Biomass research in my country started in the 1980s. Biomass energy utilization and conversion technologies mainly include physical and chemical conversion technology, biochemical conversion technology and thermochemical conversion technology [7]. Among them, thermochemical conversion technology is the research hotspot and main utilization path of biomass conversion technology. Biomass thermochemical conversion technology is divided into biomass pyrolysis technology, biomass gasification technology and biomass liquefaction technology. As a common form of biomass energy utilization, biomass gasification technology can efficiently utilize biomass resources and produce clean fuels. Compared with direct combustion, it has good environmental benefits and resource utilization. Biomass gasification uses air, oxygen and water vapor as gasification agents, and undergoes drying, pyrolysis, oxidation and reduction reactions at high temperatures to convert into gaseous products (CO, H2, CH4, H2O and various light hydrocarbons, etc.), liquid products (tar, bio-oil, gasoline, etc.) and solid products (coke). The syngas after gasification can be used for FT synthetic oil production and IGCC technology to generate electricity. At present, the largest application of biomass gasification is cogeneration. The world's first IGCC power plant was built by Sydkraft AB in Varnamo, Sweden[8]. Using wood as raw material and pressurized circulating fluidized bed gasification technology, the power generation efficiency is 32%.

2.New biomass gasification technology

2.1 Plasma gasification technology

Plasma is the fourth state different from solid, liquid and gaseous forms, also known as ionized "gas", which is electrically neutral as a whole. Plasma gasification technology is mainly suitable for the treatment of urban solid waste, including raw material pretreatment equipment, plasma gasification furnace, purification equipment, etc., using the high temperature characteristics of plasma to provide a high temperature reaction environment of 4000~7000℃, organic compounds are thermally decomposed and converted into tar-free, high-quality synthesis gas, greatly improving the reaction rate. The advantages of this technology are low raw material pretreatment requirements, low pollutant content in synthesis gas, short reaction time, and easy scale-up; the disadvantages are that the melted material is easy to solidify in the pipeline, and the maintenance and operation costs are high[9]. A furnace-type plasma furnace[10] built by the Institute of Plasma Physics of the Chinese Academy of Sciences generates a high-temperature arc through discharge. The high-temperature arc heats the gas medium flowing through it to generate plasma. Under the conditions of oxygen deficiency and high temperature, complex, toxic and harmful solid wastes are completely decomposed, opening up a new way for the treatment of solid wastes. 2.2 Supercritical water gasification technology Supercritical water gasification technology (SCWG) is a technology for converting high-water content biomass into high-quality synthesis gas, and is a new direction for efficient and high-yield hydrogen production. It was proposed by the Massachusetts Institute of Technology in the mid-1970s. Water has a strong solubility in the supercritical state of 374.15℃ and 22.12MPa. It dissolves various organic matter in biomass and generates a high-density, low-viscosity liquid through high-temperature decomposition, isomerization, dehydration, cracking, concentration, hydrolysis, steam reforming, methanation, water-gas conversion and other reaction processes. It is rapidly gasified under high temperature and high pressure reaction conditions and finally generates a hydrogen-rich mixed gas. The advantage is that wet biomass can be directly gasified without going through a drying process, saving costs; the disadvantage is that the process is complicated and difficult to control, and there is still a gap between experimental and industrial scale. Demirel et al. [11] studied the effect of reaction parameters on hydrogen production by supercritical gasification of fruit pulp under the action of KOH catalyst. KOH catalyst promotes biomass decomposition through water-gas transfer reaction, inhibits tar production, and increases gas production. At the same time, KOH captures CO2 and transfers water gas to the direction of generating H2. 2.3 Microwave pyrolysis gasification technology Microwave is an electromagnetic wave with a wavelength of 1mm to 1m and a frequency of 300MHz to 300GHz, which is between radio waves and infrared radiation. It has strong penetrating power and can penetrate deep into the material [12]. Traditional pyrolysis technology is from the outside to the inside, using heat conduction, convection and radiation to heat. The transfer process will lose heat and the heating rate is slow. Microwave pyrolysis technology is different from traditional pyrolysis technology. The heat source is that the reactants inside the material absorb microwave heat and then rotate, collide and rub themselves [13] to convert microwave energy into heat. At present, microwave heating has been successfully applied to the processing and utilization of biomass raw materials such as oil palm shells, switchgrass, straw, sludge and pine sawdust [14]. The advantages are fast heating rate, short reaction time, high thermal efficiency, reduced CO2 content in gasification products, and increased H2 and CH4 content. The disadvantages are low oil yield, unstable properties and slow commercialization process. Vecten et al. [15] used pure steam as plasma working gas in a microwave plasma reactor for the first time and studied in detail the process of converting it into combustible gas using plasma gasification technology. At the highest microwave power of 6kW, the biomass carbon conversion rate reached more than 98%, and the hydrogen content in the synthesis gas was rich, with a volume fraction of 45% to 65%. The steam that did not participate in the reaction was condensed to produce synthesis gas with a calorific value range of 10.5 to 12.0 MJ/m³. The principle of microwave gasification technology is that the air pump sends a certain amount of air into the microwave reactor, and the biomass and microwave adsorbent become fluidized and rapidly undergo gasification reaction. The gas products such as H2, CH4, CO2, H2O, etc. are collected by the collection device through the condenser [16-17].

3.Characteristics of biomass cogeneration

In the 1980s, the famous scientist Mr. Wu Zhonghua proposed the concept of total energy system based on the basic law of energy conversion. Under the guidance of the principle of comprehensive cascade utilization of chemical energy and physical energy, he advocated the energy utilization principle of "temperature matching and cascade utilization" to match the supply of energy of different qualities. Biomass gasification cogeneration technology is proposed based on these principles. From the perspective of the cogeneration system, the integration theory of biomass gasification cogeneration technology includes three aspects: system concept, system integration ideas and system design principles [18]. The process route is shown in Figure 1. It takes gasification technology as the core, integrates and optimizes the efficient subsystems in independent units, and produces combustible gas, extract and biochar, which improves the waste of by-product resources faced by the single-production system and is considered to be an efficient use of energy.