Green Alchemy: Turning agricultural residues into feed and fuel
Worldwide, farms and other agricultural operations annually produce billions of tons of inedible plant matter, crop byproducts with high levels of difficult-to-digest cellulose. Regarded as having little economic or nutritional value, corn husks, wheat stems, soybean leaves, rice straw, sugarcane leaves and tops, and many other cellulosic materials are usually left on the ground after harvest. That is likely to change, however, thanks to recent technological advances by Michigan State University (MSU) and the Michigan Biotechnology Institute (MBI), a wholly owned subsidiary of the Michigan State University Foundation.
For every year that cellulosic biomass is discarded, the pressure to produce new sustainable fuel sources increases. The rising cost of conventional fuels such as crude oil, coupled with the instability of fuel sources, has resulted in many proposed alternatives. To date, however, the only U.S. agricultural product to find widespread acceptance has been corn-derived ethanol. Corn ethanol’s utility as a fuel source has been limited by corn’s importance as a source of both food and livestock feed. The technologies being pioneered by MSU and MBI stand to help alleviate this pressure by turning discarded plant material into new sources for both biofuels and animal feed.
MSU AgBioResearch scientist Bruce Dale has devoted much of the past 30 years to developing a process to turn cellulosic biomass into an economical source of biofuel and animal feedstock. A process invented by Dale and scaled up for commercial use by MBI, ammonia fiber expansion (AFEX™), allows for the conversion of cellulose-rich plant matter into animal feed and biofuels such as ethanol.
“It’s been a major focus of my career,” said the MSU professor of chemical engineering. “I invented AFEX over 30 years ago. However, it’s one thing to carry out a process in a lab and quite another to do it economically on a very large scale.”
Bulky, unwieldy and quick to decompose in its raw form, cellulosic biomass is difficult to store and transport, and that has been a major obstacle in making it commercially viable. Dale’s AFEX treatment helps resolve this issue by making it much easier to turn the treated material into pellets.
“During AFEX, ammonia penetrates the plant material and dissolves some of the linkages between structures in the plant cell wall,” Dale explained. “The ammonia is hot and under a lot of pressure during this process. When you release that pressure, the mixture of ammonia and dissolved cell wall materials migrates from the inside of the cell wall to the outside. The ammonia evaporates and leaves the rest behind, similar to what happens with paint when the carrier liquid evaporates and leaves the color behind. The cell wall now looks like Swiss cheese; the ammonia produced a lot of holes in it, which means there’s a lot more surface area available to digest.”
The AFEX process renders the biomass into a slightly sticky substance that can be easily compressed into pellets. Reducing the material to pellet form is an important key to making AFEX economical. The pellets are not only easier to transport and store, they are expected to be more digestible for animals as well.
“We don’t know for sure how much more digestible the AFEX-treated pellets are than untreated material or how they compare to the digestibility of corn, so the very next step is to investigate that question,” Dale said.
In September 2013, MBI began a 160-day feed trial to test the efficacy of the pellets for beef cattle feedstock. The pellets are produced by MBI’s new Department of Energy-funded AFEX pilot plant near the MSU campus. If the trial shows promise, Dale and his lab have envisioned a network of regional biomass processing depots where farmers could bring crop residues such as corn stover or wheat straw.
“We’re hoping to develop a series of depots, so that farmers could take their bales of straw and other material 5 or 10 miles down the road, where it could be AFEX-treated and converted to pellets,” Dale said. “You eliminate a lot of the problems with transport and storage by bringing the facilities close to the source.”
The pilot plant is capable of producing up to 1 ton of biomass pellets each day. The commercial-level facilities necessary in Dale’s model would be able to produce 200 tons daily.
If AFEX is successfully commercialized, Dale said it will benefit many people. Farmers would gain an additional source of low-cost livestock feed, as well as a new source of revenue by selling their previously discarded crop residues. The energy industry stands to reap substantial benefits by having a new biofuel source.
“The energy industry can piggyback on what will be, by then, an already existing system for processing and storing the animal feed product,” Dale said. “We must have long-term, sustainable replacements for oil used to produce liquid fuels. The only option for that at low cost and with a low carbon footprint is biofuels made from cellulosic biomass.”
Click to subscribe to our e-publications:Subscribe