A solution for getting rid of Oregon’s mounting piles of wood waste while sparing Oregonians lungs may come in the form of mobile air curtain incinerators (ACI). About the size of a camper trailer, ACIs are essentially open metal boxes. Woody debris is placed in the burn box and set alight. Fans then blow air over the top of the material, forcing it to burn at much higher temperatures – as much as 2,500 degrees Fahrenheit. That’s about one-fourth as hot as the surface of the sun.
At those blazing temperatures, smoke emissions are reduced by more than 90 percent compared to pile burns. Tests done by the Oregon Department of Environmental Quality in spring 2023 verified that ACIs burn much cleaner than open burning This makes them ideal for disposing of wood waste near cities and towns and the Willamette Valley.
Those airsheds have stringent state protections against wood smoke. These rules limit pile burning to narrow windows of time outside of fire season when weather conditions are just right for carrying smoke away from the protected area and dispersing it high in the sky.
“By contrast, air curtain incinerators can dispose of wood waste relatively close to population centers without significantly impacting air quality,” said Cody Holthouse with the Oregon Dept. of Agriculture. “The need for this is growing for a number of reasons beyond traditional logging slash.”
Holthouse cited growing amounts of woody debris from:
- Land clearance for housing and farms
- Limb pruning in expanding numbers of orchards
- More and more trees killed by drought, diseases, and wildfires
- Tree-killing pests like emerald ash borer
Each of these is increasing the volume of wood people need to dispose of, said Holthouse, who chairs the state’s Interagency Task Force on Emerald Ash Borer. “Leaving wood waste unburned in big piles allows infested plant materials to be moved for firewood use, potentially spreading pests or pathogens (e.g., emerald ash borer, Mediterranean oak borer, sudden oak death) to novel locations.
Matt Mills, an Emerald Ash Borer Specialist with the Oregon Dept. of Forestry, said “Smaller volumes can be chipped and spread on site or used as mulch, but it’s not always desirable to spread chips from diseased trees on the ground.”
“Burning in an ACI is a great option for disposing of pest-infested trees.,” Mills said. “The mobile unit can be taken to where trees are being felled so pest-infested wood that’s under quarantine doesn’t have to be transported, eliminating the risk of spreading EAB.”
Mills said any firewood from hardwood trees is quarantined in counties where EAB has been confirmed (Clackamas, Marion, Washington, and Yamhill), as is the following material from ash trees, olive, and white fringetrees (Chionanthus virginicus):
- Nursery stock
- Scion wood
- Bud wood
- Logs
- Roots and branches
- Stumps
- Green lumber
- Chips and mulch
Mills said ODF has contracted with the Canby-based firm Valley Environmental and Greenside Construction, Inc., of Happy Valley to bring their mobile ACI to a series of demonstration burns this spring.
“For example, the Columbia Soil and Water Conservation District has been working to clear invasive hawthorn and blackberries from land at the Scappoose Airport. They wanted to see how well an ACI could handle the debris, so we’re holding a burn there with it on May 13,” said Mills. “Airplanes need good visibility to land and takeoff, so this will be a good demonstration of the ACI’s ability to burn cleanly.”
Lastly, instead of leaving a pile of ash, burning wood in an ACI at high temperatures produces a usable product called biochar. Biochar is a highly porous, carbon-rich residue resembling charcoal. It can be used as a soil amendment to improve water absorption, especially in quick-draining, sandy soils.
