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Show Unmixed Combustion for Efficient Heat and Mass Transfer in Chemical Processing Systems American Flame Research Committee 1997 Fall International Symposium The question arises as to w h y there are any N O x emissions at all, since no true combustion takes place. The answer is not yet known, but there are two immediately plausible explanations. The first is that the catalyst may itself catalyze the formation of N O x from the nitrogen and oxygen in air. If this is the case it may be possible to design a catalyst with sufficient selectivity to avoid this result. The second is that there may in fact be an interface between the fuel and air during the switch over from one to the other, and that a limited amount of combustion takes place at this interface. The discussion so far has focussed on copper/copper oxide as the unmixed combustion catalyst. However, there are a large number of metals that can be practically used for this purpose, each with different operating characteristics and different operational temperature ranges. These include iron, nickel, molybdenum, manganese, silver and tin, among others. In most cases, the only limitation on the applicability of a metal as an unmixed combustion catalyst is its volatility at its operational temperature. For example, in recent tests using zinc the tests had to be stopped because the zinc evaporated during the reduction step and recondensed in the exhaust of the apparatus, shutting down the experiment. POTENTIAL APPLICATIONS FOR UNMIXED COMBUSTION It is possible to imaging two rather different development pathways for unmixed combustion. One path would involve trying to develop an unmixed combustor as a drop-in replacement for conventional combustion processes in existing applications: a residential water heater, for example. The other path would attempt to take advantage of the special properties of unmixed combustion such as the potentially high heat transfer rates, and low emissions characteristics. The latter path has proven the more fruitful in our development efforts. Unmixed Combustion for Incinerator Emissions Control Figure 6a illustrates the basic features of a two-stage incinerator with a starved-air pyrolysis chamber and a secondary oxidation chamber followed by an exhaust stack. This design is intended to ensure that emissions to the environment are essentially benign. In normal operation, the starved-air pyrolysis unit operates in a quiescent mode, with just enough air and auxiliary fuel being introduced to sustain pyrolysis and gasification of the waste. This starved-air, quiescent operation also suppresses open combustion (fire) and reduces the amount of soot and ash that is conveyed to the secondary chamber. The products of waste pyrolysis and gasification have a significant fuel value in most cases, and so, as they enter the secondary chamber the addition of burnout air is generally sufficient to complete their combustion. In order to ensure complete destruction of potentially hazardous trace species there are both residence time and temperature requirements imposed on the operation of the secondary oxidation chamber. Depending on the local jurisdiction, the minimum exit temperature of the secondary chamber may be from 925 to 1100 °C, and the minimum residence time will usually be from 1 to 2 seconds. Fairmont Hotel Chicago, Illinois September 21 -24 1997 Page 5 |