Carbon char combustion porous chars

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... 

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... 

There is a very spectacular reaction with the charring of lead tartrate. When this residue is freshly prepared, it combusts spontaneously in air by throwing up showers of sparks. When it is rubbed on a sheet of paper, or put in the hand and thrown, the reactions of this residue are more violent. It is thought that this is due to the catalysis by lead oxide of the combustion of carbon formed, which is very porous. 

The interest in the properties of the chars derived from cellulosic or biomass solid.s extends beyond those associated with thermal transport in the char. Insofar as the char residue from a pyrolysis process must typically be burned, gasified, or put to use as an activated carbon product, there is also a need to examine the porous nature of the char, bi acbvated carbons, the pore structure is key to adsorption performance. In combustion or gasification, the porosity can play a role in determining conversion kinetics in the intrinsic rate controlled or pore diffusion controlled regimes. 

The porous carbon, or char, is left after the volatile matter is released from the coal. The combustion of carbon in the gaseous oxygen phase is a complex set of heterogeneous reactions for both the gas and the solid. It may occur on the surface or inside the pores of the char and is controlled by gas diffusion and kinetics, reaction temperature, char structure. Generally, the reactions are expressed by three heterogeneous reactions and one gas-phase reaction. 

---