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Show WASTE FUEL AIR --- Figure 1 - Process STAGE 1 REDUCTION 2200°F QUENCH MEDIUM STAGE 2 COOUNG 1400°F AIR SlAGE3 RE-OXIDA TlON 18000F CLEAN FLUE GAS ~ Because waste compositions are not known accurately and vary often, it is not always possible to maintain a certain stoichiometric ratio by feeding the necessary quantity of combustion air into the system. It is, however, possible to measure the combustibles by using analytical instruments. The concentration of combustibles is then used as a measure of the reducing atmosphere in the furnace and is controlled by modulating combustion air to the burner. High temperature and the substoichiometric conditions cause the organic compounds to dissociate, producing free nitrogen. The free nitrogen and the combustibles, competing for the limited quantity of oxygen available, keep the nitrogen from being oxidized to NOx. In the case of waste streams containing NOx, one can assume, perhaps as an oversimplification, that the NOx reacts with the hydrocarbons in the burner chamber and with CO and H2 in the furnace to produce elemental nitrogen. The products of combustion from the first stage, which contain quantities of CO, H2, an, N2, H20 and residual hydrocarbons, are cooled in the second stage to a temperature at which autoignition in the third stage would be viable with the addition of oxygen. It is necessary to cool the gases so that the re-oxidation of combustibles in the final stage occurs at a moderate temperature. A higher temperature could result in the formation of thermal NOx. The cooling is generally accomplished by injecting a medium substantially free of oxygen into the first stage flue gas. Examples of the cooling medium are water, steam or recycled flue gas. Water is relatively inexpensive and produces a lesser bulk: of gases downstream. Steam is generally too expensive to use but may be preferred in a unit where chlorinated hydrocarbons are being burned to shift the reaction towards Hel formation. Recycled flue gas is used in applications where there is heat recovery equipment downstream. The major advantage with flue gas recycling is that the amount of heat recovered in a boiler downstream is the highest of any medium used. One minor disadvantage is the potential for corrosion in the recycle flue gas ducting in systems burning chlorinated hydrocarbons. The third stage, takes place in what is called a re-oxidation funace, and involves the oxidation of carbon monoxide and hydrogen produced in the fll'St stage and any residual hydrocarbons. Enough 2 |