Pyrolysis characteristics and kinetic modeling of Artemisia apiacea by thermogravimetric analysis

Chinese herbal medicine plays an important role in disease treatment and world medicine market. Its fire safety, especially pyrolysis in storage deserves more attentions due to low bulk density. As an herbal medicine representative and malarial drug, Artemisia apiacea pyrolysis was investigated based on thermogravimetric analysis in an inert atmosphere at different heating rate from 10 to 50 K min(-1). Two model-free methods of Kissinger-Akahria-Sunose (KAS) and Flynn-Wall-Ozawa (FWO) were employed to estimate pyrolysis activation energy. A model-fitting method called Coats-Redfern (CR) was used to predict the reaction mechanism for pyrolysis of Artemisia apiacea stem and leaf. For stem, a single peak can be observed at a range from 584 to 604 K, while two shoulders and a single peak exist for leaf. With increasing the conversion rate, activation energy of stem pyrolysis increases but leaf pyrolysis activation energy first decreases, then increases and lastly decays. So the cellulose plays a significant role in stem pyrolysis, while hemicellulose, cellulose and lignin together do in leaf pyrolysis. Correspondingly, the average values of activation energy were also estimated for both stem and leaf by three above methods. The diffusion-Valensi model or diffusion-Ginstling model can be used to describe stem reaction mechanism. Furthermore, a detailed pseudo-mechanistic model serves for leaf reaction mechanism.