Mild hydrotreating of bio-oils with varying oxygen content produced via catalytic fast pyrolysis

Authors: Mullen, Charles; Boateng, Akwasi

Submitted to: Fuel
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/6/2019
Publication Date: 6/1/2019
Citation: Mullen, C.A., Boateng, A.A. 2019. Mild hydrotreating of bio-oils with varying oxygen content produced via catalytic fast pyrolysis. Fuel. 245:360-367. https://doi.org/10.1016/j.fuel.2019.02.027.
DOI: https://doi.org/10.1016/j.fuel.2019.02.027

Interpretive Summary: Biomass, such as crop residues, herbaceous grasses and woody materials, are the largest cellulosic based source available and can be readily converted to liquids, known as bio-oil, by rapid heating in the absence of air, otherwise known as fast pyrolysis. Bio-oil is considered a potential intermediate to hydrocarbon drop-in bio-fuels that could be produced at petroleum refineries, but bio-oils are not yet completely compatible with crude oils or refinery infrastructure. This is mostly due to the high concentration of reactive oxygenated compounds found in the bio-oil that make it thermally unstable. One method for reducing the amount of these compounds in the bio-oil is to perform the pyrolysis process in the presence of a material, called a catalyst, which alters the chemical reactions occurring to remove oxygen as carbon oxide gases and/or water, leaving behind hydrocarbons comparable to those found in crude oil. This process is called catalytic fast pyrolysis (CFP). The more active the catalyst, the more oxygen that is removed from the bio-oil, but also in lower yield. The next step in refining bio-oils is often a hydrogenation step, this stabilizes the bio-oil for further refining under higher temperature and pressure conditions to produce renewable liquid fuels. In this work we compared how different bio-oils produced by CFP, at different levels of catalyst activity performed in hydrogenation. It was found that bio-oils produced at a mid-level of catalyst activity from CFP, those with about 25 wt% oxygen, were better hydrogenated than those produced with a higher activity catalyst (had less oxygen content) and those with a less active catalyst (higher oxygen content). Use of mid-level deoxygenation during CFP also produced a 2-3 fold yield increase in stabilized (hydrogenated) bio-oil compared with those more severely deoxygenated during CFP. This information will be important for anyone designing catalytic fast pyrolysis based biorefineries.

Technical Abstract: Several biomass catalytic fast pyrolysis oils produced over HZSM-5 at varying levels of catalyst of deactivation, and therefore exhibiting oxygen contents ranging from 6 wt% to 33 wt% were subjected to mild hydrotreating over Ru/C at 140 degrees C. The purpose of this study was to evaluate the performance of these catalytic pyrolysis oils in this process, which is often used as a stabilization step prior to exposure of the bio-oil to more severe hydrodeoxygenation conditions, at higher temperatures and pressures, in pyrolysis based biorefinery concepts. High liquid product recoveries were achieved for the mild hydrotreating in all cases, meaning the yield of hydrogenated liquids from biomass was nearly exclusively dependent on the catalytic pyrolysis step. However, a trend was observed that the bio-oils with increasing oxygen content were more responsive to the hydrogenation. For example, no aromatic ring hydrogenation was observed for the bio-oils with <20 wt% oxygen content but ring saturation was observed for bio-oils with oxygen contents of 25-30 wt%. The bio-oils with about 25%-30% oxygen content had the largest increase in H/C ratio upon hydrogenation. This trend was attributed in part to a solvent effect, where the more polar bio-oils acting as their own solvents, helped promote hydrogenation. An increase in the average molecular weight of the bio-oils with >25 wt% oxygen content was observed for exposure to the conditions, suggesting some oligomerization occurred prior to hydrogenation in these cases. In summary, bio-oils with about 25 wt% oxygen content responded well to the mild hydrotreatment, were stable to the conditions, and offered a 2-3 fold increase in carbon yield from biomass compared to the bio-oils that were more severely deoxygenated during catalytic pyrolysis.