably affected. On the other hand, H2S formation was favored by the rise in temperature up to 850 °C, while the HCl concentration was not significantly affected. Heavy metals supplied with the fuel were mostly retained in solid residues, with Pb and Hg being the most volatile at 850 °C. However, the leachability of these metals was found to be below the analytical detection levels, and only small quantities of SO4 2- and Cl- were released.

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