Deoxygenation of biomass pyrolysis vapors via in situ and ex situ thermal and biochar promoted upgrading

Authors: RAYMUNDO, LUCAS - Federal University Of Rio Grande Do Sul; Mullen, Charles; Strahan, Gary; Boateng, Akwasi; TRIERWELLER, JORGE - Federal University Of Rio Grande Do Sul

Submitted to: Energy and Fuels
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/2019
Publication Date: 2/19/2019
Citation: Raymundo, L.M., Mullen, C.A., Strahan, G.D., Boateng, A.A., Trierweller, J.O. 2019. Deoxygenation of biomass pyrolysis vapors via in situ and ex situ thermal and biochar promoted upgrading. Energy and Fuels. 33:2197-2207. https://doi.org/10.1021/acs.energyfuels.8b03281.
DOI: https://doi.org/10.1021/acs.energyfuels.8b03281

Interpretive Summary: Renewable sources of liquid fuels must be developed to extend or replace nonrenewable fossil fuels. A major potential source of renewable energy is biomass, such as crop residues, woody materials, and animal wastes. Biomass can be converted into a liquid (bio-oil) via pyrolysis, in which the biomass undergoes rapid heating in an oxygen-free environment. However, the liquid produced is of low value as a feedstock for producing fuels due to its high acidity and oxygen content. Tail Gas Reactive Pyrolysis (TGRP), a patented process developed by the USDA-ARS, utilizes gases formed during pyrolysis for the production of higher quality bio-oil that has lower oxygen content, without the addition of external catalysts or reagents. This higher quality bio-oil can be more easily refined to transportation fuels and/or renewable chemicals. To better understand the effects of various process variables during TGRP, a laboratory scale reactor was developed that could perform the process while systematic changes were made to temperatures, reaction times and the possible reaction with of bio-char, a co-product of the process itself. Temperatures at various points in the system indicated that high temperature excursions above the 500 degrees C normally used for pyrolysis (up to 750 degrees C) could be partially responsible for the initial observations made that TGRP can deoxygenate bio-oils. Furthermore, the addition of pre-made bio-char in a secondary reactor system revealed that the same deoxygenation effect that only occurs at 750 degrees C without bio-char addition can be observed at 600 degrees C in the presence of bio-char. This indicates that accumulation of bio-char within the heated zones of the reactor may also play a major role in deoxygenation during TGRP. This information will be valuable to those considering the development of pyrolysis based biorefineries.

Technical Abstract: Our group has previously reported that deoxygenation of pyrolysis vapors occurs in an atmosphere partially consisting of recycled tail gas. This process, called tail gas reactive pyrolysis (TGRP), was further studied in a new laboratory scale pyrolysis system to better understand the factors affecting vapor deoxygenation. Temperature excursions from the fast pyrolysis temperatures of about 500 degrees C and/or the catalyzing effect of accumulated bio-char were hypothesized to be potential key parameters for deoxygenation during TGRP. Therefore, experiments were executed with process temperatures in the 500 - 750 degrees C range in both the fluidized bed pyrolysis reactor (in-situ) and in a secondary chamber (ex-situ), post removal of bio-char. Based on this arrangement, the oxygen content of bio-oils produced varied with temperature changes as follows: 31wt% -> 30wt% -> 19wt% for in-situ temperatures at 500 degrees C, 600 degrees C, 700 degrees C , and 31wt% -> 27wt% -> 23wt% -> 19wt% for ex-situ temperatures of 500 degrees C, 600 degrees C, 700 degrees C and 750 degrees C, respectively. The discrepancy between the results of the in-situ and ex-situ cases at 700 degrees C, could suggest an effect of bio-char present in the fluidized bed reactor. To further analyze the effect of bio-char accumulations as a contributing factor in deoxygenation via TRGP, experiments with bio-char pre-loaded in the ex-situ chamber were performed at 500 degrees C and 600 degrees C. At 600 degrees C, the oxygen content of bio-oil produced was as low as 19 wt%, similar to that obtained with only quartz beads in the ex-situ chamber at 750 degrees C. This suggests that bio-char could have a catalytic deoxygenation effect during TGRP particularly in a fixed or plugged bed, motivating further exploration of biochar as a catalyst for bio-oil deoxygenation.