Wood Material Science and Engineering, 2010; 5: 104-109
Heat sterilization of ash (Fraxinus spp.) firewood: Heat-treating options, temperature monitoring and thermal verification
X. WANG1, R. D. BERGMAN1 & T. MACE2 1
USDA Forest Products Laboratory, 1 Gifford Pinchot Drive, Madison, WI 53726 2398, USA, 2Wisconsin Department of Natural Resources, Madison, WI 53726-2398, USA
Abstract Because of the potential risk associated with moving emerald ash borer (EAB)-infested firewood, the interstate movement of all hardwood firewood in the USA is currently restricted under the Federal quarantine. Communities and firewood producers are now faced with decisions on how to treat their firewood for interstate commerce. The new US Federal regulations for heat sterilization of ash firewood require holding a core temperature of 718C for 75 min, which is higher than current international standard for heat treating solid wood packaging materials (ISPM 15). A study funded by the US Forest Service Wood Education and Resource Center examined the efficacy of different heat-treatment schemes for meeting the new regulations and developed empirical models for estimating heating times under various heating conditions. This paper addresses some practical issues of the heat-treatment process in terms of meeting the current heating standard for EAB, monitoring temperature changes during heating process and providing thermal verification after the heat-treatment operations.
Keywords: Ash ﬁrewood, emerald ash borer (EAB), heat sterilization, heat treatment.
Introduction Emerald ash borer (EAB), Agrilus planipennis, is a non-native bark- and wood-infesting insect of Asian origin that poses an enormous threat to North American urban and rural forests. Since its discovery in south-eastern Michigan in the summer of 2002 (Haack et al., 2002), EAB has killed millions of ash (Fraxinus spp.) trees in Canada and the USA. As of April 2010, the infested area includes two Canadian provinces (Ontario and Quebec) and 13 states (Illinois, Indiana, Kentucky, Maryland, Minnesota, Michigan, Missouri, New York, Ohio, Pennsylvania, Virginia, West Virginia and Wisconsin) in the USA (http://www.emeraldashborer.info). The US Forest Service estimates that if EAB is not contained or eradicated, it has the potential to cost local govern ments and homeowners approximately $7 billion (present dollars) over the next 25 years to remove and replace dead and dying ash trees (USDA, 2008). This scenario would also result in extensive environ mental damage and long-term changes in the North
American forest structure. To stop further spread of EAB, quarantines are currently in place to prevent infested ash firewood, logs or nursery trees from being transported and starting new infestations. Heat sterilization is a practical and environmen tally friendly treatment to kill pests in solid wood materials and prevent the transfer of pests between regions and states. Current international regulations for heat sterilization of solid wood packaging materials require a minimal core temperature of 568C for 30 min (FAO, 2006; CFR, 2004). To deal with the potential of EAB moving in firewood, the US Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) has implemented a new heating standard for treating ash firewood. The new US Federal regulations require that the core of the firewood be heated to at least 718C for a minimum of 75 min (APHIS, 2010). Compared with heat treating wood packa ging materials, the firewood industry is faced with bigger challenges in heat treating firewood for EAB. A recent study funded by the US Forest Service
Correspondence: X. Wang, USDA Forest Products Laboratory, 1 Gifford Pinchot Drive, Madison, WI 53726 2398, USA. E-mail: [email protected]
(Received 5 January 2010; accepted 12 May 2010) ISSN 1748-0272 print/ISSN 1748-0280 online # This material is declared a work of the United States Government and is not subject to copyright protection:
approved for public release; distribution is unlimited.
Heat sterilization of ash firewood 105 on the heat-treating method, energy resources avail able and the cost of the energy.
Wood Education and Resource Center (WERC) examined the efficacy of different heat-treatment schemes in meeting the new regulations and devel oped empirical models for estimating heating times in various heating conditions (Wang et al., 2009). This paper addresses some practical issues of fire wood heat-treatment process in terms of meeting the current heating standard for EAB, monitoring temperature changes during the heating process and providing thermal verification after the heattreatment operations.
Heating medium The temperature and humidity of the heating medium significantly affect the heating times. Higher heating temperatures yield shorter heating times and heating wood in saturated steam (wet heat) results in the shortest heating times. When the heating med ium is air that is not saturated with steam, there is a wet-bulb depression (the relative humidity is less than 100%) and drying occurs as water evaporates from the wood surface. As the heating medium changes from wet to dry heat, the time needed to reach the required temperature increases.
Factors affecting heat treatment From a practical standpoint, the times required for the centre of a piece of firewood to reach the kill temperature are dependent on many factors, includ ing the type of energy source used to generate the heat, the medium used to transfer the heat (e.g. wet or dry heat), the effectiveness of the air circulation in the heating facility, the species and physical proper ties [configurations, specific gravity, moisture con tent (MC), initial wood temperature] of the firewood. These factors will determine whether the heat-treatment standard can be met and how quickly the treatment process can be completed.
Air circulation Maintaining adequate air circulation is also important in the heat-treatment process. The circulating air performs two functions as it does in kiln drying: it carries heat to the wood to effect evaporation and it removes the evaporated water vapour. Good air circulation insures uniform heat distribution in the chamber and keeps the wood surface temperature high so that the surface-to-center temperature gradi ent is as high as possible. This is usually accomplished with fans and baffles in a treatment chamber. Poor air circulation is one of the reasons for some heating facilities failing to pass heat-treatment certification.
Heat energy Energy is the amount of heat supplied during the heat-treatment process. Heat-treating chambers ty pically employ systems that use steam, hot air (direct fire), electricity and hot water or hot oil as mechan isms to generate the heat necessary to sterilize the wood. The choice of heat energy primarily depends
Heat-treating options There are three possible options to heat treat firewood in field operations. Selection of the heat-treating
Table I. Heating times to a core temperature of 718C for green ash ﬁrewood using dry heat schedules. Actual kiln temp. (8C) Kiln temp. setting (8C)
Heating times (min) Initial temp. of firewood (8C)
-11 2 21 -3 6 21 -10 1 17 -12 1 21 -9 2 18
471 411 396 251 232 198 198 161 144 133 126 118 142 139 129
370 290 333 199 199 176 164 143 94 92 109 89 103 104 83
672 635 484 325 271 257 231 208 173 185 157 138 185 159 146
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methods depends on the type of heating facility, energy sources and the market needs. To examine the efficacy of different heat-treatment options, a series of heating experiments was conducted to address the most important factors that influence the heat sterilization process (Wang et al., 2009). The laboratory heating experiments were conducted un der three heating schemes: (1) dry heat schedule for heat-treating green ash firewood with moisture re duction; (2) wet heat schedule for heat treating green ash firewood without moisture reduction; and (3) dry heat schedule for heat treating seasoned (B20% MC) ash firewood. Approximately 28 m3 of split, barked ash firewood in 40-46 cm lengths was provided by two local firewood producers. Fifteen experimental kiln runs were conducted. Each kiln run contained 1.8 m3 of ash firewood, including various forms and sizes that are normally produced using a firewood splitting machine. During each kiln run, the core temperatures of 30 largest firewood samples were measured using thermocouples inserted to the geo metric centre of each piece of firewood. Detailed procedures of the heating experiments can be found in Wang et al. (2009). Heat treating green firewood with moisture reduction (dry heat schedules) This strategy integrates the heat-treatment proce dures with a kiln-drying process and is considered as a primary option by many firewood producers who have dry kiln facilities. The heating medium used is typically dry heat. The firewood pieces are first heated to the target core temperature of 718C and held for at least 75 min (heat-treatment stage). After the heating standard is met, the firewood loads are kiln dried until the MC of the firewood reaches 20% or below (kiln drying stage). The heating capacity of dry kiln facilities for firewood businesses varies widely. Max imum kiln temperature can range from below 718C to over 1388C, depending on the type of kiln and energy source used. Table I summarizes the heating times to the core temperature of 718C at various heating conditions. The heating time for ash firewood ranged from a few hours at kiln temperatures of 116 and 1388C to over 10 h at a kiln temperature of 778C. The effect of kiln temperature on heating time is also shown in Figure 1. This laboratory study showed that initial tempera ture of firewood had a practical effect on heating times when the kiln temperature was 938C and below (Figure 1). The lower the initial wood temperature, the longer the heating time, as was
Figure 1. Average heating times for green ash ﬁrewood with dry heat at varying kiln and initial wood temperatures.
expected. This implies that heat-treating operations in winter should take into account initial firewood temperature and plan for longer heating times than in warmer seasons. Heat treating green firewood without moisture reduction (wet heat schedules) This heating strategy applies to situations where firewood only needs to be heat treated to meet the US Federal regulations and no drying is required. Heating with wet heat (no or low wet-bulb depres sion) is the most efficient way to accomplish the heat-treatment goal in this scenario. Heating with wet heat yields much shorter heating times than heating with dry heat. Table II shows heating times for green ash fire wood under several different wet heat conditions: three temperatures (71, 77 and 828C) and two wetbulb depressions (08C and 5.68C). The heating times to a core temperature of 718C were generally in the range of 2-4 h in a fully saturated heating Table II. Heating times to a core temperature of 718C for green ash ﬁrewood using wet heat schedules. Kiln temp. (8C)
Heating time (min)
Initial wood temp (8C)
71 71 77 77 82 82
0 5.6 0 5.6 0 5.6
24 23 18 12 13 16
200 229 152 205 151 159
149 165 114 160 119 124
239 273 196 241 189 197
Note: wbd=wet bulb depression.
Heat sterilization of ash firewood 107 Table III. Heating times to a core temperature of 718C for seasoned ash ﬁrewood using dry heat schedules. Heating times (min.) Kiln temp. setting (8C)
71 77 82
Initial temp. of firewood (8C) -6 6 19 20 18
Note: N/A=not applicable.
temperature of 718C, compared with 5.6-8 h for green ash firewood. Figure 2. Average heating times for green ash ﬁrewood with dry heat and wet heat schedules. Dry-bulb temperature of heating medium =778C; wbd= wet-bulb depression.
Temperature monitoring and thermal verification
Heat treating seasoned firewood (dry heat schedules)
APHIS Plant Protection and Quarantine (PPQ) enforcement regulations require that a heat-treating facility be inspected and certified by a qualified PPQ official for initial qualification. Certified heattreatment facilities are required to monitor the core temperature of the firewood during heat-treatment operations and provide a temperature history re cord of each heat-treatment run as a thermal verification. The sensors used to monitor firewood tempera ture need to be calibrated and read within 90.58C of the treatment temperature. Sensors should be properly inserted into the largest firewood pieces being monitored and reach the centre of the crosssection. The firewood samples monitored are required to be placed in the coldest areas of the
Another scenario for commercial firewood producers is heat treatment of firewood after it has been airdried (B20% MC). This procedure would only be used to meet the US Federal regulations in order to move firewood freely from EAB quarantine areas. Although both dry heat and wet heat schedules can be used to heat treat seasoned firewood, dry heat schedules are more likely to be used as firewood producers who air-dry firewood typically do not have a traditional steam kiln. The experimental heating times for seasoned fire wood with dry heat schedules are shown in Table III. The comparison of heating times between green and seasoned firewood is illustrated in Figure 3. The results indicate that heating time for seasoned fire wood is significantly less than that for green firewood (p50.009). In the case of 778C kiln runs, it took 24.8 h to heat the seasoned ash firewood to a core
Figure 3. Average heating times to a core temperature of 718C for green and seasoned ash ﬁrewood.
medium with a kiln temperature of 71-828C. Heat ing time increased significantly as wet-bulb depres sion increased from 0 to 5.68C (p50.012). Figure 2 shows the effect of heating medium on average heating times to a core temperature of 718C for green ash firewood at 778C kiln temperature. Note that under similar initial wood temperature condi tions, the heating times for wet-heat schedules (0 and 5.68C wet-bulb depression) were significantly lower than those for dry-heat schedule (pB0.001), and the heating medium with 08C wet-bulb depres sion resulted in the shortest heating time.
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Figure 4. Temperature monitoring system in a ﬁeld heat-treatment operation.
chamber. The internal wood temperature should be collected at least once every 5 min and stored in a data file. Most firewood producers in the USA are facing challenges in meeting these requirements owing to lack of a reliable temperature monitoring system in the heating facilities. Currently, there is no temperature equipment readily available for such applications. In a production trial of the study, a temperature monitoring system was designed and installed in a commercial kiln to demonstrate the
temperature monitoring and thermal verification process. Figure 4 shows a temperature monitoring system used in a field heat-treatment operation. The facility in this operation includes a hot water boiler (fuelled with waste wood) and a dry kiln with a capacity of holding approximately 51 m3 of firewood. The temperature monitoring system consists of four thermocouple wires, a four-channel data logger and a laptop computer. These four thermocouple wires are connected with the data logger and the data logger can be initiated before a heat-treatment run. One of the thermocouple wires is used to measure air temperature (kiln temperature) and the other three wires are used to measure the core temperatures of the firewood samples in three baskets located in the cold area (back row) of the kiln. Figure 5 shows the temperature data recorded during a heat-treatment run and demonstrates that the treatment has met the heating standard for EAB. This temperature history chart, along with the original temperature data file, can serve as proof of a successful heat treatment for EAB. Acknowledgement
Figure 5. Temperature record of a ﬁeld heat-treatment run. EAB=emerald ash borer.
The work upon which this article is based was funded in part through a grant awarded by the Wood Education and Resource Center, Northeastern Area State and Private Forestry, Forest Service, US Department of Agriculture.
Heat sterilization of ash firewood 109 References APHIS (2010). Treatment schedules T300--Schedules for miscella neous plant products. p. 5-4-38. Retrieved April 8, 2010, from http://www.aphis.usda.gov/import_export/plants/manuals/ports/ downloads/treatment_pdf/05_04_t300schedules.pdf CFR (2004). Rules and regulations*Importation of wood packa ging material (7 CFR Part 319). Federal Register, 69 (179), 55719-55733. FAO (2006). International standards for phytosanitary measures: Guidelines for regulating wood packaging material in interna tional trade (Publ. No. 15., 2002, with modiﬁcations to Annex 1, 2006). Rome: Food and Agriculture Organization of the United Nations.
Haack, R. A., Jendek, E., Liu, H., Marchant, K. R., Petrice, T. R., Poland, T. M. & Ye, H. (2002). The emerald ash borer: A new exotic pest in North America. Newsletter of the Michigan Entomological Society, 47 (3-4), 1-5. http://www.emeraldash borer.info. Accessed April 8, 2010. USDA (2008). Emerald ash borer: Control may be on the horizon. USDA Forest Service Northern Research Station Research Review, 2. Wang, X., Bergman, R., Simpson, W. T., Verrill, S. & Mace, T. (2009). Heat-treatment options and heating times for ash ﬁrewood (Gen. Tech. Rep. FPL-GTR-187). Madison, WI: Forest Products Laboratory, Forest Service, US Department of Agriculture.
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Heat sterilization of ash (Fraxinus spp.) firewood: Heat-treating options, temperature monitoring and thermal verification
X. Wanga; R. D. Bergmana; T. Maceb a USDA Forest Products Laboratory, Madison, WI, USA b Wisconsin Department of Natural Resources, Madison, WI, USA Online publication date: 03 August 2010
To cite this Article Wang, X. , Bergman, R. D. and Mace, T.(2010) 'Heat sterilization of ash (Fraxinus spp.) firewood: Heat-
treating options, temperature monitoring and thermal verification', Wood Material Science and Engineering, 5: 2, 104 — 109 To link to this Article: DOI: 10.1080/17480272.2010.496535 URL: http://dx.doi.org/10.1080/17480272.2010.496535