Experiment on atomization of pyrolyzed tar from carbon-gas co-production through straw pyrolysis

More than 800 million tons of residual straw produced in China each year, about half of which have not been reused effectively. Alternatively, a biochar-gas-tar polygeneration can transform agriculture residues, such as straw into pyrolysis gas, biochar, pyrolysis tar, and other products, indicating the full use of renewable resources to reduce environmental pollution. However, pyrolysis tar is a sort of inevitable by-product in the process of biochar-gas-tar polygeneration. The treatment of pyrolysis tar has become a prominent problem restricting the development of technology. The mass concentration of volatile phenol in pyrolysis tar is 156~312 mg/L, rich in aldehyde, benzene, phenol, hydrocarbon, ammonia, and heavy metals. At present, there are four main methods to treat the pyrolysis tar: high temperature cracking, catalyst cracking, plasma, and combustion. The former three methods can only reduce the emission of pyrolysis tar, but cannot eliminate or completely realize the use of tar. Moreover, there are still some problems, such as high requirements for equipment material, and high temperature bearing capacity, high cost, low efficiency, high energy consumption or catalyst deactivation. Since the energy density of pyrolysis tar is relatively high, the clean energy utilization of pyrolysis tar can be realized by burning pyrolysis tar as liquid fuel. However, the physical and chemical properties of pyrolysis tar are quite different from those of common fuel tar, such as high moisture content, high viscosity, and poor effect of atomization combustion. Therefore, it is necessary to study the atomization characteristics of pyrolysis tar under different conditions, and thereby to reveal the atomization law of pyrolysis tar, further to realize clean and efficient combustion of pyrolysis tar. In this study, an atomization test of pyrolysis tar was designed using two technologies of fluid atomization and scattering particle size analysis, in order to ensure the stable combustion and atomization of pyrolysis tar. The single factor and orthogonal experiments were carried out to compare the effects of tar temperature, gas temperature, and gas pressure on the particle size and its distribution in the steady state free spray of tar. The results showed that the optimal effect of tar atomization was achieved, when the temperature of tar exceeded 60℃, the temperature of gas exceeded 80℃, and the pressure exceeded 0.3MPa. With the increase of tar temperature, air temperature and air pressure, the SMD of tar droplets decreased. The influence of each factor on atomization effect was ranked in a descend order: tar temperature, gas pressure, and gas temperature. The small particle rate of pyrolysis tar accounted for more than 85%, where the optimal atomization pyrolysis tar temperature was 100℃, gas temperature was 120℃, and the gas pressure was 0.5MPa, indicating a good atomization effect on pyrolysis tar. The proposed system can provide a potential data support for clean combustion and comprehensive utilization of pyrolysis tar. Since this experiment mainly studied the pyrolytic tar atomization, the next step can be set to carry out the investigation on pyrolytic oil atomization combustion combined with atomization parameters.