Submitted to: Energy and Fuels
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 3, 2008
Publication Date: June 6, 2008
Citation: Mullen, C.A., Boateng, A.A. 2008. Chemical composition of bio-oils produced by fast pyrolysis of two energy crops. Energy and Fuels. 22:2104-2109. Interpretive Summary: The two technologies used for the conversion of biomass into liquid fuels are called the "biochemical" and the "thermochemical" processes. Biochemical conversion involves fermentation of biomass feedstock by use of a biological organism (brewers yeast) into fuel 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 can be further processed into transportation fuels or chemicals. However, bio-oil is a complex mixture of chemical compounds that vary with biomass feedstock. Therefore, the inter-relation between the bio-oil composition and the identity of biomass is important for the evaluation of the utility of various energy crops available for thermochemical conversion. We examined the composition of bio-oils produced from two biomass materials considered potential energy crops, switchgrass and alfalfa, and found that they were different. Alfalfa bio-oil had more compounds associated with nitrogen than bio-oil from switchgrass which relates to the protein-rich feedstock. Hence, bio-oil produced from alfalfa is more likely to produce nitrous oxide emissions if burned “as is” than switchgrass bio-oil. Also, alfalfa bio-oil had less sugar-related compounds than that from switchgrass. The information will be useful to energy crop breeders, potential producers and refiners of bio-oil, extension agents who advise farmer groups, and ultimately the crop producers who are considering use of these two energy crops as feedstock for liquid fuels using the thermal conversion route.
Technical Abstract: Fast pyrolysis of switchgrass and alfalfa stems (from two stages of development: bud and full flower stages) has been carried out in a 2.5 kg/hr fluidized bed reactor. Pyrolysis experiments were conducted at 500 deg C under a nitrogen atmosphere. The liquid product, bio-oil (pyrolysis oil or pyrolysis liquid) was recovered (after removal of char via cyclones) in a series of 4 canisters cooled in water/dry ice bath followed by an electrostatic precipitator. The bio-oils were analyzed via wet-chemical methods, GC-MS and HPLC. A total of 62 chemical species were positively identified in the liquids. Of the identified compounds 27 have been quantified. The chemical compositions of the bio-oils are compared with what is known about the compositions of the feedstocks. Noted is the presence of more nitrogen containing compounds in bio-oils produced from alfalfa stems than from switchgrass. Also noted is a marked decrease in levoglucosan and hydroxyacetaldehyde concentrations among bio-oils from alfalfa stems compared with bio-oil from switchgrass. Compared to composition of bio-oils from woody feedstocks previously characterized by others, bio-oils from these herbaceous species had similar concentrations of most major components.