Technology Roadmap: Lumber and Value-Added Wood Products
4.12 Sorting of Green Lumber

Existing Technology

Green lumber sorting has become more sophisticated in recent years. With competition increasing on the dry wood market, producers were forced to tighten up quality standards to improve finished product appearance (straighter wood) and to obtain a better moisture content distribution. Mills involved in value-added production, whether in the form of engineered or appearance products, need better quality drying (reduced moisture content variations among pieces, reduced moisture content gradient within pieces). Value-added being more species-specific than commodity production, these mills also wish to sort lumber into species categories.

In view of the problems involved in drying SPF species (subalpine or balsam), as a group, species sorting is being used to decrease the proportion of over-dried pieces; it also helps meet standard requirements regarding percentage of wet boards, and reduces costs significantly by shortening overall drying times. Some mills have been able to implement programs to sort the raw material by species, and this is usually done in the bush where the material is most easily differentiated by eye. Many mills find it impractical to consider sorting in the bush as the varying log sizes result in inefficiencies at the sawmill. As there are also variations in drying properties within species, mills have shown an interest in technology to sort by some other characteristics related to the drying behaviour ("dryability") of the material. Dryability is particularly affected by the presence of "wetwood" in some species such as subalpine and balsam fir, aspen and poplar. Wetwood occurs randomly within a tree, and it is believed to be the result of bacterial infection. Wetwood zones dry much more slowly than normal wood, and often result in wet pockets within otherwise dry boards.

The pre-sorting techniques currently available to the industry include those based on weight, radiofrequency (RF) power measurements, proximity sensors, MPB (laser) and specific heat. In a recent sampling of mills in Alberta and BC, approximately 25 percent of the mills had installed some form of pre-sorting equipment. Scanners for pre-sorting systems are installed along the green chains of sawmills, and the information is used to physically sort the material at the bin sorter. One of the major constraints for existing, as well as new mills, is the need to install extra sorting capacity. Extra bins and the corresponding building space quickly become the major part of the investment when installing pre-sorting equipment. Mills have minimised the impact on sorting systems by limiting pre-sorting to one or two lumber dimensions or, in some cases, mixing two or more lengths in a sort. Other mills have been looking at alternative solutions, like adding an eight-foot sorter adjacent to the existing 5–metre (16–foot) sorter. The mixing of lengths causes other problems as most piling equipment in softwood mills is not designed to pile such material in a proper manner for drying.

Sorting relies on the human eye, and no commercial system allows effective sorting of similar species such as those in the SPF group. A few years ago, a technique based on the "sniffer" technology used for drug detection was explored, but, for a variety of reasons, the project was never concluded. More recently, a system based on the physicochemical properties of wood (acidity rate, pH) was developed and it is being tested in industry. It is quite likely that the eventual solution in most areas of the country will involve a combination of techniques rather than a single principle: for example, a species sorting system combined with either a weight or RF-power measurement system. As pre-sorting offers the potential to both reduce drying degrade and increase productivity, there will continue to be much interest in this area. Research is under way and should continue on characterising the drying performance of various species and searching out alternative pre-sorting as well as drying methods. In addition to product criteria (commodity vs. value-added specifications), sorting criteria could involve such factors as lumber size and length, species, grade, moisture content, density and dryability (for example, a combination of factors that describes the ability of a piece of wood to dry under specific conditions, or, in other words, a specific relationship between drying conditions and drying rate). Forintek developed OASiS software allows the mills to sort lumber into lots with similar dryability characteristics, thereby limiting the need for more bins; software of this type should be further developed and complemented with economic analysis modules.

Incremental Technological Innovation

• Investigate and develop other methods (multi-step sorting) to deal with expanding product categories.
• Develop and implement fast and accurate lumber sorting systems, based on live input, to allow for more efficient drying and reduced drying degrade. Sorting criteria could include species, moisture content, weight, and other factors identified as affecting drying quality (dryability).
• Provide reliable calibration procedures or correlations between on-line moisture meters, handheld resistance meters and hand-held RF-power meters so that all moisture content measurements in the mill may be comparable and usable in common databases.
• Develop and implement methods to accurately measure the moisture content of frozen lumber.
• Develop and implement a system for detecting zones in rough green lumber that might result in drying degrade.
• Pursue the development and implementation of species sorting technology.
• Re-evaluate drying strategies (technical and economic feasibility) in view of expanding product diversification.

Breakthrough Technological Innovation

• Further refine the concept of dryability as a means of identifying products that can be dried in a single batch, thus limiting the number of product categories to be pulled out on the green chain. Develop system, based on a combination of sensors (density, moisture content, weight, species, etc.) to sort products into groups of equal dryability.