The state of calcium as a char gasification catalyst — a temperature-programmed reaction study

Abstract

The purpose of this study was to characterize the calcium (nature, amount of various species and state of division) in carbonaceous materials such as low-rank coal char or a model char (obtained by pyrolysis of a polymeric resin at 1273 K under an inert gas). Ca loading (<6 wt%) was performed by impregnation or ionic exchange of various char samples with an acetate solution. The experimental method involved performing temperature-programmed reactions (TPR) with Ca-containing char samples, under a low partial pressure of CO 2 in an inert gas stream, by increasing and then decreasing the temperature in the range 300–1200 K. Samples were previously outgassed by temperature-programmed desorption (TPD) under He up to 1200 K. The various CO 2 and CO peak curve features (apart from the gasification reaction) were interpreted as follows. Desorption of physisorbed CO 2 on char takes place at a low temperature (< 380 K), and is followed by CO 2 chemisorption on CaO, and then bulk carbonation of CaO. Finally, above the equilibrium inversion temperature, T i , of the carbonation reaction, the carbonate formed in the preceding steps is decomposed. During cooling, a trough related to the gasification of carbonaceous matter is first observed before recarbonation of CaO. Samples of either pure CaO of known specific surface area, or mechanical mixtures of CaO or CaCO 3 and char, were also studied for reference. The temperatures of the troughs in the increasing temperature parts of TPR spectra allow us to distinguish between CaO species resulting from ion-exchanged calcium and those formed from deposited carbonate crystals during the preparation. The behaviour of sintered CaO from both origins is also different. This method may possibly be extended to other Ca C systems that can undergo thermal treatments.