(495e) The Effects of Pretreatments in Improving the Quality of Bio-Oil Products from Non-Catalytic and Catalytic Pyrolysis of Lignocellulosic Biomass

Lignocellulosic biomass is a promising feedstock for the production of chemicals and fuels since it is available in vast amounts. One promising approach that has gained interest for converting lignocellulosic biomass in the past few years is fast pyrolysis. Fast pyrolysis is a thermochemical conversion process in which biomass is decomposed to produce a liquid (bio-oil), a solid (biochar), and a non-condensable gas mixture (syngas) by exposing biomass to high temperatures, typically between 400 and 600^oC, in the absence of oxygen. The liquid mixture of bio-oil is composed of wide variety of chemical compounds having different physicochemical properties. These chemical compounds include aromatics, carbohydrates, phenolic compounds and oxygenated compounds, such as aldehydes, ketones and organic acids. To be used as a feedstock for the production of liquid transportation fuels, bio-oil ideally must contain high amounts carbohydrates, aromatics and phenolic compounds and low presence of oxygenated compounds. High presence of oxygenated components in bio-oil is undesirable since it causes bio-oil to be chemically unstable, highly polar, acidic and low in heating value. The high polarity of bio-oil makes it immiscible with crude oil, making it difficult for using bio-oil as a co-feedstock in petroleum refineries. Furthermore, while many chemical constituents in bio-oil are valuable, their contents are low, making their recovery technically difficult and costly.

Bio-oil can be upgraded to make it more suitable for use as a liquid transportation fuel by using zeolites as a solid acid catalyst. Catalytic upgrading of biomass using H-ZSM-5 has been shown to produce a product mixture with lower oxygen content and higher amounts of single ring aromatics. Unfortunately, the increased aromatic content from catalytic pyrolysis lowers the overall liquid bio-oil yield due to higher production of non-condensable gases (primarily CO₂).

We have found one way to improve bio-oil quality while still increasing the yield of aromatics is to carry out acid hydrolysis pretreatment of the biomass. In the experiments, acid hydrolysis pretreatment steps were conducted by using phosphoric acid at concentrations of 0, 2, 4, 6.5, and 9 wt% in boiling water under reflux for one hour. Non-treated and some of the acid-treated biomass feedstocks were subjected to torrefaction process, in which biomass was heated under inert atmosphere at 220^oC for 30 minutes. All fast pyrolysis experiments were conducted by using a micropyrolyzer system which was connected to a GC/MS for analyzing the chemical composition of bio-oil products and a TCD gas analyzer for analyzing the composition of non-condensable gas products. To evaluate the effects of the type of feedstocks, pinewood and switchgrass were used to represent woody and grassy type lignocellulosic biomass, respectively.

The experimental studies indicated that acid hydrolysis pretreatment on both biomass feedstocks have a significant effect in increasing selectivity to aromatics and sugars when pyrolyzed. 2 wt% phosphoric acid was the most effective concentration for pretreating both biomass feedstocks. Surprisingly, the pretreatment improved aromatic yields using a standard non-catalytic pyrolysis of pinewood without the addition of the HZSM-5. Although still a minor component of the bio-oil, in the absence of HZSM-5, there was over five times the aromatics produced. A similar improvement was found for the switchgrass. The selectivity towards undesirable low molecular weight oxygenated compounds was decreased up to 60% compared to that of bio-oil produced from non-treated biomass. For catalytic pyrolysis, there was about a 3-fold increase in selectivity towards the desired aromatics and a 60% decrease in undesirable oxygenated compounds. Since catalytic pyrolysis with HZSM-5 is known to produce significant yields of single ring aromatics, the 3-fold improvement is noteworthy.

The increase of product selectivity towards sugars and aromatics on non-catalytic and catalytic pyrolysis of acid-treated biomass may be attributed to the breakdown of the lignin-hemicellulose bonds surrounding the cellulose structure making the cellulose more available for further conversions to sugars and aromatics. Acid hydrolysis also reduces the amount of inorganic compounds in the solid biomass, which lowers the degree of inorganic metal-catalyzed secondary reaction giving undesired low molecular weight oxygenated compounds.

Torrefaction treatment on acid-hydrolyzed biomass did not significantly improve the selectivity for aromatics or sugars. The treatment can be used as a biomass drying method which improves the overall process for producing bio-oil in an actual plant. Torrefaction on the acid hydrolyzed biomass produces a dry feedstock which can be easily stored and feed continuously to a process without the complications arising from having high water content in the feed or resulting bio-oil product. Overall, the combined pretreatment/torrefaction treatment improves both the processing and desired aromatics production from a plant carrying out catalytic pyrolysis.

*Contact: Dr. Justinus Satrio (justinus.satrio@villanova.edu)