December 22, 2008 | General

Forest Thinning Strategies For Biomass Utilization

BioCycle December 2008, Vol. 49, No. 12, p. 32
Projects examine how to reorganize forestry operations in the West to better use slash as a biomass feedstock.
Rhodes Yepsen

IN the Rocky Mountain region of the U.S., utilizing woody biomass has a unique set of barriers. Slash, tops and limbs from logging and forest thinning operations traditionally have been viewed as waste material, and thus piled and burned. The sites are remote, with steep and rough access roads, leading to high shipping costs for woody biomass.
Despite the obstacles, forest thinning operations on national forest lands provide opportunities for biomass utilization. Several projects are testing new equipment and management practices that treat slash as a marketable commodity. Projects are often combined with research on profitability and best practices to educate loggers to utilize slash from their operations, and to promote use of woody biomass.


With the downturn in the construction industry nationwide, sawmills have less by-product, creating a dearth of wood waste for many markets. “With the price of fossil fuel going up, and the opportunity to utilize this material as fuel, we’re seeing a highly dynamic market evolve quickly in this region,” says Dave Atkins, Economic Action Program Coordinator/Fuels for Schools for the U.S. Forest Service’s Northern and Intermountain regions. “There will be an increasing specialization, with different grades of wood fuel, processed differently and sold at various price levels.”
Atkins has been involved with establishing woody biomass heating systems at several schools in the region. These schools often have smaller biomass systems that cannot handle contaminants like stones, or oversized pieces of wood. “Industrial systems may have a screening system, but a lot of schools projects aren’t budgeted for that extra equipment,” says Atkins. Schools therefore tend to be more concerned with a consistent sized fuel, compared to industrial applications.
Even with industrial systems, however, oversized pieces will get stockpiled and eventually need to be either reground or shipped out. “In a typical setup, a biomass facility has a storage building, and the bottom has an auger moving across the floor to pull material on to a conveyor, which goes up to a metering bin, through feed augers and into the boiler itself,” describes Atkins. “Oversized pieces become an issue because they can jam the metering bin or be too big for the conveyor.”
Choosing a woody biomass-fueled boiler, whether for a school or factory, depends largely on what feedstocks are locally available, as well as the anticipated annual consumption. For instance, pellets are more expensive to buy, but the boiler system and storage facility are smaller and less expensive. “Pellets are dry, at about 6 to 9 percent moisture content, which means very little water vapor is released in the combustion container, requiring a smaller boiler room, and smaller overall footprint,” says Atkins. “Conveyance and storage for pellets is simpler and smaller too, more like a grain bin, gravity fed with a simple flex auger and a tiny motor.”
The trade-off for this initial low cost is a higher fuel cost. Pellets make sense for small-scale operations, like residences and small schools, because they won’t burn enough fuel to offset the cost of a large storage and conveyance system needed for wood chips. “A larger facility, however, will purchase enough yearly tonnage that the low-cost wood chips will amortize the extra cost of storage conveyance,” observes Atkins. The difference between chipped and ground wood can be significant as well. “Chipped material flows better, whereas ground material causes more friction,” he continues. “Augers need to have bigger motors to handle ground material, with stronger springs.”
While schools and factories may be choosing systems based on local availability of material, biomass producers are adapting methods of processing woody biomass to meet the growing market. “How to best harvest, collect, process and ship that material hasn’t been totally sorted out for the Rocky Mountain region,” says Atkins.


Whether the application is timber sale harvest, forest fire fuel reduction or storm debris clean up, the pile/burn method is traditionally the cheapest method of slash disposal. For instance, a large windstorm in November 2007 hit Custer National Forest in Montana, leaving material around recreation and cabin areas. “This material had to be cleaned up before the summer recreation season,” recalls Atkins. “However, we were only able to utilize saw logs and post and pole quality wood, because there was no market close enough for the tops and small diameter wood.” Although there is a composting facility in Billings, Montana, about 60 to 80 miles away, it would not pay for the hauling. So the slash and undersized wood was piled and burned.
Now that there is incentive to handle the waste slash differently, logging and thinning operations need a new strategy. “You need to plan your whole operation with the lowest value product in mind,” says Craig Thomas of CKYBER Enterprises, Inc., a logging company located in Stevensville, Montana. “Because it’s the lowest value, you can’t afford to waste any movement handling or processing slash.” Instead of piling limbs and tree tops in a jumble, which would create more work down the line, they are stacked in the same direction so that a front end loader or knuckle boom can get a big bite, and feed a grinder or chipper more continuously, for a high production rate. “It’s part of changing the traditional logging mindset, where the waste product is now marketable,” says Thomas.

A forest thinning project at Hebgen Lake in Montana, which began in the fall of 2007, attempted to both reduce fire risks near recreation cabins, while utilizing a high degree of forest slash for biomass fuel. “At Hebgen Lake, we sought to identify benefits and problems with accessing and handling slash more efficiently,” says Atkins, who helped organize the project. The main goal was to establish a dollar per ton cost for processing slash at a thinning operation.
“Prior studies had been conducted on forestry slash collection, processing and shipping at conventional logging operations,” says Brad Blackwelder, a Senior Scientist for the Idaho National Laboratory (INL) Bioenergy Program. “In those settings, wood is harvested and processed for lumber. At Hebgen Lake, the scenario was different, addressing a fuel reduction strategy, thinning the forest and undergrowth near recreation areas, campsites and private homes in an effort to reduce forest fires.” The cost for forest thinning and fuel reduction has traditionally been high, with little to no saleable product.
The study looked at methods of collecting undergrowth and slash, transporting it with roll-off containers to a staging area, grinding, and then selling it as biomass fuel. “If the process can prove to be at all profitable, it might change the dynamic of forest thinning operations,” says Blackwelder. “With projects like this, we’re trying to come up with a model – how much material can we expect to be harvested, and at what price? These are the type of figures that a cellulosic ethanol facility, for instance, wants to know before siting a facility in the area. Obviously, the price point is nebulous, depending on supply and demand, shipping distances, etc.”
Forest thinning removes smaller, overstocked trees that cause unruly wildfires, as well as unhealthy forests. “We removed 24 tons/acre on about 58 acres at Hebgen Lake,” says Thomas, who was hired for the project. “Because natural wildfires have been controlled in national forests, there may be as many as 15,000 stems/ acre, when it should be more like 50 to 70 stems/acre for a healthy forest.” CKYBER used a log max processor at Hebgen, a conventional logging machine that grabs the tree, strips the limbs and measures lengths for the mill. Tops, branches and other slash were placed into customized roll-off bins. “About half of the timber went for post and pole roundwood, and the other half was either slash or undersized material,” he adds.
One of the primary barriers to utilizing slash in the West is accessing material at remote sites. Chip vans, typically used for transporting ground wood, have difficulty navigating the narrow and winding mountain roadways that go to logging sites. At Hebgen Lake, custom-made roll-off containers were used to collect slash. These bins can be hook-lifted onto a forwarder, a multipurpose logging vehicle, as well as onto a semitruck. “The forwarder can haul a bin on its back, filling it with slash from the thinning operation,” says Thomas. “When several bins are full, a semitruck comes to pick them up, towing three at a time to market or for further processing.”
The roll-off containers were tested side by side with piling and burning by hand. “We had done some similar tests in Eureka, Montana, which was predominantly Douglas fir and ponderosa pine trees,” Atkins explains. “But at Hebgen it was primarily lodge pole pine, and we wanted to test the concept in a different setting, with different conditions.”
Roll-off containers were hauled to the nearby West Yellowstone composting facility, about five miles away, where material was unloaded and allowed to dry for six months. Osler Logging was contracted to process the slash, and brought in a Vermeer HG6000 horizontal grinder. The resulting biomass fuel was sold to Basic American Foods, a food processing plant about 75 miles away. “We wanted to compare the cost of the pile and burn method to the cost of using this equipment method, with the hope that utilizing the slash will be seen as a viable option,” says Atkins. Final numbers are still being calculated from collected data.


Deciding when, where and how to process slash into fuel will impact the economics of an operation, as well as determine the embodied energy in the biomass. A 2007 study done by the Rocky Mountain Research Station examines the energy cost versus energy return of using woody biomass at a large industrial boiler plant. It primarily looked at the diesel costs of collecting, grinding and hauling slash, and compared it to the energy output of the wood. “The results were positive, with the energy output of the biomass, compared to energy input of diesel (process and shipping), ranging from 17:1 to 22:1, depending on hauling distance, which is a very attractive ratio,” Atkins says. The study used a model to examine the Bitterroot Valley, in southwestern Montana, as the area producing the woody biomass, and the Smurfit-Stone pulp mill, near Missoula, as the existing delivery point.
Because the wood can be processed and delivered as wood chips, directly converted to energy in a boiler, these localized slash to biomass operations are quite energy efficient. Hauling distance becomes the variable for energy efficiency, along with the decision to either grind the slash before shipping, or ship unprocessed. “By grinding or chipping slash immediately, material could go right into a collection bin, eliminating a step and lowering the chances of contamination,” Atkins says. “However, the downside is that you’ll end up hauling a lot of water, because the moisture content is very high in green wood. Undoubtedly there is a break even point, a distance at which it becomes more efficient to dry out material first.”
Although the final numbers from the Hebgen Lake project are still being tabulated, Blackwelder notes that one of the immediate upshots of the project is the “depot grind concept,” in which unprocessed slash is hauled a short distance to a staging area before grinding and further shipping. “The West Yellowstone transfer station and composting facility happened to be close by in this case, but a depot-level processing area could be established easily,” says Blackwelder. “It would be on a small footprint, in a centralized location to feedstocks, and would allow drying and preprocessing steps to increase the value of the materials, and reduce shipping costs. This might be the future of woody biomass processing in the West.”
The depot staging area also encourages better use of grinding equipment, which would only need to be brought in once enough material was stockpiled, allowing maximum efficiency. “Grinders are typically oversized for forestry operations, working below capacity because not enough material can be gathered in one place,” explains Blackwelder. Atkins also commented about the need for properly sized grinding equipment. “There are a lot of areas with piles of biomass that are too small to low-boy in a big grinder, where you could instead tow in a medium grinder or chipper behind a 2-ton truck,” he says.


Although large quantities of woody biomass are available in the West, most of it is on public land, in national forests and parks. “Unlike a private tree plantation in Georgia, there are many restrictions on the type of equipment and acceptable practices for harvesting wood on public land in the West,” observes Blackwelder. “There are environmental sensitivity considerations in the national forests, which mean extensive research has to be done on the sustainability of biomass collection, and the impacts on wildlife. However, there is a chance to have our cake and eat it too, by actively managing forests to reduce forest fire potential, and securing a steady flow of woody biomass.”
“I’m a believer in woody biomass as a green technology,” says Blackwelder optimistically. “When you compare agricultural crops, which have to be planted and harvested annually, to woody biomass, which grows naturally and is only harvested every 60 to 100 years, sustainability issues are greatly reduced. Also, only about half of the slash can be effectively collected, leaving the other half on the forest floor, returning nutrients back to the soil.”
Although there are some early adopters utilizing woody biomass fuel in the Rocky Mountain region, Thomas of CKYBER Enterprises suggests that the state of Montana could be a leader in this arena. “Public reaction to woody biomass is positive on the whole, but as actual customers, they still view biomass fuel as inconvenient,” says Thomas. “I’d like to see more state leadership, and not just with legislation. The government buildings in Missoula could lead by example and heat with woody biomass, publicizing both the cost savings and the green benefits.”
ASH produced from woody biomass boilers is high in nutrients. “Some schools in the Fuels For Schools program spread the ash on their football fields as fertilizer,” reports Dave Atkins, Economic Action Program Coordinator/Fuels for Schools for the Forest Service’s Northern and Intermountain regions. “However, larger volumes of ash, such as from commercial boilers, could be hauled to a compost facility, instead of being landfilled.”
The high nutrient level in slash, and subsequent ash, can actually be a problem for the boilers. The needles from the tops of trees have a particularly high concentration of minerals and nutrients (when compared to the wood and bark). “Boilers burn hot and clean, liquefying some of the minerals, which causes problematic formations, referred to as clinkers, slag or moon rock,” Atkins explains. In a recent field trial, grinding methods were tested to separate the problematic fines and coniferous needles from the finished wood chip. A flap on the discharge chute of a chipper was opened, allowing the fines to blow onto the ground, instead of accompanying the wood chips into a collection bin. “These fines and needles could be collected and sent to a compost facility, where they would add valuable nutrients, instead of causing problems for boilers,” Atkins says.
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The specific goal for DOE’s biomass initiative is the replacement of petroleum-based transportation fuels. “Along the way, our research may influence other developments, such as combustion technologies,” says Blackwelder. “But, liquid transportation fuels are expensive, and currently are the biggest use of imported petroleum. Direct combustion of woody biomass is often replacing coal, not petroleum. Although this is an important step for now, wood is ultimately too valuable to burn, so combustion technologies are not long term.”

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