Pulverized coal char oxidation

Because of the variability in deduced reaction orders for different experiments and carbon types, a general expression for the kinetic rate that includes the oxygen dependence could not be determined. 

In reality, it is believed that the oxidation of carbonaceous surfaces occurs through adsorption of oxygen, either immediately releasing a carbon monoxide or carbon dioxide molecule or forming a stable surface oxygen complex that may later desorb as CO or C02. Various multi-step reaction schemes have been formulated to describe this process, but the experimental and theoretical information available to-date has been insufficient to specify any surface oxidation mechanism and associated set of rate parameters with any degree of confidence. As an example, Mitchell [50] has proposed the following surface reaction mechanism  

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Soot can be oxidized by molecular oxygen, through the process described previously for pulverized coal char oxidation, but in combustion systems the dominant oxidation of soot is performed by radical species, especially the hydroxyl radical, OH. This discovery was made by Fenimore and Jones... 

A limited amount of oxidant (oxygen or air) is introduced into the reactor and is mixed with crushed/pulverized coal feed (either dry or as slurry) to allow volatile products and some of the char reacts with oxygen to form carbon dioxide and carbon monoxide. The basic reactions for the CO and CO2 formation are the partial and total combustion of C, respectively... 

The combustion gases, residual char, and fly ash pass into a boiler chamber where burnout is completed and there are various stages of heat exchange and heat recovery, producing steam to drive the turbine and generator. As a result of the intense combustion conditions, nitrogen oxide (NOx) formation tends to be considerably higher than in a pulverized coal combustor. 

WEW Process (24,25). Vereingte Elektrizitatswerke Westfalen is a German electric utility that has developed a unique dry feed, entrained-flow gasification process. The VEW concept relies on partial gasification with preheated air to produce low-Btu-coal-derived gas and char. After waste heat recovery, steam generation, and sulfur removal, the coal gas is fired in a combustion turbine. The char is fired in a pulverized coal boiler with the hot flue gas from the combustion turbine serving as the preheated combustion oxidant. This design requires additional cleanup of the pulverized boiler flue gas to meet emission requirements. 

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