Carbon gasification oxide catalysts

Oxide catalysts are known to be effective for oxidation reactions. In this study, we wanted to produce carbon monoxide through partial oxidation of the biomass, as this could be expected to lead to a conversion of carbon monoxide into hydrogen via the water-gas shift reaction. An oxidization of the tarry product is also expected. By these two effects, improvement of the efficiency of the gasification is expected. Oxide catalyst is expected to enhance the oxidation reaction needed for this scenario. Since oxide catalyst is considerably cheaper than nickel catalyst, its use would make the whole gasification process more economical. Hence, we decided to examine the effect of oxide catalysts on gasification with partial oxidation using cellulose as a model compound. 

The carbon gasification efficiency was 68% with nickel catalyst, 31% without any catalyst, and 31% with the addition of SSC catalyst. This shows that the oxide catalyst does not effectively enhance the gasification or the oxidation of tarry... 

Gasification assisted with partial oxidation is effective for cellulose gasification and a carbon gasification efficiency of 68% was obtained for nickel catalyst at a reaction temperature as low as 400 C and reaction time as short as 5 minutes. 

Oxide catalysts do not effectively enhance the partial oxidation or increase the carbon gasification efficiency, but they do enhance the water-gas shift reaction, especially when mixed with nickel catalyst. 

This mechanism was not able to explain the observed experimental features of the carbon gasification catalysed by iron. The oxygen-transfer mechanism assumes that the catalyst undergoes a cycle between two oxidation states such as metal and oxide or lower oxide and higher oxide. Walker et al extended the views of previous workers and proposed the following equation as representing the mechanism for the case of C-CO2 reaction ... 

The observed catalytic effect of the alkali metal carbonates and oxides and the alkaline earth oxides upon the gasification of the ESC deposit in water vapour again most probably derived from successive oxidation and reduction processes. A possible cycle carbonate-metal-hydroxide could be feasible at this temperature, at least for sodium, potassium and lithium (3) and conceivably also for cesium and rubidium. For barium and strontium the cycle could be between a higher and a lower oxide. Calcium, in contrast to barium and strontium, does not form a peroxide by oxidation of calcium oxide and in any case this would not be stable above 200°C, which could explain why calcium oxide was not an active catalyst. 

Of these inorganic materials it is the salts, oxides and metals of the series of alkali metals, alkali earth metals and transition metals which are particularly effective as catalysts. The efficiency of a catalyst is dependent on the size of the catalyst particle and its mode of distribution within the carbon. To emphasize the importance of catalysis it can be said that about 100 ppm of for example a lead salt increases the rate of oxidation of carbon by molecular oxygen by a factor of about 10 when compared with a pure carbon. It is thus possible to appreciate an earlier statement that it is very difticult to study a carbon gasification reaction which is not catalyzed in some way. The preparation of pure carbons must be in conditions as strict as those of an aseptic laboratory or a radio-chemical laboratory. 

It was proposed by Roh et al. [37] that the presence of partially oxidized Ce sites in Ce,cZri 02 suppresses CH4 formation by acetaldehyde decomposition, thus optimizing the hydrogen yield. In addition, Ce Zri c02 promotes noble metal and transition metals for the water-gas shift reaction (Eq. (24.3)) [38]. Moreover, in the reduced state, CexZii x02 niay reduce water to directly yield hydrogen [39]. Finally, Ce Zri, (02 improves the catalyst stability by (i) limiting the formation of ethylene and (ii) promoting carbon gasification [40]. 

Fig. 11. The loss of carbon rapidly increases with the increase of temperature. Heating of the catalysts in open air for 30 minutes at 973 K leads to the total elimination of carbon from the surface. The gasification of amorphous carbon proceeds more rapidly than that of filaments. The tubules obtained after oxidation of carbon-deposited catalysts during 30 minutes at 873 K are almost free from amorphous carbon. The process of gasification of nanotubules on the surface of the catalyst is easier in comparison with the oxidation of nanotubes containing soot obtained by the arc-discharge method[28, 29]. This can be easily explained, in agreement with Ref [30], by the surface activation of oxygen of the gaseous phase on Co-Si02 catalyst.

Another important factor affecting carbon deposition is the catalyst surface basicity. In particular, it was demonstrated that carbon formation can be diminished or even suppressed when the metal is supported on a metal oxide carrier with a strong Lewis basicity [47]. This effect can be attributed to the fact that high Lewis basicity of the support enhances the C02 chemisorption on the catalyst surface resulting in the removal of carbon (by surface gasification reactions). According to Rostrup-Nielsen and Hansen [12], the amount of carbon deposited on the metal catalysts decreases in the following order ... 

The catalytic conversion of heavy hydrocarbons, such as heavy oil or sulphurous organic residues, from the oil industry via steam reforming is not feasible because solid carbon starts to be deposited at temperatures above 800 °C, which renders the catalyst inactive in a short period of time and, furthermore, blocks the gas flow in the reactor. Heavy hydrocarbons are, therefore, converted to hydrogen using partial oxidation (POX). Note that in refineries the term gasification is more commonly used partial oxidation is the scientific terminology. 

Gasification by low-temperature steam-reforming reactions, the heart of die MRG process, is carried out between liquid hydrocarbons and steam over catalyst to fonn methane, hydrogen, and carbon oxides. In order to increase the calonfic value of product gas to the values similar to natural gas, methanation reactions are required. Hydrogen in product gas is reacted with C02 and CO to form methane, with only a small portion unconverted. Methanation reactions are ... 

Methanol as Source ofSNG. Methanol can be produced from a large range of feedstocks by a variety of processes. Natural gas. liquefied petroleum gas (LPG), naphthas, residua] oils, asphalt, oil shale, and coal are in the forefront as feedstocks to produce methanol, with wood and waste products from farms and municipalities possible additional feedstock sources, hi order to synthesize methanol, the main feedstocks are converted to a mixture of hydrogen and carbon oxides (synthesis gas) by steam reforming, partial oxidation, or gasification. The hydrogen and carbon oxides are then converted to methanol over a catalyst. 

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