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Biomass energy is a clean, renewable energy source. Its environmental protection and renewable nature make it increasingly important in the competition with fossil fuels that seriously pollute the environment and have limited reserves. Biomass energy utilization technology is becoming increasingly mature and has huge advantages in processing urban domestic waste, industrial organic waste liquid, agricultural waste straw, etc., so it is increasingly valued by countries around the world. Our country has abundant biomass resources and should actively develop practical technologies with independent intellectual property rights, develop biomass clean energy, and catch up with the world's advanced level. Biomass utilization technologies can be divided into three basic types: direct combustion, thermochemical and biochemical. This article analyzes the technical, economic and social characteristics of the main methods in each basic type, and compares their respective advantages and disadvantages. 1 Direct combustion process Direct combustion is a common basic process for converting biomass into energy. The heat and steam produced by the combustion process can be used to generate electricity or to provide heat where it is needed, such as industrial processes of all sizes, space heating, cooking, and home heating. Since most biomass has high water content and complex composition, and the combustion process is unstable, it is difficult for boilers using biomass to achieve an efficiency comparable to conventional boilers in terms of cost. Ways to improve combustion efficiency are: (1) Reduce the water content to keep the water content in the biomass at an appropriate level, which can not only weaken the adverse effects of water evaporation on the rise in fuel temperature, but also use the hydrogen produced by water decomposition at high temperatures to improve the combustion effect; (2) Change the size to minimize the size of the fuel particles and improve the rate, stability, and adequacy of combustion; (3) Keep the temperature in the combustion chamber above a certain temperature. In order to achieve this condition, the flue gas can be used to preheat the air, so that the waste heat of the flue gas can be fully utilized at the same time; (4) Increase the air input rate and maintain a certain air margin; (5) Combined combustion does not require major changes to existing equipment, but can also provide opportunities for optimal mixing of biomass and fossil fuels. More practical methods include burning biomass on the grate assembled in the furnace of a coal-fired boiler and gasifying biomass in a gasifier, using gas as boiler fuel, etc. For cities, the environmental protection effect of direct combustion of biomass is greater than the utilization of its heat energy. A city with a population of one million produces thousands of tons of garbage every day. The main ways to deal with this garbage are landfill, biodegradation and incineration. The scope of application of biodegradation is limited. Landfilling will bring about a series of problems such as occupying urban land, contaminating groundwater, and potential explosion risks. Incineration not only disposes of garbage, but also recovers part of the energy, and has good development prospects. Currently, Zhejiang University and others are studying this technology. Biomass slurry proposed by Liu Xinzhi of Nanjing University of Science and Technology is a promising way to utilize biomass energy. The combustion stability of biomass fuel processed through the pulverizing and pulping process is greatly improved. At the same time, because the slurry-forming agent can be water, oil and organic waste liquid, the slurry fuel can meet different requirements of combustion environments such as high reliability, cleanliness and environmental protection. It has high comprehensive competitive advantages in terms of cost, storage and transportation, safety, and environmentally friendly performance, and is suitable for large-scale commercialization. However, some technologies are not yet mature, such as preventing slurry from becoming moldy. 2 The thermochemical process converts biomass into more valuable or more convenient products. The basic thermochemical process is pyrolysis, which can be divided into gasification, pyrolysis and direct liquefaction. Pyrolysis is a basic thermochemical process that converts biomass into useful fuels. Combustion of biomass when heated in the absence of oxygen or in the presence of partial oxygen can produce hydrocarbon-rich gas mixtures, oily liquids, and carbon-rich solid residues. . The advantage of pyrolysis is that the equipment is simple and it can produce carbon and a variety of chemical products. The disadvantage is that the utilization rate is low and the applicability is small. Gasification is also a type of pyrolysis. It is mainly a gas with the best yield at high temperature. The gas produced mainly contains CO, H2 and CH4, as well as a small amount of CO2 and N2. The main advantage of gasification is that after it is converted into combustible gas, it has a high utilization rate and is widely used. However, since the generated gas is not convenient for storage and transportation, special users and supporting facilities must be provided; in addition, the gas generated is generally low or medium calorific value, and high calorific value gas is rarely produced. If the process is changed, that is, instead of directly pyrolyzing and gasifying the raw materials, but going through pretreatment → compression molding → carbonization granulation → water gas biogasification, it is expected to obtain gas with a high calorific value higher than 10,000 kJ/m3. Since biomass solidification and molding technology is relatively mature, how to obtain high-quality carbon particles is the key to this process. Liquefaction is the decomposition of raw material macromolecules into small molecular compounds under the action of a suitable catalyst, and the small molecular compounds are then repolymerized into oil compounds with appropriate molecular weights. Water is generally used as the solvent. Under supercritical conditions, water as the medium has good reaction characteristics such as accelerating the reaction, being environmentally friendly, and easy to separate the product. It is a biomass liquefaction agent and reaction medium with potential for development and application. The main advantage of liquefaction is that biomass can be burned into oil as a substitute for petroleum, its application range and added value * * The main disadvantage is that the technology is complex and the current cost is still high. 3 Biochemical process The biochemical process uses the biochemical effects of raw materials and the metabolism of microorganisms to produce gaseous fuels and liquefied fuels. Because it can minimize the environmental damage caused by the use of biomass energy, it has more development prospects in today's increasingly stringent environmental protection requirements in the world. Common products are biogas, hydrogen and alcohol fuels represented by bioethanol. Biogas (main component is CH4) is produced by organic matter under the action of anaerobic bacteria. These anaerobic bacteria are usually methanogen-producing primitive bacteria, and they are quite specific. Methanolysis of biomass generally has three main steps. The three main steps of methane fermentation. The biomass raw materials of biogas fermentation are mainly domestic wastes, waste liquids and industrial and agricultural wastes, which can be used as a simple and very feasible way of waste disposal. Because the methane it produces can solve the problem of rural energy shortage to a large extent, it is currently developing rapidly in rural areas. According to reports, the most successful promotion and application in the world is in rural areas of my country, with more than 5 million biogas digesters in use. Alcohol fuels are also receiving more and more attention, and Brazil has been more successful in promoting and applying bioethanol technology. At present, Brazil produces 12 billion liters of ethanol every year, accounting for 62% of the country's automobile fuel. The United States also attaches great importance to replacing fossil fuels with alcohol fuels. In 2000, the sales volume of fuel ethanol in the United States reached 5.59 million tons, with an average annual growth rate of 20%. Bioethanol is mainly produced through microbial fermentation. The most famous ethanol-producing microorganism is Saccharomyces cerevisiae. Saccharomyces cerevisiae produces different amounts of ethanol in different processes. The raw material of traditional yeast fermentation is mainly grain biomass, which is relatively expensive. Using a mixed fermentation method (mixed saccharification of inorganic acids and fungal enzymes) to ferment lignocellulose to produce ethanol to further reduce the cost of nutrients and improve biocatalysts is the direction of future research. Hydrogen, as a clean energy source, is favored by many countries * * Attention with researchers. Microbial fermentation to produce hydrogen can not only utilize chemical bonds in biomass, but also utilize light energy. Hydrogen is usually produced as part of energy metabolism. Experiments have found that each mole of glucose can produce up to 4molH2. There is still a lot of work to be done to achieve the goal of high-efficiency photolysis preparation, and an important step is to improve the light saturation level of the photosystem. In short, rapid and efficient hydrogen production requires suitable microorganisms and optimized reaction conditions. Using H2 liquid-gas transfer technology to separate the produced H2 during the non-autobic fermentation process can make the H2 production rate 80 times higher than the thermodynamic equilibrium value. 4 Conclusion (1) Direct combustion of biomass has a low heat utilization rate and will also produce secondary pollutants. However, due to its simple technology and low investment cost, it is still in use, especially after being transformed by new ideas such as biomass slurrying and new technologies such as combined combustion, it will produce good economic and social benefits. As an effective method of garbage disposal, garbage incineration is also a wise move under current conditions. (2) The product obtained by the thermochemical method is gaseous or liquid fuel, which not only greatly increases the energy flow density of biomass, improves the energy quality, and expands the scope of use of biomass energy, but also because its cost is lower than that of biochemical methods for obtaining organic fuels. Therefore, although there are still some problems (such as technical complexity, inconvenient storage and transportation, etc.), the use of biomass by thermochemical methods will have a great competitive advantage both now and in the foreseeable future. According to my country's national conditions, gaseous fuel with higher calorific value can be produced by solidifying and shaping straw and then pyrolyzing and gasifying it. (3) Biochemical methods are the most promising way to utilize biomass energy. Its biggest advantages are the warm fuel production conditions and the environmentally friendly characteristics of producing no or little secondary pollution.