An experimental investigation of heat-transfer limitations in the flash pyrolysis of cellulose

A new experimental system is presented to investigate the fast pyrolysis of solid fuels, in the absence of heat- and mass-transfer limitations. It consists of an electrically heated furnace, where a thin layer of powdered solid is exposed, on both sides, to radiative heating. A PID temperature controller is programmed for two different working conditions: the usual constant furnace temperature (A) and a constant sample temperature (B). Cellulose pyrolysis is investigated in the temperature range 523-699 K. It is shown that significant heat-transfer limitations 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 intraparticle secondary reactions of primary vapors. External heat-transfer limitations, due mainly to endothermic reaction energetics, are also avoided through proper variation in the intensity of the external radiative heat flux. Consequently, conversion occurs under exactly determined temperature conditions. A cold helium flow carries away from the reaction environment volatile products so that the activity of extra-bed secondary reactions is hindered as well. Cellulose weight loss and temperature curves are applied to evaluate the global degradation kinetics and to study the influences of heat- and mass-transfer limitations.