WO2015015042A1

WO2015015042A1 - Method for the combustion of a gas, in fixed bed, with an oxidized solid and associated installation

Info

Publication number: WO2015015042A1
Application number: PCT/ES2014/070633
Authority: WO
Inventors: Juan Carlos ABANADES GARCÍA; Jose Ramón FERNÁNDEZ GARCÍA
Original assignee: Consejo Superior De Investigaciones Científicas (Csic)
Priority date: 2013-08-02
Filing date: 2014-08-01
Publication date: 2015-02-05
Also published as: ES2530546A1; ES2530546B1

Abstract

The present invention pertains to the field of the generation of energy from combustible gases, incorporating the capture of carbon dioxide for use or permanent storage and, specifically relates to cyclical methods of gas combustion with oxidized solids (chemical looping processes), in fixed bed, for solving the problem of controlling temperature in the combustion of gaseous fuels in fixed beds of metal oxides operating at high pressures, and also the associated installation.

Description

PROCEDURE FOR THE COMBUSTION OF A GAS IN FIXED BED WITH A SOLID OXIDIZED AND ASSOCIATED INSTALLATION

D E S C R I P C I O N

OBJECT OF THE INVENTION

The present invention falls within the field of obtaining energy from combustible gases incorporating capture of carbon dioxide for use or permanent storage. In particular, the present invention relates to cyclic processes of combustion of gases with oxidized solids (processes of "chemical looping") in fixed bed to solve the problem of temperature control in the combustion of gaseous fuels in fixed beds of metallic oxides operating at high pressures and temperatures.

BACKGROUND OF THE INVENTION

CO2 is the main gas responsible for climate change and the increase in its concentration in the atmosphere is mainly due to the use of fossil fuels for the generation of energy. The capture of CO2 in industrial processes and power generation, and its subsequent transport and permanent geological storage, is presented as a valid alternative to mitigate the greenhouse effect in the medium term. The main objective of the CO2 capture and storage technologies is to obtain a gaseous current with high CO2 purity and this entails an energy penalty on the energy generation processes without CO2 capture already existing. The interest in developing new technologies of capture of CO2 that can reduce the energy penalty and the cost of the necessary equipment with respect to the known processes is increasing. There is a family of processes, usually called "chemical looping", or combustion with oxygen transporters, which make use of oxidation / reduction reactions of a transition metal (Fe, Ni, Cu, Mn, Co, etc.) or of other redox equilibria (CaS / CaS0 ₄ , etc) to carry out the combustion of a fuel without it having direct contact with the air. These are gas / solid reactions at high temperature where the metal (or CaS) is oxidized with air in the oxidation reactor or "air reactor". This oxidation step is highly exothermic, yielding a metal (or CaS0 ₄₎ oxide and a stream of oxygen - depleted air at high temperature (and in many cases high pressure) that can be used for power generation or other energy applications quality. Next, the solid oxidizes the fuel in the reduction reactor or "fuel reactor", generating a mixture rich in CO2 and water vapor. The oxide reduction is usually endothermic (except in some particular cases such as CuO reduction or when H ₂ and / or CO are used as fuels), so it is necessary to provide external heat to carry it out. The concept of "chemical looping" for the generation of energy from a hydrocarbon is described for example in patents US5447024A and US5509362. A recent review of the state of the art of this family of processes can be found in Adanez et al. (Progress in Energy and Combustion, 38, 215-282, 2012).

The processes of "chemical looping" have been developed to date mainly in configurations composed of two or more interconnected fluidized beds. This arrangement favors the transport of the oxidized solid towards the fuel reactor and its subsequent regeneration in the air reactor. The small size of the particles results in a good gas / solid contact, favoring the kinetics of the redox reactions involved. In addition, the hydrodynamics characteristic of fluidized beds allows good control of temperature in the reactors, an aspect that is fundamental in chemical looping processes, where there are highly exothermic or endothermic reactions. But the use of fluidized beds for chemical looping processes is also associated with well-known drawbacks. One of the most important is the need to operate at high pressure when the fuel is a quality gas fuel (such as natural gas), so that the high energy efficiencies of natural gas combined cycles (NGCC) can be exploited. There are theoretical studies (see for example: Brandvoll and Bolland in Inherent CO2 capture using chemical looping combustion in a natural gas fired power cycle; Journal of Engineering for Gas Turbines and Power, 126, 316-321, 2004) demonstrating the advantages of "chemical looping" systems with interconnected fluidised beds operated at high pressures, upstream of a gas turbine. But at present these systems can be considered highly exotic and of uncertain development, since there are no experiences demonstrating their practical feasibility.

For all this, to carry out the combustion under pressure of a gas by means of "chemical looping" has also been described the possibility of using fixed bed reactors, where the solid in the form of pellets (to avoid attrition and reduce the fall of gas pressure in the bed) remains static inside the reactor. Kumar, Cole and Lyon in "Unmixed reforming: an advanced steam reforming process, Preprints of Symposia, 218 ^th ACS National Meeting, New Orleans, 44, 894-898, 1999" combine the exothermic oxidation reaction of Fe, Cu materials, Ni or Co in a fixed bed with an endothermic reaction of calcination of CaCÜ3 in the same bed. In their work they demonstrate experimentally that the reactions take place in narrow reaction fronts perpendicular to the gas flow, allowing the design of cyclic operations in the same bed of solids, alternating the supply of gaseous reactants and the conditions of pressure and temperature. But it is necessary to emphasize that this process does not solve the problem of CO2 emissions associated with the combustion of a fuel, since the CO2 generated in the decomposition of CaCÜ3 leaves the oxidation reactor very diluted in nitrogen.

The international application WO2006123925A1 describes a combustion device by "chemical looping" in fixed beds on a small scale (for application in greenhouses) where the small size of the beds favors the exchange of heat outside the reactor and the moderation of temperatures in the oxidation stage. But this process would not be applicable in large-scale power generation systems.

Noorman, van Sint Annaland and Kuipers (Packed bed reactor technology for Chemical looping combustion; Ind. Eng. Chem. Res. 46, 4212-4220, 2007), demonstrate the viability of a combustion concept by "chemical looping" on a large scale carried out in a system of parallel fixed beds for the generation of power, with High efficiency of CO2 capture. The rapid kinetics of oxidation and reduction in the conditions used in the process make it possible for the reactions to take place in narrow reaction fronts, which allows to effectively deplete the beds in each stage and in turn generate continuously a hot product gas stream capable of being fed to a gas turbine for power generation. In this process, the solid bed contains a metal that is oxidized with air (21% O2), so that the front of the oxidation reaction proceeds faster than the gas / solid exchange front. In this way, the combustible gas reaches the reaction front heated by the already oxidized solids, which have also been heated before the oxidation reaction front passes. These authors show that to moderate the maximum temperature reached at the oxidation front, the bed material must contain only a small amount of active phase of the metal that is oxidized (normally less than 15% by mass), dispersed in an inert support ( alumina, silica, zirconia, etc.). In this way, the high heat flux evolved by the oxidation of the metal with air is used to raise the temperature of a large mass of inert solids present in the reactor.

Related to the previous one, patent EP2514516A1 describes a way to fill a fixed bed with particles of inert material (silica, glass, ceramic material) mixed with the oxygen carrier to carry out a process of "chemical looping" and avoid excessive peaks of temperature that lead to the sintering of the material. The inert solids have a larger particle diameter than the oxygen carrier solid so that their inclusion in the bed does not excessively increase the pressure drop during the oxidation and reduction operation.

However, the use of fixed beds with high inert content implies a much higher reactor volume to convert the same fuel gas flow fed into the system. Large pressure reactors entail greater investments in equipment, greater wall thicknesses and greater security requirements. In addition, the excess of inert solids as thermal ballast requires the presence of additional stages of heat exchange between beds to condition the temperatures of the gases and solids present in the process.

Document US201092898 describes a third configuration for carrying out a combustion process by chemical looping in fixed beds consisting of a bed of rotating solids in the shape of a circular corona to which two gas streams, one air and another, are fed in different sectors. of fuel, which flow radially from the shaft to the outside of the bed. This configuration presents serious doubts regarding its development on an industrial scale and requires solving important technical challenges, mainly the one related to sealing between its moving parts.

Patent EP2305366B1 discloses a power generation method that also makes use of fixed beds, where the heat evolved in the reduction of CuO with a combustible gas (exothermic reaction) is used for the calcination of CaCÜ3 (endothermic reaction) formed in a previous stage of absorption of a combustion gas or of reforming a carbonaceous fuel. This procedure makes it possible to carry out the regeneration of the CO2 sorbent efficiently, since both reactions occur in the same bed. The control of the temperature in the reaction front during the necessary oxidation of the Cu to CuO (highly exothermic) is solved by recirculating part of the nitrogen obtained at the exit of the stage and cooling it before re-feeding it to the reactor.

Therefore, the combustion processes by "chemical looping" using fixed beds at high pressures and temperatures present a double disadvantage that is not definitively resolved in the state of the art: the difficult temperature control in the reaction fronts when fast and highly effective reactions take place. exothermic and the need for heat exchange to the bed of solids when the initial state of the bed is of solids at low temperature and / or the reaction that takes place in the bed is endothermic. In particular, the high exothermicity of the oxidation reaction makes the control of the temperature during the Operation in a fixed bed is a critical aspect in its design. Oxygen transporting materials lose activity when subjected to high temperatures and their thermal resistance is related to their melting temperature. Most of the materials used in chemical looping processes should not be subjected to temperatures higher than 1200 ° C.

A state-of-the-art technique relevant to the object of this invention is the known way of extracting or supplying large quantities of heat to fixed beds of solids through the use of gaseous streams that pass through the bed in a cyclic manner (heating-cooling), in systems commonly known as "regenerative beds" (see for example Zarrinehkafsh and Sadrameli, "Simulation of fixed bed regenerative heat exchangers for flue gas heat recovery, Applied Thermal Engineering", 24, 373-382, 2004). The efficiency in the heat transfer between gaseous streams and these fixed beds of solids can be very high depending on the operating conditions and characteristics of the gases and the bed, forming clear fronts of heat exchange in a cross section of the bed perpendicular to the gas flow, where there is a jump of gas and solids temperatures in a very short space of bed. These clear heat exchange fronts advance along the bed until all bed solids reach the gas inlet temperature, while the gas leaves the bed at the initial temperature of the solids until the heat exchange front it reaches the exit point of the reactor.

Other techniques of the state of the art relevant to the object of this invention are common to other systems of fixed-bed reactors with alternating pressures or temperatures (PSA and TSA in its acronym in English). In particular, the valve systems for alternating the air and fuel streams in a synchronized manner to different reactors in various operating states are similar to those of other state-of-the-art processes (see for example US2010 / 0196259A1). The management of intermediate stages of "rinsing" between oxidation and reduction, injecting some inert gas as water vapor, can also be considered part of the state of the art.

It is clear from the description of the current state of the art that the processes that until today have been raised to carry out the concept of combustion of a gas with "chemical looping" for power generation are in practice not feasible in the conditions of high energy efficiency (high pressures and temperatures) required for combustion efficient gas The procedure described in this invention tries to solve this problem, revealing a method of operation and a specific sequence of reaction and heat exchange stages with gas recycling that allows to carry out continuous processes of combustion of a gas at high pressure and temperature with integrated CO ₂ separation and power generation in a combined gas cycle. This procedure can provide substantial cost reductions and lower energy efficiency penalties compared to other natural gas combustion processes with C0 ₂ capture. DESCRIPTION OF THE INVENTION

The present invention relates to a process for the combustion of a gas in a fixed bed with an oxidized solid, which takes place in several stages, with the aim of obtaining a separate stream of CO ₂ during the combustion of a gaseous carbonaceous fuel in a of the stages and continuously supply a gas free of CO ₂ at high pressure and at high temperature to a gas turbine to generate power. The process requires several identical fixed-bed adiabatic reactors, but operated simultaneously and synchronously in different ways in the different stages.

The method of invention comprises:

i) a first stage in which the fuel gas is fed to a fixed bed reactor containing initially oxidized solids and with a part of them hot located at the fuel gas inlet at a first temperature T1, where this first stage ends with the completely reduced solids.

The solids at high temperature allow rapid kinetics of the reaction of reduction of the solid with the fuel. Also, in the event that the reaction of reduction is endothermic, the sensible heat contained initially in the oxidized solids at high temperature of the bed is used to maintain a high temperature in the front of reduction reaction, which allows the advance of said reduction reaction front along the bed .

The first stage further comprises feeding a recycle of the gas product of the first stage together with the fuel gas fed during the first stage. The feeding of this recycle of a part of the gases rich in CO ₂ and H ₂ 0 (steam) generated in the first stage, once cooled and conditioned, allows to moderate the temperature jump in the reduction reaction front and to do more fast the advance of the heat exchange front. The recycle flow is designed so that in no case the exchange front reaches the reaction front before the complete reduction of the bed of initially oxidized solids. When the solid reduction reaction is endothermic, this prevents the reaction from being quenched by lack of sensible heat in the bed solids. When the solids reduction reaction is exothermic (for example, when copper oxide is reduced with methane or synthetic gas is used as a fuel), this recycling of CO ₂ and H ₂ 0 also allows temperature control on the front of the plant. reduction reaction. In both cases, the presence of H ₂ 0 in the recycle has as an added advantage to avoid the formation of coke on the reduced solids. The procedure also comprises:

I) a second stage of oxidation of the solids with a mixture of air and recycled nitrogen that enters through the gas outlet of the first stage and lasts until the nitrogen leaving the reactor does it at a maximum temperature; i¡¡) a third stage in which the reactor continues to feed recycled nitrogen or a mixture of air and recycled nitrogen if the oxidation of the solids has not ended and in which the nitrogen stream leaving the reactor at a temperature maximum is fed to the turbine of a combined cycle of gas;

iv) a fourth stage in which recycled nitrogen or a mixture of air and recycled nitrogen continues to be fed to the reactor if the oxidation of the solids has not ended and in which the nitrogen stream leaving said reactor at a maximum temperature cooled slightly to the temperature required for the high temperature solids in the first stage i);

v) a fifth stage in which the high temperature nitrogen stream leaving the fourth stage iv) is fed to the reactor containing cold and completely oxidized solids resulting in the fourth stage iv).

The choice of both the materials to be used in the bed, both the oxygen carrier and its support, as well as its degree of aggregation, usually in the form of pellets to reduce the pressure drop of the gases in their passage through the fixed beds , is considered part of the state of the art of fixed-bed gas combustion technology using oxygen transporters ("chemical looping"). In addition, all the individual components and equipment used in the inventive method can be considered part of the state of the art (compressors of the fuel and air stream, fans or cold recycle gas blowers, heat exchangers, adiabatic reactors of fixed bed ). The control and instrumentation techniques necessary for the sequential and synchronized operation of the different stages are also considered part of the state of the art, through the controlled opening and closing of valves (which can be cooled and / or constructed of materials that withstand high temperatures). and pressures) to alternate the entry and exit of reactants to the reactors in their different stages. It may also be necessary to purge gas stages in the reactors ("rinse") known in the state of the art.

In general there will be many vanantes of the invention procedure that will be evident to a person trained in the state of the art of the combustion of gases by "chemical looping" in fixed bed and other industrial processes that makes use of alternating pressures and / or temperatures in fixed bed reactors (PSA or TSA processes). DESCRIPTION OF THE DRAWINGS

An assembly of drawings is included as an integral part of this invention, in which the following has been represented with an illustrative and non-limiting character:

Figure 1: Shows a schematic of the preferred configuration of the method of the invention, with a snapshot of the reduction reaction front (oxidized solids in the scratched area and reduced solids in white) and of the different temperature zones expected in the reactors, indicating with jumps of temperature the different fronts of gas-solid heat exchange.

Figure 2: Shows a schematic of another preferred configuration of the inventive method following the same notation as in Figure 1. Figure 3. Shows a flowchart of the preferred configuration of the inventive method, using NiO / Ni as an oxygen transporter, according to Example 1.

Figure 4. Shows an example of conceptual design of a system with the preferred configuration of the invention method, using NiO / Ni as an oxygen transporter, according to Example 1, where on the abscissa the number of reactors is indicated .

Figure 5. Shows an example of a conceptual design of a system with the preferred configuration of the invention method, using ilmenite as an oxygen transporter, according to Example 2, where on the abscissa axis the number of reactors is indicated. PREFERRED EMBODIMENT OF THE INVENTION

By way of illustration but not limitation, the main steps of the inventive method are described in more detail below. For simplicity, it has been considered that the gases move in piston flow and that both the gas-solid reactions and the gas-solid heat exchange process occur in clear reaction fronts, as observed in similar reactions and operations described in the state of art. In the preferred embodiment of the method of invention, the first stage of the cycle (100) shown in Figure 1 comprises the oxidation of a gaseous fuel stream (10) in a fixed-bed adiabatic reactor (101) containing initially oxidized solids, with a part of them at high temperature (T1) and with another part of them at low temperature (T4) as a result of a previous oxidation stage of said solids (to be described later). The high temperature solids (T1) are initially located near the fuel gas inlet to the reactor. The reduction of the solid is usually endothermic, so to sustain the advance of the reaction front of solid reduction and gas combustion, it is necessary that the bed initially contains sufficient sensible heat, which is provided by the solids at high temperature ( T1). A compressor (102) compresses and heats the fuel gas (10). Said already compressed and heated gas (11) is passed through a heat exchanger (103) to adjust the temperature to that required to start the first stage (100), obtaining a current (12) at the outlet of the heat exchanger (103). In this preferred embodiment of the invention, the first stage contains a gas recycle (16) of the gas stream to the outlet (14) of the reactor (101), where a blower or fan (104) pumps said gas recycle ( 16), rich in CO2 and H ₂ 0, to join it to the fuel stream (12) at the outlet of the exchanger (103).

The rate of advance of the reduction front (inferred scratched area / non-scratched area in Figure 1) depends on the composition of the fuel gas and the oxidized solid, the stoichiometry of the reaction, the flow of recycle gas (17) after the blower or fan (104), the fraction of metal oxide in the bed and the difference between the molecular weights of the gas and the solid. This reduction front can be advanced at a higher speed than the two gas / solid exchange fronts (the first one marked by the transition between a gas temperature at the reactor inlet (T3) and a gas temperature after the exchange of heat with the solid bed (T2), which will be described next, and the second one marked by the transition between the temperature of the solids at high temperature (T1) and the temperature of the solids at low temperature (T4) in Figure 1). Under these conditions, the inlet stream (13) of the fuel gas enters the reactor at temperature T3 and exchanges heat with the solid bed to settle at the temperature of the gas after heat exchange with the solid bed (T2) resulting from the front. of reduction, which is at a higher temperature.

Then, and for simplification, use will be made of T1, T2, T3 and T4 to describe the embodiment examples more clearly and simply. Upon the exchange of heat between the gas and the solid bed in the fixed-bed adiabatic reactor (101), a first heat exchange front (marked as transition between T3 and T2 in Figure 1) is established which advances to the right as the first stage (100) proceeds. As the reaction front was faster (inferred scratched area / non-scratched area in Figure 1) said front moves at all times downstream of the first exchange front (transition between T3 and T2). In the reaction front there is also a temperature jump (between T1 and T2), whose magnitude is a function, among other factors, of the enthalpy of the reduction reaction. The reduction reaction can be endothermic, where T1> T2 (as in Figure 1) or can be exothermic, where T2> T1, as it would be in the case of CuO reduction with methane or in some cases where the gaseous fuel is H ₂ and / or CO.

The favorable gas-solid heat exchange conditions in fixed beds allow the gas stream at the outlet (14) of the reactor (101) to always do so at the temperature of the solids near the outlet of said reactor, i.e. to T4, until the exchange front marked as transition between T1 and T4 reaches the end of the reactor.

The gas recycle stream (16) can be designed so that the arrival of the exchange front T1 -T4 at the end of the reactor (101) coincides with the arrival at the end of the reactor (101) of the reaction front. In this way, the gas stream at the outlet (14) of the reactor (101) leaves the reactor (101) at all times at T4. On the other hand, the progressive approach between the reaction and exchange front T1 -T4 increases the temperature jump that occurs in the reduction front, that is, the difference between T1 and T2, which makes the initial temperature of the hot part of the bed or temperature of the solids at high temperature (T1) have to be high enough not to leave the bed already reacted too cold, at temperature T2 to start a second oxidation step which will be described below (). In cases where the solid reduction reaction is exothermic (for example when copper oxide is reduced with methane or synthesis gas is used as fuel gas), the proper design of the inert gas recycle, mainly CO2 and H ₂ 0 , allows to separate the advance of the reaction and heat exchange fronts during the operation, achieving a better dissipation of the generated heat of reaction and avoiding the appearance of unwanted hot spots in the reduction front.

The process comprises a second stage (200) where the oxidation of part of the solids contained in a reactor (201), which have been previously reduced in the first stage (100), is carried out. . To achieve said oxidation, the reactor (201) is fed with a mixed stream of air and recycled nitrogen (23) so that it was the outlet of the first stage (100), so that the second stage (200) starts rusting solids at temperature T2. The nitrogen resulting from the exothermic oxidation reaction progressively heats the solids downstream of the reaction front at temperatures above T2, leaving solids at temperature T4 above the reaction front, which is equal to that of the air mixture stream and of recycled nitrogen (23). The oxidation of the bed solids (200) is a highly exothermic reaction and carried out with air could cause the appearance of very hot spots in the reaction front that entail unwanted reactions and the irreversible loss of solid activity. Therefore, the recirculation of part of the nitrogen obtained as a reaction product makes it possible to regulate the maximum temperature in said oxidation front. Thus, the air (20) necessary for the second stage (200), once compressed in a compressor (202), is passed through a heat exchanger (203) to adapt its temperature to that required to carry out the second one. stage (200).

In addition, part of the output stream (24) of the reactor (201), composed mainly of nitrogen, is recirculated, as will be seen below, which it allows decreasing the oxygen content of the stream (23) to the reactor inlet (201). Under these conditions the gas / solid exchange front, marked as transition between T5 (will be described below) and T2 in Figure 1, advances ahead and more quickly than the reaction front, marked as inferred striped area / white zone in the second stage (200). The recycled stream of air and nitrogen (23) comes at the reaction front at the inlet temperature T4 at all times and is suddenly heated up to a maximum temperature (T5) that reaches the mixed stream of air and nitrogen recycled at the front of the reactor. reaction due to the exothermic oxidation reaction. The heat generated in said reaction front is transported by the product gas (mainly nitrogen) from the reaction front and ends by heating the solids downstream of the bed (exchange fronts marked by the temperature jump T5-T2 and T2-T3). By means of a recirculation flow of cold nitrogen (28) and the appropriate choice of temperature T4 of the air mixture stream and recycled nitrogen (23) it is achieved that the maximum temperature (T5) reached in the reaction front is not exceeds the permissible operating limits. On the other hand, at the beginning of the second stage (200), the outlet current (24) of the reactor is at the lower temperature of the gas in the reactor (T3), which corresponds in this case to the initial temperature of the solids located at the exit of the reactor (201) at the beginning of said second stage (200). When the gas / solid heat exchange front, marked as transition between T3 and T2 reaches the end of the reactor, the output stream (24) of the reactor (201) comes out at temperature T2. This outlet temperature will be maintained until the second gas / solid exchange front (marked as transition between T2 to T5) reaches the end of the bed (201). At this time, the second stage is considered finished

(200) .

During the entire second stage (200), the fraction of the recirculation stream of cold nitrogen (28) must have a low temperature, preferably T4, whereby the excess sensible heat of the output stream (24) of the reactor

(201) must be extracted in a heat exchanger (204). This heat can be used in a steam cycle, for the production of power (not shown) in Figure 1 for simplicity). To compensate for the pressure loss through the reactor (201) the cold pressure nitrogen (25) at the outlet of the heat exchanger (204) is fed to a fan or blower (205). The fraction of the nitrogen stream (26) not recirculated to the reactor (201) is fed to a reactor (301) operating in a third stage (300) described below.

The third stage (300) of the preferred embodiment of the method of invention is also shown in Figure 1. In it the further oxidation of a part of the reduced solids in the first stage (100) is carried out with a mixture of air and recycled nitrogen (33), where the nitrogen stream produced (34) at high pressure and at the temperature T5 is fed to the gas turbine (303) of a combined cycle for power generation. This stage starts with a reactor (301) partially oxidized (coming from the second stage 200) and without exchange fronts apart from the temperature jump from T4 to T5, which exists in the oxidation reaction front. The air (30) required for the third stage (300) is compressed in a compressor (202), and passed through a heat exchanger (302) to adjust its temperature to T4. The produced nitrogen stream (34) leaving the bed (301) at temperature T5 and under pressure is expanded and cooled in a combined cycle with a gas turbine (303) and a heat recuperator (304). It is interesting, in order to maximize the electricity generation efficiency in the combined cycle, to increase the temperature of the gas entering the turbine to typical input values of said turbine (1400-1450 ° C). The nature of the solids used in the state of the art as oxygen transporters does not allow such high operating temperatures. In this case a small additional amount of fuel (60) is burned with a fraction of the compressed air (64) for the product obtained (65) to raise the temperature of the gases fed to the turbine (35). This option means that a small part of the carbon contained in the additional fuel is not captured and is emitted as CO2 into the atmosphere in the exhaust gases (38) of the gas turbine (303).

As in the case of the second stage (200), to moderate the temperature T5, the concentration of oxygen in the mixture of air and nitrogen must be reduced recycled (33) entering the reactor (301). To do this, part of the output gas stream (37) of the combined cycle is recycled (61) and recompressed in a compressor (307). An additional heat exchanger (308) is required so that the gas recycle stream after the compressor (307) is fed at temperature T4 to the reactor (301).

In order to minimize the part of the output gas stream of the combined cycle that is recycled (61), there are fed back to the reactor inlet (301) two streams of cold nitrogen under pressure (27.53) surpluses of the rest of the stages that make up the present procedure. A system of valves (305, 306) allows to regulate at the output of the combined cycle the part of the gas stream of the combined cycle that is recycled (61) to maintain constant the mixture of air and recycled nitrogen (33) that enters the reactor (301), modifying the flow rate of the recycle nitrogen stream (63) after the exchanger (308) as a function of the flows of cold nitrogen streams under pressure (27, 53).

In the fourth stage of the cycle (400), the oxidation is carried out in a reactor (401) of the final part of the solids reduced in the first stage (100) and not yet oxidized in the second and third stages (200, 300). For this, a mixture of a stream of compressed and cooled air (42) to T4 and a stream of recycled cold nitrogen (54) is fed to the reactor (401) and a stream (44) of nitrogen is generated at the outlet of the reactor. reactor at temperature T5. At the end of the fourth stage (400) the reactor (401) is completely oxidized and is at the temperature of the inlet gas T4.

The fifth and final stage of the cycle (500) has the purpose of heating the bed of completely oxidized and cold solids, initially to T4, resulting from the end of the fourth stage (400), in order to restart a new cycle with the first stage (100 ). For this, the nitrogen stream (44) leaving the reactor (401) in the fourth stage (400), is cooled slightly in a heat exchanger (502) to the temperature T1 and fed to the reactor (501), which has totally oxidized solids and initially at temperature T4. These solids are heated progressively at temperature T1 as the exchange front advances, marked as transition between T1 and T4. The cold nitrogen stream (51) leaving the reactor (501) at the initial temperature of the bed T4 is fed to a fan or blower (503) to compensate for the pressure drop suffered by the gas as it passes through the reactor (501). Part of the stream (52) at the outlet of the fan or blower (503) is recirculated to the inlet of the reactor (401) as recycled cold nitrogen stream (54), reactor (401) which is operating at that time in the fourth step (400), where the rest of the non-recycled nitrogen (53) to the fourth stage (400), is fed to the reactor (301), which is operating at that time in the third stage (300).

The device described in the process steps of the present invention allows adjusting through the degree of recycling of recycled gases (CO2 and steam in the case of the first stage (100), and nitrogen in the second, third, fourth and fifth stages). (200), (300), (400) and (500) oxidation,), the speed of displacement of the reaction fronts and heat exchange inside the reactors in each of the stages. As will be shown in the embodiments of the invention, the practical implementation of this procedure when using solids with high oxygen transport capacity (such as the NiO / Ni system) requires an installation where at least eight are used adiabatic fixed-bed reactors operated simultaneously but in different phases of the five stages described above.

As will be shown in the embodiments of the invention, the practical realization of this procedure when using solids with low oxygen transport capacity, such as the Fe ₂ T ₅ 0 / FeT 0 3 (ilmenite) system, requires an installation where use is made of at least seven adiabatic fixed-bed reactors operated simultaneously in the various stages of the invention process. When this type of solids with low oxygen transport capacity (or beds with a high proportion of inert solid and a low proportion of oxygen transporting solids) is used, complete oxidation of the solids is achieved during the second stage (200), leaving the third, fourth and fifth stages (300, 400, 500) as stages where only fronts exist of gas-solid heat exchange advancing in a bed of completely oxidized solids (see Figure 2 and Example 2).

EXAMPLES

Example 1: Preferred embodiment of the method of the invention using a solid with a high oxygen transport capacity: NiO / Ni.

In this example, the conceptual design of the cyclic procedure represented in Figure 1 is carried out, which has as its object the combustion of a combustible gas (CH ₄ in this example) with an oxide with a high oxygen transport capacity (NiO in this example) . The example has been proposed for the continuous combustion of 10 kg / s (0.63 kmol / s) of pure CH ₄ (10) in a first stage (100), carried out in a fixed-bed adiabatic reactor (101) to 20 pressure bar, in which is found a solid whose composition in percentage by weight is in this example: 60% NiO and 40% AI2O3. To perform the calculations, gas piston flow and average thermal properties of the solid materials that make up the bed and of the inlet gases and the reaction products have been assumed for all stages. The enthalpy of reduction of NiO with CH ₄ is ΔΗ = 134 kJ / mol CH ₄ . The heat capacity of the gas varies between 2.20 kJ / kg K at the reactor inlet (101) and 1.75 kJ / kg K at the outlet, due to the change in temperature and composition experienced by the gas during the first stage (100) In the same way, the average heat capacity of the solids is initially 0.98 kJ / kg K and once the first stage (100) is 0.91 kJ / kg K. The composition of the fuel gas and the oxidized solid, the stoichiometry of the reaction (1 mole of CH ₄ reduces 4 moles of NiO), the metal oxide fraction in the bed and the difference between the molecular weights of the gas and the solid, make the NiO reduction front with CH ₄ (inferred gray / white in Figure 1) advance to the right 1 .6 times faster than the gas / solid exchange front marked as transition between T3 and T2 in Figure 1. The increase of gaseous moles in the NiO reduction reaction with CH ₄ causes the gas / solid exchange front marked as transition between T1 and T4 to advance 1.46 times faster than that marked as T3-T2 transition and 0.9 times faster than in front of reduction.

The compressor (102) supplies the inlet stream (13) of the fuel gas at 20 bar and 150 ° C (T3 in Figure 1). The recirculation of part of the reactor outlet stream (101) allows to decrease the concentration of CH ₄ in the inlet stream (13) of the fuel gas and increase the mass flow of gas through the bed. In this way, the velocities of the reaction fronts and the gas / solid heat exchange are approximated. The gas recycle (16) is designed so that the first stage (100) can be started with a part of the bed at low temperature (150 ° C, T4 in Figure 1) and that the arrival of the reaction front at the end of the reactor coincides with the arrival of the T1-T4 exchange front and not before. In this way the quenching of the reaction front is prevented and complete reduction of the bed can be carried out. Since the oxidation of CH ₄ with NiO is highly endothermic, it is necessary that the bed of solids contain sufficient amount of sensible heat to sustain the advance of the oxidation front of CH ₄ (and reduction of the solid) until the bed is exhausted. The approach between the T1-T4 exchange front and the reduction front increases the temperature jump (difference between T1 and T2) in the reduction front, so that the initial temperature of the hot part of the bed (T1) has to be high enough so that the reduction front remains at temperatures above 650 ° C at all times and thus guarantee a high reaction speed and a clear reduction front. Nickel is a thermally stable material with a high melting temperature (1453 ° C), which makes it possible to operate with it at very high temperatures without any loss of solid activity or agglomeration. This assumes a bed temperature of oxidized solids at T1 (see Figure 1) of

1200 ° C.

In this example, the recirculation of 52% of the product gas from the reactor (101), gas recycle (16), allows to carry out the complete reduction of the NiO starting from a bed with 45% of it at 1200 ° C (T1). in Figure 1) and 55% at 150 ° C (T4 in Figure 1). The inlet stream (13) of the fuel gas, fed to the reactor (101) at 150 ° C (T3) and 20 bar, contains 2.72 kmol / s with a volume content of: 23% CH ₄ , 26% CO2 and 51% of H ₂ 0. For the operating conditions of this example, the temperature jump resulting from the reduction of NiO with CH ₄ is 550 ° C, so that the temperature T 2 is 650 ° C. Once the reduction of the nickel oxide with the fuel gas has been completed, 55% of the bed remains cold at the temperature T3 of the inlet gas (150 ° C), and the rest remains at a temperature T2 of 650 ° C (T1 - 550 ° C).

In Figure 3 the flow diagram of the process of the invention is represented when NiO / Ni is used as an oxygen carrier. As will be explained later, of the total of reactors constituting said procedure, only a first reactor (1) operates as a first stage (100). During the duration of the same, a valve (615) at the entrance of the first reactor (1) remains open, while another valve (616) at the entrance of the first reactor (1) is closed, allowing to feed continuously the methane required for the first stage (100), avoiding the entry of air. Downstream of the first reactor (1), other valves (617, 623, 624, 625) remain closed, while another valve (618) remains open, as well as other valves (621, 626) that regulate the outlet of steam and CO2 from the system and recycle, respectively. In the second stage (200) a compressor (202) feeds a flow of 1.86 kmol / s of air (20) through the exit point of the first stage (100), where there is higher temperature (650 ° C, T2 in Figure 1), which allows a clear oxidation front to be achieved from the start where the oxygen fed is completely converted and NiO is produced. The oxidation enthalpy of Ni is ΔΗ-452 kJ / mol O2, the composition of the air in the calculations of this example is considered 21% of O2 and 79% of N ₂ (composition by volume), the molecular weight of the air is 29 g / mol and the initial average molecular weight of the solids in the reactor (201) is 73 g / mol. The heat capacity of the gas is maintained at around 1 .13 kJ / kg K and that of the solids is initially 0.91 kJ / kg K and once the second stage (200) is 0.98 kJ / kg K. Due to the high exothermicity of Ni oxidation with air, the temperature on the reaction front would quickly reach values well above 1200 ° C, which would cause the irreversible loss of activity of the nickel material. In order to regulate the maximum temperature on the oxidation front and not to exceed the limit value of 1200 ° C fixed in this example (T5 in Figure 1), 80% of the product gas from the reactor (201) is recirculated, which is a recirculation current of cold nitrogen (28) (5.80 kmol / s), which is mixed with the pressurized air (22), reaching the stream of air and recycled nitrogen

(23) a total flow of 7.67 kmol / s, with a composition by volume of 5% O2 and 95% _N2. Under these conditions, due to the low concentration of oxygen in the gas phase and high mass flow of inert gas, the gas / solid exchange front T5-T2 advances 3.2 times faster than the reaction one. Therefore the gas reaches the reaction front at the inlet temperature (T4 = 150 ° C) and experiences a temperature increase of 1050 ° C due to the exothermic oxidation reaction. The heat generated is transmitted downstream transported by the hot gas leaving the reaction front. Therefore, during the course of this step along the reactor (201) two fronts of gas-solid heat exchange (marked as transition between T3 to T2, and between T2 to T5 in Figure 1) are moved from right to left. ) and a reaction front (inferred gray / white), which advances from behind. During the Ni oxidation reaction the number of moles in the gas phase decreases, but since the concentration of oxygen is very low (5% by volume), the advance speeds of the gas / solid heat exchange fronts are very similar . In the specific case of the present example, the exchange front marked as transition T3-T2 advances 1 .05 times faster than the front marked as transition T2-T5. The output stream (24) of the reactor (201) corresponds to a flow of 7.27 kmol / s, of which 57% (by volume) does so at a temperature of 150 ° C (T3), corresponding to the time in that the first exchange front (T3 to T2) has not yet reached the exit, while the remaining 43% (in volume) does so at 650 ° C (T2), corresponding to the time in which the second exchange front (T2 to T5) is still inside the reactor (201). As the recirculation current of cold nitrogen (28) has to have a temperature of 150 ° C, the excess of sensible heat in the output current

(24) of the reactor is extracted in a heat exchanger (204). To compensate the pressure drop produced in the reactor (201) a blower (205) is introduced to recompress up to 20 bar the recirculation current of cold nitrogen (28). Finally, the non-recirculated gas stream (27), corresponding to a flow of 1.47 kmol / s of nitrogen under pressure, is fed to the inlet of another reactor operating at that moment in a third stage (300) of oxidation that outlined below.

Of the total of reactors constituting the process of the invention (see Figure 3), a second and a third reactor (2, 3) operate as two second stages (200). During the duration of the same, the valves (628) and (639) at the entrance of the second and third reactors (2, 3) remain open, while other valves (627, 638) at the entrance of the second and third reactors ( 2, 3) are closed, which allows feeding the required air to the second and third reactor (2, 3), preventing the entry of methane to them. At the output of both reactors (2, 3), valves (630, 641) remain open and other valves (629, 640) are closed, since in the second stages (200) no product gas is sent to the combined cycle. Likewise, other valves (633) and (644) remain closed because during the second stages (200) no CO2 and water vapor are generated. Another valve (634) is open to send the nitrogen product from the second reactor (2) not recirculated to a fourth reactor (4), which at that time operates as a third stage (300), while another valve (636) remains closed because it is not required to send nitrogen not recirculated to the third reactor (3), since this reactor (3) also operates as a second stage (200). Other valves (635, 645, 646) are closed because no external nitrogen is needed for the second and third reactors (2, 3), while other valves (637, 648) remain open to pass the recycled N ₂ taken to the output of the second and third reactors (2, 3), respectively. Another valve (647) is open to send to the fourth reactor (4), which operates as a third stage (300), the nitrogen produced in the third reactor (3) and not recirculated.

In the third stage (300) the additional oxidation of a part of the solids reduced in the first stage (100) is carried out with air and with recycled N ₂ , where the nitrogen produced in the reactor (301), goes out to high pressure and maximum oxidation temperature of 1200 ° C (T5 in Figure 1) and is fed to a gas turbine (303) of a combined cycle where it expands and cools producing power. For this, a flow of 2.98 kmol / s of compressor air (202) is fed. Since the concentration of oxygen at the entrance of the reactor (300),, must be 5% to not exceed 1200 ° C on the oxidation front (T5 in Figure 1), part of the exhaust gases of the heat recovery (304) of the combined cycle, are recirculated (61) and recompressed up to 20 bar in the compressor (307) and adjust its temperature to 150 ° C in the heat exchanger (308) before being mixed with the compressed air stream (32). The total inflow in the third stage (300) is 12.26 kmol / s and that of the produced nitrogen stream (34) of 1 1.64 kmol / s. In order to minimize the gas recycle stream (61), the cold nitrogen streams at excess pressure of the rest of the steps that make up the present process (27, 53) are also fed back to the reactor inlet (301). In this example, the excess gas stream (27) from the second stage (200) assumes a flow of 1.47 kmol / s, while the excess gas stream (53) from the fifth stage (500) is from 0.87 kmol / s. In this example, a system of valves (305, 306) collected in the state of the art allows to regulate at the outlet of the combined cycle the gas recycle stream (61) to maintain the mixture of air and recycled nitrogen at all times. (33) a constant flow of 12.26 kmol / s with 5% O2.

Of the total of reactors constituting the process of the invention (see Figure 3), a fourth, a fifth and a sixth reactor (4, 5, 6) operate as third stages (300). During the duration of the same, some valves (650, 660, 672) remain open, while other valves (649, 660, 671) are closed, which allows to supply the required air to the fourth, fifth and sixth reactor (4, 5, 6), avoiding the entry of methane to them. At the output of said reactors, other valves (651, 661, 673) remain open and other valves (652, 662, 674) are closed, since in the third stages (300) all the product gas is sent to the combined cycle. Likewise, other valves

(655, 665, 677) remain closed because during the third stages (300) CO2 and water vapor are not generated. Other valves (657, 669, 680) are open to allow the passage of the N ₂ recompressed and fed back to the third stages (300), taken from the output of the combined cycle. Other valves (658, 670, 681) are open to allow the passage of recirculated N ₂ , while other valves (656, 666, 667, 668, 678, 679) remain closed.

It may be interesting, in order to maximize the efficiency of electricity generation in the combined cycle, to increase the temperature of the product gas (34) of the third stage (300), to adapt it to the typical inlet temperatures of the gas turbines ( 1450 ° C). The enthalpy of combustion of CH ₄ with oxygen is 800 kJ / mol CH ₄ . In this case it would be necessary to add to the system an additional quantity of methane under pressure (60) of 0.1 1 kmol / s that would be burned at the entrance of the turbine with an air flow also under pressure of 1.07 kmol / s, taken from the output of the compressor (202), with the stoichiometric oxygen content to raise the resulting current (35) feeding the turbine (303) to 1450 ° C. To carry out this operation, valves (613, 614), represented in Figure 3 and corresponding to a methane and air line, respectively, remain open at all times. The impacts of this addition of fuel gas on the recycle compositions and the inert gases accompanying the oxygen entering the reactor (301) during the third stage (300) are not discussed in this example, as evident.

It is evident that in the third stage (300) is where more useful energy is obtained from the system, because it is the only stage where the hot high pressure nitrogen generated in the reactor (301) expands and cools in a combined cycle. However, in this example, the third stage must be interrupted before completing the oxidation of the bed and thus leave sufficient sensible heat to restart a new cycle with the first stage (100). Otherwise, the bed of solids would be completely oxidized and all the solids in the bed at the feed temperature of the air mixture and recycled nitrogen (33) (150 ° C), which would make it impossible to restart a new cycle with the first stage (100) endothermic.

Therefore, a heat balance to define the heat requirements in the first stage (100), allows to calculate the transition point between the third stage (300) and the fourth stage (400). In the fourth stage (400) of this example, the reactor (401) starts from an initial state with 80% of the oxidized solids. The oxidation of 100% of the solids in the reactor (401) is completed using a mixture of air (1.1 kmol / s, stream 40) and recirculated nitrogen (54) (3.45 kmol / s), obtaining as a product a nitrogen stream (44) of 4.32 kmol / s at 1200 ° C (T5) and leaving a completely oxidized bed at 150 ° C (T4) upstream of the reaction front.

Of the total of reactors represented in Figure 3, only a seventh reactor (7) operates as a fourth stage (400). During the duration of the same, one valve (683) remains open, while another valve (684) is closed, which allows to feed the seventh reactor (7) air and not methane. Another valve (685) remains open, but other valves (684, 688) are closed because N ₂ is not sent to the combined cycle nor is generated in the fourth stage (400) CO2 and water vapor. Another valve (700) remains open to drive all the N ₂ produced in the seventh reactor (7) to the eighth reactor (8), which operates as a fifth stage (500). Therefore, another valve (689) is closed. Other valves (690, 691) remain open to allow the passage of part of the N ₂ obtained in an eighth reactor (8), which is recirculated to the seventh reactor (7) to meet the balance of matter at the entrance of said reactor.

Once the bed has been completely oxidized in the fourth stage (400), which has left the solids at a temperature T4 (150 ° C in this example), the reactor starts a fifth and final stage (500) destined to heat with the nitrogen coming from from the fourth stage (400) a part of the oxidized solids of the reactor (501) up to the temperature of 1200 ° C (T1 in Figure 1). Therefore, the heat exchanger (502) is not necessary for this example, with (44) and (50) being the same gas stream. In this particular example, the temperature T5 has been matched to the temperature T1 set in the first stage (100). Therefore, the stream (50) is introduced into the oxidized reactor (501) and its sensible heat allows 45% of the fixed bed to be heated up to 1200 ° C, a fraction required to carry out a new first reduction stage (100) .

Of the total of reactors represented in Figure 3, only the eighth reactor (8) operates as a fifth stage (500). Being a stage dedicated to the transmission of heat from a hot gas to a bed of solids, the contribution of methane and air is not necessary and therefore, valves (692, 693) are closed. A valve (695) remains open, but the other valves (694, 698) are closed because N ₂ is not sent to the combined cycle nor is CO2 and water vapor generated in the fifth stage (500). Another valve (699) remains open to allow passage of the N ₂ leaving the eighth reactor (8) and being sent to other reactors that are operating in the third or fourth stage (300, 400). Another valve (701) is closed and another valve (702) open, which allows to feed the N ₂ obtained in the seventh reactor (7) to the eighth reactor (8). To synchronize the five previous stages, allowing a change between them at the same time, it is necessary to use a certain number of identical reactors that operate in the different phases or stages of the process. Each of these reactors operates at each stage a certain time before moving on to the next. In this example, a reference time of 5 minutes has been chosen to carry out the first reduction step (100). The choice of lower cycle times leads to lower reactor volumes, but also to a greater frequency of change of each stage and to the need for more reactive materials to guarantee the existence of clear reaction fronts. Considering a feed to the process of 0.63 kmol / s of pure CH ₄ and given the established cycle time, the material balance indicates that a mass of solids (N¡O + AI ₂ O3) is required for the first stage (100). Approximately 100000 kg. Assuming that these solids are in the form of pellets with an average density of 1700 kg / m ³ and an equivalent diameter of 0.01 m, and considering a bed porosity of 0.5, a bed volume of solids of 58 m ^{3 is obtained.} example, by similarity with the commercial reforming reactors, a length of 5 m is adopted for each reactor, which implies a reactor cross-sectional area of approximately 10 m ^2. For this example, this area is compatible with the existence of a single reactor of reduction. In addition, since during the first stage (100), at the entrance of the reactor (101) 2,717 kmol / s are fed (23% by volume of CH ₄ ), the gas is considered to circulate at a maximum surface velocity of 1. 4 m / s, resulting in a pressure drop of only 0.32 bar at the reactor outlet. . Fixed the dimensions of the reactor of the first stage (100), it remains to define the additional number of reactors of the same dimensions that operate in the remaining stages. In the second stage (200), 7.66 kmol / s (5% by volume of O2) are fed to the reactor (201) also for 5 minutes. To limit the pressure drop of this gas to less than 1 bar during this stage, the second and third reactors (2, 3) are used in this example operating simultaneously and under identical conditions as reactor (201) in the second stage (200 ). Under these conditions, the gas passes at a maximum surface velocity of 2.3 m / s, causing a pressure drop of 0.85 bar. In the same way, to the third stage (300) 12.26 kmol / s are fed (5% in volume of O2) during 5 minutes until completing the same one. To limit the pressure drop of the gas to less than 1 bar during this stage, three reactors (4, 5, 6) which operate simultaneously under identical conditions as reactor (301) are considered. Under these conditions, the gas circulates at a maximum surface velocity of 2.4 m / s, causing a pressure drop at the outlet of each reactor of 0.95 bar. To complete the oxidation of the solids in the fourth stage (400), 4.55 kmol / s (5% by volume of O2) are fed for 5 minutes. A single reactor, in this case the seventh (7) is sufficient under the conditions of this example. The gas circulates at a maximum surface velocity of 2.6 m / s, causing a head loss of 1 .10 bar. Finally, the 4.32 kmol / s leaving the fourth stage (400) are fed to a single reactor, in this case the eighth reactor (8), which operates as a reactor (501) in the fifth stage (500). After 5 minutes, 45% of the solids in the bed are heated to maximum temperature (1200 ° C), thus achieving the conditions required to start a new cycle in the first stage (100). In the fifth stage (500) the gas circulates at a maximum superficial velocity of 2.5 m / s, causing a loss of load at the exit of

1 bar In view of these results, with the concrete assumptions for this example, a number of 8 adiabatic reactors of 10 m ² of cross-sectional area and 5 m of length operating in parallel is required (Figure 4): 1 in the first stage of reduction (100), 2 in the second stage of partial oxidation (200), 3 in the third stage of partial oxidation (300), which continuously produce hot gas under pressure for the combined cycle, 1 in the fourth stage (400) finishing its oxidation and 1 in the fifth stage (500) conditioning its temperature to restart the cycle in the first stage (100). The duration of a complete cycle is 40 minutes. Obviously, this example is only one of the many possible ways of executing the inventive method applied to this Ni / NiO reaction system.

Example 2: Preferred embodiment of the method of the invention using a solid with low oxygen transport capacity: Fe ₂ Ti05 / FeT03 (ilmenite). In this example, the conceptual design of the cyclic process represented in Figure 2 is carried out, which has as its object the combustion of a combustible gas (pure CH ₄ in this example) with an oxide with low oxygen transport capacity (ilmenite in this example). ).

The illustrative example has been proposed for the combustion of 10 kg / s (0.63 kmol / s) of CH ₄ (10) in a first stage (100), carried out in a fixed bed adiabatic reactor (101) at 20 bar of pressure, in which a solid is found whose composition as a percentage by weight is: 60% Fe ₂ TiOs and 40% AI ₂ O 3. It has been assumed for all stages that the gas flows in piston flow. The reduction enthalpy of Fe ₂ TiOs to FeT03 with CH ₄ is 106.5 kJ / mol CH ₄ and the oxidation of FeT03 to Fe ₂ TiOs with oxygen is -454.4 kJ / mol O2. Ilmenite is a thermally stable material with a high melting temperature (1365 ° C), which allows it to be operated at very high temperatures without any loss of solid activity or agglomeration. The composition of the fuel gas and the oxidized solid, the stoichiometry of the reaction (1 mole of CH ₄ reduces 4 moles of Fe ₂ T05), the fraction of metal oxide in the bed and the difference between the molecular weights of the gas and of the solid, make the reduction front (inferred striped area / white zone in Figure 2) advance to the right 1 .8 times faster than the gas / solid exchange front (marked as transition between T3 and T2 in Figure 2) ). The increase of gaseous moles in the reduction reaction of Fe ₂ TiOs with CH ₄ makes the gas / solid exchange front marked as transition between T1 and T4 advance 1.3 times faster than the marked as transition T3-T2 and 0.7 times with respect to the reduction front. The value of the reduction enthalpy of Fe ₂ T05 with methane is moderate (ΔΗ = 106.5 kJ / mol CH ₄ ) and the oxygen transport capacity of the ilmenite is low, so that the decrease in temperature in the front of reduction is just 55 ° C. In addition, the Fe ₂ T05 is a material moderately reactive with CH ₄ , which makes the initial temperature of the hot part of the bed (T1 in Figure 2) have to be at least 750 ° C to guarantee a high reaction rate and a clear reduction front. The inlet stream (13) of the fuel gas is fed at 20 bar and 400 ° C (T3 in Figure 2) after passing through the compressor (102) and the heat exchanger (103). The recirculation of 67% of the output stream of the reactor (101) allows to approximate the speeds of the fronts of reduction and heat exchange gas / solid so that the complete reduction of the Fe ₂ TiOs can be carried out starting from a bed with 30% of it at 755 ° C (T1 in Figure 2) and 70% at 400 ° C (T4 in Figure 2). The input stream (13) of the fuel gas contains 4.46 kmol / s with a volume content of: 14% CH, 29% C0 ₂ and 57% H ₂ 0. For the operating conditions of this example, the Temperature jump resulting from the reduction of Fe ₂ T¡Os with CH ₄ is 55 ° C, so the temperature T2 is 700 ° C. Once the reduction of the oxide with the fuel gas has been completed, 51% of the bed remains cold at the temperature T3 of the inlet gas (400 ° C), and the rest remains at a temperature T2 of 700 ° C (T1 -55 ° C). In the second stage (200), a compressor (202) feeds an air flow of 5.95 kmol / s (21) at 20 bar and 400 ° C (T4 in Figure 2) through the exit point of the first stage (100). ), which is 700 ° C (T2), allowing achieved from the outset a front crisp oxidation FeT¡0 ₃ _Fe2 T¡Os where all oxygen fed reaches complete conversion. The moderate exothermicity of the oxidation reaction of ilmenite with air causes the maximum temperature increase in the reaction front with a solid with the composition indicated in this example 2 (60% active phase) to be close to 400 ° C. The recirculation (28) of around 25% of the output stream of the reactor (201), makes it possible to accelerate the advance of the exchange front T5-T4 and bring it closer to the oxidation front, which is ahead moving 5.3 times more fast (see Figure 2). As there are less solids between both fronts to dissipate the heat generated in the oxidation reaction, there is a greater temperature increase in the reaction front (T5-T2), specifically 500 ° C, which means that for the initial conditions of the second stage (200) is reached a maximum temperature (T5) of 1200 ° C (limit value assumed in this example to ensure that the ilmenite does not suffer loss of activity or agglomeration phenomena). The current air mixture recycle nitrogen (23) reactor inlet (201) resulting is 7.39 kmol / s, with a composition by volume of 17% O2 and 83% _N2. During the course of the second stage (200), a swap front T3-T2 moves ahead of the oxidation front (201) and another swap front T5-T4, indicated above, moves behind it. During the oxidation reaction of FeT03, the number of moles in the gas phase decreases when the O2 fed is consumed, so that the T3-T2 exchange front advances slower, specifically for the conditions of this example 0.83 times the speed of the T5-T4 exchange front. The output current (24) of the reactor (201) corresponds to a flow of 6.14 kmol / s, of which 12% (by volume) do so at a temperature of 400 ° C (T3 in Figure 2), which correspond at the time in which the first exchange front T3-T2 has not yet reached the exit, while the remaining 88% does so at 700 ° C (T2), corresponding to the time on the oxidation front still inside the reactor (201). The excess sensible heat is extracted in an exchanger (204) and the resulting current (25) is introduced into a blower (205) to compensate for the pressure drop in the reactor (201). The non-recirculated gas stream (27), corresponding to a flow of 4.71 kmol / s of nitrogen under pressure, is fed to the inlet of another reactor operating at that time in another third stage (300). The second stage (200) ends when the oxidation front reaches the end of the reactor (201), which implies that all the ilmenite present has been oxidized to Fe ₂ T05. At that moment the exchange front T5-T4 is still inside the reactor (201), which makes 53% of the bed remain at 1200 ° C

(maximum oxidation temperature, T5) and the remaining 47% at 400 ° C (inlet gas temperature, T4). In the third stage (300), nitrogen is introduced at 20 bar pressure and at 400 ° C (T4) to extract a large part of the sensible heat retained in the oxidized solids in the second stage (200) and obtain a nitrogen flow at the outlet (34) under pressure and at the maximum oxidation temperature of 1200 ° C (T5 in Figure 2), which is fed to a gas turbine (303) of a combined cycle where it expands and cools producing power. For this, a flow of 10.05 kmol / s of nitrogen (33) is fed, which is a mixture of 4.70 kmol / s of the non recirculated gas stream (27) in the second stage (200) and of 5.35 kmol / s (stream 63), which is a part of the exhaust gases of the heat recovery unit (304) of the combined cycle, which have been recirculated and recompressed in a compressor (307) up to 20 bar. As in example 1, it may be interesting, in order to maximize the electricity generation efficiency in the combined cycle, to increase the temperature of the gas stream (34) from the third stage (300), to adapt it at the typical inlet temperatures of gas turbines (1450 ° C). In this case it would be necessary to add to the system a flow of methane (60) at pressure of 0.1 kmol / s that would burn at the entrance of the turbine with a flow of air also under pressure of 0.92 kmol / s (taken from the outlet of the compressor (202), with the stoichiometric oxygen content to raise the nitrogen flow stream (34) to 1450 ° C.

However, not all the sensible heat retained in the oxidized solids can be used for power generation in the combined cycle, since part of this sensible heat is necessary to restart a new cycle with the first stage (100). A heat balance allows to calculate the transition point between the third stage (300) and the fourth stage (400). In the fourth stage (400) the bed starts with 30% of it at 1200 ° C and the remaining 70% at 400 ° C. The reactor (401) is fed with 13.00 kmol / s of nitrogen (43) at 20 bar and 400 ° C, recycled from the fifth stage (500) and recompressed in the blower (503), obtaining at the exit that same flow (44) at 1200 ° C.

As the temperature required to start from the hot side of the reactor (101) of the first stage (100) is 755 ° C (T1 in Figure 2), the sensible heat excess of the output stream (44) of the fourth stage (400) is extracted in the heat exchanger (502) prior to being fed to the reactor (501) of the fifth stage (500). The fifth stage (500) ends when 30% of the solids that make up the bed have been heated up to 755 ° C, a prerequisite for starting a new first stage (100) of reduction.

To synchronize the five previous stages, allowing a change between them at the same time, it is necessary to use a certain number of identical reactors that operate in the different stages of the process. In this example, a time of 5 minutes has been set to carry out the first reduction stage (100). Considering a feed to the process of 0.63 kmol / s of pure CH4 and given the established cycle time, the material balance indicates that a mass of solids is required for the first stage (100) (Fe ₂ T05 + AI ₂ 03 ) approximately 460000 kg. Assuming that these solids are in the form of pellets with an average density of 1800 kg / m ³ and an equivalent diameter of 0.01 m, and considering a bed porosity of 0.5, a bed volume of solids of 250 m ^{3 is obtained.} example, by similarity with the commercial reforming reactors, a length of 5 m is adopted for each reactor, which implies a reactor cross-sectional area of approximately 50 m ² For this example, it is considered that this area is compatible with the existence of two reduction reactors of 25 m ² each. Since during the first stage (100), at the entrance of the reactor (101) 4.46 kmol / s is fed (14% by volume of CH ₄ ), the gas is considered to circulate at a maximum surface velocity of 0.4 m / s , resulting in a pressure drop of only 0.03 bar at the reactor outlet. Fixed the dimensions of the reactors of the first stage (100), it remains to define the additional number of reactors of the same dimensions that operate in the remaining stages.

In the second stage (200), the reactor (201) is fed 7.39 kmol / s (17% by volume of O2) also for 5 minutes. Under these conditions, two reactors are used operating simultaneously and in identical conditions as reactor (201). The gas passes at a maximum surface velocity of 0.9 m / s, causing a pressure drop of 0.15 bar. In the same way, to the third stage (300) 10.05 kmol / s of nitrogen are fed during 5 minutes until completing it. A single reactor operates in this stage (300). Under the conditions of this example the gas circulates at a maximum surface velocity of 2.4 m / s, causing a pressure drop at the output of the reactor of 0.95 bar. To carry out the fourth stage (400), 13 kmol / s of nitrogen are fed to the reactor (401) for 5 minutes. A single reactor operating as reactor (401) is sufficient under the conditions of this example. The gas circulates at a maximum surface velocity of 3.1 m / s, causing a pressure drop of 1.6 bar. Finally, the 13 kmol / s of nitrogen leaving the fourth stage (400) are fed to a single reactor that operates as a reactor (501) in the fifth stage (500). After 5 minutes, 30% of the solids in the bed are heated to 755 ° C, thus achieving the conditions required to restart a new cycle in the first stage (100). In the fifth stage (500) the gas circulates at a maximum superficial velocity of 2.1 m / s, causing a loss of load at the outlet of 0.75 bar. In view of these results, with the concrete assumptions for this example, a number of 7 adiabatic reactors of 25 m ² of cross-sectional area and 5 m of length operating in parallel is required (Figure 5): 2 in the first stage (100 ), 2 in the second oxidation stage (200), 1 in the third stage (300) of gas / solid heat exchange, which continuously produces hot gas under pressure for the combined cycle, 1 in the fourth stage (400 ) of heat exchange gas / solid and 1 in the fifth stage (500) conditioning its temperature to restart the cycle in the first stage (100). The duration of a complete cycle is 35 minutes. Although the number of reactors is lower (seven) than in Example 1 (eight), the total volume of solids required for the process using ilmenite as an oxygen carrier is almost double that in the case of using nickel oxide.

Claims

R E I V I N D I C A C I O N S

1 .- Procedure for the combustion of a gas in a fixed bed with an oxidized solid comprising: a) a first stage (100) in which the fuel gas (13) is fed to a fixed bed reactor (101) containing solids initially oxidized and with a part of them hot located at the fuel gas inlet (13) at a first temperature (T1), where this first stage (100) ends with the solids completely reduced. characterized in that the first stage (100) further comprises feeding a recycle of the gas (16) product of the first stage (100) together with the fuel gas (13) fed during the first stage.

2 - Process for the combustion of a gas in a fixed bed with an oxidized solid according to claim 1, characterized in that it comprises:

a) a second stage (200) of oxidation of the solids with a mixture of air and recycled nitrogen (23) that enters through the gas outlet of the first stage (100) and lasts until the nitrogen stream (24) that leaves the reactor (201) at a predetermined maximum temperature (T5);

b) a third stage (300) in which recycled nitrogen or a mixture of air and recycled nitrogen (33) is continued to be fed to the reactor (301) if the oxidation of the solids has not ended and in which the nitrogen stream (34) that leaves the reactor at the predetermined maximum temperature (T5) is fed to the turbine of a combined gas cycle; c) a fourth stage (400) in which recycled nitrogen or a mixture of air and recycled nitrogen (43) is continued to be fed to the reactor (401) if the oxidation of the solids has not ended and in which the nitrogen stream (44) that leaves said reactor (401) at the predetermined maximum temperature (T5) cools slightly until the first temperature (T1) required for the high temperature solids in the first stage (100);

d) a fifth stage (500) in which the nitrogen stream (44) is fed at high temperature leaving the fourth stage (400) to the reactor (501) which presents cold and completely oxidized solids (50) after the fourth stage stage (400).

3. Process for the combustion of a gas in a fixed bed with an oxidized solid according to any of the preceding claims, characterized in that the first stage of the cycle (100) comprises a step of compressing and heating a fuel gas (10) by means of a compressor (102), where on said gas already compressed and heated (11) a heat exchange is carried out to adjust the temperature to that required to start the first stage (100), obtaining a current (12) at the outlet of the heat exchanger (103), and where the gas recycle feed (16) of the gas stream to the outlet (14) of the reactor (101) is carried out by pumping said gas recycle (16) to connect it to the fuel stream (12) at the outlet of the exchanger (103). 4. Process for the combustion of a gas in a fixed bed with an oxidized solid according to claim 3, characterized in that in the second stage (200) air is used (20) which is compressed in a compressor (202), for later adjusting its temperature to that required to carry out the second stage (200) by means of a heat exchanger (203)

5. Process for the combustion of a gas in a fixed bed with an oxidized solid according to claim 4, characterized in that part of the output stream (24) of the reactor (201) is recirculated and the mixed stream of air and recycled nitrogen ( 23) arrives at the reaction front at all times at a temperature of the solids at low temperature (T4) in the reactor (201), and is suddenly heated up to the predetermined maximum temperature (T5) which reaches the air-nitrogen mixture stream. recycled on the front of reaction

6. Process for the combustion of a gas in a fixed bed with an oxidized solid according to claim 5, characterized in that the output stream (24) of the reactor (201) is at the lower temperature of the gas in the reactor (T3) , which corresponds in this case to the initial temperature of the solids located at the exit of the reactor (201) at the beginning of said second stage (200). 7. Process for the combustion of a gas in a fixed bed with an oxidized solid according to claim 6, characterized in that the nitrogen stream produced (34) at high pressure and at the maximum pre-established temperature (T5) produced in the third stage (300 ) is fed to the gas turbine (303) of a combined cycle for power generation.

5

8. - Process for the combustion of a gas in a fixed bed with an oxidized solid according to claim 7 characterized in that part (61) of the output nitrogen stream (34) of the combined cycle is recycled and recompressed in a compressor (307 ) and cooled to the temperature of the solids at low or temperature (T4) of the reactor (301).

9. Process for the combustion of a gas in a fixed bed with an oxidized solid according to claim 8, characterized in that two streams of cold nitrogen under pressure (27 and 53) 5 surpluses of the second stage are fed back to the reactor inlet (301). (200) and the fourth stage (400), respectively, that make up the present process.

10. - Process for the combustion of a gas in a fixed bed with an oxidized solid according to claim 9, characterized in that in the fourth stage of cycle 0 (400) the oxidation of the final part of the reduced solids is carried out in the first stage. stage (100) and not yet oxidized in the second and third stages (200, 300) by feeding a mixture of an air stream compressed and cooled (42) and a stream of recycled cold nitrogen (54) to the reactor (401) that remains completely oxidized and at the temperature of solids at low temperature (T4) at the end of said fourth stage (400). 1. Procedure for the combustion of a gas in a fixed bed with an oxidized solid according to claim 10, characterized in that in the fifth stage (500) part of the bed of completely oxidized and cold solids is heated, which initially are at the temperature of solids at low temperature (T4) resulting from the end of the fourth stage (400), in order to then restart a new cycle with the first stage (100).

12. Process for the combustion of a gas in a fixed bed with an oxidized solid according to claim 1 characterized in that the nitrogen stream (44) leaving the reactor (401) in the fourth stage (400), is cooled slightly in a heat exchanger (502) up to the first temperature (T1) required for the solids at high temperature and is fed to the reactor (501), where the solids are progressively heated to the first temperature (T1) required to restart a new cycle with the first stage (100). 13. Process for the combustion of a gas in a fixed bed with an oxidized solid according to claim 12, characterized in that the cold nitrogen stream (51) leaving the reactor (501) at the temperature of solids at low temperature (T4) is it feeds a fan or blower (503) and part of the stream (52) at the outlet of the fan or blower (503) is recirculated towards the reactor inlet (401) as recycled cold nitrogen stream (54), reactor

(401) that is operating at that time in the fourth stage (400), where the rest of the non-recycled nitrogen (53) to the fourth stage (400), is fed to the reactor (301), which is operating at that time in the third stage (300). 14. Installation that uses the method described in any of claims 2-13 characterized in that it comprises at least eight adiabatic fixed-bed reactors operated simultaneously but in different phases of the five stages described above when using solids with high oxygen transport capacity, among which are those of a NiO / Ni system. 15.- Installation that makes use of the procedure described in any of claims 2-13 characterized in that it comprises at least five adiabatic fixed-bed reactors operated simultaneously in the five stages of the procedure described above when using solids with low transport capacity of oxygen, among which is the system Fe ₂ Ti0 ₅ / FeTi0 ₃ (ilmenite).