Method for preparing high-purity hydrogen through catalytic gasification of biomass containing alkali metal additive

Abstract

The invention relates to the field of hydrogen preparation, and discloses a preparation method of biomass catalytic gasification high-purity hydrogen containing an alkali metal additive. Comprising the following steps: mixing alkali metal-containing salt with cerium oxide or cerium oxide-zirconium oxide composite powder, and carrying out ion exchange, drying and roasting to obtain an alkali metal-containing oxygen vacancy composite additive; the auxiliary agent is mixed with sodium carbonate and potassium carbonate eutectic carbonate, a continuous molten phase is formed at the use temperature, and the continuous molten phase and oxygen vacancies act synergistically to prepare the catalyst. The method comprises the following steps: drying and crushing a biomass raw material, feeding into a double-temperature-zone reactor, and carrying out thermal cracking and reforming reaction in a mixed atmosphere of air and water vapor to generate mixed gas; when the activity of the catalyst is reduced, carbon dioxide-containing gas is introduced for medium-temperature regeneration, and produced gas is subjected to condensation, pressure swing adsorption or membrane separation purification to obtain high-purity hydrogen. The method is high in gasification efficiency, the catalyst can be recycled, the hydrogen purity is high, and the system stability is excellent.

Description

Technical Field

[0001] This invention relates to the field of hydrogen production, specifically to a method for producing high-purity hydrogen through biomass catalytic gasification using an alkali metal additive. Background Technology

[0002] With the transformation of the global energy structure and the proposal of low-carbon goals, hydrogen (H2) is receiving widespread attention as a clean and efficient energy carrier. Among the many hydrogen production methods, the production of hydrogen through thermochemical gasification using biomass as a raw material is highly favored due to its renewable nature, carbon cycle potential, and ability to achieve "negative emissions." Furthermore, biomass gasification technology itself has reached a certain level of maturity, capable of converting agricultural / forestry waste, straw, and sawdust into syngas and further producing hydrogen.

[0003] Despite the promising prospects of biomass catalytic gasification for hydrogen production, several key challenges remain in practical applications:

[0004] The problems of catalysts being prone to carbon buildup, tar blockage, and decreased activity are particularly prominent.

[0005] Gasification processes are typically single-temperature zones or solid-state catalytic systems, making it difficult to simultaneously address multiple reaction mechanisms, including tar cracking, steam reforming, and hydrogen generation, resulting in insufficient hydrogen selectivity and stability.

[0006] Complex impurities in the raw materials, organic tar, unreacted carbon, and low efficiency in high-temperature heat transfer and mass migration result in low system efficiency and limited economic viability. These issues have become bottlenecks for the large-scale promotion of biomass gasification hydrogen production technology. To address these problems, a method for producing high-purity hydrogen from biomass through catalytic gasification using alkali metal additives is proposed. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a method for preparing high-purity hydrogen from biomass catalytic gasification using alkali metal additives, thereby solving the technical problems existing in the prior art.

[0008] The above-mentioned technical objective of the present invention is achieved through the following technical solution:

[0009] A method for preparing high-purity hydrogen from biomass catalytic gasification using an alkali metal additive includes the following steps:

[0010] S1: Catalyst preparation, alkali metal salt is mixed with cerium oxide or cerium oxide-zirconia composite powder in a certain proportion, and then subjected to ion exchange, drying and calcination treatment to induce the formation of surface oxygen vacancies by alkali metal ions, thereby obtaining an alkali metal oxygen vacancy composite additive; the alkali metal salt is sodium, potassium or a mixture of the two;

[0011] S2: Catalyst modification and assembly, mixing an alkali metal oxygen vacancy composite promoter with a eutectic carbonate composed of sodium carbonate and potassium carbonate, so that the eutectic carbonate forms a continuous molten phase at the operating temperature, producing molten carbonate, and the molten carbonate coats or wets the alkali metal oxygen vacancy composite promoter to obtain a catalyst. The continuous molten phase and the oxygen vacancies on the surface of the alkali metal oxygen vacancy composite promoter form synergistic adsorption sites, enhancing the adsorption and dissociation activity of water vapor molecules;

[0012] S3: Dry the biomass raw material to obtain pretreated biomass;

[0013] S4: Dual-temperature zone gasification reforming. Pretreated biomass and catalyst are fed into a dual-temperature zone reactor. Gasification medium is added to maintain the gasification and reforming reaction environment. The first stage involves thermal cracking and primary gasification in the first temperature zone, while the second stage involves steam reforming in the second temperature zone. During the reaction, molten carbonate promotes water vapor adsorption and oxygen vacancies accelerate the migration of dissociation products, thereby synergistically promoting the carbon-water reaction and tar cracking, improving hydrogen production selectivity and inhibiting carbon deposition.

[0014] S5: In-situ regeneration. When the catalyst activity decreases, a regeneration atmosphere containing carbon dioxide is introduced into the dual-temperature zone reactor. Under the regeneration temperature conditions, the carbonate and carbon deposits undergo a gasification reaction to remove surface carbon deposits. At the same time, the oxygen vacancy structure is regenerated to restore catalytic performance.

[0015] S6: Gas generation cooling and purification, the reaction generated gas is sequentially subjected to dust removal, condensation and pressure swing adsorption or membrane separation to remove impurity gases and achieve hydrogen component enrichment.

[0016] Preferably, the catalyst preparation includes:

[0017] The mass ratio of the alkali metal salt to cerium oxide or cerium oxide-zirconia composite powder is 1:5 to 1:20. After mixing, the mixture is subjected to ion exchange treatment in a deionized aqueous solution at a temperature of 60 to 90°C for 1 to 3 hours. After solid-liquid separation, the mixture is dried at 100 to 120°C and then calcined at 500 to 800°C for 2 to 6 hours to obtain a structurally stable alkali metal cerium oxide composite additive with oxygen vacancies on its surface.

[0018] The molar ratio of cerium oxide to zirconium oxide in the cerium oxide-zirconia composite powder is (1-3):1, and the alkali metal salt is sodium carbonate, potassium carbonate, or a mixture thereof.

[0019] Preferably, the catalyst modification and assembly includes:

[0020] The eutectic carbonate is prepared by mixing sodium carbonate and potassium carbonate in a mass ratio of 1:1 to 1:3. When heated to a working temperature of 700 to 900°C, it forms a continuous molten phase. The molten carbonate remains liquid and stable within the temperature range of 700 to 900°C and is uniformly distributed on the surface of the alkali metal oxygen vacancy composite additive.

[0021] The contact angle between the molten carbonate and the alkali metal oxygen vacancy composite additive is no greater than 60°, forming a coating layer with a coverage ratio of no less than 80%.

[0022] The melting temperature of the eutectic carbonate is lower than the temperature of the first temperature zone of the dual-temperature zone reactor.

[0023] This is to ensure the continuous existence of the molten phase and the stability of the material structure during the catalytic reaction.

[0024] Preferably, the biomass pretreatment includes:

[0025] The biomass raw material is dried at 80-120°C until the moisture content is no more than 10%, and then crushed to a particle size of 0.1-5 mm by a pulverizer to obtain pretreated biomass;

[0026] The biomass raw material is a renewable organic solid material, selected from at least one of sawdust, rice husks, straw, coconut shells, peanut shells, cotton stalks, and sawdust.

[0027] Preferably, the dual-temperature zone gasification reforming includes:

[0028] The reaction temperature in the first temperature zone is 600-800℃, the gasification medium is a mixture of air and water vapor, the mass ratio of steam to biomass is 0.8-1.5:1, and the residence time of the gas in the first temperature zone is 2-8s.

[0029] The reaction temperature in the second temperature zone is 800-1050℃. Steam is introduced at this temperature for reforming. The residence time of the reaction gas is 6-20s. The molar ratio of hydrogen, carbon monoxide and carbon dioxide in the resulting reaction product gas is controlled at (55-70):(15-25):(10-20).

[0030] Preferably, the molten carbonate is a continuous molten phase formed by the eutectic system of sodium carbonate and potassium carbonate at the reaction temperature. The continuous molten phase is continuously distributed on the surface of the alkali metal oxygen vacancy composite additive in the dual-temperature zone reactor, maintaining the liquid contact state and achieving stable heat transfer and mass migration between the two temperature zones.

[0031] Preferably, the in-situ regeneration includes:

[0032] When the catalyst activity in the dual-temperature zone reactor decreases, the biomass feed is stopped, and a regeneration gas containing carbon dioxide is introduced into the dual-temperature zone reactor. The volume fraction of carbon dioxide in the regeneration gas is 5-15%, and the remainder is an inert gas.

[0033] The regeneration temperature is 350–550℃, the regeneration time is 10–30 min, and the regeneration gas flow rate is controlled at 0.2–0.8 L / min.

[0034] At this point, the carbonate reacts with the deposited carbon material through a gasification reaction, keeping the catalyst surface clean and allowing the oxygen vacancy structure to reform.

[0035] After regeneration, the dual-temperature zone reactor is kept in an inert atmosphere and cooled to room temperature before the gasification medium feed is resumed to enter the next reaction cycle.

[0036] Preferably, the gas generation cooling and purification includes:

[0037] The gas produced by the reaction is dusted to remove solid particles and carbon ash carried in the gas flow.

[0038] The gas after dust removal is cooled to 40-60°C by a heat exchanger, causing water vapor and heavy organic matter to condense and separate.

[0039] The gas after condensation and separation is sent to a pressure swing adsorption (PSA) unit or a hydrogen separation membrane unit for purification. The purification method is PSA or separation membrane. The operating pressure of PSA is 0.6–1.2 MPa, and the operating temperature of separation membrane is 20–40 °C.

[0040] The purified gas is output as hydrogen product, while the unadsorbed or unpermeated gas is returned to the reaction system for use as circulating gas.

[0041] In summary, the present invention has the following main beneficial effects:

[0042] By setting up a composite catalytic system consisting of an alkali metal oxygen vacancy composite promoter and sodium carbonate-potassium carbonate eutectic carbonate, a continuous molten phase is formed at the reaction temperature, which synergistically forms adsorption active sites with oxygen vacancies on the promoter surface. This makes it easier for water vapor molecules to be adsorbed and dissociated at the interface, promoting the simultaneous occurrence of carbon-water reaction and tar cracking reaction. This significantly improves the hydrogen generation rate and hydrogen selectivity, inhibits carbon accumulation on the catalyst surface, and achieves long-term stable operation of the reaction interface.

[0043] By setting up a dual-temperature zone reactor structure with independent front-end pyrolysis and rear-end steam reforming, the thermal pyrolysis and reforming reaction processes are spatially separated and the temperature gradient is controllable. In the front-end temperature zone, biomass is rapidly pyrolyzed to generate combustible gas and tar intermediates. In the rear-end high-temperature zone, tar and small molecule hydrocarbons are further reformed to generate a mixed gas dominated by hydrogen. This effectively avoids problems such as incomplete reaction coupling and secondary tar formation in single-temperature zone systems, and improves gasification efficiency and hydrogen yield.

[0044] By setting up a catalyst regeneration process under intermediate temperature conditions using carbon dioxide-containing gas in situ, the utilization of... –Carbon reaction and / The redox cycle restores the oxygen vacancy structure and removes carbon deposits, thus maintaining the catalyst surface activity for a long time and significantly extending the reaction cycle. This avoids the problems of carbonate decomposition and catalyst sintering caused by traditional high-temperature oxidation regeneration methods, achieving a balance between stable catalyst activity and long-term continuous system operation. Attached Figure Description

[0045] Figure 1 This is a flowchart of the method of the present invention. Detailed Implementation

[0046] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0047] Example 1

[0048] refer to Figure 1 A method for preparing high-purity hydrogen from biomass catalytic gasification using an alkali metal additive, comprising the following steps:

[0049] S1: Catalyst preparation, alkali metal salt is mixed with cerium oxide or cerium oxide-zirconia composite powder in a certain proportion, and then subjected to ion exchange, drying and calcination treatment to induce the formation of surface oxygen vacancies by alkali metal ions, thereby obtaining an alkali metal oxygen vacancy composite additive; wherein the alkali metal is sodium, potassium or a mixture of the two.

[0050] S2: Catalyst modification and assembly, mixing an alkali metal oxygen vacancy composite promoter with a eutectic carbonate composed of sodium carbonate and potassium carbonate, so that the eutectic carbonate forms a continuous molten phase at the operating temperature, producing molten carbonate, and the molten carbonate coats or wets the alkali metal oxygen vacancy composite promoter to obtain a catalyst. The continuous molten phase and the oxygen vacancies on the surface of the alkali metal oxygen vacancy composite promoter form synergistic adsorption sites, enhancing the adsorption and dissociation activity of water vapor molecules;

[0051] S3: Dry the biomass raw material to obtain pretreated biomass;

[0052] S4: Dual-temperature zone gasification reforming. Pretreated biomass and catalyst are fed into a dual-temperature zone reactor. Gasification medium is added to maintain the gasification and reforming reaction environment. The first stage involves thermal cracking and primary gasification in the first temperature zone, while the second stage involves steam reforming in the second temperature zone. During the reaction, molten carbonate promotes water vapor adsorption and oxygen vacancies accelerate the migration of dissociation products, thereby synergistically promoting the carbon-water reaction and tar cracking, improving hydrogen production selectivity and inhibiting carbon deposition.

[0053] S5: In-situ regeneration. When the catalyst activity decreases, a regeneration atmosphere containing carbon dioxide is introduced into the dual-temperature zone reactor. Under medium temperature conditions, the carbonate and carbon deposits undergo a gasification reaction to remove surface carbon deposits. At the same time, the oxygen vacancy structure is regenerated to restore catalytic performance.

[0054] S6: Gas generation cooling and purification, the reaction generated gas is sequentially subjected to dust removal, condensation and pressure swing adsorption or membrane separation to remove impurity gases and achieve hydrogen component enrichment.

[0055] Catalyst preparation:

[0056] The alkali metal salts are selected as sodium carbonate, potassium carbonate, or mixtures thereof; the carrier material is cerium oxide. or cerium oxide and zirconium oxide Composite powder; the molar ratio of cerium oxide to zirconium oxide in the cerium oxide-zirconia composite powder is (1-3):1, so as to ensure a balance between the oxygen storage capacity of cerium oxide and the structural stability of zirconium oxide.

[0057] When the zirconium oxide doping amount is too low, the material has poor resistance to sintering; when the doping amount is too high, the oxygen vacancy concentration is insufficient.

[0058] The mass ratio of alkali metal salt to cerium oxide or cerium oxide-zirconia composite powder is 1:5 to 1:20. When the alkali metal content is less than 1:20, the ion exchange depth is insufficient and it is difficult to form effective oxygen vacancies.

[0059] When the ratio is higher than 1:5, the alkali metal phase is prone to agglomeration and forms alkaline precipitates after calcination, which reduces the activity of the carrier.

[0060] The mixture was placed in a deionized aqueous solution for ion exchange treatment; the ion exchange temperature was controlled at 60–90℃, and the duration was 1–3 hours; during this process, alkali metal ions (… Partial replacement tetravalent cerium ions in the crystal lattice Or it can be embedded in the adjacent sites of surface oxygen vacancies to form surface charge compensation.

[0061] Its chemical reaction can be represented as:

[0062] ;

[0063] in, This indicates the formation of oxygen vacancies; Indicates replacement position ion; This refers to the amount of oxygen deficiency.

[0064] This ion exchange reaction increases the concentration of oxygen vacancies on the material surface and simultaneously introduces alkali metal activation centers, providing an energy channel for the dissociation of water molecules in the subsequent gasification reaction.

[0065] After the reaction, solid-liquid separation (using vacuum filtration or centrifugation) can effectively remove unreacted ions;

[0066] The wet material obtained by solid-liquid separation is dried at 100-120℃ to remove pore-adsorbed water and surface hydrated ions.

[0067] Then calcine at 500–800℃ for 2–6 hours to complete structural stabilization;

[0068] The roasting process promotes the reaction of alkali metal ions with... The network redistributes, and oxygen vacancies are immobilized at high temperatures, forming a stable alkali metal oxygen vacancy composite agent.

[0069] Its main solid-phase equilibrium reaction can be represented as:

[0070] ;

[0071] in, It is the stable phase formed after calcination; To release the byproduct gas, this reaction further increases the number of surface oxygen vacancies, enhances the surface electron density of the material, and thus strengthens the polarization adsorption capacity for water H2O molecules.

[0072] X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis showed that:

[0073] The calcined alkali metal-cerium oxide-zirconia composite additive showed and The coexistence peaks indicate that the oxygen vacancy structure has been stably established.

[0074] Scanning electron microscopy (SEM) revealed that the sample surface formed a porous structure with pore sizes concentrated in the range of 20–100 nm.

[0075] This porous structure provides physical anchors in the subsequent catalyst melt coating stage, enabling the eutectic carbonate to be uniformly wetted on its surface.

[0076] Therefore, this additive not only has good oxygen storage and release capacity (from...) / Reversible support), and also has a basic center ( High polarization adsorption activity induced by )

[0077] Catalyst modification and assembly:

[0078] Eutectic carbonates are composed of sodium carbonate ( ) and potassium carbonate ( It is prepared by mixing in a mass ratio of 1:1 to 1:3.

[0079] in, and All of them are alkali metal carbonates, and their eutectic system can form a stable liquid phase at high temperatures, with a eutectic point of approximately 710°C.

[0080] when When the ratio is less than 1:1, the eutectic point rises to about 740°C, which makes it impossible to form a continuous molten phase in the low-temperature vaporization section;

[0081] when When the ratio is higher than 1:3, the viscosity of the melt increases significantly (from 4.1 mPa·s to more than 8 mPa·s), which is not conducive to wetting and diffusion.

[0082] Therefore, a mass ratio of 1:1 to 1:3 can ensure that molten carbonates maintain a low viscosity and a stable liquid phase in the range of 700 to 900°C.

[0083] Mix and When heated to an operating temperature of 700–900°C, according to the eutectic phase diagram, the solid phase completely transforms into the liquid phase within this temperature range, forming a continuous molten carbonate.

[0084] Molten carbonates can be uniformly distributed on the catalyst surface and wet the surface of the aforementioned alkali metal oxygen vacancy composite additive.

[0085] The thermal stability of molten carbonates can be expressed by the following reaction:

[0086] ;

[0087] in, Indicates solid state; It indicates a liquid state; above 700℃, the reaction equilibrium shifts to the right, forming a stable liquid-phase mixed melt.

[0088] Molten carbonate reacts with the surface of the additive within this temperature range. Lattice oxygen vacancies ( No chemical reaction occurs, but an electrostatic adsorption interface is formed through ion polarization, making... It exhibits high ion mobility in the liquid phase;

[0089] The wetting behavior of molten carbonates on the surface of additives can be characterized by the contact angle, which is the physical meaning of the equilibrium state of the droplet on the solid surface.

[0090] Under thermodynamic equilibrium conditions, the wetting angle satisfies:

[0091] ;

[0092] in, Solid-gas interfacial tension; For solid-liquid interfacial tension; Liquid-gas interfacial tension; The equilibrium contact angle between a droplet and a solid surface; when the contact angle At a temperature <60°, a liquid can spontaneously spread on a solid surface, which is called good wetting.

[0093] The contact angle between molten carbonate and alkali metal oxygen vacancy composite additive is no greater than 60°, indicating that the liquid can spread fully and form a continuous coating layer;

[0094] Under high temperature conditions, molten carbonate flows and spreads along the pores and grain gaps on the surface of the additive, forming a liquid thin film coating layer.

[0095] The coverage ratio of the coating layer is defined as:

[0096] ;

[0097] in, Coverage rate; The effective surface area covered by molten carbonate; The total specific surface area of ​​the additives; when When the coating rate is ≥80%, the molten layer forms a continuous coverage, which can ensure the heat transfer and reaction interface stability of the catalyst surface; the coating rate of the catalyst of the present invention reaches 80-95%, which is a completely continuous coating state;

[0098] The melting temperature of the eutectic carbonate system (approximately 710°C) is lower than the reaction temperature of the first temperature zone in the front section of the dual-temperature zone reactor (600–800°C), ensuring that the molten carbonate remains in a liquid state and is continuously distributed throughout the entire reaction process.

[0099] In the second temperature zone (800–1050 °C), the molten carbonate is in a fully fluid state, which can form a stable liquid film heat transfer interface between the surface of the catalytic particles and the gasification reactants.

[0100] This liquid film acts as both a physical barrier (preventing sintering of additives and volatilization of alkali metals) and allows for the passage of substances through its interior. , Ion migration enables the dissociation of water vapor molecules into products ( The rapid migration of carbon-water reaction and tar cracking synergistically promotes the rapid migration of carbon-water reaction and tar cracking.

[0101] Oxygen vacancies on the surface of the composite additive containing alkali metal oxygen vacancies in the molten carbonate layer. A dual-interface reaction system is formed:

[0102] The molten layer provides ion channels ( migrate);

[0103] The oxygen vacancy layer provides electron channels ( / (Reversible).

[0104] Under reactive conditions, this composite interface forms a charge-mass cooperative migration field, the microscopic process of which can be represented as follows:

[0105] ;

[0106] in, Represents water molecules in the gas phase; Lattice oxygen to fill oxygen vacancies; To adsorb hydrogen radicals; the H• generated by adsorption migrates in the molten carbonate layer and participates in... The activation reaction promotes the generation of hydrogen.

[0107] Biomass raw materials are renewable organic solid materials, specifically including at least one of sawdust, rice husks, straw, coconut shells, peanut shells, cotton stalks, and sawdust.

[0108] The main component of these raw materials is carbon ( ),hydrogen( ),oxygen( ), and also contains a small amount of nitrogen ( ),sulfur( ) and ash components.

[0109] Taking typical rice husks as an example, its elemental composition is approximately: 46%, 5.8%, 42%, 0.8% ash content 5.4%.

[0110] The selection principles for biomass raw materials are:

[0111] Thermal stability: It does not produce violent cracking in the temperature range of 600 to 900℃;

[0112] Low ash content: ash content ≤8%, to avoid slag blockage;

[0113] Moderate alkali metal content: to prevent interference with subsequent reactions with the molten carbonate system;

[0114] Particle size and moisture uniformity: Ensure uniform heating and gasification within the dual-temperature zone reactor.

[0115] The drying process is carried out at a temperature of 80–120°C to remove adsorbed water and some bound water from the biomass feedstock.

[0116] The product is dried until the moisture content is no higher than 10%, a threshold determined through long-term experiments.

[0117] When the moisture content is higher than 10%, the heat absorbed by water evaporation is large, which will form a local cold zone in the initial stage of gasification, resulting in a decrease in pyrolysis efficiency.

[0118] When the moisture content is too low (below 3%), the powder is easily too dry, causing uneven conveying.

[0119] The drying process follows the classic convection drying heat transfer equation:

[0120] ;

[0121] in, The mass of water in the sample; Drying time; The mass transfer coefficient; The heated area; The partial pressure of water vapor on the surface of the raw material; This represents the partial pressure of water vapor in the dry gas. The rate of change of sample moisture content per unit time;

[0122] According to the equation, increasing the drying temperature and gas flow rate can accelerate the moisture diffusion rate, thereby shortening the drying time.

[0123] In actual operation, the drying medium is air or nitrogen, the linear velocity is 0.5 to 1.2 m / s, and the drying time is 30 to 60 min.

[0124] After drying, the moisture content can be detected by the Karl Fischer method or an infrared moisture meter.

[0125] The dried biomass is pulverized in a mechanical pulverizer with a target particle size range of 0.1–5 mm.

[0126] Particle size control is closely related to the uniformity of the gasification reaction; when the particle size is >5 mm, the heat conduction rate decreases, and a temperature gradient exists inside the particle, resulting in incomplete gasification.

[0127] When the particle size is <0.1mm, it is easily carried out of the reaction zone by the airflow, increasing the dust load of the system;

[0128] Particle size distribution can be described by the Rosin–Rammler equation:

[0129] ;

[0130] in, Larger than particle size The particle mass fraction; This refers to the actual particle size; Characteristic particle size; Distribution coefficient;

[0131] It can be seen from the sieving process that ≈1.5mm, ≈0.9 indicates that 90% of the particles have a diameter of less than 5 mm. Particles within this range can form a stable bed packing density of about 0.35 g / cm³ in the gasification reactor, which ensures both uniform flow of the reaction gas and uniform conduction of reaction heat.

[0132] The pretreated biomass has an increased surface area and reduced moisture content after drying and pulverizing, which allows for more thorough contact with the aforementioned alkali metal oxygen vacancy composite additive.

[0133] In the subsequent dual-temperature zone gasification reaction, the moisture content is controlled below 10% to prevent the formation of a condensation film on the catalyst surface, thereby ensuring the synergistic reaction between the molten carbonate coating layer and the oxygen vacancy active centers.

[0134] The specific synergistic mechanism is as follows:

[0135] When dried biomass comes into contact with molten carbonate, no vigorous vaporization occurs; a moderate amount of moisture (approximately 5%) is converted into steam during the vaporization process. Participation in the response:

[0136] ;

[0137] in, This represents the carbon element in biomass;

[0138] generated Further dissociation occurs at oxygen vacancies:

[0139] ;

[0140] Among them, the biomass pretreatment step is chemically directly coupled with the functional structure of the catalyst, and indirectly improves the gasification reaction rate and hydrogen production selectivity by controlling moisture and particle size.

[0141] The first temperature zone mainly performs the thermal pyrolysis and preliminary gasification reactions of biomass.

[0142] The reaction temperature is controlled at 600-800℃. This temperature range can promote the decomposition of organic components and prevent ash melting.

[0143] The vaporization medium is a mixture of air and water vapor, and its volume fraction is controlled to maintain micro-oxygen conditions (oxygen volume fraction ≤5%), which avoids complete combustion and provides the necessary partial oxidation exothermic heat to maintain the system's self-heating balance.

[0144] The mass ratio of steam to biomass should be controlled between 0.8 and 1.5:1. If it is too low, the hydrogen supply will be insufficient, and if it is too high, it will lead to a cooling effect and reduce the gasification efficiency.

[0145] Within this temperature range, the following reactions mainly occur:

[0146] ;

[0147] in, It is the main organic component of biomass; Indicates tar components; Represents solid carbon;

[0148] The tar and carbon generated by thermal cracking are partially converted into oxygen and vacancy composite additives under their catalytic action. and ;

[0149] The main reactions include:

[0150] ;

[0151] ;

[0152] The residence time of the reactant gas in the first temperature zone is 2 to 8 seconds to ensure that the pyrolysis reaction is complete and to avoid secondary polymerization of tar.

[0153] The reaction temperature in the second temperature zone is 800–1050°C. This high-temperature zone mainly undertakes the functions of steam reforming and tar cracking.

[0154] In this stage, high-temperature steam is introduced into the reactor, and under the combined action of alkali metal oxygen vacancy composite additives and molten carbonates, the tar, methane and other intermediate gases are further reformed.

[0155] Methane steam reforming reaction:

[0156] ;

[0157] Water-gas shift reaction:

[0158] ;

[0159] Tar steam reforming reaction:

[0160] ;

[0161] in, Indicates tar or heavy hydrocarbon molecules; The gas represents water vapor; the residence time of the reactant gas in the second temperature zone is controlled at 6–20 seconds to ensure complete tar cracking and sufficient hydrogen generation; the molar ratio of hydrogen, carbon monoxide, and carbon dioxide in the final reaction product gas is controlled at (55–70):(15–25):(10–20); to verify the gasification performance of this catalytic system, experiments were conducted in comparison with conventional catalyst systems, and the results are shown in Table 1:

[0162] Table 1 Comparison of Hydrogen Production Performance of Biomass Gasification

[0163]

[0164] The composite structure of molten carbonate and oxygen vacancy in the catalytic system of this invention significantly improves the adsorption and dissociation rate of water vapor, and promotes the carbon-water reaction and tar cracking reaction.

[0165] The molten carbonate is a eutectic system composed of sodium carbonate and potassium carbonate.

[0166] Under dual-temperature reaction conditions, the eutectic system begins to melt at approximately 710°C and remains liquid throughout the 600–1050°C range.

[0167] Molten carbonates are continuously distributed on the surface of the alkali metal oxygen vacancy composite additive in the reactor, forming a liquid coating layer with a thickness of about 10 to 50 μm.

[0168] This continuous molten phase plays a triple role between the two temperature zones:

[0169] Stabilizing heat transfer: The thermal conductivity of the liquid layer is significantly higher than that of the gas medium, which can achieve self-balanced heat transfer between the upper and lower temperature ranges and reduce the difference in thermal stress.

[0170] Mass migration: in the liquid phase , High ion mobility can promote The dissociation of molecules and the diffusion of intermediate products increase the reaction rate and hydrogen selectivity.

[0171] Structural stabilization function: The molten layer can prevent alkali metal agglomeration and sintering of additives, prolong catalyst life and maintain the activity of the reaction interface.

[0172] The continuous liquid contact state ensures that the two temperature zones are physically independent and that heat flow is continuously transferred, thus achieving a reaction equilibrium with local high temperature and overall steady state.

[0173] To further illustrate the synergistic effect of molten carbonate and oxygen vacancies, the overall rate of the gasification process can be described by a comprehensive reaction rate equation:

[0174] ;

[0175] in, The gasification reaction rate; Pre-exponential factors; Apparent activation energy; Gas constant; The reaction temperature; Water vapor partial pressure; Activity of solid carbon; , The reaction order is [number]. This is a function of oxygen vacancy concentration.

[0176] The regenerated gas consists of carbon dioxide and an inert gas, wherein... The volume fraction is 5-15%, with the remainder being inert gases;

[0177] when When the volume fraction is less than 5%, the carbon gasification rate is too slow and the deposited carbon cannot be effectively removed.

[0178] when When the volume fraction is higher than 15%, the reaction releases too much heat, which may lead to catalyst sintering or molten carbonate volatilization.

[0179] The regeneration gas flow rate is controlled at 0.2–0.8 L / min and is constantly input through a mass flow meter to ensure uniform distribution of the regeneration gas in the reaction bed.

[0180] The regeneration temperature is set to 350–550℃, and the regeneration time is 10–30 min.

[0181] Lower limit (350℃) guaranteed – The initial rate of the carbon reaction;

[0182] The upper temperature limit (550℃) is lower than the decomposition temperature of sodium carbonate and potassium carbonate (about 600℃), which avoids the destruction of the carbonate structure; the time range of 10 to 30 minutes ensures complete removal of carbon deposits while preventing excessive oxidation; the regeneration process is controlled by both atmosphere and temperature to ensure that carbon removal and oxygen vacancy regeneration are carried out simultaneously.

[0183] The core of the regeneration stage is The Boudouard reaction is a gasification reaction with carbon deposits.

[0184] ;

[0185] This reaction is endothermic and is accelerated within the medium temperature range of this invention by the ion migration of the molten carbonate phase.

[0186] During the reaction, carbonate ions in the molten carbonate system can react with surface carbon, achieving carbon-oxygen exchange at the solid-liquid interface:

[0187] ;

[0188] in, Indicates sodium or potassium, alkali metal elements; promotes carbon removal and formation The gas re-exposes the catalytic active sites on the surface; simultaneously, the following oxygen vacancy regeneration reaction occurs in the alkali metal oxygen vacancy complex promoter:

[0189] ;

[0190] in, These are reduced cerium ions in cerium oxide; The ions are in the oxidized state (cerium ions). This reaction is a charge-balanced process, which can reactivate the redox cycle on the catalyst surface. After in-situ regeneration... / When the ratio returns to the initial level and the oxygen vacancy concentration returns to more than 80% of its original value, it indicates that the catalytic performance has been effectively restored. After regeneration, the regeneration gas is stopped and replaced with an inert gas. The gas is then cooled to room temperature (25±5℃) in an inert atmosphere to prevent air from entering and causing high-temperature oxidation.

[0191] After cooling, the gasification medium (a mixture of air and water vapor) is reintroduced, and the next gasification reaction cycle begins. Table 2 shows the activity retention of the catalyst after multiple in-situ regenerations.

[0192] Table 2 Comparison of catalyst activity retention rate during regeneration

[0193]

[0194] The catalyst of this invention retains over 90% activity after five in-situ regenerations, significantly outperforming traditional nickel-based catalysts. This result demonstrates that by introducing an alkali metal oxygen vacancy composite promoter and a molten carbonate coating structure, carbonate and carbon deposits can undergo a reversible gasification reaction during the regeneration stage, thereby removing deposited carbon and restoring surface active sites, achieving long-term stable operation.

[0195] Under the same conditions, the tar residue and carbon deposit formation of conventional catalysts and the catalyst of the present invention are compared as shown in Table 3:

[0196] Table 3 Comparison of Tar and Carbon Deposit Formation

[0197]

[0198] As shown in Table 3, the tar residue and carbon deposition in the catalytic system of this invention are significantly reduced, indicating that the molten carbonate-oxygen vacancy interface promotes the conversion of tar cracking products to the gas phase and effectively inhibits carbon deposition. This synergistic mechanism enables the system to have higher carbon conversion rate and catalytic stability.

[0199] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for producing high purity hydrogen from biomass catalytic gasification with an alkali metal promoter, characterized in that, The method comprises the following steps: S1: catalyst preparation, mixing alkali metal salt and ceria or ceria-zirconia composite powder in proportion, ion exchange, drying and calcination treatment, inducing alkali metal ions to form surface oxygen vacancies, obtaining alkali metal oxygen vacancy composite catalyst; the alkali metal salt is sodium, potassium or mixture of the two; S2: catalyst modification and assembly, mixing alkali metal oxygen vacancy composite catalyst with eutectic carbonate composed of sodium carbonate and potassium carbonate, making eutectic carbonate form continuous molten phase at use temperature, producing molten carbonate, molten carbonate coating or wetting the alkali metal oxygen vacancy composite catalyst to obtain catalyst, the continuous molten phase and the surface oxygen vacancy of the alkali metal oxygen vacancy composite catalyst form synergistic adsorption sites, enhancing the adsorption and dissociation activity of water vapor molecules; S3: drying the biomass raw material to obtain pretreated biomass; S4: two-temperature-zone gasification and reforming, feeding pretreated biomass and catalyst into two-temperature-zone reactor, adding gasification medium to maintain gasification and reforming reaction environment, the front section performs thermal cracking and primary gasification in the first temperature zone, and the rear section performs steam reforming reaction in the second temperature zone; during the reaction process, the molten carbonate promotes water vapor adsorption, and the oxygen vacancy accelerates the migration of dissociation products, thereby synergistically promoting carbon-hydrogen reaction and tar cracking, improving hydrogen production selectivity and inhibiting carbon deposition; S5: in-situ regeneration, when the catalyst activity decreases, a regeneration atmosphere containing carbon dioxide is introduced into the two-temperature-zone reactor, and the carbonation and carbon deposition occur under the regeneration temperature condition, so that the surface carbon deposition is removed, and the oxygen vacancy structure is regenerated to restore the catalytic performance; S6: gas cooling and purification, the reaction gas is sequentially subjected to dust removal, condensation and pressure swing adsorption or membrane separation treatment to remove impurity gas and realize hydrogen component enrichment.

2. The method for producing high-purity hydrogen from biomass catalytic gasification with alkali metal additive according to claim 1, characterized in that, The catalyst preparation comprises: The mass ratio of the alkali metal salt to the ceria or ceria-zirconia composite powder is 1:5-1:20, ion exchange treatment is performed in a deionized water solution after mixing, the ion exchange temperature is 60-90°C, the duration is 1-3h, solid-liquid separation is performed, drying is performed at 100-120°C, and calcination is performed at 500-800°C for 2-6h to obtain the alkali metal-containing ceria composite catalyst with stable structure and surface oxygen vacancies; The molar ratio of ceria to zirconia in the ceria-zirconia composite powder is (1-3):1, and the alkali metal salt is sodium carbonate, potassium carbonate or a mixture thereof.

3. A process for the production of high purity hydrogen from biomass catalytic gasification with alkali metal promoter as claimed in claim 2, wherein, The catalyst modification and assembly comprises: The eutectic carbonate is prepared by mixing sodium carbonate and potassium carbonate in a mass ratio of 1:1-1:3, a continuous molten phase is formed at a use temperature of 700-900°C, the molten carbonate remains liquid stable in the temperature range of 700-900°C and is uniformly distributed on the surface of the alkali metal oxygen vacancy composite catalyst; The contact angle between the molten carbonate and the alkali metal oxygen vacancy composite catalyst is not more than 60°, forming a coating layer with a covering ratio of not less than 80%; The melting temperature of the eutectic carbonate is lower than the temperature of the first temperature zone of the two-temperature-zone reactor. To ensure the continuous existence of the molten phase and the stability of the material structure during the catalytic reaction process.

4. The method for producing high-purity hydrogen from biomass catalytic gasification with alkali metal additive according to claim 3, characterized in that, The biomass pretreatment comprises: The biomass raw material is dried at 80-120℃ to a moisture content of not higher than 10%, and is crushed by a crusher to a particle size of 0.1-5mm to obtain the pretreated biomass; The biomass raw material is renewable organic solid material selected from at least one of sawdust, rice husk, straw, coconut shell, peanut shell, cotton stalk, sawdust.

5. The method for producing high purity hydrogen from biomass catalytic gasification with alkali metal additive according to claim 4, characterized in that, The dual-temperature zone gasification reforming comprises: The reaction temperature of the first temperature zone is 600-800℃, the gasification medium is a mixed gas of air and steam, the mass ratio of steam to biomass is 0.8-1.5:1, and the residence time of the gas in the first temperature zone is 2-8s; The reaction temperature of the second temperature zone is 800-1050℃, steam is introduced at the temperature for reforming, the residence time of the reaction gas is 6-20s, and the molar ratio of hydrogen, carbon monoxide and carbon dioxide in the obtained reaction generated gas is controlled to be (55-70):(15-25):(10-20).

6. The method for producing high-purity hydrogen from biomass catalytic gasification with alkali metal additive according to claim 5, characterized in that, The molten carbonate is a continuous molten phase of a eutectic system of sodium carbonate and potassium carbonate formed at the reaction temperature, which is continuously distributed on the surface of the alkali metal oxygen vacancy composite additive in the dual-temperature zone reactor, maintains a liquid contact state and realizes stable heat transfer and mass transfer between the two temperature zones.

7. The method for producing high-purity hydrogen from biomass catalytic gasification with alkali metal additive according to claim 6, characterized in that, The in-situ regeneration comprises: When the catalyst activity in the dual-temperature zone reactor decreases, the biomass raw material feeding is stopped, and a regeneration gas containing carbon dioxide is introduced into the dual-temperature zone reactor, the volume fraction of the carbon dioxide in the regeneration gas is 5-15%, and the rest is inert gas; The regeneration temperature is 350-550℃, the regeneration time is 10-30min, and the flow rate of the regeneration gas is controlled to be 0.2-0.8L / min; At this time, the carbonates and the deposited carbon materials undergo a gasification reaction, so that the catalyst surface is kept clean and the oxygen vacancy structure is reformed; After the regeneration is completed, the dual-temperature zone reactor is maintained in an inert atmosphere and cooled to room temperature, and the gasification medium feeding is restored to enter the next round of reaction cycle.

8. The method for producing high-purity hydrogen from biomass catalytic gasification with alkali metal additive according to claim 7, characterized in that, The gas production cooling and purification comprises: The reaction generated gas is dedusted to remove the solid particles and carbon ash carried in the gas stream; The dedusted gas is cooled to 40-60℃ by a heat exchanger cooler, so that the water vapor and heavy organic matter are condensed and separated; The gas after condensation and separation is sent to a pressure swing adsorption unit or a hydrogen separation membrane unit for purification, the purification mode is pressure swing adsorption or separation membrane, the operating pressure of the pressure swing adsorption is 0.6-1.2MPa, and the operating temperature of the separation membrane is 20-40℃; The purified gas is output as a hydrogen product, and the gas not adsorbed or permeated is returned to the reaction system as a circulating gas.

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