Effect of Pressure on Soot Formation During Biomass Pyrolysis in a Pressurized Fixed Bed Reactor

Biomass is a renewable resource that is important for mitigating the greenhouse effect and addressing the energy crisis. Pressurized combustion has received increasing attention due to its higher efficiency. During the biomass combustion, soot is unavoidably produced, which not only lowers combustion efficiency but also has negative effects on the environment, human health, and climate change. In order to regulate soot emission during pressurized combustion, it is crucial to disclose the effect of pressure on the soot formation. However, the study of biomass soot formation at elevated pressure is rather limited. In this study, a pressurized fixed bed reactor was employed to study soot formation during pressurized pyrolysis. The results show that the soot yields increase with increasing pressure and subsequently drops, with the maximum yields occurring at 3 bar. As pressure rises, the yields of gas and char increase, while the tar yields drastically decrease. H₂ yields increase as pressure rises, whereas C₂H₂ yields rise first and then decrease. According to the tar analysis, 2-rings PAHs are more prominent in wheat straw tar, whereas 4-rings PAHs are dominant in wood tar. As the pressure increases, PAHs expand from 2-rings to 4-rings. XRD analysis shows that the soot crystal stack height rises as pressure rises, which is due to that the high pressure suppresses small soot crystals and promotes its vertical growth. However, the increased pressure also reduces tar formation and slows down the synthesis of PAHs, resulting in a decrease of the plane length. The wood soot particle’s inner multi-core and outer shell structures were observed by TEM, suggesting that the initial PAHs are very important for the soot growth. This work provides a better understanding of the soot formation and control at elevated pressure.