| OCR Text |
Show The FG-DVC model is also capable of predicting fluidity. In principle such information could be used to predict cenosphere structures; high fluidity gives thin-walled cenospheres that tend to explode, and low fluidity gives thicker-walled structures. The main heterogeneous chemical reactions for char burnout are the reaction of carbon oxygen and carbon dioxide. Also occurring is the homogenous chemical reaction of C O with 02. Some of these reactions may occur in the kinetic region inside porous particles but at high temperatures (>1800 K) most of reactions will be in the diffusion regime. Therefore the effect of particle boundary conditions on char oxidation in the surrounding reactive gas flow become an important parameter to be considered. Experimental evidence indicates that the late stages of char combustion have a significant on the degree of carbon burn-out. Chars are progressively deactivated during high temperature combustion resulting in reduction in carbon conversion at a late stage of char combustion. The deactivation process at high carbon conversion can be related to a number of factors such as interaction between carbon and inorganic compounds (ash), particle macroporosity and morphology and more importantly to the mesoscopic nano-scale rearrangements in the crystal lattice and ultra-fine structure of the organic matter. The simple global char oxidation models discussed previously can not predict the low observed at high carbon conversion, and therefore the carbon burnout can not be predicted accurately. In order to overcome these uncertainties in char combustion modelling, the advanced coal combustion model developed by the authors includes statistical kinetics for carbon burnout analysis developed recently. This model describes the variations in char reactivity as variations in single particle global pre-exponential factor with apparent activation energy and reaction order held constant across the particle population. Gamma distribution is used for analytical approximation of the reactivity distribution function: -BA FAAr) = W)Arlf*" (U) 2.4. Char NOx The conversion of char nitrogen to N O is assumed to occur by -CN +V202 >NO + -Cf A conversion factor for char-bound nitrogen may be calculated as [NO]_ ^ [[CO] + '[cCha0r2 ]}(N/Qt This conversion factor is very sensitive to the reactivity of the char and decreases with increasing combustion reaction rates. The conversion efficiency (r|) is directly related to the balance 9 |
