The Odd Decouple

National laboratories argue for a paradigm shift in the industry’s approach to biomass handling and preparation for cellulosic ethanol biorefineries and other biomass applications. In-depth reporting in the November EPM.
By Tim Portz | October 19, 2016

In Iowa, corn stover bales sit unprotected in vast piles, exposed to sun and rain, until they are picked off the pile and carried by a forklift to a bale receiving machine. They are then shredded, milled, and passed immediately into pretreatment for conversion into cellulosic ethanol. In Minnesota, urban wood waste is shredded, stored in piles in a woodyard, loaded onto trucks and delivered to a downtown heat-and-power facility for feeding into a boiler.    

These approaches, characterized by few, if any, additional efforts to better prepare the feedstock for conversion, are common throughout the biomass-to-energy industry. According to researchers at the nation’s national laboratories, this low-tech approach must evolve if the biomass-to-energy industry is going to significantly increase its contributions to the nation’s power, heat and fuel portfolios.

This approach, where biomass handling, sizing and drying all occur at the conversion facility, is referred to as a direct couple, and researchers at Idaho National Laboratory, among others, are urging the industry to move away from a practice they say carries too much operational risk. “A conventional system that you see the guys building today are mostly passive systems, with very little active control in them at all, including active moisture management,” says Richard Hess, director of the Idaho National Laboratory Energy Efficiency and Renewable Energy Program. “The grinder is directly coupled to the pretreater. If the grinder plugs and shuts down, it shuts the whole plant down.”

In other industries that handle vast quantities of solid materials, Hess notes, it is rare that preprocessing and conversion are coupled to the degree that they currently are at many biomass-to-energy facilities. “If you look at the Budweiser plant out here, its receiving station is five miles away, and all their malt formulations are done there,” Hess says. “Once that is done, a train car rolls out and drops it all in the malt plant. It’s completely decoupled.”

Hess recognizes that biorefineries, heat plants and biomass power plants are all capable of particle-size reduction, but he and others are calling for more measures, preferably happening at some distance  from the conversion facility. “Taking a corn stover bale, grinding it up and changing it from an 8-pound-per-cubic-foot rat’s nest to a 2-pound-per-cubic foot rat’s nest that still won’t flow is not an acceptable level of decoupling,” says Hess.  “This material still won’t work in the next process.”

Commercial Relevance
Hess’s assertions are an extension of an approach that the U.S. DOE has been championing for years. Most recently, the concept received more attention and articulation in the 2016 Billion-Ton Report released by the DOE in July. In chapter 6 of the report, “To the Biorefinery: Delivered Forestland and Agricultural Resources,” the authors call for increased innovation along the biomass feedstock supply chain, noting that current feedstock systems have “few to no active control strategies.” The report calls for feedstock streams that transform “raw feedstocks that are aerobically unstable and highly variable into a high-density, flowable format that can be traded as a commodity.” 

For Hess and his colleagues at the DOE, the solution is to create a series of regional biomass depots where raw biomass streams of many kinds can be received, sized and upgraded for downstream conversion. The resulting material, often referred to as an intermediate, will be a conversion-ready material that can flow seamlessly through the material handling equipment at biorefineries, biomass power facilities or district heating installations. “The biomass depot is really a business model to implement a concept, but the fundamental concept is we’re moving from a passive feedstock supply system to an active one,” Hess explains. “What is active in that feedstock supply system is very feedstock- and regionally specific. We can list many things from moisture, all the way down to quality control, blending or leaching.”

Hess says that the processes the depots would be capable of performing would vary by region, owing to the varied biomass streams available throughout the country. The active controls required to prepare corn stover for conversion into cellulosic ethanol will likely vary greatly from the steps necessary to make forest residues ready for a power plant cofiring biomass.  “The point is, the materials leaving a biomass depot will be conversion-ready,” he stresses.

Value-Added Upgrading
For Hess, the vision of a biomass depot goes beyond merely getting biomass streams into a conversion-ready product that will flow easily into and around conversion facilities. The promise of a biomass depot, Hess notes, is really beginning to drive some increased value into biomass by increasing the ease of its later conversion. “We need to get beyond beating on things with hammers and start looking at different types of preprocessing treatments,” he says. “Kinetic energy is really the lowest, poorest way to tear stuff apart. If you start coming in and putting a little heat here and a little bit of pressure there, your finesse and ability to take things apart into a better and more uniform product is much improved.”

Hess says that value-added upgrading, like the biomass depot concept, will vary based on the feedstock and the ultimate downstream uses of the material. “Mild torrefaction approaches that don’t go all the way to torrefaction, but more closely resemble coffee roasting, can do significant things to reduce the grinding energy, for instance,” he says. “If you can start to apply those processes on a fractional basis, then you can apply those treatments to the different issues and you can get a much better product.”

The continued evolution of the biomass industry hinges on the evolution of our ability to better preprocess the feedstock streams it intends to convert. “Right now, we’re just using a hammer to take apart a complex composite,” Hess adds. “When you do that, your quality control over the deconstruction of that product, and the particle-size distribution, is just very broad and very random. That creates a lot of challenges when you start talking about heat transfer and the processes that the biomass will undergo in the various conversion reactors these streams will end up in.”

Author: Tim Portz
Executive Editor, Ethanol Producer Magazine
[email protected]