The Denser the Better
Researchers in three states are making headway in developing pretreatment and densification methods to better process cellulosic materials for biofuels production. Funded largely by the North Central Sun Grant Center, the $1.1 million research project is scheduled to be completed next June.
“The overarching goal of this project is to develop and validate the performance of an integrated biomass pretreatment and densification process that will reduce the logistical hurdles facing second-generation biofuels,” says Kasiviswanathan Muthukumarappan, a professor at South Dakota State University’s agricultural and biosystems engineering department. He and other researchers believe logistical issues could be alleviated. “We think it’s better to think in terms of what we call ‘regional biomass processing depots,’” says Bruce Dale, a chemical engineering professor at Michigan State University and collaborator. “The idea is to pretreat the biomass at these regional centers and densify it there so that you can preserve and make the biomass more uniform as close as you can to the field. This gives the opportunity for local interests, farmers or co-ops, to own one of these regional depots and thereby participate more in the value chain.”
In the project, SDSU researchers gather samples of switchgrass, cordgrass and corn stover and mill them to specified sizes. The samples are then shipped to Dale at MSU, where they are subjected to the university’s ammonia fiber expansion pretreatment process (AFEX). The treated samples are then sent west again, this time to Fargo, N.D., where they are compacted using a device designed by engineers at Federal Machine Co.
Michael Flaherty, project manager, says Federal Machine engineers were first inspired to design the pelletizing machine by the obvious need for alternatives. “With fuel prices and energy prices escalating, we looked for an opportunity to develop equipment that would service people in need of energy,” he says. Typical pelletizing machines are bulky and require a large amount of horsepower to operate. He saw a need for a smaller machine that could produce up to a ton per hour of compacted biomass more efficiently than the existing devices. “That is what our machine does,” he says. The machine being used to compact biomass samples for this project is the second-generation, and a third-generation, commercial model is now being designed.
After SDSU compacts the biomass samples, the density of the paks increases from about 7 pounds per cubic foot (similar to straw) to 45 to 50 pounds per cubic foot, which more closely resembles the density of corn or other grains. This is good, because it means conventional grain handling equipment should be able to also handle the paks, Dale says. The project has also shown that larger-sized particles can be compacted than previously thought. “What it means is that it’s going to require less energy in pre-processing biomasses,” Muthukumarappan says. Biomass pieces up to 12 millimeters in size (nearly one-half inch) have worked successfully in the process. Muthukumarappan adds they have not yet discovered what the size limit will be. “However, I know that we can compact alfalfa that has been run through a hay grinder producing 50 millimeter-long pieces.”
Multiple university labs in North Dakota and South Dakota are employing various techniques to process the compacted biomass samples. At North Dakota State University, assistant professor Scott Pryor is using separate hydrolysis and fermentation to convert the samples into ethanol. William Gibbons, a professor in SDSU’s biology and microbiology department, is using simultaneous saccharification and fermentation. Lew Christopher, an associate professor at the South Dakota School of Mines and Technology, is exploring whether microbes obtained from the Homestake Gold Mine in western South Dakota could be useful for conversion to biofuels. Finally, Michael Twedt at SDSU and Chris Saffron, an assistant professor at MSU, are evaluating the technical and economic feasibility of the entire biomass processing operations.
Other issues being addressed by this extensive collaborative study include how long the densified paks can be stored, how well the material will flow when it is unloaded at a processing facility, and how much biofuel the densified paks can produce as compared to uncompacted biomass. —Kris Bevill