Professor Xu Guangwen's Team from Shenyang University of Chemical Technology: Solving the Problem of Biomass Gasification Tar in the Green Methanol / Aviation Fuel Synthesis Technology Chain

Global low-carbon development is forcing the energy structure to shift from fossil energy to renewable energy, with the expectation of reducing carbon dioxide emissions while decreasing dependence on fossil energy. 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-rank biomass into high-energy-density fuel oil and gas, effectively replacing fossil fuel oil and gas. This process significantly accelerates the circulation speed of carbon dioxide in the plant growth cycle, produces no additional carbon dioxide, and embodies “carbon neutrality” attributes. The utilization of green hydrogen, derived from photovoltaic water electrolysis, can further adjust the hydrogen-carbon ratio of biomass syngas to serve as raw material gas for methanol synthesis, green aviation fuel synthesis, hydroformylation, etc., and can also recycle carbon dioxide in the production and utilization of chemical products. This process not only extends the carbon dioxide cycle, but also greatly enhances the utilization rate of “green carbon”, reduces the consumption of biomass raw materials, and effectively contributes to the national  “carbon neutrality” goal through enhanced biomass productivity.

1. The traditional gasification technology cannot meet the technical requirements of green methanol/aviation fuel synthesis industry.

Using biomass as a green “carbon” source, gasification has become the only technical path choice to convert biomass raw materials into syngas. For large-scale gasification, coal gasification technology with good industrial practice can be considered. However, due to the low energy density of biomass itself, high volatile content, high oxygen content, high alkali metal content, and SiO2-based ash content, coal gasification technology often fails to meet the needs of large-scale biomass gasification. The specific performance is as follows: (1) The fixed bed gasification technology of the Lurgi gasifier has high requirements on the strength and thermal stability of raw materials, and biomass is difficult to meet these requirements. (2) The synthesis gas produced by circulating fluidization bed gasification technology has high tar content, so it is necessary to install a two-stage cracking furnace to remove biomass tar, but the cracking furnace needs to be heated to above 1100 °C, which results in high energy consumption and poor adaptability to biomass raw materials with low ash melting points. (3) The micron-sized grinding powder required by the entrained flow bed technology cannot be directly applied to biomass, so it is necessary to carbonize biomass into biomass particles, resulting in the complex and difficult pretreatment process, and at the same time, silica-alumina ratio of ash is high, making it difficult to achieve “anti-slag with slag” when the flow bed is running.

2. Fluidization two-stage gasification technology innovation is applied to solve the biomass gasification “tar” problem

In order to make the gasifier fully used to the greatest extent and solve the world-universal problem of by-product “tar” in biomass gasification process from the root, the team led by Professor Xu Guangwen from Shenyang University of Chemical Technology innovated the fluidization two-stage gasification technology, and realized the two-stage gasification process of low tar by using fluidized reactor (Fig.1). The team, through the reaction decoupling of biomass pyrolysis and semi-coke gasification, coordinated the effects of biomass preoxidation reducing tar generation, biomass tar thermal cracking and high temperature catalytic cracking of semi-coke for deep tar removal. Meanwhile, according to the product gas quality, the team optimized and regulated the reaction temperature and conditions of pyrolysis and gasification/tar understanding respectively, and effectively realized the technical advantages of the formation and release of ultra-low tar from medium and low temperature gasification in the process of easy large-scale fluidization reaction.

Fig.1 Process flow diagram of fluidization two-stage gasification for preparation of ultra-low tar syngas.

Fluidization two-stage gasification has been successfully applied to the production of gas from the air gasification of biomass waste in light industrial processes such as Chinese medicine residue and distiller’s grains. In Henan, Sichuan, Shandong, Anhui and other provinces, multiple sets of industrial application projects for the treatment of industrial biomass waste ranging from 10,000 to 50,000 tons per year have been established (Fig.2 is a representative application), which effectively achieves the international advanced technical index of tar content of less than 50 mg/Nm3 in gasification gas (Bioresource Technology 2016, 206, 93–98; Carbon Resources Conversion 2018, 1(2),109–125; Carbon Resources Conversion 2020, 3, 1–18).

Fig. 2 Representative application engineering photos of fluidization two-stage gasification technology.

3.Fluidization two-stage gasification technology has been highly appraised in academic and industrial circles at home and abroad.

The fluidization two-stage gasification technology, along with its technical characteristics and advantages in achieving low tar, has received significant recognition and positive evaluations from academic and industrial entities worldwide. For instance, M. A. Rosen, an academician of the Canadian Academy of Engineering and former President of the Canadian National Academy of Engineering, identified fluidization two-stage gasification as the leading promising technology globally in his review paper on biomass gasification technology (Biofuels, 2017, 8(6): 725-745). In 2021, Western Economics Diversification Canada and Natural Resources Canada funded over CAD 4 million in projects for UBC and FPInnovations to support the evaluation and verification of the technical processes and low tar characteristics of fluidization two-stage oxygen-rich synthesis of biomass syngas (Materials: Confidentiality agreement between Shenyang University of Chemical Technology and the three parties, and Canadian project description provided to Professor Xu Guangwen by Professor UBC). In terms of the industrial application of this technology in numerous pharmaceutical factories, which effectively utilize the energy from high moisture content Chinese medicine residues, the China Light Industry Federation recognized this technology as “international leading” and awarded it the first prize of the Science and Technology Award of the China Light Industry Federation. The related technical inventions and basic research achievements also won the first prize of Liaoning Province Technological Invention Award and the first prize of China Chemical Industry Society Research Achievement Award respectively

4. The fluidization two-stage biomass pure oxygen (oxygen-enriched) gasification achieved technical verification.

(1) Operation stability and syngas quality. The principle of “two-stage” ultra-low tar gasification in fluidization reactor system that is easy to scale up in industry achieving reaction classification, makes it more suitable for gasification and conversion of small particle fuels with high water content, including biomass and low rank coal. On the basis of successful industrialization application of air gasification technology, in response to the significant need for large-scale low-tar gasification technologies for biomass gasification to produce syngas in Green synthesis”, Xu Guangwen’s team from Shenyang University of Chemical Technology recently built and operated a laboratory-scale fluidization two-stage gasification process device for biomass oxygen-rich gasification to produce low-tar syngas. Systematic experiments were conducted on pine biomass pellet fuel with high tar yield, and long-term continuous and stable operation was achieved. The results are shown in Figure 3 and Figure 4. Using O2/CO2-rich oxygen gasification, the system operates stably for nearly 50 hours (Figure 3), and the effective gas (H2 + CO + CH4) content in the product gas reaches 70%. However, due to the long-term experiment in the laboratory, the steam gasification agent is not used, and the riser reactor is short while electric heating is utilized, leading to low H2/CO and high methane content. There is significant room for improvement in the actual project-scale higher temperature O2/CO2/steam gasification.

Fig. 3 Changes of long-term operating parameters of biomass fluidization two-stage O2 / CO2 gasification process in laboratory

(2) The tar content in the synthesis gas was extremely low. During the aforementioned long-term experiment, the tar content in the gas was tested by washing the product gas sample stream through the three-stage acetone solution every 5 hours. The tar content in the product gas decreased gradually over time of operation. By 28 hours, the acetone wash solution was clear and transparent (Figure 4). Then it was difficult to detect the presence of tar, indicating the achievement of ultra-low tar synthesis gas. Additionally, the temperature curve in Figure 3 showed that the temperature of the gasification reaction formed by the electrically heated experimental setup was not high (~800°C). In an industrial-scale gasifier, this gasification reaction temperature will stabilize above 900°C and will be suitable for a certain ratio of steam as a gasification agent. This makes it more conducive to the thermal cracking, catalytic cracking, and catalytic reforming of tar in the gasification reactor, ensuring the effective production of ultra-low tar synthesis gas.

Fig.4 Changes in the tar content in the product gas of long-cycle pine particle O2/CO2 gasification corresponding to the reaction conditions of Figure 3

 5.Development of Technology for Producing Ultra-Low Tar Syngas through Biomass Oxygen Enrichment Gasification

 At present, the global pursuit of the “carbon neutrality” goal has urgently proposed the requirements for the production of green methanol and green aviation coal. Both domestic and international large-scale energy and chemical companies have initiated numerous large-scale industrialization projects, conducting in-depth research on their preliminary feasibility. The results show that the whole technology chain of green synthesis based on biological “carbon” lacks industrialized biomass syngas production technology at home and abroad, namely biomass pure oxygen (oxygen-enriched) gasification technology. Various large-scale fluidization bed gasification technologies applied in coal gasification are difficult to be effectively used for biomass fuel due to differences in fuel quality and ash characteristics. Gasification based on fluidization beds is becoming the preferred technology for achieving industrial-scale synthesis gas production in biomass gasification, but it needs to solve the problem of “tar” in biomass fluidization gasification.

The fluidization two-stage technology process, through its industrial application in the production of gas by air gasification and the validation of its long-term operation in a rich oxygen environment in the laboratory device for deep tar removal, fully demonstrates and reveals the effectiveness and feasibility of this process in ensuring the low tar characteristic of biomass gasification technology. This technology is simple, based on a dual fluidization bed reaction system, and has excellent equipment and process industrialization practices both domestically and internationally. Therefore, it exhibits excellent industrial scaling capabilities, making it a strong contender for supporting the “green methanol/aviation fuel supply chain” biomass gasification technology in the future.

Recognizing the advanced nature and the industrialization possibility of this technology, government departments of Canada set up special projects and signed a technical source declaration and confidentiality agreement with Professor Xu Guangwen, the person in charge of technical invention. Canada will allocate funds to support UBC and FPInnovations to carry out the technical feasibility verification and practice of low-tar biomass syngas production by fluidization two-stage gasification. Professor Xu Guangwen’s team will actively promote cooperation with large domestic and international enterprises, conducting research on the process and equipment for pressurized fluidization two-stage gasification production of biomass low tar syngas. This is aimed at overcoming the current challenges in the green synthesis technology process based on biomass raw materials.