(36d) Effect of Particle Size on Supercritical Water Gasification of Biomass

Introduction
Supercritical water gasification of biomass is exactly in the scope of reactor engineering of biomass feedstocks. Specific to biomass is the very complex reaction compared to conventional fossil fuel conversion in chemical industry. Biomass is more reactive than coal and oil, and thus undergoes various reactions at supercritical temperature (600 °C) which is much lower than conventional fossil fuel conversion temperature (1000 °C). Biomass is fed as solid, but it is known that most of biomass species can get dissolved in supercritical water, and then furthermore decomposed in the molecular scale. This dissolution-decomposition model has been long mentioned when supercritical water gasification of biomass is discussed. However, apparently, complete dissolution should be difficult when particle size is sufficiently large. Chemical engineering textbooks tell us dissolution is usually mass transfer limited phenomena, and when diffusion or mass transfer is slow, complete dissolution takes time. When temperature inside the particles gets high before dissolution, pyrolysis inside the particle should take place. Then, the question arises: what is the diameter needed for complete dissolution for usual biomass gasification in supercritical water? The purpose of our study was to answer this question for effective commercialization of supercritical water gasification technology.

Experimental
A laboratory scale continuous flow reactor for supercritical water gasification was employed. Reaction temperature and pressure were 600 °C and 25 MPa, respectively. Feedstock was slurry of particles of biomass or cellulose. The size of these particles was set to several to sub- micrometer using special apparatus developed in Nara Machinery Co., Ltd. So far no one has worked on these fine particles for supercritical water gasification.

Results and discussion
For EFB particle, the effect of particle size is observed. If complete dissolution is achieved, this should not happen. Thus, it is found that 3.5 μm is not sufficiently small to achieve complete dissolution.
Cellulose particle size in the range of much smaller size was prepared. To achieve this sub-micrometer size, we had to employ flow separation of particles. Interestingly, effect of particle size disappeared for small particles. This implies complete dissolution.

Conclusion
We successfully found the criteria of complete dissolution of particles in a laboratory scale continuous supercritical water gasification reactor. This result shows that when actual biomass is used in the continuous reactor, dissolution is not complete, and behavior of solid particles should be considered. This is the new and important finding in terms of supercritical water gasification of biomass.