KILN SCHEDULE CERTIFICATION FOR

KILN SCHEDULE CERTIFICATION FOR INDUSTRIAL DRYING OF RADIATA PINE Rubeń A. Ananıás*{ Professor Departamento de Ingenierıá en Maderas Universidad del Bıó Bıó Avenida Collao 1202, Casilla 5-C—CP: 4081112 Concepcioń, Chile E-mail: [email protected]

Rodrigo Venegas Production Leader CMPC Maderas S.A. Km 1,5 camino a Laja, Cabrero Bucalemu, Chile E-mail: [email protected]

Linette Salvo Research Associate Departamento de Ingenierıá en Maderas Universidad del Bıó Bıó Avenida Collao 1202, Casilla 5-C—CP: 4081112 Concepcioń, Chile E-mail: [email protected]

Diego Elustondo Research Associate FPInnovations Lumber Manufacturing Department 2665 East Mall Vancouver, Canada, V6T 1W5 E-mail: [email protected] (Received June 2012) Abstract. This is a summary of a study carried out in Chile to certify industrial kiln drying of radiata pine to comply with the international phytosanitary standard ISPM 15. The drying tests were performed in 100-m3 industrial kilns located at four different sawmills of the VIII region in Chile. The objective was to develop a standard protocol to certify industrial drying of radiata pine depending on the drying schedule and wood thickness. In part, the results were used to develop a multiple regression equation that permits sawmills to select their drying schedules in such a way that the lumber can be officially stamped as both kiln-dried and heat-treated for international trade. Keywords:

Kiln drying, heat treatment, drying schedules, ISPM 15, Pinus radiata. INTRODUCTION

Industrial drying of radiata pine in Chile is typically performed in conventional kilns. There are almost 1000 sawmills in Chile drying more * Corresponding author { SWST member Wood and Fiber Science, 45(1), 2013, pp. 1-7 # 2013 by the Society of Wood Science and Technology

than 60% of the wood produced in the country, and 95% of the total volume (approximately 6 Mm3 per year) comes from plantations of radiata pine (INFOR 2011). Conventional kilns work by forcing hot air to flow throughout layers of lumber separated by thin stickers (approximately 19 mm thick).

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WOOD AND FIBER SCIENCE, JANUARY 2013, V. 45(1)

As the air flows through the lumber layers, the hot air stream transfers heat to the lumber surfaces and it removes vapor coming out from the wood. Because the drying rate depends on the lumber moisture content, kiln conditions (air temperature, humidity, and velocity) are typically adjusted as the lumber dries, and when the lumber reaches approximately 14% MC, the process is finished and the lumber is said to be kiln-dried. In recent years, however, the international standards for phytosanitary measures (ISPM), an international agreement supervised by the Food and Agriculture Organization of the United Nations, recommended sterilizing wood materials in international trade (FAO 2009). Wood sterilization was initially intended for packaging material, which is frequently made of green wood and may contain infectious agents, but the same phytosanitary measures are now required for export of kiln-dried lumber to some countries. The need for phytosanitation was first recognized when nematode parasites were found in softwood lumber shipments from North America to Europe in the mid-1980s (Rautapaa 1986). Nematodes are microscopic worm-like organisms found in almost every insect, plant, and animal. A nematode such as Bursaphelenchus xylophilus is common and relatively inoffensive in North American softwood species (Steiner and Buhrer 1934), but it could have devastating effects in European and Asian forests.

steps. The first step is called warm-up, and it consists of increasing the air temperature as fast as possible and maintaining it at almost saturated conditions (wet-bulb equal to dry-bulb) for approximately 4 h. After warm-up, the schedule moves into the drying stage by gradually increasing the difference between dry-bulb and wet-bulb temperatures with time. In average numbers, dry-bulb/wetbulb temperatures for drying 40-mm radiata pine could reach approximately 70/50 C for a 72-h conservative schedule, 90/60 C for a 36-h accelerated schedule, and 120/70 C for a 12-h hightemperature schedule. The dried lumber is then allowed to cool down for about 1 h, assessed with a handheld meter for average moisture content, and finally exposed to a conditioning stage in which air humidity is maintained as close as possible to saturation for another 4 h. The effectiveness of heat treatment has been demonstrated in the past (Newbill and Morrell 1991; Tomminen and Nuorteva 1992; Simpson 2002; Simpson et al 2002), and it has been demonstrated that the relationship among heat treatment time, wood thickness, and kiln temperatures can be predicted through both analytical and numerical methods (Simpson 2004; Simpson and Illman 2004). In particular, it has been demonstrated that multiple regression analyses provide a reasonable estimate of heat treatment times (Ananıás and Venegas 2005; Simpson 2006).

Because sterilization with methyl bromide had a negative impact on the ozone layer, the Food and Agriculture Organization recommended sterilizing with heat treatment instead (FAO 2008). As defined in ISPM 15, heat treatment is “a specific time–temperature schedule that achieves a minimum temperature of 56 C for a minimum duration of 30 continuous minutes throughout the entire profile of the wood (including at its core).”

Here we report the methods and results of a study carried out in Chile that allowed kiln-drying operations to officially stamp radiata pine as both heat-treated and kiln-dried according to the HT-KD56/30 regulation set by the Chilean Ministry of Agriculture (SAG 2006).

The question that remained to be answered in Chile was if the typical kiln-drying schedules used to dry radiata pine were also capable of fulfilling the heat treatment requirements of ISPM 15. Typical drying schedules for radiata pine in Chile can be divided into four basic

The experiments were designed to fully comply with the regulation set by the Chilean Ministry of Agriculture for heat treatment of wood and packages in conventional kilns (SAG 2006). This regulation requires measuring the temperature in the core of the lumber for at least three

MATERIALS AND METHODS

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Ananıás et al—CERTIFYING INDUSTRIAL DRYING OF RADIATA PINE

boards placed randomly on the side of the kiln that is exposed to exit air when drying starts. The Chilean regulation does not require measuring the lumber green moisture content, which is supposed to be approximately 145% for typical green radiata pine (Ananıás et al 2012). The kiln-drying experiments were performed in the VIII region of Chile using four 100-m3 conventional kilns installed at different sawmills. After stacking the green lumber on the kiln tracks (ready to be pushed into the kiln), four K-type thermocouples were inserted in the core of four random boards located in the side of the kiln that was going to be initially exposed to the exit air. The thermocouples were inserted through holes into the lumber and sealed with high-temperature silicon adhesive. Additionally, thermocouples were placed beside the kiln’s dry-bulb and wet-bulb sensors. The thermocouple cables were made of two commercial chromel and alumel wires electrically insulated with a first layer of plastic around each wire and mechanically protected with a second layer of plastic around the two wires together. Because of this set-up, the thermocouple cables were not cylindrical but rather flat with a width of approximately 3 mm. Temperature data were collected with a data logger every 1-15 min (depending on the test) and stored in a computer for subsequent analysis. Two sets of drying experiments were performed in this study. The first set comprised a total of 36 runs involving four saturated air temperatures for the warm-up phase (70, 80, 90, and 100 C), three wood thicknesses (16, 24, and 37 mm), and three replications. The significance of air temperature and wood thickness in the minimum drying time required to achieve heat treatment conditions (tmin) was statistically determined through the least significant difference test, assuming completely random design and a significance level (p) of 0.05. The second set involved measuring the minimum drying time required to achieve heat treatment for 92-mm-thick timber by using K-type thermocouples and the standard in-kiln electronic RTD sensors simultaneously.

The in-kiln RTD sensors were inserted into the core of the lumber through 5-mm holes drilled from one of the lateral faces of the boards, and they were calibrated before each test by comparing with the thermocouples. For the two sets of tests, sticker thickness was approximately 20 mm and air velocity was 4 m/s. The experimental data were then used to develop and validate a multiple regression equation that was also reported in a previous study (Ananıás and Venegas 2005): tmin ¼ 109:3 0:78 T þ 1:7 e

ð1Þ

where tmin ¼ minimum drying time to complete heat treatment (min), T ¼ saturated air temperature ( C), and e ¼ lumber thickness (mm). RESULTS AND DISCUSSION

Table 1 shows tmin as a function of wood thickness for 16-, 24-, and 37-mm-thick lumber. It was found that average tmin significantly increased from 66 to 104 min as wood thickness increased from 16 to 37 mm, probably because the heat traveled a longer distance from surface to core. For illustration purposes, Fig 1 shows the experimental dry-bulb, wet-bulb, and lumber temperature recorded in 6 of the 36 industrial drying runs measured in this study. Lumber temperature was higher than dry-bulb in some instances, but this was simply a consequence of measuring both dry-bulb and lumber temperature on the exit side of the kiln when the airflow was reversed. Similarly, Table 2 shows tmin as a function of the saturated air temperature from 70 to 100 C. It was found that average tmin significantly decreased from 74 to 62 min as the saturated air temperature increased from 70 to 90 C. Average tmin also decreased from 62 to 57 min as the saturated air Table 1. Average tmin to complete heat treatment as function of lumber thickness. Thickness (mm)

Runs

Time (min)

16 24 37

12 12 12

66 91 104

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Figure 1. Temperatures (T) measured with thermocouples during drying of 16-, 24-, and 37-mm radiata pine. Black, gray, and dotted lines indicate dry-bulb, wet-bulb, and lumber temperatures, respectively.

temperature increased from 90 to 100 C, but the effect was not significant from the statistical viewpoint. At 100 C, tmin could not be proven statistically lower than at 90 C although the drying process was faster.

It was also found that the experimental tmin measured with the calibrated thermocouples could be predicted with the multiple regression equation in Eq 1. Assuming 8.1% standard deviation in the difference between actual and predicted values


Table 2. Average tmin to complete heat treatment as function of air temperature. Temperature ( C)

Runs

Time (min)

70 80 90 100

9 9 9 9

74 68 62 57

(Ananıás and Venegas 2005), it was deduced that a drying schedule longer than 1.2 times the tmin predicted with Eq 1 will achieve heat treatment conditions in almost 99% of the cases. In addition, the temperature at the surface of the lumber during drying can never be lower than the dew point temperature (approximately equal to the web-bulb), thus most drying schedules will have the same tmin even if the dry-bulb is increased but the wet-bulb is maintained equal to or higher than the saturated air temperature used to calculate tmin. Table 3 shows tmin for the 92-mm-thick lumber as measured with both K-type thermocouples and in-kiln electronic RTD sensors. It was found that the difference between the tmin determined with the calibrated thermocouples and the tmin determined with in-kiln RTD sensors was between 32 and 41%. Because the RTD sensors were calibrated before each run, it is believed that these differences were caused by the measurement method. Compared with the thermocouple cables, the RTDs had a larger diameter, uniform cylindrical shape, and an external metallic cover. This may have increased the contact area between the sensor and wood, decreased the net distance from the contact area to the external air, and added additional heat into the wood flowing longitudinally through the metal. These assumptions, however, were not validated in this particular study. Table 3. Minimum drying time to complete heat treatment measured for 92-mm-thick lumber. Saturated air temperature ( C) tmin measured with in-kiln RTD (min) tmin measured with thermocouples (min) Error (%)

70 144

80 125

90 118

100 111

212

203

196

187

32

35

40

41

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Finally, Fig 2 shows the dry-bulb, wet-bulb, and lumber temperatures measured during the first few hours of drying 92-mm-thick radiata pine. As can be observed, there was a delay of approximately half an hour at the beginning of the schedule in which the core lumber temperature remained unaffected by the drying process, but then the lumber core temperature rapidly increased and tended asymptotically toward the temperature of the saturated air. It is also observed that the lumber core temperature remained higher than the wet-bulb temperature after the wet-bulb depression was increased to start the drying phase. Although air velocity was not considered as a variable in this study, it is known that decreasing air velocity decreases the heat transfer coefficient, thus, in theory, tmin should also increase. Similarly, if the dry-bulb temperature is decreased during the heat treatment process or the air is not constantly maintained close to saturated air conditions, then tmin would probably increase. This means that Eq 1 is only applicable to the kiln conditions used for the calculation, and any other kiln conditions with lower dry-bulb, wet-bulb, or air velocity should be certified on a case-by-case basis. CONCLUSIONS

This study reports the methodology developed in Chile to certify industrial kiln-drying of radiata pine to comply with the international phytosanitary standard ISPM 15. Currently, 15 sawmills in Chile operating a total of 40 kilns have adopted this method to certify their drying processes. It was found that the minimum drying time required to achieve heat treatment conditions significantly increased from 66 to 104 min as the wood thickness increased from 16 to 37 mm and it significantly decreased from 74 to 62 min as the saturated air temperature increased from 70 to 90 C. Minimum drying time also decreased from 62 to 57 min as the saturated air temperature increased from 90 to 100 C, but the effect was not significant from the statistical viewpoint. The study concluded that drying times longer than 1.2 times the value predicted with a multiple

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Figure 2. Temperatures (T) measured with thermocouples during drying of 92-mm radiata pine. Black, gray, and dotted lines indicate dry-bulb, wet-bulb, and lumber temperatures, respectively.

regression equation will complete heat treatment in almost 99% of the cases, and most drying schedules will still achieve heat treatment conditions in the same time if the wet-bulb temperature is maintained equal or higher than the temperature used to calculate minimum drying time. It was also found that the difference between minimum drying time determined with the calibrated thermocouples and the time determined with in-kiln RTD sensors was between 32 and 41%. This study showed that typical drying schedules used for radiata pine in Chile are likely to meet the international phytosanitary standard ISPM 15. ACKNOWLEDGMENTS

We appreciate the financial support of the National Commission of Scientific & Techno-

logical Research (Conicyt) of Chile (MEC-PAI No. 80110012). REFERENCES

Ananıás RA, Ulloa J, Elustondo D, Salinas C, Rebolledo P, Fuentes C (2012) Energy consumption in industrial drying of radiata pine. Drying Technol 30(7): 774-779. Ananıás RA, Venegas R (2005) Industrial drying of radiata pine. Heat sterilization time and temperature development. Maderas-Cienc Tecnol 7(3):179-188 [in Spanish with abstract in English]. FAO (2008) Replacement or reduction of the use of methyl bromide as a phytosanitary measure. International Plant Protection Convention, Food and Agriculture Organization of the United Nations, Rome, Italy. www.ippc.int/ index.php?id=1111098 (1 June 2012). FAO (2009) International standards for phytosanitary measures: Regulation of wood packing materials in international trade. Food and Agriculture Organization of the United Nations, Rome, Italy. 18 pp.


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