Char particles

The gas, along with entrained ash and char particles, which are subjected to further gasification in the large space above the fluid bed, exit the gasifier at 954—1010°C. The hot gas is passed through a waste-heat boiler to recover the sensible heat, and then through a dry cyclone. SoHd particles are removed in both units. The gas is further cooled and cleaned by wet scmbbing, and if required, an electrostatic precipitator is included in the gas-treatment stream. 

The stmcture of residual char particles after devolatilization depends on the nature of the coal and the pyrolysis conditions such as heating rate, peak temperature, soak time at the peak temperature, gaseous environment, and the pressure of the system (72). The oxidation rate of the chat is primarily influenced by the physical and chemical nature of the chat, the rate of diffusion and the nature of the reactant and product gases, and the temperature and pressure of the operating system. The physical and chemical characteristics that influence the rate of oxidation ate chemical stmctural variations, such as the... 

Porous Media Packed beds of granular solids are one type of the general class referred to as porous media, which include geological formations such as petroleum reservoirs and aquifers, manufactured materials such as sintered metals and porous catalysts, burning coal or char particles, and textile fabrics, to name a few. Pressure drop for incompressible flow across a porous medium has the same quahtative behavior as that given by Leva s correlation in the preceding. At low Reynolds numbers, viscous forces dominate and pressure drop is proportional to fluid viscosity and superficial velocity, and at high Reynolds numbers, pressure drop is proportional to fluid density and to the square of superficial velocity. 

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. 

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. 

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. 

Groeneveld M.J. and Swaaij W.P.M., Gasification of Char Particles with CO2... 

Xia and Ball (1999) measured sorption isotherms for a series of chlorinated benzenes and PAHs for an aquitard material (foc = 0.015 kg oc kg-1 solid) from a formation believed to date to the middle to late Miocene. Hence, compared to soils or recent sediment POM, the organic matter present in this aquitard material can be assumed to be fairly mature and/or contain char particles from prehistoric fires. A nonlinear isotherm was found for TeCB (fitting Eq. 9-2) and the following Freundlich parameters were reported XreCBF = 128(mg g 1)(mg mL 1) "T CBand "TeCB = 0.80. For partitioning of TeCB to this material (linear part of the isotherm at higher concentrations), the authors found a Kioc value of 4.2 x 104 L kg oc. 

De Soete, G. G. "Mechanisms of Nitric Oxide Reduction on Coal and Char Particles," Report on EERC Subcontract No. 8318-6, (Internal Report Institut Francais du Petrole, Ref. No. 28136), 1980. 

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... 

Particle Temperature Overshoot. The temperature of the burning char particles will run hotter than that of the bed by amounts that depend upon particle size, reactivity, bed temperature. It is determined in part by the heat released at the particle surface due to reaction and in part to the additional heat released by carbon monoxide oxidation near the particle surface (54-58). Measurements for 1.8 to 3.2 millimeter size coke particles burning in a fluidized band of sand at 1173 K increased from the bed temperature at low oxygen concentrations to values 150 to 200 K above the bed temperature for oxygen concentrations approaching that of air (72). Estimation of this temperature rise is important for purposes of evaluating the NO/C reaction and also for prediction of the burnout times of fines. 

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