Canadian Developments in Kiln Drying
The objective of this discussion is to summarize recent trends related to industrial kiln drying with especial emphasis on the drying of softwood dimension lumber. According to an relatively recent report, between 1995 and 2007 the number of softwood sawmills in the United States and Canada was reduced by 24.5%. The same authors predicted that the prospects for the lumber industry in the next few years are uncertain due to over capacity, cyclical downturn of the housing market and the mountain pine beetle epidemic that is currently devastating most of the pine in Western Canada.
Another cause for concern for the wood industry is the apparent steady decline in student enrolment in disciplines related to wood technology. Despite these pessimistic predictions, some analysts believe that once the over-supply issue is resolved and the lumber market returns to its normal course, the sawmills that managed to succeed in those difficult times will be in excellent conditions for years to come. They will fulfill the demand for lumber products for traditional residential markets, for growing remodelling markets and for the potential expansion of the non-residential markets.
In addition to strong economic activity, other components that will fuel future trends in kiln drying technology include, skill set development; increased pressure from environmental groups; growing customer demand for better quality; growing pressure for reducing costs; increased competition from other materials; challenges associated with current wood species; utilization of alternative species; potential new products and applications; increased stringent regulations regarding product performance; and technology adaptation and innovation.
Global market is a clear reality. Companies must be aware of the subtle changes in public perception and react accordingly to the new trends related to public demand. Educational institutions must work closely to industry leaders in the design of the curricula to prepare future generations so that new approaches to wood manufacturing can be developed. It has been argued that the introduction of new control systems for dry kilns could reduce the average energy consumption by 18%, and if such energy efficient systems were installed in 50% of the dry kilns operating in Canada that use fossil fuels, then the energy decline would be equivalent to decreasing by 332 kT per year the amount of CO2 released into the atmosphere. Thus, there is great potential for saving energy and at the same time minimize the impact on the environment. In addition to the increase in energy costs, labour costs have also been increasing since lumber producers must constantly compete with other industrial sectors to attract skilled workers. Increased demand for high quality has spread to all types of wood products. With the advent of home centres, customers can inspect the product before buying and therefore be selective regarding their choices. Customers aware of potential degrade risks associated with inadequate moisture are demanding stringent specifications in the purchase orders.
Dimensional stability is one of the main challenges faced by producers. Wood is constantly changing its moisture content with the environment in order to reach what is known as the equilibrium moisture content (EMC). Dimensional changes associated with moisture content tend to affect product performance in two ways: excessive shrinkage or excessive swelling and the development of drying defects such as checking and/or warping.
Final moisture content for dimension lumber normally exhibits a large variation (~10 to 19%). Thus, higher tolerances related to dimension changes are allowed. However, in view of customer demand for higher quality products, tighter tolerances for these products are now common in several markets. Wood is still a preferable material for many applications, but manufacturers of lumber products must ensure that the final moisture content of their products is compatible with the normal service conditions.
The main challenge faced by kiln operators drying spruce-pine-fir (SPF) is the difficulty associated with the drying of wetwood present in subalpine fir (Figure 1) and other species. Wetwood (wet pockets), as the name implies, are areas in the wood with abnormally high moisture contents. The presence of wetwood is believed to be associated with bacteria that causes changes wood structure resulting in slower drying rates. At the end of drying, wetwood regions are still wet and the product is downgraded because of excessive moisture.
The current MPB infestation is affecting lumber quality and drying characteristics. Once the tree is attacked by the MPB it starts to dry very quickly. Most MPB lumber being currently processed exhibits moisture contents below 22-25% prior to being kiln dried. Although mills are able to significantly reduce drying times (and conserve energy) when drying MPB lumber, quality of the final product may be compromised due the presence of checks observed before logs enter the sawmill.
It is expected that industry diversification will stimulate the use of species in non-traditional applications. For example, due to decline of Lodgepole pine because of the MPB infestation, other species may be considered for certain applications where pine has been the predominant species. Potential new markets such as China will expand the possibilities of new products/applications for construction and value-added products. Changes to drying will stimulate new approaches to quality control as well the development of technology (drying process, control strategies and drying monitoring techniques).
Kiln drying will greatly benefit from other areas such computer technology, electronics and sensor technology. Research related to fundamental wood properties allowed adapting technologies for assessing moisture content, development of drying stresses, detecting drying degrade development and drying rate characteristics.
Developments and trends
Products & Markets – In western Canada, as the supply of high quality Lodgepole pine decreases due to the MPB infestation, mills will need to rely on additional volumes of spruce and Subalpine fir to continue supplying to North America markets.
Subalpine fir dries considerably slower than spruce and pine. In addition, even with longer drying times, final moisture content variation for subalpine fir is too high (more than 5%). The challenges associated with subalpine fir are twofold: 1) to design drying schedules to maximize drying rates and 2) to design strategies to reduce final moisture content variability. In addition to green sorting, mills may have to employ different drying technologies and/or combination of technologies to eliminate the observed problems with current conventional drying of Subalpine fir. In short, new approaches related to drying subalpine fir and other species that exhibit wetwood (wet pockets) is urgently needed.
Products such as thick hem-fir timber (more than four inches) take too long to dry more than 14 days), which may hinder their economic feasibility. Those products are for the most part dried in conventional kilns, which limit the maximum drying rates that can be achieved without compromising the quality of the final product. Technologies such as superheated steam vacuum (SS/V) and radio-frequency (RF) drying have the potential for producing shorter drying times (lsess than seven days), but capital costs, especially under the present economic situation, may delay the adoption of these technologies in North America.
Today’s market pressures coupled with rising energy costs, will stimulate researchers and producers to re-visit non-conventional drying technologies.
Drying systems – Presently, drying systems available to the industry offer the main components of: variable speed drives (air velocity); humidification control (low pressure steam /cold water spray); in-kiln moisture meters; multi-zone control systems; equilibrium moisture content sensors; moisture content gradient sensors; and temperature drop control systems.
In view of the potential opportunities, it is expected that the area of drying schedule development will soon see some significant progress to allow mills to optimize their drying schedules and realize gains in productivity, energy conservation and product quality. Thus, based on potential markets, new products, species and technical and quality requirements, the main trends related to developments in kiln drying in North America are:
- Training and skill development
- Drying process (development of more
- sophisticated drying schedules; evaluation of air drying for certain products, species and season; combination of air and conventional techniques; and re-drying sorted wet lumber.)
- Kiln equipment (control systems using artificial intelligence; use of progressive kiln systems; SS/V and RF drying for specialty products; in-kiln moisture content monitoring systems; fans systems designed to produce high air velocities; and heat recovery systems.)
- Energy systems (the replacement of
- traditional energy sources by co-generation systems; direct-fired kilns using wood residues; and use of biomass as primary heat source for thermal oil and steam kilns.)
- Sorting technologies (sorting prior to drying, by weight, electrical properties, gamma radiation; post sorting using combinations of technologies such as weight and DC-resistance; and species sorting or pH sorting system).
- Process control (temperature drop across the load; multi-zone systems and continuous drying; and exiting air temperature based control.)
- Heat treatment (international standards; and heat treatment schedule.)
- Drying research (high temperature of western softwoods; monitoring drying stresses; log drying; wood-moisture
- relations; quality control; heat treatment/phytosanitary; energy modelling; properties of wetwood; drying schedule for hardwoods; three-dimensional simulation of drying/ stress development; sorting prior to drying; radio-frequency; high temperature heat pumps; drying MPB; and pre-sorting and post-sorting strategies
There are probably many other areas of research interest not listed above. However, based on the research areas listed it is possible to be optimistic about the future of kiln drying and expect developments that will address many challenges that are currently affecting producers.
Opportunities for drying process optimization – A typical sawmill in British Columbia dries between 950 and 1,150 kiln charges per year. Theoretically, a significant amount of data is available and therefore strong trends related to moisture content, moisture content variation, drying times and drying degrade could be developed. Kiln supervisors are not always equipped with tools for analysis or with necessary training to carry out a comprehensive analysis of drying data. The opportunity for process optimization is significant and should be the primary focus for improving the profitability of the drying operation.
In addition to product quality and energy consumption, other performance indicators include, drying times, final moisture content variation, kiln uptime, downtime reason, and moisture variation at the planer mill.
Performance indicators allow kiln supervisors to detect problems and opportunities. Thus, independently of new developments in kiln drying technology, performance indicators and data analysis can be useful for mills aiming to optimize their drying processes without substantial investments in capital and other resources.
This article is partially based on the paper “Canadian developments in kiln drying” by the authors, published in Drying Technology 2012, 30(15): 1792-1799.
Luiz C. Oliveira, Ph.D., and Diego Elustondo, Ph.D., are scientists, wood drying, with FPInnovations’ Lumber Manufacturing Department. Arun Mujundar, Ph.D., is a professor with National University of Singapore’s Department of Mechanical Engineering and Ruben Ananias, Ph.D., is a professor with University of Bio-Bio’s Lumber Engineering Department.
February 20, 2013 By Luiz Oliveira and Diego Elustondo FPInnovations and Arun Mujundar and Ruben Ananias
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