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Wood Drying and Heat Treating

A forklift carrying a pallet of fried lumber out of a drying kiln.

Do I need a Permit for my Wood Drying or Heat-Treating Operation?

In most cases, an air permit will need to be obtained for operations utilizing a boiler or burner to supply heat for a kiln or rotary dryer to produce dimensional lumber, wood fuel, pellets, or other dried wood products. Kiln drying of cordwood is less of a concern due to the lower surface area causing less volatile organic compounds (VOCs) to be released. Heat treating of wood for pest control is also less of a concern for VOCs, but instead would be evaluated based on the equipment supplying heat to the operation. Please see our boiler source category webpage for information regarding the permitting thresholds for boilers.

There are two main source of air pollution from drying wood:

  1. VOCs that are liberated from the wood during the drying process, and
  2. the air pollutants resulting from combustion of fuel to produce process heat.  

VOCs are of concern because they contribute to ground level ozone, one of the main components of “smog”.

Kiln Dried Lumber Facilities

The quantity of VOCs released from a lumber drying operation will depend on several factors, including wood species, size of kiln, and drying schedule. Facilities that produce kiln dried lumber for commercial sale should already have a good understanding on these parameters, as they will affect the final quality of the finished product. Typically lumber drying kilns are collocated at a sawmill with other forms of air pollution, including particulate matter (PM) generated from sawing and sanding operations, as well as air contaminants from combustion equipment (stationary engines, boilers, etc). These operations will typically trigger the need for an air permit regardless of the lumber drying operations.  

Emission of VOC from the lumber drying kilns will be estimated based on the following emission factors:

VOC Emissions from Kiln Drying Dimensional Lumber

Lumber Type VOC Emission Rate (lb/1000BF)1 REFERENCE
Hardwood 1.44 Forest Product Journal article "Estimated VOC Losses During the Drying of Six Eastern Hardwood Species" Oct. 2006
Softwood 2.26 NCASI Technical Bulletin 718 “A Small-Scale Kiln Study on Method 25A Measurements of Volatile Organic Compound Emissions from Lumber Drying” July 1996

1lb/1000 BF is a pound of VOC released per 1,000 board feet of lumber dried. A board foot is a unit of volume for timber equal to 144 cubic inches.

Utilizing the emission factors above, a lumber drying operation would be able to dry up to approximately 6.5 million board feet of hardwood or approximately 4 million board feet of softwood per 12-month period before exceeding the five ton per year threshold and be required to get an air permit.

Due to the fugitive nature of VOC releases from a kiln dried lumber operation, commissioning stack tests are not feasible. In addition, there are no air pollution control technologies applicable for this type of operation. Therefore, facilities are encouraged to adopt permit-enforceable limits on the throughput of their kilns to avoid Non-attainment New Source Review (NNSR).

Pellet Plants

Pellet plants typically employ a process in which a wood burner exhausts heat and gases through a rotary dryer that drives off the moisture from wood chips. This mixture of gases resulting from the burning of fuel are mixed with the moisture and VOCs driven off from the chips and are then exhausted through a central stack. This exhaust will contain several air pollutants including, particulate (PM), volatile organic compounds (VOC), carbon monoxide (CO), oxides of nitrogen (NOx), and hazardous air pollutants (HAPs). Dried chips will then subsequently be processed through a hammermill and pelletizer to achieve the final sizing of the desired pellet which results in the additional emissions of PM.

Emissions of air contaminants from the burner/dryer will depend on several factors including, the burner design, quantity of material dried, species of wood, and inlet and outlet temperatures. Permittees will be required to install monitoring equipment to calculate hourly average inlet and outlet temperatures of the dryer. In addition, a commissioning stack test will be required for facilities to demonstrate compliance with permitted emission limits. Air pollution control equipment for pellet plants are typically cyclones, multiclones, and fabric filters for PM. Larger plants may choose to employ wet scrubber or electrostatic precipitators (ESPs) for additional PM control. Other technologies exist to control VOCs and HAPs, such as afterburners or thermal oxidizers, but these are typically cost prohibitive for the size of plants seen in Vermont.  

Kiln Dried Cordwood and Heat Treating

In Vermont, kiln drying or heat treating for control of pests of cordwood are relatively small operations, and are typically not collocated with a larger sawmill or pellet operation. Emissions of VOCs from the drying of cordwood are expected to be lower due to the reduced surface area of the material being dried, lower drying temperatures and higher final moisture content.  Emissions of concern are more likely to be associated with the fuel burning equipment used to provide heat to the drying process. For a facility using propane or distillate oil to dry cordwood, a permit is not required unless the design capacity heat input to the fuel burning equipment exceeds 10 million British Thermal Units (BTUs) per hour. For facilities using wood fuel, a permit would be required if allowable emissions equal or exceed five tons per year, or if the design capacity heat input to the wood fuel burning equipment exceed 90 boiler horsepower (900 square feet of boiler heating surface area). 

State Regulations for Wood Drying Operations

Regardless of the size of your wood drying operation, any fuel burning installation in Vermont is subject to PM emission limits (§5-231), opacity limits (§5-211), and restrictions on the type of fuel being burned (§5-221). If wood is used as a fuel, Section 5-204 of the Regulations requires that certain wood-fired heating equipment is EPA certified. Facilities that emit or propose to emit air contaminants exceeding 5 tons per year are also required to register their emissions with the Agency annually, pursuant to Subchapter VIII of the Regulations. Applicants are encouraged to familiarize themselves with the Regulations and to contact an environmental consultant when preparing an application for an air permit.

What Do I Need to do to Obtain an Air Permit?

Your first step should be to talk with one of the permitting engineers – it might turn out that a permit is not required.  If you want to read more about the air permitting process, then please refer to our Permit to Construct Application Guidance webpage for more information.  Note that if a permit is required for a proposed project, you cannot commence construction until the permit has been issued.

What Other Permit or Requirements May Be Applicable to My Project?

The Department of Environmental Conservation Environmental Assistance Office has permit specialists that can provide assistance in determining what other state permits or programs may be applicable to your project. In addition, the Agency has developed a Permit Navigator that can assist in determining what permits you may need for your project.