I have heard of air curtains, but had not seen one being used until today. After Hurricane Andrew devastated a large section of south Florida in 1992, an air curtain was used to dispose of a huge pile of debris (we lovingly called Mount Trashmore) which for a while was the highest point in that part of the state.
An air curtain is not terribly sophisticated. It simply pumps air into the area where material is burning. The extra air flow and oxygen greatly shortens the time it takes to burn the fuel.
Wednesday I visited a site near Custer, South Dakota where Bayfield, Colorado based NRG Consulting Services is conducting contract work on private land. They are thinning and reducing the fuel around structures as well as cutting and burning beetle-infested trees.
Some of the thinned and bug trees they are cutting are green, partially green, or at least have a moisture content high enough to make it impossible to burn in a conventional slash pile right away. But if they wait for the wood to dry out enough to burn, the beetles will hatch and disperse to other trees.
The brand new portable unit they were using Wednesday has a movable ceramic-lined firebox mounted to a trailer. The box is raised and lowered by a hydraulic lift system. Once on site, the fire box, which has no bottom or floor, is lowered to the ground. So the burning occurs on the ground, surrounded by the box. If you notice the line of holes in the wall of the box in the photo below, that’s where the air comes out at a fairly high velocity. A small diesel engine powers the blower and the hydraulics that raises and lowers the fire box.
While I was at the site the system was producing virtually no smoke. The photo below shows what was left after a full day of use. At the end of the day they let it sit overnight, then the next morning they may raise the fire box and move it to a different location.
While a fire in an air curtain is less likely than a burn pile to escape, there is some fire hazard associated with the system. At one point I happened to look down at the sleeve of my cotton shirt and saw a small burning ember about the size of a gnat. I brushed it off and saw that it left a small burn mark on my shirt. Then I noticed that the cotton shirt of the crew boss had at least a dozen holes just like the new one in my shirt. The gentleman closest to the machine, the person feeding it, was shrewdly wearing a Nomex shirt. I looked around and didn’t see any spot fires in the area. A burning permit is required to operate an air curtain in the Black Hills of South Dakota, which these folks had.