Self-ignition Behavior of Metal Hyperaccumulator Plants: Influence of Metal Content on their Thermal Stability

Abstract

The thermal stability of hyperaccumulator plant storages was investigated. Isothermal basket tests were performed according to EN 15188 on ground and sieved samples of Alyssum murale, containing approximately 1 %w Ni. Some plants were washed and dried to reduce Ni concentration to 0.2 %w. Tests were also performed by mixing the plants after Ni removal with iron powders to highlight the influence of metal addition. Thermogravimetric analyses and experiments in Grewer oven were run to determine the onset temperature, the combustion enthalpy and the oxidation kinetics using Flynn-Wall-Ozawa and Kissinger-Akahira-Sunose methods. The self-ignition temperature was significantly decreased by the presence of Ni within the plants (approximately 20 K less when Ni was still present). Complementary results were obtained by mixing iron powders and plants. These trends were confirmed by the evolution of the onset temperature. They could neither be related to a porosity change, nor to Biot number modification, as the experimental thermal conductivity was only slightly affected by the presence of 1 % of metals. Frank-Kamenetskii model was applied to the various tested materials and bring out an activation energy in good agreement with that obtained by thermogravimetry. The influence of the diffusion-controlled reaction was highlighted and the influence of metal accumulation on the thermal stability of hyperaccumulator plants was demonstrated.