Polyethylene film, commonly called "black plastic," has been widely used in fuels management programs to cover piled slash to keep it dry until proper burning conditions are available. Because it is difficult, time consuming, and often hazardous to remove the plastic, it has usually been left in place and burned with the pile. However, due to air quality concerns, open burning of plastics has been prohibited in many areas. In addition, questions have been raised as to the health effects on employees and the effects on the environment when polyethylene film is burned or left to decay. The purpose of this report is to present an overview of slash pile burning and to provide alternative methods for accomplishing fuels management objectives.
Polyethylene is a polymer of ethylene. It is a lightweight thermoplastic that is produced by the end-to-end linkage of ethylene molecules, CH2CH2. A typical "molecule" of polyethylene consists of several thousand ethylene units linked together into a long hydrocarbon chain.
Most of the simplest polymers are synthetic and are known as "plastics." Naturally-occurring polymers have a wide variety of structural variations. This accounts for the differing properties of natural materials such as wood, paper, and cotton. All of these are cellulosic materials that are based on the glucose molecule.
Employee Safety and Air Quality Concerns
Relative to employee health and air quality concerns, the effects of burning polyethylene fall into a grey area that can be characterized by this question: It may not be good, but how bad is it?
The National Fire Protection Association's Fire Protection Handbook states: "Acrolein is a particularly potent irritant, both sensory and pulmonary, which has been demonstrated to be present in many fire atmospheres. It is formed from the smoldering of cellulosic materials and also from the pyrolysis of polyethylene." (Pyrolysis is a chemical change brought about by heating.)
The acrolein produced by the small volume of polyethylene covering the slash pile may not be significant compared to that which is produced by the cellulosic material (slash). Proper techniques must be used to ensure rapid burning (to avoid smoldering and pyrolysis), and workers should use good common sense and always remain upwind of the drifting smoke to reduce exposure.
Air quality concerns have led to prohibitions on the open burning of plastics in many areas of the country. However, burning polyethylene with slash is allowed in some areas for two reasons: (1) Slash burning usually takes place far from populated areas and is done when smoke will not reach these areas so the general public is not exposed or concerned, and (2) air quality officials realize that using polyethylene to cover slash will result in lower emissions overall because less petroleum products are needed for ignition, and more complete and rapid burning can be achieved when slash is dry. The key here is that fuels management personnel should be aware of local restrictions and should work closely with local air quality officials to achieve their objectives.
To Burn or Not to Burn
Following most timber harvesting today, slash is treated in some fashion. Burning is prevalent after hand or machine piling; however, the amount of broadcast burning has declined. Using an air curtain destructor is one alternative to piling and burning that has been used successfully. An air curtain destructor is basically a large air blower. With this system, slash is fed into a pit and the blower provides a great volume of air enabling the fuel to be burned rapidly at very high temperatures so that burning is complete and little smoke is emitted.
The current trend in utilization standards is toward smaller and smaller piece sizes and some advocate using "all" fiber from the forest. This would certainly reduce the need for slash burning. However, during the past decade, the trend has been away from the total cleanup concept. Concerns about nutrient recycling, seedling protection, and wildlife habitat are resulting in more and more material being left on the ground after harvesting.
An all-encompassing, integrated, interdisciplinary approach, which fits well with current ecosystem management policies, is being used on National Forest lands. It is important that fuels management personnel work closely with other disciplines to explore tradeoffs in accomplishing their desired outcomes.
For instance, the methods and equipment used in harvesting operations can greatly affect the amount and condition of the remaining slash. Cut-to-length systems leave the limbs and tops crushed into the skid trails. Whole-tree harvesting puts the slash on the landing, where it can be chipped and hauled or spread onsite; or loggers can pile it for use as firewood. Sites close to urban areas will probably be visited by firewood cutters. If reducing the fire hazard is the only objective, only the smaller pieces and fine material should be burned, while the larger pieces are left for other uses. The key is for fuels management personnel to look for ways to reduce the need for slash burning and to reduce the amount of slash that must be burned.
Slash Pile Burning Techniques
When burning slash piles is necessary, a determination must be made on whether covering is required and, if so, what type is best. Conditions such as location, climate, aspect, elevation, access, etc., will dictate requirements for covering and burning. In some areas, covering piles is unnecessary since the slash will dry out eventually without it. When slash is piled by hand, crews can be trained to build piles that shed water and will dry without the use of covering materials. Instructions to piling crews must stress that tight, compact, high piles, with butts oriented toward the center, are required.
Various techniques in covering and burning slash piles can be used to reduce the size or amount of cover needed. One consideration is whether the covering will be burned with the pile or will be removed prior to burning and reused on another pile. If it is to be reused, the covering must be made of a durable material that can withstand long-term exposure to the elements, does not puncture readily, and is easy to handle.
A small cover (say 3 ft by 3 ft) (0.91 m by 0.91 m) can be used to cover a section of smaller pieces to be used as kindling. Larger pieces can be stacked around it in teepee fashion over the covered kindling to help shed some precipitation and keep the cover in place. (See figure 2.) Dark-colored covering material, oriented toward a southern exposure, if possible, will increase the amount of solar heating and, therefore, decrease drying time.
Other techniques that may work include covering only the windward or weather side, or covering only the top of the pile to shed precipitation. In these cases, larger branches should be used to hold the cover securely in place.
If a hot fire is needed for complete burning, light the pile on the downhill side and/or the side where the wind will carry the fire across the pile and rising heat will dry out the material prior to igniting. If exposed residual trees nearby could be scorched by a hot fire, pile slash on a flat area and light on the downwind side or on all sides at once. Use the least amount of petroleum products necessary and just a match, if possible, to ignite. Propane is a clean-burning fuel and propane-burning igniters are available.
Cover Material Alternatives
To choose the best material for covering slash piles, consider the local open-burning restrictions, site-specific characteristics (i.e., how much precipitation can be expected and what kind: rain or snow), time of year burning will be done (and, therefore, how long the cover is expected to last), ignition and piling techniques to be used, and whether the cover will be removed prior to burning.
Polyethylene film has been widely used because it is virtually moistureproof, lightweight, easy to handle, readily available, and relatively inexpensive. In areas where the open burning of plastics is not restricted, this may be the preferred material. Its drawbacks are that it can be punctured easily and is difficult to remove and reuse. When the polyethylene is burned, crew members must remain upwind from the drifting smoke.
In areas where open burning of plastics is prohibited and the polyethylene film is removed prior to burning, concerns have been raised about the long-term effects on the environment from dumping large volumes of plastic. Polyclean™, produced by Archer Daniels Midland, is a cornstarch-based product that provides a system for the degradation of polyethylene. It is an additive which is used by the producers of biodegradable polyethylene films, such as compost bags. The Polyclean™ contains several components and works via two degradation pathways-biological and oxidative. Both the biodegradation of the starch and the auto-oxidation processes will work independently, but together they lead to a synergistic effect under conditions such as composting, greatly speeding up the degradation process.
Another environmentally sensitive product that could be used (and removed prior to burning for reuse) is heavy plastic or vinyl sheeting made from recycled materials. The number and types of products produced from recycled materials has grown rapidly in the past few years. Companies like Amazing Recycled Products (Denver, CO) produce plastic products (plastic sheeting and spill containers, for example) made from 100 percent post-industrial black polyethylene and vinyl sheets made from automobile interiors.
Ordinary kraft paper, available through GSA (NSN 8135-00-966-2533), is currently being used successfully for covering slash piles. Paper cover will work for short durations and/or if heavy rains are not expected. It is relatively inexpensive, readily available, and can be burned with the pile. Cardboard has similar qualities but is more durable. If more moisture resistance is needed, paper or cardboard, coated on one or both sides with a thin layer of polyethylene, is also available. Wax-coated paper is available at only a slightly higher cost than plain paper and provides good moisture resistance. Another option is to purchase paraffin wax at the local hardware store and paint it on cardboard sheets. This method is also currently being used successfully in several areas.
Fortifiber Corporation produces a product that is made specifically for covering slash piles. Fortifiber Slash Cover grade 207, also referred to as Fibreen 207, consists of two sheets of heavy-duty natural kraft paper sandwiching an asphalt layer that is reinforced with synthetic fibers. In some areas this material may be burned with the slash, but will usually result in excess smoke and odor from the asphalt laminate. Because it is a durable material, it may also be reusable.
A. Work closely with local air quality officials.
B. Be alert for ways to reduce the need for slash burning and reduce the amount of slash that must be burned. Be involved in the timber sale planning process.
C. Teach proper burning techniques to slash crews. Stress the importance of tight, compact, high piles, with butts oriented toward the center.
D. Do not cover piles unless it is necessary.
E. Remove covers prior to burning and reuse if possible.
F. If covers are burned with the pile, use the smallest covers that will do the job.
G. Use paper or cardboard to cover. Coat with wax to get more moisture resistance and longevity.
H. Reduce the amount of petroleum products used for ignition. Use a match only or use cleaner burning propane torches.
For Additional Information Contact:
Fire Management Program Leader
San Dimas Technology and Development Center
444 East Bonita Avenue, San Dimas CA 91773-3198
Phone 909-599-1267; TDD: 909-599-2357; FAX: 909-592-2309
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