An ash fusion temperature increasing agent principally comprising a silicon and/or titanium compound, such as SiO.sub.2 and/or TiO.sub.2 is mixed with an ash-containing fuel comprising a pumpable liquid hydrocarbonaceous material and/or petroleum coke to produce Mixture A. In one embodiment, the ash fusion temperature increasing agent comprises about 55-100 wt. % of silicon compound selected from the group consisting of fine silica or quartz, volcanic ash, and mixtures thereof; and the remainder, if any, of said ash fusion temperature increasing agent comprises a member of the group consisting of aluminum compound such as Al.sub.2 O.sub.3, magnesium compound such as MgO, and mixtures thereof. Mixture A is reacted with a free-oxygen containing gas in a free-flow refractory lined reaction zone of a partial oxidation gas generator. A hot raw effluent gas stream comprising H.sub.2 +CO along with fly-ash having an increased initial deformation temperature are produced at a higher temperature. Alternatively, Mixture A may be first introduced into a coking zone and converted into petroleum coke in which the ash fusion temperature increasing agent is dispersed throughout. The petroleum coke entrained in a liquid or gaseous carrier is then introduced into the partial oxidation gas generator where a hot raw effluent gas stream comprising H.sub.2 +CO is produced along with fly-ash having an increased ash fusion temperature. The fly-ash is readily separated from the effluent gas stream; and, the gas generator may be operated in the fly-ash mode at a higher temperature, thereby yielding higher carbon conversions while extending the life of the refractory lined reaction zone.