OCR Text |
Show 5.9.14 Energy is imparted to the combustion air of the combustor. This permits more rapid mixing of the air (oxygen) with the fuel droplet. In many burners this turbulence creates an internal recycling of the hot products of combustion which transfers heat to the atomized droplets and raises them to the ignition point. (7) Important in these designs where the superheated liquid reaches ignition temperature is that air (20-23% oxygen) is available to complete the oxidation reaction. Problems occur if the vaporized hydrocarbon contacts a low oxygen level stream (see Fig. 7). Pyrolysis results, cracking the hydrocarbon into carbon, carbon monoxide and hydrogen, and time for completion of combustion is increased. A secondary reaction will occur when these combustion gases eventually reach the proper oxygen supply. In a true thermal oxidizer, the most rapid oxidation occurs when single stage combustion is designed into the system. If the vaporized liquid contains solids, the design must allow that the solids be carried into the gas stream without agglomeration. A high swirl or a cyclonic design may force reagglameration of these solids into larger particles which become more difficult to burn. Proper design of air mixing and nozzle location are important to permit the solids to be carried into the gas stream and oxidized. Again proper air (oxygen) at the surface of these solids is needed to insure a gradual oxidation to occur. Since they are solid, they will burn at the surface and heat will be transferred inwardly to the core of the droplet. Sufficient time should be provided to permit complete burnout of the solid in suspension. If inert materials are mixed with the liquid as it is vaporized, these become inert particulates in the gas stream. Depending on the type of atomizer, the composition of the solid, and the temperature of the oxidizer, a percentage of the solids will become sub-micron in size and be carried with the gas stream. The heavier particulates will became molten and agglomerate. The combustor design should provide collection of these particulates without plugging the flow passages of the system. |