Fiber: Feedstock of the Future

Realizing its full potential requires a better understanding of fiber’s fickle characteristics when used as a cellulosic ethanol feedstock. In-depth reporting in the November EPM.
By Susanne Retka Schill | October 17, 2016

In years to come, ethanol producers will be learning a lot about fiber, as many plants diversify from the starch-only, first-generation production process into the next generation of biorefining technology. The first step for many will be doing more with the corn kernel already being processed. Realizing the full potential of cellulosic ethanol, though, requires taking the next step into better understanding the fickle characteristics of the wide range of biomass fiber. 

Converting corn kernel fiber is seen as the low-hanging fruit by many. Up to now, ethanol dry mills have focused on optimizing the grinding of whole corn in the process. Wet millers and a handful of dry millers, though, have long separated kernel components. Neal Jakel got his start in the wet mill industry before working in fuel ethanol production and now in strategy and technology at Fluid Quip Process Technologies. “One of my first projects out of college 25 years ago was trying to convert corn fiber from a wet mill to ethanol. It’s not a new concept, it’s been around for a long time,” Jakel says. “[Fiber] is very difficult to separate out into individual C6 or C5 molecules because of the way they’re bound together. It’s very chemically and energy intensive to make a clean separation, which is the complete opposite of how starch is set up. When you think about it, that fiber is meant to protect the seed, that kernel, so that it’s not easily broken down by mold or anything, whereas the starch is readily available to be broken down for energy to feed that new seed when it’s planted to grow. That’s why the starch is much easier to convert to sugars, but the fiber is not.”

Kernel Fiber
Tapping the potential of corn kernel fiber requires understanding the kernel. The outer hull, the pericarp, is the most predominant source of kernel fiber and the easiest to separate and remove due to its large particle size. The pericarp protects the starch and the germ that contains most of the protein and oil. The big dry millers separate the fiber and bran, Jakel explains, which typically retains a fair amount of starch, producing food items like corn flakes and brewers grits.

Wet mills use a different process to maximize the separation of kernel components—the fiber, protein, starch and oil. “Wet mills are intensely focused on getting those four components in as pure form as possible,” Jakel says. The kernel is soaked in water and sulfur dioxide or a similar chemical, which softens the outer hull and disrupts the protein matrix surrounding the starch granules. The process allows the whole germ to break free and the protein to be washed from the starch granules. The protein, now called corn gluten meal, is 60 percent protein or better and used in poultry, pet and acquaculture feeds. The oil is separated, refined and goes into the vegetable oil market. The remaining corn gluten feed is 60 percent or more fiber, typically dried and pelletized to increase bulk density to be shipped overseas. The starch is further processed into paper coatings or for hairspray, converted to high-fructose corn sugar or low-cal sweeteners or fermented into ethanol or other products such as dextrin or citric acid. FQPT is adapting the wet mill approach for the fuel ethanol industry, Jakel says, developing bolt-on versions of the proven technologies. Three fiber by-pass systems have been installed at plants so far. “We see a huge opportunity, the fiber stream being just one of the many streams that we can separate and do more with in an ethanol plant.”

While fiber separation has long been done, using it in other processes and not just drying it for feed is relatively new. “When processing a pure fiber stream, it has to be very dilute in order to pump it,” Jakel says. A 50 to 70 percent fiber stream needs to be around 10 percent solids or less in order to move through a pipe. “There’s no doubt fiber has a great ability to bind with water and to structures. It’s a great mechanism to plug up heat exchangers, valves and nozzles, just because of the 3D structure of the fiber—it’s got a lot of edges and points to grab in the process.” 

Handling Issues
Overcoming the challenge of handling fiber has been a big part of ICM’s research. Like FQPT, ICM is working with front-end separation technologies, but with a different focus, integrating its select milling and fiber separation technologies into traditional dry mill ethanol plants with an eye on the transition to cellulosic ethanol production in what it calls its Gen 1.5 process, converting corn kernel fiber into cellulosic ethanol. The lessons learned in handling kernel fiber are being applied to developing a Gen 2 process for converting feedstocks such as stover, biomass sorghum and switchgrass.

One of those lessons is the optimal solids loading. “Normal Gen 1 corn mashes are going to run in a 32 to 35 percent solids range. That’s still quite a liquid,” says Jeremy Javers, director of technology development at ICM’s St. Joseph, Missouri, demonstration plant. “If you jumped into a tank of corn mash, you’d sink. If you jumped into a tank of 35 percent corn fiber solids, you’d probably get the middle of your calves wet and your knees would still be dry.” 

There’s a big difference between the feedstocks under consideration for cellulosic ethanol processes. “By far, corn fiber is the easiest. It’s the one we can push the solids higher, and it still acts like a liquid,” Javer says.  There’s another advantage in the handling and cleaning of corn kernels. “We have well-established systems like scalpers and sieves and air classification systems that can help take things like rocks and chains, bolts and hitch pins out of the corn that gets brought in. We’re very good at getting that cleaned up. Occasionally, something gets through and busts a hole in a slurry screen.”

The systems for cleaning second-generation feedstocks aren’t fully developed yet. “When you bring in a bale of corn stover, you get the rocks, the bolts, the twine, the dead animals coming in. And when you’re processing 1,000 tons a day, 2,000 bales material a day, it becomes an issue,” he says. “You’ve got to figure out a way to get the stuff out. If you don’t, it’s not busting a hole in a slurry screen, it’s starting a fire or breaking a piece of equipment or causing serious damage to the equipment inside the process—wearing out bearings, plugging things, breaking things, and on a high-pressure system.” 

Once ground, getting the fiber to flow through the process was the primary challenge to overcome, Javers acknowledges. “We spent millions of dollars trying to get that material from the first tank through the first 25 feet of pipe. That’s where we spend the majority of development dollars.”  In the early years of research, the team experimented with multiple iterations of process conditions and equipment configurations. At one point, he says with a laugh, the key performance indicator was not how long the system would run, but how quickly they could take it apart and unplug it for the next iteration.  “You’d spend all day getting ready to start it up, then you’d run for an hour and everything would plug up,” he says. The system would be dismantled, unplugged, power washed and sometimes even needed chiseling to get the material out. Javers commends the ICM team and the equipment providers who hung in there through multiple modifications in the process. Today, the system starts up smoothly within a couple of hours and runs continuously, he says.

Getting the fiber to flow was the biggest challenge, but not the only one. The work with switchgrass illustrates another dimension of fiber conversion, Javers says. “The switchgrass fields were no more than 15 miles apart but the levels of water soluble materials were variable. That variation in just water soluble ash and other components could have a serious impact on conversion efficiencies.” The researchers focused on ways to equalize the incoming feedstock.  “One way we did that was through applying washing technology,” he says. “Getting certain components out of the process did a few things. Some of them act like bases in the pretreatment system, and we run a dilute acid pretreatment, so the more equalized the feedstock, the more efficient our acid doses were and the more consistent conversion we got field to field. So it’s not just about milling and mixing and pumping, it’s also about preparation of the feedstock and presenting it to the system and pre-wetting the feedstock.” Even the pre-wetting step presents a challenge, he explains, as the lignin content in some feedstocks means the chopped material does not absorb water easily, instead floating on the top of the tank.

Javers says ICM now has five fiber separation systems (FST) operating, with a sixth starting up soon. Initially, the fiber is being removed, increasing throughput in the starch process, and then reintroduced at the backend. Discussions are under way to determine which will be the first to add the Gen 1.5 system to convert the fiber into ethanol. The process also will modify and diversify coproducts with high-protein distillers, modified syrup and a potential new fiber stream.  

“Every one of these startups of FST have had their unique challenges and opportunities,” Javers says.  “I’m really proud of the customer, the team and the company for being able to respond to every one of those challenges and find solutions and have successful startups.” 

ICM and FQPT are just two of many companies, each with its own approach to tapping into the potential of fiber biorefining. Javers echoes a comment heard from many of them. “We want to see others succeed. At the end of the day, the vision of success is so great that we can’t afford to fail. The value this industry, the bioprocessing industry is going to bring to agriculture is going to be great.”

Author: Susanne Retka Schill
Managing Editor, Ethanol Producer Magazine
[email protected]