THERMAL EFFECTS IN CELLULOSE PYROLYSIS - RELATIONSHIP TO CHAR FORMATION PROCESSES

The thermochemistry of cellulose pyrolysis has been studied by a combi nation of differential scanning calorimetry and thermogravimetric anal ysis. Additionally, the vapor pressure and heat of vaporization of lev oglucosan have been determined by an effusion method. The cellulose py rolysis has been carried out under inert gas at heating rates from 0.1 to 60 K/min. The main cellulose thermal degradation pathway is endoth ermic, in the absence of mass transfer limitations that promote char f ormation. The endothermicity is estimated to be about 538 J/g of volat iles evolved. It is concluded that this endothermicity mainly reflects a latent heat requirement for vaporizing the primary tar decompositio n products. Pyrolysis can be driven in the exothermic direction by cha r-forming processes that compete with tar-forming processes. The forma tion of char is estimated to be exothermic to the extent of about 2 kJ /g of char formed. Low heating rates, in concert with mass transfer li mitations, serve to drive the pyrolysis in this direction. The enthalp y of cellulose pyrolysis is thus seen to be a sensitive function of th e pyrolysis conditions. Pyrolysis appears to initially follow a common thermal pathway (in terms of enthalpy required per mass of volatile l oss), irrespective of heating rate. Only at some finite level of conve rsion does the ''thermal trajectory'' of the process follow a heating rate dependent path, as significant char formation begins to occur.