AN EXPERIMENTAL INVESTIGATION OF HEAT-TRANSFER LIMITATIONS IN THE FLASH PYROLYSIS OF CELLULOSE

A new experimental system is presented to investigate the fast pyrolys is of solid fuels, in the absence of heat- and mass-transfer limitatio ns. It consists of an electrically heated furnace, where a thin layer of powdered solid is exposed, on both sides, to radiative heating. A P ID temperature controller is programmed for two different working cond itions: the usual constant furnace temperature (A) and a constant samp le temperature (B). Cellulose pyrolysis is investigated in the tempera ture range 523-699 K. It is shown that significant heat-transfer limit ations cannot be avoided with the modality A, unless very slow heating rates, as in the classical TGA systems, are applied. In the modality B (global heating rates 19-56 Ws), the independence of the char yields from the sample thickness, for values of this below a critical value, indicates negligible spatial temperature gradients and activity of in traparticle secondary reactions of primary vapors. External heat-trans fer limitations, due mainly to endothermic reaction energetics, are al so avoided through proper variation in the intensity of the external r adiative heat flux. Consequently, conversion occurs under exactly dete rmined temperature conditions. A cold helium flow carries away from th e reaction environment volatile products so that the activity of extra -bed secondary reactions is hindered as well. Cellulose weight loss an d temperature curves are applied to evaluate the global degradation ki netics and to study the influences of heat- and mass-transfer limitati ons.