Gasification reactions rates

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

However, actual high-temperature gasification reaction rates are not known, and the residence time at a given reactor temperature is usually determined empirically based on small-scale gasifier experiments. It is also generally accepted that reaction rates vary with coal type. 

There is another possible explanation for the apparent lack of effect of metals on the steam gasification reaction rate. 

Effect of Substrate on Coke Gasification Rate. The effect of substrate variations on the steam-carbon reaction rate at 1500°F is shown in Table II and in Figure 3. The substrates can be classified into two categories according to their effect on coke gasification reaction rates ... 

When calculating the primary pyrolysis and the char combustion and gasification reaction rates, the variation of the particle size (char) by the combustion and gasification reactions is to be accounted for. 

As it is nearly impossible to make any general kinetic description of the gasification processes as a practical design basis, constrained equilibrium calculations offer a useful tool for comparative studies [51]. If gasification reaction rates are considered as a rate of approach to chemical equilibrium, increasing residence times lead to gasification products near equilibrium [52]. This is especially true for fluidized-bed and entrained-flow processes. Because the equilibrium state does not depend on the path used to achieve it, a process simulator such as Aspen Plus [49] can use a hypothetical reactor to decompose coal into its elements. The subsequent equilibrium calculation can be carried out, including other feed streams. 

The thermal degradation of mixtures of the common automotive plastics polypropylene, ABS, PVC, and polyurethane can produce low molecular weight chemicals (57). Composition of the blend affected reaction rates. Sequential thermolysis and gasification of commingled plastics found in other waste streams to produce a syngas containing primarily carbon monoxide and hydrogen has been reported (58). 

The gasification is performed usiag oxygen and steam (qv), usually at elevated pressures. The steam—oxygen ratio along with reaction temperature and pressure determine the equiUbrium gas composition. The reaction rates for these reactions are relatively slow and heats of formation are negative. Catalysts maybe necessary for complete reaction (2,3,24,42,43). 

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. 

Pyrolysis processes were discussed in previous sections. Suffice it to say that if the heating rate is fast, a solid particle is heated to high temperatures in a short period of time, and then the gas-gas phase and solid-gas phase gasification reactions take place simultaneously. 

Equation (2) can now be shown to be consistent with at least two mechanisms where carbon monoxide retards the gasification reaction. Mechanism A applies where the rates of the back reactions of reaction (1) and (2) are negligible. 

If the pore diameter and process conditions are well defined, the rates of internal and film diffusion can be calculated. The temperature dependency of the rate can be presented in the form of an Arrhenius plot, that is, log rate vs. reciprocal absolute temperature. Gasification rates can be divided into three zones, depending on whether reaction rate is controlling,... 

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