DOE webinar explores conversion of raw biomass into feedstock
Biomass leaching paired with torrefaction yields a densified biomass fuel ideally suited in many ways for biopower production, according to Luis Cerezo of the Electric Power Research Institute.
Cerezo was one of several speakers participating in a lengthy U.S. DOE webinar Aug. 30 that left nothing out in its exploration of converting raw biomass materials into efficient feedstocks for liquid biofuels and biopower. The webinar was composed of technical presentations that were given at “Transforming Biomass into Feedstock,” a biomass preconversion and densification workshop event hosted by DOE’s Biomass Program, the Office of Science and the Advanced Research Projects Agency. The workshop was held Aug. 23-24 at the Idaho National Laboratory in Idaho Falls, Idaho.
The theme for the informative event was based on the fact that biomass in its raw form is not an optimized feedstock for biorefineries or power plants, so speakers discussed producing and transforming it into high-quality, on-spec and densified feedstocks. They also addressed the impacts of the technologies performing those functions on supply chain logistics and feedstock performance. Broken up into sessions, the webinar agenda featured a summary of the workshop findings, presentations discussing fundamental science and transformative advances, and preconversion process information with an emphasis on mechanical, thermal and chemical preconversion.
In his summary of findings from “Transforming Biomass into Feedstock,” J. Richard Hess of the Idaho National Laboratory outlined the three mission areas: improve biomass’s bulk and energy densities, as well as stability and infrastructure capability; improve feedstock quality and end-use performance; and increase accessible biomass quantities, diversity and supply stability.
“As we discussed these areas, we realized we have some vision constraints,” Hess said, citing costs, energy balance, sustainability and greenhouse gas emissions. He walked webinar attendees through research and development pathway constraints also, including the crucial need to provide an appropriate benefit today, while removing barriers for tomorrow.
Workshop participants generally agreed that increasing performance and variability by converting raw biomass into feedstocks will be important for developing a national-scale bioenergy industry, Hess reported during the webinar. Concern was raised about how to bridge the vertically integrated systems of today with the commodity feedstock supply system of tomorrow. Feedstock formulation was a new idea for many workshop participants and gathered the most interest and intrigue of all the workshop areas, Hess said. A recurring theme was the need for information on feedstock characterization, performance and specifications. Another consistent theme was that densification of raw biomass simplifies conversion and is necessary. Not surprisingly, Hess said workshop participants shared the concern that the transition to commodity feedstock will be difficult.
“Raw biomass materials are NOT solid fuel,” read the slide that kicked off a presentation by Troy Runge, assistant professor at the University of Wisconsin and researcher at the Great Lakes Bioenergy Research Center. “Biomass materials must be processed and refined into fuel,” it continued. The fact is not as obvious for solid fuels as it is for liquid fuels, he told webinar listeners. “Biomass can be burned, but it doesn’t make it a good solid fuel.”
Runge said feedstocks can be refined through two main processes: thermal and biological. He explored the possibility of an extraction step, citing testing he and his colleagues have conducted with poplar, miscanthus, corn stover and switchgrass. The team extracted water and acid, creating solid fuels with nitrogen reduction of 50 to 75 percent, chlorine reduction of 35 to 55 percent; and little reduction in overall ash without excessive washing, he said. The team wanted to know if extractions would give the pelletized feedstock unique pellet properties, and in many cases, it seemed to, he explained. Pellets made with material that endured water extraction held together better than the control group when put in water, Runge said. Certain water and acid extractions made for better and higher density pellets, depending on what was extracted, results showed.
But the added step will cost money and Runge walked his webinar audience through advantages to offset that cost. “After we did the extraction, we found a significant reduction in the energy required to do the size reduction,” he said. Post-extraction, the materials were also easy to dry and were more stable in storage. “A little bit more work to do, but it is promising,” Runge said.
In addition, he added that leaching can improve pellet durability and chemical characteristics. Cerezo focused on leaching and washing of raw materials during his entire presentation, saying advantages of the chemical preconversion pathway include its relatively low cost; its ability to increase ash fusion temperatures and reduce volatility; and its improvement of combustion properties. Leaching/washing can eliminate low-grade feedstock problems with high-efficiency conversion, and it can expand the use of short-rotation sustainable biomass energy processes, he said.
Cerezo and his Electric Power Research Institute team tested bout 10 feedstocks, including wheat straw, switchgrass, rice straw, rice hulls, olive residue and many others. Pilot and pre-commercial demonstration projects are scheduled to validate the performance and optimize production costs of leaching and washing pretreatments, Cerezo said.
A combination of leaching and torrefaction is ideal for both biofuels and biopower, he said, adding that the leaching process can be combined with that or pelleting to produce clean biocoal from low-grade biomass.