Pilot-scale fluidized-bed pyrolysis of switchgrass for bio oil production

Authors: Boateng, Akwasi; DAUGAARD, DAREN - UNIV OF TEXAS,SAN ANTONIO; Goldberg, Neil; Hicks, Kevin

Submitted to: Industrial and Engineering Chemistry Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2007
Publication Date: 4/3/2007
Citation: Boateng, A.A., Daugaard, D.E., Goldberg, N.M., Hicks, K.B. 2007. Bench-scale fluidized-bed pyrolysis of switchgrass for bio oil production. Industrial and Engineering Chemistry Research 46, p.1891-1897.

Interpretive Summary: One major challenge in using energy crops such as switchgrass as a feedstock for fuel ethanol production is their low density. Bales of switchgrass are light and very bulky and require too much space on trucks or rail to make transportation to a central processing facility economically feasible. Other challenges are difficulties associated with breaking down the complex carbohydrates in switchgrass to make simple sugars that can be converted into ethanol through the process of fermentation. These difficulties result in the current estimate that ethanol from switchgrass costs about twice as much as ethanol from grain crops, such as corn. But there are other methods that have the potential to mitigate these challenges. One of them is by heating the biomass in an absence of oxygen to produce a liquid intermediate called pyrolysis oil or bio-oil, which can be easily transshipped to central refineries and upgraded to fuels and chemicals. We have now built a unique pilot-scale reactor that uses a hot sand medium (called a fluidized-bed reactor) to convert perennial grasses to bio-oil and have now tested the reactor on switchgrass. The reactor was able to use switchgrass as a feedstock and produce a quantity of bio-oil that was 60% of the weight of the switchgrass fed into the reactor. We tested the composition and fuel properties of the produced liquid and found that the energy content was about the same as the parent switchgrass but the density was more than 2.5 X greater. The operational data collected and the test results obtained can be used to design similar tests for other grasses in the ARS energy crop program and for the design and scale-up of reactors for larger operations. The study provides useful information for companies interested in building small scale distributed pyrolysers that could be used by farmers "on the farm" to produce a pyrolytic oil. Farmers could then sell the product as a "crude oil" to oil refiners, who in turn, would convert it into transportation or heating fuels. If successful, this would lead to increased opportunities for farmers who could put marginal lands into switchgrass production and help drive a renewable fuels economy in rural America.

Technical Abstract: The US biomass initiative is counting on lignocellulosic conversion to boost the quantities of biofuels currently produced from starches in order to achieve much needed energy security in the future. However, with current challenges in fermentation of lignocellulosic material to ethanol, other methods of converting biomass to useable energy have received consideration nationally. One thermochemical technique, fast pyrolysis, is being considered by the Agricultural Research Service (ARS) researchers of the USDA for processing energy crops such as switchgrass and other agricultural residues e.g., barley hulls and alfalfa stems for bio-oil (pyrolysis oil) production. A 2.5 kg/hr biomass fast pyrolyzer has been developed at ARS and tested for switchgrass conversion. The unit has provided useful data such as energy requirements and product yields that can be used as design parameters for larger systems based on the processing of perennial energy crops. Bio-oil yields greater than 60% by mass have been demonstrated for switchgrass, with energy conversion efficiencies ranging from 52 to 81%. The results show that char yielded would suffice in providing all the energy required for the endothermic pyrolysis reaction process. The composition of the non-condensable gas produced has been initially characterized. Initial mass and energy balances have been calculated based on this system yielding useful parameters for future economic and design studies.