Page 395 - Solid Waste Analysis and Minimization a Systems Approach

POTENTIAL TECHNOLOGIES AND STRATEGIES 373

rusty, steel conical sawdust burners have for the most part vanished, as the sawdust
and other mill waste is now processed into particleboard and related products, or used
to heat wood-drying kilns. Cogeneration facilities will produce power for the opera-
tion and may also feed superfluous energy onto the grid. While the bark may be
ground for landscaping bark mulch, it may also be burned for heat. Sawdust may make
particleboard or be pressed into wood pellets for pellet stoves. The larger pieces of
wood that won’t make lumber are chipped into wood chips and provide a source of
supply for paper mills. Wood by-products of the mills will also make oriented strand
board paneling for building construction, a cheaper alternative to plywood for panel-
ing. In the United States annual cut of timber for lumber products is equivalent to
approximately 240 million trees. However, if the sawmills operated at a 70 percent
recovery efficiency, the same annual harvest of lumber could be derived from 171 mil-
lion trees. The saving would be the equivalent of 69 million trees annually if recovery
efficiency improved from 50 to 70 percent in the primary processing industry. In addi-
tion, these same 69 million trees, if permitted to grow in the forest, would continue to
absorb about 900,000 tons of carbon dioxide and produce about 650,000 tons of oxygen
each year.
Following is a brief case study describing “thin kerf” sawing as related to improv-
ing process efficiency. Today, a typical circular sawmill converts 50 percent of the
log into primary product with band mill conversion at about 57 percent. Saw kerf
averages 21 percent for the circular sawmill and as low as 12 percent for high pro-
duction band mills. Obviously, sawmill efficiencies have increased since the 1920s
but there is still much room for improvement. In addition, trends such as environ-
mental constraints on timber harvesting, smaller logs from the forest, and an
increased demand for wood products makes it imperative that we improve sawmill
efficiencies.
The good news is that technologies currently exist that can enable 70 percent or
more conversion efficiencies at sawmills. However, the adoption of new technologies
such as thin kerf sawing have not yet come state-of-the-art in many mills. A saw kerf
is the width of the path cut by the sawteeth as the saw blade moves through the log.
Thin kerf is a relative term, however, because it only has meaning when compared to
something else. If circular saws are compared to band saws, then band saws would be
considered thin kerf since they are generally 50 percent thinner than circular saws. If
today’s saw kerfs are compared with those of the past, we can generally say that today
we have thinner kerfs.
The saw kerf has a significant impact on conversion efficiency, often referred to as
lumber recovery. A crude but effective way of calculating the amount of sawdust that
develops during sawing is to determine the total wood usage per pass, the action of
logs moving or passing back and forth through the saw blade. Wood usage per pass
includes the average thickness of the piece being sawn plus the saw kerf. For exam-
ple, in cutting a board that is 1.125 in thick with a saw kerf of 0.300 in, the total wood
usage per pass is 1.425 in. Calculating the saw kerf as a percentage of the total wood
usage per pass results in 21 percent of the wood removed as sawdust or about one-fifth
of the log resource. A band saw with a kerf of 0.140 in would result in an increase in
lumber recovery of about 10 percent.