Char, reaction rate

Figure 3 Literature discrepancies in drag coefficient, in mass transfer for CFB due to meso-scale structures and in char reaction rate coefficient. For the drag coefficients, curves are adapted from Wang et al. (2010) for the mass transfer, curves are adapted from Dong et al. (2008a) for the coal reaction, different symbols refer to different coal data.

Gasification kinetics during entrained flow gasification Part II - intrinsic char reaction rate and surface area development. Fuel, 107, 653-661. 

Thus, for a successful fluorination process involving elemental fluorine, the number of coUisions must be drasticaUy reduced in the initial stages the rate of fluorination must be slow enough to aUow relaxation processes to occur and a heat sink must be provided to remove the reaction heat. Most direct fluorination reactions with organic compounds are performed at or near room temperature unless reaction rates are so fast that excessive fragmentation, charring, or decomposition occurs and a much lower temperature is desirable. 

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. 

Burning times for coal particles are obtained from integrated reaction rates. For larger particles (>100 fim) and at practical combustion temperatures, there is a good correlation between theory and experiment for char burnout. Experimental data are found to obey the Nusselt "square law" which states that the burning time varies with the square of the initial particle diameter (t ). However, for particle sizes smaller than 100 p.m, the Nusselt... 

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

Additives, such as fire retardants, can have a major effect on pyrolysis, and even trace amounts of ash have been shown to influence pyrolysis (6 ). Generally, fire retardants work by increasing the dehydration reaction rate to form more char and as a direct result give fewer flammable volatile compounds (1,3,7). Several papers have noted that phosphoric acid and its salts decrease the Efl (13,18,22,29), aluminum chloride has little effect (22) on Efl and boric acid increases the Efl (12,18). The reaction order for treated samples has been generally reported as lst-order (12,13,18,29) which is also the most commonly used rate expression for analysis of TGA data of untreated cellulose. 

The violent nature ofreactions between fluorine and hydrocarbon compounds has already been noted here, and the direct fluorination of organic polymers is not a exception it is so exothermic that if the reaction is not controlled, it generally leads to fragmentation and charring of the substrate. Moderation of the reaction rate can be effected by ... 

Conversion of polymers and biomass to chemical intermediates and monomers by using subcritical and supercritical water as the reaction solvent is probable. Reactions of cellulose in supercritical water are rapid (< 50 ms) and proceed to 100% conversion with no char formation. This shows a remarkable increase in hydrolysis products and lower pyrolysis products when compared with reactions in subcritical water. There is a jump in the reaction rate of cellulose at the critical temperature of water. If the methods used for cellulose are applied to synthetic polymers, such as PET, nylon or others, high liquid yields can be achieved although the reactions require about 10 min for complete conversion. The reason is the heterogeneous nature of the reaction system (Arai, 1998). 

In order to extrapolate the laboratory results to the field and to make semiquantitative predictions, an in-house computer model was used. Chemical reaction rate constants were derived by matching the data from the Controlled Mixing History Furnace to the model predictions. The devolatilization phase was not modeled since volatile matter release and subsequent combustion occurs very rapidly and would not significantly impact the accuracy of the mathematical model predictions. The "overall" solid conversion efficiency at a given residence time was obtained by adding both the simulated char combustion efficiency and the average pyrolysis efficiency (found in the primary stage of the CMHF). 

In electrochemistry the -> reaction rate can be determined by the measurements of the -> current flowing in the electrical circuit. Because the current is proportional to the surface area of the electrode, in order to char-... 

The extent of the activation reaction is characterized by the burn-off as determined by the change in mass of the char, expressed as the percentage weight loss of the carbonized material as a result of HTT under controlled conditions. With some chars, the bum-off increases linearly with the time of HTT at a constant temperature. This form of linear dependence has been reported by Rodriguez-Reinoso (1986) for the activation of carbonized olive stones and almond shells in C02 at temperatures around 850°C. The extensive linear relationship was a clear indication that the reaction rate was almost constant over a very wide range of bum-off (i.e. 8-80%). 

Oguma, A., Yamada, W., Furusawa, T., and Kunii, D. Reduction reaction rate of char with NO, 11th Fall Meeting ofSoc. of Chem. Eng., Japan, p. 121 (1971). 

The above calculation is quite tedious and gets complicated by the fact that the properties which ultimately control the magnitude of these fourteen unknown quantities further depend on the physical and chemical parameters of the system such as reaction rate constants, initial size distribution of the feed, bed temperature, elutriation constants, heat and mass transfer coefficients, particle growth factors for char and limestone particles, flow rates of solid and gaseous reactants. In a complete analysis of a fluidized bed combustor with sulfur absorption by limestone, the influence of all the above parameters must be evaluated to enable us to optimize the system. In the present report we have limited the scope of our calculations by considering only the initial size of the limestone particles and the reaction rate constant for the sulfation reaction. 

Selection of the cations was based on several criteria. First, the raw coal contained alkaline-earth, mainly Ca and Mg, and some alkali metals on its carboxyl groups. Also, McKee (13) and Walker et al. (12) have shown that sodium, potassium and calcium are excellent catalysts for the C-O2 reaction. Thus, these cations may have a significant effect on the char burnout rate. In addition. Mg was back exchanged on the coal since it was contained on the raw coal and, as shown by Walker et al. (12), it is a poor catalyst for the C-O2 reaction. The purpose of using this alkaline-earth metal was to determine if catalysis of the heterogeneous C-O2 reaction affected the char burnout rate. This would help to elucidate whether the char burnout step was chemically or physically rate controlled. 

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