Roland Minges

Influence of the catalyst precursor anion in catalysis of water vapour gasification of carbon by potassium: 1. Activation of the catalyst precursors

The activation of various potassium salts as raw materials for potassium catalysed water vapour gasification was studied with a natural graphite and a high volatile bituminous coal using thermogravimetry, differential thermoanalysis and gas analysis. The result of the study is a self-consistent general scheme of activation, whereby potassium hydroxide is formed from all salts. Potassium hydroxide therefore represents the key component of all the activation processes, from which finally the active species, a non-stoichiometric potassium/oxygen compound KxOy (y < x) is formed.

The influence of the catalyst precursor anion in catalysis of water vapour gasification of carbon by potassium: 2. Catalytic activity as influenced by activation and deactivation reactions

Based on a fundamental clarification of the activity of various potassium salts as precursors for potassium catalysed water vapour gasification in part 1 of this paper, the catalytic activity was studied by gasification experiments in a fixed bed flow reactor using equimolar argon/water vapour and hydrogen/water vapour mixtures. The catalytic activities found with the various potassium salts (hydroxide, carbonate, nitrate, sulphate, chloride) characterized by the onset of catalysed gasification and the maximum gasification rates during linear heating (4 K min −1), were found to be identical with the kinetics of the activation, confirming the following sequence: KOH ~ K2CO3 ~ KNO3 >K2SO4 >KCl. As potassium hydroxide represents the key component in the activation of all salts, the active species has to be formed from this compound. The active species is defined as a non-stoichiometric potassium-oxygen compound KxOy (y < x) with varying oxygen contents. It acts as a dissociation centre for water and transfers the oxygen to the carbon surface, from which carbon monoxide is finally desorbed. A high selectivity towards carbon dioxide found in argon/water vapour and a high selectivity towards methane in hydrogen/water vapour are explained by the shift reaction and methanation of primary formed carbon monoxide being catalysed by the actual active species of different oxygen content. Deactivation of the active species by sulphur does not occur. There is also strong irreversible deactivation of the active species by silicates. Inhibition by hydrogen is interpreted by blocking of active sites at the carbon surface.

Water vapour gasification of carbon: Improved catalytic activity of potassium chloride using anion exchange

The use of anion exchange techniques enabling the use of low cost inactive minerals as catalysts for water vapour gasification of carbon is discussed. KCl + CaCO3 or KCl + MgSO4 were studied as model systems with Ceylon natural graphite and a Leopold gas flame coal in a fixed bed flow reactor in an ArH2O mixture. Total pressures were 2 MPa with temperature increasing at 4 K min−1 to 900 °C (graphite) or 850 °C (coal). The thermodynamics of anion exchange reactions are presented with the gasification kinetics confirming the predicted feasibility of the exchange reactions. Both catalyst systems have a strongly improved catalytic effect compared with the use of single components of the mixtures. Results are summarized in a general reaction scheme. This new technique is not restricted to systems so far investigated but can include dolomite or gypsum where KCl is the preferred potassium source.

Catalytic activity of potassium halides in water vapour gasification of graphite

The catalytic activity of potassium halides in water vapour gasification of graphite was studied at 900 ° C and pressures up to 2 MPa. The initial step is the hydrolysis of the potassium halide which controls the catalytic activity: KF >KCl >KBr. Main steps of the catalysed gasification reaction are in good agreement with an oxygen transfer mechanism. The following general reaction scheme is proposed not only for the potassium halides, but also for K2CO3 and KNO3. Initial reactions: K2CO3 + H2O → 2KOH + CO2; KNO3 + H2O → KOH + NOx; KX + H2O → KOH + HX. Intermediate step: KOH + C, H2 → K + CO, H2O. K-catalysed gasification reactions: K + H2O → K(O) + H2; K(O) + C → K + CO; K(O) + CO → K + CO2.

Catalytic water vapour gasification of carbon: Importance of melting and wetting behaviour of the ‘catalyst’

Abstract The melting and wetting behaviour of various alkali compounds (carbonates: Li 2 CO 3 , Na 2 CO 3 , K 2 CO 3 , Cs 2 CO 3 ; halides: KF, KCl, KBr, NaCl; single potassium salts: KNO 3 , K 2 SO 4 ) without and with additives (V 2 O 5 , FeSO 4 · 7H 2 O) on a polished surface of a polycrystalline graphite of high purity was studied during heating at 4 K min −1 to 900 °C in different atmospheres. It was found that all those catalyst raw materials which showed catalytic activity in earlier studies are low-melting and wet the graphite surface. This is especially true of mixtures of alkali chlorides (NaCl, KCl) with V 2 O 5 and of K 2 SO 4 with FeSO 4 · 7H 2 O or other iron salts.

Mixtures of potassium sulphate and iron sulphate as catalysts for water vapour gasification of carbon. 2. Wettability phenomena and reactions of the catalyst precursors

The interaction between carbon and a mixed catalyst (FeSO4 with K2SO4, Fe/K mass ratio 1:5), which showed a superior catalytic activity in water vapour gasification of carbon, was studied by measuring the wettability of a fine-grained graphite, penetration into this material and the metal distribution after treatments in H2H2O mixtures at various temperatures. The decomposition and conversion reactions were analysed by measuring the relative mass losses. It was found that the catalyst mixture forms a melt phase by 650 °C, whereas the pure sulphates are still present as powders. Penetration is complete after 2 h treatment at 700 °C. EPMA studies show a homogeneous distribution of Fe and K in the graphite substrate. The same treatment causes no significant change with the pure sulphates. The relative mass losses suggest that the melt may be composed of FeS, FeO, K2S and KOH. It is assumed that the conversion of K2SO4 to K2S or even KOH may be catalytically influenced by FeS, FeO or possibly Fe. Further studies are necessary to analyse the active state of the catalyst and especially the mutual catalytic activity of Fe and K.

Catalytic activity of carbon supported potassium in the carbon monoxide shift reaction

Abstract The catalytic activity of carbon supported potassium was studied in a fixed bed flow reactor at 0.1 M Pa by using synthesis gas (CO + H 2 ) of various compositions and various water vapour contents. In the temperature range between 400 and 550 °C the carbon monoxide shift reaction: CO + H 2 O ⇌ CO 2 + H 2 , may be described by the following rate equation: r CO 2 = 2.10 3 exp (−82 500/ RT ) p CO 0.6 p H 2 O 0.4 . At 550 °C carbon supported potassium (28.3 mass parts K per 100 mass parts activated carbon) exhibits the same catalytic activity like an industrial high temperature catalyst (type K 6–10 of BASF).

Mixtures of potassium sulphate and iron sulphate as catalysts for water vapour gasification of carbon: 3. Chemsitry of the in situ activation of the catalyst

Previously it was shown that certain mixtures of K2SO4 and FeSO4 were excellent raw materials for catalytic water vapour gasification of carbon. It was suggested from the results that the iron salt catalyses the reduction of K2SO4 to K as the main catalyticly active species of gasification. This paper is concentrated on this activation process. Using TGA, DTA, ESCA, EPMA and visual observations of the melting behaviour after or during treatment of varying K2SO4FeSO4 mixtures in H2 and H2H2O atmospheres it is found that elemental iron is formed in an early stage during heating up. This then catalyses the reduction of K2SO4 to K2S, which is readily hydrolysed by water vapour to liquid KOH. K2SO4 and K2S form an intermediate eutectic (m.p. = 610 °C), which favours wettability of the carbon surface and hydrolysis of K2S. As well as FeSO4 any iron salts can be used that are easily reduced to elemental iron in the gasification atmosphere. A general reaction scheme, including activation and catalytic gasification, is proposed. The activation of K2SO4 to catalytically active KOH can be described as follows: Fe-saltγFe3K2SO4 + 8Feγ3K2S + 4Fe2O3Fe2O3 + 3H2γ 2Fe + 3H2OK2S + H2OγKHS+KOHKHS + H2OγKOH + H2S

Catalytic activity of iron in the water vapour gasification of sulphur-containing cokes

Abstract The influence of sulphur on the catalytic activity of iron in gasification with H 2 -H 2 O mixtures at different total pressures and temperatures was studied with model cokes of various sulphur contents, prepared by copyrolysis of PVC and elemental sulphur at 600 °C. Sulphur represents a strong poison which may completely inhibit the catalytic activity of iron, owing to the presence of extremely stable sulphur surface compounds. The inhibition may partly be compensated by increase of pressure or temperature. The beneficial effect of pressure results from hydrodesulphurization of the cokes in the early stages of gasification, especially at low water vapour partial pressures. High temperatures effect a reduction of the sulphur surface compounds. A general reaction scheme is proposed and it is concluded that gradual inhibition of the catalytic activity of iron is caused by partial blocking of active centres on the iron surfac.