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Show 4 There are some situations which will arise where the NOx levels as measured on a pounds per process will be similar but where the total emissions will have been significantly reduced by tbe installation of a heat exchanger. How do we accommodate such paradoxes. the chances are that they will be simply too difficult to resolve and as such the emphasis will be transferred back to cleaning up the relatively small amounts of NOx whilst doing little to promote fuel efficiency. The way that NOx is chemically formed in the combustion process gives a clue to ways that NOx may be controlled. Thought of simply it is the combustion of Nitrogen in the presence of oxygen at elevated temperatures. If there is no free oxygen around then it is possible that the amount of NOx being produced by the combustion process will be suppressed, if the Nitrogen associated with the combustion process is also limited then the amount of NOx formation will also tend to be decreased. But here we have yet another paradox. Nitrogen is present in air but it is also some times present in the fuel. Either as highly reactive atomic Nitrogen, Nitrogen molecules associated with the gas or as Nitrates and it is quite common that the majority of the NOx will be produced from Nitrogen associated with the fuel. There are three simple methods of NOx control. The first is to reduce flame temperature. The second is to add a reducing agent. The third would be to remove Nitrogen from the combustion process completely. High level oxygen enrichment has been promulgated as a potential answer to our NOx problem as combustion in pure oxygen means that Nitrogen is inhibited from contacting the flame. However there are severe practical difficulties. The first is that Nitrogen will always be present in the form of air leaks into the chamber of the furnace. There will be a Nitrogen concentration gradient across which one can expect Nitrogen to flow. ,. The second major difficulty is that in oxygen enriched atmospheres the heat of combustion drives the flame temperature much higher than in a flame burning with air as the oxidiser and these higher flame temperatur~s will tend to drive the reaction in the direction of the NOx reaction. Any Nitrogen associated with the fuel will be more efficiently converted to NOx by the higher temperature of the flame. It should also be noted that there is an energy cost associated with the production of oxygen particularly at the levels required for most industrial equipment and although it is sometimes possible to justify the use of oxygen enrichment as an energy efficiency improvement tool the net environmental impact may be quite small because the energy required to generate oxygen from the air that has to be taken into account when attempting to evaluate the environmental impact of such a proposal. |
