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.     

Mechanisms of the alkali metal catalysed gasification of carbon

The catalytic effects of alkali metal salts in the gasification of carbonaceous materials by oxygen, steam and carbon dioxide are described. The most effective catalysts are generally the carbonates, oxides and hydroxides; other active salts tend to convert to these species under gasification conditions. Current theories of the mechanism of this type of catalysis are reviewed. Thermodynamic considerations, the results of thermal analysis and the magnitude of kinetic isotope effects suggest that cyclic sequences of elementary reactions are responsible for the catalytic phenomena.     

Distinctive burning characteristics of carbon particles

Several reaction mechanisms exist in the literature for the gasification of char/carbon particles. The general procedure for modelling particle gasification is to assume a mechanism of combustion, obtain the theoretical burning rate and particle temperature, and then validate the mechanism by comparing the results with experimental data. The present work shows that the burning rate and particle temperature are independent of heterogeneous and homogeneous reaction mechanisms and their rate kinetics as long as the oxygen (O₂) and carbon dioxide (CO₂) concentrations at the burning surface are low compared to carbon monoxide (CO) concentration. Experimental data are cited which are in agreement with the model's predictions for both particle temperatures and burning times. The relevance of the results to industrial combustors is discussed.     

Catalysed carbon gasification with Ba13CO3

The interaction of barium carbonate with carbon black was studied to understand catalysed CO₂ gasification of carbon. Temperature-programmed reaction with isotopic labelling of the carbonate and the carbon showed that carbon dramatically accelerated the rate of BaCO₃ decomposition to form BaO and CO₂, which rapidly gasified carbon to form CO. Pure BaCO₃ was observed to exchange carbon dioxide with the gas-phase, and the exchange rate was increased significantly by carbon at higher temperatures, due to formation of a carbon-carbonate complex. The interaction of BaCO₃ and C to form a complex occurred well below gasification temperatures, and BaCO₃ did not decompose until after gasification began and the gas phase CO₂ concentration was low. During catalysed gasification, formation of gaseous CO from a surface oxide is shown directly to be the slow step in the reaction. The active catalyst appears to cycle between BaCO₃ and BaO (both of which interact with carbon). The rates of carbonate decomposition, catalytic gasification, and exchange with gaseous CO₂ are all slower for BaCO₃ than for K₂CO₃, indicating the large differences in carbonate-carbon interaction between alkali carbonates and alkaline earth carbonates. The two carbonates apparently follow different reaction mechanisms.     

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.     

Design of laboratory reactors for the measurement of catalytic carbon and coal gasification kinetics

A great deal of conflicting experimental kinetics results have appeared for catalytic carbon and coal gasification by water and carbon dioxide. The reason for this can in large part be attributed to inhibition of these reactions by their products and the influence of this product inhibition on the measured kinetics in different laboratory reactor systems. The measurement of differential rates and the determination of true kinetic values requires the use of feed streams with an excess of water and hydrogen for the catalytic CH₂O reaction and an excess of carbon dioxide and carbon monoxide for the catalytic CCO₂ reaction. Recommendations are put forward for the design of laboratory reactors for the measurement of catalytic carbon and coal gasification kinetics.     

Coal gasification at pressure by mixtures of carbon dioxide and oxygen

This Paper discusses results obtained from experiments on coal gasification by mixtures of carbon dioxide and oxygen under pressure. This research was carried out at the INIEX pilot station as part of the Belgian-German programme for the development of underground gasification. Tests were carried out to study the influence of the ratio of carbon dioxide to injected oxygen on the composition and the heating value of the gas obtained, the gasification efficiency and the consumption of the gasification medium. The experimental results can be satisfactorily explained by means of a model based on thermodynamic equilibria, assuming that gasification is brought about by an internal gasification agent formed by the volatile components of the coal and by an external gasification agent consisting of injected carbon dioxide and oxygen.     

Mechanisms of non-catalysed and iron-catalysed water vapour gasification of carbon

The mechanism of non-catalysed and iron-catalysed water vapour gasification was studied with Polyvinylchloride cokes of different heat treatment temperature by temperature programmed desorption of carbon monoxide, from frozen, in surface complexes. Non-catalysed gasification is controlled by the formation of extremely stable ‘unreactive’ ethers formed at 500 °C. Dissociation of water at these surface complexes represents the rate limiting step. For iron-catalysed gasification, the oxygen transfer mechanism was confirmed. Three iron oxygen surface complexes of different stability may be formed, but steady-state isothermal gasification is only possible with the most stable complex as intermediate. Dissociation of water at the metal is accelerated and the oxygen transfer from the iron to the carbon surface controls the overall rate of gasification above 700 °C.     

Temperature-programmed desorption study of Na2CO3-containing activated carbon

The technique of temperature-programmed desorption (TPD) appears to be a powerful means of studying the catalytic action of sodium carbonate as a gasification catalyst. Carbonate decomposition, enhanced by carbon, and reduction to the metallic state can be identified by separate desorption peaks. After controlled adsorption of oxygen, carbon dioxide and water vapour at relatively low temperatures, similar desorption patterns are obtained that are a measure for catalytically active species during gasification. Therefore, these patterns have a predictive value for the activity of alkali metal catalysts and give additional information on processes that control their catalytic action and the influence of pre-treatment and partial burn-off on their performance.     

Relation between the gasification rate of carbons supporting alkali metal salts and the amount of oxygen trapped by the metal

The amount of oxygen trapped by metal on the surface of carbon was measured by the flash desorption method by use of a Curie-point pyrolyser. The steam gasification rate of the carbon supporting several alkali metal salts was proportional to the amount of oxygen measured. Furthermore, the molar amount of oxygen trapped by metal on the carbon surface was found to equal the molar amount of metal supported. The F.T.-i.r. spectra indicated that the alkali metal (M) traps oxygen in the form of an MOC bond. A mechanism for the catalytic gasification, consistent with these findings, was developed. The gasification rate equation based on this mechanism was found to fit the experimental data.     

Nitric oxide reduction and carbon monoxide oxidation over carbon-supported copper-chromium catalysts

Carbon supported copper-chromium catalysts are shown to be very active for both the reduction of nitric oxide with carbon monoxide and the oxidation of carbon monoxide with oxygen. Mixed copper-chromium oxide active phases have good activity in the simultaneous removal of nitric oxide and carbon monoxide from exhaust gases. The influence of several catalyst variables has been investigated. The activity per volume of catalyst increases with increasing loading, while the intrinsic activity shows a maximum around C/M=100−50. An optimum catalyst for nitric oxide reduction and carbon monoxide oxidation has a copper/chromium ratio of 2/1. The apparent activation energy for the carbon monoxide oxidation over carbon supported copper-chromium catalysts is 77 kJ/mol, suggesting that the Cu---O bond rupture is the rate-limiting process. The reduction of nitric oxide takes place at higher temperatures. Since all catalysts have a low selectivity for molecular nitrogen formation at lower temperatures, the dissociation of nitric oxide is probably rate determining, resulting in a slightly reduced catalyst system. In an excess of carbon monoxide the reaction is first-order in nitric oxide and zero-order in carbon monoxide. Moisture inhibits the reaction by reversible competitive adsorption, whereas carbon dioxide does not. Oxygen completely inhibits the reduction of nitric oxide due to the more rapid reoxidation of the catalytic sites compared to nitric oxide. Therefore, the reduction of nitric oxide takes place only when all oxygen has been converted and, hence, is shifted to higher temperatures. As a possible consequence, the production of nitrous oxide is reduced. Nitric oxide and molecular oxygen react preferentially with carbon monoxide, so, in an excess of oxidizing component, gasification of the carbon support occurs at higher temperatures after carbon monoxide has been completely consumed.     

Carbon gasification in the presence of metal catalysts

The development of new processes for the production of gaseous fuels from carbon-containing solids is essential in meeting the energy needs of the nation. In this paper, catalysed carbon gasification is examined. The change in the reactivity of the interface between gaseous reactant (hydrogen or steam) and solid carbon has been measured in the presence of various metal catalysts. With platinum it is found that over a range of temperatures the specific rate of methane production is of the same magnitude as the rate of hydrogen atomization. The catalytic effect is interpretable in terms of an enhanced rate of hydrogen dissociation on the metal surface, followed by surface diffusion across the metal/carbon interface and reaction with carbon. The gas formation rate during the interaction of water vapour with catalyst-activated carbons has been increased by more than an order of magnitude by depositing small weight fractions of active metal catalyst on the carbon surface. At the temperatures employed in this study (975–1175 K), carbon monoxide and hydrogen are the products of the catalysed reaction for each of the catalysts examined.     

Kinetics of catalyst loss during potassium-catalysed CO2 gasification of carbon

A significant fraction of the potassium catalyst can be lost by vaporization during catalysed carbon gasification. The extent of this loss depends primarily on the reaction start-up procedure. Temperature programmed experiments show that, under inert atmospheres, both KOH and K₂CO₃ react with carbon to give a reduced form of potassium-carbon complex. The formation of this complex appears to be a prerequisite for the vaporization of potassium. The rate of vaporization at 800 °C follows a first-order expression. Under gasification conditions, only a fraction of the catalyst is in this reduced form; therefore, the rate of catalyst loss during gasification is lower than that under inert atmospheres. The effect of catalyst loss on both the initial gasification rate and the variation in rate with conversion has been determined.     

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.     

Gasification of graphite in carbon dioxide and water vapor—the catalytic effects of alkali metal salts

The catalytic effects of a series of alkali metal salts in promoting the gasification of a graphite powder by carbon dioxide and water vapor have been studied by thermogravimetry between 700 and 1100°C. Lithium salts, specifically the carbonate and hydroxide, were the most active catalysts for both reactions. Cyclic processes which may account for the observed catalytic effects were evaluated from the standpoint of thermodynamic feasibility.     

The reduction of sulphur dioxide by carbon monoxide on a La0.5Sr0.5CoO3 catalyst

The semiconducting perovskite oxide, La_0.5Sr_0.5CoO₃, is a catalyst for the reduction of sulphur dioxide by carbon monoxide. Unlike aluminia-supported metal catalysts, La_0.5Sr_0.5CoO₃ tolerates high levels of oxygen in the gas stream if sufficient carbon monoxide is present to react with all the oxygen. Water vapour (2%) does not adversely affect the reaction; unwanted byproducts H₂S and COS are reduced for contact times less than 0.25 s. A computer model is used to predict equilibrium gas compositions for systems containing oxygen and water vapour. During the reaction the catalyst itself reacts with sulphur producing metal sulphides, possibly of simple cubic structure. The perovskite sulphides LaCoS₃ and La_0.5Sr_0.5CoS₃ are theoretically unstable.     

Raman spectroscopic study of effect of steam and carbon dioxide activation on microstructure of polyacrylonitrile‐based activated carbon fabrics

This work presents the different effects of steam and carbon dioxide activation on the microstructure of an oxidized polyacrylonitrile (PAN) fabric. An investigation was conducted on a series of carbonized fabrics and two series of activated carbon fabrics. The fabrics were activated by steam and carbon dioxide using heat‐treatment temperatures of 900–1100°C. Steam and carbon dioxide developed the microstructure initially present in the PAN‐based activated carbon fabrics, but with different effects. These fabrics in the form of fabric and powder were examined by X‐ray diffraction and Raman spectrometry. This study indicated that carbon dioxide only reacted with the crystalline edges or the irregular carbon on the fiber surface and that the inside structure of the fibers was not greatly affected. When the fabrics were activated using steam, water molecules reacted not only on the fiber surface but also with the carbon at the crystal edge and/or the nonregular carbon in the fibers, which led to communicating pore structures on the surface and in the inner portions of the fiber. This activation also promoted the denitrogenation reactions. Because of these structures and reactions, the activated carbon fabrics, which were activated by steam, had the highest stacking height for carbon layer planes (L_c), the highest number of layer planes (L_c/d₀₀₂), the highest oxygen content, the largest crystal size (L_a), and the highest density over the other samples. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1090–1099, 2001     

Surface properties of carbons from poly(vinyl chloride)

Non-activated carbons were prepared by the thermal degradation of poly(vinyl chloride) (PVC) in air or nitrogen atmosphere in the temperature range 600-1000°C. Carbon dioxide-activated carbons from PVC were also obtained by gasification of non-activated carbon from PVC at 900°C burn-off (4-50%). Thermal degradation in air atmosphere gave high carbon yield because the oxygen of air increased crosslinking at lower temperature and chemisorbed on the carbon surface at high temperatures. Thermal degradation in air and gasification with carbon dioxide created carbon-oxygen surface groups which increased the hydrophilicity of the carbon surface and consequently increased water adsorption capacity. Gasification with carbon dioxide to high burn-off created new pores and widened already existing pores.     

CO2 gasification of carbon catalysed by alkali metals: Reactivity and mechanism

A systematic study of the catalytic activity of alkali metal carbonates on the CO₂ gasification of activated carbon revealed the following order: Li < Na < K < Rb < Cs. Outgassing in an inert gas results in a pronounced activity decrease for Cs, whereas the other alkali metals show a slight increase. The activated carbon itself is unaffected. Apparent activation energies for the CO₂ gasification are also changed by outgassing and decrease from Li to Cs. Upon outgassing of the samples, CO₂ and CO are released in five distinguishable temperature regions, arising from decomposition of surface complexes and carbonate-like species, gasification phenomena and reduction of oxidic species. Outgassing patterns of all alkali metals are quite similar. During alkali-metal-catalysed gasification in CO₂ two types of oxidic species are present: surface bonded -OM species of high stability and oxidic species having less interaction with the carbon surface.