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Kinetics of cellulose pyrolysis
Abstract
This work studied the process of cellulose pyrolysis under radiant heating. Dynamic thermogravimetric data were used in a mathematical model to generate useful values of the rate constant and the heat of reaction. For the first time the volume shrinkage of cellulose has been accounted for in a mathematical model describing the pyrolysis process. Cylindrical cellulose samples were suspended from an electrobalance and subjected to radiant heating. The heating source was an infra-red furnace set at temperatures ranging from 400 to 800$\sp\circ$C. The cumulative product gasses were analyzed for hydrogen, oxides of carbon, methane, ethane, ethylene and acetylene. The effect of sample size on product composition was studied, at three temperatures, by using samples ranging from 1/8 (0.3175 cm.) to 3/8 inches (0.9525 cm) in diameter. In addition to cellulose particles, samples of two actual wood material (oak and hemlock) were pyrolyzed at set temperatures of 600$\sp\circ$, 700$\sp\circ$ and 800$\sp\circ$ C. Their product gas compositions were compared to that for cellulose. A mathematical model describing the heat and mass transfer effects was derived. It included the volume shrinkage which ranges from 55-68%. The model has been solved for different values of the Arrhenius constant (A), the activation energy (E) and the heat of reaction ($\Delta$H) and the optimum values of A, E and $\Delta$H were obtained. The optimum in this case is defined as the values that give the best fit between the experimental and calculated weight loss curves. Raising the furnace temperature had two effects. It raised both the heating rate of the sample and the final temperature attained by it. Increasing the furnace temperature results in higher hydrogen and light hydrocarbon content in the cumulative product gas. The $CO/CO\sb2$ ratio also increases. Solution of the model showed that the local optimum values of A and E when plotted as ln(A) vs E, lie on a straight line. At a common temperature, obtained from the slope, the combination of the A and E yield a constant "rate constant". The common temperature is called the "compensation" temperature. The overall reaction was endothermic, the heat of reaction increasing with temperature/heating rate.
Subject Area
Chemical engineering
Recommended Citation
Ahmed, Shabbir, "Kinetics of cellulose pyrolysis" (1988). ETD collection for University of Nebraska-Lincoln. AAI8818604.
https://digitalcommons.unl.edu/dissertations/AAI8818604