Drying Energy Efficiency Through Partial Air-Drying: A Case Study

The green lumber products tend towards a state of equilibrium with the surrounding environment. The final target is the equilibrium moisture content (EMC). Whether is natural convection in the lumberyard or forced convection in the kiln drier, the road towards the final EMC consumes time and energy. The air drying operations (natural convection) are not an easy task for the drying management. This is valid especially when it comes to prove that in doing so, these operations will increase the overall drying process efficiency. A break-even analysis was conducted to understand and find if the air-drying practices (natural convection) are acceptable from accounting and drying management point of view. Material flow, pessimist, realist, and optimist scenarios are presented based on the expectations of different management level personnel. INTRODUCTION As well known, the lumber drying is the most energy consuming operation of a lumber mill. One way to reduce the energy consumption is to introduce the airdrying practice as an integrated part of the overall drying operations. A survey (Léger et al, 2002) conducted in Canadian province Quebec shows that among 73 respondents, 63 percent do include air-drying in their operations, especially for their balsam fir with high initial moisture content. The air-drying operation can be described as a process with no direct energy consumption, slow, and which demands low investment. In the same time, this process is highly dependent on the meteorological conditions. Several experimental studies (Marinescu et al, 1999, Tremblay, 2003) showed that the average air-drying rate vary considerably depending on lumber dimension, season, wood species, etc. In this study, the average air-drying rate was evaluated at 1.67 % of moisture content (MC) per day during summer conditions. This value was estimated based on heat-up times of the load in the kiln drier. The objective of this work is to find if the airdrying process of the softwood lumber (balsam fir) should be considered as a part of the overall drying operations. The goal of the new operation is to reduce the initial moisture content of the lumber without completely drying it. The main factors influencing the decision making process for implementing this strategy are the following: Air-drying system: Mother Nature Operations management: Lumber sorting Sawmill, drying kiln and planer mill rate production. Drying kiln capacity. Accounting management: Additional cost for air-drying purposes. Energy consumption considering the product type. Greenhouse gas emissions by lumber categories. 1 In this case the lumber mill has three main operations: sawing, drying, and planing. 8 International IUFRO Wood Drying Conference 2003 220 DATA INVENTORY AND ANALYSIS Mother Nature is the drying system. Therefore there is no control on air-drying conditions. In the following table, data describing meteorological conditions is presented (Météomédia, 2003). The average is representative for Quebec City region, during the summer season, when this study was conducted. Table 1 Summer meteorological conditions. Air variables Average values T [°C] 19.00 Rain [mm] 100.20 Relative humidity [%] 68.20 EMC [%] 12.13 Wind velocity [m/s] 3.67 The temperature and the EMC values (Table 1) showed that the air-drying conditions were favourable. Moreover, the wind velocities assured a certain uniformity of moisture content among the lumber in the load. Operations management is the second main factor in the decision of including or not the air-drying as a part of the lumber drying activity. Information concerning kiln drying and production management point of view was gathered. In the sawmill different dimensions of lumber are produced. In addition, the lumber is sorted in various initial weight categories. The objective of lumber sorting is to have as much uniformity in a load as possible from initial MC point of view. In the next table (Table 2) the sorting strategy of the lumber is presented. The lumber labelled Low has an average initial MC of 71.62 %, the lumber labelled Medium has an average initial MC of 106,93 % while the lumber labelled High has an average initial MC of 141.67 %. Table 2 Sawmill: Sorting strategy. Low Medium High Medium + High 2x3x8 √ √ 2x4x8 √ √ √ -