Review of biomass energy utilization technology and policy

1.3 Gas fuel technology

Existing biomass gas fuel technologies mainly include biological fermentation, high temperature pyrolysis, plasma pyrolysis, molten metal gasification, supercritical water gasification, etc. The devices mainly include fixed bed, fluidized bed and gas flow bed, which are divided into single bed and twin bed (Wang Yusheng et al., 2015; Li Ji et al., 2016). Among them, the gasification reaction of non-gasifier is called pyrolysis gasification, and the gasification with gasifier can be divided into oxygen gasification, water vapor gasification, air gasification and composite gas gasification (Tang Ying et al., 2017). Biomass gasification technology can also be applied to centralized gas supply, cogeneration, synthetic natural gas, synthetic liquid fuel and hydrogen production (Wang Zhonghua, 2016; Li Ji et al., 2016). The main problems restricting the development of biomass gasification include tar content, secondary pollution, heat value of gas production and economic benefits, etc. (Li Ji et al., 2016). Factors affecting biomass gasification products mainly include biomass material characteristics (moisture, ash, particle size, material layer structure), type of reactor, biomass pretreatment (drying method, crushing method, pickling method, alkali washing method, organic solvent method, steam blasting method, biological treatment method, ammonia explosion method, carbon dioxide blasting method, high temperature liquid water method, etc.), type of gasification agent (empty Gas, oxygen, water vapor, hydrogen, composite gas), biomass feed rate, pressure and temperature in the reactor, etc. (Tang Ying et al., 2017). Biomass gas fuel technology also includes liquefaction synthesis gas technology, which is mainly divided into direct liquefaction of biomass into syngas and rapid cracking of biomass into bio-oil and re-gasification into syngas (Yiguanlin et al., 2016). The process of biomass thermochemical gasification to produce synthetic natural gas includes biomass pretreatment, gasification, purification and adjustment, methanation and gas purification, among which methanation is the key technology, mainly involving methanation reactor and catalyst. There are two types of reactors used for syngas methanation, fixed bed and fluidized bed, and the frequently used methanation catalyst is the transition metal on the loaded and oxide carrier (Dong Ming et al., 2017). In fermentation technology, biogas production involves the need to give living microorganisms a specific environment to perform their optimal function, so a distinction is often made between normal temperature and high temperature production, wet pulp production and dry production. Among them, wet technology refers to the treatment method of liquid organic matter with the dry matter content of raw materials less than 8%, and dry technology refers to the treatment method of solid biomass with the dry matter content of raw materials between 20% and 40% (Luo Zhigang, 2016). At present, the gas fuel technology is mainly biogas technology, and the technology of various countries has basically developed and matured, and China has also begun to operate some large-scale biogas production projects. The latest research progress is algal biomass gasification to produce methane. At present, countries have successively carried out technological research and development, such as China's research on cyanobacteria fermentation to produce methane, Japan's research on algal fermentation to produce methane, Spain's research on algae cracking to produce methane, etc. (Sun Shujing et al., 2017).

1.4 Power production technology

At present, the three main biomass technologies used in electricity production are: direct combustion, co-combustion and gasification. Direct combustion is the use of biomass as the sole fuel in power plant furnaces. The average industrial efficiency of these plants is 25%. Because these factories rely mainly on raw materials, they often have very small production volumes. The second technique, co-combustion, involves replacing part of the coal with biomass in an existing power plant furnace, usually in the range of 5 to 30 percent. When the ratio is right, the efficiency level of co-combustion plants can rival that of all-coal combustion, which is typically around 35%. The third technique, gasification, involves heating biomass until it forms a flammable gas. This biomass gas can be purified and used in combined cycle power generation systems with an efficiency of up to 60%. Of the three technologies, co-combustion is expected to be the one with the most economic potential to meet energy demand growth in the short term. As technology costs fall, gasification will become a more promising technology in the future. In addition, a common application of biomass power is a co-generation plant that provides both heat and electricity (Emily et al., 2012). At present, these three production technologies have been basically developed and mature, and countries have built some biomass straw direct combustion power generation, biomass and coal co-combustion power generation and biogas power generation projects according to their respective resource endowments. In 2007, the total installed capacity of biomass power generation in the United States has exceeded 10 GW, and there are more than 350 biomass power stations (Lu Xudong et al., 2009).