The invention relates to a process for removing carbon dioxide from hot gas streams. The production of carbon dioxide in energy generation processes is becoming more and more of a problem worldwide. The increased concentration of carbon dioxide in the atmosphere results in reduced heat radiation from the Earth, which causes the temperature on Earth to rise and a number of life-determining processes to be perturbed. A solution for this problem can be sought in limiting the consumption of energy and, in addition, in forms of energy generation which do not yield carbon dioxide, such as combustion of hydrogen, nuclear energy and solar energy. The effect of solutions of this type is limited, certainly in the short and medium term, so that it is, at most, a question of reduced increase of the production of carbon dioxide.

When fossil fuels are gasified, the main products are hydrogen and carbon monoxide, the most desirable component of the two being hydrogen. The shift from carbon monoxide to hydrogen can be effected, for example, by treatment with water (steam). This again produces carbon dioxide which causes the problems outlined above.

Methods for removing carbon dioxide from gas streams do exist, but they have the drawback that they result in a considerable decrease

(approximately 10%) of the efficiency of the energy generation (see

K. Blok and CA. Hendriks, "Voorstellen voor de integrerende studie ter afsluiting van het SOP C0 ₂ " [Proposals for the Integrating Study at the close of the Coherent Research Package C0 ₂ ], State University of

Utrecht, December 199 ; Techniques for Absorption or Fixing of C0 ₂ ,

Technieuws Tokyo, volume 29 (1). May 1991. publication of the Dutch

Ministry of Economic Affairs. Japanese Patent Application 51~**090 discloses a process for removing water and C0 ₂ from hot gases, in particular in the manufacture of steel, by treating the gases with CaO and/or MgO. The used oxide can be regenerated, according to this process, by hot a r being passed through. In US-A-3, 16,808 a process is described for regenerating calcium oxide which has been used for removing C0 ₂ . According to Dutch

Patent Application 82.02061 (= GB-A-2,103.645) , C0 ₂ and H ₂ S can be removed in a wet process from gases such as hydrocarbons by treatment

with a solution of a tertiary amine and a physical absorbent. Japanese Patent Application 61-146344 discloses a process wherein C0 ₂ and water are removed from inert gases by utilising a zeolite bed which is regenerated by heating with combustion gases of a catalysed combustion of propane.

A process has now been found by means of which, using any of the current carbon-containing sources of energy such as bituminous coal, petroleum and natural gas, carbon dioxide can be removed, effectively and regeneratively, from the combustion gases or gasification gases, and can be compressed, and in which the decrease in the efficiency can be kept within acceptable limits (less than approximately 5%) •

The process according to the invention is characterised in that the gas stream is successively: a) used for heating and/or desorbing a solid absorbent loaded with carbon dioxide; b) optionally used for generating energy; c) passed through an absorber in which carbon dioxide from the gas stream is absorbed on the absorbent, and: d) is discharged; and wherein: e) the absorbent from step c) is desorbed in a desorber at least partially and optionally under elevated pressure and is then returned to step c) . According to this process, the gas stream is stripped of carbon dioxide by the absorbent at a temperature at which the absorption heat liberated can be used in the energy-producing process. The loaded absorbent is then regenerated, with the aid of heat present in the energy-producing process, at an elevated temperature, possibly also at an elevated pressure, carbon dioxide dissociating off. The carbon di¬ oxide liberated during the desorption can, if necessary, be compressed further, before it is utilised or discharged. In this process the energy generated in the energy-producing process is thus utilised for the separation, and optionally the compression, of the carbon dioxide. The efficiency of the energy-producing process does not, however, drop as a result, as the heat liberated in the absorption can be utilised if a suitable absorbent is chosen. It is true that there may be drops in the efficiency as a result of pressure differentials across the

absorber and as a result of heat losses from the apparatus used, but these remain within acceptable limits (less than 5%) •

The most important advantage achieved by means of the process according to the invention is the reduced energy loss compared to processes employing wet scrubbing. The absorption step takes place at so high a temperature that the liberated heat can still be used for generating energy.

In the process according to the invention, the absorbent used is preferably calcium oxide and/or magnesium oxide, optionally in the presence of calcium carbonate. More preferably, magnesium oxide is used, especially half-calcined dolomite (CaC0 ₃ .Mg0) .

The heat exchange in step a) preferably takes place at a temperature of 500-900 ^β C, especially 600-800°C. The absorption in step c) preferably takes place at a temperature of 150-500°C, especially 200-350 ^β C. The desorption in step e) preferably proceeds at a temperature of 300-850 ^* C, especially at 400-700 ^β C.

For the purpose of the absorption and/or the desorption, one or more fluidised beds connected in parallel can be used. The heat liberated during the absorption is preferably used for generating energy.

The process according to the invention can be used both for removing carbon dioxide from flue gases and for removing carbon dioxide from fuel gases. In the first instance this involves the treatment of gas streams which are produced in the combustion of fossil fuels (petroleum, natural gas, bituminous coal, lignite and the like) . In the second instance this involves the treatment of gas streams arising in the production of gases intended for combustion, for example in the gasification of powdered coal. The mixture of carbon monoxide and hydrogen formed in the gasification of coal can be shifted, by adding steam, to a mixture containing carbon dioxide instead of carbon monoxide, according to the following reaction equation:

CO + H ₂ 0 - C0 ₂ + H ₂ By removing the carbon dioxide or a large part thereof, a fuel gas mainly consisting of hydrogen is obtained which upon use (combustion) produces only harmless water. The shift from carbon monoxide to carbon dioxide can advantageously take place in the absorber and can be cata¬ lysed by the absorbent itself (such as CaO or MgO) or by a catalyst

added to the absorbent, for example a transition metal such as iron, chromium, copper or zinc, for example on alumina as a support.

In the process according to the invention, it is possible to add, before or after the removal of carbon dioxide, a provision known per se for removing other noxious components in the gas streams, such as sulphur dioxide. Sulphur dioxide can also be removed simultaneously by adding a second absorbent, for example calcium oxide.

The invention also relates to a system in which the process for removing carbon dioxide from gas streams can be carried out. The system comprises a heat exchanger (1) which is provided with a gas supply (4) and gas discharge (5) and a solids supply (6) and solids discharge (7). a desorber (2) which is provided with a discharge (10), a solids supply (7) and solids discharge (6) communicating with the heat exchanger (1) and a solids supply (8) and solids discharge (9). and an absorber (3) which is provided with a gas supply (5) communicating with the heat exchanger and a gas discharge (11) and a solids supply (9) and solids discharge (8) communicating with the desorber. The heat exchanger (1) and the desorber (2) may be integrated in the form of, for example, a tube bundle in the desorber, in which case the solids lines (6) and (7) are omitted. If very hot gases, such as combustion gases, are being treated, a boiler section

(12) may be connected between the heat exchanger (1) and the gas discharge ( ).

The process and the system according to the invention are explained in more detail with reference to the accompanying Figures 1 and 2.

Figure 1 depicts diagrammatically a system according to the invention which is suitable for removing carbon dioxide from flue gases. The flue gases (pressure approximately 1 bar, T = approximately 1,000 ^* C) emanating from the furnace (14) are passed through a heat exchanger via (4), in which the gases are cooled, either by means of a tube bundle in the desorber (2) , or directly by means of one or more fluidised beds connected in parallel (only one bed is drawn (1)), to a temperature of approximately 700°C and absorbent is heated for the purpose of desorption. The flue gases then pass through the (conventional) boiler section (12) , in which cooling of the gas to approximately 30 ^β C takes place and in which steam can be generated. The flue gases are then passed via (5) into the absorber (3)

(comprising one or more beds connected in parallel) , in which the carbon dioxide is largely absorbed from the flue gases at a temperature of 200-350°C (depending on the absorbent) . The flue gases are now discharged via line (11). The absorption heat liberated is rejected by means of a tube bundle (13) to water which is heated in the process and may turn into steam. The absorbent loaded in the absorber (3) is passed via (8) to the desorber (2) (comprising one or more beds connected in parallel) , in which it is heated. C0 ₂ is liberated in the process and is discharged via (10). The unloaded absorbent is returned via (9) to the absorber (3) . It is possible to work under pressure in the desorber so that less work has to be performed for the compression of the carbon dioxide liberated. If magnesium oxide or half-calcined dolomite is used as the absorbent, a pressure of approximately 60 bar can be achieved at a desorption temperature of approximately 650 ^β C. The absorption heat liberated or desorption heat supplied, respectively, is approximately 60 kJ/ ol when magnesium oxide or half-calcined dolomite are used as the absorbent. This means that for a 1,000 MWel power station the heat flow involved in absorption and desorption is approximately 350 MW. Figure 2 diagrammatically depicts a system according to the invention which is suitable for removing carbon dioxide from fuel gases. The fuel gas (pressure about 20-40 bar, T = about 1,000\'C) emanating from the gasifier is passed through heat exchanger (1) in which the gases are cooled by means of a tube bundle in desorber (2) , or directly by means of one or more fluidised beds connected in parallel (only one bed is drawn (1)), to a temperature of approxim¬ ately 700°C and absorbent is heated for the purpose of desorption. The fuel gas is then mixed with steam via supply (16) and passed into the absorber (3) (comprising one or more beds connected in parallel). The absorber here at the same time serves as a reactor for shifting the gas mixture to carbon dioxide/hydrogen. To this end, the bed optionally comprises a catalyst for said shift reaction. At a temperature of 300-350 ^β C (depending on the absorbent) carbon monoxide is converted into carbon dioxide, and the carbon dioxide formed is largely absorbed from the fuel gas. The reaction heat liberated and the absorption heat is rejected by means of a tube bundle (13) to water which is heated in the process and may turn into steam. The fuel gas stripped of carbon dioxide may be passed via (11) to a gas turbine

where it is burned with the formation of flue gas consisting of water vapour. The flue gas (T = approximately 600 ^β C) may be passed through a tube bundle through the desorber (2) where the loaded absorbent is regenerated at a lower temperature. In the process, C0 ₂ is liberated which is discharged via (10). It is possible to work under pressure in the desorber so that less work has to be performed for the compression of the liberated carbon dioxide. The liberated absorption heat or supplied desorption heat, respectively, is approximately 60 kJ/mol when magnesium oxide or half-calcined dolomite are used as the absorbent. This means that for a 600 MWel power station the heat flow involved in absorption and desorption is approximately 200 MW.