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New county technology burns fire fuels with little-to-no smoke
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New county technology burns fire fuels with little-to-no smoke


In the middle of autumn, a prescribed burn caused plumes of smoke to be seen above the tree tops in Flagstaff. But when the wind did not deliver as forecast, the smoke settled in the streets of downtown Flagstaff and caused unhealthy levels of smoke to waft into the homes of residents, jabbing at the health of locals and tourists in the area.

As forest officials continue to create a more healthy ponderosa pine ecosystem by cutting thick sections of forests and conducting controlled burns of pine needles on the forest floor, fire and smoke will be a fact of life for people who live around the four forests of northern Arizona.

On the other side of the coin, forest thinning efforts often face vehement opposition on the issue of smoke. For years, no alternative has existed beyond asking healthy and at-risk residents to stay inside and shut their doors and windows.

But now Coconino County is testing its newly purchased technology, called an Air Curtain Incinerator. The air curtain is prized for its ability to burn with less smoke, cleaner than normal pile burns, which county managers are considering using as a tool to keep large amounts of smoke out of communities.

To the untrained eye, the air curtain looks like a dumpster. Once leftover tree bits, or biomass, begin to burn inside the incinerator at over 1,500 degrees, a fan acts like a lid and stops the smoke from escaping the incinerator until the harmful smoke is burned away. Once the air curtain warms up, it can burn tons of biomass per hour without smoke escaping the top, said forest director Jay Smith. The air curtain cannot burn at the same rate as pile burns can accomplish, but its value is in how little smoke it produces.

“When you put slash in and it breaks that [air] curtain, it will let a little smoke out. Otherwise, you don’t even know it’s burning,” Smith said. “You’ll see heat waves, and that’s about it.”

Smith said the technology is not new. Incinerators are used to burn municipal waste in landfills, or excess waste after hurricanes, but is now just getting into the forestry field. The incinerator costs $120,000 and an extra $10,000 for training and additional equipment.

Particulately dangerous

Particulate matter, or PM-2.5, is what concerns Arizona Department of Environmental Quality officials the most. ADEQ monitors smoke impacts on communities in areas like Flagstaff.

Erin Jordan, spokesperson for Arizona Department of Environmental Quality, said smoke impacts people with healthy lungs and respiratory systems differently than young children, elderly or people with pre-existing conditions.

Being exposed to particulate matter for hours and days can aggravate lung disease, causing asthma, acute bronchitis, increase susceptibility to respiratory infections, and an increased risk of heart attack or irregular heartbeat, Jordan said. Long-term exposure, over years, has been associated with problems like reduced lung function and developing chronic bronchitis.

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According to the U.S. Forest Service studies, the air curtain incinerators' ability to reduce the amount of airborne particulate matter is more than just smoke and mirrors. Pile burns can cause 25.5 pounds per ton of particulate matter emissions, compared to an average prescribed burns causing 36 pounds per ton of emissions. The air curtain only creates 1.1 pound per ton worth of emissions.

“ADEQ supports any proactive efforts by permittees to better protect public health and the environment,” Jordan wrote in an email. “Since these incinerators can reduce the impacts of smoke on nearby residents compared to open burns, ADEQ supports their permitted use.”

The benefits this technology could have on communities has led Aaron Green, district manager of the Arizona Department of Forest and Fire Management, to work on finding creative solutions to expand their use across northern Arizona and regions with ponderosa pine forests.

“They could be amazingly beneficial for helping communities doing firewise work inside communities having homeowners bring their slash down to these units that are stationed in community for a weekend or two, not running into any difficulty,” Green said.

Optimizing new technology

If the piles of tree bits are not burned, they need to be taken elsewhere, said Coconino County Supervisor Art Babbott. He said leaving leftover biomass on the forest is no longer considered a best practice after the Museum Fire, which spread and burned nearly 2,000 acres, creating risks of post-fire flooding in Flagstaff due to leftover biomass.

If not burned, the piles would likely be hauled to Phoenix to be made into pallets, a viable but costly alternative for biomass at this point. A single haul costs around $700 to $1,000 for material that has relatively little value, leading to high costs for private businesses, Smith said.

“If we had a power plant that was within 30 miles of Flagstaff, that could change the scenario quickly,” Babbott said. “But [the air curtain] could be used in small communities.”

The county is required to obtain a permit from the Arizona Department of Environmental Quality before starting a burn and is getting permits for 35 tons of biomass per day; however, the air curtain’s capacity is much higher. As burning goes, 35 tons of tree bits a day is not a lot -- large pile burns can go through up to 100 tons of dead and downed branches and needles in an hour.

Jordan told the Arizona Daily Sun that permits beyond the 35-ton limit exist, but require a more complex permitting process, like modeling to show the smoke would not exceed federal or state regulations.

The county had not tried the more complex permit system, Smith said, but was open to exploring its future use. When the county was testing the air curtain in early December, Smith said the county was considering buying larger air curtains or multiple air curtains to work within the bounds of the 35-ton permit.

At this point, only time will tell where the air curtain will fit in the forest thinning toolbox, and whether it can become an avenue for cleaner burns near people’s homes.

Scott Buffon can be reached at, on Twitter @scottbuffon or by phone at (928) 556-2250.


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