Surveying Storage Solutions

Selecting the best system for storing woody biomass for power or heat generation is not a new challenge, but as renewable energy installations increase, so is interest in weighing different options.
By Keith Loria | September 01, 2014

Selecting the best system for storing woody biomass for power or heat generation is not a new challenge, but as renewable energy installations increase, so is interest in weighing different options.
The effects wood material characteristics have on a pile in storage include absorption, heat buildup, dissipation and more. The number of fines in a respective fuel pile influences how much water is absorbed, how the pile heats up and how air flows through a pile.

Isaac Slaven, applied engineering technology faculty in the School of Technology at Eastern Illinois University, says the easiest and best method to identify best storage practices is first-in, first-out (FIFO) inventory management for the same particle size and shape, material, and moisture content. “This way, the material loss from fungal and bacterial infestation is reduced by not allowing as much time for these infestations to occur,” he says. “Additionally, any effect from contaminates is reduced. For a company that has a variety of materials, moisture contents, and particle sizes and shapes, known (or tested) rates of degradation based upon these factors may require a combined schedule to reduce overall material and financial loss can be optimally reduced. Furthermore, the seasonality of some materials (such as corn stover or switchgrass) may complicate this management.”

Since there are so many identifiable variables that can cause chip pile fire, it is difficult to single out only a few things, but Slaven says it is safe to say: Keep it as dry as feasible, keep it as free of contaminates as possible, and move it though inventory as quickly as possible. In a paper he wrote at Purdue University, “Properties of Wood Waste Stored for Energy Production,” Slaven notes that how chips deteriorate in wood chip piles is not completely understood, but the observed changes in the chips and chip piles are dry matter weight loss, temperature changes, moisture content, heat of combustion, and pH. 

There is no single, easy formula for designing an appropriate storage solution. While most fuel yards share many common elements, some tailoring is almost always required because of differences in fuel type, fuel quality, energy plant operating requirements and local harvesting methods/practices.“Although most larger plants in North America rely on wood chips for fuel, smaller heat or CHP projects in the U.S. and a variety of plants in Europe rely on wood pellets,” says Chris Lindsey, associate principal at the Antares Group, headquartered in Lanham, Maryland. “Fuel yard considerations are very different between these fuels. Some plants rely on very custom yards because of specialty fuels, such as industrial processing residues, material recovered from construction and demolition streams, that they may be using in conjunction with clean wood chips.”

A Sound Strategy

Industry insiders note that having a fuel storage and inventory management strategy can be a prerequisite to project financing, and therefore must be carefully considered even prior to final design. “Deciding how large the storage should be is primarily, but not necessarily entirely, a factor of the size of the energy project being supplied and local climate conditions,” Lindsey says. “Plants usually shoot to have two to four weeks of fuel on hand to help get through periods when suppliers may have trouble delivering fuel in harsh or muddy weather. Smaller heating plants or smaller pellet fuelled facilities may play their supplies tighter, it just depends on how sensitive they are to interruptions due to temporary supply shortages.”

It is typical for larger plants to rely on piles that are actively managed with  loaders, dozers to minimize fire hazards and maintain fuel quality. According to Lindsey, FIFO approaches are normal. Pile reclaim may be done automatically, (under pad reclaim) or manually using loaders and reclaim chutes and hoppers. “It is common for storage to include day silos or bins (live storage) in conjunction with piles,” he says. “The silos or bins allow for one to two days of live storage that can be used to reliably feed the energy conversion unit regardless of upstream issues and allows for yard operations to be shut down for maintenance or operating personnel days off. At smaller, institutional heating plants, conveyance may be simpler and include at-grade or below-grade fuel bunkers, which may be primarily indoors.”

 Typical elements to be designed into the system are yard truck traffic control—sufficient area for truck delivery and turnaround, truck receiving and unloading, either by tippers or walking floor truck, and incorporating scales, initial conveyance to classification stand—sorting for size and debris, resizing via milling, rejection—and conveyance to pile storage or live storage. Systems will also include tramp metal removal and real-time weighing at various points to determine flows to storage or, at a minimum, the amount of material leaving live storage to the energy conversion system, usually a boiler. Fuel sampling stations may also be included.

The View From Alaska

Amanda Byrd, biomass specialist for the Alaska Center for Energy and Power at the University of Alaska, says the best storage systems have to be covered out of the rain and snow, and have good ventilation for drying, or a drying bin. Keeping the wood off the ground is also important for air circulation and reducing rocks and other unwanted debris. “Having a day bin is common practice, and in some places it is a ‘week bin’ holding enough fuel for a week. Sometimes the wood is chipped at the plant site, and other times it is chipped where felled and the chips transported,” she says. “We have had some of our biggest wood storage issues happen in winter. For instance, a school in Southeast Alaska received wood chips from a local mill, but the chips were wet and when stored turned to blocks of ice. They since installed a drying bunker for the chips to dry out before being augured into the boiler.”

In Alaska, projects are often very diverse, and one solution is not going to be the best fit for everyone, she adds. “We are still in our development stages in Alaska. Often times to keep the costs down, we must use what is available and affordable. So, using a rarely used ice-hockey rink to store wood chips can be the easiest solution at the time,” Byrd says. “We have economies of scale issue in Alaska—our needs are quite small, and building large amounts of infrastructure often is not economically feasible.”

Smaller Solutions

For smaller systems and institutional settings, where the owner or the person who’s operating the system wants to spend as little time as possible messing with the fuel storage, Timothy Maker, president and CEO of Community Biomass Systems Inc. in Montpelier, Vermont, says “unquestionably, a below-grade bin is the way to go. A concrete bin that can hold at least one-and-a-third tractor-trailer loads of fuel. The bigger the facility, the more storage you want to have,” he says. “The person who is in charge of the system doesn’t want to be worrying about running out of fuel when it’s cold. They want to know they’ve got a full bin and they’ve got a few days of elbow room.”

He adds for indoor storage, the storage facility should not be heated and it should have plenty of ventilation. “The reason why you don’t want it heated is that in cold weather you don’t want to make it more likely that you’re going to have moisture leaving the fuel and filling up the air,” Maker says. “So if the space is well-ventilated when the moisture goes off from the fuel, when it evaporates into the air from the fuel, then it gets out instead of being stuck inside this bin building.”

Challenges Abound

When dealing with storage solutions, climate is a very important consideration, especially places prone to severe icing or snow. “Snow removal must be a factor in the yard traffic design and icing must be considered on any inclines. This includes truck ramps and even conveyor angles,” Lindsey says. “In the case of the latter, even relatively modest slopes can be problematic at times as conveyor belts will harden up and material may slide back into unwanted places. Lower conveyor angles mean more linear length and higher cost, so there is a balance to be struck. Covered conveyors are also a good idea in places with lots of annual precipitation.”

The polar vortex this past year wreaked havoc at many plants simply because the cold weather pushed conditions beyond design and made it a real challenge for some operations to maintain output. This was not only true of the equipment design, but of maintenance.

 For most biomass fuels, fuel dust is a nuisance factor that can be mitigated, but usually not completely eliminated. Provisions in the design for dealing with dust removal and cleanup of fines throughout the fuel handling and processing system should be considered. This includes clean-out ports in strategic locations and enclosures to protect systems that are sensitive to dust. 

Fuel consistency and quality is always a critical factor in operating a plant. Pelletized fuels have an advantage in this regard. Some plants put in multiple bins or silos to allow fuel blending to maintain desired characteristics.

Final Thoughts

Plan on the system requiring fairly regular maintenance and be sure that multiple redundancies are built into the system to deal with unexpected failures in the reclaim/conveyor systems. “From my experience, the best systems are the ones where a lot of care was taken during the design process to involve the operator of the system,” Maker says, “so the operator knows what they’re getting. They not only get a chance to talk to the system supplier, but they’re actually working with the system supplier during the installation of that equipment, so that they know how to operate it, they know how to troubleshoot it. If all of that is done carefully during the design and the early stages of implementation, when it’s actually being constructed, then you get a smooth project.”

Lindsey adds that fuel moisture in most large plants in North America is managed by fuel spec and pile management. Volatile organic compounds do evolve from piles and can change fuel quality over time. “Preferably, wood residues stored outside and destined for sustainable energy production should have adequate airflow and should be protected from the rain and snow; however, the costs of protection may be prohibitive,” he says. “In a world of increasing energy demands, proper management and rapid inventory turnover can help make wood a part of a renewable and sustainable energy portfolio.”

Author: Keith Loria
Freelance writer, Biomass Magazine