This report analyses softwood-drying practices in Canada and identifies the R&D efforts required in this field. These issues need to be examined in order to address environmental concerns and implement solutions that will improve energy efficiency and reduce greenhouse gas emissions.
Developing advanced softwood-drying control systems would reduce energy use and enhance product quality. According to some researchers, the potential reduction in energy use by kilns in Canada would be 5.5 PJ per year, or 335 kT per year in carbon dioxide (CO2) emissions. Furthermore, it is estimated that CO2 emissions could be reduced by an additionnal 90 kT per year through a decrease in the amount of lumber that is downgraded.
This study aims to give an overview of the main trends in developing control systems and to identify barriers to their introduction. It will also serve as a starting point for launching and directing projects on control procedures for lumber-drying in cooperation with the industry, universities, private and public laboratories, manufacturers and users.
In keeping with this objective, researchers for this study surveyed members of the Quebec Lumber Manufacturers' Association and a few mills in British Columbia. The main findings are as follows:
- Industry opinion is that its facilities are sufficiently modern to meet current market needs.
- Industry opinion is that quality (grade reduction / rejection rate) is the most significant factor when evaluationg drying systems.
- Because it is not easy to measure the quality of the drying process, drying time is the most often used to evaluate drying performance.
- Although quality was identified as the main variable in the drying process, the proportion of under-dried and over-dried lumber units was 9 and 16 percent, respectively.
- Operators play a significant role in drying operations (they manage the process before, during and after drying), and their actions affect the results of the process considerably.
- The decision to purchase a drying-control system is driven more by the acquisition of a kiln than by requirements related to the process itself.
The researchers reviewed current technical knowledge of the main dry kiln control systems by considering two types of controls : air temperature control when drying ; and setting up drying programs. The figures in this report illustrate the use of these two approaches along with various other control methods employed in the industry.
There are five softwood kiln controller manufacturers in Canada, which together account for 75 percent of the Canadian market. Although they use similar controllers, there are differences in how drying programs are set up and how changes in moisture content are measured during drying. In spite of recent technological advances, proper drying operations still depend on operator expertise.
R&D on new measurements instruments and mathematical models has not resulted in advanced kiln controllers so far. Innovation in this area has not kept pace with the advances in other leadings sectors. One technical problem that has not been resolved is that of measuring moisture content. In spite of more than 20 years of effort, mathematical models are still being developed in the scientific community, and few applications resulting from this work have benefited the industry other than those supporting operator training.
The research community and the industry acknowledge that the development of an advanced controller represents a promising avenue for improving the lumber-drying process. Unfortunately, problems in modelling the drying process and measuring moisture content remain represent major obstacles to the development of high-efficiency controllers.
Another obstacle relates to the difficulty of evaluating the financial benefits that would accrue from potential advances with the necessary speed and accuracy. These, then, are the key factors hindering the introduction of new drying technologies. They also explain why length of drying time is still the most frequently used control variable, despite the fact that the industry considers finished product quality more important. Furthermore, it appears that operators' actions significantly affect what happens not only in the kiln but at all stages in the process, from sawmill to shipping.
In view of this, we believe that a system for monitoring the entire drying process is worth investigating. Such a system would :
- serve to collect all data generated by measuring instruments at all stages in the process, from the sawmill to the planing mill
- help to establish productivity and quality indicators for measuring the monetary value of process enhancements introduced by operators
- make it possible to provide a rationale for other promising research approaches such as multivariate analysis and experimental design
This approach would make it possible to enhance control of the drying process and process quality while also revealing potential energy savings.
This project was initiated to provide technical assistance to the Alberta wood drying industry. The specific objective was to identify opportunities to improve product quality through modification of the drying schedules. Seven mills representing almost 50% of the solid, softwood lumber production in the province were selected for the project. All mills provided a great deal of cooperation and commitment to the project was excellent. In general, lumber drying operations in Alberta are in good physical condition and operating personnel have a sound knowledge of basic drying concepts.
In general, drying schedules were found to be quite harsh. the specific concerns at most mills related to too rapid a heat-up rate and extremely low relative humidity at the end of the drying cycle. Most of the schedule modification called for more gradual and controlled heat-up rates with higher wet-bulb temperatures. The objective of this modification is to avoid setting up conditions othat promote variability in moisture content from board to board. Higher relative humidity is required at the end of the drying cycle to avoid over-drying faster drying boards. Achieving a reduction in final moisture content variability and a higher overall average moisture content should be the objective of drying schedule modifications.
Mill visits were used not only to review drying schedules but also to conduct a brief inspection of drying practices and equipment. It would be unproductive to identify schedule modifications if there were obvious shortcomings in other areas of the operation that would make it difficult to implement or over-shadow the effect. The primary concern with drying equipment is the leakiness of the structures. A common recommendation to mills was to tighten up kiln doors and walls in order to retain more moisture in the kiln environment. Another area of concern was related to lumber handling operations. Most problems in this area could be addressed through educating and training staff working at stackers, handling material in the yard, or preparing loads for the kiln.
Logging and log storage practices at all of the mills visited has a serious and detrimental impact on the drying operations. At most times of the year, operators are having to deal with a wood supply that has a mix of initial moisture content conditions. Most mills seem to manage the small percentage of balsam fir in their mix effectively. Some future gains may be achieved through refined presorting techniques that take into account initial MC variability as well as differing drying characteristics between species.
The purpose of this study on tamarack (Larix laricina) was to propose a drying technique adapted to the end use, to document the colour change after drying, and to assess the rot resistance of the various drying processes used. This study was divided into two parts. In the first part, 5/4’’ thick lumber was dried using superheated steam/vacuum (SS/V) drying in order to compare drying results with the results of a study conducted at Université Laval. The Université Laval study compared drying results obtained with three conventional drying schedules: high temperature (115 °C), elevated temperature (90 °C), and standard temperature (82 °C). The study compared drying time, final quality, colour change, and rot resistance. The second part of the study involved drying 7/4’’ thick lumber using SS/V drying.
It was clearly shown that drying process does not affect rot resistance of tamarack. However, drying process and operating temperature affect colour after drying. High temperature drying resulted in the greatest post-drying colour change. The SS/V process provided post-drying results that closely matched those before drying. The drying time obtained from the best SS/V test, with a time/quality trade-off, was longer than in the Université Laval high-temperature conventional test (0.9 factor). Compared to conventional drying schedules, the SS/V process was 1.4 times faster than the elevated temperature schedule, 2 times faster than the standard temperature schedule, and 3.4 times faster than schedules used in the industry.
Drying times required to reach a final moisture content of approximately 12% in the four 7/4” lumber tests ranged from 132.2 hours to 175 hours. Compared to current industrial results, drying by SS/V is approximately 2.6 to 3.1 faster. Warping was better controlled during the 7/4’’ tests. Winter conditions during the tests made it difficult to maintain conditions in the SS/V kiln. Nonetheless, the use of concrete dead loads on the charges and high temperature conventional kiln drying appear to provide good possibilities for Canadian manufacturers.
NRCan Value to Wood Program which discusses Larix laricina; Seasoning - Vacuum, Steam and Kiln drying
This project evaluated a number of opportunities to coastal producers related to kiln drying issues such as drying practices related to high-value products, drying with superheated steam vacuum and internal core temperature monitoring for large timbers during the heat-up phase. In summary, this project included several laboratory studies to evaluate the using superheated steam/vacuum (SS/V) for drying 7/8”x 6, green western red cedar lumber, and 8x8 and 5x(5,6,7,8,9,10,12) Douglas-fir timbers. SS/V drying yielded faster drying schedules when compared to the results obtained in industrial conventional kilns. The results obtained from the SS/V drying of WRC indicated the potential benefits of technology for drying specialty products especially when compared to drying times obtained with conventional drying (longer than 7 days). However, the results obtained also emphasize the importance of green sorting that is, sorting prior to drying to optimize drying times and reduce the variation of final moisture content.
For large cross section Douglas-firs the drying times were between 3 and 14 days depending on the severity of the drying schedule and initial moisture content distribution. The influence of moisture content and cross section during the early and late stages of the heating process were evaluated on 5x5, 6x6 and 8x8 Douglas fir timbers. Thermodynamic equilibrium was reached after 20 hours regardless of moisture content or cross section size. The knowledge is intended to be used to design conventional drying schedules for large cross section timbers.
This manual is intended to serve as an educational resource and working tool for people actively involved in the drying of Spruce-Pine-Fir (SPF) lumber. The manual covers subject matter from the basic principles of drying through to the application of techniques specific to the drying of this species group. The range and depth of information presented has been selected to meet these objectives.
Disponible en français: https://library.fpinnovations.ca/en/permalink/fpipub7507
Ce projet consiste à faire la démonstration à l’échelle semi-industrielle d’un procédé de séchage de précision par haute fréquence en continu développé pour améliorer la qualité des produits séchés à la fin de la première ou au début de la deuxième transformation du bois. Plus spécifiquement, les objectifs du projet sont de :
Ø Concevoir, fabriquer et démontrer le fonctionnement d’un séchoir par haute fréquence en continu à l’échelle préindustrielle qui peut être intégré dans les usines de rabotage ou à l’entrée des usines de deuxième transformation;
Ø Transférer la technologie à un intégrateur devenant le fournisseur de l’équipement pour l’industrie canadienne de la transformation du bois.
La technologie de séchage par haute fréquence en continu développée par FPInnovations et Hydro-Québec a récemment été démontrée à l’échelle semi-industrielle (précommerciale) (Lavoie et al. 2015). Les essais de séchage ont porté principalement sur des applications de produits à valeur ajoutée. La technologie est viable techniquement et peut répondre à des besoins de séchage de précision pour des applications spécifiques. La technologie a également le potentiel de resécher des pièces demeurées humides (volontairement ou involontairement) lors de la production de bois d’œuvre.
Lumber warp is the primary cause for drying degrade. Over the past decade, Canadian producers have been paying increasing attention to the box-store market and that of engineered wood products such as wood I-joists and glued-laminated beams (glulam). One characteristic of these markets is that they require straight and stable lumber. The objective of this study was to determine the effect of sticker spacing on stickering costs and lumber quality.
To address this objective, we conducted tests in three stud mills. Stickers were added as required for the assessment of 48-, 32-, 24- and 16-inch spacings, i.e. 3, 4, 5 and 7 stickers respectively with 8-foot lumber. The bundles of test lumber were dried in a single load in all mills. After drying, the lumber was graded by a grading agency inspector. He determined the potential grade before drying and the actual grade after drying for each piece of lumber. The moisture content (MC) of the test lumber was also determined on a sample basis.
In Mill 1, we observed that drying degrade for the entire sample was reduced from 2.2 to 1.3% in the bundles spaced at 24 inches, which represented a gain of $3.21/Mbf. As for lumber meeting the requirements for the special grade, degrade was reduced from 71.3 to 49.5%, which is a gain of $10.41/Mbf.
In Mill 2, drying degrade decreased from 32.1 to 26.3% when sticker spacing was reduced from 48 to 32 inches, for an approximate gain of $1/Mbf. In this particular mill, a 16-inch spacing failed to improve performance over a 32-inch spacing.
As for Mill 3, reducing sticker spacing from 48 to 24 inches decreased drying degrade in the special grade lumber by half, leading to a $5.33/Mbf gain. Drying degrade decreased regularly from 48 to 32 inches and from 32 to 24 inches. Closer spacing benefited lumber quality at both the top and the bottom of the stacks.
Optimal spacing in a sawmill should be based on species and stickering costs. As costs vary widely from mill to mill, the report provides information to help users calculate additional stickering costs. We observed significant gains from reduced sticker spacing. Twenty-four-inch spacing should become standard practice in the manufacture of quality lumber. As a rule, closer spacing requires only limited investments, i.e., the acquisition of additional stickers and, occasionally, minor modifications to the stickering equipment.
