(47w) Comparison of Different Methods to Determine the Activation Energy of Flammable Dusts Mixed with Inert and Inhibitory Materials

Comparison of different methods to determine of the activation energy of flammable dusts mixed with inert and inhibitory materials

C. Wanke, B.Binkau, P.Rettig, U.Krause

Porous flammable media often undergo spontaneous combustion and the spread of smoldering flame fronts. Adding inert or inhibitory substances, the self-ignition and combustion behavior can be influenced. In a series of experiments the activation energy of flammable dusts (lignite, carbon) mixed with inhibitors (ammonium sulfate, ammonium phosphate) was determined and compared to the pure flammable dusts.

To determine the activation energy self-heating methods (isoperibolic and adiabatic hot storage tests) and temperature programmed methods (thervogravimetric (TG) and differential scanning calorimeter (DSC) tests) can be used. The common method in Europe is the isoperibolic hot storage test according to DIN EN 15881. This method is very time and material consuming, due to the fact, that the self-ignition temperature of three to four different sample sizes must be analyzed. An alternative is the adiabatic hot storage test where the activation energy can be determined from just one single experiment. The disadvantage is the difficult analysis of the resulting Arrhenius plot, where the activation energy is the steepest slope of the logarithmic temperature rise. To find this value a Matlab tool was developed.

The advantage, using TG and DSC is the small amount of sample material. The analysis of the mass loss rate from the TG experiment results in an Arrhenius plot as well, which can be evaluated by the mentioned Matlab tool. Using the DSC, three experiments with different heating rates have to be executed. In spite of the slow heating rates, this method is still faster than the isoperibolic test. 

Summing up, the activation energy of pure substances, as well as mixtures, can be calculated by the methods above. The isoperibolic test lead to the highest activation energies, the DSC and TG experiments lead to the lowest values. This can be traced back to the different experimental procedures. Isoperibolic tests start at the self-ignition temperature (SIT), adiabatic tests at the onset temperature, which is below the SIT, and DSC and TG start at ambient temperature. The reactions below the starting temperature can’t be taken into account for the determination of the activation energies, therefore the results of DSC and TG are lower.