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.
An automated process has stringent tolerance requirements for product dimensions, such as maximum allowable wane and warping. Products that do not meet these tolerances create problems in the process flow, and hence reduce productivity and quality. In addition, the assembly of building components is moved from on site to off site. This move calls for the integration of different processes. It requires adjustments in raw material characteristics. Therefore, the forestry industry can gain from the opportunity to adapt its processes to provide dimensionally stable products to the construction industry.
FPInnovations a effectué un essai en laboratoire afin d’étudier la teneur en humidité (TH) du bois lamellé-croisé (CLT) découlant du coulage de chapes de béton, et l’efficacité avec laquelle un enduit imperméabilisant et une membrane permettent de de prévenir cette humidification.