Porous char particle

Figure 1. Gas-solid reactions in a porous char particle containing CaO/CaS...

By making use of the physical structure and chemical rate expressions described above it is possible to formulate mass and energy balances for the gasification of a single porous char particle. The mass balances for the diffusing species take the form ... 

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

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. 

Because coal chars are highly microporous, most of the gasification reactions take place inside the char particles. Therefore, diffusion of gas into, and products out of, porous particles is required. The overall diffusion process can be described by the following steps (1) diffusion of the reactant from the bulk gas to the solid surface (film diffusion) (2) diffusion of the reactant from the particle s surface to its interior (internal diffusion) (3) diffusion of the product from the interior to the particle s surface (internal diffusion) and (4) diffusion of the product from the surface to the bulk gas (film diffusion). 

In contrast to the slag, char particles collected in cyclones working with moving grate installations and the cyclone material from fluid bed combustors, although free form volatiles, appear to burn well even at 800 C, which can probably be attributed to their highly porous character and consequently a large effective surface area available for reaction [13 ]. Data for fluidized bed volatile char are included in Table 2. 

Since the carbon particle is porous and also char particle in a stagnant environment will leave a residual ash uninterrupted, the diffusion coefficient involves the resistance to flow of oxygen and combustion products through the ash layer and through the porous carbon itself. The rate of reaction of the carbon with oxygen at the reaction front can be expressed in terms of the oxygen partial pressure at the surface and the surface reaction rate constant as... 

From the above it appears that in PS-and PP-clay nanocomposites, both the homogeneity of the clay residue and the presence of additional carbonaceous char are important mechanistically in providing a continuous coverage of effective heat-transfer material and thereby reducing the flammability. It should be kept in mind that since the carbonaceous char yield is low, the primary modes in which char affects the flammability are by prevention of reagglomeration of the clay particles and to provide additional insulating properties to the porous char. 

The pore structure can markedly affect char reactivity. Most coals in general, and coal chars in particular, are highly porous and contain a polymodal pore size distribution. Pores normally are classified into macropores (>500 A in diameter), transitional pores (20-500 A in diameter), and micropores (< 20 A in diameter). Upon pyrolysis, pores in coals open up but still contain microporosity. Coal chars, in general, and lignitic chars, in particular, retain the polymodal pore distribution. The surface areas of coal chars can range between 100 and 800 m /g. Most of this surface area and therefore the active surface area resides inside the char particles so the accessibility of the reactive gases to the active sites is very important. 

Step one is, oxygen diffusion in the porous system of the particle inwards to the char combustion front and the reaction site, (2) adsorption of oxygen to the active sites on the intraparticle char phase, (3) oxidation reaction with carbon, and (4) desorption of... 

The conversion of char can be described using two different physical particle models, the shrinking-pa Hide model and the porous-particle model, where p is the density of the particle ... 

The sensible heat of the inlet flows, char and gases, is absorbed in endothermic reactions of C + CO2 and C + H2O producing H2 and CO while CH4 and N2 remain unchanged. The rate of these processes can be limited by 1) mass and heat transfer from the gas phase to the particles 2) diffusion of the gaseous reactants into the porous particle 3) chemical reaction at the active surface of the particle 4) diffusion and convection of the gaseous products out of the particle to the gasphase. 

Uses Film-former for applies, such as coating or surf, treatment for porous and nonporous substrates, adhesives for difficult substrates, frothable interlayers or coatings, nonwoven saturants and binders Features Solventless produces light-stable films low VOC Properties Milky wh. liq. char, odor 2 p particle size sp.gr. 1.07 dens. 8.9 Ib/gal vise. 400 cps pH 7-9 surf. tens. 42 dynes/cm VOC 10.8 g/l Film props. tens. str. 6000 psi tens, elong. 600% (ultimate) anionic 62% solids... 

---