The composition of tar affects the conversion process of pyrolysis, but no matter what composition, the final product of pyrolysis is similar to the composition of gasification gas, so tar pyrolysis has no obvious effect on the quality of gasification gas, but the quantity increases. For most tar components, water vapor plays a key role in the cracking process, because water vapor can react with some tar components to generate gases such as CO and H2, which not only reduces the generation of carbon black, but also increases the generation of combustible gases.
② Characteristics and selection of catalysts. The principle of catalytic cracking of biomass tar is similar to that of petroleum, so the choice of catalyst can be inspired by petroleum industry. However, because the added value of tar catalytic cracking is small and its cost requirement is very low, people have studied many low-cost materials, such as limestone, quartz sand and dolomite, in addition to catalysts in the petroleum industry. o
③ Process conditions of catalytic cracking of tar. Catalytic cracking of tar requires not only suitable catalysts, but also strict process conditions. Like other catalytic processes, the most important factors affecting the catalytic effect are temperature and contact time, so the process conditions are determined according to this requirement.
④ The key to meet the requirements of FCC process. For an ideal dolomite catalyst, the first condition of tar cracking is a sufficiently high temperature (above 800℃), which is close to the operating temperature of fluidized bed gasifier. Relevant experiments show that adding dolomite directly into fluidized bed gasifier can control tar to some extent, but it can't completely solve the problem. This is mainly due to insufficient contact between tar and the catalyst in the gasifier (because tar is mainly produced at the feed inlet, but even in the circulating fluidized bed, the amount of catalyst above the feed inlet is unlikely to be much). Therefore, in order to achieve the expected effect, gasification and tar cracking generally need to be carried out in two independent reaction furnaces, which makes the following problems appear in practical application.
A. The gas temperature at the gasifier outlet has dropped to about 600℃. In order to keep the temperature of the cracking furnace above 800℃, it is necessary to increase the heat source or partially burn the gas (the general combustion share is 5%- 10%), which will make the quality of the gasified gas worse and increase the sensible heat loss. B whether the cracking furnace adopts a fixed bed or a fluidized bed furnace, ash or carbon particles in the gasification gas may block the inlet of the cracking furnace. Therefore, it is necessary to add a gas-solid separation port between the cracking furnace and the gasifier, but the gas temperature cannot be reduced too much, which makes the system more complicated.
C. Because tar cracking needs an independent device, and because of the requirement of high temperature, the cracking device needs to run continuously (otherwise the efficiency is too low), so catalytic cracking technology is only suitable for large gasification systems, which limits the technology and applicability.
Therefore, the key of tar catalytic cracking application is to design different cracking furnaces according to different gasification characteristics, so as to reduce the energy consumption of cracking furnaces as much as possible and improve the thermal efficiency of the system.