The subject of the invention is a heat generator able to burn high-sulfur fuels and which, in the form of a compact assembly, enables the production of useful heat to be separated from the desulfurization of the flue gases. 
Strict regulations in protected zones govern emissions of sulfur oxides in the gaseous effluents from heat generators and forbid the use of high-sulfur fuels which, however, have definite economic advantages: this is the case for certain coals related to lignites, and for oil residues from refining processes. 
Aside from downstream processes for treating fumes which generally apply to very-high-power facilities, in certain fossil-fuel-burning thermal units desulfurization is accomplished in the course of combustion by directly injecting a calcium-based absorbent (limestone, lime, dolomite, etc.)  into the hearth. 
This in situ desulfurizing process is considered principally for solid fuels, and its efficiency (between 30 and 60%) is a great contributor to the temperature distribution in the combustion chamber, while requiring substantial lime consumption (Ca/S ratio=Calcium/Sulfur ratio on the order of 3 to 4 moles/mole). 
A different method consists of using so-called "dry ash" fluidized-bed boilers which operate at about 800.degree.-900.degree.  C. and in which fuel and absorbent are placed in intimate contact. 
In particular, within a "fast" or "circulating" fluidized bed having systematic recirculation of the solid particles, a very high rate of desulfurization can be obtained (85-90%) with relatively modest Ca/S ratios (1.5 to 2 moles/mole). 
However, the self-desulfurizing circulating-bed heat generator poses a number of technological problems. 
In particular, its reliability is closely linked to the strength of heat-exchanging tube bundles and to abrasion and corrosion phenomena. 
The device proposed has the essential advantage of being reliable since it can be implemented by using tested techniques.  Moreover, the generator according to the present invention is compact and takes up very little space. 
The basic idea is based on the combination of three principal elements arranged such that the exchange surfaces are protected from the rapid flow of solid particles which are often the cause of rapid deterioration of these surfaces. 
Thus, the generator proposed consists essentially of a hearth or combustion chamber, preferably with cold walls, a recovery boiler capturing the sensible heat of the flue gases, and an intermediate circulating bed with an insignificant internal exchange surface, whose function is to desulfurize the gases passing between the hearth upstream and the exchanger downstream. 
"Cold wall" is understood herein to mean that the wall has means for extracting heat. 
In general, the present invention relates to a great generator with a combustion chamber, a circulating bed, and a recovery boiler.  According to the present invention, the circulating bed and combustion chamber have a common wall. 
This common wall may have at least one orifice for feeding into the circulating bed a stream of primary fluid and/or at least one orifice for feeding into the circulating bed a stream of secondary fluid. 
This common wall may be a cold wall.  Likewise, other walls of the combustion chamber may be cold walls. 
The various cold walls may have provision for circulation of a fluid. 
According to the present invention, the circulating bed and the recovery boiler may have a common wall. 
Likewise, the combustion chamber and the recovery boiler may have a common wall. 
The walls of the circulating bed may have a coating made of a heat-insulating material. 
The desulfurizing circulating bed whose entrained solid material is essentially, the absorbent, uses the hot gases coming from the hearth as a working fluid. 
Since the temperature of the gases may vary with the generator load, the bed may be maintained at the optimum desulfurizing temperature (800.degree.-900.degree.  C.) by injecting a makeup fuel into the reactor, whereby combustion takes place with the excess oxygen from the upstream hearth, possibly with additional fresh fuel. 
The compactness of the generator according to the invention is achieved by original spatial distribution of the three main elements disposed vertically.  This compactness facilitates its prefabrication.  The present invention will be better understood and its advantages will emerge more clearly from the description hereinbelow of particular non-limitative examples illustrated by the attached figures.