A Solar-Powered Methane Chemical Chain Dry Reforming System and Method Based on TiO2 / TiC Carbon Deposition Utilization Oxygen Carrier

Abstract

The application discloses a solar methane chemical looping dry reforming system and method based on TiO2 / TiC carbon deposition utilization type oxygen carrier, the system comprises a solar heat collecting device, a reduction reactor, an oxidation reactor, a CO / CO2 separator and an oxygen carrier circulation loop, the solar heat collecting device is respectively connected with the reduction reactor, the oxidation reactor, a material preheating channel of the reduction reactor and a material preheating channel of the oxidation reactor in heat connection; a first heat exchanger is connected between an output end of the reduction reactor and a methane input pipeline, a second heat exchanger is connected between an output end of the oxidation reactor and a CO2 input pipeline; the oxygen carrier circulation loop is connected with a solid phase output end of the reduction reactor and a solid phase input end of the oxidation reactor, and a solid phase output end of the oxidation reactor and a solid phase input end of the reduction reactor. The application effectively solves the carbon deposition problem in long-period circulation while realizing higher solar fuel efficiency.

Description

Technical Field

[0001] This invention relates to a methane chemical chain dry reforming cycle system and method, specifically to a solar-powered methane chemical chain dry reforming system and method based on TiO2 / TiC carbon deposition utilization oxygen carrier. Background Technology

[0002] Solar energy is the largest exploitable renewable energy source on Earth; the amount of solar energy reaching the Earth's surface in one hour exceeds the total energy consumed by humankind in a year. How to rationally utilize this enormous solar energy resource is a crucial issue facing modern societal development. The inherent intermittency, volatility, and low energy density of solar energy limit its large-scale application; therefore, developing efficient solar energy storage technologies is essential for the advancement of solar energy utilization technologies.

[0003] Among various solar energy storage methods, thermal energy storage (through sensible heat, latent heat, and chemical pathways) is generally considered the best choice for solar thermal utilization. The two-step solar thermochemical CO2 / H2O cracking technology can directly convert CO2 / H2O into fuel CO / H2, enabling both long-term solar energy storage and long-distance transportation, as well as the resource utilization of CO2 / H2O into fuel. It is considered a solar energy storage technology with significant future application potential. Traditional two-step solar thermochemical CO2 / H2O cracking mainly utilizes the redox cycle of an oxygen carrier to achieve energy and material conversion, primarily including two steps: reduction and oxidation. In the reduction step, concentrated solar energy is typically used as a heat source to drive the reduction reaction of multivalent metal oxides (MOx) at high temperatures, releasing oxygen from the material. Then, in the oxidation step, the reduced metal oxides usually react with carbon dioxide or water at a relatively low temperature. The metal oxides release CO / H2 while returning to their initial state, thus realizing the energy conversion and storage process from solar energy to fuel.

[0004] The main advantage of the two-step solar-powered thermochemical redox cycle is that it separates the O2 and CO / H2 produced from the cracking of CO2 / H2O into two different processes, avoiding the energy consumption required to separate high-temperature gases and greatly reducing the risk of combustion or explosion between products, thus improving the safety and operability of the cycle. Steinfeld's team at ETH Zurich used porous cerium oxide as an oxygen carrier in a 4.1 kW solar-powered thermochemical reactor at 1500 °C. Reduction at 0.1 mbar, 900°C Oxidation at 1 bar achieved a highly efficient two-step solar-powered carbon dioxide decomposition process. This study achieved a solar-fuel efficiency of 5.25%, the highest energy efficiency reported to date in the field of solar-powered two-step thermochemical cycles. However, this still falls short of the 20% energy efficiency requirement for commercialization of solar thermochemistry. One factor contributing to this efficiency penalty cannot be ignored: To improve the thermodynamic driving force of the CO2 / H2O cracking process, the reduction step is generally carried out at a higher temperature, while the oxidation step is carried out at a relatively lower temperature. The large temperature fluctuations between reduction and oxidation, while providing the thermodynamic driving force, also result in significant energy losses due to periodic temperature rises and falls, thus affecting the improvement of system efficiency. Therefore, how to reduce the reduction reaction temperature and narrow the temperature difference between the oxidation and reduction reactions while ensuring CO2 / H2O cracking efficiency is currently a hot research topic in this technology.

[0005] Among the many measures to lower the reaction temperature of the reduction step and reduce the temperature difference in the thermochemical cycle, introducing a reduction sacrificial agent, represented by CH4, to construct a solar-powered methane chemical chain dry reforming cycle is one of the most effective methods. Taking the methane chemical chain dry reforming reaction as an example, in the reduction reaction, the lattice oxygen released by the oxygen carrier can selectively oxidize methane partially into syngas (H2 and CO); in the oxidation reaction, the reduced oxygen carrier reacts with CO2, replenishing the lattice oxygen and releasing CO.

[0006] The reduction reaction in a methane-driven solar thermochemical cycle significantly lowers the reduction temperature for solar thermochemical fuel production, thereby improving system energy efficiency. Several researchers have conducted experimental studies on methane-driven solar thermochemical cycles. H. Zhang et al. used a solar simulator to drive dry reforming of the methane chemical chain. This study achieved a peak methane conversion rate of 90% at a reduction temperature of only 1150 K, with a throughput of 23.5 mL / min. -1 ·g -1 The peak hydrogen yield was 11.6 mL·min. -1 ·g -1 Peak carbon monoxide yield was achieved, with a net energy efficiency of 1.93% for solar-to-fuel conversion. S. Chuayboon et al., within a temperature range of 900-1050 °C, conducted research on methane solar-chemical reforming to syngas (H2 / CO) by optimizing the structural parameters of cerium dioxide. This optimized the volumetric absorption of concentrated solar radiation, improved temperature field uniformity, reduced solar energy consumption, and achieved a solar-to-fuel efficiency of 5.6%. M. Zuber et al., within a temperature range of 800-1000 °C, successfully demonstrated feasible dry methane reforming using cerium dioxide as a medium in a concentrating solar tower under redox reforming and co-feed operation modes, achieving a maximum system thermal efficiency of 27%.

[0007] The core of methane chemical chain dry reforming cycle technology is the redox coupling reaction between methane and an oxygen carrier. Compared to traditional dry / wet methane reforming, methane chemical chain dry reforming has lower energy consumption and can directly produce high-purity syngas with an ideal H2 / CO ratio, reducing costs and improving safety. However, its cycle conversion process largely depends on the reactivity of the oxygen carrier. In particular, when the oxygen release rate of the oxygen carrier is lower than the activation rate of the CH bond in methane, some methane may fail to be oxidized in time and directly crack, leading to the formation of solid carbon deposits on the oxygen carrier particles. Carbon deposits cover the active sites on the surface of the oxygen carrier, blocking mass transfer channels and significantly reducing the reactivity of the oxygen carrier. Solving the carbon deposit problem in the solar methane chemical chain dry reforming cycle is of great significance for further improving the energy conversion efficiency of solar methane chemical chain dry reforming.

[0008] Researchers have attempted to suppress carbon formation by improving the matching between oxygen migration and methane activation rates in oxygen carriers through methods such as elemental doping, loading with active metals, and further reducing reaction temperatures. Zhang et al. promoted FeO6 octahedral distortion by doping Ce into orthorhombic perovskite LaFeO3, improving the bulk oxygen migration rate and surface oxygen exchange capacity of the oxygen carrier. After 40 consecutive cycles at 850℃, the carbon deposition selectivity decreased from 37% to 4.9%. Carrillo et al. improved the reducibility of CeO2 by doping it with metal cations, loading the noble metal Ru onto the CeO2 surface, increasing CO selectivity from 62% to 99% while significantly suppressing carbon deposition. Chen et al. achieved 96% CO selectivity while significantly suppressing carbon deposition by using a CeO2−LaFeO3 redox catalyst and lowering the reduction temperature below 850℃. However, these studies show that while carbon deposition can be suppressed, it is difficult to completely eliminate it. In actual operation, variations in operating conditions may further exacerbate the carbon deposition problem, thus affecting the overall system efficiency. Summary of the Invention

[0009] This invention breaks through the traditional approach of carbon deposition suppression, innovatively proposing to use carbon deposits as reactants in the cyclic reaction. Based on the high-performance TiO2 / TiC novel oxygen carrier, it provides a solar-powered methane chemical chain dry reforming system and method based on the TiO2 / TiC carbon deposit utilization oxygen carrier. This invention achieves high solar-fuel efficiency. At the same time, it effectively solves the problem of carbon accumulation in long-cycle cycles. Through the TiO2 / TiC methane chemical chain dry reforming cycle, CH4 can be completely converted and TiO2 can be basically completely recovered. The system has good long-term cycle stability.

[0010] The objective of this invention is achieved through the following technical solution:

[0011] A solar-powered methane chemical loop dry reforming system based on TiO2 / TiC carbon deposition utilization oxygen carrier includes a solar thermal collector, a reduction reactor, an oxidation reactor, a CO / CO2 separator, and an oxygen carrier circulation loop, wherein:

[0012] The solar thermal collector is used to provide reduction heating for the reduction reactor and the oxidation reactor. and oxidation heating amount It also provides energy for preheating methane and CO2. , ;

[0013] The solar thermal collector is thermally connected to the reduction reactor, the oxidation reactor, the material preheating channel of the reduction reactor, and the material preheating channel of the oxidation reactor, respectively, to provide the main thermal energy input required by the system.

[0014] The reduction reactor is used to receive samples preheated to the reduction temperature. Methane reacts with TiO2 oxygen carrier in a reduction reaction to produce TiC oxygen carrier and a high-temperature H2 / CO mixture. =1550K;

[0015] The oxidation reactor is used to receive TiC oxygen carrier generated by the reduction reactor and oxygen preheated to the oxidation temperature. The CO2 undergoes an oxidation reaction, regenerating the TiO2 oxygen carrier and generating a high-temperature CO2 / CO mixture. =1550K;

[0016] The gas-solid heat exchange unit includes a first heat exchanger and a second heat exchanger. The first heat exchanger is connected between the output end of the reduction reactor and the methane input pipeline, and is used to preheat the input methane using the high-temperature H2 / CO mixed gas produced by the reduction reaction. The second heat exchanger is connected between the output end of the oxidation reactor and the CO2 input pipeline, and is used to preheat the input CO2 using the high-temperature CO2 / CO mixed gas produced by the oxidation reaction.

[0017] The CO / CO2 separator is used to receive the CO / CO2 mixture after it has been cooled by the second heat exchanger, and to separate the CO product and the CO2 that is recycled back to the oxidation reactor.

[0018] The oxygen carrier circulation loop connects the solid phase output end of the reduction reactor to the solid phase input end of the oxidation reactor, and the solid phase output end of the oxidation reactor to the solid phase input end of the reduction reactor, in order to realize the closed-loop circulation of TiO2 and TiC oxygen carriers between the two reactors.

[0019] A method for solar-powered methane chemical chain dry reforming based on the above system and utilizing TiO2 / TiC carbon deposition as an oxygen carrier includes the following steps:

[0020] Step S1, Solar thermal energy supply and material preheating: The reduction heating is provided through a solar thermal collector. and oxidation heating amount Simultaneously, the first heat exchanger is used to preheat methane with the waste heat from the reduction reaction, and the second heat exchanger is used to preheat CO2 with the waste heat from the oxidation reaction.

[0021] Step S2, Reduction Reaction Step: Preheated methane is introduced into the reduction reactor and undergoes a reduction reaction with TiO2 oxygen carrier to generate TiC oxygen carrier and a high-temperature H2 / CO mixture. The high-temperature H2 / CO mixture is cooled by the first heat exchanger; part of it is output as fuel, and the other part combines with CO from the oxidation reactor. The feed ratio for the reduction reaction is... ;

[0022] Step S3, Oxidation Reaction Step: The TiC oxygen carrier generated from the reduction is fed to the oxidation reactor, where it undergoes an oxidation reaction with preheated CO2, regenerating the TiO2 oxygen carrier and generating a high-temperature CO / CO2 mixture. The high-temperature CO / CO2 mixture after the reaction is cooled by a second heat exchanger and then fed to a CO / CO2 separator. The feed ratio for the oxidation reaction is... ;

[0023] Step S4, Gas-Solid Separation and Circulation: The cooled CO / CO2 mixture is separated by a CO / CO2 separator to obtain CO product. The separated CO2 is recycled back to the oxidation reactor to continue participating in the reaction.

[0024] Step S5, closed-loop circulation of oxygen carrier: The TiO2 oxygen carrier regenerated from the oxidation reactor is circulated back to the reduction reactor to continue the reduction reaction with the newly introduced methane, completing the entire cycle.

[0025] Compared with the prior art, the present invention has the following advantages:

[0026] This invention addresses the carbon deposition problem in methane chemical chain dry reforming cycles by proposing an innovative approach that incorporates carbon deposits as reactants. Focusing on methane chemical chain dry reforming to syngas technology, and based on a novel TiO2 / TiC oxygen carrier, this invention employs thermodynamic quantitative analysis to quantitatively calculate and analyze the migration patterns and energy changes of carbon-based substances in different reaction stages of the cycle using the TiO2 / TiC oxygen carrier. The introduction of CH4 during the reduction reaction significantly lowers the reduction temperature of TiO2, making the process compatible with concentrated solar power (CSP). Compared to traditional two-step solar-powered thermochemical redox cycles, the novel TiO2 / TiC methane chemical chain dry reforming cycle provides greater thermodynamic driving force, achieving a more efficient chemical chain reforming cycle. Thermodynamic equilibrium analysis of the methane conversion steps indicates that the optimal reduction conditions for the TiO2 / TiC methane chemical chain dry reforming cycle are... =3、 =1550 K. Further analysis follows. =3、 The degree of cyclic re-oxidation and the conversion rate of carbon dioxide in the carbon dioxide cracking step at 1550 K. =3 drives the reaction heat of the carbon dioxide cracking step ( The study examined the carbon buildup and found that introducing excess CO2 during the oxidation reaction was beneficial for increasing fuel production and ensuring stable cycle operation. Regarding... The condition of =3 was adjusted by an energy boosting factor. and solar-fuel efficiency Research revealed that efficient solar energy storage is impossible when CO2 is insufficient. Gas-phase sensible heat recovery improves solar-fuel efficiency. This represents a significant improvement and has substantial value for the energy conversion and utilization of solar thermochemical cycles. Following the principles of the Box-Behnken central composite experimental design, it focuses on solar-fuel efficiency. As response variables, for three key factors , , Response surface methodology experiments were conducted, revealing the solar-fuel efficiency... The order of the significance of the influence of each factor is as follows: > > Considering the advantages of isothermal solar redox cycles, and taking into account factors such as the degree of re-oxidation, carbon buildup, and solar-fuel efficiency, Based on the analysis results of the response surface methodology, determine =3, =4.5, = Isothermal operation at 1550 K represents the optimal condition for the TiO2 / TiC methane chemical-loop dry reforming cycle. Considering 80% gas-phase sensible heat recovery, the maximum solar-fuel efficiency of the TiO2 / TiC methane chemical-loop dry reforming cycle is... It reached 53%. Attached Figure Description

[0027] Figure 1 Diagram of the dry reforming cycle of methane using TiO2 / TiC carbon deposition-utilizing oxygen carriers;

[0028] Figure 2 Gibbs free energy as a function of temperature for TiO2 / TiC methane chemical loop dry reforming cycle (blue), TiO2-based methane reforming cycle (red), and TiO2 direct decomposition (black);

[0029] Figure 3 A schematic diagram of the thermodynamic system of methane chemical chain dry reforming for the preparation of solar fuels using the TiO2 / TiC redox pair;

[0030] Figure 4 For the methane conversion process and The changes are shown in (a) (b) ;

[0031] Figure 5 For the methane conversion process and The changes are shown in (a) (b) Y C (c) Y CO (d) Y TiC ;

[0032] Figure 6 For the methane conversion process and The changes shown by (a)S C (b) S CO (c) S TiC ;

[0033] Figure 7 for =3 and The carbon dioxide cracking process differs at 1550K. and (a) Degree of re-oxidation and (b) CO2 conversion rate;

[0034] Figure 8 for =3 Thermal energies driving the carbon dioxide cracking step (kJ / mol): (a) =3、(b) =4.5、(c) =6、(d) =7.5;

[0035] Figure 9 for Carbon deposition ratio at =3: (a) =3、(b) =4.5、(c) =6、(d) =7.5;

[0036] Figure 10 for =3, =1550 K different and Lower energy enhancement factor ;

[0037] Figure 11 for =3 Energy Enhancement Factor (a) =3、(b) =4.5、(c) =6、(d) =7.5;

[0038] Figure 12 for =3 solar-fuel efficiency (a) =3、(b) =4.5、(c) =6、(d) =7.5;

[0039] Figure 13 for When the value is 3, the solar-fuel efficiency after heat recovery should be considered. (a) =3、(b) =4.5、(c) =6、(d) =7.5;

[0040] Figure 14 For (a) and (b) and (c) and Solar-fuel efficiency The response surface affected;

[0041] Figure 15 for =3, = =1550K different The degree of re-oxidation in the cycle. Detailed Implementation

[0042] The technical solution of the present invention will be further described below with reference to the accompanying drawings, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention that do not depart from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention.

[0043] This invention constructs a "carbon deposit utilization" methane chemical chain dry reforming cycle system based on the high-performance TiO2 / TiC oxygen carrier. The oxygen carrier can promptly convert the generated carbon deposits and utilize them in the oxidation step. The specific reaction process of the chemical chain dry reforming cycle is as follows: Figure 1 As shown, the reduction process begins with the partial oxidation of methane by lattice oxygen in the metal oxide: TiO₂ + CH₄ = TiO + CO + 2H₂. As the oxygen release rate of the metal oxide gradually slows down, when the migration rate of lattice oxygen cannot match the activation rate of methane, the "excess" methane will not be further oxidized and will directly decompose into hydrogen and carbon (CH₄ = C + 2H₂). At this point, unlike traditional reactions that easily form carbon deposits, based on the TiO₂ / TiC carbon deposit-utilizing functionalized oxygen carrier, and based on the selective control of the reaction products by the reactant ratio and reaction temperature, the metal oxide can be further promoted to react with the carbon on its surface to form metal carbides: TiO + 2C = TiC + CO. The reduction process can promptly react with the carbon produced by the decomposition of "excess" methane as a reactant to convert it into metal carbides, thus eliminating the need to inhibit the formation of carbon deposits. For the oxidation reaction, at the oxidation temperature, the metal carbide is first oxidized by CO2 to a lower valence metal oxide: TiC + 2CO2 = TiO + 3CO; at the same time, the generated lower valence metal oxide and the unreacted lower valence metal oxide in the reduction reaction will be oxidized to a higher valence metal oxide in the initial state of the cycle under the continuous action of CO2, and CO will be released at the same time: TiO + 3CO2 = TiO2 + 4CO, thus forming a complete redox cycle. The overall reaction equations of the methane chemical chain dry reforming cycle based on TiO2 / TiC are shown in equations (1) and (2):

[0044] (1)

[0045] (2)

[0046] To confirm the thermodynamic feasibility of the TiO2 / TiC methane chemical chain dry reforming cycle, this invention comparatively calculated the Gibbs free energy of the reduction reaction in three different thermochemical cycles based on TiO2 oxygen carriers. The results are as follows: Figure 2 As shown in the figure, the calculation results of the chemical looping dry reforming cycle show that, like the oxide-based redox cycle, the TiO2 / TiC methane chemical looping dry reforming cycle can significantly reduce the TiO2 reduction reaction temperature. Simultaneously, at the same reduction temperature, the novel cycle can provide a greater thermodynamic driving force, potentially further reducing the reduction temperature while achieving carbon deposition conversion and utilization, thus creating conditions for constructing an efficient solar isothermal cycle.

[0047] To comprehensively evaluate the application potential of the novel TiO2 / TiC methane chemical chain dry reforming cycle, this invention focuses on the solar-powered methane chemical chain dry reforming cycle. From a thermodynamic perspective, it conducts a comprehensive analysis and calculation of the TiO2 / TiC methane chemical chain dry reforming cycle, deeply investigating the influence of various thermodynamic parameters, such as reaction temperature, reactant ratio, heat dissipation, and heat recovery, on the thermodynamic equilibrium composition. By changing the reaction temperature and reactant ratio, the changes in product selectivity, reactant conversion rate, carbon deposition utilization rate, and solar-fuel efficiency are analyzed. The research results are expected to provide a new perspective on addressing the carbon deposition problem in hydrocarbon fuel chemical chain reforming.

[0048] The system structure and analysis methods are explained in detail below.

[0049] I. Thermodynamic Systems and Analytical Models

[0050] This invention studies the thermodynamic process of solar-powered methane chemical chain dry reforming cycle based on TiO2 / TiC by establishing a reaction thermodynamic analysis model. The following are the idealized assumptions used in the analysis process:

[0051] (1) It is assumed that all reactions are carried out under standard atmospheric pressure;

[0052] (2) It is assumed that the reaction can be carried out without kinetic constraints;

[0053] (3) Assume that the solar reactor is an insulating blackbody absorber;

[0054] (4) Assume that product separation does not consume additional power and the calculation is based on steady-state operation.

[0055] Using the commercial thermodynamic calculation software HSC Chemistry 10.0, the thermodynamic composition and equilibrium composition of TiO2 / TiC oxygen carriers at different reaction stages of the cycle were calculated, revealing the behavior of the system under different reaction conditions. This provides a basis and guidance for the regulation of key characteristics such as product selectivity, reactant conversion rate and cycle stability, and optimizes the thermodynamic conditions of the solar thermochemical fuel production system to achieve the goal of eliminating the impact of carbon deposition and simultaneously achieving the highest solar-fuel efficiency.

[0056] Figure 3 The diagram illustrates the process of a TiO2 / TiC methane chemical chain dry reforming cycle, which involves both material transformation and energy transfer and conversion. Regarding material transformation, when the reaction reaches equilibrium, multiple substances are produced, with the main equilibrium products being CH4, CO, CO2, C, H2, TiO2−δ, and TiC. The formation and transformation of these equilibrium components are crucial for thermodynamic analysis. This invention studies a chemical chain dry reforming cycle based on the Gibbs free energy minimization method to calculate and analyze the components at thermodynamic equilibrium. To comprehensively evaluate the system's performance, the conversion rate of reactants TiO2 and CH4 is defined (…). The yields of products such as H2, CO, C, and TiC ( ) and production selectivity ( The corresponding calculation equations are shown in equations (3) to (11). These equations not only help the present invention to quantify the formation of various products, but also provide an important basis for optimizing reaction pathways and improving the selectivity of target products. In the above equations, the subscripts ini and eq represent the initial state and equilibrium state, respectively.

[0057] (3)

[0058] (4)

[0059] (5)

[0060] (6)

[0061] (7)

[0062] (8)

[0063] (9)

[0064] (10)

[0065] (11)

[0066] II. Thermodynamic Energy Analysis of Solar Energy-Fuel Efficiency

[0067] Besides in-depth analysis of the material transformation during the cycle, the transfer and conversion of system energy are also crucial from the perspective of solar energy utilization and storage. Among these, solar-fuel efficiency is a key indicator for evaluating the thermodynamic performance of this cycle, helping to determine its potential in practical applications. For example... Figure 3 As shown, the total energy required for the two main reaction steps—methane conversion and carbon dioxide cracking—in the TiO2 / TiC methane chemical loop dry reforming cycle is provided by concentrated solar power. This energy is mainly used to heat the reactants, drive the thermochemical reactions, and mitigate the efficiency penalties associated with the reactions, heat transfer, and separation. The energy sources will be described in detail below.

[0068] Before methane conversion, the system first needs to heat the introduced methane to its reduction temperature using concentrated solar power. The energy consumed in heating CH4 is proportional to the CH4 feed ratio in the methane conversion step. (defined as the amount of CH4 added in the methane conversion step / the amount of TiO2 added), the specific expression is shown in equation (12).

[0069] (12)

[0070] According to the principle of reaction thermodynamics, the net energy required to drive the methane conversion step to a certain chemical equilibrium at a certain temperature can be calculated according to equation (13).

[0071] (13)

[0072] In the formula, and This represents the molar amounts of all possible products in the methane conversion step at equilibrium and their corresponding standard molar enthalpy of formation. Indicates ambient temperature ( The standard molar enthalpy of formation of methane at K = 298. This indicates that TiO2 is at the oxidation temperature ( The standard molar enthalpy of formation at )

[0073] Similarly, before carbon dioxide cracking, the introduced carbon dioxide also needs to be heated to its oxidation temperature. The amount of heat consumed is also related to the feed ratio in the carbon dioxide cracking step. (Defined as the amount of CO2 introduced in the carbon dioxide cracking step / the amount of TiC). The energy required to heat carbon dioxide in one cycle is shown in Equation (14).

[0074] (14)

[0075] After the methane conversion step is completed, the gaseous products separate naturally, while the solid products remain in the reactor and are further cooled to the reaction temperature of the carbon dioxide cracking step. Then, carbon dioxide is introduced to carry out the cracking reaction. Generally speaking, carbon dioxide cracking is an exothermic oxidation reaction process, but for the sake of calculation rigor, this invention still calculates and analyzes the energy change of the oxidation reaction process. The energy driving the carbon dioxide cracking reaction is shown in Equation (15).

[0076] (15)

[0077] In the formula, and It represents the molar amounts of all possible products in the carbon dioxide cracking step at equilibrium and the corresponding standard molar enthalpy of formation. and This refers to the equilibrium molar amounts of all solid substances (C, TiO, Ti, Ti2O3, etc.) and the standard molar enthalpy of formation of the product at the oxidation temperature, prior to the carbon dioxide cracking step, i.e., at the end of the reduction reaction. A negative result indicates an exothermic reaction. Considering the significant limitations and challenges of solid-phase heat recovery, the recovery of this portion of the solid product's residual heat back into the system is no longer considered. However, when... When the value is positive, it indicates that the carbon dioxide cracking reaction is also an endothermic reaction, and the reaction needs to absorb heat from concentrated solar energy to be driven.

[0078] After the carbon dioxide cracking step is completed, the unconverted CO2 introduced needs to be separated from the product CO. The change in work is minimized when the flow system undergoes a reversible isothermal and isobaric change. Therefore, for an isothermal and isobaric process, the minimum work required for the separation process is... The change in work done by the product stream and the feed stream is equal to the change in Gibbs free energy. The calculation is shown in equation (16).

[0079] (16)

[0080] in, Represents a stream molar flow rate, Represents a stream Middle substances The molar concentration is set at 99.99%, and the separation purity is set at 99.99%. Equation (14) gives the minimum energy required to separate the mixture, but this is only the thermodynamic limit. The actual separation process must consider energy dissipation. Based on the current research progress in gas separation, the separation efficiency is... The ratio of minimum power consumption to actual power consumption is set to 15%. This represents the energy required to separate the CO2 and CO mixture. The calculation method is shown in equation (17).

[0081] (17)

[0082] After a TiO2 / TiC methane chemical chain dry reforming cycle, in order to continue the next cycle, all solid products need to be reheated to the reduction temperature to prepare for the reduction reaction in the second cycle. The energy required to heat the solid from the oxidation temperature to the reduction temperature (…) As shown in equation (18).

[0083] (18)

[0084] in, and These represent the equilibrium molar amounts of all solid-phase products after the carbon dioxide cracking step and before the methane conversion step, and their standard molar enthalpy of formation at the reduction temperature. and Let represent the equilibrium molar amounts of all solid-phase products after the carbon dioxide cracking step reaches equilibrium, and represent the standard molar enthalpy of formation of these substances at the oxidation temperature. The difference between these two values ​​is _____. It should be noted that, while the reduction temperature is generally higher than the oxidation temperature, this invention, for the sake of rigorous and comprehensive calculations, also considered the case where the oxidation temperature is higher than the reduction temperature. In this case, [further details may be needed]. The case where the value is negative, but the study cannot include it in energy recovery, in this case... Record it as 0.

[0085] In thermochemical redox cycles, heat dissipation is crucial to the utilization of thermal energy. In solar reactors, heat loss from the walls to the environment is directly proportional to the energy required for solar-driven reduction, heating of carbon dioxide, and warming of solids. This loss can be assessed using a loss factor. To quantify. Heat loss energy due to heat dissipation ( The calculation of ) is shown in equation (19).

[0086] (19)

[0087] In the formula, This represents the heat lost through the wall via conduction and convection. The variation range is usually set between 0 and 0.3. In this invention, it is taken as... =0.15.

[0088] Besides heat loss, the energy driving the TiO2 / TiC methane chemical chain dry reforming cycle is provided by solar energy, and radiation loss, a crucial factor that cannot be ignored, must also be considered. In the process of utilizing solar energy, the solar absorption efficiency of the solar reactors and heaters required for the methane conversion step and the carbon dioxide cracking step is calculated as shown in equations (20) and (21).

[0089] (20)

[0090] (twenty one)

[0091] in, Solar irradiance, This represents the ratio of solar flux concentration to total solar flux concentration. This is the Stefan-Boltzmann constant. Based on previous reports of thermodynamic analyses of thermochemical cycles, [the value is...]. and The values ​​are set to 3000 suns and 1000 W / m 2 .

[0092] With radiation loss taken into account, the solar energy required to drive the methane conversion and carbon dioxide cracking steps for the solar reactors and heaters is calculated as shown in equations (22) and (23).

[0093] (twenty two)

[0094] (twenty three)

[0095] Based on the above energy composition analysis, the total solar energy consumption for driving the TiO2 / TiC methane chemical chain dry reforming cycle can be calculated as shown in equation (24).

[0096] (twenty four)

[0097] This invention focuses on solar-driven dry reforming technology for methane chemical chains, and therefore employs an energy-enhancing factor. (Energetic upgrade factor) and solar-to-chemical energy conversion efficiency Two key indicators, solar-to-fuel efficiency, are used to conduct a preliminary assessment of the conversion of solar energy to chemical energy. The calculation method is shown in equations (25) and (26).

[0098] (25)

[0099] (26)

[0100] In the formula, For high heat generation, and These represent the total amounts of H2 and CO produced in the TiO2 / TiC methane thermal redox cycle, respectively. (Fuel and reducing agent) They are respectively: =283 kJ / mol =286 kJ / mol =890 kJ / mol.

[0101] In solar-powered chemical loop dry reforming cycles, heat recovery is also a key factor affecting thermal energy utilization efficiency. For the TiO2 / TiC methane chemical loop dry reforming cycle studied in this invention, gas-phase heat recovery technology is used to recover heat from the high-temperature gaseous products (H2, CO, CO2) in the methane conversion and carbon dioxide cracking steps, and the impact of the recovered heat energy on solar-fuel efficiency is investigated. The impact of heat recovery. Here, it is assumed that after heat recovery, the temperature of the gaseous products drops to ambient temperature. =298K, with This indicates the amount of heat recovered. For heat recovery efficiency. According to reported results, current technology can achieve a maximum efficiency of nearly 90% for gas-phase heat recovery, here... The value is taken as 80%. The amount of heat recovered is shown in equation (27).

[0102] (27)

[0103] Solar-fuel efficiency after considering the factors affecting heat recovery The calculation formula is shown in formula (28).

[0104] (28)

[0105] III. Results and Discussion

[0106] 1. Thermodynamic equilibrium analysis of the reduction process based on temperature and reactant ratio

[0107] The TiO2 / TiC methane chemical chain dry reforming cycle is based on the redox reaction of TiO2 / TiC. From a theoretical perspective, the reactant ratio of the methane conversion step ( ) and reduction temperature ( The reactant ratio has a significant impact on the degree of reduction and thermodynamic equilibrium. To determine the optimal reactant feed ratio for the methane conversion step... ) and reduction temperature ( This invention details the conversion rates of reactants (TiO2 and CH4), the yields of products (H2, CO, C, and TiC), and their selectivity in the methane conversion step, varying with the reactant ratios in the methane conversion step. ) and reduction temperature ( The changing trend of ).

[0108] The upper limit of fuel yield (including CO and H2) in a methane chemical loop dry reforming cycle depends largely on the conversion rate of the reducing agents (CH4 and TiO2). For example... Figure 4 As shown in (a), in the methane conversion step, the methane conversion rate is not affected by the reactant feed ratio within the reduction reaction temperature range of 600-2000 K. The effect of reduction temperature is significant, and it varies with the reduction temperature. The changing patterns of methane are almost identical. At around 600 K, the reaction with TiO2 becomes increasingly significant, reducing the high-valence metal oxide TiO2 to a lower valence state. At around 1200K, the conversion rate of methane slows down, at which point methane begins to undergo a cracking reaction. With increasing reduction temperature... When the K level is raised to around 1550 K, the methane conversion rate approaches 100%. Figure 4 (b) shows the conversion rate of TiO2 at the reduction temperature. At 600K, TiO2 begins to transform, reacting with methane and being reduced to lower-valence metal oxides. At around 1400 K, the conversion rate of TiO2 slows down. At this point, the oxidized form of titanium reacts with elemental carbon to form TiC. The conversion rate of TiO2 varies with the reactant feed ratio (…). Increased by ) When it is 3, when At 1500K, the TiO2 conversion rate reaches 100%, with TiO2 almost completely converted to TiC. According to the reaction equation TiO2 + 3CH4 = TiC + 2CO + 6H2, theoretically, CH4 and TiO2 should achieve a 100% conversion rate at the same temperature, but... At 1500K, TiO2 can be completely converted, but CH4 cannot achieve a 100% conversion rate. This conversion varies with the reactant feed ratio (…). As the amount of CH4 continues to increase, the conversion rate of TiO2 reaches 100%, theoretically making it impossible to oxidize CH4 further using the oxygen atoms released from TiO2. However, when the amount of CH4 increases, the conversion rate remains the same, indicating that excess CH4 can still undergo conversion. This suggests that CH4 may have other conversion pathways at this point. Therefore, this invention compares and analyzes the yields and selectivity of the products (H2, CO, C, and TiC) of the methane conversion step, and explores the conversion pathways of CH4 in depth.

[0109] H2 yield is related to reactant feed ratio ( Insensitive. Regardless Regardless of the changes, the H2 yield curve maintains the same pattern of change. At temperatures <1400 K, the yield of H2 increases with increasing reduction temperature, indicating that methane continuously converts to H2 with rising reduction temperature. When At approximately 1400 K, the yield of H2 is nearly 100%, and as... With further increases in [amount], the yield of H2 no longer changes. Combined with [other factors]... Figure 4 Analysis of the results in (a) shows that the methane conversion rate reached 100%, indicating that all hydrogen atoms in the methane were ultimately converted to H2, with no H2O generated. It should be noted that the H2 may originate from the partial oxidation reaction of CH4 with metal oxides releasing oxygen, or it may originate from the direct cracking of CH4 at high temperatures. To study the specific process of CH4 conversion from a thermodynamic perspective, this invention analyzed the yield variations of C, CO, and TiC. For example... Figure 5 As shown in (b), with increasing reduction temperature, in all Under these conditions, the yield of C showed an overall trend of first increasing and then decreasing, and the yield of C remained at a relatively high level in the temperature range of 1000-1400 K. Combined with... Figure 4 Calculations show that when the reduction temperature reaches 1000 K, over 80% of the methane has been converted, while the conversion rate of TiO2 is less than 10%. This phenomenon indicates that the oxygen release rate of the oxygen carrier is lower than the activation rate of the CH bond in methane, resulting in some methane being directly cracked due to insufficient oxidation, with a large amount of CH4 being converted to C. When the reduction temperature continues to rise to 1400 K and above, the conversion rate of the oxygen carrier TiO2 increases significantly, promoting the conversion of CH4 to CO. When the reduction temperature reaches 1550 K and... When C is ≤3, the yield of C will approach 0. Figure 5 (c) It can be seen that at this time, more of the C in CH4 will be converted into CO. However, when the initial amount of CH4 is further increased, the oxygen atoms released by the reduction of TiO2 cannot convert all of the CH4 into CO. Excess CH4 will still produce carbon deposits through cracking, which will hinder the stable operation of the cycle.

[0110] To avoid the impact of carbon deposits on thermochemical cycle performance, this invention innovatively constructs a novel carbon deposit utilization cycle. Therefore, the core of the research is to control the reaction conditions to further convert C into TiC. TiC can convert and store the carbon deposits that would otherwise be generated separately. A higher TiC yield is more beneficial for fuel generation in subsequent oxidation reactions. Figure 5As shown in (d), regardless of the amount of CH4 added, the initial reduction temperature for TiC formation is approximately 1400 K. With increasing reduction temperature, the yield of TiC increases rapidly, indicating that high temperature favors TiC formation. At 1550 K, the yield of TiC gradually stabilized and no longer changed with further increases in reduction temperature. Figure 5 From (b) and (c), we can see that the reduction temperature At 1500 K, the yield of C decreases slightly, while the yield of CO increases slightly, indicating that at this temperature, C in CH4 thermodynamically tends to undergo partial oxidation, converting into CO and TiC. However, as the reaction temperature increases, the thermodynamic driving force of the CH4 cracking reaction becomes stronger, and the yields of CO and TiC decrease accordingly.

[0111] To investigate the specific conversion pathways of carbon in the methane conversion steps of the TiO2 / TiC methane chemical chain dry reforming cycle, this invention compares and analyzes the selectivity variations of C, CO, and TiC. For example... Figure 6 As shown, in the same and At this temperature, the sum of the selectivities for C, CO, and TiC equals 1. With increasing reduction temperature, the selectivity for C generally decreases, while the selectivities for CO and TiC increase. At around 1000K, the selectivity of C begins to decrease slowly from 1, while the selectivity of CO begins to increase slowly. CH4 begins to be converted to CO in small amounts. At 1400K, the selectivity for C decreases significantly, the selectivity for CO increases substantially, and the selectivity for TiC also begins to increase. At this point, TiC begins to form. ≤3, At 1550 K, the selectivity for C, CO, and TiC gradually stabilizes and no longer changes with further increases in reduction temperature. It is worth noting that when excess methane is introduced ( When >3), a slight increase in the selectivity of C can be observed in the range of approximately 1550-1600K, while a slight decrease in the selectivity of CO and TiC is observed. This is due to the competitive nature of different reactions involving C in CH4 during the conversion process.

[0112] When the feed ratio of the methane conversion step =3, reduction temperature At ≥1550K, the conversion rate of reactants (CH4 and TiO2) reaches 100%, and the yields of products (H2, C, CO and TiC) tend to stabilize. =3, At K = 1550, the yield and selectivity of C are 0, while the selectivity of CO and TiC reaches its maximum. All C in methane is converted to CO and TiC, with no carbon deposits formed. Considering the reactant conversion rate, product yield, and selectivity in the methane conversion process, the optimal conditions for the methane conversion step in the TiO2 / TiC methane chemical loop dry reforming cycle are determined to be: =3, =1550 K. Therefore, this invention will be based on =3, Under the reduction reaction conditions of 1550 K, we will continue to conduct thermodynamic analysis on the subsequent TiO2 / TiC methane chemical chain dry reforming cycle carbon dioxide cracking steps.

[0113] 2. Thermodynamic equilibrium analysis of the oxidation process based on temperature and reactant ratio

[0114] For the oxidation reaction in the TiO2 / TiC methane chemical loop dry reforming cycle, its key performance aspects are twofold: firstly, to maximize the conversion of CO2 to CO for efficient fuel production; and secondly, to re-oxidize previously generated reduction products such as TiC, Ti2O3, and TiO back to TiO2 for the next cycle, ensuring stable and continuous operation of the thermochemical cycle. Therefore, the CO2 conversion rate and the degree of re-oxidation in the cycle were first investigated. Figure 7 (a) is shown as =3, Under reduction reaction conditions at 1550 K, the degree of re-oxidation in the cycle, i.e., the ratio of TiO2 generated after the oxidation reaction to the initial TiO2 input in the cycle, increases with oxidation temperature. Feed ratio of carbon dioxide cracking step The changes. For example Figure 7 As shown in (a), the CO2 input has a significant impact on the degree of recycling and re-oxidation. When When the concentration is ≤3, complete re-oxidation of TiO2 can only be achieved when the oxidation reaction temperature is below 1200 K. However, when the amount of CO2 passing through ( When >3), in Complete reoxidation of TiO2 can be achieved across the entire range ≤1600 K, thus ensuring stable progress in the next thermochemical cycle. Regarding CO2 conversion, the study revealed a very interesting phenomenon. In common two-step thermochemical cycle reactions, reduction reactions are generally endothermic, while oxidation reactions are generally exothermic. Therefore, lowering the oxidation temperature thermodynamically promotes the cracking of CO2 or H2O, thereby increasing fuel yield. However, in the TiO2 / TiC methane chemical chain dry reforming cycle studied in this invention, regardless of the CO2 feed ratio, the CO2 conversion rate decreases to varying degrees as the oxidation temperature decreases. When the value is 1.5, CO2 can be completely converted at reaction temperatures above 1200 K. When When the ratio is 3, theoretically the requirement for the CO2 feed ratio in the oxidation reaction equation TiC + 3CO2 = TiO2 + 4CO is met, but at this point the CO2 conversion rate is... It reaches its peak at around 1250K, but does not achieve 100% CO2 conversion. When Further increases will lead to a slight decrease in CO2 conversion rate. When excessive CO2 is introduced... When the temperature is ≥3), the carbon dioxide conversion rate reaches its peak at approximately 1000 K, and further increasing the oxidation temperature at this point will not affect the CO2 conversion rate.

[0115] The endothermic and exothermic characteristics of oxidation reactions not only affect the thermodynamic equilibrium of the oxidation reaction, but also reduce the energy conversion efficiency of solar-to-fuel reactions because additional energy is required to drive the endothermic oxidation reaction. To further investigate the impact of the endothermic and exothermic characteristics of oxidation reactions on cycle performance, this invention... Under the condition of 3, the endothermic and exothermic characteristics of the oxidation reaction change with oxidation temperature. Feed ratio of carbon dioxide cracking step The variation pattern was calculated and analyzed, and the results are as follows: Figure 8 As shown, it can be clearly observed that within the studied oxidation temperature range, the oxidation reaction does not solely exhibit exothermic characteristics, and is endothermic for the vast majority of the oxidation temperature range. On the other hand, the endothermic / exothermic characteristics of the oxidation reaction are relevant to the feed ratio in the carbon dioxide cracking step. Insensitive to changes, different The endothermic and exothermic patterns are basically the same. Oxidation reactions tend to exhibit exothermic characteristics at higher reduction temperatures and lower oxidation temperatures. When At K ≥1550, the oxidation reaction is exothermic regardless of the oxidation temperature. Similarly, when At K ≤750, the oxidation reaction is also exothermic. Based on the above analytical conclusions, Figure 7(b) The reaction condition under which CO2 decreases with decreasing oxidation temperature should be: ≥1550 K, and At ≤750K, the oxidation reaction is exothermic. Lowering the oxidation temperature should favor CO2 conversion, thus increasing the CO2 conversion rate. However, due to… Figure 7 (b) The results show that a lower oxidation temperature Feed ratio compared to carbon dioxide cracking step This is detrimental to improving the efficiency of the cycle, and the large temperature difference between the reduction and oxidation reactions also leads to significant energy loss in the cycle. This analysis indicates that the key reason for the reduced CO2 conversion rate is not the change in the endothermic and exothermic characteristics of the oxidation reaction, but rather the competition between other reactions and the CO2 cracking reaction, thus reducing the CO2 conversion efficiency. Thermodynamic analysis shows that almost no CO is generated at lower oxidation temperatures. Therefore, it is speculated that a disproportionation reaction between TiC and CO may occur at lower oxidation temperatures (TiC + 4CO = TiO2 + 4C + CO2). The CO generated from CO2 conversion in the oxidation reaction will react with TiC to generate C and CO2 at lower oxidation temperatures. Therefore, this invention studies the generation of C during the oxidation reaction.

[0116] like Figure 9 As shown, the ratio of C generated after the oxidation reaction to the initial CO2 input in the oxidation reaction is defined as the carbon deposition ratio of the oxidation reaction. When When the ratio is 3, introducing different proportions of CO2 will produce carbon deposits during the oxidation reaction, and the reduction temperature has little effect on the proportion of carbon deposits in the oxidation reaction. When =3, At K ≤1200, all oxidation reactions produce carbon deposits. Furthermore, carbon deposits become increasingly severe with decreasing reduction and oxidation temperatures. As the proportion of CO2 increases, the oxidation temperature range for carbon deposit formation significantly narrows, and the proportion of carbon deposits also gradually decreases. This may be because introducing excess CO2 can effectively inhibit the disproportionation reaction between TiC and CO, and convert some of the generated C into CO, promoting fuel production. Therefore, introducing excess CO2 during the oxidation process is crucial for increasing fuel yield and ensuring stable cycle operation.

[0117] 3. Solar-fuel efficiency analysis

[0118] This invention utilizes solar energy to drive a dry reforming cycle of methane. The energy conversion characteristics of the cycle are crucial to the entire system and are the focus of this invention. On one hand, in the dry reforming cycle of methane, CH4 and CO2 participate in the cycle reaction as feedstocks and are converted into H2 and CO gaseous fuels. Clearly, the energy of the H2 and CO generated in the cycle should be higher than the energy of the input CH4 for the material conversion involved in the cycle to be meaningful. On the other hand, the cycle in this invention is driven by solar energy, and the energy conversion efficiency from solar energy to fuel chemical energy is also a core parameter for evaluating cycle performance. Therefore, based on thermodynamic equilibrium analysis, the energy conversion efficiency of the cycle is calculated when… When =3, different and the energy enhancement factor at the cyclic reaction temperature and solar-to-fuel energy conversion efficiency To evaluate the energy conversion characteristics of the two aspects mentioned above in the cycle.

[0119] Energy booster The value reflects the relative amount of fuel produced in a cycle to the energy input into the fuel system, and also reflects the amount of solar energy stored in the form of chemical energy, for example... =1.20 means that 20% of solar energy can be stored as chemical energy for the recycling of fuels. For example... Figure 10 As shown, energy enhancement factor Overall, with oxidation temperature and the feed ratio of the carbon dioxide cracking step The increase is due to the rise in the feed ratio of the carbon dioxide cracking step. Energy-enhancing factors The impact is significant, when When set to =1.5, regardless of oxidation temperature How it changes, its The value is much smaller than the same , The case of ≥3 indicates that effective solar energy storage cannot be achieved under conditions of insufficient CO2. This phenomenon may be attributed to... The lower degree of re-oxidation at a ratio of 1.5, due to insufficient CO2, severely limits the production of carbon monoxide in the carbon dioxide cracking step. The condition of 1.5 is insufficient to drive deep oxidation of TiC and is not suitable for increasing U.

[0120] like Figure 11 As shown When the energy enhancement factor U is 3, When ≥3, as long as the oxidation temperature is... >1000K can enable the energy enhancement factor. >1, achieving effective solar energy storage. For When =3, when The maximum energy boost factor under this condition can be achieved at 1250 K. =1.27. At this point, the energy enhancement factor will not increase further as the oxidation temperature increases. Combined with... Figure 7 (b) It can be seen that this is because increasing the oxidation temperature at this point will not promote the further conversion of CO2 into CO, therefore the energy enhancement factor cannot be further increased. When the feed ratio of the carbon dioxide cracking step... When the value is greater than 3, its effect on energy enhancement factor is... The impact is relatively small; as the input CO2 continues to increase, the energy enhancement factor... It can only be increased slightly to 1.28. This is because when the provided CO2 can completely oxidize TiC, further increasing CO2 does not promote the generation of fuel CO, and therefore does not provide further benefit for solar energy storage. Oxidation temperature and reduction temperature The effect of energy-enhancing factors can be further explored through... Figure 11 This is reflected in the fact that when... Oxidation temperature <1550K The effect on U is significant, depending on the feed ratio at different carbon dioxide cracking steps. Below, with oxidation temperature The increase in energy-boosting factor A significant increase, when Oxidation temperature at ≥1550 K Energy-enhancing factors The impact is significantly reduced. To allow more solar energy to be stored and utilized, the cycle should be completed as much as possible. ≥3, ≥1550 K and Operate under conditions of ≥1250 K.

[0121] The above analysis shows that effective solar energy storage cannot be achieved when CO2 is insufficient. Therefore, this section analyzes... =3、 The energy conversion efficiency of solar energy to fuel is ≥3. For example... Figure 12 As shown, when The TiO2 / TiC methane chemical loop dry reforming cycle has the highest efficiency when the value is 3. With... The continuous increase in solar-fuel energy conversion efficiency The peak value gradually decreases. Therefore, it can be inferred that although increasing the amount of CO2 used is beneficial for CO generation in the carbon dioxide cracking step, the additional energy consumption from heating excess CO2 will reduce solar-fuel efficiency. Solar energy-fuel efficiency With reduction temperature The increase in efficiency and decrease in efficiency are due to the fact that reducing methane to CO requires less energy than reducing it to TiC. (Solar-fuel efficiency) With oxidation temperature The overall trend of the increase is first increasing and then decreasing, according to Figure 7 (b) and Figure 9 It is known that at lower oxidation temperatures, a significant portion of carbon dioxide is decomposed into carbon, which consumes more energy than the decomposition of carbon dioxide into CO. Therefore, in the oxidation temperature range where the carbon dioxide conversion rate does not reach 1, the solar-fuel efficiency is relatively low. With oxidation temperature It increases with the rise in temperature. According to... Figure 8 It can be seen that at the oxidation temperature At higher temperatures, with increasing oxidation temperature As the temperature rises, the oxidation reaction requires more heat to absorb, thus affecting the solar-fuel efficiency. It will vary with oxidation temperature The efficiency decreases as the solar-fuel efficiency increases. However, it's important to note that achieving the calculated solar-fuel efficiency requires... To address this, the problem of carbon buildup must be solved. Therefore, it is necessary to confirm the reduction temperature. During the oxidation process, it is crucial to maximize the yield of TiC in the methane conversion step, thereby converting and storing as much of the carbon deposits that would otherwise be generated during the reduction reaction as possible. This requires confirming the oxidation temperature. When operating within the specified range, it is crucial to ensure that the CO2 conversion rate in the carbon dioxide cracking step reaches 1, with no carbon buildup. Combined with... Figure 5 (d) Yield of TiC Figure 7 (b) CO2 conversion rate and Figure 9 The carbon deposition ratio, represented by the red line segment, indicates the solar-fuel efficiency. Areas without carbon buildup are marked on the cloud map. Under the condition that =3, = 1550 K, Solar-fuel efficiency of TiO2 / TiC methane chemical loop dry reforming cycle at 1300 K The maximum value of 48% was achieved. It should be noted that, according to... Figure 7 (a) It can be seen that when the oxidation temperature... At K ≥1200, the degree of re-oxidation in the cycle cannot reach 100%, meaning that TiO2 cannot be completely restored to its initial state. Although the above cyclic reaction conditions can achieve the highest energy conversion efficiency in the first cycle, incomplete re-oxidation will lead to inefficient and unstable operation in subsequent cycles, thus reducing energy conversion efficiency. When =4.5, = 1550 K, Solar-fuel efficiency of TiO2 / TiC methane chemical loop dry reforming cycle at 1050 K The maximum value of 45% was obtained under this condition.

[0122] The above analysis shows that the TiO2 / TiC methane chemical loop dry reforming cycle can achieve high solar-to-fuel energy conversion efficiency at high reduction and oxidation reaction temperatures without carbon deposition. Therefore, a significant amount of sensible heat is stored in the unreacted gaseous reactants and the generated gaseous products. Effective recovery of this sensible heat would help further improve the overall energy utilization efficiency of the system. This is based on... Figure 12 Further analysis was conducted on the impact of gas-phase sensible heat recovery on the cycle energy in reaction conditions without carbon deposition. For example... Figure 13 As shown, the energy conversion efficiency under all reaction conditions was significantly improved after heat recovery was added, indicating that gas-phase sensible heat recovery has significant value for the energy conversion and utilization of solar thermochemical cycles. The solar-fuel efficiency of the first cycle, after heat recovery was included, was... Still =3, =3, = 1550 K and The maximum solar-fuel efficiency is achieved at 1300 K. It can significantly increase from 48% to 58%. When When =4.5, at =1550 K、 Within the K range of 1050-1300 K, the maximum solar-fuel efficiency remains at around 54%, representing an improvement of approximately 10% compared to the state without heat recovery. The graph shows that the oxidation temperature... Compared to reduction temperature Feed ratio compared to carbon dioxide cracking step For solar-fuel efficiency The impact is relatively small, therefore in =3, Under isothermal cycling conditions at 31550K, the system's solar-fuel efficiency It can still reach 57%, when When the efficiency is 4.5, the system's solar-fuel efficiency is... It can still reach 53%. Isothermal operation avoids the repeated heating and cooling of solid materials between different reduction and oxidation temperatures in the redox cycle, greatly reducing the complexity of the cycle equipment and operation. Given the significant advantages of isothermal solar redox cycles, a small sacrifice in solar-fuel efficiency is made for greater system operational flexibility. It is also worth noting that... =4.5 and reduction temperature Under the condition of 1550 K, oxidation temperature Within a certain range, the energy conversion efficiency remains high. This indicates that the TiO2 / TiC methane chemical-loop dry reforming cycle differs from traditional thermochemical cycles. On the one hand, it can achieve a higher isothermal cycle energy conversion efficiency, and on the other hand, it can effectively reduce the impact of cycle temperature difference on energy conversion efficiency, which is of great significance for the actual operation of the system. When the cycle operates under isothermal conditions, even when the system experiences a large cycle temperature difference due to operational issues, the energy conversion efficiency does not decrease significantly. The TiO2 / TiC methane chemical-loop dry reforming cycle exhibits strong resistance to temperature fluctuations under these conditions. However, when the CO2 input further increases, the cycle's peak energy conversion efficiency gradually decreases, dropping to 52% (…). =6) and 50% =7.5). In addition, the system's resistance to temperature fluctuations is significantly weakened. For example, with a value of 7.5, when the oxidation temperature varies between 1100-1550 K, the energy conversion efficiency will decrease from 50% to 47%, significantly increasing the requirements for system operation.

[0123] 4. Analyze the optimal operating conditions of the loop based on the response surface methodology.

[0124] The above analysis results demonstrate the energy conversion law of the system under different reaction conditions, and reveal three main factors in the TiO2 / TiC methane chemical chain dry reforming cycle. , , Solar-fuel efficiency The mechanism of action of the system is understood. However, due to the discontinuity of variable values ​​and the limitations of the analytical methods, determining which influencing factor is more dominant and whether energy conversion efficiency can be further improved through reaction condition optimization are crucial factors for the future stable and efficient operation of the system. Therefore, this invention addresses the solar-fuel efficiency... Response surface methodology was used to investigate the effect of various operating parameters on solar-fuel efficiency. The significance of the impact and the mutual influence between the factors.

[0125] Table 1 Response Surface Experimental Design

[0126]

[0127] Table 2 Results of Box-Behnken Response Surface Experiment

[0128]

[0129] Table 3. Analysis of Variance for Regression Models

[0130]

[0131] Note: P<0.05 indicates significance, and P<0.01 indicates extremely significant.

[0132] Response surface methodology remains unchanged. The condition of 3 follows the principles of the Box-Behnken central composite experimental design, using solar-fuel efficiency as the basis. As response variables, for three key factors , , Response surface methodology experiments were conducted, and the detailed experimental design is shown in Table 1, which considers three key factors. , , Three numerical factors were set as experimental parameters, each with three levels and a center point count of 5, resulting in 17 control experiments. The results of the Box-Behnken multiple regression fitting experiment are shown in Table 2, and the solar-fuel efficiency was obtained based on the experimental results. The equation for the ternary quadratic regression model is:

[0133] =534.68425-0.715755A+0.173595B-12.70800C+0.000073AB+0.006000AC-0.000667BC+0.000177A 2 -0.000095B 2 +0.222556C 2

[0134] The results of the analysis of variance are shown in Table 3. The significance level of this model is... A value below 0.0001 indicates that the regression model has a highly significant fit. In the equation, both the linear and quadratic terms are significant, indicating that the influence of each specific factor on the response value is not a simple linear relationship, suggesting a high degree of complexity in the operation of the TiO2 / TiC methane chemical chain dry reforming cycle system. (Coefficient of determination) =0.9960, Adjusted coefficient of determination =0.9907, Predicted coefficient of determination =0.9484, Adjusted coefficient of determination With the predicted coefficient of determination The difference is less than 0.2, indicating that the equation has high reliability and accuracy, and the model has a good fit; the lack-of-fit term... =0.0871, which is greater than 0.05, indicating that the lack-of-fit term is not significant. This suggests that the experimental error is small, and therefore this model can be used to measure solar-fuel efficiency. Prediction and analysis of optimization methods.

[0135] , , Significance levels of the three factors The values ​​are all below 0.05, which is detrimental to solar-fuel efficiency. The impact is significant, among which and Significance level of the two factors A value below 0.01 indicates a solar-fuel efficiency. The most significant influencing factors. The magnitude of the value can also directly reflect the degree of influence of each factor on the response value. The larger the value, the more significant the influence of that factor on the response value. (Based on the comparison table 3...) The magnitude of the value determines the order in which the influence of various factors on the solar-fuel efficiency η is ranked: (C)> (A)> (B). Further analysis of the lack-of-fit terms in AB, AC, and BC. ,Discover and The interaction between them is quite significant, while and and and The interaction between them is not obvious.

[0136] To delve deeper into the interactions between various factors and to summarize experimental methods for confirming the optimal operating conditions of a solar thermochemical redox cycle system, this invention is based on solar-fuel efficiency. The ternary quadratic regression model equation is obtained as follows: , , Three key factors interact in pairs to affect solar-fuel efficiency. The response surface plot shows the impact of various factors on solar-fuel efficiency. The steepness of the response surface directly reflects the effect of each factor on solar-fuel efficiency. The steeper the response surface, the greater the impact of this factor on solar-fuel efficiency. The greater the impact, the better. Figure 14 As shown, solar-fuel efficiency and , and Both showed a negative correlation. Response surface ratio of factors The response surface of the factor is steep. Factors affecting solar-fuel efficiency The degree of influence is greater than Factors. According to Figure 14 (b) and (c), regarding The response surface of the factor is very steep. Solar-fuel efficiency The impact far exceeds and Factors. Further analysis. Figure 14 From (a) to (c), we can see that regarding and The response surface of the factors is relatively flat, solar-fuel efficiency It is relatively insensitive to temperature changes. Factors on The effect is particularly significant, and With reaction temperature and The interaction effects are significant. Through response surface methodology, the study found that improving the solar-fuel efficiency of the solar thermochemical redox cycle is crucial. First, an appropriate reduction reaction temperature range needs to be determined experimentally. Then, a suitable feed ratio needs to be determined. Based on this, the oxidation reaction temperature can be further adjusted. This method helps the system better control the reaction process and find the optimal operating conditions for efficient solar energy utilization.

[0137] In the previous analysis, =3, At 1550K, when the feed ratio of the carbon dioxide cracking step is... Solar-fuel efficiency at =3 The highest. However, this condition only applies when the oxidation temperature is... Carbon deposits will only disappear at ≥1250K. At ≥1200K, the degree of re-oxidation in the cycle cannot reach 100%, therefore The condition of 3 is unacceptable; the optimal feed ratio for the carbon dioxide cracking step is... The oxidation temperature should be searched within the range of 3 to 4.5. The results of response surface methodology confirm this. Solar-fuel efficiency The impact is relatively small, combined with Figure 13 Solar-fuel efficiency after heat recovery The analysis suggests that increasing the oxidation temperature to allow the cycle to run under isothermal conditions, while appropriately sacrificing some solar-fuel efficiency, is necessary to obtain the significant advantages of isothermal cycling. =3, = The isothermal condition of 1550K is the most suitable cycling temperature. To confirm the optimal feed ratio for the carbon dioxide cracking step under this condition... ,right =3, = =1550K different The degree of re-oxidation under the following cycles was analyzed. For example... Figure 15 As shown, under this condition only when A pH of 4.5 is required to achieve 100% re-oxidation and ensure efficient and stable operation of the cycle. Therefore, the optimal operating conditions for the TiO2 / TiC methane chemical loop dry reforming cycle are: =3, =4.5, = =1550K, maximum solar-fuel efficiency =53%.

Claims

1. A solar-powered methane chemical looping dry reforming system based on TiO2 / TiC carbon deposition utilization oxygen carrier, characterized in that... The system includes a solar thermal collector, a reduction reactor, an oxidation reactor, a CO / CO2 separator, and an oxygen carrier circulation loop, wherein: The solar thermal collector is used to provide reduction heating for the reduction reactor and the oxidation reactor. and oxidation heating amount It also provides energy for preheating methane and CO2. , ; The solar thermal collector is thermally connected to the reduction reactor, the oxidation reactor, the material preheating channel of the reduction reactor, and the material preheating channel of the oxidation reactor, respectively, to provide the main thermal energy input required by the system. The reduction reactor is used to receive samples preheated to the reduction temperature. Methane reacts with TiO2 oxygen carrier and undergoes a reduction reaction to produce TiC oxygen carrier and a high-temperature H2 / CO mixture; The oxidation reactor is used to receive TiC oxygen carrier generated by the reduction reactor and oxygen preheated to the oxidation temperature. The CO2 undergoes an oxidation reaction, regenerating the TiO2 oxygen carrier and generating a high-temperature CO2 / CO mixture; The gas-solid heat exchange unit includes a first heat exchanger and a second heat exchanger. The first heat exchanger is connected between the output end of the reduction reactor and the methane input pipeline, and is used to preheat the input methane using the high-temperature H2 / CO mixed gas produced by the reduction reaction. The second heat exchanger is connected between the output end of the oxidation reactor and the CO2 input pipeline, and is used to preheat the input CO2 using the high-temperature CO2 / CO mixed gas produced by the oxidation reaction. The CO / CO2 separator is used to receive the CO / CO2 mixture after it has been cooled by the second heat exchanger, and to separate the CO product and the CO2 that is recycled back to the oxidation reactor. The oxygen carrier circulation loop connects the solid phase output end of the reduction reactor to the solid phase input end of the oxidation reactor, and the solid phase output end of the oxidation reactor to the solid phase input end of the reduction reactor, in order to realize the closed-loop circulation of TiO2 and TiC oxygen carriers between the two reactors.

2. The solar-powered methane chemical looping dry reforming system based on TiO2 / TiC carbon deposition utilization oxygen carrier according to claim 1, characterized in that... The =1550K.

3. The solar-powered methane chemical loop dry reforming system based on TiO2 / TiC carbon deposition utilization oxygen carrier according to claim 1, characterized in that... The =1550K.

4. A method for solar-powered methane chemical chain dry reforming based on a TiO2 / TiC carbon deposition utilization oxygen carrier using the system described in any one of claims 1-3, characterized in that... The method includes the following steps: Step S1, Solar thermal energy supply and material preheating: The reduction heating is provided through a solar thermal collector. and oxidation heating amount Simultaneously, the first heat exchanger is used to preheat methane with the waste heat from the reduction reaction, and the second heat exchanger is used to preheat CO2 with the waste heat from the oxidation reaction. Step S2, Reduction Reaction Step: Preheated methane is introduced into the reduction reactor and undergoes a reduction reaction with TiO2 oxygen carrier to generate TiC oxygen carrier and high-temperature H2 / CO mixture; after the reaction, the high-temperature H2 / CO mixture is cooled by the first heat exchanger, part of which is output as fuel product and the other part is combined with CO from the oxidation reactor. Step S3, Oxidation reaction step: The TiC oxygen carrier generated by reduction is transported to the oxidation reactor, where it undergoes an oxidation reaction with preheated CO2 to regenerate the TiO2 oxygen carrier and generate a high-temperature CO / CO2 mixed gas; the high-temperature CO / CO2 mixed gas after the reaction is cooled by the second heat exchanger and then transported to the CO / CO2 separator. Step S4, Gas-Solid Separation and Circulation: The cooled CO / CO2 mixture is separated by a CO / CO2 separator to obtain CO product. The separated CO2 is recycled back to the oxidation reactor to continue participating in the reaction. Step S5, closed-loop circulation of oxygen carrier: The TiO2 oxygen carrier regenerated from the oxidation reactor is circulated back to the reduction reactor to continue the reduction reaction with the newly introduced methane, completing the entire cycle.

5. The solar-powered methane chemical looping dry reforming method based on TiO2 / TiC carbon deposition utilization oxygen carrier according to claim 4, characterized in that... In step S2, the feed ratio for the reduction reaction... .

6. The solar-powered methane chemical looping dry reforming method based on TiO2 / TiC carbon deposition utilization oxygen carrier according to claim 4, characterized in that... In step S3, the feed ratio for the oxidation reaction... .

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