Ignition studies on cotton cellulose by DTA

Abstract

Differential thermal analysis is used to investigate the spontaneous combustion of cotton cellulose in an atmosphere of flowing air. The temperature at which the onset of spontaneous ignition occurs, T i, is determined as a function of sample mass, air flow rate and heating rate. For sample masses between 1 and 15 mg, T i is constant although complete sample ignition occurs only for masses in excess of 8 mg where char residues fall below 5%. Air flow rates between 50 and 300 cm 3 min −1 do not significantly influence T i recorded for cellulose samples having a given mass and subjected to a known heating rate, H R. However, for heating rates ranging from 4 to 20°C min −1, T i is observed to increase according to a simple power law T i = 295 H 0.0579 R°C. The overall reproducibility of the technique shows T i = 350.5 ± 1.5°C when 10 mg cellulose samples are subjected to a heating rate of 20°C min −1 and air flowing at 200 cm 3 min −1. A simple steady-state model of pyrolysis and subsequent combustion predicts that the function (2 log T i-log H R) will be linearly dependent upon 1/ T i. From the slope, an upper limit value for the activation energy of pyrolysis, E p, is found to be 146 kJ mole −1. This value compares well with measurements of E p made by other workers using a variety of alternative methods.