The trend of green hydrogen substitution is gradually emerging
Hydrogen, as a secondary energy source, needs to be extracted from substances such as coal, hydrocarbons, and water through an energy conversion process. There are various ways to prepare hydrogen, which can be divided into "gray hydrogen", "blue hydrogen" and "green hydrogen" according to the carbon emissions in the hydrogen production process.
"Grey hydrogen" refers to the production of hydrogen through the reforming of fossil fuels such as coal, oil, and natural gas, and industrial by-product hydrogen represented by coke oven gas, chlor-alkali tail gas, propane dehydrogenation (PDH), etc., which releases a large amount of carbon dioxide in the production process, but is the current mainstream hydrogen production method due to mature technology and low cost; "Blue hydrogen" is the capture, utilization and storage (CCUS) of CO2 by-products on the basis of gray hydrogen, reducing carbon emissions in the production process and achieving low-carbon hydrogen production; "Green hydrogen" is the production of hydrogen through renewable energy (such as wind power, Hydropower, solar energy) hydrogen production, biomass hydrogen production and other methods of hydrogen production, the production process basically does not produce carbon dioxide and other greenhouse gases, to ensure that the production process of green hydrogen zero emissions.
According to the latest public statistics of the International Energy Agency (IEA), the global hydrogen production in 2021 will be about 94 million tons/year, and the hydrogen energy production will mainly come from fossil energy hydrogen production, accounting for 81%, of which natural gas hydrogen production accounts for 62%, coal hydrogen production accounts for 19%, low carbon emission hydrogen production accounts for only 0.7%, and water electrolysis hydrogen production production is only 35,000 tons, accounting for only 0.04%. As fossil energy hydrogen production can introduce low-cost hydrogen sources for the industry, natural gas hydrogen production accounts for a relatively large proportion in the past 10 years, and the annual output of hydrogen in mainland China is about 33 million tons, which is mainly composed of fossil energy hydrogen production and industrial by-product hydrogen, of which coal hydrogen production accounts for 62%, natural gas hydrogen production accounts for 19%, and industrial by-product hydrogen accounts for 18%, which is consistent with the energy characteristics of the mainland "rich in coal, poor in oil and less gas", and the scale of renewable energy hydrogen production is still in its infancy, accounting for a small proportion. In the context of carbon peaking and carbon neutrality, the development of clean energy will be accelerated, and hydrogen production by water electrolysis will gradually dominate, and in the future, global hydrogen will be gradually transformed into hydrogen production by electrolysis of renewable energy.
Green hydrogen production
Hydrogen production from water electrolysis from renewable energy is the most mature path
Green hydrogen production technologies include the use of wind power, hydropower, solar energy and other renewable energy sources to produce hydrogen by electrolysis of water, solar photolysis of water to produce hydrogen and biomass hydrogen production, among which renewable energy water electrolysis hydrogen production is the most widely used and the most mature technology.
Hydrogen production by electrolysis of water
Hydrogen production by electrolysis of water is the process of splitting water into hydrogen and oxygen through electrical energy, and this technology can use renewable energy to produce electricity without the emission of CO2 and other toxic and harmful substances, so as to obtain "green hydrogen" in the true sense. The raw material for hydrogen production by electrolysis of water is water, the process is pollution-free, the theoretical conversion efficiency is high, and the hydrogen obtained is of high purity, but the hydrogen production method needs to consume a large amount of electric energy, of which the electricity price accounts for 60%~80% of the total hydrogen cost.
Water electrolysis technologies for hydrogen production mainly include alkaline electrolyzed water (ALK), proton exchange membrane electrolyzed water (PEM) and solid oxide electrolyzed water (SOE) and other water electrolysis technologies. The rationale for the first three is shown in the figure below.
Basic schematic diagram of water electrolysis technology for hydrogen production
Alkaline water electrolysis (ALK) hydrogen production refers to the process of hydrogen production by electrolysis of water in an alkaline electrolyte environment, and the electrolyte is generally 30% mass concentration KOH solution or 26% mass concentration NaOH solution.
Compared with other hydrogen production technologies, alkaline water electrolysis can use non-precious metal catalysts to produce hydrogen, and the electrolyzer has a long service life of about 15 years, so it has cost advantages and competitiveness. Alkaline water electrolysis hydrogen production technology has decades of application experience, in the middle of the 20th century to achieve industrialization, high commercial maturity, rich operation experience, some domestic key equipment main performance indicators are close to the international advanced level, single-tank electrolysis hydrogen production capacity, easy to apply to the grid electrolysis hydrogen production. However, the electrolyte used in this technology is a strong alkali, which is corrosive and the asbestos diaphragm is not environmentally friendly and has certain hazards.
The alkaline water electrolysis hydrogen production system mainly includes the alkaline electrolyzer main body and the auxiliary system (BOP). The main body of the alkaline electrolytic cell is assembled by the end pressure plate, the sealing gasket, the plate, the electric plate, the diaphragm and other parts, the electrolytic cell includes dozens or even hundreds of electrolytic chambers, and these electrolytic chambers are pressed together by the screw and the end plate to form a cylindrical or square, and each electrolytic cell is bounded by two adjacent plates, including six parts: positive and negative bipolar plates, anode electrodes, diaphragms, sealing gaskets, and cathode electrodes.
Diagram of the structure of an alkaline electrolyzer
The main cost components of alkaline electrolyzers are electrolytic stack components (45%) and system auxiliary equipment (55%), and 55% of the cost of electrolyzers is diaphragm and membrane modules. According to the data analysis of many mainstream manufacturers in the industry, the main technical parameters and investment levels of alkaline electrolyzers in 2025 and 2030 are as follows:
Technical parameters and prospects of alkaline electrolyzers
Proton Exchange Membrane (PEM) water electrolysis technology refers to the hydrogen production process that uses proton exchange membrane as a solid electrolyte to replace the diaphragm and liquid electrolyte (30% potassium hydroxide solution or 26% sodium hydroxide solution) used in alkaline electrolyzers, and uses pure water as the raw material for hydrogen production from water electrolysis.
Compared with alkaline water electrolysis hydrogen production technology, PEM water electrolysis hydrogen production technology has the advantages of high current density, high hydrogen purity, fast response speed, etc., PEM water electrolysis hydrogen production technology has higher work efficiency, easy to combine with renewable energy consumption, and is an ideal solution for hydrogen production by water electrolysis. However, because the PEM electrolyzer needs to operate in a highly acidic and oxidizing working environment, the equipment needs to use electrocatalysts containing precious metals (platinum, iridium) and special membrane materials, resulting in high costs and poor service life as alkaline water electrolysis hydrogen production technology.
At present, there is still a certain gap between the development of PEM electrolyzer in China and the level of foreign countries, and the maximum hydrogen production scale of a single tank of PEM electrolyzer produced in China is about 260 standard cubic meters per hour, while the maximum hydrogen production scale of a single tank of PEM electrolyzer produced abroad can reach 500 standard cubic meters per hour.
The PEM water electrolysis hydrogen production system consists of a PEM electrolyzer and an auxiliary system (BOP). The PEM electrolyzer is assembled from components such as a proton exchange membrane, a catalyst, a gas diffusion layer, and a bipolar plate. The most basic unit of an electrolyzer is the electrolytic cell, and a PEM electrolyzer contains dozens to hundreds of electrolyzers.
Diagram of PEM electrolyzer structure
45% of the cost of the proton exchange membrane electrolyzer is the electrolytic stack, 55% is the system auxiliary, 53% of the electrolytic stack cost is the bipolar plate, and the cost of the membrane electrode is composed of four elements: metal Pt, metal Ir, perchlorosulfonic acid membrane and preparation cost. Since the proton exchange membrane of the PEM electrolyzer needs 150-200 microns, it is more likely to swell and deform during processing, and the swelling rate of the membrane is higher, and the processing is more difficult, mainly relying on foreign products. According to the data analysis of many mainstream manufacturers in the industry, the main technical parameters and investment levels of PEM electrolyzers in 2025 and 2030 are as follows:
Technical parameters and prospects of PEM electrolyzer
The high-temperature Solid Oxide Electrolysis Cell (SOEC) water electrolysis technology for hydrogen production is still in the stage of technology demonstration and system testing, including three methods: proton-solid oxide, oxygen ion-solid oxide and carbon dioxide combined electrolysis. SOEC uses solid ceramics as electrolytes, which need to react at high temperatures of 500~1000°C, and the kinetic advantage allows it to achieve or approach 100% conversion efficiency, and the catalyst used does not depend on precious metals. The SOEC electrolyzer is fed with water vapor, and if carbon dioxide is added, syngas (a mixture of hydrogen and carbon monoxide) can be generated, which can then be further produced to produce synthetic fuels (e.g. diesel, jet fuel). Therefore, SOEC technology is expected to be widely used in CO2 recovery, fuel production and chemical synthesis, which has been the focus of EU research and development in recent years. The electrochemical performance of the hydrogen production process of this technology is significantly improved and the efficiency is higher. However, the shortcomings of this technology include: (1) the mechanical properties of the electrode are not stable enough at high temperatures, (2) the high temperature will also shorten the life of glass-ceramic sealing materials in the electrolyzer, and (3) the heating rate of high-temperature reaction conditions needs to be broken through in terms of matching with renewable energy with high volatility and unstable output. These shortcomings restrict the selection and large-scale promotion of application scenarios of this technology.
Other water electrolysis technologies, such as Anion Exchange Membrane (AEM) water electrolysis technology, are fundamentally different from PEM in that the exchange ions of the membrane are replaced by protons to hydroxide ions. The relative molecular mass of hydroxide ions is 17 times that of protons, which makes them migrate much slower than protons. The advantage of AEM is the absence of metal cations and the absence of carbonate precipitation to clog the hydrogen production system. The electrodes and catalysts used in AEM are non-precious metal materials such as nickel, cobalt, and iron, and the hydrogen production is of high purity, good air tightness, and fast system response, which is very compatible with the characteristics of current renewable energy power generation. However, the mechanical stability of AEM membranes is not high, and the electrode structure and catalyst kinetics in AEM need to be optimized. AEM water electrolysis technology is in the kilowatt stage of development, and globally, several research organizations/institutions are actively working on the development of AEM water electrolyzers, and there are still some innovations/improvements needed in order to expand the commercial application of this technology.
According to the data disclosed by the IEA, by the end of 2022, the global installed capacity of hydrogen production by water electrolysis reached 700MW, with ALK leading the way in hydrogen production, accounting for nearly 60%, followed by PEM electrolysis hydrogen production, accounting for more than 30%, and other electrolysis hydrogen production methods accounted for a relatively low proportion.
Photolysis of water to hydrogen
In 1972, Japanese scholars Fujishima A and Honda K reported for the first time that the experimental research on the production of hydrogen by photolysis of water with TiO2 single crystal electrode opened up a new way to produce hydrogen by photolysis of water, and hydrogen production through solar photolysis of water is also considered to be the best way to produce zero-carbon hydrogen in the future.
Photolyzed water, also known as photocatalytic water splitting, can be understood as an artificial photosynthesis. The scientific principle is the photoelectric effect of semiconductor materials - when the energy of incident light is greater than or equal to the energy band of the semiconductor, the light energy is absorbed, and the valence band electrons transition to the conduction band, producing photogenerated electrons and holes. Electrons and holes migrate to the surface of the material and undergo a redox reaction with water to produce oxygen and hydrogen. The production of hydrogen from photolysis of water mainly includes three processes, namely, light absorption, photogenerated charge migration and surface redox reaction.
Schematic diagram of photolysis of water to hydrogen
The industrialization of photolysis depends on the efficiency of solar-to-hydrogen (STH) energy conversion. Photolysis of water is divided into three technical routes, one is photocatalytic water splitting, using nanoparticle suspension system to produce hydrogen, which is low cost and easy to scale up, but the STH efficiency is low (about 1%). The key issues in the follow-up research of this route are photocatalysts with high efficiency and wide spectral response, high efficiency charge separation strategies, new high-efficiency cocatalysts, and new methods and materials for gas separation; second, the STH efficiency of photocatalytic water splitting has exceeded 2.0% in some typical photoanode semiconductor materials (BiVO4 and Ta3N5, etc.); third, the photovoltaic-photoelectric coupling system has the highest STH efficiency among the three pathways, and has exceeded 10% in multiple experimental systems. The newly reported STH efficiency of multi-junction GaInP/GaAs/Ge cells coupled with Ni electrocatalysts can reach 22.4%, which has met the requirements of industrial applications. However, the cost of photovoltaic cells (especially multi-junction GaAs solar cells) greatly limits their large-scale application, so it is also the current highest-cost technical route (about 300-400 yuan/kg).
The U.S. Department of Energy (DOE) has been conducting research on photocatalysis for many years, and in 2011 set the target for hydrogen production by photocatalysis and photovoltaic-photoelectric coupling systems. According to the relevant literature of the China Hydrogen Energy Alliance Research Institute, the cost, STH efficiency and hydrogen production rate of photocatalysis and photovoltaic-photoelectric hydrogen production have not yet made significant breakthroughs, and the overall level is still maintained at the level of 2015.
Xi'an Jiaotong University is one of the earliest teams in China to start the research of solar photocatalytic water splitting for hydrogen production, and took the lead in establishing the first direct solar continuous flow large-scale hydrogen production demonstration system, with the system running stably for more than 200 hours, and at the same time formulated the GB/T 26915-2011 "Energy Conversion Efficiency and Quantum Yield Calculation of Solar Photocatalytic Water Splitting Hydrogen Production System". The research team of Li Can from the Dalian Institute of Chemical Physics, Chinese Academy of Sciences has been exploring the demonstration of large-scale application of solar hydrogen production. Drawing on the idea of large-scale crop planting on farms, the team proposed and verified the "Hydrogen Farm Project" (HFP) strategy based on the powder nanoparticle photocatalyst system for large-scale solar water splitting for hydrogen production, with an STH efficiency of more than 1.8%, which is the highest STH efficiency of photocatalytic water splitting based on powder nanoparticles reported internationally.
At present, the solar-hydrogen conversion process is limited by many kinetic and thermodynamic factors, and there is still a big gap between the highest solar energy conversion hydrogen efficiency achieved by semiconductor materials and the practical application requirements. The development of high-efficiency photocatalysts for hydrogen production is the core issue of the large-scale application of photolysis of water to hydrogen production technology, and it is necessary to strengthen basic theoretical research to promote the development of this field.
Biomass hydrogen production
Biomass hydrogen production technology refers to the use of biomass as raw material to prepare hydrogen through chemical reactions or biological reactions. Biomass hydrogen production technology has a wide range of raw material sources and high hydrogen yields, which can be crop straw, wood, waste, animal manure, etc., which can only be regarded as waste in the traditional sense. Biomass hydrogen production enables waste gas and biomass to be recycled, reduces environmental pollution, and can also provide more options for energy transition, which is a hydrogen production technology with development potential and prospects. Biomass hydrogen production technology is mainly divided into two paths: thermochemical hydrogen production and biological hydrogen production, among which biomass thermochemical hydrogen production technology is relatively mature.
Thermochemical hydrogen production refers to the decomposition of substances at high temperatures to produce gas, and then the gas is decomposed into hydrogen through the action of catalysts. The advantages of this method are a wide range of raw materials, high efficiency in hydrogen production, and a variety of useful by-products, such as methanol, ethanol, acetic acid, etc., can be obtained. However, due to the coking and carbon deposition phenomenon that is easy to occur under high temperature conditions, it is necessary to take the method of high temperature rapid reaction to solve it.
Biological hydrogen production, also known as microbial degradation method and biomass fermentation method, is the catalytic degradation of water molecules and organic substrates in biomass into hydrogen through two key enzymes: hydrogenase and nitrogenase. Common technologies include biophotolysis for hydrogen production, photofermentation, dark fermentation, light-dark coupled fermentation, cell-free enzyme biotransformation, and other subdivision technologies. The advantage of this method is that it does not require a high-temperature reaction, does not produce coking and carbon deposits, and at the same time can obtain useful by-products such as organic fertilizers. However, since the growth of microorganisms is affected by environmental factors, the reaction conditions need to be controlled to ensure the efficiency of hydrogen production.
In October 2022, the mainland's first biomass gasification hydrogen production polygeneration application research pilot project was successfully ignited in Ma'anshan, Anhui Province. The cost of the whole process of the project is far lower than that of the current general water electrolysis hydrogen production project, with a purity of 99.99% and an annual hydrogen output of 110,000 square meters. The hydrogen produced can be used for fuel cell power generation and multi-format hydrogen commercial applications, with an energy utilization rate of more than 90%.
Although some breakthroughs have been made in biomass hydrogen production, most of the current biomass hydrogen production process is completed on small equipment, and there are still certain challenges in using it for large-scale industrial production. First of all, the biomass conversion process is complex and requires high technical support. Secondly, due to the characteristics of biomass and its changes in the reaction process, the quality of hydrogen produced may be affected to a certain extent, and it is necessary to further study and optimize the reaction process to improve the yield and quality of hydrogen. Realizing the controllability of the hydrogen production process, improving the hydrogen production rate and efficiency, saving production costs, and accelerating the industrialization process are the urgent problems to be solved in biomass hydrogen production. From a global perspective, the development of biomass hydrogen production technology is still in its infancy. Although the mainland biomass hydrogen production technology started late, it has developed rapidly in recent years and has great development potential.
Green ammonia production
The technology is mature and continues to explore new routes
Ammonia synthesis is one of the largest ways of hydrogen consumption. As the second largest chemical in the world, synthetic ammonia is one of the most important chemical products in modern society. Ammonia is an important raw material for the manufacture of nitric acid, fertilizers, explosives, ammonia is quite important to life on earth, it is an important ingredient in all food and fertilizers. It is also a direct or indirect component of all medicines. Due to its wide range of uses, ammonia is one of the most produced inorganic compounds in the world, with more than eighty percent of ammonia used to make fertilizers.
Ammonia synthesis is a synthesis of ammonia produced by the reaction of hydrogen and nitrogen under the action of a catalyst, and gaseous hydrocarbons are used as raw materials. The process methods adopted by various companies in the world are different, but the basic production process has not changed significantly, and the process flow is basically the same. At present, most of the processes used in China are imported from abroad, such as Kellogg, Topsoe, Casale, Braun, ICIAMV, ICILCA, KBR KAAP and other processes, and the design concept of the synthesis process is to improve the net ammonia value and energy saving as the ultimate goal.
Synthetic ammonia produced from green hydrogen and nitrogen separated from the air is called green ammonia, and green ammonia is prepared from renewable energy as raw materials, which can be truly sustainable and carbon-free. In terms of synthesis principle and technical route, there is no essential difference between green ammonia synthesis and traditional ammonia synthesis in terms of process flow, key equipment, design and operation indicators.
At present, most of the preparation methods of green ammonia are based on the Haber-Bosch synthesis method, which uses green hydrogen and nitrogen to synthesize green ammonia under the action of a catalyst, and the process is mainly divided into three parts: hydrogen and nitrogen compression, ammonia synthesis and condensation separation, and ammonia compression and freezing.
Hydrogen nitrogen compression
After mixing the qualified nitrogen with the qualified hydrogen produced by the electrolysis of water in proportion (hydrogen:nitrogen = 3:1), it is compressed by a syngas compressor from low pressure (taking 2.2 MPa as an example) step by step, and compressed together with the circulating gas from the synthesis cold exchanger at the final stage, raising the pressure to 14.5 MPa, and sending it to the ammonia synthesis process.
Ammonia synthesis and condensation separation
Ammonia is synthesized and produced in a fixed-bed ammonia synthesis tower, using a process synthesized under a design pressure of 15 megapascals, with two-stage ammonia cooling and secondary ammonia separation to reduce refrigeration power consumption. The internals of the ammonia synthesis tower are two-axis and two-diameter, and the tower pot is directly connected, and the waste pot recovers heat and produces 2.5 megapascal medium-pressure steam.
The synthesis column uses a two-stage thermal, intercooled design, and each bed is filled with 1.5 mm-3 mm iron-based synthesis catalysts. Iron-based catalysts are usually solid particles made of iron, aluminum, potassium and other elements, with many micropores on the surface to increase the contact area with reactant molecules, which can accelerate the reaction rate and reduce the reaction activation energy. The make-up and circulating gas from the syngas compressor is preheated to approximately 236 degrees Celsius by means of a feed/outlet heat exchanger with the outlet before entering the synthesis tower. The concentration of ammonia entering the synthesis column is about 3.8 molar percent.
Ammonia compression freezing
The gas ammonia of different pressure levels from the first and second ammonia coolers in the ammonia synthesis process enters the ammonia compressor inlet separator respectively, and the gas ammonia at the outlet of the three-stage separator enters the first to third stage inlets corresponding to the ammonia compressor respectively, and the ammonia at the outlet of the ammonia compressor is increased to 1.6 megapascals and then enters the ammonia condenser for condensation, and the condensed liquid ammonia enters the liquid ammonia receiving tank. The liquid ammonia condensed in the liquid ammonia receiving tank is divided into two strands, one of which is exchanged with the liquid ammonia of the product through the ammonia heater, and after cooling, it provides cooling capacity for the first ammonia cooler in the ammonia synthesis process, and the gas ammonia enters the three-stage inlet separator, and the other directly enters the liquid ammonia storage tank. After recovering the cold capacity, the circulating gas is mixed with hydrogen and nitrogen and re-enters the synthesis tower. The process flow of a typical ammonia synthesis is shown in the figure below.
Process flow chart of green hydrogen to green ammonia
The synthesis of ammonia by means of renewable energy to prepare green hydrogen, the production of 1 ton of ammonia theoretically requires the consumption of 0.18 tons of hydrogen, and the cost of preparing green hydrogen accounts for a large proportion of electricity and equipment investment costs, the current comprehensive cost of green ammonia is about 3500 yuan/ton, and the comprehensive cost of green ammonia in the future mainly needs to be further reduced with the decline of green hydrogen preparation cost.
The biggest challenge faced by green hydrogen to green ammonia is to consider the fluctuations in renewable energy supply and market demand, and develop a green hydrogen ammonia production process that fully considers operational safety and process economy, including ammonia synthesis towers, compressors, gas separation, heat exchange networks and other adaptation schemes and collaborative control, so as to realize the mutual assistance of cooling, heating and power, improve system flexibility, and improve comprehensive conversion efficiency.
In the domestic large-scale synthetic ammonia industry, such as large-scale air separation, domestic mature technology can be adopted, and low-pressure synthetic ammonia technology has also entered the international advanced level, and many large-scale synthetic ammonia bases have been built, and a large number of enterprises with high technical level and large production scale have emerged, such as Yuntianhua, Hubei Yihua and Hualu Hengsheng.
Under the background of the dual carbon policy, the synthesis of green ammonia by renewable energy has been developed rapidly, and three generations of ammonia synthesis technology have been roughly formed, the first generation is the traditional Haber method ammonia synthesis technology, the second generation is low-temperature and low-pressure ammonia synthesis technology, and the third generation is a variety of technical routes, mainly including: direct electrocatalytic ammonia synthesis, plasma combined catalyst ammonia synthesis and low-temperature and atmospheric pressure ammonia synthesis. For example, Chen Ping's team from the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences published a new study in Nature Catal. to synthesize ammonia using ternary Ru complex hydride at 300 degrees Celsius and atmospheric pressure. In addition, the Korea Institute of Machinery and Materials uses low-temperature plasma to synthesize ammonia directly from water and nitrogen, and the ammonia volume concentration reaches 0.84%, and the ammonia production rate reaches 120 micromoles per second. On the whole, the domestic renewable energy synthetic ammonia technology can basically be on par with foreign countries.
Green alcohol production
The technology is diversified to be verified by industrialization
Methanol is another avenue for hydrogen applications. As a basic organic chemical raw material, methanol has a wide range of uses. Methanol can be used in chemical products such as synthetic fibers, formaldehyde, plastics, pharmaceuticals, pesticides, dyes, synthetic proteins, etc., and can also be used as a liquid fuel for methanol fuel cells (DMFCs) and methanol engines. Methanol can also release hydrogen through cracking, thus becoming a carrier for hydrogen storage and transportation. There is no unambiguous definition of green methanol in the world. According to the International Renewable Energy Agency, green methanol requires all feedstock sources to meet renewable energy standards. At present, there are two main ways to produce green methanol: one is biomass methanol, which is produced from bio-based raw materials, and the other is green electricity to produce methanol.
Diagram of the production pathway of green methanol
Biomass to green methanol
The mainland has abundant biomass resources, such as straw, straw, wood chips, wood chips, corn cobs, rice husks, etc., which are efficiently converted into liquid fuel methanol through thermochemical conversion and biotransformation, which is not only a way to achieve the green development of biomass resources, but also an effective means to replace traditional fossil energy.
There are two main ways to produce methanol from biomass: one is the way of biomass gasification - syngas, and the other is the fermentation of biomass to methane and then methanol. Among them, biomass gasification technology has the potential for sustainable production of green methanol.
The biomass gasification to methanol includes two parts: biomass gasification and syngas to methanol, the first is the biomass gasification to form carbon-rich syngas, and then the gas is reintegrated into methanol. Among them, biomass gasification technology is one of the most promising key processes to convert biomass into high-quality syngas, and the technical principle of syngas to methanol is similar to that of coal-to-methanol, which has a history of 80 years, and the process route has been mature and stable.
Biomass gasification is a very complex thermochemical reaction process, which usually includes four processes: drying, pyrolysis, oxidation and reduction. The biomass raw material is pretreated and then enters the gasifier, and under the action of heat, the surface moisture is precipitated, and the main drying stage is at 200~300 °C. When the temperature rises above 300°C, the pyrolysis reaction begins. At 300~400 °C, biomass can release about 70% of the volatile components, and the volatile components precipitated by pyrolysis reaction mainly include water vapor, hydrogen, carbon monoxide, methane, tar and other hydrocarbons. The oxidation process is mainly from the pyrolysis of biomass some combustible gas and substances in the state of limited O2 combustion and partial combustion reaction, mainly C and H oxidation, are exothermic reactions, and provide energy for biomass drying and pyrolysis, the temperature rises rapidly to more than 1000 °C, the process is generally carried out at the temperature of 1000~1500 °C. The reduction process is more complex, including two processes: pyrolysis and oxidation, the gas mixture interacts with coke to form the final syngas, which has endothermic and exothermic reactions, which are generally carried out at 600~1000 °C. It also includes tar reforming, a process in which small molecule hydrocarbons are formed from large tars, and the tar is removed to prevent the catalyst from being deactivated to obtain syngas with suitable methanol synthesis properties.
The research on biomass gasification technology in China focuses on three key aspects: gasification technology, equipment and principle. Key equipment includes a biomass gasifier, a steam change chamber and a methanol synthesizer. The key factors are the biomass gasification equivalent ratio, steam shift temperature, hydrogen cycle ratio, etc.
The production of methanol by biomass fermentation is the use of microorganisms to produce biogas by anaerobic fermentation of biomass, which is converted into hydrogen and carbon dioxide to synthesize methanol, or the carbon dioxide in it is separated, hydroreformed, and biomethanol can also be synthesized. Limited by biomass gasification technology, large-scale industrial application has not yet been realized.
Green electricity to methanol
The production of methanol from green electricity mainly uses CO2 as raw material, and its technical routes are divided into: green electricity to green hydrogen coupled with CO2 to methanol, and CO2 electrocatalytic reduction to methanol. Among them, there are still some key challenges in the industrialization of CO2 electrocatalytic reduction to methanol, compared with CO2 hydrogenation to methanol, which proves to be the most feasible and large-scale route.
The hydrogenation of carbon dioxide to synthesize methanol is one of the important ways to realize the resource utilization of carbon dioxide, and it is also a realistic choice to solve the greenhouse effect, develop green energy and achieve sustainable economic development, and play an important supporting role in the development of CCUS industry chain.
The synthesis of green methanol from green hydrogen and renewable carbon dioxide requires the use of "renewable carbon dioxide", i.e. carbon dioxide produced from biomass or captured from the air. Green hydrogen and renewable carbon dioxide are synthesized into green methanol at high temperature and high pressure, although carbon dioxide will be produced when methanol is burned in the future, but because these carbon emissions are captured through circulation, the carbon emissions of green methanol in the whole life cycle are zero.
A large number of studies have been carried out on the hydrogenation of carbon dioxide to methanol at home and abroad, and the principle is that carbon dioxide and hydrogen are adsorbed on the surface of the catalyst and gradually converted into gaseous methanol. Most of the catalysts used are copper-based Cu-Zn-Al catalysts.
The process of carbon dioxide hydrogenation to methanol is mainly divided into three parts: hydrogen preparation, carbon dioxide capture, methanol synthesis and rectification. Hydrogen uses green hydrogen produced by electrolysis of water, carbon dioxide is mostly captured and separated by solvent absorption, pressure swing adsorption, membrane separation, liquefaction separation and other processes, and H2 and CO2 are according to the molar ratio of 31. It is mixed into syngas, pressurized to a certain pressure by the compressor and enters the methanol reactor, under the condition of high temperature and pressure, through the action of a highly selective catalyst, the reaction generates crude methanol (a mixture of methanol and water), and finally the methanol product with high purity is obtained by distillation.
The research focus of carbon dioxide catalytic hydrogenation to methanol includes catalyst preparation and process route design. There are mainly copper-based catalysts, palladium-based catalysts, indium-based catalysts and oxide solid solution catalysts. Among them, copper-based catalysts have been industrialized and are the most widely used because of their simple preparation and economical raw materials. The process route is mainly developed according to different catalyst systems, and a number of process routes have been formed based on different catalysts in China, and a number of demonstration devices have been built.
Shanghai Advanced Research Institute of Chinese Academy of Sciences and Offshore Oil Fudao Company have completed a 5,000 tons/year carbon dioxide hydrogenation to methanol demonstration plant; Dalian Chemical Physics Research Institute of the Chinese Academy of Sciences has completed a 1,000-ton liquid solar fuel synthesis demonstration project in the Green Chemical Engineering Institute of Lanzhou New Area, and will continue to carry out a 100,000-ton liquid sunlight industrialization demonstration project in the future; Southwest Chemical Research and Design Institute Co., Ltd. and Luxi Chemical Group Co., Ltd. have developed and put into operation a 5,000 tons/year methanol production test pilot plant;
The Icelandic Carbon Cycle International Company (CRI) is a leader in the commercialization of the CO2 direct to methanol process, and the world's first carbon dioxide hydrogenation to methanol plant has been built in Iceland and has achieved commercial operation, with a methanol production capacity of 4,000 tons/year of the demonstration plant, and it is said to have the ability to promote technology of 50,000~100,000 tons/year.
Geely has been researching methanol vehicles and methanol engines since 2005. At present, we have mastered the relevant influence mechanism of methanol fuel on automobiles and engines, and through the research and analysis of the corrosiveness, swelling, cleanliness and other characteristics of methanol, we have successfully solved the industry problems such as alcohol resistance and durability of methanol vehicles, formed more than 200 patents, and the cumulative sales of methanol vehicles have exceeded 30,000 units, with the highest mileage exceeding 1.2 million kilometers and nearly 10 billion kilometers traveled.
At present, the technical route of carbon dioxide hydrogenation to methanol has been opened up, and the pilot demonstration has been realized, and the technology needs to be further improved to solve the problem of industrialization, and the industry will focus on the development of low energy consumption and high stability of water electrolysis catalysts, and the development of catalysts for carbon dioxide hydrogenation to methanol with high activity, high selectivity and high stability.
CO2 hydrogenation to methanol process technology combines renewable energy water electrolysis to hydrogen production technology and carbon dioxide resource utilization, which can reduce carbon dioxide emissions while producing green methanol with a wide range of uses, realizing a new way from renewable energy to green liquid fuel methanol production. With the progress of technology, the cost of key equipment such as photovoltaic panels and electrolysis tanks will be gradually reduced, and the performance of catalysts will be further improved, and the green methanol industry will usher in a broader development prospect.
There is no limit to the development of green hydrogen-based energy
In addition to being used as basic chemical products, ammonia and methanol can also be used as new fuels and hydrogen carriers, and can be used as an effective solution for the local consumption of renewable energy such as wind power and solar energy. Although the current production cost of green ammonia and green methanol is higher than that of traditional synthetic ammonia and methanol, driven by the "dual carbon" policy stimulus and capital investment, green hydrogen-based energy production technology will develop rapidly and mature, and the output of green ammonia and green methanol is expected to increase significantly, and the future development prospects will be very broad, and it will have the potential to become the main form of replacing traditional fossil energy in the future.
We believe that although the road to green hydrogen-based energy development is long and difficult, it is coming. As long as we continue to do it, the future of the industry will be extremely bright.
Source: General Institute of Hydropower and Water Conservancy Planning and Design