As a renewable energy source, biomass gasification has good development prospects. However, due to the high tar content in the crude gas produced by gasifiers and the lack of effective treatment methods, it has become a major obstacle to the promotion of this technology in rural areas. Tar is an inevitable by-product of biomass gasification. It is in a gaseous state at high temperatures and gradually condenses into a liquid state as the temperature decreases. Because the tar components are very complex, more than 200 types can be analyzed, of which there are more than 20 main components. The ones with higher contents are benzene, naphthalene, toluene, xylene, styrene, phenol and indene, etc. It is difficult to effectively separate or process it from the gas [3-4]. Small household biomass gasification devices often use updraft gasifiers, which have higher tar content than downdraft gasifiers. Although gas is used as cooking gas and does not require cooling and can be used directly, when the gas temperature is lower than 200e, tar is easy to condense into a liquid state, and combines with water, ash, etc. to block gas pipelines or valves, seriously affecting the long-term stable operation of the gasification device. If it enters the stove in a gaseous state, it will be difficult to achieve complete combustion, and it will easily produce particles such as carbon black. Since tar accounts for 5% to 15% of the total energy of crude gas, if it is not effectively treated, it will have obvious adverse effects on the environment and gasification efficiency.
Currently, there are two types of available technologies for controlling tar content in gasification units: one is the cracking method, and the other is the ordinary method. The cracking method is divided into thermal cracking and catalytic cracking, which decomposes tar into small molecular combustible gases by providing higher temperatures and other conditions; ordinary tar removal can be divided into two types: wet method and dry method.
1 Thermal cracking method to reduce tar content
Converting tar into combustible gas through cracking method can not only improve the gasification efficiency, but also reduce the tar content in the gas and solve the harmful impact of tar on the environment and equipment operation. The thermal cracking method is based on the biomass gasification process. The amount of tar products mainly depends on the conversion temperature and the gas phase residence time. Generally, the tar product is the most when the biomass is around 500e; and the longer the gas phase residence time is at the same temperature, the more complete the tar thermal cracking will be. Therefore, during the gasification process, the temperature and gas phase residence time should be increased as much as possible to thermally crack the tar and decompose the tar into permanent gas for use together with combustible gas, thereby reducing the amount and type of tar. The thermal cracking method can achieve higher conversion efficiency above 1100e. The gasification temperature of small biomass gasification devices (such as fixed beds) is generally around 900e, which is difficult to increase even if some technical measures are adopted. Therefore, it is difficult to apply the thermal cracking method in practice.
2 Catalytic cracking method to reduce tar content
Thermal cracking of tar requires very high temperatures, but if the tar is catalytically cracked with the help of certain catalysts, not only can the tar cracking temperature be reduced to 750~900e, but the cracking efficiency can also be improved. Therefore, catalytic cracking is currently the most promising technology for reducing tar content. The principle of biomass tar cracking is similar to the catalytic cracking of petroleum. Domestic and foreign research has found that catalysts that can be used for tar conversion include dolomite, alkali metal and other metal-based catalysts, nickel-based catalysts, etc. If low-cost requirements are met, limestone, charcoal and quartz sand can also be used as catalysts. Among them, dolomite (CaCO3·Mg2CO3) has received widespread attention due to its high catalytic efficiency and low cost [5].
Whether tar catalytic cracking can achieve the desired results depends on whether the required process conditions are met in terms of temperature and contact time. Depending on the location and method of adding the catalyst, it can be roughly divided into two types: one is to directly mix the catalyst and biomass before gasification, so that gasification and catalytic conversion of tar operate under the same working conditions. For example, tar cracking by dolomite only has a high cracking rate when the temperature reaches 800e or above [6] (as shown in Figure 1). This temperature is similar to the gasification temperature of biomass, so adding a catalyst to the furnace for cracking reaction can easily meet the required temperature conditions. The other is to set up another reactor at the outlet of the gasifier so that tar cracking can be carried out in a separate reactor. Because the gas temperature at the outlet of the gasifier has often dropped to 500e, the temperature is often raised through an external heating source or partial combustion of the gas, making the catalytic cracking technology more suitable for larger gasification systems.
The presence of water vapor can also play an important role in the catalytic cracking process. Water vapor can react with certain tar components to generate gases such as CO and H2, which not only reduces the production of carbon black, but also increases gas production.
