Reducing the impacts of smoke should be considered when planning a prescribed fire. Smoke management is not really about the reduction of emissions, but the redistribution of emissions, which is done by choosing wind direction, atmospheric conditions (such as mixing height), and ignition techniques to manage the convection. Perhaps less discussed are ignition techniques themselves. Following are some basic guidelines when planning ignition techniques to reduce smoke emissions. We must be aware, however, of potential tradeoffs between reduced smoke emissions and achieving ecological goals with fire.
How to influence smoke
Smoke includes a mix of chemicals including bits of the unconsumed fuel. Unconsumed fuel on the ground leads to smoldering phase burning. Smoldering produces more smoke than burning in the flaming stage of ignition. Ignition techniques that lead to more complete combustion should produce less smoke than other techniques. Here are some examples.
1. Backing fire. Backing fires (fires that burn against the wind) consume fuels more completely than head or flanking fires; thus, backing fires generally produce less smoke. Tradeoffs include increased time to complete burns because backing fires move more slowly, and flame lengths are relatively short so may not provide sufficient heat to kill some plants.
2. Headfire. Headfires (burn with the wind) move more quickly, produce more heat, generally consume fuel less completely, and produce taller flame lengths. Incomplete fuel combustion results in greater smoke production than backfires. Although headfires may produce more smoke, they may be used to complete a fire more quickly, thereby reducing the overall burn time. Furthermore, the speed and intensity of headfires releases more heat that can help the smoke to rise in a column, facilitating dispersal at higher levels in the atmosphere. Headfires can also be more challenging to conduct safely if used exclusively.
3. Ring headfire. This technique uses a combination of back, flank, and head fires. Backfires and flanking fires are used around the perimeter to create a safe black zone to send the headfire towards. This technique is often used because of its safety and compromise in completion time. The backfire portion of the fireline may be less smoky, but generates steady amounts of heat so when the headfire begins, smoke forms a column and is dispersed aloft. The placement of the head and backfire zones can be adjusted to avoid smoke in sensitive areas and apply the right flame length/residence time combination to treat fuels in different parts of the burn unit. Ring headfires are generally not recommended when wildlife is a consideration because animals may be entrapped.
4. Additional techniques, such as chevron, single point ignition, and strip head-firing, are often used to speed the development of a safe zone around a burn unit perimeter. These techniques can also be applied to create pulses of heat to lift smoke aloft. Keep in mind that head fires typically produce more smoke through incomplete ignition and subsequent smoldering, but generate high levels of heat. Size of the strips used with these techniques can make a huge difference. Large strips may produce more smoke, but because of increased heat production, the smoke is lofted higher more quickly than with smaller strips. A quick completion of a burn can reduce the time of potential exposure to smoke though generating more smoke in the short period.
When choosing an ignition technique, consider many factors, including smoke, and prioritize them with respect to objectives and risks.
Resources: NWCG Fire Use Working Team. 2001. Smoke management guide for prescribed and wildland fire. NFES 1279, PMS 420-2 or an older version from Feb. 1985.
NWCG. 2012. Glossary of wildland fire terminology. PMS 205. http://www.nwcg.gov/pms/pubs/glossary/pms205.pdf