Steam reaction, char

It should be noted that there has been some previously published work dealing with these reactions. Studies at Union Oil Research in the early 1970 s appear to be among the first to attempt the exploitation of the steam-char reaction and led to the development of the SGR retorting process( ). Later, Burnham at Lawrence Livermore Laboratories(4, J5,6j conducted both non-isothermal and isothermal experiments and obtained reaction rate expressions for the rate of char consumption due to both reactions... 

The role of CaO as a catalyst for the steam-char reaction can be inferred from the results obtained during mixed CO2-H2O gasification (figures 3 and 4). In many respects it is desirable to prevent the decomposition of CaCOj to CaO because of the high endothermic heats of reaction associated with this reaction. However, as we have shown, the steam gasification of oil shale char is about ten times slower when CaO is not present. A better evaluation of the importance of CaO requires a knowledge of the dependence of the reaction rate on the quantity of CaO present. 

Steam-Char Reaction. The rate of the steam-char reaction in the first phase is represented by ... 

Effect of Partial Pressure of Steam. Contradictory conclusions have been reported in the literature regarding the effect of the partial pressure of steam. The reaction has been variously reported to be zero order (31), fractional order, first order (12, 15,17,18, 21), and between first and second order (21,31) with respect to steam. As shown in Figures 5, 6, 7, and 8, the rate of the steam-char reaction is not affected by the partial pressure of steam, so far as detectable by the experiment. This agrees with the previous investigators (13, 26, 27, 30). 

Figure 5. Rate of steam-char reaction during the first-phase reaction at 1700° F.

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

Figure 10. Effective mass transfer factor for steam-char reaction in second phase indicating the possibility of diffusion-control mechanism...

The char-hydrogen reaction and the steam-char reaction are considered as taking place independently so that the rates of the reactions are additive. 

Therefore, the steam gasification reaction rate of the gingko nut shell-char can be represented by the following kinetic equation as ... 

In Great Britain, the Imperial Chemical Industries, Ltd., has developed a fluidized coal gasification process (32) using combustion of part of the fuel with air to supply the heat necessary for the endothermic steam-carbon reaction. This procedure is divided into three steps carbonization, air-blowing, and steam-blowing. The first and third steps operate with fluidized beds, and the second with a dilute suspension. Powdered coal is fed into the carbonization vessel, where it is heated by combustion of some of the carbonization gases. The powdered char is blown into a feed hopper connected with the air-... 

An important variant of the Fluid Bed system is under development. This variant eliminates use of air or oxygen in the actual gasifier. Steam and coal are the reactants. Since we know from Table 3 that the reaction of steam with coal is endothermic, a heat source must be provided. Hot solids in the form of char are heated in a combustor and are transferred to the gasification reactor as one these processes. In another, hot alkaline oxides react with the carbon dioxide in the gas to form carbonates. The exothermic reaction of carbonate formation supplies the heat requirements of the steam-carbon reaction. Both of these processes depend on a reactive coal or char to implement the steam-carbon reaction. 

Hydrogen inhibits the steam gasification reaction. The char gasification reaction with steam and hydrogen can be modelled based on Langmuir-Hinshelwood kinetics. The model fits well the results. 

Reaction of steam with carbon is one of the basic processes involved in the gasification of coals or chars to produce clean fuels. The industrial importance of this reaction is considerable, particularly at this time of spiraling energy costs. In addition, the steam-carbon reaction finds other important industrial applications, such as preventing or minimizing the coking of olefin-plant cracker-tubes. Because of its commercial importance, the steam-carbon reaction has been studied extensively, and excellent reviews are available (1,2). In addition to reaction kinetics and mechanisms, the effects of carbon structure, catalysis by metals, and impurities (anions) have been investigated (3,4, .5) ... 

Because it requires many possible variables, such as temperature, pressure, the nature of chemical reaction, and the character of the solid surface, and because it incorporates many constants which require experimental evaluation, the general mathematical model to estimate the product gas distribution for different levels of carbon conversion can become exceedingly complicated. Practical application of this model is particularly difficult when a choice has to be made between reaction mechanisms, each of which can generate complex functions with a sufficient number of arbitrary constants to fit any given experimental curve. The purpose of the work discussed in this paper was to study the influence of temperature and the partial pressure of hydrogen and steam on the rate of steam-hydrogen and coal char reactions based on the previous pilot plant data obtained at IGT (10, 11) and to develop a correlation to estimate the performance of a hydrogasification reactor in terms of its product gas distribution for different levels of carbon conversion. 

The coal char reaction with a hydrogen and steam mixture is represented by the first-phase and the second-phase reactions. The first-phase reaction is related to the amount of volatile matter and may be regarded as a volume (or mass) reaction. The second-phase reaction is characterized by the heterogeneous reaction at the carbon surface—a reaction which seems to be controlled by gas diffusion for the range of operating conditions in the continuous moving-bed reactor. 

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