(634c) Ex-Situ Catalytic Fast Pyrolysis of Biomass over HZSM-5 in a Two-Stage Fluidized-Bed/Fixed-Bed Combination Reactor

Catalytic fast pyrolysis (CFP) has been developed as one of the most promising technologies to convert biomass into high quality fuels and chemicals. Depending on the different contacting types between pyrolysis vapors and catalyst, CFP of biomass can be categorized into in-situ and ex-situ CFP, as shown in Fig. 1. In recent years, numerous studies have been conducted over various aspects of in-situ CFP, while limited research on ex-situ CFP of biomass over HZSM-5 has been reported. In order to get a deeper understand about the reaction mechanisms of CFP and the potential of ex-situ CFP in the generation of olefins and aromatics, a systematic study on ex-situ CFP was performed in a two-stage fluidized-bed/fixed-bed combination reactor. Ex-situ and in-situ CFP of biomass over HZSM-5 were compared first. Both these two types of CFP gained similar carbon yield of aromatics + olefins (around 20%) , while ex-situ CFP produced much more olefins than in-situ CFP (10.2% vs. 5.7%). Meantime, less char + coke and C₉₊ aromatics were generated in ex-situ CFP. These results can be explained by the thermal cracking of produced unstable organic vapors before catalysis in ex-situ CFP process and the different characteristics of heat and mass transfer and hydrodynamics of the pyrolysis reactor in ex-situ and in-situ cases. The effects of catalyst loading, carrier gas flow rate, pyrolysis temperature and catalysis temperature on ex-situ CFP were further studied. Pyrolysis vapors were deoxygenated inadequately with insufficient catalyst, while excessive catalyst led to an undesired cracking of target products. Flow rate of carrier gas did not significantly influence the product distribution, in spite that an increase of aromatics + olefins was observed at higher carrier gas flow rate. Pyrolysis temperature and catalysis temperature are the most important parameters for ex-situ CFP. Both pyrolysis temperature and catalysis temperature were proved to have an optimal value for the generation of target products, which were 550 °C and 600 °C, respectively. A moderate increase in pyrolysis temperature and catalysis temperature would promote the formation of light oxygenates which can diffuse into the pores of HZSM-5 catalyst to be convert to aromatics and olefins. However, higher temperature would also enhance the cracking of organic vapors into permanent gases. Increased pyrolysis temperature was effective to inhibit the formation of char and coke. In addition, olefins and aromatics selectivities were highly influenced by catalysis temperature. When catalysis temperature reached to 700 °C, the carbon selectivity of ethylene and benzene boosted to 80.8% and 59.8%, respectively.

Fig. 1. The schematic of the continuous feeding two-stage fluidized-bed/fixed-bed combination reactor for in-situ and ex-situ CFP of biomass.