Carbon-oxygen reaction

Recently, Ahmed and Back (14) have proposed a mechanism for carbon-oxygen reaction. The simple mechanism was described as follows... 

Equation (39) represents the reaction rate in Zone III. The reaction is clearly first order with respect to the reactant concentration in the main gas stream. This is clearly shown by Day (24) for the carbon-oxygen reaction, as shown in Fig. 7. 

Fio. 7. Illustration of first-order kinetics for the carbon-oxygen reaction in Zone III. [After R. J. Day, Ph.D. Thesis, The Pennsylvania State University, 1949.1... 

Day 2 ), who apparently is completely in Zone III for his studies on the carbon-oxygen reaction, confirms the small dependence of reaction rate on temperature, as shown in Fig. 8. Between 1227 and 2027°, the activation energy is less than 8 kcal./mole at all flow velocities used. 

For a particular gas-carbon reaction. Equation (39), with one reservation, leads to the conclusion that under identical reaction conditions (i.e., Cg, Dfree, and S are constant), the rate of reaction in Zone III is independent of the type of carbon reacted. The reservation is that in the carbon-oxygen reaction, the nature of the carbon may affect the CO-CO2 ratio leaving the surface and hence the reaction rate per unit of oxygen diffusing to the surface. Unfortunately, little data are available on reactivities of different carbons where the reaction has been conducted completely in Zorn III. Day (2Ii) reports that the reaction rates of petroleum coke, graphitized lampblack, and graphitized anthracite rods agree within 12 % at a temperature of 1827° and at a constant gas velocity. 

Thermodynamic data have also been calculated for carbon—oxygen reactions in fused salts [7, 8], The oxidation of solid carbon principally yields carbon dioxide at low temperature and carbon monoxide at high temperature. In this case, at constant temperature, the CO/CO2 concentration ratio at solid carbon depends on pressure. The carbon—oxygen electrode is used as reference to investigate cryolite—alumina melts at c. 1000°C [9] and molten slags at higher temperatures. 

Laine, N.R., Vastola, F.J., and Walker, P., Jr (1963). The importance of active surface-area in the carbon-oxygen reaction./ Phys. Chem., 67, 2030-4. 

As we will see in Chapter 13, to describe a chemically reacting system it is not necessary to consider all the chemical reactions that can occur between the reactant species, only the independent reactions. Furthermore, the molar extent of reaction for any chemical reaction among the species can be computed from an appropriate linear combination of the known extents of reaction for the set of independent chemical reactions (see Sec. 13.3). Thus, in the carbon-oxygen reaction system just considered, only two reaction variables (and the initial mole numbers) need be specified to completely fix the mole numbers of each species the specification of a third reaction variable would be redundant and may result in confusion. 

Figure 1.12 Proposed mechanisms for CO2 formation in the carbon-oxygen reaction la Ib Ic, oxygen insertion (dissociative chemisorption) route 2a 2b 2c, nondissociative chemisorption route on the carbene edge sites.

Each of these reactions is strongly exothermic and some of the heat released promotes the initial carbon-oxygen reaction at what is usnally the hottest part of the fuel bed. At this stage, the oxygen concentration is very much depleted and the carbon dioxide is reduced by the Boudouard reaction in the next layer of fuel ... 

Accurate measurement of reaction rate of char particles is necessary to predict the rate-controlling mechanism in fluidized bed. Most researchers assume that carbon-oxygen reaction is first order with respect to oxygen, i.e., n = 1 in Equation 33. This leads to the simple mathematical expression of burning rate in fluidized beds. Recent studies indicate fractional order of reaction. This will lead to a more complicated equation of burning rate and may require numerical solution. Table 2 depicts reactivities of various fuels. 

Essenhigh, R.H., Rate equations for the carbon-oxygen reaction An evaluation of the Langmuir adsorption isotherm at atmospheric pressure. Energy Fuels. 5( I). 41 -46 (1991). 

Although oxidation has been used widely to purify carbon materials, carbon-oxygen reactions have also been shown to drastically alter the physiochemical properties of nanostructures, particularly their wettability and adsorption/desorption characteristics. Moreover, oxidation potentially induces damage to carbon nanomaterials or even destroys the structures under improper conditions. To fully utilize the selectivity of the oxidation process at the nanoscale, a comprehensive understanding of the chemical and physical nature of a material and the structure dependence of the oxidation kinetics is required. For the latter, one must systematically study the interactions of the different carbon nanostructures with gaseous and liquid oxidizer, monitor changes in structure and composition, and analyze the reaction kinetics in greater detail. 

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