Rapid pyrolysis of biomass generates a liquid with properties that are particularly attractive for production of hydrocarbons that could be substituted for liquid fuels derived from petroleum. However, the high oxygen content of the biomass derived liquids presents a number of problems because of the high water content and the considerable concentration of carboxylic acids. Measurements of total acid number (TAN) of pyrolysis oil (bio-oil) samples show that values in the 90-100 range are fairly common. This level of acidity has been shown to cause corrosion problems that have to be addressed in the selection of structural materials that are used in the production, subsequent processing, storage and transport of the pyrolysis oils. Chemical analyses have been performed and laboratory corrosion studies have been conducted in order to assess the aggressiveness of the raw pyrolysis oil from several sources as well as the corrosion caused by a bio-oil that has been treated to reduce the acid and oxygen content. Components of biomass pyrolyzers have also been fabricated from various candidate alloys, and these components have been exposed for extended periods during operation of the pyrolyzers. This paper will report on results of these analyses and corrosion studies.
Thermochemical processing of biomass can produce solid, liquid and/or gaseous products depending on the temperature and exposure time used for processing. The liquid product, known as pyrolysis oil or bio-oil, offers the potential of a replacement for imported petroleum, but it also introduces other problems. As produced pyrolysis oil contains a significant amount of oxygen, primarily as components of water, carboxylic acids, phenols, ketones and aldehydes. As a result of these constituents, these oils are generally quite acidic with a Total Acid Number (TAN) that can reach levels as high as 100.