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standard equipment trailer, the larger units are oversized loads for most roads. So the logistics are more complicated than towing a trench burner.
The trench burner can be easily towed behind a truck to the jobsite. These units will handle more uneven terrain than the fireboxes and can be mobilized quickly. The trench allows the operator to easily see the fire and load the pit without the need to raise the fuel up over the wall. The real minus for the trench burner is in the construction of the trench. The trench must be dug correctly or the efficiency of the ACD goes down, increasing emissions and decreasing thru-put (burn rate). If the trench is omitted or too shallow, the principle of the air curtain is lost. Ground and soil conditions become a big factor. A high water table can create flooding in the trench and cause trench walls to lose integrity if the soil is too soft. Additional safety factors also must be considered. Precautions must be taken to alert personnel to the pit's location to avoid inadvertently failing in or perching heavy loading equipment too near the edge of the pit causing the walls to collapse.
Operation
Skid-mounted or trench burner ACDs are simple, easy, and almost identical to operate. ACD operations follow three stages: startup, full operation, and burndown. For startup, the trench or firebox is partially loaded with layers of fine and easily burnable forest slash. An accelerant (typically diesel fuel) is applied over the layers and covered with heavier logs to just under the manifold. Fusees could be used for ignition. The fan is started once the heavier materials burn, and the fan speed is gradually increased to full capacity. Smoke will be produced during startup, but will decrease as the fan speed increases and the process approaches full operation. Startup burning takes about 1 hour and is complete when a base of hot coals and burning material is established. During full operation, slash is fed to the fire at a steady rate using a front-end loader or an excavator with a bucket and thumb. The last stage, burndown, typically takes about 1 to 2 hours. The air is slowly decreased as the last load burns down. After burndown, hot coals may remain for several days under an insulating blanket of ash. The ash may be left in place, disposed of, or used as a soil amendment by mixing it with the soil onsite or at other locations.
Skid-mounted ACDs are designed to run for approximately 24 hours before the ash needs to be removed. Long burns are generally more efficient (having lower emissions) than shorter burns. The efficiency starts to drop once the ash pile reaches approximately 1/4 to 1/3 the depth of the firebox or pit.
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