Gasification rate, carbon

Carbon Gasification Rates. Because the reforming rates we observed during this work were often controlled by diffusion, it was not possible to. determine individual reaction rates and rate constants. However, from the TPSR measurements we were able to estimate rate constants for the gasification of catalyst and noncatalyst carbons. These rates are listed in Table VII along with selected results taken from the literature (29, 30, 31). We found that the catalyst carbon gasification rates were first order in carbon amounts up to equivalent (CO adsorption) monolayer... 

Carbon gasification rates were directly measured in the fluid-bed tests conducted with Disco char (1,2,3) and values of /l t and a can be... 

The experimental results of the screening tests conducted in unit A are presented in Tables Ha and lib, and for unit B they are given in Tables Ilia and Illb. The specific gas production rates and gasification rates are based on the approximate 4-hr reaction time at 850°C. The results indicate that methane production rates as well as carbon gasification rates can be increased significantly if certain compounds are admixed with the coal feed. 

Figure 5. Effect of temperature on carbon gasification rate using sprayed Raney nickel. Test conditions pressure, 300 psig flow, 2000 std cc/hr N2 water, 5.8 and 1.16 grams/hr.

Thermodynamically, the formation of methane is favored at low temperatures. The equilibrium constant is 10 at 300 K and is 10 ° at 1000 K (113). High temperatures and catalysts ate needed to achieve appreciable rates of carbon gasification, however. This reaction was studied in the range 820—1020 K, and it was found that nickel catalysts speed the reaction by three to four orders of magnitude (114). The Hterature for the carbon-hydrogen reaction has been surveyed (115). 

Even at 1,500 F, equilibrium eonstants for the first two reactions are high enough (about 10) to expect reaction to go essentially to completion except for kinetic-rate limitations. The reaction zone might be expected to be sized by volume of rabbled carbon bed, considering that the carbon gasification reactions that occur in it are governed by kinetics and are reaction-rate limited. Actually, it is sized by hearth area. The area exposed to the gases controls mass transfer of reactants from the gas phase to the carbon and heat transfer to support the endothermic reactions. 

This paper describes a simulator which has been developed at the Alberta Research Council and permits gasification in a two-ton coal block. Initial gasification experiments with air, steam and carbon dioxide are summarized, and data for product gas composition, heat propagation through the coal block, and gasification rates as functions of the geometry of the reaction channel are presented. 

Unfortunately, insofar as a clear understanding of the true orders of gas-carbon reactions is concerned, the problem is made more difficult when the gasification rate is affected by product retardation and by the rate of mass transport of reactants to the surface of the solid. Product retardation can result in the obtaining of orders of reaction which are too low, while mass transport retardation can either raise or lower the apparent order depending upon the true order of reaction and the nature of the mass transport control. In Secs. V and VI, these complicating factors will be discussed in more detail. In the remainder of this Section, pertinent references on the orders of gas-carbon reactions will be given. 

Predicted Relative Rates of Carbon Gasification in Reaction Zone III for Similar Shapes of Carbon Specimens and Constant Linear Gas Flow Rate... 

The reaction is described by two basic routes in (391) they are chosen so that carbon enters the reaction only in route A 1, and route N<2) results in the conversion of a part of CO formed in route N(1) into C02. With such a choice of the basis of routes, r u, the rate of the reaction along route Nil) is the rate of carbon gasification i.e., corresponds to symbol r(1) in (390). 

In order to establish the optimum chemisorption temperature, a series of isothermal chemisorption experiments were performed at different temperatures between 200 and 300 °C. The sample was first outgassed in Ar (15 °C min, 1000 C, 5 hours). The temperature was then lowered to the chemisorption temperature, and a flow rate of 75 mL min of oxygen was introduced to the TGA. In this way, an optimised chemisorption temperature of 250 °C was found, so that equilibrium could be achieved in a reasonable period of time, and simultaneous carbon gasification could be avoided. 

Kannan, M.P. Richards, G,N.(1990). Gasification of biomass chars in carbon dioxide dependence of gasification rate on the indigenous metal content. Fuel,... 

Cerfontain et ai. also show that the gasification rate for activated carbon only depends on the ratio Pco/Pco2 and not on Pco2 or Pco. 

Many researchers have tried to correlate the kinetic rates of carbon gasification to the many physical and chemical properties of the different materials used. Despite of this a universal rate expression does not exist. Furthermore it is difficult to find data operational for reactor simulations in the relevant temperature and partial pressure ranges. In particular the variation of the reactivity with conversion due to structural variations is not dealt with, i.e. the structural profile is seldom explicitly given. 

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