R. Meijer

Catalyst loss and retention during alkali-catalysed carbon gasification in CO2

Characterization of the catalyst/carbon samples by outgassing yielded reproducible and consistent results, provided the amounts of CO2 and CO released are related to the amount of alkali metal actually present. The catalytically active cluster is anchored to the carbon by phenolate groups and is capable of chemisorbing one CO2 molecule per three to five alkali metal atoms. Between KC = 0.02 to 0.04 the alkali cluster size increases, but the specific gasification rate, rK, remains unaffected. During potassium catalysed CO2 gasification initially 20% to 40% of the potassium was lost; the rest remained present throughout the gasification process. The amount of catalyst that can be stabilised on the carbon increases in the order Na < K < Cs and depends on the amount of oxygen in the carbon. The latter is reduced by heat treatment of the applied carbon, and hence a smaller amount of catalyst can be stabilised. The differences between the alkali metals arise from their ability to migrate into the carbon matrix and their interaction with lattice oxygen at which they are trapped. This migration of Cs and K into the carbon matrix explains the burn-off profiles of catalyst/carbon samples after TPD treatment. Initially only part of the catalyst is active for gasification, but gradually more catalyst becomes available, resulting in an increasing gasification rate. This behaviour is absent for sodium.

Alkali-catalyzed carbon gasification in CO/CO2 mixtures: An extended model for the oxygen exchange and gasification reaction

Based on the results of reactivity measurements and step-response experiments with labeled molecules an extended model for the oxygen exchange and gasification of carbon in CO/CO/sub 2/ atmospheres over alkali carbonate/carbon systems is given. The alkali catalysts studied (Na, K, Cs) contain considerable amounts of chemisorbed CO/sub 2/, nearly temperature and pressure independent, but easily exchangeable with gas-phase CO/sub 2/. For all three catalysts the gasification rate correlates well and in the same way with the amount of chemisorbed CO/sub 2/. Gasification is envisaged as the decomposition of C(O) complexes which are formed by interaction of the catalyst and the carbon. Oxygen exchange is catalyzed by the alkali catalyst species. Carbon-oxygen complexes, C(O), are not involved in this process. The proposed model can account for the observed pressure dependences; i.e., oxygen exchange is only proportional to the CO pressure and the gasification rate is proportional to the ratio of the CO and CO/sub 2/ partial pressures. In both reactions chemisorbed CO/sub 2/ is the important intermediate.

The interaction of H2O, CO2, H2 and CO with the alkali-carbonate/carbon system : a thermogravimetric study

Abstract 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 CO 2 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 H 2 O or CO 2 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, H 2 O is capable of oxidizing or gasifying the catalyst/carbon sample, but no H 2 O chemisorption is observed. The alkali-catalysed oxygen exchange reactions in H 2 O-, CO 2 -, 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 CO 2 (CO 2 - ∗ ) intermediates are involved. The H 2 O/H 2 oxygen exchange proceeds through (O- ∗ ) and ( ∗ ) intermediates, whereas the CO 2 /CO oxygen exchange proceeds through the CO 2 - ∗ intermediate. The inhibiting role of CO 2 on all oxygen exchange rates can be explained by the presence of CO 2 - ∗ 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.

Burn-off behaviour in alkali-catalysed CO2 gasification of bituminous coal char : a comparison of TGA and fixed-bed reactor

A comparative study has been performed on the kinetics of the alkali-catalysed gasification reactivity in CO2 of a bituminous coal char (Westerholt) at elevated pressures in a fixed-bed reactor (FBR) and a thermobalance (TGA). Both apparatus essentially yield the same kinetic description, taking into account the integral behaviour of the fixed-bed reactor. The parameters obtained for the kinetic model have physical significance and show good agreement with values reported in the literature. In TGA the intrinsic burn-off profile is observed, because the complete system is gradientless. In the FBR this information is averaged out due to the integral reactor behaviour. The burn-off behaviour of the K2CO3/char Westerholt can be described by shrinking sphere gasification based on the grainy pellet model, taking into account catalyst migration. Between approximately 40 and 80% burn-off an apparent order in carbon of −13 is observed, which can be rationalised by the grainy pellet model and a homogeneous dispersion of the catalyst in the grains. Characterization by CO2 and N2 adsorption and scanning electron microscopy provide additional support for this model.

Methane formation in H2,CO mixtures over carbon-supported potassium carbonate

Abstract The rates of CH 4 and CO 2 formation over K 2 CO 3 /carbon have been studied in H 2 ,CO mixtures as a function of temperature (600–1000 K), total pressure (1.5–10 bar), H 2 /CO feed ratio (0.3–8), and the K 2 CO 3 loading (0–20 wt%). In H 2 ,CO mixtures both the rate of CH 4 and CO 2 formation are enhanced by the presence of the potassium catalyst and show the same dependence on the alkali loading as that observed for gasification of carbon in H 2 O and CO 2 . Below 900 K the CH 4 formation has an apparent order of ≌1.2 in p H 2 and ≌0.3 in p co . Under most experimental conditions the rate of CO 2 formation ( E a (app) = 40–50 kJ mol −1 ) is higher than that of CH 4 formation ( E a (app) = 130–150 kJ mol −1 ), resulting in carbon deposition. Only at pressures above 5 bar can CO be selectively converted with H 2 into CH 4 and CO 2 above 900 K. Hydrogenation of the support and of the carbon deposited by the CO disproportionation is not catalysed by potassium. A reactive carbon intermediate is proposed, formed by catalysed dissociation of CO. This can either react with hydrogen to form methane or form the carbon deposit. The oxygen is removed by CO in a manner similar to that in potassium-catalysed oxygen exchange reactions. The observed deactivation is ascribed to a combined effect of blocking of the active sites by carbon deposition and migration of metallic potassium into the carbon matrix, leading to H 2 adsorption on the liberated carbon edge sites, as revealed by temperature-programmed desorption. Potassium is hardly lost at all during the methanation experiments.

Active Sites in Carbon Gasification with CO2 Transient Kinetic Experiments

A transient kinetic study has been conducted of the uncatalysed and potassium catalysed gasification with CO2. Step response experiments using labelled CO2 indicated that in both cases at least two types of oxygen complexes are present at the carbon surface that both yield CO. One decaying within seconds, the other decaying on a minute time scale. Moreover, CO chemisorption or insertion takes place above 1000 K. The active site in catalysed gasification contains at 1000 K distinct amounts of chemisorbed CO2 and reactive oxygen. The total amount of oxygen in the cluster equals that of the potassium and can be easily exchanged with CO2. At higher temperatures the O/K ratio decreases and CO2 chemisorption is no longer observed.

KINETICS AND MECHANISM OF THE ALKALI CATALYSED GASIFICATION OF CARBON

Publisher Summary This chapter discusses the kinetics and mechanism of the alkali catalyzed gasification of carbon. It presents a unified kinetic and mechanistic model that accounts for the phenomena that take place during gasification, together with the role of the catalyst therein. The results of a thermogravimetric study of the interaction of a potassium carbonate/carbon sample with various gases, viz., CO2, CO, H2, and H2O, indicated the presence of three types of catalyst intermediates that are significant in the kinetic modeling. Although gasification takes place at high temperatures, the catalyst remains on the carbon surface. The general picture is that the alkali catalyst is anchored to the carbon surface via a phenolate type of bonding. The removal of the carbon oxygen by high temperature treatment reduces the amount of catalyst that initially can be anchored. At low loadings, the potassium catalyst is highly dispersed.