Assessing moisture content of piled woody debris: implications for burning

Transcription

1 Assessing moisture content of piled woody debris: implications for burning Greg Baxter Researcher, Wildland Fire Operations Group, FPInnovations Feric Keywords Harvest debris, woody debris, fuel moisture, debris piles, moisture meters, harvesting, coastal British Columbia 1

2 Abstract The main objective of this study was to monitor the moisture content of post-harvest, piled woody debris on eastern Vancouver Island, British Columbia over 10 months in 2008/09 to determine time required for the moisture content to drop to 25%. This level allows for the most efficient burning, and thus the most complete disposal and the least smoke production. A secondary objective was to compare the operational use of two types of digital meters for measuring moisture content of wood: a time-domain reflectometer and a Protimeter Timbermaster. The woody debris was piled immediately following harvesting during January to March 2008, and 5 months elapsed before the debris reached the level of 25% moisture content. However, this 5-month period coincided with the summer season when forest fire risk is elevated due the dry conditions. Moisture content of the woody debris increased significantly during the subsequent fall and winter, which is the rainy season in coastal British Columbia. The timedomain reflectometer was found to be more accurate but the Protimeter Timbermaster was easier to use. Introduction The results of a study conducted by FPInnovations Feric in 2007/08 (Baxter in preparation) showed that protecting a pile of post-harvest woody debris from precipitation by covering it with large plastic tarps for a period of time prior to burning helped the pile burn more quickly and with less smoke than piles that were not covered. Incidental to this study, our research is showing that a moisture content threshold affects the ease of ignition and burning of the debris. This threshold is ~25%, i.e., debris containing 25% total moisture content appears to be at the threshold for burning and offers advantages in terms of ease of ignition. 2

3 After Feric completed the plastic tarp study, its members became interested in better understanding the moisture content cycle of harvested debris so that pile burning can be conducted as efficiently and promptly as possible. This understanding would potentially allow harvesting operations to utilize the first available post-harvest burn window rather than waiting out the subsequent rainy months for the next window. Historically the forest industry burned debris piles relatively soon after harvesting in order to reduce the fire hazard. Today, however, burning can be conducted only when the venting is good. This limits burning activities to certain seasonal windows, which in coastal British Columbia is usually November to January/February and co-incides with the rainy season. Because the number of suitable burning days are limited, the number of unburned debris piles increase during years when unfavourable weather limits venting even further. Knowing when the moisture content of the debris reaches the optimal 25% level may allow industry to make the best use of the limited burning windows. This in turn would complete activities on harvesting blocks sooner, thus saving the costs of maintaining site access. Also, ensuring that piles are burned when the moisture content of the debris is low will help the debris burn more intensely, leading to a more complete disposal and less smoke. FPInnovations Feric therefore initiated another study to determine the time required for the moisture content of piled, post-harvest woody debris to drop to 25% moisture content. Over 10 months in 2008/09, FPInnovations used two types of digital instruments to measure the moisture content of woody debris in piles at five harvesting sites on Vancouver Island, British Columbia. This report presents the results of the moisture content readings and discusses the feasibility of using such readings operationally. 3

4 Objective The main objective of this study was to monitor the moisture content of post-harvest, piled woody debris over time to determine the time required for the moisture content to drop below 25%, which is the approximate level for the most efficient burning, and thus the most complete disposal and the least smoke production. A secondary objective was to compare the operational use of two types of digital meters for measuring moisture content of wood. Methods The study was conducted on southeastern Vancouver Island, British Columbia. In March 2008 ten piles of post-harvest woody debris were selected for monitoring two piles on each of five sites that had been harvested during the period January to March In all cases, the piles had been built during or immediately after the completion of harvesting operations (Figure 1). The mean size of the piles was 10 m long x 8 m wide x 3.5 m high. The mean diameter of the debris was 14.4 cm with a standard deviation of 5.8 cm. The pieces selected were marked with spray paint at the sampling location. Data were collected from these pieces on five occasions over a 10-month period: in May, August, October, and November/December 2008, and in March A total of 914 measurements were taken on pieces located in the piles (~95% of the measurements taken) as well as on pieces located on the ground within 30 m of a pile (~5% of the measurements). A minimum of 20 pieces from each pile and 10 surface pieces were measured during each visit. During each of the five site visits, moisture content data were collected by means of two digital meters: a time-domain reflectometer and a Protimeter Timbermaster. 4

5 The time-domain reflectometer determines the moisture content of wood by averaging the time it takes for an electronic signal to travel through a piece of debris and then converting this time into moisture content. Because the meter s two 12.5-cm probes (Figure 2) penetrate to the middle of a debris piece, the resulting measurement is the mean moisture content from the middle to the surface of the piece. Prior to the study, the time-domain reflectometer was calibrated specifically to take readings of Douglas-fir and western redcedar (Ewart, 2003). Originally designed for use in a mill or wood yard, it is a relatively bulky piece of equipment that includes a 1-m drill with probes on one end, a meter box, and a display unit. It runs on large rechargeable batteries or a small generator, and is pre-calibrated to take readings of a variety of species. The Protimeter Timbermaster is a hand-held meter that is small enough to fit in a pocket (Figure 3). Its two 1.3-cm probes can measure the moisture content of a piece of woody debris to a depth of about 1 cm. The Timbermaster is pre-calibrated for over 50 species. On the first site visit, each sample piece was marked with spray paint, and the probes of the time-domain reflectometer and the Protimeter Timbermaster were inserted in separate locations in the wood near that mark. On subsequent visits, the probes were inserted about 2.5 cm from the previous insertion point for that meter. All the measurements were taken within 12 to 15 cm of the spray-painted mark. Only data collected from Douglas-fir and western redcedar pieces are reported here. Other species were represented in the piles, but the number of readings was inadequate to be included in the analysis. Two piles were burned in November 2008, and two were burned in March 2009 (Figure 4). On March 10, 2009, BC Environment used a nephelometer to sample and measure the particulate 5

6 matter <2.5 micrometers at a distance of 10 m to 400 m from two burning piles one in the Nanaimo Lakes Road area, and one in the Malahat area. The venting on this day was good. Results Figures 5 to 9 show the moisture content of Douglas-fir and western redcedar woody debris located in piles and on the ground surface, as determined by the time-domain reflectometer and the Protimeter Timbermaster, over a 10-month period in 2008/09. The results show that the moisture content of debris pieces decreased rapidly from May to August (Figures 5 to 8), and over half into October. In most cases, moisture content was >25% in May 2008 when the first measurements were taken. Approximately 5 months (using February as a harvest) were required for the debris to achieve 25% moisture content. During the summer months, the moisture contents of the piled debris and surface debris were similar. The rates of moisture loss for the two types of debris were also similar. The moisture content of both types of debris increased when the rains arrived in the fall, but surface debris incurred a greater increase than debris in piles, probably because surface debris was in contact with the wet ground surface. The mean values of the readings taken by the hand-held Protimeter Timbermaster were higher than those obtained by the time-domain reflectometer. The Protimeter Timbermaster, with its short probes, is designed to take measurements to only a 1-cm depth. The readings of this meter are therefore slightly skewed because the exterior of a piece contains more moisture during the wet season. However, the probes of the time-domain reflectometer penetrate a piece up to 12.5 cm and the meter averages the readings from the middle to the surface. For this reason the time-domain reflectometer is the more accurate tool. However, it is 6

7 cumbersome and requires calibration, so users may find the hand-held, pre-calibrated Protimeter Timbermaster more convenient. Data for Douglas-fir show a very good relationship between the readings of the time-domain reflectometer and those of the Protimeter Timbermaster (Figures 5 and 8). The time-domain reflectometer s readings of western redcedar pieces (Figure 6) were low relative to those of Douglas-fir pieces (Figure 5), and relative to those taken by the Protimeter Timbermaster (Figures 7 and 8). For Douglas-fir pieces in piles, the moisture content trends recorded by the two meters were quite similar (Figure 9). The readings of western redcedar pieces by both meters (Figures 6 and 8) were less consistent than for the Douglas-fir (Figures 5 and 7). A much larger range in the data occurred for western redcedar, producing trends that showed large increases and decreases in moisture contents. The state of the wood at the time of measurement may have caused this. For example, western redcedar is susceptible to interior rot, which could skew the readings. Also, the readings would be skewed if the western redcedar pieces were either extremely wet or extremely dry because neither type of meter works as well in those conditions. When the study piles were burned in November 2008 and March 2009, the moisture content of the Douglas-fir pieces was very near or below 25% (Figures 5 and 7). All piles ignited easily by means of a propane torch and they burned down quickly with minimal smoke production. Although BC Environment collected particulate samples from only two sites on one day, some interesting preliminary results emerged that indicate pile structure had an influence on the recorded data. The pile in the Nanaimo Lakes Road area was tall and cone shaped. This pile burned very well and the mean value for particulate matter <2.5 micrometers was 6.3 ug/m 3. The pile at in the Malahat area was loosely built, was much longer than it was tall, and included 7

8 a large stump covered with soil. The mean reading at the Malahat site was 180 ug/m 3. These are very preliminary results, but they suggest that pile structure is an important factor in smoke production. Conclusions The study indicates that debris piled immediately after harvesting during the period January to March 2008 was dry enough ( 25% moisture content) for burning about 5 months later. Douglas-fir piles were dry enough to burn from June on. However, the chances of conducting a burn during the summer are low due to the elevated fire risk of the surrounding fuels and forests at that time. If especially good planning including moisture monitoring, and effective control measures are in place, the benefits of disposing of the piles sooner rather than later could be realized. If debris piled in late winter or early spring remains piled until the fall or winter then the moisture content will again rise to levels that would lead to greater smoke production. Regular monitoring of the moisture content of piled debris would help ensure that the coincidence of optimal moisture content and venting windows are not missed. Some cost would be incurred with regular monitoring, but this could be reduced by sampling during other necessary visits to the site. Historically, the timing of burning was determined by the surrounding fire hazard. Now it is also determined by air quality and the venting index thus restricting debris disposal by burning. Alternatives to burning, or different burning practices may be required. The effects of snow on moisture content are thought to influence peak moisture contents in spring. The snow coats the debris in the piles and tends to freeze overnight and thus does not allow the debris to absorb as much moisture as when it is exposed to rain. As part of another study in the same region, FPInnovations monitored moisture contents of woody debris over two 8

9 years. Peak moisture contents in March 2008 were significantly higher than those documented in March 2009 because the weather in March 2008 was far wetter (Baxter 2009 in press). Fewer snowfall events occurred in the earlier study, but rainfall amounts were higher. In March 2009 the moisture content of the piled debris was below 25%, so the piles burned better and more easily than the piles burned in March The moisture content meters could also be used to monitor roadside debris to determine the best time to transport them for use as biofuel. Dry debris weigh less than freshly harvested debris, therefore truck fuel costs could be lower. Or, in the case of smaller diameter logs, it is possible that the truck could haul more stems per load than if the logs were green. Implementation and Recommendations Digital monitoring of the moisture content of piled woody debris can benefit the forest industry by enabling harvesting operations to plan pile burning during a period with a low moisture content in the debris, safe burning conditions, and good venting in order to achieve efficient and fast burning and minimal smoke production. In this study, the debris harvested during January to March 2008 was ready for burning 5 months after being piled, i.e., in summer But the fire hazard in summer in coastal British Columbia would likely be high; conducting a burn at this time of year would require extensive precautions. The benefits of disposing of piles relatively soon after piling would need to be weighed against the risk of an escape, and against the cost of leaving the debris in piles and maintaining access until the next opportunity to burn arises. The readings of the time-domain reflectometer and the Protimeter Timbermaster showed similar trends in the moisture content of Douglas-fir woody debris. Either meter can be used to monitor Douglas-fir, but the TimberMaster is the better tool for this species because it is very portable and easy to use. The Protimeter Timbermaster can be used in Douglas-fir if 5% is 9

10 subtracted from the mean moisture content. The Protimeter Timbermaster should be tested on different wood species and in other climate types to determine if it can be calibrated for use in various conditions. Proper piling of debris, combined with conducting burning when the debris is dry ( 25% moisture content) can reduce the smoke and particulate production. Proper techniques can be found in a Woody Debris Management A Guide to Fuel Hazard Abatement and Smoke Reduction (Johnstone and Clark 2009) produced in this study area. The particulate data collected by BC Environment during this study support the advantages of good piling and burning practices. Acknowledgments The author thanks Gary Dakin for assisting in all the data collection; Ed Proteau (Feric) for consulting with the forest companies (TimberWest Forest Corp. and Island Timberlands LP), selecting sites, and handling all the logistics; Ed Korpela (British Columbia Ministry of Forests and Range) for consulting with BC Environment and for providing the Protimeter Timbermaster; and Dave Clark for igniting all the piles and assisting in some data collection. FPInnovations thanks Island Timberlands LP and TimberWest Forest Corp. for their assistance and for providing access to their cutblocks. Finally, thanks to BC Environment for partially funding this project through its Clean Air Initiative and the Debris Management Program. References Baxter, Greg. In preparation. The effect of moisture content in harvest debris on smoke management on east Vancouver Island (working title). FPInnovations Feric. Advantage Report. 10

11 Ewart, J Determining log moisture using time domain reflectometry: case study results. Feric Advantage Report: Vol. 4. No. 12, Johnstone, S. and D. Clark Woody debris management a guide to fuel hazard abatement and smoke reduction. TimberWest Forest Corp., Vancouver, BC. Figure 1. A typical debris pile, located along Nanaimo Lakes Road on the eastern side of central Vancouver Island, British Columbia. 11

12 Figure 2. Components of the time-domain reflectometer. Above left: 12.5-cm probes mounted on a 1-m drill, being inserted into a piece of woody debris, insertion of the moisture probes in holes created by the drill (above right) Bottom: digital display. Figure 3. The Protimeter Timbermaster, with its 1.3-cm probes inserted into a piece of woody debris. Figure 4. Light smoke from the burning of post-harvest debris piles at Loon Lake, near Port Alberni on the northwest coast of Vancouver Island, March

13 Moisture content (%) Moisture content (%) Douglas-fir May June July Aug Sept Oct Nov Dec Jan Feb Mar 2008/09 Pile pieces Surface pieces Figure 5. The moisture content of post-harvest Douglas-fir debris pieces, over a 10-month period, as measured by the time-domain reflectometer. Western redcedar May June July Aug Sept Oct Nov Dec Jan Feb Mar 2008/09 Pile pieces Surface Figure 6. The moisture content of post-harvest western redcedar debris pieces, over a 10-month period, as measured by the time-domain reflectometer. 13

14 Moisture content (%) Moisture content (%) Douglas-fir May June July Aug Sept Oct Nov Dec Jan Feb Mar 2008/09 Pile pieces Surface pieces Figure 7. The moisture content of post-harvest Douglas-fir debris pieces over a 10-month period, as measured by the Protimeter Timbermaster. Western redcedar May June July Aug Sept Oct Nov Dec Jan Feb Mar 2008/09 Pile pieces Surface pieces Figure 8. The moisture content of post-harvest western redcedar debris pieces over a 10-month period, as measured by the Protimeter Timbermaster. 14

15 Moisture content (%) Douglas-fir May June July Aug Sept Oct Nov Dec Jan Feb Mar 2008/09 Time-domain reflectometer Protimeter Timbermaster Figure 9. An overall comparison of the moisture content readings taken by the time-domain reflectometer and the Protimeter Timbermaster, for post-harvest Douglas-fir debris pieces in piles only. 15