The U.S. renewable fuels standard (RFS2) requires an increase in the domestic supply of alternative fuels to 36 billion gallons by 2022. This consists of 15 billion gallons from corn-based ethanol and 21 billion gallons of advanced biofuels from lignocellulose biomass. Over the past two decades much research has been undertaken to find suitable upgrading processes to render bio-oil produced from fast pyrolysis of biomass substitutable for petroleum products. Although bio-oil may look promising, its oxygenated structure makes it unsuitable for use as a fuel; hence it needs to be deoxygenated. Presently the most traditional route for deoxygenation of bio-oil is catalytic hydro-deoxygenation (HDO), which uses hydrogen at high pressure and temperature in the presence of a catalyst to remove oxygen. One of the limitations of the HDO process is that it requires a significant amount of hydrogen, which is predominantly made from the steam reforming of natural gas or other hydrocarbons (fossil resources).
To address cost and environmental performance of bio-oil deoxygenation, Ceramatec, a research and manufacturing facility in collaboration with Pacific Northwest National Laboratory (PNNL) is developing an electrochemical cell to deoxygenate bio-oil compounds, a process known as electrochemical deoxygenation (EDOx) (Elangovan et al, 2015). The EDOx process applies an electrical potential across a gas tight, electrically insulating ceramic membrane, having a high conductivity for oxygen ions. In Elangovan et al (2015), separate deoxygenation tests were done with aqueous phase bio-oil obtained from PNNL along with bio-oil model compounds syringol and guaiacol using two types of EDOx configurations, a button and stack cell. The feed and product streams were analyzed with a gas chromatograph to compare the percent oxygen concentration in both streams and determine the oxygen weight percent reduction in the liquid stream.
A life cycle assessment (LCA) was conducted to compare the state-of-the-art HDO process described by Zacher et al. (2014) and Jones at al. (2013) to the proposed EDOx process. An LCA study that was carried out by Hsu, 2012 based its analysis on the PNNL report with its focus on greenhouse gas intensity of gasoline and diesel produced via fast pyrolysis. On the other hand, this present study evaluates the energy consumption and environmental impacts associated with the EDOx and HDO processes. A functional unit of 1tonne of deoxygenated bio-oil produced was used to provide a reference for the comparison of the two processes. Although, the deoxygenation of the bio-oil with HDO and EDOx removes 25% of the oxygen, this study further optimizes the process to 60% oxygen removal and an average energy consumption of 0.18 joules. There are limitations to comparing the not fully oxygen-free EDOx product to the fully deoxygenated diesel product from HDO. The system boundaries for the HDO process include the energy balance of the condenser from the fast pyrolysis unit, construction and operation of reactors, compressors, a pre-heater, and the equipment required for hydrogen formation. The boundaries for the EDOx process include the production and operation of the electrochemical cell, integrated in the pyrolysis unit immediately after the cyclone and prior to the condenser, and the energy balance of the condenser.
Major differences in view of the impact assessment are the unit operations associated with the pressure vessels and pre-heater required for the HDO process. These are not needed in the EDOx process due to its integration in the pyrolysis unit. Furthermore, apart from electrolysis of water from the bio-oil, in the EDOx process reactive hydrogen is also generated in the deoxygenation reactions themselves, whereas in the HDO process hydrogen production has to be produced externally via steam reforming of natural gas and hence is included in the life cycle inventory (LCI). A sensitivity analysis reveals how much this affects the process both economically and environmentally. In view of the economic outlook, in the EDOx process, pure oxygen gas is directly formed as a co-product, adding economic value to the process. Results from this study, can be used to optimize the deoxygenation of bio-oil and potentially open avenues for commercialization of bio-oil upgrading into transportation fuels.
Elangovan, S., Larsen, D., Hartvigsen, J., Mosby, J., Staley, J., Elwell, J., Karanjikar, M. Electrochemical upgrading of bio-oil. Accepted for publication in ECS Transactions, 2015
HSU, D.D. Life cycle assessment of gasoline and diesel produced via fast pyrolysis and hydroprocessing. Biomass and Bioenergy 2012; 45:41-47
Jones SB, Meyer P, Snowden-Swan L, Padmaperuma A, Tan E, Dutta A, Jacobson J, Cafferty K. Process Design and Economics for the conversion of lignocellulosic biomass to hydrocarbon fuels. Richland WA: Pacific Northwest National Laboratory; 2013. Nov. 97 pp. Report No.: PNNL-23053
Schnepf, R., Yacobucci, B.D. Renewable Fuels Standard (RFS): Overview and Issues. Congressional Research Service; 2013. March. 35 pp. Report No: R40155
Zacher, A.H., Olarte, M.V., Santosa, D.M., Elliott, D.C., Jones, S.B. Green Chem. 2014; 16: 491-515