Currently most research focused on the one-step catalytic fast pyrolysis where the catalysts are mixed with the feedstock in the pyrolysis reactor (Xie et al., 2015; Zhang et al., 2017). However, the one-step process has several disadvantages, including fixed cracking and catalytic upgrading temperature, and high coke formation on the catalyst causing rapid catalyst deactivation, etc. In the present study, a new two-step approach combining a fast microwave-assisted pyrolysis (fMAP) step for bio-crude production and a separate packed-bed catalysis step for bio-oil upgrading in one system was developed. In this two-step system, the catalyst bed is placed between the pyrolysis and condensation processes outside of the microwave cavity. The cracking and catalytic upgrading operations can be flexibly and independently controlled, thus enabling the optimum catalyst performances targeted at desired products under optimized reaction temperature and residence time. This also prevents the unreacted char and mineral components from contacting the upgrading catalyst, and can extend the life of the catalyst and reduce the catalyst loading and therefore the cost. In this study, the effects of fMAP temperature, catalyst bed temperature and catalyst-to-biomass ratio on product fractional yield and selectivity were investigated. Eight major components in bio-oils were identified and quantified with an external standard method. The comparison between catalytic and non-catalytic pyrolytic bio-oils provided insights into the possible reaction pathway of the sequential two-step catalytic fMAP. The results showed that the optimum fMAP temperature and catalyst bed temperature were 550 and 425 , respectively. The XRD results showed that the crystal structure and crystallinity of the HZSM-5 catalyst were intact before and after the pyrolysis and regeneration process, indicating the HZSM-5 had good stability in the catalytic upgrading process. In addition, the two-step process was superior to the one-step process in terms of the amount of catalyst needed to achieve similar bio-oil quality. Thus, the newly developed two-step fast microwave-assisted pyrolysis process allowed more flexibility for the high quality bio-oil production that meets the requirements for downstream processing and applications.
Xie, Q.L., Addy, M., Liu, S.Y., Zhang, B., Cheng, Y.L., Wan, Y.Q., Li, Y., Liu, Y.H., Lin, X.Y., Chen, P., Ruan, R. 2015. Fast microwave-assisted catalytic co-pyrolysis of microalgae and scum for bio-oil production. Fuel, 160, 577-582.
Zhang, Y., Chen, P., Liu, S., Peng, P., Min, M., Cheng, Y., Anderson, E., Zhou, N., Fan, L., Liu, C., Chen, G., Liu, Y., Lei, H., Li, B., Ruan, R. 2017. Effects of feedstock characteristics on microwave-assisted pyrolysis - A review. Bioresour Technol, 230, 143-151.