Thermal decomposition and gasification of cellulose and wood wastes

Abstract

The thermal decomposition of jack pine bark, white birch sawdust and cellulose has been studied by both isothermal weight-change determinations and dynamic thermogravimetry mostly in inert atmospheres up to 900°C. Gaseous products formed in helium and helium/water vapour atmospheres from 350 to 650* and 101 to 2533 kPa were analysed by gas chromatography. The effects of a number of additives, alkali metal and transition metal compounds, on the kinetics of the reaction and on the gas yields have been examined. The effect of pressure on the chemical balance and distribution of the gaseous products was also studied. Residues were analysed for CHN contents and any structural changes on heating were detected by scanning electron microscopy. Both kinetic data and gaseous product analysis showed that cellulose was more susceptible to the influence of additives than the wood samples. Isothermal decomposition data for bark and sawdust were similar to results expected for diffusion-controlled processes. The addition of potassium carbonate shifted the kinetics of decomposition of cellulose from a description consistent with a phase-boundary controlled model to that for a diffusion-controlled process. The apparent activation energy for cellulose pyrolysis was lowered by the addition of both potassium carbonate and iron (Ill) oxide. Gaseous products analysis suggested that the addition of iron (III) oxide catalyzed the water-gas reaction in the overall gasification while zinc (XI) chromite behaved as an inhibitor in this regard. Potassium carbonate appeared to catalyze the carbon-steam reaction and there was some indication that more gases and tar could be formed from the secondary reactions of tars when the pressure of the system was raised to 2533 kPa. In the gasification of cellulose and jack pine char, the addition of water vapour, potassium carbonate, iron (III) oxide and zinc (II) chromite was found to increase the total gas yields which in turn Increased the heat content of the mixtures.