Thermal degradation and spontaneous ignition of paper sheets in air by irradiation

The temperature and surface-density histories of a radiantly heated thermally thin filter-paper sheet held freely in air were measured in order to study the dynamics of the ignition of paper. Analyses of these histories indicate that the chemically complex degradation reactions can be approximately represented for fire dynamics purposes by two competitive first-order reactions with Arrhenius kinetics as observed by Tang [3]. One of these reactions with a preexponential factor 5.9 × 10 6 sec −1 and an activation energy 26 kcal/gm-mole is dominant at less than about 655°K. At higher temperatures, the other reaction with a preexponential factor 1.9 × 10 16 sec −1 and an activation cnergy 54 kcal/gm-mole is dominant. The heat-transfer rates to and from the test sheet were measured in order to estimate the energetics of the reactions. The data were insensitive to the small heat of the low-temperature reaction. Assuming this heat to be −88 cal/g (endothermic), based on DTA measurements of Tang and Neill [8], the heat of the high-temperature reaction is estimated to be about 444 cal/g (exothermic). An approximate formula is developed to predict the spontaneous ignition of a thermally thin sheet under known heating and cooling conditions, provided the Arrhenius kinetics and the heat of a first-order reaction in the sheet are known. Using the measured kinetics and heat of the high-temperature reaction in this formula, the results are compared with the measured data as well as with Martin's [9] ignition data.