【Ten Years of Rapid Development in Chemical Engineering Equipment】1866 – Innovative two-stage fluidized gasification technology solves the problem of \"tar\" in biomass gasification
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Safety first. Prevention first – let’s give credit and support to China’s achievements in chemical engineering technology and equipment. **********************【Ten Years of Rapid Development in Chemical Engineering Equipment】A continuously updated summary post is available; feel free to join the discussions at https://bbs.hcbbs.com/thread-3576046-1-1.html (Source: Haichuan Chemical Engineering Forum) ***************** Global efforts toward low-carbon development are pushing the energy structure to shift from fossil fuels to renewable sources, with the goal of reducing dependence on fossil fuels while also cutting carbon dioxide emissions. As the largest organic group in nature, biomass is the only solid resource that possesses characteristics similar to fossil fuels. Biomass thermal conversion technology can transform low-grade biomass into fuels and gases with high energy density, which serve as alternatives to fossil fuels and gases. This process accelerates the cycle of carbon dioxide during the plant growth cycle without generating additional carbon dioxide, making it a process with \"carbon-neutral\" properties. Green hydrogen produced via photovoltaic water electrolysis can be used to further adjust the hydrogen-to-carbon ratio in biomass syngas; it serves as a feed gas for methanol synthesis, the production of green aviation fuel, and hydroformylation processes. Carbon dioxide can also be recycled into the production and use of chemical products, which not only extends the cycle of carbon dioxide utilization but also significantly increases the use rate of \"green carbon\" while reducing the consumption of biomass raw materials, thereby making a meaningful contribution to achieving the **\"carbon neutrality\" goal** through innovative forms of productivity. 1. Traditional gasification technologies cannot meet the technical requirements of the green methanol/aerosol fuel synthesis industry. By using biomass as a green \"carbon\" source, gasification becomes the only technical approach for converting biomass feedstocks into syngas. For large-scale gasification, it is possible to draw on coal gasification technologies with proven industrial experience. However, due to the low energy density of biomass, its high volatile content, high oxygen content, high alkali metal content, and ash composition dominated by SiO2, its properties as a fuel differ significantly from those of coal. As a result, gasification technologies often fail to meet the requirements for large-scale biomass gasification, as evidenced by the following: (1) Fixed-bed gasification technologies such as the Ruhr process require high strength and thermal stability in the feedstock, which biomass finds difficult to satisfy ； (2) The syngas produced by circulating fluidized bed gasification technology has a high tar content; therefore, a two-stage pyrolyzer is required to remove the biomass tar. This pyrolyzer needs to be heated to above 1100°C, which results in high energy consumption and poor suitability for biomass feedstocks with low ash melting points ； (3) The micron-level grinding required by fluidized bed technology cannot be directly applied to biomass; it is necessary to carbonize the biomass into biomass pellets. This preprocessing process is complex and difficult, and moreover, the silica-alumina ratio of the ash is high, making it challenging to achieve \"using ash to counteract ash\" during fluidized bed operation. 2. The innovative fluidized two-stage gasification technology addresses the problem of “tar” in biomass gasification. In order to enable gasifiers to process coarser feedstocks to the greatest extent possible and to resolve fundamentally the global issue of tar formation as a by-product of biomass gasification, Professor Xu Guangwen’s team from Shenyang University of Chemical Technology developed a fluidized two-stage gasification technology. This technology utilizes fluidized reactors to achieve a low-tar two-stage gasification process (as shown in Figure 1). By decoupling the reactions of biomass pyrolysis and semi-coke gasification, it combines the effects of biomass pre-oxidation to reduce tar formation with those of thermal cracking of biomass tar and catalytic cracking of high-temperature semi-coke to effectively remove tar. The reaction temperatures and conditions for pyrolysis and gasification/tar cracking are optimized based on the quality of the product gas, thereby enabling the technical advantage of ultra-low tar generation and release during medium- and low-temperature gasification within a fluidized process that is suitable for large-scale application.
Two-stage fluidized gasification has been successfully applied in the production of gas from air gasification for biomass wastes from light industry processes such as traditional Chinese medicine residues and liquor distillation residues. Multiple industrial-scale plants with annual processing capacities ranging from 10,000 to 50,000 tons of industrial biomass waste have been built in regions such as Henan, Sichuan, Shandong, and Anhui (Figure 2 shows a representative example). This approach enables the achievement of internationally advanced technical standards, with a tar content in the gas produced at less than 50 mg/Nm3 (Bioresource Technology 2016, 206, 93–98; Carbon Resources Conversion 2018, 1(2),109–125; Carbon Resources Conversion 2020, 3, 1–18).
3. The fluidized two-stage gasification technology has received high praise from the academic and industrial communities both at home and abroad. The process of fluidized two-stage gasification, along with its technical advantages in reducing tar production, have been widely recognized and highly appreciated by scholars and industry professionals worldwide. For example, M. A. Rosen, a member of the Canadian Academy of Engineering and former president of the same academy, listed two-stage gasification with fluidization as one of the most promising technologies in the world in his review paper on biomass gasification technology (Biofuels, 2017, 8(6): 725–745). In 2021, agencies such as Western Economics Diversification Canada and Natural Resources Canada in Canada also allocated over 4 million Canadian dollars to UBC and FPInnovations to support research on the technical processes for producing biomass syngas through fluidized two-stage oxygenation, as well as on the evaluation and verification of low-tar characteristics (Sources: confidentiality agreements between Shenyang University of Chemical Technology and the relevant parties, and documentation provided by a UBC professor to Professor Xu Guangwen regarding the Canadian funding). Regarding the industrial application of this technology in enabling the energy utilization of traditional Chinese medicine residues with high water content in various pharmaceutical factories, it was rated as \"internationally leading\" by the China National Light Industry Council, and it won the first prize in the Science and Technology Award of the China National Light Industry Council. The relevant technical invention achievements and fundamental research results also won the First Prize for Technical Invention in Liaoning Province, as well as the First Prize for Research Achievements from the Chinese Chemical Society.
4. Technical verification of two-stage fluidized biomass pure oxygen (oxygen-enriched) gasification (1) Operational stability and syngas quality. Fluidized two-stage gasification utilizes the principle of \"two-stage\" ultra-low calorific value gasification, which enables reaction staging in fluidized reactor systems that are easy to scale up industrially; this makes it more suitable for the gasification of small-particle fuels with high water content, including biomass and low-rank coal. Building on the successful industrial application of air gasification technology, and in response to the growing demand for biomass gasification to produce syngas as part of \"green synthesis\" processes, Xu Guangwen’s team from Shenyang University of Chemical Technology has recently developed and put into operation a laboratory-scale fluidized two-stage gasification system for the production of low-tar syngas from biomass using oxygen-enriched gas. Systematic experiments were conducted on pine biomass briquette fuel with high tar yield to achieve long-term continuous and stable operation, and the results are shown in Figures 3 and 4. By using O2/CO2 for oxygen-enriched vaporization, the system operated stably for nearly 50 hours (Figure 3), with the content of useful gases (H2+CO+CH4) in the product gas reaching 70%. However, since no water vapor was used as a vaporization agent during the prolonged experiments in the laboratory, and because the riser reactor was short in length and electric heating was employed, the H2/CO ratio was low while the methane content was high. There is significant room for improvement in actual engineering-scale vaporization processes that use higher temperatures along with O2/CO2/water vapor.
(2) The syngas has an extremely low tar content. During the aforementioned long-duration experiment, the product gas sample stream was washed with tertiary propylamine every 5 hours to determine the tar content in the gas. The tar content in the product gas gradually decreases over time; after 28 hours, the propylene wash liquid had become clear and transparent (Figure 4), making it difficult to detect any tar present, indicating that ultra-low tar vaporization has been achieved. At the same time, the temperature curve in Figure 3 shows that the gasification reaction temperature in the experimentally set up electrically heated device is not high (around 800°C). In industrial-scale gasification furnaces, this gasification reaction temperature remains above 900°C, and a water vapor gasifying agent in a specific ratio is used; this facilitates the thermal cracking, catalytic cracking, and catalytic reforming of tar within the gasification reactor, thereby ensuring the efficient production of ultra-low-tar syngas.
5. Technical development in the production of ultra-low tar synthesis gas through biomass-rich oxygenation. Currently, the global push toward carbon neutrality has created an urgent need for the production of green methanol and green aviation fuel; large energy and chemical companies around the world have launched numerous large-scale industrial projects to conduct in-depth studies on their feasibility. The results show that for the entire green synthesis technology chain based on biological \"carbon\", there is a lack both domestically and internationally of an industrializable biomass syngas production technology, namely biomass pure oxygen (oxygen-enriched) gasification technology. Various large-scale fluidized-bed gasification technologies used in coal gasification are difficult to apply effectively to biomass fuels due to differences in fuel quality and ash content characteristics. Fluidized-bed gasification is becoming the preferred technology for biomass gasification to produce syngas on an industrial scale, but it is necessary to overcome the problem of \"tar\" in biomass fluidized gasification. The industrial application of the fluidized two-stage technology for gas production via air gasification, along with the verification of effective tar removal achieved through prolonged operation of the oxygen-enriched gasification system in the aforementioned laboratory setup, fully demonstrate and confirm the effectiveness and feasibility of this process in ensuring low-tar characteristics in biomass gasification. This process is simple in technology; it is based on a dual-fluidized bed reaction system. There are extensive experiences in terms of equipment development and process industrialization both domestically and internationally, which gives it excellent potential for scale-up. It is therefore a strong competitor for biomass gasification technologies that will support the \"green methanol/aerosol fuel industry chain\" in the future. It is precisely because of the recognition of the technological advancement and industrialization potential that Canadian government agencies established special programs, signed technology source statements and confidentiality agreements with Professor Xu Guangwen, the leader of the technical innovation, and provided targeted funding to support UBC and FPInnovations in carrying out feasibility studies and practical tests on the use of fluidized two-stage gasification to produce biomass-based low-tar syngas. Professor Xu Guangwen’s team will make every effort to advance cooperation with large enterprises at home and abroad, carrying out research on the processes and equipment required for two-stage gasification using pressurized fluidization to produce biomass-based synthetic gas with low tar content, in order to overcome the technical challenges faced by current green synthesis technologies based on biomass feedstocks.