Woodchips and sawdust


Woodchips are a medium-sized solid material made by cutting, or chipping, larger pieces of wood. Woodchips may be used as a biomasssolid fuel. They may also be used as an organic mulch in gardening, landscaping, restoration ecology and mushroom cultivation. According to the different chemical and mechanical properties of the masses, the wood logs are mostly peeled and the bark chips and the woodchips processed in different processes.

Woodchips are made in the process of woodchipping with a woodchipper.

different grades of wood chips. Author: Florian Gerlach (Nawaro)

different grades of wood chips

Woodchips as raw material

Woodchips are used primarily as a raw material for technical wood processing. In industry, processing of bark chips is often separated after peeling the logs due to different chemical properties.

Woodchips Sources

Possible Methods of Woodchip Conveyance

As mulch

Woodchips are frequently the byproduct of pruning of trees. When used as a mulch at least three inches thick, they conserve soil moisture, moderate soil temperature and suppress weed growth. In climates conducive to growing trees woodchips are readily produced, and may even be available for no additional cost beyond transport.

As fuel

Traditional use of woodchips is as a solid fuel for heating in buildings or in energy plants for generating electric power from renewable energy.

The newer fuel systems for heating use either woodchips or wood pellets. The advantage of woodchips is cost, the advantage of wood pellets is the controlled fuel value. The use of woodchips in automated heating systems, is based on a robust technology.

In a number of cases, coal power plants have been converted to run on woodchips. This is fairly straightforward to do, since they both use an identical steam turbineheat engine, and the cost of woodchip fuel is comparable to coal.

Solid biomass is an attractive fuel for addressing the concerns of the energy crisis and climate change, since the fuel is affordable, widely available, and is carbon neutral and sustainable as long as the crops are allowed to regrow. In most cases, biomass is not carbon neutral as wood is not regrown and the efficiency of biomass operations produce more pollutants than the processes they replace. Compared to coal and nuclear fuels, woodchip biomass does not have waste disposal issues, since wood ash can be used directly as a mineral-rich plant fertilizer. The fertilizer argument is misleading, however, since the minerals are removed from the fertile fields where the trees grow. If the minerals are not returned, the yields from these fields decrease and therefore more greenhouse gas is produced.

Automated Handling of Solid Fuel

Unlike the smooth, uniform shape of manufactured wood pellets, woodchip sizes vary and are often mixed with twigs and sawdust. This mixture has a higher probability of jamming in small feed mechanisms. Thus, sooner or later, one or more jams is likely to occur. This reduces the reliability of the system, as well as increasing maintenance costs. Despite what some pellet stove manufacturers may say, researchers whom are experienced with woodchips, say they are not compatible with the 2 inch (5 cm) auger used in pellet stoves.[1]

Micro Combined heat and power

Wood is occasionally used to power engines, such as steam engines, Stirling engines, and Otto engines running on woodgas. As of 2008, these systems are rare, but as technology and the need for it develops, it is likely to be more common in the future. For the time being, wood can be increasingly used for heating applications. This will reduce the demand for heating oil, and thereby allow a greater percentage of fuel oil to be used for applications such as internal combustion engines, which are less compatible with wood based fuel and other solid biomass fuels. Heating applications generally do not require refined or processed fuels, which are almost always more expensive.

Comparison to other Fuels

Woodchips are less expensive than wood pellets. Also, woodchips are theoretically more energy efficient than pellets, because less energy is required for manufacturing, processing, and transportation; however, this assumes that they are consumed in an appropriately designed burner, and as of 2008, these are mostly only available in large systems designed for commercial/institutional use. In contrast to the lack of residential systems, commercial heating installations have been very successful in terms of performance, cost, reliability, and efficiency. [2]

Woodchips are also less expensive than cordwood, because the harvesting is faster and more highly automated. Also there is a greater supply, partly because all parts of a tree can be chipped, whereas small limbs and branches can require too much labor to be worth converting to cord wood. Woodchips are similar to wood pellet, in that the movement and handling is more amenable to automation than cord wood, particularly for smaller systems. Cordwood generally needs to be "seasoned" or "dry" before it can be burned cleanly and efficiently. On the other hand, woodchip systems are typically designed to cleanly and efficiently burn "green chips" with very high moisture content of 43-47% (wet basis).[1] (see gasification and woodgas)

Environmental Issues

If woodchips are harvested through sustainable forestry practices, then this is considered a source of renewable energy. On the other hand, it is clear that some harvesting practices, such as clearcutting large areas, are often highly damaging to forestecosystems.Theoretically, whole-tree chip harvesting does not have as high a solar energy efficiency, as compared to short rotation coppice; however, it can be an energy-efficient and low-cost method of harvesting. In some cases this practice may be controversial when whole-tree harvesting may often associated with clear cutting, and perhaps other questionable forestry practices.

Woodchips for waste processing

Bark chips, not woodchips, are used as a bulking agent in industrial composting of municipal biodegradeable waste, particularly biosolids.Woodchip biomass does not have the waste disposal issues of coal and nuclear power, since wood ash can be used directly as a mineral-rich plant fertilizer.

Forest Fire Prevention

Woodchip harvesting can be used in concert with creating man made firebreaks, which are used as barriers to the spread of wildfire. Undergrowth coppice is ideal for chipping, and larger trees may be left in place to shade the forest floor and reduce the rate of fuel accumulation.

Market Products, Supply and Demand

Currently, domestic or residential sized systems are not available in products for sale on the general market. Homemade devices have been produced, that are small-scale, clean-burning, and efficient for woodchip fuels. Much of the research activity to date, has consisted of small budget projects that are self-funded. The majority of funding for energy research has been for liquid biofuels.

Woodchip prices in the United States

"Wood chip costs usually depend on such factors as the distance from the point of delivery, the type of material (such as bark, sawmill residue or whole-tree chips), demand by other markets and how the wood fuel is transported. Chips delivered directly to the (powerplant) station by truck are less expensive than those delivered ... and shipped by railcar. The range of prices is typically between $18 to $30 per (wet)-ton delivered."[3]In 2006, prices were $15 and $30 per wet-ton in the northeast. [4]In the 20 years leading up to 2008, prices have fluctuated between $60-70/oven-dry metric ton (odmt) in the southern states, and between $60/odmt and $160/odmt in the Northwest. [5]

European Perspective

In several well wooded European countries (e.g. Austria, Finland, Germany, Sweden) wood chips are becoming an alternative fuel for family homes and larger buildings due to the abundant availability of wood chips, which result in low fuel costs. The European Union is promoting wood chips for energy production in the EU Forest action plan 2007-2011 [6].

See also
  1. ^ abcVTHR Green wood Chip Furnace
  2. ^US Dept. of Energy report,... Fuel Handling Equipment
  3. ^Woodchip price factors for a power Generating Station in Burlington, VT, US
  4. ^Vermont Heat Research - An Experimental Wood Chip Furnace
  5. ^First quarter wood chip costs up almost 50% in western US, but pulpmills in the US South experienced only small upward price adjustments
  6. ^[1]

External links


Sawdust is a by-product of cutting lumber with a saw, composed of fine particles of wood. It can present a hazard in manufacturing industries, especially in terms of its flammability.

Sawdust made with hand saw. Author: Rasbak

Sawdust made with hand saw

Sawdust has a variety of practical uses, including serving as a mulch, as an alternative to clay cat litter, as a fuel, or for the manufacture of particleboard. Until the advent of refrigeration, it was often used in icehouses to keep ice frozen during the summer. It has been used in artistic displays, and as scatter. It is also sometimes used to soak up liquid spills, allowing the spill to be easily collected or swept aside. It is used to make Cutler's resin. Mixed with water and frozen, it forms pykrete, a slow-melting, much stronger form of ice.

Science of sawdust

The main by-product of sawmills, unless reprocessed into particleboard, burned in a sawdust burner or used to make heat for other milling operations, sawdust may collect in piles and add harmful leachates into local water systems, creating an environmental hazard. This has placed small sawyers and environmental agencies in a deadlock.

Questions about the science behind the determination of sawdust being an environmental hazard remain for sawmill operators (though this is mainly with finer particles), who compare wood residuals to dead trees in a forest. Technical advisors have reviewed some of the environmental studies, but say most lack standardized methodology or evidence of a direct impact on wildlife. They don’t take into account large drainage areas, so the amount of material that is getting into the water from the site in relation to the total drainage area is minuscule.

Other scientists have a different view, saying the "dilution is the solution to pollution" argument is no longer accepted in environmental science. The decomposition of a tree in a forest is similar to the impact of sawdust, but the difference is of scale. Sawmills may be storing thousands of cubic metres of wood residues in one place, so the issue becomes one of concentration.

Water-borne bacteria digest organic material in leachate, but use up much of the available oxygen. This high "biological oxygen demand" can suffocate fish and other organisms. There is an equally detrimental effect on beneficial bacteria, so it is not at all advisable to use sawdust within home aquariums, as was once done by hobbyists seeking to save some expense on activated charcoal.

But of larger concern are substances such as lignins and fatty acids that protect trees from predators while they are alive, but can leach into water and poison wildlife. Those types of things remain in the tree and, as the tree decays, they slowly get broken down. But when sawyers are processing a large volume of wood and large concentrations of these materials get out into the runoff, they cause toxicity and are toxic to a broad range of organisms.[1]

Ogatan, Japanese charcoal briquettes made from sawdust. Author: DryPot

See also

Dust collection system

  1. ^Canadian Geographic Online

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