Review of biomass energy utilization technology and policy

1 Main technical paths and application progress of biomass energy utilization

The world's more mature technology to achieve large-scale development and utilization of biomass energy utilization methods mainly include biomass power generation, biological liquid fuel, biogas and biomass molding fuel. The conversion technologies of biomass energy utilization mainly include: direct combustion technology, dense molding technology, gasification technology, pyrolysis, vegetable oil esterification technology, municipal landfill gas power generation and heating, biomass fermentation ethanol technology, carbonization technology, biogas power generation technology, etc. (Zhou Yanru et al., 2011). According to the division of biomass energy products, biomass energy technology research is mainly focused on solid biofuels (biomass molding fuel, biomass direct power generation/heating), gas biofuels (biogas and vehicle methane, biological hydrogen production), liquid biofuels (fuel ethanol, biodiesel, BTL) and alternative petroleum-based products such as bio-based ethylene and ethanol derivatives. The products that have been marketized are mainly biomass power generation/heating, biogas and vehicle methane, fuel ethanol and ethanol downstream products, biodiesel and related chemical products. At present, the EU countries have formed a mature technology system and industrial model from raw material collection, storage, pretreatment to fuel production, distribution and application of the entire industrial chain, the technical system of developed countries is also increasingly perfect, less developed countries still need to carry out technological research in key areas.

1.1 Solid fuel technology

In the aspect of solid biofuel, China has developed the technology of biomass molding at room temperature.  Solid fuel technology mainly includes biomass molding fuel technology and biochar technology, among which biomass molding fuel technology mainly includes biomass pellets, biomass blocks and molding equipment manufacturing technology. The main factors affecting the biomass solidification molding are raw material type, particle shape, water content, temperature, etc. They are the main influencing factors determining the development of biomass molding fuel technology. Biochar refers to a class of highly aromatic refractory carbon-rich substances produced by pyrolysis of biomass under the condition of complete or partial hypoxia at high temperature, mainly including ash, fixed carbon and volatile components. According to the carbonization method, biomass carbonization technology is generally divided into hydrolytic carbonization, pyrolytic carbonization and flash carbonization technology (Lv Haohao et al., 2015). At present, European countries are developing the fastest in biomass solid fuels, so overall the development of solid fuel technology is the most mature. Japan's dense forming technology has led the world. China also needs to tackle the key technologies of biomass solid molding fuel in order to achieve large-scale development.

1.2 Liquid fuel technology

In terms of liquid biofuels, there are two technologies, biodiesel and fuel ethanol, among which biodiesel has conventional base (acid) catalysis technology, high pressure alcoholysis technology, enzyme catalysis technology, supercritical (or subcritical) technology. Fuel ethanol mainly includes cassava ethanol, sweet sorghum ethanol and cellulosic ethanol. In general, starch (including sugar), lignocellulose and oil are the three types of biomass resources mainly suitable for the preparation of liquid fuels. Among them, starch and sugar are mainly used for the preparation of fuel ethanol by fermentation, and oil is mainly used for the preparation of biomass-based diesel or gasoline by thermal cracking, transesterification or catalytic hydrogenation. Lignocellulosic liquid fuels are mainly prepared through fermentation, gas-Fischer-Tropsch synthesis, liquefaction-refining, and selective synthesis of platform compound intermediates (Zhang Jiren et al., 2013). In the United States, it is more inclined to graduate the first generation of raw materials and second generation of raw materials for ethanol production (respectively starch biomass and wood fiber biomass), at the current level of technology, the United States and the world's ethanol scale production is mostly from the former. Cellulosic raw materials, such as agricultural and forestry residues, seasonal pastures, woody plants and stationary waste, have advantages because they do not have to compete directly with food, feed and fibre production and require fewer inputs (such as water, nutrients and land) than corn and other commercial crops. However, due to the immaturity of technology, the production cost is still high, and it is not suitable for large-scale industrial production. At present, European countries, the United States and China have basically developed mature liquid fuel technology with food as raw materials. In the cellulose as a raw material of liquid fuel technology, the United States, Europe has made great progress, China has also made some achievements, but still with the United States and Europe have a certain distance. At present, sweet sorghum for ethanol production can be grown in 18 provinces in northern China, but cellulosic ethanol technology faces three major technical bottlenecks: efficient straw plant biomass pretreatment technology; The enzymes that degrade cellulose to glucose are costly; Lack of high conversion of pentose and hexose to produce ethanol microbial strains. In terms of biodiesel, the current technology, which is mainly based on herbal oil as raw material, has matured and is shifting to the development of fuel technology based on woody oil.