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Watching Wood Dry
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Adequately drying wood chip fuel is an important component of maximizing heat and power production, and techniques vary widely.
Moisture content in wood chips used for energy can significantly influence system performance. Consequently, equipment and methods used to dry wood chip fuel are constantly evolving, as companies strive to perfect their respective processes, meet the needs of system operators and maximize plant efficiency. Drying methods today include simple, passive drying techniques to the advanced and energy-intensive methods.
Adam Sherman, executive director of the Vermont-based Biomass Energy Resource Center, says that moisture content in wood fuels is a universal problem, and over the past couple of years, the trend has been importing European boilers to help with the job. “In order to meet the efficiency and emissions standards that get more stringent each year, fuel quality becomes more important, and moisture content is really a big deal for efficiency because of latent heat loss, and also because the moisture vapor is often times a carrier for ultrafine particulates from an emissions standpoint,” he says. “We’re seeing some producers of wood chips realize there is an emerging market where a drier chip is desired and has value.”
Sophia Ren, a wood pellet plant consultant for Azeus Pellet Mill, manufacturer of pellet machines for biomass, says drying methods can run the gamut from using forced ambient air and residual heat from electricity generation to using larger-scale dryers. “Drying is done through various methods, including passive evaporation from airflow and from active heating,” she says. “Air drying will occur if the ambient air passing through has a lower relative humidity than the wood chips. Evaporation of water can also be achieved through heating, generally with forced hot air. Wood chip piles can also self-dry, where compacted piles reach temperatures above 40 degrees Celsius (104 degrees Fahrenheit) through microbial action.”
Drying wood increases combustion value and lowers storage costs, as wood chips have less sensitivity to natural harmful effects such as microbiological degradation. “Dried wood chips will perform better,” Ren says. “If the chips are too wet, it may be impossible to even keep the flame lit. With dry chips, the flame burns hotter and more evenly. Also, a smaller quantity of ash is produced, reducing the cost of ash disposal.”
For wood chips with a moisture content (MC) of 45 percent, the maximum boiler efficiency with standard equipment is about 74 percent. If the same stand and equipment is burning dry wood (10 to 15 percent MC), the efficiency can be as high as 80 percent. The BioMax 15, one of the latest projects made by TMU, the USDA Forest Product Laboratory Technology Marking Unit, is a unique example of a system that uses residual heat to dry fuel. The prototype combined-heat-and-power system was developed by the Community Power Corp. and operates by feeding wood chips with MC up to 25 percent from a hopper to a conveyor belt. “The conveyor belt moves the chips though a dryer, which is heated by excess heat from the internal combustion engine,” Ren says. “After the chips dry to 15 percent moisture content, they are fed into the gasification hopper. The chips flow downward through the gasifer (operating at 1,472 F). Airflow through the wood chips is limited, so they are combusted under starved-oxygen conditions.”
Mark Froling, president of Froling Energy, says the company dries its screened chips in a batch process utilizing a custom kiln that circulates hot air through the green wood chip pile until the moisture is removed. The company installed nearly a dozen biomass boilers capable of reaching a half million Btu or more, which have been engineered to accommodate burning either wood pellets (5 to 10 percent moisture) or PDCs (20 to 30 percent moisture). “We can dry about 40 tons of green material from 50 percent down to 25 percent in about 48 hours,” he says. “Others utilize a continuous process with a rotary drum dryer, fluidized bed dryer or belt dryer.”
This drying process raises the net usable energy content of the material from about 8 MMBtu to about 12 MMBtu per ton. It also provides a longer storage life for the material as degradation comes into play—for green chips, after about four months, rot and mildew issues are concerns. “Furthermore, removal of the water makes transportation a bit easier during cold weather, as the chips won’t freeze as quickly to the truck body during long transport, or when trucks need to be parked overnight,” Froling says. “The same is true for the chip storage. The drier the chips, the less clumping will occur during chip storage (freezing of chips in uninsulated bins) and the better the fuel will flow through the material handling systems.”
In recent years, Froling says he has noticed that some of the larger boilers utilized for wood pellets are also able to burn dry wood chip materials. After exploring various ways of manufacturing these dry chips, the company settled on its current process and started production of such chips. “The economics are in favor of the dry chips by almost 30 percent once you get to a certain scale, but in some cases, the existing infrastructure or logistics still point to using pellet boilers,” he says. “It is very convenient that these boilers are dual fuel boilers. It gives our customers some sense of security from a supply chain standpoint.”
For larger-scale wood chip dryers, different from the simple and traditional ways of drying, Ren says industrial production tends to choose newer dryers that make the production process more mechanized and efficient. “Larger-scale dryers are expensive and commonly run using waster or byproduct heat in larger installations,” she says. “Dryer types used in drying biomass fuels include rotary, conveyor, cascade and flash dryers.”
Rotary dryers have been used for a long time in drying biomass fuels and are the most common dryer type in the existing large-scale bioenergy plants. “The advantages of rotary dryers are they are less sensitive to particle size and can accept the hottest flue gas of any type of dryer,” Ren explains. “They also have low maintenance costs and the greatest capacity of any type of dryer. However, the material moisture is hard to control in rotary dryers, due to the long lag time for material in the dryer.”
Conveyor dryers can handle a wide range of materials, making them attractive for biomass feedstocks. “The uniformity of drying using this method is strong, due to the shallow depth of material on the belt,” Ren says. “The advantages are that they are better suited to take advantage of waste heat recovery opportunities, since they operate at lower temperatures than rotary dryers. The lower temperature also implies less of a fire hazard and lower emission of volatile organic compounds.
Flash dryers, usually combined with a cyclone, are normally used for small particles, and its gas stream velocity must be higher than the free fall velocity, according to Ren “For wet or sticky materials, some of the dry material can be recycled back and mixed with the incoming wet material to improve handling. Meanwhile, the recirculation of the materials can also shorten the drying time. The main advantages are that it can dry thermolabile materials, due to the short contact time and parallel flow, and that the capital and maintenance costs are low.”
Cascade or sprouted dryers are used extensively in Nordic countries. “This method operates at intermediate temperatures between those of rotary and conveyor dryers and have a smaller coverage area,” Ren continues.
Bertil Stromberg, vice president of biofuels at Andritz Inc., which touts its belt drying system, says by reducing the biomass water content to 10 to 15 percent, its calorific value is increased from 2 kilowatt hours (kWh) per kilogram (kg) to approximately 4.5 kWh/kg. This cuts transport and storage costs and creates ideal conditions for direct firing or optimum pelletizing properties, both for industrial and high-grade wood pellets. As a result of the drying process, less fuel input is required to generate energy, which also reduces the pollutant emissions caused by the combustion system.
Other examples of larger-scale dryers on the market include Saimatec’s vertical silo type dryer, and STELA Laxhuber GmbH’s low-temperature belt dryer.
Sherman says that a number of U.S. firms are looking at more passive drying methods, such as letting wood sit outside and dry before it’s chipped. Some places in Europe use a grapple head and pincher, so as the wood is loaded off a truck, it’s split lengthwise so there’s more surface for drying.
While this method can be used in the western U.S. and in states like Montana where sitting outdoors for a month or two is no problem, it doesn’t work in the Northeast.
Charles A. Levesque, president of Innovative Natural Resource Solutions LLC in Antrim, New Hampshire, says the company did research for a number of years on the best drying solutions and determined that the cost of some of the high-tech rotary and conveyor dryers couldn’t be justified, so they are now air-drying feedstock before its chipped.
“We have researched the best stacking methods and species to understand what it takes to get something down to the moisture content we are looking for—which is sub-30 percent,” he says. “We do thousands of tons and it takes a lot of preplanning, but it can be done, and done well. We know if we ever get to the point of working with tens of thousands of tons, we will need some conventional drying, but this helps us reduce cost per unit of wood on a Btu basis.”
Sherman says there’s also been an increase in demand around kiln-dried firewood, especially in the Northeast U.S.
Austrian company Cona-Solar is being championed for a low-tech method of predrying chips, Sherman notes. It works by blowing warm air to the Trocknungsbox, where the wood chips are dried on pitched and flat grates that have a drying capacity approximately 10,000 cubic meters. Through the construction of an underground heat storage stone, solar energy can also be used when the sun is not shining.
Cona-Solar’s solar method currently have 49 chip-drying systems in Austria, with a total solar area of 4000m² and can dry approximately 80,000 cubic meters of wood chips per year, removing nearly 14 million liters of water from wood chips. “They have installations all around the world (in 17 countries), none in the U.S., but it’s a simple solution that people could deploy here,” Sherman says.
He adds that the years ahead should bring out plenty more wood drying innovation for the wood chip and pellet industry.