Ignition techniques can reduce smoke

The main focus of smoke management is often more related to the redistribution of emissions rather than the reduction of emissions.  This is achieved by choosing wind direction, atmospheric conditions such as mixing height, and ignition techniques to manage the convection.  Perhaps less discussed are ignition techniques themselves.  The documents linked at the end of this article provide some basic guidelines for planning ignition techniques to reduce smoke emissions.  We must be aware however, of potential trade-offs between reduced smoke emissions and achieving ecological goals with fire. 

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.  Backfire.  Backfires (fires that burn against the wind) consume fuels more completely than head or flanking fires, resulting in less smoke.  Tradeoffs include: increased time to complete burns, as backing fires move more slowly, and relatively short flame lengths may not provide heat at the proper height for treating some plants.

2. Headfire.  Headfires (fires that burn with the wind) move quickly, produce intense heat, consume fuel incompletely, 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, reducing the overall burn time fuels.  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 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 with which 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 that as 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.

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 difference.  Large strips may produce more smoke, but because of increased heat production the smoke is lofted higher more quickly than with smaller strips.  Quick completion of a burn can reduce the time of potential exposure to smoke although it may generate more smoke in the short period.

When choosing an ignition technique, consider many factors, including smoke, and prioritize 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.