Is there a critical fuel moisture content for flammability?

Abstract

The moisture threshold for ignition has attracted land managers and has been the main subject of discussion for several decades. The critical moisture of extinction is still a subject of extensive studies, and an endothermic/exothermic phase transition is expected to occur near the critical region. Ignition tests were realized for live pinus halepensis needles using a cone calorimeter. In order to ensure the live character of the fuel and avoid seasonal effects and changes in their chemical structure during ignition, the samples were picked the same day from the same tree, then partially dried using a microwave and a desiccator. By definition, phase transitions occur when a small variation of an external control parameter leads to a qualitative modification of the system. In order to determine whether the moisture content is the control parameter of the endothermic/exothermic phase transition or not, we had to verify the existence of a moisture content threshold for ignition by analysing the dependence of ignition time on moisture content. The experimental data have shown no critical moisture for the fuel ignition. We pointed out that the observed critical region corresponds to an ignition critical heat flux rather than a critical moisture.