Even the most modern sawmills are hard pressed to turn half the volume of a log into lumber, creating huge quantities of waste wood. Depending on the geographic location of the mill there is a wide range of opportunities for utilising this waste, including pulp for use in paper production and energy recovery.
by Jack Lutz
In the 19th century, sawmills - like almost everything else in the world - were steam-powered. They generated this steam by burning their waste wood, but generated more waste than they needed for their steam boilers. The mills piled some of it but in North America much of the excess was burned in tall, conical burners known as teepee or wigwam burners in the U.S. and beehive burners in Canada.
In the early 20th century North American mills began converting to electrically powered machinery and installing kilns to dry their lumber. While some of the wood that had produced steam for power was now burned to produce steam to heat the kilns, increasing quantities of waste were being burned in the teepee burners.
In a bid to better use this material, pulp mills were built in western North America and the larger solid pieces of waste wood were chipped and processed into pulp. For most of the 20th century those pulp mills were furnished almost entirely with sawmill chips.
However, sawmill capacity was reduced as the U.S. National Forests were closed to timber harvesting in the 1990s. Add to this an increase in the use of smaller, second growth timber and as a result, western pulp mills started using roundwood pulpwood.
Additionally, the clean air laws introduced in the 1970s severely restricted the ability of sawmills to burn wood waste and resulted in an intensive effort to find other outlets for these materials. Sawmills in other parts of North America also had to develop markets for their residuals. Pulp mills in the East, which had historically purchased small logs that they chipped themselves, added equipment to handle sawmill chips.
The history of sawmill waste utilisation in other parts of the world is similar to that of North America. In the initial stage of developing a lumber industry, sawmill residuals are piled or burned (or sold in local markets for fuel or animal bedding). Some products (e.g., chips) require mills of a minimum size to produce because the cost of the equipment needed to produce pulp-quality chips requires a sufficient volume of material to justify the investment.
There is also no sense in installing chippers to make pulp-quality chips unless there are pulp mills in the area that can pay a good price for them. When the lumber industry is large enough to produce significant volumes of these materials, facilities that consume them will be built.
Residue values vary greatly around the world. It is estimated that about 15% of a lumber mill's revenues are from residues. There are both industrial and retail markets for residues.Fuel for a co-generation plant at Aurora Forestal sawmill in Uruguay
Chips for pulp, panels and residues for wood pellets and hog fuel move through industrial markets. Bark, hog fuel and shavings can be sold through industrial markets for fuel or retail markets for mulch and animal bedding. These are often sold bagged in garden centres or farm supply stores or by the pick-up truck load at the mill.
Pulp-quality chips usually bring the highest prices, so lumber mills located near pulp mills will usually realise higher revenues for their residues. Lumber mills without access to chip markets will earn less from their residues.
Prices for sawmill chips vary with location and market conditions. Since pulp markets and lumber markets are not perfectly correlated, the supply of chips and the demand for them is often not in equilibrium. When lumber prices are high, sawmills will squeeze every piece of lumber out of a log that they can, which reduces the volume of chips they produce.
When lumber prices are low and chip prices are high, mills may send some of the material that would make low quality lumber to the chipper. Very poor lumber markets result in sawmills eliminating shifts or the number of days of operation, lowering lumber (and chip) production. This pushes chip prices up and keeps some sawmills operating when lumber prices alone would suggest the mills should close. This is important in some rural areas where a sawmill is the primary employer and closing it would cause significant problems in the local economy.Product Quality
Large, high-volume sawmills can afford to install machinery that allows them to increase their production of lumber and the quality of the by-products produced. Yet even small sawmills may not be able to take full advantage of local markets for certain products even if they do exist.
There are small sawmills all over the world. These mills usually do not remove the bark before sawing their logs, so their slabs and edgings still have the bark attached and chipping this material would produce inferior-quality chips that most pulp mills would not want to buy. They also do not produce enough solid wood waste to justify the cost of installing chipping equipment.
Small sawmills often saw a variety of species, which may also be a problem for a pulp mill, especially if the sawmill saws both hardwoods and softwoods. So small sawmills may not be able to take advantage of the pulp chip markets even if they are located within delivery distance of a pulp mill. They have to dispose of their waste in other ways.Case Studies
The value that can be obtained from sawmill waste depends on local markets and the sawmill itself. Markets also change over time as new uses are developed for the by-products. The differences (and changes) in markets across the world can be illustrated by looking at timber processing investments held by the Phaunos Timber Fund (PTF) - a Guernsey based investment fund traded on the London Stock Exchange. PTF has timber-related investments in nine countries across six continents, including four sawmills and two pole plants - which turn long logs into utility poles.
Located in Oregon, PTF's GTFF investment includes a sawmill that processes hybrid poplars grown on the investment. The mill is located in an area with a highly developed economy and a highly developed forest products industry, so it has well-developed markets for all of its by-products. Clean chips from the GTFF sawmill are currently sold to four pulp mills in Oregon and Washington. Some sawdust is sold to two other pulp mills and the remainder of the sawdust is sold for animal bedding. Bark and coarsely-ground solid wood waste are processed into hog fuel. Three of the six pulp mills buy that hog fuel to burn in their boilers for heat and power. Additional hog fuel is sold for animal bedding.
Another PTF investment located in northern Uruguay, Aurora Forestal (AF), includes a medium-sized sawmill which saws pine grown on its timberland. The forest products industry is less developed in Uruguay than in Oregon, so it has fewer markets for some of its by-products. While the pulp industry in Uruguay is expanding, the AF facility is located too far from the new pulp mill on the border between Uruguay and Argentina for AF to make and transport chips to that mill.
This sawmill has been burning its waste wood to generate steam for its dry kilns, but that has not kept up with the volumes produced and has had to pile the waste.
To better cope with the situation the site has recently built a combined heat and power (CHP) plant, which burns wood residues to produce steam for the kilns and electricity that is both used on-site and sold into Uruguay's power grid. This CHP plant will consume the vast majority of wood waste of all types produced by the sawmill.
"The biomass cogeneration plant is the perfect complement to Aurora Forestal's activities, leveraging synergies within its forest industrial complex. In addition, it helps diversify markets by meeting the growing demand for power in Uruguay," comments Helizander Brecailo, director of Investments and Acquisitions at FourWinds Capital Management.
Meanwhile, in Africa PTF's Green Resources AS (GRAS) investment operates two sawmills and a pole plant in Tanzania and a pole plant in Uganda. The sawmills in Tanzania process softwoods and the pole plants process eucalyptus. Some of the slabs are chipped and sold to the local pulp mill but the mills do not have the capacity to chip all the slabs. Some of the unchipped slabs are reprocessed into short pieces for pallets or sold into the local market for fencing or other similar uses.
The sawmills currently have no markets for their sawdust, so it is burned but GRAS is looking for alternative uses for this material. Shavings are sold to chicken farms for bedding. Bark at the mills in Tanzania is ground and combined with rice husks to produce growing medium. At the pole plant in Uganda the bark is removed in the forest, where it decomposes and provides nutrients for the next tree crop.
The GRAS facilities operate in a part of the world where charcoal is a commonly used industrial and domestic fuel. The ends trimmed off the logs are processed into charcoal. Prior to 2012, the waste and trimmings from the pole plant in Uganda were sold to a nearby panel plant that either chipped them to make particleboard or burned them to make steam.
"Not only does charcoal production from waste wood provide a significant revenue to Green Resources, it also alleviates pressure on native forests and provides a cleaner source of fuel for the rural poor," explains Kristen Kleiman, director of Investments and Acquisitions at FourWinds Capital Management.Summary
Only around half of a log gets turned into lumber at a sawmill. The 'waste' from the process is utilised to produce a wide range of materials used for pulp, panels, pellets and energy production. The markets for these products and their value depend largely on where the sawmill is located and the condition of the economy and forest products industry in the area.
As new technologies are developed the possibilities for using this resource have increased. The by-products of the timber industry which were once simply burned for convenience have the potential to not only increase the sustainability of the industry but to boost its bottom line.
Jack Lutz, PhD. is a forest economist at Forest Research Group and consultant to FourWinds Capital Management.
This article is on-line.Sawmill Waste Explained
Bark is removed from the log before it is sawn into lumber. It has a number of uses, such as in the production of Hog fuel, where it is combined with other waste products and coarsely ground ready either to be burned for heat, or used as animal bedding. Bark can also be mixed with other materials such as sawdust to produce wood pellets. However pellets with bark added make more smoke than those without. Another common use for bark is as landscape mulch.
Sawdust is created as individual boards are sawn from the log. Sawmills may use circular saws or band saws. Circular saw blades are thicker than band saw blades and thus create more sawdust with each cut. As with bark, uses for sawdust include hog fuel, wood pellets and animal bedding. It is also occasionally used by pulp mills.
Solid pieces of residues such as offcuts and chunks are created at several steps in the lumber production process:
These extra centimeters may be trimmed from the log before it enters the sawmill, or they may be trimmed off the individual boards at the trimmer. Logs may also be delivered tree-length. In this case, the small end of the logs may be too small to make lumber, so they are cut off and sent to the chipper.Slabs are the round parts of the log that are sawn off the outside as lumber is being produced.Edgings are the round parts on the side
of many boards as they come off the head rig (main saw). The edging process produces boards of standard width and a narrow strip of waste wood with a rounded side.Trim is wood cut off the ends of individual boards. The trimming process produces boards of standard length and in some cases can create a more valuable board by removing low-quality material from the end.
One of the main uses for offcuts in areas with pulp mills or panel plants is as chips. Chips are usually produced to tight size specifications and are more uniform and of higher value than hog fuel. As the offcuts are chipped, some of the resulting material will be too small (fines) or too large (overs) to meet the chip specifications. These fines and overs are sent to one of the other byproduct lines for further processing.
Another use for offcuts is in the production of charcoal. In some developing economies, chucks are processed into charcoal for local markets where it is used for domestic cooking and heating. Offcuts are also used to produce hog fuel and pellets.
Shavings are produced when dried lumber is smoothed and shaped into its final form in the planer mill. Shavings differ from other sawmill wastes in that they are produced from dried lumber and have a higher energy content.
Other materials must usually be dried before they can be burned or further processed.
These shavings are often used as fuel for dry kiln boilers, as animal bedding and in the production of wood pellets.