What Is an Air Curtain Burner — and How to Make Biochar With One

What an air curtain burner does — and how crews use it to make biochar

An air curtain burner is a machine that burns clean wood waste fast and clean. A diesel fan blows a high-velocity wall of air across the top of a steel burn box. That air curtain traps smoke and unburned particulate, pushes it back down into the fire, and lets the pile burn hot and near-smokeless — often north of 1,800°F. You get roughly the same disposal you'd get from an open burn pile, minus most of the smoke complaints and with the burn rate cranked up.

That's the first job: making a mountain of stumps, limbs, and slash disappear on-site instead of paying to haul it. The second job — the one more land clearers are waking up to — is that the same machine can leave you with biochar worth selling.

How the machine works

Most units land in one of three forms:

Above-ground firebox — a refractory-lined steel container. The workhorse for land-clearing and forestry crews. Depending on size, these chew through anywhere from about 1 ton to 9-plus tons per hour.
Trench burner — the air-curtain manifold mounts on a frame over an excavated pit. Best when you're feeding oversized material like full logs and root balls that won't fit a box.
Portable/towable unit — trailer-mounted, smaller throughput, built to move site to site every few days.

Feedstock matters. These burners are for clean, untreated wood and vegetative debris — no painted lumber, no treated wood, no trash. Burn rules and permits vary by state, and federal rules for air curtain incinerators live under 40 CFR Part 60, so check your air quality authority before you light off. That's on the operator, not the machine.

Where the biochar comes from

Here's the part most people miss. An air curtain burner naturally splits into two zones. Up top, oxygen is plentiful and everything combusts. Down at the bottom of the bed, the char pack starves the fire of oxygen. That oxygen-starved heat is pyrolysis — gases and moisture cook off and leave a porous, carbon-rich solid behind. You don't need a sealed chamber. The bed makes char on its own; the trick is pulling it out before it burns to ash.

Three common ways crews recover it:

Quench and screen. Let the burn build a char bed, then hit it with water to stop combustion. Rake it out and run it over a screen to separate solid char from fine ash. Simplest method, most hands-on.
Smother. Kill the fan and cap the box — some crews use roughly 10 inches of soil. Retained heat converts the remaining fuel to char overnight instead of letting it ash out.
Automated recovery. Purpose-built systems mechanically pull and quench char while the burner runs. The CharBoss, developed with the U.S. Forest Service, is the best-known example and produces on the order of a few hundred pounds an hour.

Realistic yield sits around 11 to 25 percent of dry feedstock weight, with carbon content commonly in the 70–90 percent range. Quenching isn't optional — dry char holds heat and can reignite hours later, so wet it down.

Is it worth the extra step?

Depends on your situation. If you're already running the burner to clear a site, recovering char is nearly-free upside — you were burning that wood anyway. If you're buying a machine purely to make biochar, an air curtain burner is a blunt tool; a dedicated kiln or retort will give you cleaner, more consistent char. We break that trade-off down in our companion post on biochar production equipment.

RPG Equipment brokers the grinders, screeners, and processing iron that sit on either side of a burner — the grinder that sizes your feedstock, the trommel or shaker screen that finishes your char into a sellable, graded product. If you're weighing a burn-and-char setup or want to move a machine you've outgrown, we can point you at the right units on the listings page or source one you don't see.

Have questions about air curtain burners and biochar? Contact RPG Equipment — we work with buyers and sellers across the US and Canada.