THE INTERACTION OF H2O, CO2, H2 AND CO WITH THE ALKALI-CARBONATE CARBON SYSTEM - A THERMOGRAVIMETRIC STUDY

The mass changes of an alkali-carbonate/carbon sample were studied in various gas mixtures during temperature programmed gravimetric analysis (TPGA), isothermal adsorption and desorption and temperature programmed reduction and desorption experiments at (sub)gasification temperatures. Both CO2 and CO show a strong interaction with the alkali/carbon system, resulting in reversible mass changes, which are ascribed to changes in the catalytically active alkali species present on the carbon surface. The extent of reversible mass change is strongly dependent on temperature, gas phase composition and pretreatment of the catalyst/carbon sample. In the presence of H2O or CO2 addition or removal of H-2 shows no significant effect on the sample mass, whereas in the absence of an oxidizing agent H-2 acts as a strong reducing agent. As is known, H2O is capable of oxidizing or gasifying the catalyst/carbon sample, but no H2O chemisorption is observed. The alkali-catalysed oxygen exchange reactions in H2O-, CO2-, H-2- and CO-containing gas mixtures, e.g. the water gas shift reaction, can be described by a three step model in which empty (*), oxidized (O-*) and chemisorbed CO2 (CO2-*) intermediates are involved. The H2O/H-2 oxygen exchange proceeds through (O-*) and (*) intermediates, whereas the CO2/CO oxygen exchange proceeds through the CO2-* intermediate. The inhibiting role of CO2 on all oxygen exchange rates can be explained by the presence of CO2-* sites. The model proposed provides a basis for the kinetic modelling of the steam gasification process, taking into account changes in catalytic activity in various gas mixtures.