Wood Yard Wisdom

Many factors come into play when designing the ideal wood yard and fuel handling system for a bioenergy or wood pellet operation.
By Anna Simet | September 03, 2018

Upfront capital budgets, operating budgets, climate, space, fuel quality, local regulations, facility size and even more factors come into play when choosing a bioenergy or pellet manufacturing facility’s wood yard and fuel handling system, and each scenario presents its own complex set of design challenges. When John Saucier of ProcessBarron is explaining wood yard options to customers, he explains these factors and the degree of influence they should have on decision-making, laying out three different categories for potential options—fully automatic, semi-automatic and manual.

“For fully-automated from the time the trucks come in until the fuel is burned in the boiler, it is never manually handled, other than maybe a truck tipper that has to be operated by the driver,” Saucier explains. “All of the storage is stacked out and reclaimed, and there is basically no mobile equipment needed.”

Facilities with lower capital budgets may not opt for fully-automated wood yards, he adds, but those located in cold, wet climates tend to go this route, due to the manpower required and efficiency losses that occur when operating facilties in harsh climates. “This scenario is very labor-conscious, and tends to be the route of large-scale facilities with a high throughput of biomass, or facilities in northern climates. Or, if you get into urban areas where dust or noise is a factor, where there are space constraints, or even just the visual effect, it might drive you toward an automated wood yard.

Everything is contained in silos or storage buildings that are more compact, whereas a manual wood yard tends to be spread out on a large acreage.”

Although they are smaller-scale, universities tend to go this route, according to Saucier. “We’ve done three university projects, and all were fully automated, and up north,” he says. “Being on campus, they’re landlocked and in that urban environment, they all had a tighter space, and university labor costs tend to be more expensive.”

On the opposite side of the spectrum is a fully manual wood yard, a labor-intensive option that might have as many as three or four pieces of mobile equipment. “From the time the trucks come in and dump fuel, it’s manually stacked up into piles, and then pulled back out of the piles to be reclaimed and sent to the boilers,” Saucier says. “Mostly, it’s guys out there doing the work. Conversely, this option is more popular at smaller mills that have lower throughputs. Anyone who can operate with one or two frontloaders will most often go this route. Smaller plants in the South do as well, if it’s a rural area, if labor is cheap, and weather isn’t a factor.

Finally, semi-automated is a middle-ground solution, Saucier says, that doesn’t lean heavily on operational or capital costs. “With semi-automated, you will see some automation in the stack-out or reclaim end of the fuel-feed chain, and this can keep your upfront investment and operational costs in check.”

Once a wood yard layout is selected, there are plenty of considerations for equipment selection. For pellet manufacturing operations sizing fiber on-site, the most important part of equipment is the chipper, according to Dave Evans, area sales manager at Andritz. 

Designs Dos, Don’ts
“We’re a firm proponent of making minichips, or some people call them microchips,” Evans says. “This is reducing the wood logs into small particles that can be dried immediately, instead of receiving bigger pulp mill chips, and having to reduce those further to get them to a size appropriate for the dryer,” Evans says, adding that Andritz’s HHQ mini chipper is a popular choice for its pellet manufacturing customers. “If you’re getting residual chips from local sawmills, chip ends, some logs, some dry material and some not, a real mishmash of material, it wouldn’t apply, but it does to mills being built with its raw material supply being logs.”

Bringing in logs instead of chips or sawdust may be a cost equation, as some wood is cheaper to buy in its whole form, but it may also be an economic decision, Evans says. “With a chipper, you have much bigger economies of scale, when you can produce your minichips directly from round wood, rather than buying individual truck loads from raw material from either an in-woods chipper or a sawmill. You will also always make a high-quality, uniform thickness and length material that will also dry uniformly, and thus give you the fiber you need at the lowest horsepower energy per ton. Smaller mills will buy residual materials, but a mill that’s built to produce 250,000 dry tons of pellets or more, they are looking at wood to supply their raw material needs.”

After a chipper, Evans says, a crane to handle the wood is an important component. “A crane can store a lot of wood in a relatively small area, and also provides a way to very evenly feed the raw material through the process so it’s chipped at an even rate.”

On the bioenergy facility side, the main equipment factor for consideration is not necessarily any particular component, but rather, how it’s designed, according to Justin Price, principal at Evergreen Engineering. “Your mass flow through the system—it’s critical in how it’s delivered, but also the volumetric flow through the system. Along with this is consideration of peak demand, average demand, and the difference between the two. If you design on the average, your average is great, but if you’re constantly getting the hot and the cold, you sort of miss the design. So don’t just design for average flow rate conditions; design for peak flow rates, and consider frequency and durations of those.”

For pellet facilities, Price says aside from the same raw material mass flow considerations for bioenergy facilities, what’s sometimes overlooked is handling of the actual pellet, once it’s made. “You really have to minimize impact to that pellet, because of durability and generation of fines,” he explains. “Big drops into silos, long transitions, those things can cause degradation of the wood pellets—a lot more dust and damage to product. More time should be spent on focusing on good transitions, and proper handling of the product.”

Improper transitions between conveyors and leakage, spillage over the belts, improper seals and inadequate loading zones are other design flaws Price sees at bioenergy facilities. “At pellet facilities, we see issues with the design of silos, storage bins and intermediate storage such as hoppers and bins—a lot of bridging issues. If you don’t get good material flow through those silos, or uniform flow, it will cause plugging, or rat holing. Those are the most common, but other issues that we see, though not as often, are wrong-sized screening applications and hammer mills, and a lack of understanding of feedstock size and variation.”

That might mean a producer specifies three-inch minus chips, but gets in softball-sized material. “Biomass fuel standards that have come out have come a long way in helping define that product, but there really has to be an understanding of where the material is coming from, potentials for size variation and lumps,” Price says. “In a wintery climate, it might be frozen wood, and you can get things as big as a football.

Saucier reiterates Price’s remarks. “Wood yards are often designed too light of duty for what’s perceived as an easier-to-handle, cleaner, better quality fuel that what you’ll really have,” he says. “A common mistake is to believe that when a supplier brings you an initial sample of material, that’s what you’ll be getting for the next 20 years. The most common thread in what we see in wood yards is that they’re underdesigned to handle difficult fuel that they will receive down the road—stringy, dirty, not as homogenous and doesn’t flow as well. In two years, you might have a very different fuel than when you started.

“So what’s the difference between an underdesigned fuel handling system, and an adequate one? “The difference is motor horsepower, the drive design, and general strength of all of the components—shafting, bearings, chains, belts, and sometimes, even the structural components of the system,” Saucier says. And the cost difference? It might be 20 to 40 percent of the overall job cost, according to Saucier. “If you’re talking fully automated, you could be talking millions more, and for totally manual, maybe the hundred thousands range.”

Specifically on underdesigning, Price says not to skimp on belt width and feed. “The rest of it tends to work itself out,” he says. “Screens are second—make sure you have the right type, and the right screen particle size separation.”

For those looking for improvements or upgrades to existing wood yards, there are some features to incorporate—not necessarily sweeping changes, but small, technological improvements that have made a big difference over time.

Innovation, Evolution
Wood yard and fuel handling technology has evolved over decades, Evans says, and most innovation is based off what already existed, such as the microchipper. “The pellet chipper, specific for pellet facilities, came around in the early 2000s, and that really was a development of a standard pulp chip-type chipper. Pulp chips might be three-fourths of an inch long. Mini chips are a third of that, so you’re looking at making quarter inch chips instead of three-fourths inch chips—it’s not revolution, it’s evolution in existing design.”

Stackout and reclaim systems have also been adapted from the pulp and paper industry, and “most have been out and around for 40 to 50 years,” Evans says, but small changes have made a difference. “New technology in screening for these chips might be something like an Andritz jet screen, which screens chips via an air knife,” he says. “It’s a relatively new process for wood chips, whether pulp mill or minichips—one of these is installed at Georgia Biomass.”

Price says he is seeing more belt monitoring systems for both biometric and flow rates and tension on the belt for wear. “There are new products coming out, and I’m seeing more focus on operators for cleaning and maintenance, which I think stems from having dust hazard analyses (DHA) conducted. Due to upcoming OSHA DHA requirements, we’re seeing more attention being paid to combustible dust. Even though the product might be 50 percent moisture or considered wet fuel, that material that does sit on the ledges and in the hoppers will dry out, and then become combustible dust.”

Conveyor technology systems are another bright spot, Price says. “Some manufacturers are doing a better job of understanding demands of the product, and building more robust machines,” he says. “I’m seeing a lot more use of en-mass conveyors, drag chains like Tramco’s, and there are companies doing a great job bringing new products to the market. Bruks is another—those guys have been doing conveyors for a long time, and they’re doing a really good job incorporating some unique designs into the features of en mass conveyors—a lot of them now have explosion panels built into them, and things like that.”

The bottom line is, wood yards and fuel handling systems should be designed to handle the lowest-quality fiber a plant might receive. “You might plan to have a minimum of three-inch material, and end up with smaller than one inch,” Evans adds. “So you’ll have waste or potential jam issues—stuff falls through onto the bark belt, and ends up jamming up your process. You plan that you’ll have the best, but you need to expect that you’ll get some of the worst.”

Author: Anna Simet
Editor, Biomass Magazine