Separation processes to improve utilization of bio-oil from switchgrass pyrolysis

Abstract

Bio-oil produced from biomass pyrolysis, a thermochemical decomposition process, has potential as a biorenewable energy source. Its challenging properties, including high acidity and high moisture content, however, hinder its applications. The main objective of this study is to separate components of switchgrass pyrolysis bio-oil through various processes in order to improve its utilization. The research targets the acidity of bio-oil and explores separation processes such as solvent extraction, water addition, pH neutralization, and capacitive deionization (CDI). A standard total acid number (TAN) analysis was employed to better understand the acidity of aqueous bio-oil. Aqueous and organic components of switchgrass bio-oil were separated via solvent extraction, water addition, and pH neutralization in order to produce an aqueous bio-oil phase that has high concentrations of organic acids and low concentrations of heavy organic compounds. Optimal phase separation occurred after the pH of bio-oil was raised to 6. The aqueous bio-oil phase is suitable for microbial electrolysis to produce hydrogen as an energy source; hydrogen is also needed to further upgrade bio-oil via hydrodeoxygenation. TAN analysis of aqueous bio-oil revealed that some organic acids (e.g., vanillic acid) that act as polyprotic acids have a stronger influence on the acidity of bio-oil than acids (e.g., acetic acid) that act as monoprotic acids. Process-intensification devices, including a static mixer and a centrifugal contactor, were employed as continuous-flow pH-neutralization reactors. These continuous-flow systems were found to be effective, and the results were comparable to those from batch systems with long reaction times demonstrating a potential to scale up pH neutralization of bio-oil. In addition, the study involves removal of acidic components via water extraction to reduce the acidity of the organic bio-oil phase, which can be used to produce fuel or other products such as resin. Acids removed by water were recovered through CDI for use in microbial electrolysis. This research is an intermediate step between biomass pyrolysis for bio-oil production and microbial electrolysis for hydrogen generation and further upgrading of bio-oil. It is a necessary step toward closing the carbon cycle of the overall bio-oil production and upgrading processes, so that bio-oil can become a carbon-neutral energy source.