Reactor for Heating a Gas and Uses Thereof

Abstract

This invention discloses a reactor and methods for heating of a gas as it reacts with a solid. The reactor contains gas conducts that are empty of solids and that cross through a region packed with solids. The wall of the gas conducts has orifices to make it permeable but not selective to gases, while effectively separating the solids from the gas. In the reactor, the heat source to heat up the gas is generated by the exothermic reaction of the solids with one active component of the gas. The region packed with the reacting solids is at temperatures ranging from 500° C. to 1500° C., to promote the heat transfer towards the gas and the high reactivity of the solids with the active components of the gas, that is forced to diffuse from the conduct through the orifices of the conduct wall.

Claims

1. Reactor for heating a gas by reaction with solids, said reactor comprising gas conducts crossing through a region packed with the solids, said gas conducts having walls that are permeable but not selective to gases, characterized in that: i. the walls of said gas conducts present orifices with a length between 1 and 3 mm, while the void fraction of the walls range from 0.1 to 0.5; ii. the ratio between the length of said gas conducts and their effective diameter ranges from 200 to 1000; iii. the temperature of the region packed with the solids is between 500-1500° C.

2. Reactor for heating a gas according to claim 1, wherein the gas is air, and the reactive component of the gas is oxygen.

3. Reactor for heating a gas according to anyone of claim 1 or 2, wherein the packed solids form a bed which comprises a reactive component selected from a carbonaceous fuel, a transition metal, a partially oxidized transition metal or calcium sulfide.

4. Reactor for heating a gas according to claim 3, wherein the transition metal is selected from Fe, Ni, Cu, Mn or Co.

5. Reactor for heating a gas according to anyone of claim 3 or 4, wherein the packed solids comprise an additional solid material with a high thermal conductivity.

6. Reactor for heating a gas according to claim 5, wherein the additional solid material is selected from metal alloys and silicon carbide.

7. Reactor for heating a gas according to anyone of claim 1 or 6, wherein the wall of the gas conducts is a perforated high temperature metal alloy.

8. Reactor for heating a gas according to anyone of claims 1 to 7 wherein the wall of the gas conducts contains fins penetrating the region packed with the solids.

9. Reactor for heating a gas according to anyone of claims 1 to 8, wherein the ratio formed by the product of the length of said gas conducts and the void fraction of the conduct walls, divided by the product of the effective diameter of said gas conducts and the length of the orifices in the conduct walls, is comprised between 50000 and 300000 m.sup.−1.

10. Reactor for heating a gas according to claim 9, wherein the ratio is comprised between 100000 and 200000 m.sup.−1.

11. Reactor for heating a gas according to anyone of claims 1 to 10 wherein the gas comprises gas active components that are forced to diffuse from the gas conducts through the orifices of the gas conduct walls.

12- Use of the reactor for heating a gas as defined in claims 1 to 11, wherein the reactor is the gas heater part of a Brayton cycle for power generation, with the air inlet at a pressure comprised between 10 and 30 bar and temperatures comprised between 500° C. and 700° C., and gas exit temperatures comprised between 900° C. and 1500° C. and a pressure drop below 10% of the inlet.

13. Use of the reactor according to claim 12 which further comprises the following cyclic steps: charging of a batch of reacting solids preheating of the reacting solids cooling of the reaction products and discharge of the solid reaction products. wherein the heating of the gas by reaction with solids takes place after the preheating step and before the cooling step.

14. Use of the reactor according to claim 13 characterised in that it comprises an additional regeneration step of the solids within the reactor by using a reducing fuel gas selected from hydrogen or natural gas.

15. Use of the reactor according to claim 14 wherein the reactor is the air reactor of a chemical looping combustion process for power generation characterised in that it comprises a series of cyclic steps where the oxidation of the reacting solids by air is followed by a regeneration of the reacting solids by their reaction with a fuel gas to produce a concentrated stream of CO.sub.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0064] A set of drawings is attached wherein, with illustrative and non-limiting character, the following has been represented:

[0065] FIG. 1: shows a general scheme of the reactor of this invention comprising empty gas conducts manufactured with a material permeable to gases, and regions occupied by a bed of packed solid material with a high affinity to a reactive component in the gas, all enclosed in an housing of insulating material and non-porous wall. In the bottom-right of the figure, a detail of an orifice in the gas conduct wall, and the schematics of the gas diffusion mechanism in the wall orifice, is provided.

[0066] FIG. 2: shows a general scheme of the reactor integrated in the gas heating part of a Brayton cycle

[0067] FIG. 3: shows a general scheme of a chemical looping system using at least two reactors of this invention (top: oxidising the oxygen carrier with gas; bottom: reducing the oxygen carrier with a fuel gas).

PREFERRED EMBODIMENTS OF THE INVENTION

[0068] The main components of the reactor are represented in FIG. 1. The gas heater device (100) is fed by a flow of gas (1) that leaves the reactor in (2) at high temperature. The inlet gas (1) is first distributed in a multiplicity of gas conducts (3). The conduct walls remain at high temperature because of an exothermic gas solid reaction taking place in the reactor. There is a layer of insulating material (4) to prevent heat losses and a non permeable wall (5) that can withstand high pressures and that is protected from the high temperatures in the reactor by (4).

[0069] The conducts have walls with orifices (6) separating the gas flow from a bed of solids (7) with solid particles (8) with a high affinity to react at high temperature with an active component in the gas. As a result, a certain flow (9) of the active gas component diffuses through the pores of the conduct wall (6) to react with the solids (8). As a result of the gas solid-reaction, removing the active gas component from the gas phase, and the diffusion control of the flux of active gas component by the orifices, such active gas component is depleted (10) in the void regions between the particles that make the bed of solids (7). The gas (1) is heated in contact with the conduct walls with the heat (11) originally generated during the reaction between the active solids and the active gas component. Such heat must flow through the bed of solids (7) in the vicinity of the conduct walls to the wall. To facilitate the transfer of such heat towards the conduct walls, the bed of solids (7) may contain inert particles or filaments (12) of high thermal conductivity or be penetrated by fins (13) connected to the conduct wall.

[0070] A particular application of the preferred embodiment of FIG. 2 is the oxidation in gas of oxygen carriers used in chemical looping combustion, CLC, and reforming applications [a family of metals (Ni, Cu, Mn) or partially oxidised metals (FeO, Fe3O4) or sulphides (like CaS)]. An important advantage of these materials respect to carbonaceous fuels is that their oxidation step does not emit CO.sub.2. Example 1 illustrates a design of an emission-free intermittent power generation device using as a heat source the oxidation of an initial batch of CaS to CaSO.sub.4. The oxidation of CaS in the reactor of this invention is integrated in a Brayton cycle (for simplicity, in FIG. 2, only the heater part of the cycle is shown). CaS solids must be preloaded through a suitable port (10) and preheated to temperatures over the ignition temperatures, before the oxydation process starts. Air (1) goes first to a compressor (101) and enters the reactor (100) at 400-700° C. For simplicity, preheating stage that may be needed between (101) and (100) are omitted in the figure. During the time period required for oxidation of the solids in (100), the heated air depleted in oxygen (2) is expanded in the gas turbine (102). After the oxidation step is completed, the resulting CaSO.sub.4 can be reduced again to CaS within the reactor, for example by feeding a flow of renewable hydrogen. If needed, it can also be extracted from the reactor (11) and reduced to its initial form (CaS in the Example 1) in a separate reactor, using a suitable fuel gas and suitable methods described in the state of the art processes of chemical looping combustion and reforming. Example 1 illustrates that the system of FIG. 2 can find applications as an intermittent power generation device, for example to back up large scale power generation systems. It could also be applied at much smaller scales, for disperse, smaller applications, including transport, as illustrated in Example 2.

[0071] In another preferred embodiments of the invention, represented in FIG. 3, the reduction step of the oxygen carrier is carried out in the same reactor (100), arranged to be able to operate under reducing conditions by receiving a feed of a gas fuel (13) to reduce the solids back to their original state and generate a gas stream (14) that mainly contains CO.sub.2 and H.sub.2O and that can be used or stored to avoid emission of CO.sub.2 (for example by geologically storing the CO.sub.2). Although the reduction strategy of the solids is not part of the present invention, it is interesting to note that the reactor can also operate as a “fuel reactor” to undertake these reduction reactions, that are usually endothermic (except in the case of the CuO to Cu redox system), typically slower in what refers to reaction rates, and requiring much lower gas flow rates (of fuel gas) than in the gas reactor. The reactor can be arranged to use the porous conduct walls as a gas distributor of the fuel gas, in a way that the full gas enters into contact with the solids. This can be achieved in many different combinations (see bottom of FIG. 3) using suitable plugs for of the gas conducts to facilitate a flow of fuel gas with a component perpendicular to the gas conducts and in intimate direct contact with the solids. In general, since the bed of solids and the gas conduct walls are permeable to gases, a wide diversity of mechanical solutions, as described in the state of the art of packed bed reactors, can be implemented for fuel gas distribution (13) through the bed of reacting solids, while evacuating the reaction products from the reactor (14). Example 3 illustrates one of such possible systems, using as reactive solid a batch of oxygen carrier materials containing Ni (7), that oxidises at high temperatures to NiO with the O2 contained in the air (1).

[0072] Another particular application of the preferred embodiment of FIG. 2 is when the solid with high affinity to oxygen is a carbonaceous fuel. For the combustion in the reactor of this invention of a solid carbonaceous fuel using a Brayton cycle (for simplicity, in FIG. 2, only the heater part of the cycle is shown), the fuel must be preloaded through a suitable port (10) and preheated to temperatures over the ignition temperatures, before the combustion process starts. The solids residues resulting after combustion can be discharged from the reactor through a second port (11). The fuel is preferably char, pet coke or any other solid fuel with low ash and volatile content. Low volatile content fuel is preferred in the operation of the reactor in order to minimise the intensity of hot spots when volatiles diffuse from the bed of solids and burn in the gas conducts. Example 4 illustrates that the system of FIG. 2 can find applications as an intermittent power generation device, for example to back up large scale power generation systems. It could also be applied at much smaller scales, for disperse, smaller applications, including transport, as illustrated in Example 2 for the CaS oxidation reaction.

[0073] In another preferred embodiments of the invention (see FIG. 3), the solid with affinity to react with a gas is CaO and the reactive gas is the CO.sub.2, contained in a combustion flue gas or in a synthetic mixture of air and CO.sub.2 from a reservoir or CO.sub.2 storage site that is part of an energy storage system. The carbonation reaction step of the CaO particles is carried out in the reactor (100), to heat up the gas stream containing CO.sub.2 (1) previously compressed in (101) and expand the gas with reduced content in CO.sub.2 (2) in (102). The CaO particles (reference 7 in FIG. 1) are located in the space between the conducts (reference 3 in FIG. 1). The heat generated in the carbonation reaction of CaO with the CO.sub.2 (reference 5 in FIG. 1) through the conduct walls (4) heats the gas (1) by heat transfer from the wall (such heat flow is represented by the arrow (6) in FIG. 1). In general, the reactor and the use thereof as a heater using the exothermic carbonation of CaO with CO.sub.2 does not differ from the reactor and and the use thereof described above for the case of oxidation reactions of a solid. The skilled in the art will be able to generate design rules similar to those applied for the examples involving solid oxidation reactions. The reactor and the uses thereof can be part of thermochemical energy storage systems using CaO/CaCO3 chemical loop, by regenerating the CaO (decomposing CaCO3 into CaO and CO2) in a different step using renewable energy or renewable H2.

[0074] In another preferred embodiments of the invention, the solid with affinity to react with a gas is CaO and the gas is the H.sub.2O(v), contained in a combustion flue gas or in any other synthetic mixture with sufficient fraction of H.sub.2O vapour. The skilled in the art will be able to generate design rules similar to those applied for the examples involving CaO/CaCO3 reactions. The reactor and the use thereof can be part of thermochemical energy storage systems using CaO/Ca(OH).sub.2 chemical loop, by regenerating the CaO (decomposing Ca(OH).sub.2 into CaO and H2O) in a different step using renewable energy or renewable H2.

[0075] In general, the use of the reactor of this invention can be implemented in a wide variety of systems described in the state of the art to generate thermal power from the reaction of a solid with a gas. In particular, the reactor is most suitable for the combustion of solid fuels at high pressure and for chemical looping combustion systems, chemical looping reforming systems or combinations thereof at high pressure as well as for the energy discharge part in thermochemical energy storage systems operating at high pressures and involving reversible gas-solid reactions: oxidation/reduction, carbonation/calcination, hydration/dehydration etc. For those skilled in the art, other devices, advantages and characteristics of the invention may occur in view of the description or uses of the invention. Therefore, the following examples involving oxidation reactions only are provided for illustrative purposes only, and are not intended to limit the scope of this invention.

EXAMPLES

[0076] The particular examples of design of the reactors described in the following sections have been solved for certain solid oxidation reactions only, using a range of material properties of solids and gases that are available in the state of the art and in textbooks such as “Perry's Chemical Engineering Handbook, sixth edition”. Furthermore, a set of assumptions are adopted for simplicity, that are summarised below. These can be refined in future applications, but do not alter the object of the invention: [0077] It is assumed that the oxidation reactions of the solids are extremely fast, so that the oxygen partial pressure in the vicinity of the reacting surfaces is close to zero. Furthermore, there is sufficient porosity between and within the solids particles as to allow free diffusion of oxygen in the gas phase. [0078] The controlling step of the overall process of solid oxidation is the diffusion of oxygen through the small orifices in the conduct wall. The oxygen diffusion flux (mol O.sub.2/m.sub.2s) follows Fick's law, as it is assumed to be proportional to the void fraction of the wall, f.sub.o, the oxygen diffusivity in gas, D.sub.O2,z, at the average local temperature and pressure conditions in both sides of the wall, the local concentration of oxygen in the gas conduct C.sub.O2,z, (for the purpose of estimating the oxygen flux, the concentration of O.sub.2 in the other side of the wall is assumed to be zero), and inversely proportional to the thickness of the wall or the length of the orifices traversing the conduct wall, l.sub.o. [0079] The transfer of heat from the conduct wall to the gas flowing in the gas conduct is proportional to the local temperature difference between the wall and the gas. The proportionality constant is a single heat transfer coefficient, h.sub.z, that can be calculated from the local average gas properties between the wall and the gas as it is proportional to the Nusselt number and inversely proportional to the equivalent internal diameter or characteristic distance of the gas conduct. The conduct wall is assumed to have very high thermal conductivity. The bed of solids containing reacting particles is assumed to have a certain effective thermal conductivity, that can be tailored within certain values by choosing the ceramic support for the active oxygen carrier or by introducing a third solid to enhance the effective thermal conductivity (a third solid with high thermal conductivity >20 W/mK). The maximum temperature difference between the solid surfaces where oxidation reaction is taking place and the wall is therefore assumed to be relatively small. [0080] Axial thermal heat transfer by conduction along the longest dimension of the reactor that coincides with the direction of the gas flow is assumed to be negligible. This is mainly because the very large aspect ratio between the length of the reactor and the half of the distance separating two neighbouring conducts, that determines the maximum thickness of solids associated to each gas conduct. [0081] Pressure drop of gas flowing in the gas conducts of the reactor can be calculated with standard correlations for the Fanning friction factor for turbulent flows (equation 5-66 in “Perry's Chemical Engineering Handbook, sixth edition”). [0082] All the examples are calculated assuming conducts of cylindrical geometry for the gas conducts and for the complete reactor. Indeed, horizontal flat sheets conducts or channels with rectangular shapes may more suitable when operating reactors that use carbonaceous fuels with low ash content, so that the bed of fuel is standing on the porous flat sheet an reduces in height as the fuel is consumed, while keeping the reacting solids very close to the permeable wall. However, this and other geometries typical in the state of the art of gas solid contact can be calculated following similar methodology as below, using available correlations for heat transfer and mass transfer specific for these geometries. [0083] For simplicity, the length of the orifices is assumed to be identical to the conduct wall thickness.

[0084] A critical design target for the reactors is to be able to reach a steady state where, at any distance from the entrance of the gas, z, the heat transferred from the wall to the mass flow of gas, m.sub.g, during its advance from z to a point in its proximity located downstream at z+dz, is equal to the rate of generation of heat by the exothermic oxidation of the solids. Since the rate of the oxidation reaction is controlled by diffusion of oxygen in the orifice, as long as the assumptions stated above are held, a heat and mass balance between z and z+dz yields:

m.sub.gC.sub.pg(dT.sub.g,z/dz)=h.sub.zπd.sub.c(T.sub.w,z−T.sub.g,z)=−D.sub.O2,zπd.sub.cf.sub.oC.sub.O2,z□H.sub.r/l.sub.o  [2]

[0085] where T.sub.g,z and T.sub.w,z are the temperatures of the gas and the wall respectively, d.sub.c is the equivalent diameter of the gas conduct and □H.sub.r is the enthalpy of oxidation of the solid per mol of O.sub.2.

[0086] A common design objective in the examples below is to maintain as much as possible a stable temperature output of the gas at the exit of the reactor, by maintaining stable axial temperature profiles in the reactor. This steady state is achieved at a temperature in which the heat generation rate by reaction is equal to the heat transferred from the porous conduct wall to the gas flowing in the conduct. The design of the reactor must be such as to control the heat generation rate using the porous properties of the gas conduct wall. For this purpose, it is important to exploit the fact that the term in the right hand size of the equation [2] is largely insensitive to changes in the conversion of the solids. It is also insensitive to pressure for a volume fraction of O.sub.2 in the gas, because the opposite effects of pressure on D.sub.O2,z and C.sub.O2,z. Furthermore, since the enthalpies of oxidation are high, and the gas is typically preheated before entering the reactor, a heat balance in the reactor reveals that only a few percentual points of the 21% initial volume fraction of oxygen will be required to heat up the gas by a few hundred degrees when reacting with the solids. This means that the oxygen volume fraction tends to decrease only slightly with the bed length, z. On the other hand, diffusivity of oxygen increases with temperature (proportional to T.sup.1.724 in the correlation of footnote 2 of Table 1) but the concentration C.sub.O2,z (in mol/m.sup.3) is inversely proportional to temperature. This, together with the decrease in volume fraction of O.sub.2 with the bed length, z, allows for a moderate increase in the oxygen flux along z for a given set of parameters d.sub.c, f.sub.o and l.sub.o. However, these three parameters can be chosen in practice with a large degree of freedom (see Examples below) so that a certain axial profile effective oxygen flux (and hence power generation) per unit of length of the reactor can be predefined. In other words, it should become evident at this point to those skilled in the art that the flow of oxygen diffusing per unit of length (and therefore the thermal power output from reaction per unit of length of an air conduct in the direction of the gas flow) can be tuned to be nearly constant during a period of stable operation from an initial state of solids fully reduced to a final state of solids almost oxidised, as long as the initial state of the solids is at a temperature sufficiently high as to allow the overall oxidation reaction to be controlled by the diffusion through the conduct wall orifices.

[0087] Other relationships and trends coming from the application of the equation [1] and [2], or its refinements, will be apparent to those skilled in the art of packed bed reactor design and heat transfer. The three examples below illustrate some practical applications of the basic design rules described above, for four different problems with three different materials with affinity to oxygen. Table 1 compiles the design targets, the number of estimated gas conducts and the main dimension of the calculated reactors, along with the input and output conditions in the gas, assumed to be unchanged at the beginning and at the end of each of the oxidation process of the batch of reduced solids.

Example 1 Back-Up Power Generation Device or Energy Storage Device, Oxidising a Mass of CaS to CaSO.SUB.4 .and Using the Released Energy in a Brayton Cycle

[0088] In this example (see FIG. 2) the reactor of this invention (100) is applied as a heater of a Brayton cycle to produce 100 MW of thermal output during 77 hours, by oxidising during this period a large batch of CaS previously loaded into the reactor containing 1300 kg of active CaS per m.sup.3 of bed of packed solids in the reactor (which is about 50% of the volume in the bed of packed solids). At the end of the operational period, the resulting CaSO4 can be discharged or regenerated with a suitable reducing gas (like electrolytic renewable hydrogen). The reactor can therefore be used during the energy discharge cycle of a thermochemical energy storage system involving CaS/CaSO4. The list of the main operating variables and parameters in the device is listed in the column Example 1 of Table 1. They are a combination of assumed values and calculations from the solution of equation 1 and 2, solved in an iterative manner as described above for small finite elements in the direction of the main gas flow, using as inputs the outlet conditions of previous element.

[0089] The reactor of Example 1 is fed with compressed gas at 20 bar in a compressor (101) and preheated to 500° C. (1) to generate a flue gas stream (2) at 1266° C. to be expanded in a turbine (102). The batch of CaS is assumed to be preheated over ignition temperatures before the oxidation process starts, for example by using state of the art techniques of regenerative heat exchange in solid beds or by burning a small portion of a gas fuel in the reactor. Therefore, when the air stream (1) enters the reactor, there is a high affinity to oxygen in the region of the reactor occupied by the CaS and the process of CaS oxidation to CaSO.sub.4 is assumed to be controlled by the diffusion of O.sub.2 through the gas conduct wall orifices. For simplicity, it is omitted in the solution of this example the role of decomposition reactions of CaSO.sub.4 to give CaO and SO.sub.3, that could translate into a certain diffusion of trace quantities of SO.sub.3 from the region with solids to the air stream flowing through the reactor. The mass flow rate of air is fixed to ensure the generation of 100 MW thermal in the reactor under adiabatic conditions, when heating from 500° C. to the exit temperature of the flue gas. The gas velocity through the gas conducts is assumed to be 20 m/s at the entrance of the reactor. The gas (1) is then distributed in 93 pipes having a diameter of 0.1 m and a total length of 300 m. The cross section of the conducts does not change along the axis of the reactor so that gas velocity increases with the increasing temperature of the gas flowing into the reactor up to 38.0 m/s. Other conduct shapes, in particular flat sheets or other rectangular shapes can be equally or more convenient depending to facilitate loading (10) and discharge (11) of solids or their regeneration in contact with a reducing gas. Straight lines with no bends are considered in the Figures and in the Examples, but other arrangement are also possible involving bends, spirals or u-turns of the conducts in order to accommodate the total length of the gas conducts (300 m in this example) into a given volume of the reactors (100). The wall thickness of the gas conducts is assumed to be 2 mm and the fraction of the wall area occupied by orifices or voids is assumed to be 0.12. As indicated by the solution of equation [1] and a mass balance on O.sub.2 flowing through the conducts, these wall properties allow for a transfer of oxygen by diffusion towards the bed of CaS, so that the oxygen volume fraction starts with 0.21 and exits the device with 17.1% vol.

[0090] The heat transfer coefficient between the gas and the wall at the inlet of the conduct is estimated to be 418 W/m2K from the Nusselt number equation of footnote 1 of Table 1, calculated using the average temperature between the gas (500° C.) and the wall (524.6). Diffusivity of oxygen in the porous wall is calculated at the temperature of the wall with the equation noted in the footnote 2 of Table 1.

[0091] In this particular example, and with this simplified problem solving procedure, the reactor will have 2.9 m diameter, without including the layer of insulating material and the non-permeable wall to complete the reactor. The reactor of this example would be able to deliver 100 MW of thermal output in energy-efficient Brayton cycles without emitting CO2 following the scheme of FIG. 2.

Example 2. Power Generation at Small Scale from CaS Oxidized to Drive a Brayton Cycle

[0092] In this example the reactor of this invention (100) is applied as a heater of a Brayton cycle installed in a small scale application (a building or a vehicle with sufficient space available to accommodate the reactor). The scheme is as in FIG. 2 and the design methodology as in Example 1. However, this design example has the specific target of minimization the dimensions of the reactor for a given power output. As noted in Table 1, Example 2, the reactor is in this case intended to oxidise during 6.8 hours the initial batch of CaS contained in reactor containing gas conducts 5 m long. To achieve such a short length, it is necessary to allow a higher rate of diffusion of oxygen through the porous walls of the gas conducts (assumed to have in this case 0.3 vol fraction and just 1 mm of wall thickness). The high rate of O.sub.2 towards the bed of solids is also achieved by reducing to 0.01 the diameter of the pipes used as gas conducts. This also favours an increase in heat transfer coefficients wall-gas (to 662 W/m2K at the entrance of the reactor).

[0093] The reactor has 0.74 m r total diameter and is able to deliver 1 MW of thermal power. Such reactor could therefore find application in a wide range of distributed power generation systems, including large transport vessels.

Example 3. Chemical Looping Combustion System

[0094] In this example the reactor of this invention (100) is applied as the air reactor in a chemical looping combustion system of a gaseous fuel (like natural gas etc). The chemical looping combustion system is integrated in a Brayton cycle to deliver maximum energy conversion efficiencies from the original energy contained in the gas fuel. This particular example uses nickel as oxygen carrier to become NiO in the oxidation step, while reducing back to Ni during the fuel combustion step, with NiO oxidising the fuel gas to CO.sub.2 and H.sub.2O. The packed bed of solids contains 1780 kg of active Ni per m.sup.3 of bed of packed solids (which occupy about 20% of the volume in the bed of packed solids).

[0095] In this example, when the reactor is in oxidation mode as at the top of FIG. 3 is designed to operate with high thermal output (100 MW) targeting shorter cycle times (62 minutes in this example) for the oxidation reaction stage. A total reactor length of 8 m and an equivalent diameter of 3.4 m (without taking into account insulation and closure wall) while the aspect ratio of the individual gas conducts remains 400 (ratio of the diameter of an individual gas conduct to the length of the conduct). Since the Ni/NiO material is known to be highly reactive and reversible in a wide range of temperatures, the reactor has been arranged to have a large number (3003) of small diameter pipes (0.02 m of internal diameter) with other properties at listed in Table 1. The calculation methodology to solve equation [1] and [2] is as in Example 1. A multiplicity of other designs can be considered to attempt a more adequate integration of the reactor to the specific requirements of the overall chemical looping combustion system. However, this example indicates that the reactor of this invention is able to solve the problem of high pressure, high temperature oxidation of a bed of oxygen carrier with an inherent control of maximum temperatures in the solids, by limiting the oxygen flow through the permeable gas conduct wall and by relying on the effective heat transfer from such wall to the flowing air in that conduct.

Example 4 Back-Up Power Generation Device, Burning a Carbonaceous Solid Fuel in a Brayton Cycle

[0096] In this example the reactor of this invention (100) is applied as a heater of a Brayton cycle to burn during 37 hours and 30 min a batch of char previously loaded into the reactor through port (10). At the end of the operational period, the fuel is burnt-off and the remaining solid residues (ash, if any), are discharged through port (11). The list of the main operating variables and parameters in the device is listed in the column Example 4 of Table 1.

[0097] The reactor of Example 1 is fed with compressed gas at 20 bar and preheated to 600° C. (1) to generate a flue gas stream (2) at 1385° C. to be expanded in a turbine. The batch of char is assumed to be preheated over ignition temperatures before the oxidation process starts, for example by using state of the art techniques of regenerative heat exchange in solid beds or by burning a small portion of the solids in the reactor. Therefore, when the gas in stream (1) enters the reactor, there is a high affinity to oxygen in the region of the reactor occupied by the char and the process of char combustion is assumed to be controlled by the diffusion of O.sub.2 through the gas conduct wall. The mass flow rate of gas is fixed to ensure the generation of 100 MW thermal. The air is distributed in 406 pipes, with a diameter of 0.05 m and 100 m long and the pressure is estimated to drop from 20 bar to 18.9. The wall thickness of the gas conducts is assumed to be 2 mm and the fraction of the wall area occupied by orifices in the conduct's wall is assumed to be 0.2. As indicated by the solution of equation [1] and a mass balance on O2 flowing through the conducts, these wall properties allow for a transfer of oxygen by diffusion towards the bed of char, so that the oxygen volume fraction starts with 0.21 and finishes with 16.0% vol. Such system illustrates a possible application of the reactor in back up electric power generation systems, that would be able to use solid fuels (including biofuels like char) in energy-efficient Brayton cycles.

TABLE-US-00001 TABLE 1 Compilation of the design targets, number of estimated gas conducts and the main dimension of the calculated reactors, along with the input and output conditions in the gas during the oxidation process with air of a batch of reduced solids. Example Example Example Example 1 2 3 4 Type of active solid with affin- CaS/CaSO.sub.4 CaS/CaSO.sub.4 Ni/NiO C/CO.sub.2 ity to oxygen/oxydized product Bulk density of active reduced 1300 1300 1780 700 solid (kg/m3 of packed bed) Thermal power (MW) 100 1 100 100 Duration of oxidation cycle 77 hours 6.8 hours 62 minutes 37.5 hours Inlet pressure/outlet pressure   20/18.3   20/19.6   20/19.8   20/18.9 (bar) Air velocity (m/s) inlet/outlet   20/38.0 20/36   20/33.5   20/35.7 L: Reactor length (m) 300 5 8 100 d.sub.c: Effective diameter of air 0.1 0.01 0.02 0.05 channel (m) l.sub.o: Wall thickness or orifice 0.002 0.001 0.001 0.002 length (m) f.sub.o: Wall area fraction occupied 0.12 0.3 0.3 0.2 by orifices L/d.sub.c 3000 500 400 2000 (L * f.sub.o)/(d.sub.c * l.sub.o) (m.sup.−1) 180000 150000 120000 200000 Number of air channels or air 93 104 3003 406 pipes Density of active particle 2600 2600 8908 1500 (kg/m.sup.3) Enthalpy of oxidation (kJ/mol 480.5 480.5 479.4 393.7 O.sub.2) Vol. fraction of active solids in 0.7 0.7 0.2 0.5 bed Thickness of bed of solid parti- 0.1 0.03 0.02 0.05 cles (m) Equivalent diameter of device 2.9 0.74 3.4 3.1 (m) Air temperatures T inlet/Tout-  500/1266  500/1177  600/1250  600/1385 let (° C.) Wall temperatures inlet/outlet 524.6/1299   578/1305  702/1398  634/1431 h wall-gas inlet/outlet 418/353 662/560 547/485 455/400 (W/m.sup.2K).sup.1 D.sub.O2,z inlet/outlet (m.sup.2/s × 5.39/18.3 5.39/18.1 6.64/18.0 6.64/19.2 1000000) O.sub.2 volume fraction inlet/outlet  0.21/0.171  0.21/0.176  0.21/0.177  0.21/0.160 Power per m of one air conduct 3.23/3.61 1.61/2.25 3.52/4.50 2.41/2.87 (kW/m) inlet/outlet .sup.1from Nu = 0.023 * Re.sup.∧0.8 * Pr.sup.∧0.4; .sup.2from D.sub.O2,z = 0.00000000113 * (T + 273).sup.∧1.724/P m.sup.2/s