Production of bio-oil from alfalfa stems by fluidized-bed fast pyrolysis

Authors: Boateng, Akwasi; Mullen, Charles; Goldberg, Neil; Hicks, Kevin; Jung, Hans Joachim; Lamb, Joann

Submitted to: Industrial and Engineering Chemistry Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/25/2008
Publication Date: 6/6/2008
Citation: Boateng, A.A., Mullen, C.A., Goldberg, N.M., Hicks, K.B., Jung, H.G., Lamb, J.F. 2008. Production of bio-oil from alfalfa stems by fluidized-bed fast pyrolysis. Industrial and Engineering Chemistry Research. 47:4115-4122.

Interpretive Summary: "Thermochemical" (high temperature) conversion of biomass for production of renewable fuels offers an alternative to the biochemical (fermentative) conversion of biomass to fuel ethanol and the potential problems associated with it, such as the inability of brewer's yeast to ferment many of the sugars in biomass to ethanol. The themochemical conversion process involves heating the biomass in the absence of air (pyrolysis) to convert low density biomass into high density liquids called bio-oil. Bio-oil, similar to "crude oil", can be further processed into renewable transportation fuels or chemicals at petroleum refineries. Because it is denser than biomass, transportation of bio-oil to a central refinery location is more economically feasible that trying to ship bulky biomass to a distant refinery. Alfalfa stems are being considered as a potential biomass source for thermochemical conversion in part because they do not interfere in the food/feed supply since they are left over after alfalfa leaves are used for animal feed. We studied the production of bio-oil from alfalfa stems using a pilot-scale thermal reactor. Approximately 50% of the weight of the alfalfa stems was recovered in a high density bio-oil that has energy content 65% that of conventional diesel fuel oil. Additionally, a solid charcoal product was obtained that can be used as a renewable solid fuel or as a soil additive. Both the oil and the char are potential high-valued products that could be produced at small processing facilities distributed throughout rural areas providing additional farm income. The operational data collected and the test results obtained can be used to design similar tests for other biomass sources and for the design and scale-up of reactors for larger operation. The information will be useful for companies interested in building small scale distributed (near the farm) pyrolysis systems, energy crop breeders, extension agents who advise farmer groups, and ultimately those who are considering use of energy crops as feedstock for liquid fuels using the thermal conversion route.

Technical Abstract: This study focused on the production of bio-oil from alfalfa stem material. Two alfalfa maturity stages, harvested at early bud and full flower stages of development, were examined to evaluate the impact of variation in cell wall polysaccharide and lignin content on pyrolysis oil yields, production efficiency, bio-oil and char quality in terms of their use as combustion fuel and for chemicals. Findings included a lower than average yield of bio-oil and a higher than average yield of charcoal from alfalfa stems compared to previous results for other biomass feedstocks. The bio-oil showed a decrease in oxygen content from the alfalfa stems leading to a higher than average energy content in the bio-oil. Bio-oil yields were slightly higher for the more mature alfalfa, which had higher levels of cell wall cellulose and lignin. Overall, when all the pyrolysis products were considered, energy recovery was better for the more mature alfalfa stems