The San Dimas Technology and
Development Center (SDTDC) investigated the use of air curtain destructors
(ACDs) as an efficient, environmentally friendly, and technically viable
means of disposing of slash, wood, and other burnable waste materials.
ACDs should be considered an additional alternative to current fuel reduction
methods and disposal of road clearing debris such as pile burning, chipping,
landfill disposal, and prescribed fire. SDTDC’s research of the
industry indicates that one company, Air Burners LLC, manufactures both
self-contained and trench ACDs. Their self-contained ACDs are basically
skid-mounted air curtain incineration systems including a refractory lined
firebox that does not require any setup or teardown. Their trench burners
are trailer-mounted air curtain incineration systems requiring a pit or
earthen trench that functions as a firebox. Both types of burners can
efficiently dispose of large quantities of forest waste products at very
high temperatures with very little air emission. This safe and clean method
of burning allows its operation nearly any time of year except when fire
danger is too high. In addition to burning safely and cleanly, volume
reductions of approximately 95 to 98 percent are achieved. The ash may
be used as a soil amendment that can be spread on the forest floor.
Use of ACDs for wildfire mitigation
and fuel management is growing rapidly as an alternative to current fuel
reduction methods. The use of prescribed fire as a means of slash removal
is subject to weather conditions, and in some cases, prohibited in wildland
urban interface areas. Leaving the slash on the forest floor to decompose
is another traditional alternative. However, the slash may take many years
to decompose, particularly in semiarid and cold environments. While decomposing,
the material remains a considerable fire risk. This method can also increase
the risk of unwanted insect outbreaks. Pile burning, another traditional
alternative used to remove slash, is also subject to weather/burn conditions.
Chipping, grinding, and mulching are other alternatives that still require
a means of disposal and may prove to be very costly.
ACDs can be operated safely and practically year round for disposal of
slash with only a few operating limitations such as fire conditions, required
clearance from trees (or other fuel hazards), and maximum allowable wind
conditions (see figure 1). Volume reduction of slash is approximately
95 to 98 percent and the byproduct (ash) may be used as a soil amendment
by spreading it on the forest floor.
|Figure 1- ACD in full operation (no visible smoke).
The ACDs manufactured by Air Burners LLC have been used worldwide for
several types of applications. They are used in forest fuel management
and wildfire mitigation efforts, in the construction industry to reduce
debris from land clearing and demolition operations, and at landfill sites
to maximize costly space by reducing wood waste and similar burnable waste
streams. They are also used in disaster recovery for clearing the aftermath
from storms or floods.
The main operating principle
of the ACD is the high velocity air (curtain) blown across and into the
upper portion of the combustion chamber (see figure 2). This powerful
curtain of air has two effects. First, the high volume of air causes overoxygenation
of the fire, and secondly the high velocity airflow over the combustion
chamber entraps particulates (smoke), which then completes combustion
in the combustion chamber, thus limiting emissions and smoke. The high
turbulence along with increased combustion time and temperatures in excess
of 1800 oF results in complete combustion and significantly
reduced air emissions. Reduced air emissions lower impact on nearby residents,
smoke sensitive individuals, and decrease smoke-related inversions during
fall and winter. In addition, the high temperatures and oxygen-rich environment
burn everything from green fuels to red slash.
Air Burners LLC manufactures
two types of ACDs: self-contained skid-mounted firebox systems that do
not require any setup or teardown; and trench burners that require setup
and an earthen pit or trench which functions as the firebox.
The skid-mounted ACD as shown in figure 3, is a self-contained system
that includes a refractory walled firebox, diesel engine power plant,
mechanical drive system, blower fan, and fuel tank. These ACDs are engineered
to be transportable by a lowboy or similar drop-deck trailer. The transportability
increases the flexibility of bringing the ACD to the wood waste source,
rather than hauling the waste to a fixed location for processing. These
ACDs are ready for use as soon as they are offloaded at the jobsite. The
refractory lined firebox allows for controlled burning without the need
for an earthen pit or trench. The forward equipment deck shown in figure
3 supports the diesel engine, the fuel tank, the direct drive system to
operate the fan, and the fan. An air nozzle manifold is mounted on one
side of the firebox. The wood waste is loaded over the top of the ACD
on the side opposite the manifold (see figure 4).
|Fig 2 - Operating Principle if ACD
- Air curtain burner manifold
and nozzles directing high velocity air flow
- Refractory lined wall for
self contained ACD or earthen wall for trench ACD
- Waste material to be burned
- Air flow forms a high velocity
"curtain" over fire
- Continued air flow over-oxygenates
fire keeping temperatures high and higher temperatures provide cleaner
burn and more complete burn
|Fig 3 - Skid mounted ACD in full operation (no visible
Skid-mounted systems are designed
and constructed to optimize the air curtain concept. High velocity air
is blown across and down at an optimum angle into the pit creating the
air curtain on top and a rotational turbulence within the firebox. The
high velocity air creating the rotational turbulence provides an oxygen-enriched
environment in the combustion zone that accelerates the combustion process
(similar to the effect of fanning a fire). The temperature within the
firebox is usually above 2,000 oF. The high velocity
air over the firebox creates an air curtain that traps unburned particulate
until it is completely consumed. Nearly complete combustion is achieved
with minimal amounts of escaped particulates, virtually eliminating smoke.
Vertical refractory walls aid
in the combustion process by retaining and reflecting the high temperatures
generated within the firebox. The combustion process reduces the wood
waste by approximately 98 percent, leaving about 2 percent in volume as
residual ash. Twin refractory lined panel doors at the rear of the firebox
allow for ash removal. The unit has no bottom and can be dragged on its
skids with the rear door panels open for dumping ash.
The skids and durability of
the unit allow it to be dragged around the site for repositioning or from
site to site depending upon the terrain and distance to be moved. The
ash may be left in place, disposed of, or used as a soil amendment by
mixing it with the soil at the site or other locations.
Air Burners LLC manufactures
several skid-mounted systems with burn rates ranging from 1 to 15 tons
per hour. The larger units are more difficult to transport or move around
the site. Due to their size, special permits are required for transporting
over roads. Systems can be customized to meet specific needs. The standard
units can also be leased.
Trench Burner Systems
The trench burner systems are trailer-mounted self-contained air curtain
incineration systems consisting of a power plant, mechanical drive system,
blower fan, and fuel tank (see figure 5). All of the components are either
mounted to or stored on the trailer. An earthen trench must be constructed
since the trench burners do not contain a firebox. The manifold sections
are assembled and placed along the trench edge. Carrier pipe sections
are assembled to carry the air from the power plant to the manifold, thereby
keeping the trailer-mounted components clear of damaging heat generated
from the burning operations (see figure 6). Burning operations can usually
run until the ash in the trench needs to be removed or a new trench is
needed. Air Burners LLC manufactures several trailer-mounted trench systems
with burn rates ranging from 5 to 14 tons per hour. These units can also
VS TRENCH BURNER
The self-contained firebox burners
eliminate guesswork regarding the size of the fire area. These above-ground
units avoid problems with the water table, rocks, and roots and allow
for easier ash removal. The real minus for these units is their size.
The smallest ACDs weigh over 20,000 pounds. So dragging it around in soft
soil can be difficult. While the smaller units will fit on a 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 falling in or perching
heavy loading equipment too near the edge of the pit causing the walls
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. [your edits changed the meaning] The efficiency starts
to drop once the ash pile reaches approximately 1/4 to 1/3 the depth of
the firebox or pit. For safe operation, the manufacturer recommends a
100-foot clearance around the ACD. Barring extremely high winds there
is little chance of large embers escaping the trench or firebox and burning
beyond the clear area. Very small embers can escape, but generally burn
completely before they hit the ground. Having an engine and crew onsite
further reduces the risk of fire. A patrol of the area may reduce the
clear area requirements and burn condition limitations. ACDs should not
be operated if the fire danger is too high or if people or animals are
likely to fall into the pit or climb up on the box. Should conditions
require shutdown, the fire could be extinguished in 10 to 20 minutes.
Safety should always be the number one operational consideration.
U.S. Environmental Protection
Agency regulations for New Source Performance Standards (NSPS) requires
that ACDs operate below opacity limits of 35 percent during the initial
30-minute startup and 10 percent during operation (6-minute average),
provided the material burned is restricted to 100 percent wood waste,
clean lumber, and/or yard waste. Air Burner ACDs operate well below these
limits in contrast to open burning which averages between 60 to 80 percent
SDTDC is currently planning
to operate a skid-mounted unit for evaluation purposes in Fall 2002 at
a site within national forest lands. Data will be collected on aspects
such as mobility, durability, and operability to develop Forest Service
recommendations and standards for operation. This information will be
published during FY 2003.
ACDs should be considered when evaluating alternatives to current fuel
reduction methods in urban interface areas. ACDs may not be as cost competitive
in areas where broadcast and pile burning are acceptable. Potential advantages
to ACDs include:
- Produces lower smoke emissions compared to pile or broadcast burning.
- Burns a greater variety of materials from green fuel to red slash.
- Reduces fire risk and outbreak of insect problems.
- Operates with fewer restrictions on weather and burn conditions.
- Residents in urban interface areas are more willing to accept ACD
use and remove wood waste and slash fuel hazards around their homes
if offered free disposal.
- The fire is contained and easily and quickly extinguished, if necessary.
For further information regarding
ACDs manufactured by Air Burners LLC, contact:
Brian O’Connor or Norbert Fuhrmann
Air Burners LLC
4390 Cargo Way
Palm City, FL 34990
888–566–3900 or 772–220–7303
Approximate English to Metric System Conversion
Temperature Conversion of Units
°C = (°F – 32)
ABOUT THE AUTHOR
Alan Schapiro, a licensed professional
engineer in the State of California, obtained both a bachelor’s degree
in mechanical engineering and a master’s degree in engineering from
Cornell University. He has 20-plus years of engineering and project management
experience in the development, design, startup, and operation of oil/gas,
coal, nuclear, and alternate/renewable electric generation power plants.
Alan is a project leader in the Fire and Aviation program responsible for
air curtain destructor applications, fire engine development and testing,
and foam proportioner testing. He also has responsibilities in other program
areas such as engineering roads technology, forest management, and recreation.
For Additional Information
Project Leader, Fire Management
San Dimas Technology & Development Center
444 East Bonita Avenue, San Dimas CA 91773-3198
Phone 909-599-1267; TDD: 909-599-2357; FAX: 909-592-2309
in this document has been developed for the guidance of employees of the
Forest Service, United States Department of Agriculture (USDA), its contractors,
and cooperating Federal and State agencies. The USDA assumes no responsibility
for the interpretation or use of this information by other than its own
employees. The use of trade, firm, or corporation names is for the information
and convenience of the reader. Such use does not constitute an official
evaluation, conclusion, recommendation, endorsement, or approval of any
product or service to the exclusion of others that may be suitable.
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