Mixtures of potassium sulphate and iron sulphate as catalysts for water vapour gasification of carbon. 1. Kinetic studies

The catalytic activity of K₂SO₄ and FeSO₄ · 5H₂O and mixtures of both sulphates in water vapour gasification has been studied with a PVC model coke. Mixtures of H₂ and H₂O were used at pressures of 0.2, 1 and 2 MPa, a heating rate of 4 K min^?1 and temperatures of 800–910 °C (1 h). Catalyst mixtures of approximately 20 wt% Fe and 80 wt% K (total metal amount 7 wt% of carbon) show an optimum activity, especially in equimolar H₂/H₂O mixtures. Increasing pressure enhances the total gasification rate and significantly increases the CH₄ yield.     

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 $\text{H}{}_2\text{H}{}_2\text{O}$ 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.     

Mechanism of the potassium catalysed gasification of carbon in CO2

The literature on the potassium catalysed gasification of carbon in CO₂ is critically reviewed with respect to the mechanism and the experimental ‘facts’ relevant to the mechanistic considerations. It is concluded that bulk intercalation compounds (C₈K, C₂₄K, etc.) are not present under gasification conditions; also other metallic K-species are not the major species during gasification. It is shown that the catalytic activity can be attributed to an oxygen transfer cycle with either reduction of carbon or decomposition of the oxygenated complexes as the rate determining step. In this catalytic cycle only oxidic potassium species are involved.     

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

The influence of sulphur on the catalytic activity of iron in gasification with H₂-H₂O 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.     

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 + NO_x; 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.     

Nickel-catalysed carbon gasification: Crystallographic and magnetic studies

Nickel possesses high catalytic activity for steam gasification of carbon at 810K but shows rapid deactivation at 30–50% carbon conversion. In an attempt to understand the mode of deactivation, the crystallographic and magnetic properties of finely divided nickel particles on carbon were determined as a function of gasification conditions. X-ray analyses showed that nickel particles increased in crystallite size with carbon gasification. Magnetic studies related the observed ferromagnetic moment to crystallite size and the degree of metal reduction. Catalyst deactivation was shown to be partially due to crystallite growth and loss of nickel-carbon contact. Furthermore, catalyst activity was shown to be related to the presence of oxidized nickel species. Catalyst deactivation due to nickel oxide formation or carbon dissolution has been ruled out.     

Catalyzed gasification of active carbon by oxygen: influence of catalyst type,temperature, oxygen partial pressure and particle size

A thermogravimetric study of the oxygen gasification of a commercial active carbon using Co, Ni and Cu as catalysts has been carried out. This follows a previous study of the influence of the particle size, gas volumetric flow rate, initial sample weight temperature and oxygen partial pressure an non‐catalytic gasification. The results show that for T < 500 °C; P₀₂ < 1 atm; D _p < 0.4 mm, Q _v > 50 cm³ min⁻¹ and M _o < 75 mg the gasification of the active carbon with oxygen is controlled by the chemical reaction. The presence of a catalyst increases in all cases the reaction rate, both temperature and catalyst concentration showing a very positive influence. The activity of the different catalysts follows the order: Co > Cu > Ni. A kinetic study applied to both types of gasification has allowed the activation energy and the reaction order with respect to oxygen and the catalyst to be determined. In all cases both the activation energy and the Arrhenius pre‐exponential factor are lower in the catalytic gasification. This brings about the existence of an isokinetic point for the catalytic and the non‐catalytic processes, which were also determined. © 2000 Society of Chemical Industry     

Investigations on the alkali-catalysed steam gasification of coal: Kinetics and interactions of alkali catalyst with carbon

After a reflection on the gasification-kinetics of alkali-doped coal the catalytic activities of Li, Na, K, Cs were compared. The behaviour of coke doped with alkali carbonate under inert atmosphere was also studied.     

Catalytic activity of coal minerals in water vapour gasification of coal

Possible catalytic influences of coal minerals during water vapour gasification of coal have been studied by kinetic measurements and microanalytical methods. A bituminous coal without and with various pretreatments and also model chars synthesized from PVC and PVC-sulphur mixtures were used as raw materials. Kinetic measurements were performed in a fixed-bed flow reactor at pressures between 0.2 and 2 MPa and temperatures from 880 to 1010 °C using hydrogen/water vapour mixtures as gasification agents. It was found that coal gasification at and beyond 880 °C can be decisively catalysed by the iron as constituent of mineral matter. Preconditions are elimination of inorganic sulphur and reducing atmosphere to stabilize elemental iron. The optimum pressure is in the range of 0.5 to 1 MPa. Scanning electron microscopy and electron probe microanalysis confirm that catalytic gasification starts as soon as the iron is free of sulphur. The organic sulphur of coal does not prevent but lowers the catalytic activity of iron.     

Effects of catalysts and steam gasification on e.s.r. of carbon black

The effects of temperature and steam concentration on Spheron-6 carbon black with and without impregnated K₂C0₃ and diluted with alumina have been measured by in-situ electron spin resonance (e.s.r.). For samples in flowing helium, heating from 290 to 1070 K results in a small increase in the carbon black resonance linewidth but a large and mostly irreversible increase in the resonance linewidth of carbon black with K₂C0₃ at >650 K. The effect of steam concentration on the e.s.r. spectra of the carbon black at 875 K has been studied; an effect is observed only for carbon black with K₂C0₃. Increasing the steam concentration from 0 to 6.5 vol% results in reversible narrowing of the linewidth while hardly changing the free radical concentration. The mechanism of the line-broadening of the resonance resulting from heating the K₂C0₃-impregnated carbon black may be due to one of two processes: an unresolved nuclear hyperfine interaction between the unpaired electron of the carbon free radical and potassium, or an increased mobility of charge carriers produced by the potassium. The reversible effects of steam are attributed to a modificartion of these processes and may be related to the mechanism of catalysed gasification of coal.     

Significance of the reduction of alkali carbonates in catalytic carbon gasification

The catalytic effect of alkali carbonates on carbon gasification depends not only on the initial catalyst loading but also on how these carbonates are reduced to surface complexes during the initial heating of the samples. This effect is primarily due to three important processes which take place simultaneously during the initial heat treatment: 1, catalyst re-distribution on the surface; 2, catalyst loss by vaporization; and 3, change in the substrate surface area due to carbon conversion.     

Observation of alkali catalyst particles during gasification of carbonaceous materials in CO2 and steam

Results of a microscopical examination of catalysed carbon gasification are reported. Both in CO₂ and steam, alkali catalysts show evidence of mobility. In the steam gasification of coal chars, the catalysts irreversibly combine with indigenous mineral matter. This is less pronounced in C0₂. The catalysed CO₂ gasification was observed by hot stage microscopy, where alkali carbonate catalysts achieve an apparently molten state during incipient gasification. For single crystal graphite, circular pitting, hexagonal pitting and channelling were observed. For coal chars, irregular morphologies tend to obscure direct observation of surface/catalyst interactions, though subsequent scanning electron micrographs reveal the consequences of extensive catalyst mobility.     

Catalytic coal gasification: use of calcium versus potassium

A comparative study is made of the relative catalytic effects of potassium and calcium on the gasification in air and 3.1 kPa steam of North Dakota lignitic chars prepared under slow and rapid pyrolysis conditions. It is indicated that potassium achieves relatively high catalytic activity by chemical interaction with the carbonaceous support, no matter how it is added to the lignite or its char. Deactivation of the catalytic potassium is brought about by interaction with inherent aluminosilicates. However, deactivation of calcium is related to its sintering via crystallite growth.     

Formation and gasification of carbon deposits on NiSiO2 catalysts

The deposition of carbon from CH₄ and CO on $\text{Ni}\text{SiO}{}_2$ catalysts was studied in pulse-flow experiments as well as volumetrically with a low-field magnetic permeameter. It was found that carbon, deposited from CH₄ according to: CH₄ → C + 4H, gave rise to the formation of nickel carbide, Ni₃C, only at the surface of the nickel particles (T< 300 °C). However, carbon, deposited from CO according to: 2CO → C + CO₂, led to the formation of a bulk nickel carbide as well as dissolution of carbon interstitially. The reactivity of the carbon thus deposited was studied with both H₂ and H₂O. The rate of reaction with hydrogen appeared to be a function of temperature: the rate passed through a maximum at 200 °C and dropped steeply above 300 °C. The only product of the reaction was CH₄. The reaction with H₂O produced besides CH₄, CO₂ and (at low carbon surface coverages) H₂.     

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 K_xO_y (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 K_xO_y (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.     

Catalytic effect of potassium on the rate of char-CO2 gasification

The effect of potassium on the rate of char-CO₂ gasification at 800 °C was investigated. The instantaneous rate depends on both catalyst concentration ($\text{K}\text{C}$) and the internal porous structure of the solid. At low values of $\text{K}\text{C}$ atomic ratio, the rate increases sharply with the addition of catalyst. As catalyst concentration is increased, the rate first levels off and then decreases. The levelling off is attributed to the saturation of the surface with catalytic sites. The subsequent decrease in rate seems to be due to the plugging of micropores by catalyst deposits. The reaction rate changes significantly during gasification and drops sharply before gasification is completed. The drop in rate before total conversion can be explained by catalyst accumulation and pore plugging.