Reaction char-oxygen

Figure 1 has shown that the maximum chemisorption of oxygen on chars from untreated wood occurs at HTT 450°-500°C. However, in order to understand better the effect of metal ions on the total process consisting of pyrolysis and subsequent chemisorption and oxidation of wood char, it was necessary to carry out pyrolysis, isothermal chemisorption and oxidation reactions in a single experiment. A typical overall pyrolysis, isothermal chemisorption (140°C) and oxidation curve is shown in Figure 2. The temperature program is (1) heat from 25° to 500°C at 5°C/min, (2) cool at...

The noncatalytic reduction of nitric oxide by insitu formed char is considered one of the significant reactions which control nitric oxide emission and a detailed kinetic study was carried out. (2, 3, 4) The present authors demonstrated that this reaction proceeded even under an excess air condition and that the rate is enhanced by the coexisting oxygen up to 750°C. (.5,6) Besides the noncatalytic reaction, carbon monoxide may have a significant effect on nitric oxide reduction by char. (2.) Roberts et al.(8) reported that the gas phase reactions in the nitric oxide reduction play a minor role and that the absence of a major gas phase reaction of NO and coal nitrogen into N2 requires the participation of a surface which catalyzes reactions. Char is considered to... 

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. 

Gasification of a char is mainly carried out at 800-1000 °C with carbon dioxide, steam, or a mixture of both. As stated above, oxygen is not normally used as an activating agent because the carbon-oxygen reaction is highly exothermic and this makes the reaction impossible to control unless extremely low partial pressures of molecular oxygen are used. The reactions of carbon with carbon dioxide and steam are endothermic and easy to control ... 

The mechanism of carbon combustion in FBC is assxjmed to be diffusion controlled in most of the modelling efforts as discussed in the section on Char-Oxygen Reaction, This is true only for large particles (> 300 microns) at high temperatures (> 1200 K). Feed coal contains a wide range of sizes, and assimiing a diffusion controlled kinetics for all particle sizes wo iLd lead to overestimation of the combustion rate. 

Other techniques include oxidative, steam atmosphere (33), and molten salt (34) pyrolyses. In a partial-air atmosphere, mbber pyrolysis is an exothermic reaction. The reaction rate and ratio of pyrolytic filler to ok products are controlled by the oxygen flow rate. Pyrolysis in a steam atmosphere gives a cleaner char with a greater surface area than char pyroly2ed in an inert atmosphere however, the physical properties of the cured compounded mbber are inferior. Because of the greater surface area, this pyrolytic filler could be used as activated carbon, but production costs are prohibitive. Molten salt baths produce pyroly2ed char and ok products from tine chips. The product characteristics and quantities depend on the salt used. Recovery of char from the molten salt is difficult. 

One very efficient system which may work this way was reported by Chien and Kiang (32) who found that 1.5% chromium, introduced by the tard reaction, raised the oxygen index of polypropylene to 27 (Fig. 4) and char formation was promoted. The hypotheses as to mode of action included the idea that dehydrogenation catalysis might be involved. 

The initial step is an oxidative addition of RhCI(PPh3)3 to a C-0 bond of the ester moiety and produces rhodium-carbon and rhodium-oxygen bonds. Adjacent rhodium species can undergo further reaction with the formation of anhydride linkages. This anhydride formation may occur between adjacent pairs of reactants, between pairs in the same chain, or between pairs that are present in different chains. All of these reactions are observed, and in however the last reaction is the one of interest here since this leads to cross-linking and char formation. Rhodium is present in both the chary material and in the soluble fractions. From the reaction pathway in order for rhodium elimination to occur, two rhodium-inserted... 

Values of yields for various fuels are listed in Table 2.3. We see that even burning a pure gaseous fuel as butane in air, the combustion is not complete with some carbon monoxide, soot and other hydrocarbons found in the products of combustion. Due to the incompleteness of combustion the actual heat of combustion (42.6 kJ/g) is less than the ideal value (45.4 kJ/g) for complete combustion to carbon dioxide and water. Note that although the heats of combustion can range from about 10 to 50 kJ/g, the values expressed in terms of oxygen consumed in the reaction (Aho2) are fairly constant at 13.0 0.3 kJ/g O2. For charring materials such as wood, the difference between the actual and ideal heats of combustion are due to distinctions in the combustion of the volatiles and subsequent oxidation of the char, as well as due to incomplete combustion. For example,... 

Gas phase thermal cracking of the volatiles occurs, reducing the levels of tar. Char (fixed carbon) and ash are the pyrolysis byproducts that are not vaporized. In the second step, the char is gasified through reactions with oxygen, steam, and hydrogen. Some of the unbumed char may be combusted to release the heat needed for the endothermic pyrolysis reactions. 

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