Firewood kilns are ovens, run at high temperatures for long periods of time, typically powered by burning wood waste or natural gas, and filled with combustible fuel (green or seasoned firewood). There are potential fire risks associated with firewood kilns. Kiln operators may have operational concerns about the temperatures at which the kilns are run. Energy costs are significant drivers of production costs of kiln-dried or heat-treated firewood, and kiln operators try to produce their product with the lowest possible energy costs. Sometimes, the processor will only dry seasoned firewood in the kiln in order to reduce the energy needed to reach the desired temperature or moisture content. Lower ambient temperatures outside of the kiln (whether due to winter conditions or generally colder regions) increase the amount of energy required to process firewood within the kiln. Sometimes the preferred temperature that a given kiln operates at may be below that required by for a regulatory treatment. This guide will introduce the basics of firewood heat-treatment and includes selected references.
Kiln-dried versus heat-treated firewood:
Kiln-dried and heat-treated firewood are terms that are often used interchangeably, which is problematic as they have significantly different meanings. The processes are similar, but the end product, legal implications, and commercial goals are different. Kiln-dried is an industry term without any accompanying standard, norm, or certification- it simply refers to the fact that the end product has had some level of exposure to heat within a kiln with the intent to make a low moisture content product. Kiln-dried does not necessarily mean that the wood has been disinfested. Heat-treated references a certifiable treatment sufficient to disinfest wood, with the certification overseen by a qualified entity (state, federal, or third-party). Heat-treated does not reflect on the moisture content or other quality of the resulting product. A given bundle of firewood may be kiln-dried, heat-treated, or both- but neither process technically has any bearing on the other.
Producers of kiln-dried firewood are using this process to market their firewood as a dry wood product that is more desirable by consumers for uniformity, cleanliness, ease of lighting, and reduction in smoke and creosote when burned. Generally, kiln dried firewood commands a higher price on the market than seasoned or green firewood. Once loaded into a kiln, the firewood takes a day or more to come up to sufficient temperature and then run long enough to drive the desired amount of water weight out of the firewood and render it properly kiln-dried. In many cases, but not all, kiln-dried firewood could simultaneously meet the requirements for heat-treatment, if the kiln operator obtained the appropriate documentation, Compliance Agreements, and/or certification(s).
The purpose of firewood heat treatment, or heat disinfestation, is not about the dryness of the wood, but rather that the internal wood temperature has reached levels sufficient to destroy pests and pathogens on or in the wood. When wood is heat-treated, the primary motivation is typically that the wood will comply with local or regional regulations. In some cases, secondary motivations are that the wood is more marketable to regional distributors and/or regional consumers who believe that heat treated firewood is cleaner, better, etc. A subset of kiln operators will only reach heat treatment standards for their wood and then not continue to heat it long enough to reduce the moisture content and therefore also kiln-dry it. Heat treated wood that has not been kiln-dried may retain significant water weight and will not have the same value-added benefits to the consumer as kiln-dried firewood. If the market doesn’t support the sale of kiln-dried firewood, and regional distributors are not requiring heat treatment to sell on the wholesale market, investing in a kiln to comply with heat treatment requirements may not make economic sense to a small scale firewood producer.
Certification process:
The goal of certification is to ensure that firewood that is processed in a kiln reaches a set internal wood temperature for a specific period of time in order to kill pest and pathogen life stages and thereby reduce the risk of transporting pests. Fundamentally, this means that the heat treatment facility must be set up to monitor and record temperatures within the center of large pieces of firewood, and that these temperatures and processes are auditable by regulators.
Facility certification requirements typically include that operators need to regularly monitor their processing of firewood by having temperature probes that record ambient kiln temperature, as well as temperatures within the center of three pieces of firewood placed in pre-determined cold spots within the kiln. The probes are inserted into holes drilled into firewood and secured with putty in order to measure true temperatures in the wood rather than kiln ambient temperature. Probes should be capable of recording data at least once every five minutes and data should be stored for review for at least one year. Firewood kiln operators tend to replace probes on a regular basis as malfunctioning monitoring probes do not record data.
Regulators should periodically review a kiln’s data records for heat-treating firewood as part of the certification process. It is highly recommended that regulators annually observe a kiln being loaded and probes placed to ensure that probes are being used accurately. The USDA has found that when kilns fail a certification test it is most often because; there is poor air flow because the fans are not working, packing materials or bins are restricting air flow, the facility sensors are not properly located in the center of stacks of firewood, or the facility sensors are not working and accurately recording data (Mitchell Dykstra, USDA APHIS PPQ). Due to the insulating qualities of wood, internal wood temperatures often do not come to the same temperature as the dry bulb temperature. All monitoring probes reaching the determined level at the same time, or within a very short interval, indicates that the probes monitoring wood temperature were not sufficiently isolated from the kiln’s ambient temperature.
It is recommended that certification include calibration of kiln temperature readings. Firewood kiln probes are generally not calibrated in the same way that lumber kilns are. Probes are assumed to be calibrated when they leave the manufacturer and are replaced when they malfunction. USDA federal certification has included calibrating the kiln using independent temperature thermocouples, and this is an effective way to verify the temperatures that the kiln is reaching as well as the temperatures measured within the wood. Some states have used other processes to calibrate kiln probes during certification.
Dykstra, Mitchell. 2019. Heat treatment certification programs: essential elements and how to conduct (pgs. 328-346). Presentation prepared for the 94th Annual Meeting of the Eastern Plant Board.
Heat Disinfestation
Hansen et al., 2011 provides a thorough overview of heat disinfestation. When heat is used as a disinfestation treatment, it is used to cause thermal death of the pests or pathogens that may be present. Thermal death may be caused by denaturation or coagulation of proteins or accumulation of toxic products due to a metabolic disturbance. Thermal death point can be altered by a variety of factors, including humidity, degree of starvation, temperature acclimation, or age. For invertebrates, application of heat can cause acute mortality or long-term lethal effects (chronic mortality). Firewood kilns use a dry heat strategy, which has the disadvantage of being a slow process with the surface heating before the interior of the wood.
Hansen, J.D., Johnson, J.A., Winter, D.A. 2011. History and use of heat in pest control: a review. International Journal of Pest Management. 57(4): 267-289.
Pathogens
Pathogens present additional considerations when determining a heat treatment regimen. There are some thermophilic fungi that create special structures that make them more heat tolerant. These thermophilic fungi tend to be located in the heartwood rather than sapwood. It should be noted that some of the experiments on required temperatures for fungi vary the minutes that the heat is applied from 1 min to 75 minutes or more. Fungi that survive elevated temperatures for shorter durations may not survive longer durations.
The conditions for fungi to thrive on timber/wood include:
- The temperature must be between 10˚C and 54.5˚C. The optimal temperature for fungal growth occurs between 21˚C and 32˚C. Below 10˚C, fungi are dormant and above 54.5˚C they are killed.
- There must be sufficient oxygen present.
- There must be sufficient moisture available. Generally, there is sufficient moisture when the moisture content in wood is > 22%.
- Food, in the form of sugar, must be available. There is generally inadequate food supply in the heartwood, so fungi are found predominantly on sapwood.
Elimination of one of these elements will control the fungus.
Armstrong, J.W., Brash, D.W., Waddell, B.C. 2014. Comprehensive literature review of fumigants and disinfestation strategies, methods and techniques pertinent to potential use as quarantine treatments for New Zealand export logs. A report prepared for: Scion. Plant Food Research Contract No. 279714. Milestone No. 1.3.1. Job code: P/332016/01. SPTS No. 10678. 190 pgs. See especially page 102 discussing Gan (2005).
Treatment standard information
While there are some studies that directly measure heat-disinfestation of firewood, more information exists about disinfestation of lumber and wood packing material (WPM). Results from work done in lumber can inform decision-making about standards for firewood, but firewood is a much less consistent product than lumber. Additionally, lumber and WPM is often de-barked during processing, whereas bark is generally not removed purposely from firewood. The presence of bark may impact the ability of pests and pathogens to survive heat treatment as well as make the treated product more attractive for post-treatment infestation or infection. It is critically important that when discussing the heat-treatment standards that the standard refers to the temperature and time as measured in the center of a piece of firewood, not the ambient kiln temperature. Throughout studies of heat disinfestation of wood and firewood, there is some inconsistency with whether temperatures are measured within the core of the wood. This can make it challenging to compare studies and efficacy of standards. Regulators should encourage measuring and reporting wood temperatures within the core of firewood in order to create a standard with greater reproducibility.
There are three different treatment regimens commonly referenced for heat disinfestation of wood:
- T314-b: 56˚C (133˚F) for 30 minutes as measured in the interior of treated wood (All logs (including firewood) from gypsy moth quarantine areas)
- T314-a: 60˚C (140˚F) for 60 minutes as measured in the interior of treated wood (Regulated wood articles, including Fraxinus (ash logs and firewood) and all hardwood firewood from emerald ash borer quarantine areas)
- T314-C: 71.1˚C (160˚F) for 75 minutes as measured in the interior of treated wood (Regulated wood articles: various wood pests)
T314-b: 56˚C (133˚F) for 30 minutes standard information
T314-b is the ISPM 15 standard for international shipment of wood packing material (WPM) implemented in 2002. This standard was meant to have broad efficacy against a range of pathogens and pests without the specific goal of eliminating all phytosanitary risk for every potential pest. Revisions to ISPM 15 for WPM included that wood is free of bark and that treatment was that the core temperature of the wood is maintained at 56 ˚C for 30 continuous minutes. Reviews of ISPM 15 standards found that heating wood to 56˚C for 30 min is sufficient to kill many insects, nematodes, and pathogens, with emerald ash borer noted as being more heat-tolerant than most species studied and therefore requiring a higher standard (T314-a: 60˚C for 60 min). This standard has been shown to be lethal to fungi, although other variables like an extended drying period prior to heat treatment can result in greater thermotolerance. Thermophilic fungi can tolerate and grow at temperatures higher than 50˚C, and may require heat treatment at higher temperatures for longer durations.
Haack, R.A., Brockerhoff, E.G. 2011. ISPM No. 15 and the incidence of wood pests: recent findings, policy changes, and current knowledge gaps. Paper prepared for the 42nd Annual Meeting of the International Research Group on Wood Protection. IRG/WP 11-30568.
Juzwik, Jennifer. USFS. Personal comment. Oak wilt fungus was eradicated from red oak logs taken from western Wisconsin trees using the 56˚C for 30 min treatment standard as measured at 2 inches into the wood with vacuum steam heat treatment.
NAPPO Science and Technology Documents. ST 05: Review of heat treatment of wood and wood packaging. 2014. Prepared by the members of the NAPPO Forestry Panel. 35 pages.
Ramsfield, T.D., Ball, R.D., Gardner, J.F., and Dick, M.A. 2010. Temperature and time combinations required to cause mortality of a range of fungi colonizing wood. Canadian Journal of Plant Pathology. 32(3): 368-375.
Uzunovic, A., and Khadempour, L. Heat disinfestation of MPB-affected wood. Mountain Pine Beetle Initiative. MPBI Project # 8.41. 27 pgs.
T314-a: 60˚C (140˚F) for 60 minutes standard information
T314-a is the federal emerald ash borer (EAB) standard for certifying firewood originating in EAB-regulated areas. EAB expresses heat-shock proteins making it tolerant to heat and requiring standards higher than T314-b. Because of EAB’s ability to withstand less stringent standards, T314-a should be considered the minimum standard used to certify heat-treated firewood. This standard can be effective against EAB, Agrilus bilineatus, walnut twig beetle, and the thousand cankers disease pathogen. Some fungi that have been shown to survive the T314-b standard are controlled by the T314-a standard. Highly thermophilic fungi, however, may create specialized structures that allow them to survive periodic high heat exceeding 60 ˚C.
Haack and Petrice (not yet published, personal comment): Recorded 99% Agrilus bilineatus mortality in oak logs subjected to 56˚C for 30 minutes, when ambient kiln temperature was at a constant 65˚C or less. There was 100% mortality when the ambient kiln temperature was held at 70˚C or more, or when the core temperature was recorded at 60˚C, regardless of ambient kiln temperature. This re-iterates the importance of describing the target temperature as measured within the interior of the firewood, rather than the ambient kiln temperature.
Juzwik, Jennifer. USFS. Personal comment. Oak wilt fungus was eradicated from red oak logs taken from western Wisconsin trees using the 60˚C for 60 minutes treatment standards as measured at 2 inches into the wood with vacuum steam heat treatment. Viable propagules of one or both of the rapid ohia death pathogens (Ceratocystis lukuohia and Ceratocystis huliohia) were eliminated from ohia logs during vacuum steam trials at a regimen of 60˚C for 60 min, where the temperature was monitored at 70% of log radius and logs were 12-18 inches in diameter. These pathogens form aleurioconidia (thick-walled spores).
Mayfield, III, A.E., Fraedrich, S.W., Taylor, A., Merten, P., and Meyers, S.W. 2014. Efficacy of heat treatment for the thousand cankers disease vector and pathogen in small black walnut logs. Journal of Economic Entomology. 107(1): 174-184.
Myers, S.W., Fraser, I., and Mastro, V. 2009. Evaluation of heat treatment schedules for emerald ash borer (Coleoptera: Buprestidae). Journal of Economic Entomology. 102(6): 2048-2055.
Ramsfield, T.D., Ball, R.D., Gardner, J.F., and Dick, M.A. 2010. Temperature and time combinations required to cause mortality of a range of fungi colonizing wood. Canadian Journal of Plant Pathology. 32(3): 368-375.
Sobek, S., Rjamohan, A. Dillon, D., Cumming, R.C., and Sinclaire, B.J. 2011. High temperature tolerance and thermal plasticity in emerald ash borer Agrilus planipennis. Agricultural and Forest Entomology. 13: 333-340.
T314-c: 71.1˚C (160˚F) for 75 minutes standard information
T314-c is the highest treatment standard in the USDA’s Logs and Firewood T314 series. This is the standard referenced by many of the states that were early adopters of exterior firewood quarantines. This standard could control highly thermophilic pathogens, like Schizophyllum commune, which has been shown to be able to withstand an exposure temperature of 60˚C for 60 min. The current Asian longhorned beetle regulated areas within the USA use T314-c as the required disinfestation treatment level, although a study has found T314-b to be adequate.
Myers, S.W. and S.M. Bailey. 2011. Evaluation of the ISPM-15 Heat Treatment Schedule for Asian Longhorned Beetle, Anoplophora glabripennis (Coleoptera: Cerambycidae) For. Prod. J. 61: 46-49.
Ramsfield, T.D., Ball, R.D., Gardner, J.F., and Dick, M.A. 2010. Temperature and time combinations required to cause mortality of a range of fungi colonizing wood. Canadian Journal of Plant Pathology. 32(3): 368-375.
Uzunovic, A., and Khadempour, L. Heat disinfestation of MPB-affected wood. Mountain Pine Beetle Initiative. MPBI Project # 8.41. 27 pgs.
Uzunovic, A., Khadempour, L., and Leung, K. 2008. Heat disinfestation of decay fungi found in post-mountain pine beetle wood. Mountain Pine Beetle Working Paper 2008-14.MPBI Project #8.61. 12 pgs.
Sobek, S., Rjamohan, A. Dillon, D., Cumming, R.C., and Sinclaire, B.J. 2011. High temperature tolerance and thermal plasticity in emerald ash borer Agrilus planipennis. Agricultural and Forest Entomology. 13: 333-340.
Summary
Regulators establishing heat-treatment certification standards should consider their ultimate goal. Many insects and pathogens are controlled with temperature standards of 56˚C for 30 min as measured in the interior of heat treated firewood. A 60˚C for 60 min temperature treatment standard results in additional control of thermotolerant insects and pathogens. Some highly thermophilic pathogens that form protective structures may require the 71.1˚C for 75 min treatment regimen to achieve 100% control. When reviewing established literature and when communicating about heat-treatment requirements, it is critically important to note whether temperatures are measured within the interior of large pieces of firewood or are ambient kiln temperature.