Quantification of the VOCs released during kiln-drying red oak and white oak lumber.
Drying hardwood lumber introduces a potential environmental burden--the release of volatile organic compounds (VOCs) with the hot, humid kiln exhaust. VOCs include lower atmospheric-level ozone precursors that are potentially detrimental to human and environmental health. This project was designed to estimate the amount of VOC emissions released from hardwood lumber during drying in conventional, steam-heated dry kilns. Species investigated included Quercus rubra (red oak), and Quercus alba (white oak). VOC emissions were quantified using a small laboratory oven for lumber drying, a heated sampling line, and a total hydrocarbon analyzer, as outlined in EPA Method 25 A. Species were dried using recommended standard kiln temperature schedule T4. VOC emission data from nine charges of red and white oak were collected. On average, red oak released the largest estimated amount of VOCs, ranging from 0.154 to 0.358 pounds per MBF (data range accounts for variance among charges and for the use of two different calculation methods). Average estimated release for white oak was significantly lower, ranging from 0.058 to 0.227 pounds per MBF. Most of the VOCs were released above 30 percent moisture content.
Volatile organic compounds (VOCs) are naturally occurring, carbon containing compounds which result from processes related to living tissue. Emission of VOCs is considered a potential human and environmental health problem due to the formation of ozone and other photochemical oxidants that can irritate the respiratory system as well as disturb natural processes like plant photosynthesis (Granstrom 2003). In recent years, the focus on VOC emissions by regulatory agencies has shifted to the wood products industry. In wood, VOCs are derived from complex extractive compounds (Rice and Zibilske 1999). The greatest loss of VOCs occurs with the kiln-drying of wood at elevated temperatures (Wu and Milota 1999, Milota 2000). Cumulative amounts of VOC emissions from lumber drying may be significant on an annual basis. According to EPA estimates, the wood products industry (all sectors) releases 41,423 short tons of VOCs per year (EPA 1995). Environmental regulations continue to make an economic impact on the wood industry. Since the 1990 amendments to the Clean Air Act, the wood products industry has been struggling to obtain useful information regarding VOC emissions and, if necessary, their subsequent control.
Research-to-date on lumber VOC emissions has centered mainly on softwood emissions. There are two primary reasons for this trend. First, softwood species comprise over 83 percent of the lumber dried in the United States (Rice 1995). Nearly all solid, structural lumber products used in construction are softwood species, as well as most engineered wood products. Hardwood species make up much less industrial lumber volume than softwood species because they are used primarily in aesthetic and appearance product applications for veneer overlays, cabinets, and furniture. The other primary reason is that conifer species contain, on average, larger amounts of potentially volatile extractive materials as compared to most hardwood species. Hardwood extractives contain low molecular weight organic compounds that can be easily volatilized with the evaporation of water at elevated drying temperatures (Knotts et al. 1995). Limited research has been done to determine the identity of these specific compounds. A study done by Solliday et al. (1999) revealed that the primary VOC components of dehumidification drying of red oak were acetaldehyde and acetic acid. Other organic compounds such as ellagic acid, gallic acid, and hamamelitannin may also be present in red oak kiln condensate (Rowe and Conner 1979, Nzokou and Kamdem 2004).
A National Council for Air and Stream Improvement (NCASI) study quantified total VOC emissions using Modified EPA Method 25 A for two small-scale (laboratory) kilns and two full-scale kilns drying similar charges of southern pine lumber. Researchers found that the variability in emission rates between the small-scale and full-scale charges was reasonable considering some of the difficulties encountered in testing conventional full-scale kilns (Law and Word 2002). The results of the analysis indicated that a laboratory scale dry kiln could be used as a reasonable test surrogate for a commercial lumber dry kiln when certain operating parameters are matched (Law and Word 2002). The operating parameters that must be matched include use of representative lumber samples, consistent kiln operation, appropriate sampling methodologies, and consistent emission rates and venting characteristics (Law and Word 2002). In 2003, NCASI developed modifications to EPA Method 25 A protocol. The two primary modifications were the requirement of a sample gas dilution system (to a moisture content (MC) of 10 to 20% or less moisture (by volume)) and the requirement to perform a total measurement system leak check (NCASI 2003).
Little research exists regarding the quantification of VOCs from any species of hardwood lumber. The main objective of this study (Beakler 2006) was to quantify the amount of VOCs from drying two dense, ring-porous hardwoods: white oak (Quercus alba) and red oak (Quercus rubra).
Experimental procedures Sample preparation
One "set" of lumber (99+ % heartwood) included seven green boards 8 feet in length, as shown in Figure 1. Each set is comprised of boards from a given species (either red oak or white oak), which varied in thickness (1 to 1-3/8 inches), width (5 to 9 inches), and initial MC. Figure 1 illustrates the method used to produce the samples. In short, each drying run was comprised of seven matched pairs of samples, where one matched pair was prepared from each board. Each sample in the matched pair measured approximately 3 inches by 1 inch by 16 inches. A study by Shmulsky (1998) indicated that the wood's surface area to volume ratio was not a statistically significant factor with respect to VOC emissions. Two sets of lumber (14 boards) plus samples from one preliminary run provided enough samples for nine total drying charges per species. These nine charges included lumber from up to 21 different trees (Beakler 2006).
[FIGURE 1 OMITTED]
In this analysis, red and white oak were dried using standard kiln temperature schedule T4 (Simpson 1991), which is outlined in Table 1. When MC reached the specified step changes, temperature was adjusted accordingly. Prior to analysis, sample MC was estimated using ASTM D4442-92. Online monitoring of sample mass allowed lumber MC to be calculated continuously (Beakler 2006).
VOC data collection
The Federal Register, Appendix C to Subpart DDDD of Part 63--Considerations for a Small-Scale Kiln Emission Testing Program (2006) lists several considerations for determining emissions from small-scale kilns to approximate emissions from full-scale kilns. Nearly all of the listed parameters were collected during this project. Air velocity through the charge, wet-bulb schedules, and kiln venting profiles were the only data not recorded.
A diagram illustrating the experimental set-up is shown in Figure 2. One day prior to data collection, the programmable forced air oven was cleaned according to manufacturer specifications. A non-phosphate liquid soap was used to scrub the interior of the oven followed by repeated rinses with distilled water. A similar method was used to clean the aluminum stickers. The oven was warmed to evaporate any remaining water, and the heated sampling line was heated to 220 [degrees]F to eliminate any organics and to prevent condensation of VOCs during sampling.
On the day of analysis, the forced air oven (Hotpack) was set to the desired temperature (monitored via thermocouple), the heated sampling line was set to 215 [degrees]F (Athena controller), and the total hydrocarbon analyzer (Thermo Environmental Instruments Inc. Model 51) was calibrated according to manufacturer directions. Ultra zero air (high purity air with fewer than 0.1 ppmv propane (or carbon equivalent)) was introduced into the input port of the oven (113.7 mL per minute) to maintain a positive flow out the exhaust port. Samples were loaded into the oven such that seven matched pairs were analyzed per load. One half of the matched pair was stickered on the oven floor while its mate was stickered on the weighing platform. The weighing platform was constructed of aluminum channel and attached to a stainless steel rod. The rod was placed through a small hole in the ceiling of the oven and attached to the bottom-weighing hook of an 8100-g balance (Mettler PB-S 8100). The balance was suspended on a plywood platform above the drying oven to collect weight data for average MC determination (Fig. 2). Monitoring MC was essential to determining the point of temperature change during the drying schedule as well as the point of charge termination (between 8 to 10% MC) (Beakler 2006).
[FIGURE 2 OMITTED]
The total hydrocarbon analyzer used flame ionization as the method of hydrocarbon detection and yielded results in parts per million hydrocarbons (ppm). This analysis method was in compliance with EPA Method 25 A. No dilution of the sample stream was needed since the MC of the exhaust never exceeded 2 percent by volume for any species (no flame outages were experienced).
VOCs (ppmv) * 1L propane/[10.sup.6] L air * 44g propane/mole propane
* mole propane/24L = VOCs (g/L) 
VOCs(g/L) * Oven exhaust rate (L/min) * 0.0022 (lbs/g)
* drying time (min) = Total VOCs (lbs) 
The total hydrocarbon analyzer sampled only a small portion of the exhaust, therefore calculations were made to estimate total VOCs expelled from the oven during each drying charge. First, pounds of VOC were converted from ppm (v/v propane) (Eqs. 1 and 2) and they were averaged for given schedule temperatures and moisture intervals. Next, amounts of exhaust gas had to be estimated. A known amount of ultra zero air (113.7 mL per minute) was fed into the oven, which provided a positive airflow. Charles' Law was used to calculate the expansion of air at given temperatures by assuming constant pressure (if [V.sub.1] and [V.sub.2] are volumes of the same mass of gas at absolute temperatures, [T.sub.1] and [T.sub.2]: [V.sub.1]/[V.sub.2] - [T.sub.1][T.sub.2]). The average VOCs from the samples were multiplied by the estimated exhaust volumes to obtain estimated total VOC emissions.
Exhaust calculations were verified using an Alnor hot-wire anemometer to determine air velocity out the oven's exhaust port. A shim steel tube, 11 inches in length, was constructed to match the size of the cross sectional area of the exhaust port. A small hole was drilled through the tube for placement of the sensor filament for the anemometer. The shim steel tube was placed into the exhaust port at various times during the drying schedule. During the measurement, turbulence introduced by the oven fan was alleviated by filling the tube with small straws, leading to stable airflow during the time of velocity measurement. Fifteen, l-minute interval measurements were taken at 130 [degrees]F, which yielded a weighted average air velocity. Multiple additional velocity measurements were taken at different temperatures, but the average air velocity was similar. Air volume as determined through the velocity measurements verified that the Charles' Law calculations were similar; data from both methods are reported.
Results and discussion
Table 2 includes emissions data from the nine charges of red oak. VOC data reported in Table 2 represent sums of emissions measured by the total hydrocarbon analyzer. The range of VOCs per MBF of red oak lumber was 0.0315 to 0.0607 pounds, with an average release of 0.0416 pounds per MBF ([+ or -]0.00796 pounds). The COV (COV) among all charges was 19.1 percent.
Figure 3 is a 95 percent confidence interval plot illustrating average VOC release for the nine charges at specific MCs. The data are also from the samples measured directly by the hydrocarbon analyzer for the range of BF indicated in Table 2. The trend reveals that the largest VOC release took place at the highest MC interval. Also, it is clear that average VOC release decreased significantly from 16 ppm at 65 percent MC to 6.5 ppm at the fiber saturation point. The 95 percent confidence interval limits are much wider at 15 percent and 10 percent MCs due to variation in the concentration of VOCs released at lower MC levels. Further analysis of the data revealed that the majority of VOCs are released above 30 percent MC. A minimal amount of thermal energy (110 [degrees]F) is required to remove the extractives above 30 percent MC, suggesting the majority of VOCs released by red oak have low boiling points.
An increase in emissions occurs at 15 percent MC (Fig. 3). This increase was statistically significant (at [alpha] - 0.05) when compared to emissions at 30 percent MC. Note that temperature increases at 15 percent MC, by 40 [degrees]F (Table 1). Schedule T4 calls for 10 [degrees]F temperature increases at 30, 25, and 20 percent MC, but the temperature increase at 15 percent MC is much larger, resulting in the increased emissions. A related finding is illustrated in Figure 4, which shows the spikes that are observed each time the oven temperature is raised. Emissions recorded at approximately 165 hrs correspond to the 40 [degrees]F temperature increase at 15 percent MC. Increased drying temperatures allowed larger, higher boiling point extractive compounds in the wood to escape the attractive forces that stabilized them on and within the wood substrate. The spike in emissions suggests that either a small amount of these compounds exist or the relatively high initial MC allows more compounds to be volatilized.
[FIGURE 3 OMITTED]
Relative humidity was not controlled in this study. Wu and Milota (1999) reported that temperature, not relative humidity (from changing the wet-bulb depression), had an influence on VOC emissions from drying Douglas-fir lumber.
A slightly sour odor suggested that there were traces of bacterial infection in at least some of the samples in each of the red oak charges. To our knowledge, no studies have been conducted on the effect of bacterial infection on VOC emissions. It is likely that the bacteria are depositing organics as by-products of metabolic processes. Since bacterial infection is a common occurrence, the data presented here are considered relevant to the hardwood industry.
Table 3 reveals the range of VOCs per MBF of white oak, 0.0113 to 0.0385 pounds, with an average of 0.0198 [+ or -] 0.00964 pounds. Again, it must be emphasized that these values are summed from exhaust samples; cumulative estimates of VOC emissions are discussed later. A coefficient of variation (COV) of over 48 percent indicated that there was considerable variation between the charges from experimental sets 1 and 2 (Table 4). However, within each experimental set the COV is less than 26 percent, indicating moderately low variability in VOC release. The large variability between experimental sets of white oak could be due to several factors, including varying initial lumber MC, variability in volatile extractive molecules present in the wood, and density. Recall that red oak yielded a COV of 19.1 percent. Variability in wood is common; the COVs for most mechanical properties of wood fall in the range of 10 to 34 percent (Wood Handbook 1999).
[FIGURE 4 OMITTED]
The white oak charges are similar to the red oak charges when analyzing the total percentage of VOCs lost above and below the fiber saturation point. White oak releases the majority of its VOCs above fiber saturation.
The average trend for VOC emissions from white oak via a 95 percent confidence interval plot is shown in Figure 5. Like Figure 3, these data represent a snapshot in time, not cumulative emissions over a MC interval. As observed previously for red oak, the highest concentrations of VOCs are removed above 30 percent MC. Several differences were noted when red and white oak emissions were considered (Fig. 3 vs. Fig. 5). Although a few of the white oak charges show a small response to the temperature change made at 15 percent MC (an increase to 180 [degrees]F), the average emissions data for white oak show no significant increases below the fiber saturation point. Also, the amounts of emissions for white oak are generally lower than for red oak.
[FIGURE 5 OMITTED]
Estimated total VOC emissions per charge
Table 4 contains the total estimated VOCs released (green to dry) according to two methods: 1) Charles' Law estimations and 2) estimations from measured air volume data (see experimental section). According to our calculations, red oak emissions ranged from 0.154 to 0.358 pounds of VOCs per charge (depending on the method) while the white oak ranged from 0.058 to 0.227 pounds. A Tukey multiple comparison test at the [alpha] = 0.05 level indicated that red oak released a statistically different amount of VOCs than white oak. There could be several reasons for these differences. Variability between the two species includes the possibility that different organic molecules or amounts of organic molecules are present in each species. The large volume of acetic acid known to be present in red oak was a probable precursor to its high volatile organic emission release (Solliday et al. 1999). Maximum emissions, on a per MBF basis, from red oak and white oak are greater than red alder (Milota 2006a), but less than balsam fir, white spruce, red spruce, black spruce and red pine (Rice and Zibilske 1999). Douglas-fir also releases more VOCs than either red or white oak (Milota and Lavery 2000).
Koch (1985) reported extractive weights per species based on successive extractions in petroleum ether, ethyl ether or chloroform, 95 percent ethanol, and hot water. According to those total extractive contents, our study indicated that between 0.071 and 0.248 percent of red oak extractives (weight basis) were removed as VOCs during drying, while white oak released between 0.029 and 0.116 percent of its extractives. The data suggest that emissions were inhibited in white oak, possibly by tyloses.
Comparison to existing data from softwood VOC emissions
Most of the literature regarding VOC emissions during wood drying focuses on softwoods. Generally, the drying methods for softwoods and hardwoods are very different, and thus lead to very different VOC emission profiles. Softwood kiln-drying uses higher constant dry-bulb temperatures (sometimes 200 [degrees]F or greater) to dry from green (> 100% MC) to 19 percent MC often in 24 hours. Hardwood drying schedules start at much lower initial dry-bulb temperatures to ensure that moisture is removed at an adequate pace to prevent defects. Drying for several weeks is common. Given these differences, it is clear that the amount of moisture lost per unit time differs for softwood and hardwood drying, with rates in softwood far exceeding those in hardwood. It is plausible that VOC emissions scale with moisture lost per unit time, suggesting that VOCs would be removed more rapidly from softwoods than hardwoods.
The extractive contents of various species do vary, but studies indicate this is not the most important factor impacting total VOC release. Southern pine's ovendry extractive content is not much different than the hardwood species dried in this study, but the total VOC release for southern pine has been reported up to 12.5 pounds per MBF (Rice 1995). This is a considerably higher VOC release than any of the hardwoods analyzed here, likely due to the aforementioned factors.
In both hardwoods and softwoods, higher hydrocarbon concentrations are emitted at the start of drying schedules. Studies done by Wu and Milota (1999) and Shmulsky and Ingram (2000) on Douglas-fir and southern pine, respectively, show that there is a large initial spike in hydrocarbons in the first 4 hours of drying. The largest saturated surface area exists initially, and heating and drying of the wet lumber surfaces likely causes large amounts of VOCs to be removed. As drying progresses from the surface toward the center of the boards, the saturated surface area decreases. Water and VOCs continue to be released, but at lower concentrations as their paths to the kiln atmosphere become blocked. VOCs could react with one another, or collect in the cell wall, which would decrease emissions as MC decreases.
Nature of the VOCs
Although this study provides quantitative amounts of hydrocarbons released from drying two species of oak, much work remains in order to characterize the potential impact of these VOCs on human and environmental health. At this point, we do not have any information about emission quantities for specific chemical compounds released from these hardwoods. Of particular interest are the compounds classified as hazardous air pollutants (HAPs). A recent study by Milota (2006b) found that EPA Method 25 A is not a good indicator of two important HAPS--ethanol and formaldehyde. More work is necessary to determine the chemical nature of emissions detected by the hydrocarbon analyzer. Furthermore, as Milota's work suggests, quantifying and controlling methanol emissions will be important for industry compliance to the Clean Air Act. More work is necessary to quantify methanol emissions from hardwood drying.
Red oak and white oak released statistically different amounts of VOCs, with red oak showing greater emissions. One potential reason for high emissions from red oak is the presence of large amounts of acetic acid and acetaldehyde, which are volatile with free water at low kiln temperatures. Specific VOCs from most hardwoods have not yet been identified, but it is likely that softwoods release larger compounds with higher boiling points. The estimated average release for red oak ranged from 0.154 to 0.358 pounds per MBF and the average release for white oak ranged from 0.058 to 0.227 pounds per MBF, both significantly lower than the maximum resinous softwood release. This could be an indicator that resinous softwood (such as southern pine) extractive material is more volatile (terpenes and pinenes) than extractives in red and white oak, or that the changing vapor pressures due to elevated drying temperatures cause these extractive materials to volatilize.
A temporary spike in emissions is observed immediately following each temperature increase (as specified in kiln schedule T4). In both species, the greatest instantaneous concentrations of emissions are observed at higher MCs. When drying duration is factored in, red oak showed relatively consistent amounts of emissions per MC interval, while emissions per MC interval decrease for white oak. Overall, a lower percentage of the total extractive content is removed during white oak drying (0.029 to 0.116 percent, weight basis) than red oak drying (0.071 to 0.248 percent, weight basis). It is speculated that the presence oftyloses could be limiting white oak VOC release. Some of the red oak lumber contained a bacterial infection; the impact of this on emissions remains unclear.
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Brian W. Beakler * Paul R. Blankenhorn * Nicole R. Brown * Matthew S. Scholl * Lee R. Stover
* Forest Products Society Member.
The authors are, respectively, Former Graduate Assistant (currently employed at Armstrong World Industries), Professor and Associate Director for Academic Programs, Assistant Professor, Former Graduate Assistant (currently employed at Georgia Pacific Corporation), Senior Research Associate, School of Forest Resources, The Pennsylvania State Univ., University Park, Pennsylvania (firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org). The authors would like to thank the Pennsylvania Agriculture Experiment Station and the USDA Forest Service, Northeastern Research Station for their partial funding support for the project. The authors would also like to thank Victor Dallons for his contributions to the project. This paper was received for publication in September 2006. Article No. 10254.
Table 1.--Kiln temperature settings based on Schedule T4 (Simpson 1991). MC Temperature (%) ([degrees]F) 30 (a) 120 25 130 20 140 15 180 (a) Above 30 percent MC, temperature remains at 110 [degrees]F Table 2.--Experimental data for red oak. Total VOCs Total Charge VOCs VOCs per BF sampled charge (experimental BF (pounds * (pounds * (pounds time set) [10.sup.-4]) [10.sup.-5]) per MBF) (min) 1 (1) 5.64 2.25 3.98 0.0398 14790 2 (1) 5.57 2.30 4.12 0.0412 15375 3 (1) 5.75 2.53 4.40 0.0440 14598 4 (1) 5.44 3.30 6.07 0.0607 12345 5 (1) 5.64 2.09 3.70 0.0370 14943 6 (1) 5.73 2.24 3.90 0.0390 14394.6 7 (1) 6.09 2.53 4.15 0.0415 14008.8 8 (1) 5.45 2.15 3.94 0.0394 14220 9 (a) 6.06 1.91 3.15 0.0315 10915.8 Range 5.44 1.91 3.15 0.0315 10915.8 to to to to to 6.09 3.30 6.07 0.0607 15375 (a) Temperature schedule run from the preliminary data set. Table 3. - Experimental data for white oak. Total VOCs Total Charge VOCs VOCs per BF sampled charge (experimental BF (pounds * (pounds * (pounds time set) [10.sup.-4]) [10.sup.-5]) per MBF) (min) 1 (1) 5.84 1.22 2.08 0.0208 10590 2 (1) 5.80 2.24 3.85 0.0385 12780 3 (1) 5.77 1.71 2.95 0.0295 12060 4 (1) 5.74 1.49 2.58 0.0258 10980 5 (1) 6.07 0.921 1.51 0.0151 9454.2 6 (1) 6.14 0.785 1.27 0.0127 9195 7 (1) 6.09 0.694 1.13 0.0113 8955 8 (1) 5.88 0.724 1.23 0.0123 10935 9 (a) 5.98 0.717 1.19 0.0119 9150 Range 5.74 0.694 1.13 0.0113 8955 to to to to to 6.14 2.24 3.85 0.0385 12780 * Temperature schedule run from the preliminary data set. Table 4.--Ranges in MC and total VOC emissions per MBF for the experimental sets of red and white oak (average release in parentheses). Red oak White oak White oak Range in initial MC (%) 66.2 to 90.8 55.7 to 61.8 Range in final MC (%) 7.49 to 9.85 7.87 to 10.0 Range in total pounds 0.0315 to 0.0607 0.0113 to 0.0385 of VOCs sampled by THA per MBF (0.0416) (0.0186) Range in total estimated pounds 0.154 to 0.296 0.0556 to 0.188 of VOCs released using Charles' (0.203) (0.0967) Law per MBF Range in total estimated pounds 0.185 to 0.358 0.0672 to 0.227 of VOCs released using air (0.245) (0.116) volume per MBF
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|Title Annotation:||volatile organic compounds|
|Author:||Beakler, Brian W.; Blankenhorn, Paul R.; Brown, Nicole R.; Scholl, Matthew S.; Stover, Lee R.|
|Publication:||Forest Products Journal|
|Date:||Nov 1, 2007|
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