Converting a fuel to combustible gas

A process using an apparatus comprising a unitary vessel comprising a substantially vertical peripheral wall and a roof, the interior of the vessel being divided into at least three horizontally-successive compartments by substantially vertical internal walls extending downwardly from the roof, each compartment having a respective distributor at its base which distributes a respective fluidizing gas from a respective space beneath the distributor into the bottom of each compartment and into a bed of solid particles supported on the top of the respective distributor thereby maintaining said particles in a respective dense phase fluidized bed, each dense phase fluidized bed having a substantially distinct top surface at a respective top level within the respective compartment, the roof over each compartment having a respective outlet for the exit of gas from the compartment, each compartment having an internal wall defining an underflow weir below the said top level and thereby providing communication between a lower region of the downstream end of one compartment on one side of the internal wall and a lower region of the upstream end of an adjacent compartment on the other side of the wall, and in which process, particles circulate from the lower region of the downstream end of the fluidized bed in one compartment to the lower region of the upstream end of the adjacent fluidized bed in the adjacent compartment, and wherein said process comprises:

• a. passing a gas substantially free of diluents and which contains an oxygen-transfer mediator into a first compartment containing a first dense phase fluidized bed containing particles comprising chemically reactive CaSO.sub.4 at a temperature in the range of from 850.degree. to 1150.degree. C.;
• b. passing an oxygen-containing gas into the second compartment containing a second dense phase fluidized bed of particles of chemically reactive CaS at such a rate as to convert some, but not all, of the CaS in the second dense phase fluidized bed to CaSO.sub.4 with substantially no liberation of the sulfur moiety therefrom and such that the gas leaving the second compartment via the respective outlet in the roof contains substantially no sulfur moiety and no more than 5 vol% of molecular oxygen;
• c. passing an oxygen-containing gas into the third compartment containing a third dense phase fluidized bed of particles comprising chemically reactive CaS at a temperature higher than the temperature in the first bed at such a rate that substantially all chemically-reactive CaS is exothermically converted to CaSO.sub.4 with substantially no liberation of sulfur moiety and the gas leaving the third compartment via the respective outlet in the roof is substantially free of sulfur moiety and contains no more than 5 vol% of molecular oxygen, and wherein particles circulate through all the compartments of the apparatus in one sense from the bottom region of one bed in one compartment to the bottom region of the adjacent bed in the adjacent compartment so that particles circulate through all the beds in succession, and wherein a hydrocarbon and/or carbonaceous fuel is passed into the first compartment in contact with the particles of the first dense phase bed to convert the fuel to combustible gas and to reduce chemically reactive CaSO.sub.4 to CaS with substantially no liberation of sulfur moiety as a result of the presence of the oxygen-transfer mediator.