Chemical looping reforming of hydrocarbons

Abstract

The present invention relates to a method for the thermochemical production of a fuel gas such as syngas in a reactor, via a chemical looping process that comprises a process of reforming of a hydrocarbon such as for example methane and a process of splitting carbon dioxide, as well as a device for the production of a method for the thermochemical production of a fuel gas such as syngas.

Description

[0001] TITLE

[0002] CHEMICAL LOOPING REFORMING OF HYDROCARBONS

[0003] TECHNICAL FIELD

[0004] The present invention relates to a method for the thermochemical production of a fuel gas such as syngas in a reactor, via a chemical looping process that comprises a process of reforming of a hydrocarbon such as for example methane and a process of splitting carbon dioxide, as well as a device for the production of a method for the thermochemical production of a fuel gas.

[0005] PRIOR ART

[0006] Syngas, a mixture of hydrogen and carbon monoxide, can be used in multiple applications. For example, syngas can be used for producing ammonia or methanol, as a combustible fuel or as a precursor product in the synthesis of liquid fuels via the Fischer-Tropsch processes.

[0007] Syngas is mainly produced nowadays by steam reforming or partial oxidation of a hydrocarbon fuel gas such as natural gas. Chemical-looping reforming of methane (CLRM) is a promising but less mature pathway for producing syngas that splits the methane reforming process into two reactions by utilizing a nonstoichiometric metal oxide as an oxygen carrier. The two reaction steps are: 1) endothermic reduction of the metal oxide to facilitate the partial oxidation of methane (POM), followed by 2) exothermic re-oxidation of the metal oxide via CO2 and / or H2O splitting to form additional CO and / or H2, respectively. Equations 1 and 2a / 2b summarize these reactions for a generic metal oxide (MxOy) with 5 representing the oxygen nonstoichiometry.

[0008] M XO , + \()CH, ^ M XO , + A<5CO +2A<5H,

[0009] CO2 and H2O splitting (Reactions 2a and 2b, respectively) have comparable thermodynamic and kinetic favorability for reoxidation, and thus control of the H2 / CO ratio between 1 :1 and 3:1 using the appropriate ratio of oxidants is possible without a downstream water-gas-shift reactor. The reoxidation step can also be used to remove carbon deposits formed during the POM step as additional CO.

[0010] Cerium dioxide (CeO2) is known for being capable of use as a nonstoichiometric material used for CLRM due to generally rapid redox kinetics and favorable thermodynamic properties. CeO2 readily re-oxidizes under a wide range of conditions and maintains a stable cubic fluorite structure over a wide range of nonstoichiometry, contributing to high oxygen exchange capacity.

[0011] While CeO2 is a viable redox material for CLRM at temperatures > 1000 °C, kinetics of POM slow below 1000°C and make lower temperature operation impractical for industrial production purposes. Furthermore, yield and selectivity of the thermochemical reaction drop as the number of CLMR cycles increases and the level of oxidation in the end portion of the reactor nears a stoichiometric level.

[0012] Thus, maintaining an acceptable yield as well as acceptable selectivity during the production of syngas when using cerium oxide as catalyst material is not trivial. As the number of CLRM cycles increases, the ability of the cerium oxide to provide good yield and selectivity, which is mediated by its redox state, decreases in the downstream section of the reactor.

[0013] As a consequence, it is desirable to provide a process in which the sufficient yield and selectivity is maintained over many CLRM cycles.

[0014] SUMMARY OF THE INVENTION

[0015] The present invention provides for a process for the thermochemical production of a fuel gas in which the loss in yield and selectivity can be countered, and thereby provides a solution that permits the continuous operation of a reactor over many CLRM cycles. The inventors have found that a loss in yield and selectivity can be mitigated essentially by controlling the oxygen nonstoichiometry 5 in the metal oxide in the downstream end, or the downstream end section, of the metal oxide element of the reactor. As a consequence, it was possible to increase the overall selectivity of carbon monoxide Sco from below 0.95 to above 0.95.

[0016] It is accordingly a first object of the present invention to provide a method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non- stochiometric redox state, wherein the fuel gas exits the metal oxide element in a non- stochiometric redox state at a downstream end of the metal oxide element in a non- stochiometric redox state, said method comprising the steps of: repeating cycles of chemical-looping reforming of hydrocarbon (CLRH), wherein a chemical-looping reforming of hydrocarbon (CLRH) cycle comprises o a reduction half-cycle in which the metal oxide in a non- stochiometric redox state is reduced by contacting a flow of reductant gas comprising a hydrocarbon with the metal oxide MxOy-5ox for a predetermined duration, and o an oxidation half-cycle in which the metal oxide in a non- stochiometric redox state is oxidized by contacting a flow of oxidant gas comprising carbon dioxide, steam and / or a mixture thereof, with the metal oxide MxOy-5red for a predetermined duration, wherein the method periodically comprises increasing the duration of the reduction half-cycle, increasing the flow rate of flow of reductant gas, or increasing both the duration of the reductant half-cycle and the flow rate of flow of reductant gas, during a chemical-looping reforming of hydrocarbon (CLRH) cycle.

[0017] Further embodiments of the invention are laid down in the dependent claims.

[0018] BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Preferred embodiments of the invention are described in the following with reference to the drawings, which are for the purpose of illustrating the present preferred embodiments of the invention and not for the purpose of limiting the same. In the drawings,

[0020] Fig. 1 shows the evolution of bavgover 30 CLRM cycles at 800°C on a Ni-CeCh reactor when all cycles are kept identical (lozenges) and when cycles are periodically extended according to the invention (circles). For each cycle, the value of bavgis shown oscillating between a reduction half-cycle (higher value of bavg) and an oxidation half-cycle (lower value of bavg), and where at each 5thcycle, the values of bavgare shifter towards higher values of bavg.

[0021] Fig. 2 shows the evolution of the selectivity of carbon monoxide (Sco) over 30 CLRM cycles at 800°C on a Ni-CeCh reactor when all cycles are kept identical (lozenges) and when cycles are periodically extended according to the invention (circles). When cycles are periodically extended, the selectivity is shown to progressively increase over three cycles following the periodic extension to 0.98, before decreasing again over two cycles towards 0.95, and so on. When cycles are kept identical, the selectivity plateaus around 0.95.

[0022] DESCRIPTION OF PREFERRED EMBODIMENTS

[0023] It is a first object of the present invention to provide a method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, wherein the fuel gas exits the metal oxide element of the thermochemical reactor in a non-stochiometric redox state at a downstream end of the metal oxide element in a non-stochiometric redox state, said method comprising the steps of: repeating cycles of chemical-looping reforming of hydrocarbon (CLRH), wherein a chemical-looping reforming of hydrocarbon (CLRH) cycle comprises o a reduction half-cycle in which the metal oxide in a non- stochiometric redox state is reduced by contacting a flow of reductant gas comprising a hydrocarbon with the metal oxide MxOy-5ox for a predetermined duration by increasing 5 from box tored, and o an oxidation half-cycle in which the metal oxide in a non- stochiometric redox state is oxidized by contacting a flow of oxidant gas comprising carbon dioxide, steam and / or a mixture thereof, with the metal oxide MxOy-5red for a predetermined duration, by decreasing 5 from 5red to 50X, wherein the method periodically comprises increasing the duration of the reduction half-cycle, increasing the flow rate of flow of reductant gas, or increasing both the duration of the reductant half-cycle and the flow rate of flow of reductant gas, during a chemical-looping reforming of hydrocarbon (CLRH) cycle.

[0024] In the method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state according to the first object of the present invention, the fuel gas that is obtained is preferably a mixture of carbon monoxide and hydrogen, also known as syngas.

[0025] Syngas may be used as-is as fuel, or may be further used as a feedstock in a process to provide liquid fuels via the Fischer-Tropsch process. When the method for the production of a fuel gas in a thermochemical reactor feeds directly a Fischer-Tropsch process, the preferred range of ratio between CO / H2 is of from 1 .8 to 2.5.

[0026] In the method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state according to the first object of the present invention, the predetermined duration of the reduction half-cycle may be 60 s or 120 s, and the predetermined duration of the oxidation half-cycle may be 120 s or 150 s. Thus, for example, when the predetermined duration of the reduction half-cycle is 60 s or 120 s, it may be periodically prolonged to 90 s or 150s, respectively. The predetermined duration of the reduction half-cycle and the predetermined duration of the oxidation halfcycle may be the same or different, and in particular, the duration of the predetermined duration of the reduction half-cycle may be larger than the predetermined duration of the oxidation half-cycle. Thus, for example, the predetermined duration of the reduction halfcycle is may be 180 s, whereas the predetermined duration of the oxidation half-cycle may be 150 s, i.e. the predetermined duration of the reduction half-cycle is may be about 20 to 25% larger than predetermined duration of the oxidation half-cycle.

[0027] In the method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state according to the first object of the present invention, the thermochemical reactor comprises a metal oxide element as the catalytically active material that enables the thermochemical production of the fuel gas from feed gas. For instance, the metal oxide element may be in different shapes and forms. As an example, the metal oxide element may be in the form of a monolithic unit or in the form of a packed bed of individual metal oxide subunits. As an another example, the metal oxide element may be formed by a plurality of subunits, which may be identical, such as for example a metal oxide element formed of a plurality of identical tubular or ring-shaped subunits. In general, the metal oxide is preferably provided in a form that ensures a high surface-to-volume ratio to increase the contact surface between the metal oxide and the feed gas. For example, the metal oxide is preferably a metal oxide foam such as an opencell metal oxide foam.

[0028] In one particular embodiment of the method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, the metal oxide element is an elongated metal oxide element. During operation of the thermochemical reactor, the feed gas, which is either an oxidant gas or a reductant gas, flows across the elongated metal oxide element along the longitudinal axis of the elongated metal oxide element, by entering the upstream end of the elongated metal oxide element. During its passage across elongated metal oxide element, the feed gas is transformed to carbon monoxide and hydrogen, i.e. syngas, which syngas exits the downstream end of the elongated metal oxide element. A preferred example of an elongated metal oxide element is a tubular packed bed metal oxide element.

[0029] In the method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state according to the first object of the present invention, the metal oxide MxOy-5 of the metal oxide element is in a non- stochiometric redox state. The non-stochiometric redox state is quantified by 5, which denotes a non-zero positive real number and by x, y which denote a natural integer, with the proviso that y is larger than 5.

[0030] In one particular embodiment of the method for the production of a fuel gas in a thermochemical reactor comprising, or consisting of, a metal oxide element in a non- stochiometric redox state, in the downstream end section of the metal oxide element, during the periodically extended reduction half-cycle, 5 is increased to enhance the selectivity, in particular the selectivity for CO. In particular, 5 is increased to a value of at least 0.05, preferably to a value of at least 0.1 and of up to 0.25, and preferably of up to 0.5.

[0031] In one particular embodiment of the method for the production of a fuel gas in a thermochemical reactor comprising, or consisting of, a Ni-CeCh element in a non- stochiometric redox state, in the downstream end section of the metal oxide element, during the periodically extended reduction half-cycle, 5 is increased to enhance the selectivity, in particular the selectivity of carbon monoxide. In particular, during the periodically extended reduction half-cycle, 5 is increased to a value of at least 0.05, preferably to a value of at least 0.1 and of up to 0.25, and preferably of up to 0.5.

[0032] The inventors have found that when the oxygen nonstoichiometry 5 in the metal oxide of the metal oxide element is controlled at the downstream end, or at the downstream end section, of the metal oxide element, a better yield and selectivity may be achieved. For example, when the metal oxide is ceria (CeCh-s) or Ni-ceria (Ni-CeCh-s), it is possible to increase the overall selectivity of carbon monoxide Sco by maintaining 5 at a value of at least 0.05, preferably of at least 0.1 and of up to 0.25, and preferably of up to 0.5 at the downstream end, or at the downstream end section, of the metal oxide element.

[0033] While 5 may not be directly measured, it can be determined by determining the composition of the gas exiting the downstream end, or the downstream end section, of the of the metal oxide element. For example, it is possible to analyse the composition and / or flow rates of the gas exiting the downstream end, or the downstream end section, of the metal oxide element using a mass spectrometer or an infrared spectrometer. In particular, hydrogen, carbon dioxide and methane may be determined qualitatively and quantitatively, which allows to calculate the selectivity, for example with respect to carbon monoxide (Sco), which in turn allows calculating 5 of the metal oxide at the downstream end, or the downstream end section, of the of the metal oxide element.

[0034] In the method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state according to the first object of the present invention, cycles of chemical-looping reforming of hydrocarbon (CLRH) are repeated while the reactor is operating. Chemical-looping reforming of hydrocarbon (CLRH) cycle comprises a reduction half-cycle and an oxidation half-cycle, which are induced by feeding the reactor either reductant gas or oxidant gas, respectively, as feed gas, in an alternating fashion.

[0035] In the reduction half-cycle, the metal oxide in a non-stochiometric redox state is reduced by contacting a flow of reductant gas comprising a hydrocarbon with the metal oxide MxOy-5ox for a predetermined duration. During the reduction half-cycle, the hydrocarbon of reductant gas gets oxidized to carbon monoxide, the metal oxide is reduced in its non-stochiometric redox state, and 5 (from 50Xtowards 5red) increases.

[0036] In the oxidation half-cycle, the metal oxide in a non-stochiometric redox state is oxidized by contacting a flow of oxidant gas comprising carbon dioxide, steam and / or a mixture thereof, for a predetermined duration. During the oxidation half-cycle, the carbon dioxide and / or steam of the oxidant gas get reduced to carbon monoxide and / or hydrogen, the metal oxide is oxidized in its non-stochiometric redox state , and 5 (from 6red towards box) decreases.

[0037] In the method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state according to the first object of the present invention, 5 is maintained to be in a range of from 0.05 to 0.5 at the downstream end, or at the downstream end section, of the metal oxide element by periodically increasing the duration of the reduction half-cycle, by increasing the flow rate of flow of reductant gas, or by increasing both the duration of the reductant half-cycle and the flow rate of flow of reductant gas, during a chemical-looping reforming of hydrocarbon (CLRH) cycle. When the thermochemical reactor is run repetitively over several identical cycles of chemical-looping reforming of hydrocarbon (CLRH), the selectivity of the thermochemical reactor, and consequently, 5, exhibits a drift towards lesser selectivity, or lesser values of 5. To counteract this loss in selectivity of the thermochemical reactor, the reduction half-cycle, in which the metal oxide is reduced, may be periodically prolonged to re-establish a higher value of 5 at the downstream end section of the metal oxide element, or the flow rate of the reductant gas may be increased to re-establish a higher value of 5 at the downstream end section of the metal oxide element, or both measures may be leveraged simultaneously.

[0038] After the higher value of 5 of at least 0.05, and preferably of at least 0.1 at the downstream end section is re-established, the thermochemical reactor may be run again for a number of repeating cycles of chemical-looping reforming of hydrocarbon (CLRH), until 5 at the downstream end section falls below a threshold value, i.e. below a value of 0.1 or below a value of 0.05.

[0039] In one particular embodiment of the method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, the metal oxide is a La-Sr-Mn-based perovskite, a ceria (CeCh), and preferably is Ni- ceria.

[0040] In one particular embodiment of the method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, the metal oxide is Ni°-ceria, preferably having a Ni content of from 1 to 10 wt%, more preferably of from 4 to 8 wt%, based on weight of Ni°-ceria. Ni-ceria is obtained by means of a deposition of metallic Ni on the surface of CeC>2 particles via an incipient wetness impregnation method, in particular on the surface of CeC>2 particles having a diameter of about 500 to 1400 pm. It is important to note that the deposition of metallic Ni on the surface of CeC>2 particles via an incipient wetness impregnation method yields discrete Ni° particles on the surface of the CeC>2 particles, which may have a nanometre-scale diameter and up to 10 micrometres, and results in a material that is distinct from Ni-doped CeC>2, in which Ni is incorporated into the metal oxide lattice.

[0041] In one particular embodiment of the method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, the thermochemical reactor is operating below 1000°C, preferably between 650°C and 950°, more preferably between 700°C and 800°. While the thermochemical reactor is capable of operating at temperatures below 650°C, the kinetics of the thermochemical production slow considerably to a point where it becomes impractical for fuel production.

[0042] In one particular embodiment of the method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, the periodicity is every N chemical-looping reforming of hydrocarbon (CLRH) cycles, wherein N being a non-zero integer and preferably is at least 2, preferably is of from 2 to 25, preferably of from 2 to 15. It is understood that the periodicity will depend on parameters such as the feed gas, the metal oxide, but the principle remains in that at some point, after a number of repeating cycles of chemical-looping reforming of hydrocarbon (CLRH), the higher value of 5, of at least 0.05 or at least 0.1 is re-established.

[0043] In one particular embodiment of the method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, the periodicity is every 4 to 10 chemical-looping reforming of hydrocarbon (CLRH) cycles, preferably every 4, 5 or 6 chemical-looping reforming of hydrocarbon (CLRH) cycles. It is understood that the periodicity will depend on parameters such as the feed gas, the metal oxide, but the principle remains in that at some point, after a number of repeating cycles of chemical-looping reforming of hydrocarbon (CLRH), the higher value of 5 will need to be re-established. In general, the need to increase the duration of the flow of the reductant gas, or the mass flow rate of the reductant gas, can be determined by measuring the composition of the gas exiting the reactor on the downstream side and in particular by determining the amount of carbon monoxide exiting the reactor on the downstream end of the reactor. Thus, the appropriate periodicity and extent of the reduction half-cycle can be determined for a given metal oxide and a given reaction conditions. In general, when selectivity of carbon monoxide falls below 0.95, adjusting the reduction half-cycle is appropriate, because in that case, 5 will have fallen below a value of 0.1 , or below 0.05.

[0044] In one particular embodiment of the method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, when the duration of the reduction half-cycle is increased, without changing the flow rate of flow of reductant gas, it is increased by at least 25%, preferably 50%, more preferably by 60% and most preferably by up to 100%.

[0045] In one particular embodiment of the method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, when the flow rate of the reduction half-cycle is increased, without changing the duration of flow of reductant gas, it is increased by at least 25%, preferably 50%, more preferably by 60% and most preferably by up to 100%.

[0046] In one particular embodiment of the method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, the reductant gas comprises, or consists of, a hydrocarbon gas chosen from methane, ethane, propane, butane, and mixtures thereof.

[0047] In one particular embodiment of the method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, the reductant gas comprises methane, in combination with ethane, propane, butane, and mixtures thereof, and more preferably comprises at least 50% by weight of methane, and even more preferably comprises at least 75% by weight of methane, in combination with ethane, propane, butane, and mixtures thereof.

[0048] In one particular embodiment of the method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, the reductant gas is methane. In one particular embodiment of the method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, the reductant gas comprises less than 5 % by weight, preferably less than 3 % by weight of a non-hydrocarbon gas, in particular of an inert gas. Inert gases may for example be nitrogen, as well as noble gases, such as argon or helium.

[0049] In one particular embodiment of the method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, the oxidant gas comprises, or consists of, carbon dioxide or a mixture of carbon dioxide and steam.

[0050] In one particular embodiment of the method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, the oxidant gas comprises more than 75 % by weight, preferably less than 95 % by weight of a carbon dioxide and / or steam, and in particular is carbon dioxide, steam or a mixture of carbon dioxide and steam. Further the oxidant gas comprises less than 5 % by weight, preferably less than 3 % by weight of an inert gas. Inert gases may for example be nitrogen, noble gases, such as argon or helium. Inert gases may for example be nitrogen, noble gases, such as argon or helium.

[0051] In one particular embodiment of the method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, the duration of the reduction half-cycle is increased, without changing the duration of flow of reductant gas, by at least 15 s. 20 s, 30 s or 60 s.

[0052] EXPERIMENTAL DATA

[0053] Experimental setup:

[0054] The production of a fuel gas was carried out in a horizontal thermochemical tube reactor made of an alumina tube of 4 mm diameter and comprising a metal oxide element having a weight of 1 gram and made of a Ni-CeO2, where the amount of particles of Ni° deposited on the surface of the ceria was about 5 weight percent. This redox material was synthesized by depositing metallic Ni on the surface of CeO2 particles having a diameter 500 to 1400 pm diameter via an incipient wetness impregnation method

[0055] Gas flow to the reactor was regulated via three mass flow controllers (MKS GE50A), and the operating temperature was set and maintained using a Carbolite furnace (Carbolite STF 16 / 180). All gas flow rate and temperature inputs were controlled via a custom LabVIEW program. Downstream gas analysis consisted of a mass spectrometer (HPR-20 QIC, Hiden Analytical) to measure flow rates of H2 and CO2, as well as an infrared (IR) analyzer (Siemens Ultramat 23) to quantify flow rates of CH4 and CO.

[0056] Each CLRM cycle consisted of four steps: 1) partial oxidation of methane with cpCH4 = 10 vol% balanced in Ar(g) referred to as the reduction step, 2) 10 min purge in Ar(g), 3) oxidation with cpC02 = 5 vol% balanced in Ar(g) referred to as the oxidation step, followed by 4) another 10 min purge in Ar(g).

[0057] The reactor was operated for 30 CLRM cycles at a temperature of 800°C. Total flow rate during each step was set to 100 seem, corresponding to a gas velocity of 0.135 m / s.

[0058] Reaction times for the method according to the present invention were: tred,i = 120 s, tred = 60 s, and tox= 150 s. In every 5thCLRM cycle, the reaction time tred was increased by 50%, i.e. from 60 s to 90 s.

[0059] Reaction times for the constant cycling method are: tred,i = 120s, tred = 60s, and tox= 150s.

[0060] The final toxwas set to 15 min for both experiments to ensure complete oxidation of Ni- CeO2.

[0061] Syngas was produced according to the method of the present invention for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non- stochiometric redox state, and according to a method in which the cycles are kept identical.

[0062] In Figures 1 and 2, the two methods are compared over 30 CLRM cycles at 800°C. As can be recognized, in Figure 2, the method of the present invention for the production of a fuel gas in which the duration of the reduction step is increased periodically (every 5thcycle, in green, circles) reaches a higher selectivity Sco in the ensuing cycles when compared to a method in which the duration of the reduction step is kept constant (in black, lozenges) over the 30 CLRM cycles. In particular, it is apparent that the selectivity Sco of the periodic method increases above the selectivity Sco of about 0.95 seen in the constant method after the prolongation of the reduction step during the 10th, 15th, 20thand 25thCLRM cycle.

[0063] LIST OF REFERENCE SIGNS none

Claims

CLAIMS1 . A method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, wherein the fuel gas exits the metal oxide element in a non-stochiometric redox state at a downstream end of the metal oxide element in a non- stochiometric redox state, said method comprising the steps of: repeating cycles of chemical-looping reforming of hydrocarbon (CLRH), wherein a chemical-looping reforming of hydrocarbon (CLRH) cycle comprises o a reduction half-cycle in which the metal oxide MxOyis reduced by contacting a flow of reductant gas comprising a hydrocarbon with the metal oxide MxOy-5ox for a predetermined duration, and o an oxidation half-cycle in which the metal oxide MxOyis oxidized by contacting a flow of oxidant gas comprising carbon dioxide, steam and / or a mixture thereof, with the metal oxide MxOy-5red for a predetermined duration, wherein the method periodically comprises increasing the predetermined duration of the reduction half-cycle, increasing the flow rate of flow of the reductant gas, or increasing both the duration of the reductant halfcycle and the flow rate of flow of the reductant gas, during a chemical-looping reforming of hydrocarbon (CLRH) cycle.

2. The method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, according to claim 1 , wherein the metal oxide is ceria (CeCh), and preferably is Ni°-CeC>2.

3. The method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, according to any one of the preceding claims, the metal oxide is Ni°-CeC>2, preferably having a Ni content of from 1 to 10 wt%, more preferably of from 4 to 8 wt%, based on weight of Ni°-ceria.

4. The method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, according to any one of the preceding claims, wherein the periodicity isevery 4 to 10 chemical-looping reforming of hydrocarbon (CLRH) cycles, preferably every 4, 5 or 6 chemical-looping reforming of hydrocarbon (CLRH) cycles.

5. The method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, according to any one of the preceding claims, wherein the duration of the reduction half-cycle is increased, without changing the flow rate of flow of reductant gas, by at least 25%, preferably 50%, more preferably by 60% and most preferably by up to 100%.

6. The method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, according to any one of the preceding claims, wherein the flow rate of the reduction half-cycle is increased, without changing the predetermined duration of flow of reductant gas, by at least 25%, preferably 50%, more preferably by 60% and most preferably by up to 100%.

7. The method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, according to any one of the preceding claims, wherein the reductant gas comprises, or consists of, a hydrocarbon gas chosen from methane, ethane, propane, butane, and mixtures thereof, and more preferably comprises at least 50% by weight of methane, and / or less than 5 % by weight, preferably less than 3 % by weight of a non-hydrocarbon gas.

8. The method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, according to any one of the preceding claims, wherein the duration of the reduction half-cycle is increased, without changing the duration of flow of reductant gas, by at least 15 s, 20 s, 30 s or 60 s.

9. The method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, according to any one of the preceding claims, wherein the thermochemical reactor is operating below 1000°C, preferably between 650°C and 950°, more preferably between 700°C and 800°.

10. The method for the production of a fuel gas in a thermochemical reactor comprising a metal oxide element in a non-stochiometric redox state, according to any one of the preceding claims, wherein the predetermined duration of the oxidation half-cycle is larger than the predeterminedduration of the reduction half-cycle, and preferably wherein the predetermined duration of the oxidation half-cycle is at least 20%, or about 20 to 25%, larger than the predetermined duration of the reduction halfcycle.

Images