Thermal Effects in Cellulose Pyrolysis:  Relationship to Char Formation Processes

Abstract

The thermochemistry of cellulose pyrolysis has been studied by a combination of differential scanning calorimetry and thermogravimetric analysis. Additionally, the vapor pressure and heat of vaporization of levoglucosan have been determined by an effusion method. The cellulose pyrolysis has been carried out under inert gas at heating rates from 0.1 to 60 K/min. The main cellulose thermal degradation pathway is endothermic, in the absence of mass transfer limitations that promote char formation. The endothermicity is estimated to be about 538 J/g of volatiles evolved. It is concluded that this endothermicity mainly reflects a latent heat requirement for vaporizing the primary tar decomposition products. Pyrolysis can be driven in the exothermic direction by char-forming processes that compete with tar-forming processes. The formation 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 limitations, serve to drive the pyrolysis in this direction. The enthalpy of cellulose pyrolysis is thus seen to be a sensitive function of the pyrolysis conditions. Pyrolysis appears to initially follow a common thermal pathway (in terms of enthalpy required per mass of volatile loss), irrespective of heating rate. Only at some finite level of conversion does the “thermal trajectory” of the process follow a heating rate dependent path, as significant char formation begins to occur.

Laboratory investigation of photochemical oxidation of organic aerosol from wood fires 2: analysis of aerosol mass spectrometer data

“…The AMS signal at m/z 60 and 73 has been associated with levoglucosan Schneider et al, 2006), which can be a major component of biomass burning POA . However, levoglucosan has an effective equilibrium saturation concentration (C*) of order 0.1 µg m −3 at ambient temperatures (Milosavljevic et al, 1996), which means that it should start to evaporate at temperatures around 60 • C -significantly higher than the observed evaporation temperature of species contributing ions at m/z 60 and 73. Therefore, levoglucosan may not be the dominant contributor to the AMS signal at m/z 60 and 73 in these experiments.…”

Comparison of thermal behavior of natural and hot-washed sisal fibers based on their main components: Cellulose, xylan and lignin. TG-FTIR analysis of volatile products

“…In the case of cellulose, a small exothermic effect is observed next to the 338°C endotherm, at around 400 °C, which is associated to the charring process. Milosavljevic et al [48] claim that the charring process of the cellulose can be exothermic (-2.6 kJ/g referred to residual carbon), endothermic (0.86 kJ/g) or combination of both, depending on the products generated. Similarly, exothermic effects at around 460 °C in the case of xylan, and at 480 and 650 °C in the case of lignin, are associated to carbonizing processes.…”

Modeling kinetics-transport interactions during biomass torrefaction: The effects of temperature, particle size, and moisture content

“…This commonly used temperature limit was originally chosen to avoid excessive decomposition of the relatively carbon-rich lignocellulose components-namely cellulose and lignin) [6,57]. At temperatures exceeding 300 ○ C, more complex degradation pathways may become significant such as exothermic char-forming reactions [62], secondary reactions between the volatiles and the char products [8] and thermal tar cracking [63]. Given the high sensitivity of the rates and degradation pathways to temperature, the application of these mechanisms 30-40 ○ C above the original range of fitting would strictly be considered extrapolation.…”