Assessing the changes in thermal and catalytic pyrolysis of lignocellulose when shifting from batch to continuous biomass feeding modes

Abstract

Lignocellulosic biomass pyrolysis has been earlier studied in a large variety of reaction systems. However, there is a lack of information on the effects of passing from batch to continuous feeding mode (BM vs CM) despite being a necessary step in the scaling up of the process. In this sense, the main aim of this work is to conduct a comparative study of lignocellulosic biomass pyrolysis using the same reactor but varying the biomass feeding methods under thermal and catalytic conditions and different thermal zone temperatures (350 and 500 °C). Under thermal conditions, significant differences have been observed between both feeding systems. When operating in CM, the production of char decreases, particularly at 500 °C, from 27.3 to 18.9 wt % while the generation of gases increases. This can be attributed to the differences in the feeding mode since the CM allows a higher biomass heating rate, resulting in a more efficient lignocellulose decomposition. On the other hand, the balance between the higher residence time and the lower concentration of the vapours reduces the occurrence of secondary cracking reactions. Consequently, the share of water in the liquid fraction diminishes in CM, although only a notable increase of bio-oil* (bio-oil in water-free basis) is observed at 500 °C (39.9 vs 45.0 wt %). The bio-oil* molecular composition is also modified by this shifting, leading to higher sugars and oxygenated-aromatics in the CM reactor at 500 °C. The different operation modes also affect the interaction between the vapours and the catalyst when the n-ZSM-5 is introduced into the reactor. The vapours are more diluted in CM in comparison with the batch one, allowing a more progressive and efficient catalytic conversion. As a result, less coke is deposited over the zeolite catalyst leading to an enhanced production of aromatic hydrocarbons, achieving yields of ca. 7.6 wt %.