Pretreating bulky biomass feedstocks to make them into a denser, more compact form as a first step toward making them into biofuels may require less energy than scientists had thought.
That’s the good news thus far from a $1.1 million study funded largely by the North Central Sun Grant Center at South Dakota State University (SDSU).
Scientists already know the biofuels of the future will likely require regional biomass processing centers to pretreat and densify materials such as switchgrass and cornstalks. Pretreating and densifying feedstocks will make it easier to ship biomass to processing facilities as much as 50-100 miles away. The new study explores some of the actual methods and technologies that could be used.
“Biomass is very bulky. So there are going to be a lot of logistical hurdles in moving this bulky biomass from the field to the centralized biorefining operation,” says SDSU ag engineer Kasiviswanathan Muthukumarappan.
The study – a collaboration involving researchers in South Dakota, North Dakota and Michigan – takes advantage the adhesive-like qualities of one of the chief ingredients of biomass, lignin.
SDSU scientists are collecting three kinds of biomass: switchgrass, prairie cordgrass and corn stover. They mill the biomass to get particles of specified sizes. They then send the biomass to engineer Bruce Dale of Michigan State University, where the samples will be pretreated by a technique called ammonia fiber expansion (AFEX). It brings lignin to the surface of the biomass particles so that it can be used as a natural binding agent that is already present in the material.
After pretreatment, the samples are sent to Michael Flaherty of Federal Machine Co. in Fargo, ND, where engineers have developed a device for compacting pretreated biomass. The biomass is compacted to form paks, which are then sent back to SDSU, where the physical and chemical characteristics of the samples are analyzed.
The good news from the study thus far, Muthukumarappan said, is that pretreatment processing apparently won’t have to spend as much energy slicing and dicing biomass into pieces of very small diameter.
“We found that, in terms of compacting these AFEX-treated biomass samples, we don’t have to go to lower particle sizes; we can use the larger particle sizes. What it means is that it’s going to require less energy in preprocessing these biomasses,” Muthukumarappan says.
Instead of chopping biomass into pieces with a diameter of about 2 millimeters (mm), the process will work with pieces as large as 8-12 mm. That is one small step toward lowering the cost of making new biofuels from biomass feedstocks. Scientists had been afraid that they would have to chop the biomass into particles of smaller size in order to create more surface area so that enough lignin would be available to act as a binding agent.
The project also includes a storage study to determine how long these packs can be stored without any adverse effects.
The samples will be processed by three labs. Ag engineer Scott Pryor’s lab at North Dakota State University uses a technology s called “separate hydrolysis and fermentation.” William Gibbons, SDSU microbiologist, will use a processing technique called “simultaneous saccharification and fermentation.” Lew Christopher at the South Dakota School of Mines and Technology will explore whether unique microbes from the former Homestake Gold Mine in western South Dakota – so-called “extremophiles” adapted to the extreme conditions deep underground – might serve as a biomass breaker in the process of converting biomass to biofuels.
Michael Twedt and Chris Saffron, ag engineers at SDSU and Michigan State University, respectively, also are looking at the technical and economic feasibility of whole biomass processing operations.
The study also will consider issues such as flowability, or how well the processed material will flow when it is unloaded at a processing facility, and whether the compacted biomass will produce the same amount of biofuel as uncompacted biomass.
The two-year project runs until June 2012. The North Central Sun Grant Center is funding $700,000 of the $1.1 million project. The remaining $400,000 is a cost-share match from the partners in the research.