From Recipe to Scientific Formula
Creating a pellet or briquette from energy crops may seem more like a secret family recipe than an exact process. Begin with a selected feedstock, dry accordingly, add binders if necessary and compress generously. Sometimes, even if the recipe is dead-on, the end product may turn out differently than expected.
Densifying energy crops presents unique challenges to producers seeking to use grassy feedstocks, such as miscanthus and switchgrass, for pellet production. Moisture, binder and energy requirements are just a few of the issues inherent in creating a pellet or briquette made from energy crops, says Emily Heaton, assistant professor of biomass crop production at Iowa State University. “Sometimes there is not enough moisture or enough binder, depending on the pelleting process and getting it to stick together, and then the energy that is required to get it to stick together,” Heaton says. “You can always get it to stick together, but what does it take to do that?”
Heaton says she believes the challenge involved in densifying dedicated energy crops is simply gaining enough experience and know-how—making more kinds of pellets more often, and having a reason to begin producing them at scale to evaluate in more detail.
Researchers at Penn State University are working to do just that, by engaging in a continuous study of pelletizing biomass energy crops in small-scale pellet machines. “The project is working from two directions,” says Daniel Ciolkosz, research associate at Penn State. “One direction starts at the fundamental science of particulate materials and how they behave during the densification process.”
Heading up the fundamental side of the project, researchers Apoorva Karamchandani, Hojae Yi and Virendra Puri apply their particulate compaction background to investigate solutions for problems including pelletizer clogging and crumble-prone pellets. “What we envision is to have more consistent throughput in terms of the process, and less issues during pelletization,” says Yi, research associate of powder mechanics at Penn State. By bringing more science into the equation, the user will have greater control and insight over feedstock selection, and produce a better pellet with less production issues, he adds.
The second front of the project sprang from efforts to encourage use of cottage-industry level pelletizers. Penn State researchers saw the small-scale pelletizer as an entry-level opportunity to engage people who are excited about bioenergy. The available equipment and processes, however, caused frustrations among users, prompting them to abandon their efforts. “The equipment and processes are very picky and very finicky,” Ciolkosz says. “So often, people get excited about the idea of becoming the next great pellet mogul, they purchase a small machine, they work with it for a little while and then give up in frustration. We wanted to come up with some practical recommendations for how they can have better chances at success when they’re trying to densify biomass.”
In addition to clogging issues, the feedstock’s physical properties can cause problems within the pelletizing process. Compared to woody biomass, energy crops like switchgrass, contain less lignin and tend to have more issues, such as clogging pelletizing machines, and sometimes require additional materials to create a pellet, Yi says.
Grass-derived pellets have a tendency to spring back and puff up after leaving the pelletizer, Ciolkosz explains. “The bonds that are formed during the extrusion process are broken before the pellet can firm up,” he adds. “Switchgrass is a real challenge to pelletize.”
After two and a half years, the research team nearly has switchgrass feedstock issues sorted out. “It’s a very interesting material that poses some unique challenges in terms of being effectively pelletized,” Ciolkosz says. Pellet producers may experience difficulties in creating a uniform product, even if it seems they are handling the material with the same methods every day. “It’s not really clear why that happens all the time,” he says. Slight differences, such as particle size distribution, could be affecting particle properties, he explains.
Particle orientation inside the pelletizer is another major issue that has to be addressed when working with switchgrass. By controlling the way the switchgrass particles enter the pelletizer, researchers are able to produce a better quality pellet. Mixing in dried distillers grains (DDGs) with the ground switchgrass changes the alignment of the grassy particles. Researchers initially place a small amount of switchgrass mixed with DDGs into the pelletizer, followed by pure switchgrass. The mixture acts as a dynamic plug to provide back pressure to the material while keeping the dye clear and unplugged.
Gaining an intimate understanding of energy crops and how they behave in the densification process could help producers remove some of the guesswork in creating a quality pellet or briquette. Adds Yi, “There’s no written equation or formula to produce optimal quality pellets.”
Author: Chris Hanson
Staff Writer, Biomass Magazine