Radiant flash pyrolysis of cellulose - Evidence for the formation of short life time intermediate liquid species

This paper reports the first results of experiments and modelling of the radiant flash pyrolysis of cellulose. Small samples are exposed to brief flashes of a concentrated radiation, at the focus of an image furnace operating with a 5 kW xenon lamp associated to two elliptical mirrors. The mean heat flux densities may be higher than 10(7) W m(-2). The microscopic observations of the sample after the flash reveal the presence of short life time liquid species formed for flash durations lower than about 1 s. These products which are liquid in pyrolysis conditions are solid at room temperature, where they show a good stability. They are soluble in water. For longer flashes, they give rise to vapours escaping in the gas phase, while practically no char is formed. These results show that, if in biomass pyrolysis, lignin is known to give rise to a liquid phase, it is also the ease for cellulose. A first simple modelling of these experiments is proposed. It relies on heat and mass balances at the sample level, on the Broido-Shafizadeh (BS) model and on experimentally estimated values of some of the optical characteristics of cellulose (reflectivity and absorptivity). Indeed, cellulose is a highly reflecting and weakly absorbing (semi-transparent) material. These properties must be necessarily taken into account in any predictive calculation (only a small fraction of the incoming flux is effectively absorbed by cellulose). The calculated values of the times of beginning and end of the reaction are compared with the results of the experiments. The good agreement confirms that the intermediate products have life times shorter than about 1 s at the reaction temperature, predicted to be close to 750 K. (C) 1998 Elsevier Science B.V. All rights reserved.