Submitted to: Fuel
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/25/2009
Publication Date: 6/12/2009
Citation: Boateng, A.A., Mullen, C.A., Goldberg, N.M., Hicks, K.B., Mcmahan, C.M., Whalen, M.C., Cornish, K. 2009. Energy-dense liquid fuel intermediates by pyrolysis of guayule (Parthenium argentatum) shrub and bagasse. Fuel. 88:2207-2215. Interpretive Summary: Guayule is a perennial shrub grown in the southwestern United States that produces natural latex. The United States is dependant upon imports of Hevea latex and rubber for critical industrial needs. To assure uninterrupted supply, and to provide a material safe for people with Type I latex allergy, guayule latex has recently been commercialized in the United States. However, only about 10% of the guayule plant is used for rubber production, the remainder (bagasse) is a coproduct; economic use of the material would positively impact sustainability of the US guayule latex industry. With the current need and interest for alternate fuel resources, the bagasse is an appealing feedstock for bioenergy production. Prior efforts to produce bioenergy from guayule by fermentation or gasification have been met with limited success due to low yields or high production costs. An alternate method for guayule conversion is fast pyrolysis, a conversion method involving heating the material in the absence of air, capable of converting low density biomass to high density liquids called bio-oil. Bio-oil can be further refined into transportation fuels. Fast pyrolysis units can be amenable to a smaller scale than can other conversion technologies and therefore potentially located on the same site with the guayule latex production facility. We produced bio-oil from the unprocessed guayule plant and the waste bagasse and obtained approximately 60% of the weight of the guayule feedstock. The liquid had an energy content equivalent to 75% of the energy contained in conventional diesel fuel. Additionally, a solid charcoal product was obtained that can be used as a soil additive or as a renewable solid fuel. Both the oil and the charcoal are potential high-valued products that could be produced from the guayule bagasse. This information will be useful to those in the latex producing industry looking to add value to by-products of guayule latex production, and to others considering production of liquid fuels via small scale themo-chemical conversion.
Technical Abstract: Guayule is a perennial shrub grown in the southwestern United States that is used to produce high quality, natural rubber latex. However, only about 10% of the plant material is used for latex production; the remaining biomass, called bagasse, can be used for renewable fuel production. Fast pyrolysis of guayule, both whole shrub and bagasse was performed. From both feedstocks a very viscous, high energy content (~30 MJ/kg) pyrolysis liquid (bio-oil) was produced in yields averaging over 60% without any catalyst. The properties and compositions of the bio-oils were found to be similar in the two feedstocks. Coproducts, charcoal (20-30 wt%) and non-condensable gas (5-15%), were also dense and had a high energy content. Of the two feedstocks, the whole shrub yielded higher quantities of charcoal that also had higher energy content than the charcoal produced from bagasse. As a result, the energy recovery, estimated as the percentage of the energy products, to energy input into the reactor was lower (60%) for guayule bagasse than for the whole shrub (73%). This notwithstanding, the bagasse is a more attractive feedstock for thermochemical conversion, not only because it is a waste residue from a primary process (latex extraction) that is on-site, but also because it has a high energy content. Moreover, it produces high quality pyrolysis products. Co-production of latex rubber from the whole shrub and renewable fuels from the residual bagasse by pyrolysis should improve the already positive economics of the guayule latex rubber industry.