Steam-carbon reaction

In modern gasification practice, principal reliance is placed on the carbon-steam reaction (iii) which, depending on the mode of operation of the reactor, may be variously accompanied by the "shift" reaction... 

Figures 1 to 3 present calculated equilibrium molar ratios of products to reactants as a function of temperature and total pressure of 1 and 100 atm. for the gas-carbon reactions (4), (7), and (5), (6), (4), (7), respectively. Up to 100 atm. over the temperature range involved, the fugacity coefficients of the gases are close to 1 therefore, pressures can be calculated directly from the equilibrium constant. From Fig. 1, it is seen that at temperatures above 1200°K. and at atmospheric pressure, the conversion of carbon dioxide to carbon monoxide by the reaction C - - COj 2CO essentially is unrestricted by equilibrium considerations. At elevated pressures, the possible conversion markedly decreases hence, high pressure has little utility for this reaction, since increased reaction rate can easily be obtained by increasing reaction temperature. On the other hand, for the reaction C -t- 2H2 CH4, the production of methane is seriously limited at one atmosphere pressure and practical operating temperatures, as seen in Fig. 2. Obviously, this reaction must be conducted at elevated pressures to realize a satisfactory yield of methane. For the carbon-steam reaction.

Carbon-Steam Reaction. There is general agreement (41, 46, 53-55)... 

The mechanism of the carbon-steam reaction is discussed in more detail by Long and Sykes (43). They propose that the steam molecule decomposes at the carbon surface into a hydrogen atom and hydroxyl radical both of which chemisorb rapidly on adjacent carbon sites. This is followed by the hydrogen atom on the chemisorbed hydroxyl radical joining the hydrogen atom on the adjacent carbon site and leaving as a hydrogen molecule. Therefore, a further breakdown of the steps in mechanism A may be written... 

A second mechanism for the carbon-steam reaction, similar to mechanism B of the carbon-carbon dioxide reaction, may be operative. Mechanism B ... 

Strickland-Constable (47), observing that hydrogen is not only strongly but very rapidly adsorbed on carbon, supports the view the hydrogen retardation in the carbon-steam reaction is caused by its chemisorption on active sites. 

Gadsby and co-workers (63) report that for a coal charcoal, the rate of the carbon-steam reaction is greater by a factor of about three than the carbon-carbon dioxide reaction at 800° and a pressure range of 50 to 500 mm. Hg. The results of Pilcher et al. (68) and Walker et al. (85), using the same graphitized carbon rods and apparatus, essentially agree with this finding. At 1100°, the former workers report a reaction rate of 1.6 g./hr. at a steam partial pressure of 142 mm. Hg, which can be extrapolated to 4.8... 

No data have come to the authors attention on a direct comparison of the reaction rates for the carbon-oxygen and carbon-steam reactions. 

Relative viscosities are calculated from viscosities for the individual components at 0° (II7), weighting them on a mole fraction basis. The change in diffusivities and viscosities with temperature and pressure is assumed to be independent of gas mixture. If desired, more accurate calculations of diffusivities and viscosities of gas mixtures can be made using the approaches of Wilke (IIS) and Bromley and Wilke (II0), respectively. Table V presents relative values for Dfree, m, and p across the stagnant film for the gas-carbon reactions. Substituting these values in Equation (42), the relative reaction rates in Zone III for the gas-carbon reactions are calculated and also presented in Table V. Qualitatively, the rates of the carbon-oxygen and carbon-steam reactions are predicted to be about twice the rate... 

Fig. 9. Ideally, the predicted variation in the relative rates of the carbon-oxygen and carbon-steam reactions with temperature for a porous carbon.

It is of interest to note that several workers (99, 116) have assumed AI2/to represent the rate of reaction of a carbon specimen only when the reaction is proceeding entirely on the external surface. The above reasoning shows that AR/At can be a constant and represent the over-all rate of reaction when the reaction is occurring internally and the utilized surface area is far greater than the external surface area. Graham and co-workers (116), studying the carbon-steam reaction under high-velocity conditions. 

Producer gas is manufactured by passing air and steam through a bed of hot coal or coke at a temperature of 980 to 1540°C, depending on the fusion points of the fuel ash. The primary purpose of the steam (25 to 30 % of the weight of the coke) is to employ the exothermic energy from the reaction between carbon and oxygen to supply energy (heat) to the endothermic carbon-steam reaction. 

The reason why the minimum steam ratio goes down with temperature is not known with certainty. One possibility is that the competing reactions of carbon production and consumption have such kinetics that the rate of coke consumption increases faster with temperature than the rate of coke generation, which suggests that the carbon-steam reaction has a higher activation energy than the methane cracking and carbon monoxide disproportionation reaction. 

Reactions 17.5, 17.6, and 17.7 illustrate the gasification of char by reaction with various gases. The carbon-steam Reaction 17.5 is an endothermic reversible reaction. Steam undergoes a side reaction, Reaction 17.8, called the water-gas shift reaction. This reaction, which is very rapid, is catalyzed by various impurities and surfaces. The carbon-C02 reaction, Reaction 17.6, is favored at high temperatures and low pressures, whereas the carbon-H2 reaction, Reaction 17.7, is favored at low temperatures and high pressure. Since only three of Reactions 17.5-17.9 are independent, if the equilibrium constants for Reactions 17.6, 17.7, and 17.8 are known, the... 

The water gas shift reaction is considered to be in equilibrium. However, the heterogeneous reactions are influenced by both chemical kinetics and diffusive transport of reactants. Further, in the case of the carbon-steam reaction, the inhibition by both carbon monoxide and hydrogen is also included. 

Hiittingcr and Merdes give a comprehensive description of the models proposed in the literature for the carbon-steam reaction. Basically, there are two models of the reaction mechanism the oxygen exchange model and the hydrogen inhibition model. The equations involved are ... 

Htittinger, K.J. Merdes, W.F. (1992). The carbon-steam reaction at elevated pressure formations of product gases and hydrogen inhibitions, Carbon, Vol. 30, No. 6, pp. 883-894. 

Figure 10 The rate of carbon-steam reaction vs. amount of deposited carbon on catalysts with different contents. (Ck+ =100% is equivalent to ca. 12.9% K in poly-aluminate)...

Additional literature by Cox, (8), Appell (11), Hooverman (12), Cavalier (13), Knight (14) and Love (15) serves to reinforce the evidence that alkali carbonates increase reaction rates and overall gas yields in the gasification of biomass, coal and refuse. The general consensus is that alkali carbonates catalyze the water-gas shift and carbon-steam reactions. 

Rate Constants for Carbon + Steam Reaction on Silica-Alumina Substrate... 

The apparent increase in rate may arise from the increase in surface as carbon was removed from the original particle. Although the conventional experimental rate equation for the carbon-steam reaction (6) states that steam is a part of the rate equation, it further states that steam also inhibits the rate. The test procedure used here maintains inlet steam at a fixed velocity and in large excess thus any effect of the level of steam conversion should be negligible on the integral rates calculated here. In the runs shown in Figure 2, the maximum use of steam by carbon in a 5-min. period varied from 3.6 to 8.5% of the total steam available. Of... 

The rate at which the steam-carbon reaction proceeds depends greatly on temperature (9), requiring heat above 2000°F. to approach equilibrium (II). Since hydrogasification tests are conducted at less than 2000°F. to preserve the methane formed, the carbon-steam reaction is expected to be substantially removed from equilibrium. This is shown by Figure 6 where calculated equilibrium ratios are plotted against maximum bed temperature. The curve represents true equilibrium for comparison. 

Steam-Char Reactions. The steam-char reaction during the second phase is probably similar to the carbon-steam reaction occurring at the char surface. 

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