Effects of various reactive gas atmospheres on the properties of bio-oil using microwave pyrolysis

Authors: Tarves, Paul; Mullen, Charles; Boateng, Akwasi

Submitted to: ACS Sustainable Chemistry & Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/6/2016
Publication Date: 3/1/2016
Publication URL: https://handle.nal.usda.gov/10113/62085
Citation: Tarves, P.C., Mullen, C.A., Boateng, A.A. 2016. Effects of various reactive gas atmospheres on the properties of bio-oil using microwave pyrolysis. ACS Sustainable Chemistry & Engineering. 4:930-936.

Interpretive Summary: In order to curb greenhouse gas emissions and promote U.S. energy independence, renewable energy sources must be developed to extend or replace nonrenewable fossil fuels. A major potential source of renewable energy is biomass, i.e. plant and animal matter 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 patent pending process developed by the USDA-ARS, utilizes gases formed during pyrolysis for the production of higher quality bio-oil that can be more easily refined into biofuels or chemical products. To further understand the influence of the pyrolysis gas in the TGRP process, we have employed a laboratory scale microwave reactor and performed pyrolysis of switchgrass under various gas atmospheres and characterized the bio-oils obtained. The product yields were measured and the bio-oils were characterized by a variety of analytical methods. Pyrolysis experiments performed under a nitrogen atmosphere were used as the control and then compared to experiments performed under various reactive gases (carbon monoxide, hydrogen, and methane) and a model pyrolysis gas mixture (“PyGas”). The use of hydrogen, methane, and PyGas atmospheres each provided higher quality bio-oils with lower oxygen content. The use of different particle sizes also displayed a pronounced effect on the product distribution and the quality of the bio-oils obtained. The results obtained will aid in the further development and scaled systems for the TGRP process and will be useful to those designing pyrolysis based biorefinery systems.

Technical Abstract: Fast pyrolysis of lignocellulosic biomass produces organic liquids (bio-oil), bio-char, water, and non-condensable gases. The non-condensable gas component typically contains syngas (H2, CO and CO2) as well as small hydrocarbons (CH4, C2H6, and C3H8). Tail Gas Reactive Pyrolysis (TGRP), a patent pending process developed by USDA-ARS, utilizes this tail gas under carefully tuned conditions to enable production of deoxygenated liquid products, particularly aromatic hydrocarbons. To further understand the influence of reactive gas in various pyrolysis processes, we have employed a laboratory scale microwave reactor and performed pyrolysis of switchgrass under varying gaseous atmospheres and characterized the bio-oils obtained. The batch (100 grams of biomass) microwave pyrolysis was performed at 900-1000 watts over the course of seven minutes in the presence of a microwave absorber (10 grams of activated charcoal). The products formed were quantified and the bio-oils were characterized by GC-MS, elemental analysis, Karl Fischer and TAN titrations, bomb calorimetry, and 13C NMR spectroscopy. Pyrolysis experiments performed under a N2 atmosphere were used as the control and then compared to experiments performed under various reactive gases (CO, H2, and CH4) and a model pyrolysis gas mixture (“PyGas”). The use of a CO atmosphere had a negligible effect on the quantity and quality of bio-oils produced, whereas the use of H2, CH4, and PyGas atmospheres each provided more deoxygenated products (i.e. BTEX, naphthalenes, etc.) and lower oxygen content. The use of different particle sizes also displayed a pronounced effect on the product distribution and the composition of the bio-oils obtained. The results obtained will aid in the further development and scaled systems for the TGRP process.