Comparative extinction of pine needle fires by gravity-fed release of inert cryogen or water

Abstract

Dead needle debris accumulates in pine forest floors and poses wildfire hazards. This manuscript reports on laboratory-scale research, where gravity-fed liquid nitrogen (LN₂) jets were used to suppress and extinguish pine needle fires. Minimum quantities of LN₂ needed for fire extinction were determined, and similar quantities of gravity-fed water jets or water sprays were also used to illustrate differences in the underlying extinction mechanisms. LN₂ snuffed the fires instantaneously, as its massive flash-vaporization and volumetric expansion displaced the surrounding air and starved the fires for oxygen. Concomitant cooling of the flame zone and the fuel bed also occurred by the vaporizing cryogen. To the contrary, water suppressed the fire by wetting and cooling accessible fuel surfaces. However, fuel bed surfaces which were not wetted continued to pyrolyze and smolder, and eventually reignited. In these experiments, a portion of the volatile LN₂ jet vaporized in transit, a portion vaporized while traversing the hot gases of the flame, a portion vaporized in the pine needle pile, and some reached the ground and was absorbed therein. Application of LN₂ to fires in confined spaces was most effective for fire extinction. Application of the cryogen to hot spots facilitated flash vaporization for maximum volumetric expansion.