Black Liquor Droplet Swelling Model during Pyrolysis

Abstract

Abstract Black liquor is a spent pulping liquor from kraft pulping process. The concentrated black liquor is combusted or gasified to recover the chemical energy and to recycle the pulping chemicals. It undergoes several processes, i.e. drying, pyrolysis, and combustion and/or gasification. The droplet swells slightly during drying. Conversely, it swells heavily during pyrolysis. It can swell up to 20-30 time its original volume. The swelling stops when the pyrolysis reaction ends and/or when the solid loses its viscoelastic properties. Swelling phenomena also happens in other system, e.g. polymer foam moulding and coal pyrolysis. Two key phenomena for swelling behaviour are: gas production inside a material that forms gas bubbles and the material viscoelastic properties. The bubbles grow and whole droplet then swells accordingly. For black liquor droplet swelling, some empirical models has been developed but none of them are based on the theoretical bubble growth process. In this paper, the black liquor droplet swelling modelling based on the bubble growth process is developed. The model generally in agreement with the black liquor droplet swelling experimental data from a single droplet Thermogravimetry Analyser (TGA).