In Search Of Biomass Storage Solutions

When it comes down to the best way to move biomass from the field to the factory, the dry look may be out. Wet biomass offers advantages that may send the big bale back to the ranch to feed cattle.
By Jerry W. Kram | January 10, 2008
A hot, dry wind sweeps across the prairie landscape. A spark ignitesfrom a casually tossed cigarette or some over-enthusiastic microbe deep inside a bale, no one ever knowsand hundreds of bales containing thousands of pounds of potentially valuable biomass go up in smoke. The company waiting to convert the now-combusted cellulose takes a major financial hit.

Fire from accidental ignition or spontaneous combustion is just one potential problem with gathering large quantities of dry biomass for processing into cellulosic ethanol. Biomass sources such as corn stover and straw have a low density. That means it takes a large area to store enough biomass to operate a commercial-scale plant. Low density also means it takes more trucks to move that biomass to the plant site and those trucks would be operating below their maximum weight capacity. Harvesting crop residues such as corn stover can be disrupted by inclement weather because the stover still needs to dry down after the grain is harvested, says James Hettenhaus, founder of CEA Inc. "In 1997, after Halloween it was too wet to pick up corn stalks. If you asked how you would deal with that you'd be told, Move west,'" he says.

A better way to deal with hard-to-handle biomass feedstocks is to not worry about letting them dry down at all. In fact, an extra shot of water might be just the thing to add value to biomass feedstocks, Hettenhaus says. His idea comes from the pulp and paper industry, one of the few industries with years of experience gathering, moving and storing large amounts of cellulosic biomass. Hettenhaus looked at companies that processed bagasse, the fiber left over after sugar is extracted from sugarcane. The bagasse came out of the sugar plants at about 50 percent moisture and had to be dried to prevent the fiber from rotting. Since the 1960s, the processors have been adding water to the bagasse to accomplish the same task.

In a system called the Ritter process, cellulosic fiber is mixed with water and piped into large piles. The water is collected and recirculated through the pile. Lactic acid bacteria consume the residual sugar in the bagasse and make organic acids that inhibit other bacteria that would prevent the fiber from rotting. Drainage built into the piles' pads collects dirt and solubles such as phosphorus and potassium which can be returned to farm fields as fertilizer. As the pile ages, the biomass becomes denser and more economical to handle. Hettenhaus says the Ritter process or a similar system should work as well for corn stover as it does for bagasse.

In a demonstration project in conjunction with the National Renewable Energy Laboratory, Hettenhaus and other researchers built a 700-ton pile of corn stover in Imperial, Neb. "This is the third harvest season, so the pile has been there more than two years," he says. "We've sampled the pile and sent the samples to Purdue [University] and the system works great."

By removing soluble materials, the concentration of desirable materialscellulose, hemicellulose and ligninis increased by the process. The density of the pile doubles as it matures making transportation more economical. In the samples from the demonstration piles, there was very little loss of dry matter so the cellulosic materials were intact after more than two years of storage. "There was hardly any loss because of the density and low pH," Hettenhaus says. "There was some degradation on the surface but depending on the size of the pile there will be less than 5 percent loss."

Another major advantage of a wet storage system is that it makes one-pass harvesting of corn grain and stover possible, Hettenhaus says. Corn is usually harvested when the grain is at about 15 percent moisture. The moisture level of the stover can be 25 percent higher than that or more during harvest, much too high to be safely stored in dry bales. "Instead of drying the stover, you would just store it wet," he says. Also, because there is no fire hazard, the size of the piles is limited only by the height pumps can lift the mix of biomass and water. Hettenhaus says the same amount of biomass stored wet would require one-tenth the area for the storage of dry bales.

Even with the increase in density of wet-stored biomass, a commercial-sized cellulosic ethanol plant will still take a lot of biomass to keep running. A processing plant requiring 2,000 dry tons of biomass per day would need 100 deliveries per day by truck, 350 days per year. "Whether it's baled or wet you get about the same dry tonnage in a truck," Hettenhaus says. "But trucks are a problem because you need so many of them." In a 2003 paper, he proposed that wet biomass could be stored on farms near the fields where it was harvested. By locating the ethanol plants on railway hubs and using 50 to 70 gondola railcars, the biomass could be moved more economically than by truck. Rail transport is favored by the pulp and paper industry for moving bulky fibers like bagasse.

Preprocessing in a Bag
Removing cellulose from the plant cell wall and breaking it down into fermentable sugars will be expensive. An NREL study cited by Tom Richard, director of the Penn State Biomass Energy Center, shows that pretreatment, saccharification and cellulases could make up 39 percent of the cost of making a gallon of cellulosic ethanol. That's more than the feedstock for cellulosic ethanol is expected to cost. Richard is looking at how wet storage of biomass could reduce those costs. "We are working outside the stainless steel vessels," he says. "We are doing that to look at some of the opportunities for cost reductions in the overall process. A lot of them have to do with the sugar conversion from the feedstock."

Richard, whose work is supported by the Morgan Family Foundation and the U.S. DOE Idaho National Laboratory, is enthusiastic about ensiling biomass for many of the same reasons as Hettenhaus, including fire safety and increasing the amount of biomass that can be stored in a given area. Richard believes wet storage of biomass will become dominant as the cellulosic energy industry grows. He points out that most biomass resources in the United States have a relatively high moisture level at harvest and would have to be dried for storage. Adding water to biomass is easier and more energy efficient than removing water.

Because bacteria in the wet biomass create a dilute acid environment, some of the reactions that would occur in the first step of cellulosic ethanol production could take place in a silo. "One of the things that we are excited about is that this is a way to potentially move some of the processing out of the refinery and onto the landscape in a more decentralized fashion," Richard says. "That will impact transport, logistics and the quantities of biomass you have to move. One of the nice things about weakening the structure of lignocellulosic material during ensilage is that it is a lot softer. You can compact it better, and it's cheaper to move compacted material even if you have added water to it."

One of the advantages of this line of research is that it scales well, Richard says. The same reactions occur whether the biomass is stored in sealed gallon-sized plastic bags, silage bags hundreds of feet long, or large bunker silos that could conceivably serve a cellulosic ethanol plant. Richard says he could imagine a silage system the size of Beaver Stadium, where Penn State plays its football games. "Scale doesn't matter a lot in these systems," he says. "As a matter of fact, the bigger piles perform better than the small bags in which we do most of our experiments."

Richard and his group screened different varieties of commercial cellulose enzymes to see how they worked in a silage system. They found that the enzymes had a significant effect in silage at much lower doses than would be used in a cellulosic ethanol reactor. The enzymes do degrade over time but continue to have significant activity at least 21 days into ensilage. Analysis shows that ensilage with enzymes releases mostly mannose and xylose, showing that the treatment is breaking down hemicellulose rather than cellulose. Fiber analysis of the silage led to a similar conclusion, Richard says.

One of the disadvantages of pretreating silage is that the naturally occurring bacteria that preserve the silage consume the sugar that could be converted into ethanol. If more sugar is released during storage, it could be taken up by bacteria and not be available for fermentation into ethanol. Some products produced by the bacteria are known to inhibit ethanol fermentation by yeast. Richard says that by managing the species of bacteria, the ratio of the stronger inhibitors like acetic acid can be reduced.
Silage could prove a useful feedstock for consolidated bioprocessing. In experiments with Clostridium phytofermentens, an anaerobic bacteria that can break down cellulose and produce ethanol, Richard found that the bacteria produced significantly more ethanol from silage than from unprocessed corn stover or switchgrass.

Work is ongoing in other labs that have the potential for increasing the practicality of ensiling biomass. Richard cites research being done on lignin-degrading enzymes. Several species of fungus produce enzymes that can degrade lignin, the "glue" that holds cell walls together. If the lignin can be broken down, it will be easier to get to the valuable cellulose. The downside of ligninase is that it requires oxygen to work, and ensiling is an anaerobic process. If an aerobic phase was added at the beginning of the ensiling process, the enzyme might be able to do its job. Richard says a promising line of research shows fungi that digested lignin in preference to cellulose grew very well in tests and out competed bacteria that could damage the quality of the cellulose.

Richard says ensiling biomass has a lot of potential if it can be successfully integrated into the operation of a cellulosic ethanol plant. "We can get some benefits which may not be too dramatic," he says. "We have a storage system that is relatively robust and will probably make sense for a lot of the nation. But we really haven't tackled the interaction and interface with pretreatment systems yet to try and optimize that. That's something we are putting a lot of energy into." EP

Jerry W. Kram is an Ethanol Producer Magazine staff writer. Reach him at jkram@bbibiofuels.com or (701) 738-4962.