Particle combustion, char

Adopting the approach developed above for the char particles combustion, the size distribution function of limestone particles as a result of sulfation reaction in the overflow stream which is the same as in the bed is given by. 

Test calculations have been performed for the single char particle combustion and compared with the experimental data of Van Der Honing [9], The rate constant derived by Winter et al. [7] for the char formed from sewage sludge is currently adopted in the mechanism. 

A simplified model of PC combustion includes the following sequence of events (I) on entering the furnace, a PC particle is heated rapidly, driving off the volatile components and leaving a char particle (2) the volatile components burn independently of the coal particle and (3) on completion of volatiles combustion, the remaining char particle burns. Whue this simple sequence may be generally correct, PC combustion is an extremely complex process involving many interrelated physical and chemical processes. 

With the importance of the devolatilization process to solid particle combustion and the complexity of the chemical and physical processes involved in devolatilization, a wide variety of models have been developed to describe this process. The simplest models use a single or multiple Arrhenius rates to describe the rate of evolution of volatiles from coal. The single Arrhenius rate model assumes that the devolatilization rate is first-order with respect to the volatile matter remaining in the char [40] ... 

FIGURE 9.21 Sample oxygen profiles through the char particle boundary layer and the char particle itself during combustion proceeding according to the characteristic burning zones. 

With appropriate choices of kinetic constants, this approach can reproduce the NSC experimental data quite well. Park and Appleton [63] oxidized carbon black particles in a series of shock tube experiments and found a similar dependence of oxidation rate on oxygen concentration and temperature as NSC. Of course, the proper kinetic approach for soot oxidation by 02 undoubtedly should involve a complex surface reaction mechanism with distinct adsorption and desorption steps, in addition to site rearrangements, as suggested previously for char surface combustion. 

Char oxidation dominates the time required for complete burnout of a coal particle. The heterogeneous reactions responsible for char oxidation are much slower than the devolatilization process and gas-phase reaction of the volatiles. Char burnout may require from 30 ms to over 1 s, depending on combustion conditions (oxygen level, temperature), and char particle size and reactivity. Char reactivity depends on parent coal type. The rate-limiting step in char burnout can be chemical reaction or gaseous diffusion. At low temperatures or for very large particles, chemical reaction is the rate-limiting step. At... 

The char combustion phenomenology has been reviewed by many researchers [11,26,73]. It is a very complex process and is usually divided into three char combustion regimes, namely (I), (II) and (III) [23,54,74,75]. The combustion regimes are consequences of the initial size and temperature of the char particle, see Figure 55. 

Although some controversy exists on the correlation of Sherwood Number applicable to fluidized beds, well-defined combustion experiments support the use of the Ranz and Marshall (35) or Frossling (36) correlation with an approximate correction of mf to allow for the obstruction to diffusion by the inert particles surrounding the burning char particles (37). Thus... 

The above simplified analysis was intended to provide a feel for the relative importance of the processes that govern carbon loading, and therefore carbon combustion efficiency. More complete treatments of AFBC s are available which consider the detailed population balance equations for the char particles coupled with an oxygen balance (41-50). These treatments have given results which parallel observations on operating AFBC s but... 

Carbon Monoxide Oxidation. Analysis of the carbon monoxide oxidation in the boundary layer of a char particle shows the possibility for the existence of multiple steady states (54-58). The importance of these at AFBC conditions is uncertain. From the theory one can also calculate that CO will bum near the surface of a particle for large particles but will react outside the boundary layer for small particles, in qualitative agreement with experimental observations. Quantitative agreement with theory would not be expected, since the theoretical calculations, are based on the use of global kinetics for CO oxidation. Hydroxyl radicals are the principal oxidant for carbon monoxide and it can be shown (73) that their concentration is lowered by radical recombination on surfaces within a fluidized bed. It is therefore expected that the CO oxidation rates in the dense phase of fluidized beds will be suppressed to levels considerably below those in the bubble phase. This expectation is supported by studies of combustion of propane in fluidized beds, where it was observed that ignition and combustion took place primarily in the bubble phase (74). More attention needs to be given to the effect of bed solids on gas phase reactions occuring in fluidized reactors. 

Combined Gas, Soot, and Particulate Emission In a mixture of emitting species, the emission of each constituent is attenuated on its way to the system boundary by absorption by all other constituents. The transmissivity of a mixture is the product of the transmissivities of its component parts. This statement is a corollary of Beer s law. For present purposes, the transmissivity of species k is defined as xk = 1 — Et. For a mixture of combustion products consisting of carbon dioxide, water vapor, soot, and oil coke or char particles, the total emissivity eT at any wavelength can therefore be obtained from... 

In order to evaluate the right side of Eq. 37, we will calculate the mass loss for a single pellet due to reaction and sum up such losses for all particles present in the fluidized bed. Upon combustion, char leaves behind a layer of ash having a different density than that of coke. Thus, the mass of a single char particle, w, of size in the bed is given by ... 

The first term of Eq. 80 represents the mass loss of char particles due to carbon combustion and the second term represents the mass loss of char particles due to sulfur reaction with oxygen. Using the stoichiometry of reactions 1 and 2, we can obtain the moles of oxygen used up in these respective reactions for any arbitrary height n as. 

Fig 10. Arrhenius diagram for combustion and C02-gasification of straw char in comparison to other chars (thermobalance data for a char particle size of 50 ... 

Kulasekaran S. et al, (1998) Combustion of porous char particle in an incipiemly fluidized bed. Fuel, 77,1549-1560. 

The required time for devolatilisation and the burnout time of the char particles can be calculate by eqn. (7), respectively eqn. (8), The concentratirai of Oj varies through the riser from 1.03 mole/m after combustion of the volatiles to 0.296 mole/m in the flue gases. The concentration of O2 is assumed to vary linearly during the char combustion. Results are given in Table 2. 

The recycle of the char is thus evidently needed. Since the suspension leaving the riser is qu idied in the external steam generating bed u4iere 02-levels are very low, it is reasonable to accept that no combustion occurs in this recycle loop. In the CFB, char particles of all sizes are found. At equilibrium, the mass flow of char in the recycle loop varies from 375 kg/hr to 29,500 kg/hr when feeding at a constant rate of 2500 kg/hr of the specified particle size. Results are detailed in the Table 7. 

The combustion process of wet wood chips and formation of pollutants in a biomass furnace have been investigated. Distributions of species CO, UHC, O2 where calculated numerically and compared to experimental data. It is shown that char, as flying particle, though in small amount has a significant influence on the CO emissions at the outlet. Numerical simulation indicates that half of the CO emission at the outlet is due to the combustion of flying char particles at the upper part of the furnace. Over-fire air staging has a significant influence on the residence time of panicles and gas species in the furnace, and thus the conversion of fuel and intermediate species to final products. 

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