Currently, the generation of electricity from fossil fuels is fundamentally carried out by means of combustion processes which generate considerable amounts of CO2, the main substance responsible for the climate change. Therefore, different methods for CO2 capture for capturing the CO2 released in these processes have been proposed in the state of the art, among which those based on the carbonation-calcination reaction can be emphasized:CaO+CO2═CaCO3 
U.S. Pat. No. 5,520,894 describes a method for the absorption of CO2 with regenerable solids including, among others, MgO and/or CaO. The regeneration of the carbonate formed is carried out by means of the heat obtained from the combustion gases. This solution is viable for the case of capturing CO2 with MgO, a carbonate being formed since the MgCO3 decomposition temperatures are moderate. However, for the case of CaO as a CO2 sorbent, the proposed system for regeneration by means of CaCO3 calcination is not viable in practice, since the minimum demand of heat in the calciner is very high for CO2 capture efficiencies greater than 70%. [Rodriguez et al., Heat requirements of a CaCO3 calciner when integrated in a CO2 capture system. Chemical Engineering Journal, 138, 1-3, 148-155, 2008]. In other words, at the usual combustion temperatures the minimum temperature necessary for exchanging heat with the CaCO3 calciner is not available in the combustion gases, which calciner, due to the thermodynamic equilibrium, must necessarily be operated at temperatures greater than 900° C. to obtain pure CO2 from the regenerator.
Shimizu et al. (Shimizu et al. A twin fluid-bed reactor for removal of CO2 from combustion processes Trans IChemE, 77, A, 1999) published a method for using CaO as an absorbent of the CO2 coming from the combustion gases, with regeneration by means of continuous CaCO3 calcination, to obtain a concentrated CO2 stream. The proposed system uses two interconnected fluidized beds as a carbonator and calciner. The calciner uses oxy-coal combustion to supply the necessary heat to the endothermic step of CaCO3 calcination to give CaO and CO2. The drawback of this CO2 capture system is that it requires being applied to a high-efficiency thermal power station (generating the combustion gas stream which is fed to the carbonator) to maximally reduce the energy penalty of the expensive air separation plant for producing the highly pure O2 required in the calciner.
WO 03/080223 describes a combustion method at temperatures preferably greater than 1000° C., with integrated CO2 separation by carbonation based on using CaO as a carrier of part of the heat generated in a combustion chamber, which is used in the calciner to maintain the endothermic calcination reaction, and regenerate the CaO, without needing to use an air separation plant as proposed by Shimizu et al. To transfer the heat necessary for reaching calcination temperatures greater than 900° C., using circulating fluidized beds separated by metal walls or preferably interconnected by means of a flow of inert solids in the combustion reaction, which transfer heat from the combustion chamber to the calciner, is proposed. The first proposal has been discarded due to the lack of suitable materials.
WO 2004/097297 describes a combustion method with CO2 capture which includes a pressurized bubbling fluidized bed reactor. Three simultaneous reactions are carried out in said reactor: combustion of the carbon material fed to the reactor, “in situ” CO2 capture for capturing the CO2 generated during the combustion and “in situ” SO2 capture for capturing the SO2 generated during the combustion. The last two reactions are possible as a result of the fact that the reactor is continuously fed with a CaO stream, obtained by the combustion of coal under oxy-combustion conditions. Therefore, this method has the drawback of requiring an air separation plant for producing O2. The fuel of the invention is preferably petroleum coke or any other solid fuel with low ash content, to prevent problems with the high regeneration temperatures (>1,000° C.) in the pressurized fluidized bed. This patent describes high-pressure combustion systems due to the fact that they are the only ones in which it is possible to combine high combustion rates of solid fuels with low reactivity, such as petroleum coke, and high sulfation retention and carbonation efficiencies.
Abanades et al. (Abanades, J. C.; et al. Fluidized Bed Combustion Systems Integrating CO2 Capture with CaO. Environ. Sci. Tech. 2005, 39(8), 2861; and Abanades, J. C., et al. In-situ capture of CO2 in a fluidized bed combustor. 17th Int. Conf. on Fluidized Bed Combustion, FL-USA, ASME. May 2003. paper 10) describe a method similar to that of application WO 2004/097297, but at atmospheric pressure and only valid for fuels with a high reactivity and very low sulfur content, such as biomass. The method consists of biomass combustion and simultaneous “in situ” CO2 capture for capturing the CO2 formed by carbonation working at about 700° C. However, it has not been possible to demonstrate the viability of the method (C. Salvador, et al. Capture of CO2 with CaO in a pilot fluidized bed carbonator. Experimental results and reactor model. 7th Congress on Greenhouse Gas Control Technologies-GHGT-7; Vancouver, Canada; September 2004) mainly because it is carried out in a bubbling fluidized bed, with a large segregation in the bed of the combustion reaction which prevents the necessary contact between the CO2 and the CaO absorbent particles.
Therefore, and in view of the foregoing, there is still a need in the state of the art for providing an alternative method and device for “in situ” combustion and carbonation which at least partly overcome the mentioned problems of the state of the art and are more efficient from an energy and economic point of view, and are therefore interesting for their scaling to industrial level.