Crop-Based Pellets for Power, Fuel and Ag Heat

Pellets have taken a backseat in the Midwest where crops and grasses, not trees, dominate the landscape. Biomass energy efforts there have been much more focused on the production of liquid renewable fuels, following a model of baled feedstock supply procured from a cost-effective radius around the plant. Two Missouri companies are looking to pelletize crop residues and energy crops, targeting very different markets and following divergent business models. For MFA Oil Biomass LLC, it’s all about recruiting growers and getting a new energy crop, miscanthus, established while developing multiple markets and building a supply chain from scratch. For Enginuity Worldwide LLC, it’s about developing an engineered pellet to overcome some of the limitations inherent in pelletizing nonwoody biomass such as grass or corn stover, targeting Missouri’s coal-fired power industry needs.

In some respects, woody and agricultural feedstocks are opposites. “Wood has high content of lignin and agricultural biomass is low in lignin,” explains Jaya Shankar Tumuluru, an expert on pelletizing biomass materials at the Idaho National Laboratory. “Wood is low in ash content and agricultural materials are high in ash content.”  

Wood’s lignin content at 25 percent or higher lends itself to high-quality, durable pellets due to lignin’s thermoplastic nature that essentially melts and binds the pellet together. Crop residues and perennial grasses, on the other hand, typically range between 12 and 14 percent in lignin. That lower lignin content can require more energy to pelletize herbaceous feedstocks than woody biomass, says the INL researcher, unless binders are used. And, while binders are widely used for other applications, such as using starch as the binder for pharmaceutical tablets, there is still much to be learned. “Binding is not a finely tuned science,” Tumuluru explains. “It’s still in the experimentation stage.” 

The difference in ash content can also be problematic for ag-based pellets, particularly with residential heating where the higher ash content of herbaceous pellets is undesirable. The ash content, along with varying levels of other elements such as chlorine or silica, can also create problems for power or industrial users, contributing to fouling or slagging in boilers. One possible solution for getting better-quality pellets from agricultural biomass, the INL researcher suggests, is to blend multiple feedstocks to get a consistent content in Btu, moisture and ash, and to keep undesirable components in acceptable concentrations. Including 10 to 20 percent woody biomass in the blend can act as a binder, he adds, improving the durability of ag-based pellets.

Engineering Pellets For Power
Finding a solution for durability and weatherability of ag-based pellets has been the focus of Missouri-based Enginuity. “We grew up here in this state,” says Nancy Heimann, Enginuity president. “One of our goals is to produce a fuel that enables power plants to be effective and continue to provide baseload power, but still balance the requirements of environmental regulation and renewable portfolios and make those coal plants last longer.” 

As materials engineers, the Enginuity team has examined ways to densify ag-based biomass so it can be handled alongside coal in a seamless fashion, without significant changes to handling or boiler systems. Enginuity has dubbed its process eCARB, short for environmentally continuous annually renewable biomass. The core technology is a binder that achieves 98 percent durability with a variety of ag feedstocks, a value that is higher than coal’s, says Heimann. The patented binder is comprised of all combustible material including a starch and a hydroxide as adhesives, a silicate as a viscosity agent, a preservative and a Btu additive. 

There are advantages to using the new process, Heimann says. Unlike typical pellet mills where uniform size reduction is a critical factor, she says, “our binder doesn’t work through mechanical means, it encapsulates every fiber and sticks them together. We designed the line to have no hammermills because we didn’t want the expense or the environmental footprint of the types of significant size reductions in our process.” The binder also permits the use of a variety of shapes. “Using [biomass] as a supplement to lump coal, we’re using a unique shape to make sure it flows correctly. One of the problems with three-eighths inch pellets is they fall through all the handling equipment designed for lump coal.” For power plants using pulverized coal, other shapes and sizes will be worked out and experiments conducted to see if it works better to blend the coal and biomass before or after grinding.  

Weatherability was another goal for the materials engineers. Traditional pellets must be protected from rain and weather, potentially creating an added expense for power plants considering cofiring. Heimann points to one test burn using miscanthus pellets where the power company discovered it couldn’t even store the coal and pellets together. The miscanthus acted as a dessicant, drawing moisture out of the coal, causing the pellets to swell and disintegrate. Enginuity developed a process described as roasting to  create a water resistant biomass pellet.

The engineered pellets will be tested this winter at the municipal power plant in the company’s home town in Columbia, Mo. Columbia Power & Light has worked with Missouri Corn Growers Association and Enginuity in getting grants from the American Public Power Association to help with the costs for the test burn, including the fuel and monitoring. Heimann says they intend to test multiple blends with different proportions of corn stover, miscanthus and mixed grasses. The company also learned in late November that it has received a $500,000 USDA grant through the Rural Energy for America program to continue its research and development. “We’re just thrilled,” Heimann says. “USDA has walked alongside us quite a long time as we’ve tried to understand the problem; 90 percent of solving problems is understanding them correctly.” 

Farmer-Based Development
The second Missouri-based project is following a different development path and targeting a different market. MFA Biomass is a division of the regional farm supply cooperative, MFA Oil Co. In 2008, shortly after the renewable fuels standard was passed and oil hit $147 a barrel, MFA “looked out to see what things were on the horizon, not only from renewable fuel standpoint but also from agricultural heating standpoint,” explains Jared Wilmes, director of biomass operations. After researching several possibilities, the newly formed MFA Biomass settled on miscanthus as the bioenergy crop with the most potential. “We’re doing two things at once,” Wilmes says. “stabilizing energy prices and at the same time creating some economic returns for producers, developing a feedstock that can grow into something much larger.”

With the help of a USDA grant through the Biomass Crop Assistance Program to reduce the risk for farmers in establishing the new crop, MFA Biomass now has 13,500 acres of miscanthus established. The ultimate goal is to have enough acreage to attract a liquid renewable fuels plant, which will require between 25,000 and 30,000 acres. They started with that mindset, Wilmes says, knowing that other markets would need to be developed as the base acreage grew. “You don’t just immediately pop in and grow 30,000 acres of miscanthus,” he explains. “It’s going to be a process evolving to get there.”   As part of its multiple market development, MFA Biomass purchased a small pellet mill for experiments, using it to also make bagged wood pellets for residential heating while providing a hands-on comparison of the two feedstocks. “The miscanthus processes faster through our plant, about twice as fast through our system as wood,” Wilmes says. “That has to do with the properties of the canes versus hardwood. If we were doing softwood, that wouldn’t be as dramatic.” 

The durability of the miscanthus pellet isn’t as high as wood, he adds, “but is it sufficient to burn in an agricultural setting? Yes. It doesn’t have to be a perfect pellet to burn with the furnaces we’re using.” He expects the pellets will be handled in 20-ton bulk shipments and used in multifuel furnaces to heat poultry barns and hog production facilities. “The miscanthus pellets we have tested have come in at about 7,900 Btu per pound,” he says, which compares to wood at 8,100 to 8,300 Btu per pound. “It has slightly higher content of ash than wood, but not as high as you find in switchgrass. We’re not looking to burn it in a residential setting where ash is critical.”

The key difference from wood, he points out, is that as a perennial crop, miscanthus is harvested every year. The challenge is that, unlike wood, which is essentially harvested as-needed year around, miscanthus has a harvest window and must be stored. “I see our challenges on more of the harvest and storage and logistic sides than the pelletizing side,” he says, adding that while there are nuances to pelletizing miscanthus successfully, the process doesn’t change that much from wood.      

This winter, the project will see its first major harvest, with 40,000 bales expected from the first 4,000 acres to be cut. Miscanthus is harvested in the winter, after the crop sends its nutrients into the roots and goes dormant. It stands well, without lodging, Wilmes says, and is dry. And while southwest Missouri doesn’t have severe winters, they do expect to need to learn how to deal with cold, wet conditions.      

National User Facility
Biomass users like Enginuity and MFA Biomass have a new resource to turn to as they work through the challenges of developing new biomass utilization strategies. The Idaho National Laboratory was recently designated as a Biomass Feedstock National User Facility. Built around the capabilities of INL, Kevin Kenney, lead biomass researcher, explains, “The purpose is to engage industry sooner in the research and development process.” The facility has a commercial-scale pellet mill, size-reduction equipment and the capacity to test using the full range of pretreatment systems being researched for various biomass conversion processes from power to heating, biofuels to biochemicals. 

The user facility’s goal is to help develop and evaluate improvements in biomass utilization throughout the supply chain, from farmers needing to maximize their returns while understanding the quality requirements of the emerging bioenergy market, to equipment manufacturers throughout the supply chain in preprocessing, densification, drying or other areas such as torrefaction. INL also has an extensive biomass library, with more than 60,000 samples from several dozen species that include the range of biomass from crop residues to grasses and woody biomass. The library includes characterizations of chemical and physical properties, with the ability to query the database to identify solutions to problems or match a biomass feedstock to various processes and pathways for conversion. 

While the first round of bioenergy development in the Midwest has focused on the use of crop residues and energy crops for biorefineries based on a defined feedstock supply radius and baled-materials shipping, the long-term vision of a biomass-based energy economy will require biomass feedstock to become a commodity. “One of the requirements for that,” Kenney says, “is that it has to be stable and transportable. Densification does that for us.” 

Author: Susanne Retka Schill
Managing Editor, Pellet Mill Magazine
701-738-4922
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