Production of partially deoxygenated pyrolysis oil from switchgrass via Ca(OH)2, CaO and Ca(COOH)2 co-feeding

Authors: RAYMUNDO, LUCAS - Rio Grande Do Sul State Department Of Agricultural Research; Mullen, Charles; Boateng, Akwasi; DESISTO, WILLIAM - University Of Maine; TRIERWEILER, JORGE - Federal University Of Rio Grande Do Sul

Submitted to: Energy and Fuels
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/30/2020
Publication Date: 8/31/2020
Citation: Raymundo, L.M., Mullen, C.A., Boateng, A.A., Desisto, W.J., Trierweiler, J.O. 2020. Production of partially deoxygenated pyrolysis oil from switchgrass via Ca(OH)2, CaO and Ca(COOH)2 co-feeding. Energy and Fuels. 34:12616-12625. https://doi.org/10.1021/acs.energyfuels.0c01784.
DOI: https://doi.org/10.1021/acs.energyfuels.0c01784?ref=pdf

Interpretive Summary: Renewable sources of liquid fuels and chemicals must be developed to extend or replace those produced from fossil fuels. The major potential source of renewable carbon for these applications is lignocellulosic biomass. This includes materials such as crop residues, forest thinnings, wood waste, animal wastes and purpose grown energy crops. 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, called bio-oil, is of low value as a feedstock for producing fuels and chemicals due to its high acidity and oxygen content. Therefore, there is a need to produce a higher quality bio-oil, with lower acidity and oxygen content. One possible method for doing this is to co-process the biomass with other materials that effect the chemical reactions that occur and therefore the properties of the bio-oil. Some such materials are calcium compounds, which are derived from limestone and are quite abundant. In this study, switchgrass was combined with calcium hydroxide, calcium oxide and calcium formate and pyrolyzed. The bio-oil produced had lower oxygen content, by about 45% in the best case. Conditions for this reaction were optimized for yield and quality of the bio-oil. This information will be of interest to those involved with developing biorefineires to convert renewable resources to fuels and chemicals.

Technical Abstract: Fast pyrolysis of switchgrass was performed in a fluidized bed reactor by feeding pre-made mixtures of the biomass with calcium hydroxide, calcium oxide, and calcium formate with the goal of producing deoxygenated bio-oil. Initial exploratory tests were performed by co-feeding switchgrass with Ca(OH)2 at ratios of 0.4/1 and 0.8/1 Ca(OH)2/biomass, running at reactor temperatures of 500 degree C, 550 degree C, 600 degree C, and using nitrogen or recycled pyrolysis gas as the carrier gas. Co-feeding Ca(OH)2 results in the production of bio-oils with reduced oxygen content and a lower concentration of acetic acid and levoglucosan in comparison with the biomass-only control experiments. The yield of bio-oil and water-soluble organic compounds decreased as well, while increasing yields of pyrolysis gases such as H2, CH4, CO were observed. This shift in yields was more pronounced for the higher Ca lodging or higher temperatures. Furthermore, CaCO3 is formed as a co-product, sequestering CO2, which appeared to promote additional deoxygenation via CO2. Replacing N2 with recycled reaction gases led to increased bio-oil yields for temperatures of 500 degree C and 550 degree C, but lower yield at 600 degree C. Based on these initial experiments the conditions of 550 degree C under recycled gas were chosen to screen other calcium salts, as a balance between deoxygenation and yield was accomplished at these conditions. Dry basis bio-oil yields were 7.0%, 11.5% and 9.9% from biomass for Ca(OH)2, CaO, and Ca(COOH)2, respectively, with oxygen contents of 21%, 20% and 19%. The formation of CaCO3 followed the order Ca(OH)2> Ca(COOH)2>CaO, showing that bio-oil deoxygenation might not be only related to CO2 sorption as CaCO3, but also to the catalytic activity of the Ca compounds.