A process for treating solid particles (66) of a raw carbonaceous fuel (62) such as coal having an original sulfur content comprises producing (at 68) a slurry (74) of a quantity of the raw fuel particles and a liquid medium (70) comprising water, at least one alkali metal compound including a substantial amount of sodium or potassium sulfide or polysulfide or a combination thereof, and a catalytic agent (72) comprising calcium or magnesium oxide or carbonate, or dolomite. The slurry is subjected (in 78) for an effective period of time to elevated temperature and pressure effective with the alkali metal compounds and water to cause the medium to penetrate the microscopic structure of the particles and to chemically and physically incorporate a substantial amount of the catalytic agent into the structure. The readily separable medium is separated (at 84) from the fuel particles and the particles are washed (at 84,106) to produce a particulate fuel product (88,110) containing the incorporated catalytic agent. A substantial portion (90,94) of the separated medium is added (at 70) to the slurry produced as above. A sufficient quantity of the catalytic agent is added (at 72) to the medium or the slurry to replace the catalytic agent removed from the slurry with the particulate fuel product. More raw fuel particles (66) are added to the medium or the slurry. The foregoing steps are carried out continually with a multiplicity of new additions of the raw fuel particles and with a multiplicity of reuses of the separated medium to produce fuel product particles (88,110) containing a quantity of sulfur that is not less than the original sulfur content, in addition to the catalytic agent.