| OCR Text |
Show 7 This has the effect of reducing the overall flame temperature and therefore has an effect in reducing the rate of reactions which form the NOx. This has been shown to be particularly effective on burner systems where there are high temperature pre-heats however it is normally employed to reduce the preheated air burner to NOx levels similar to that expected with an ambient air burner. There are obviously limits to the amounts of flue gas which can be recirculated with the burner, for instance the fact that the flame temperature is reduced also means that the combustion is not as efficient as it fonnally has been. Therefore very high levels of flue gas recirculation would negate the effect of the preheater and therefore there would be no point either to the expense of the recuperator or the expense of the equipment controlling the gas recirculation rate. As an interesting aside to the technique of flue gas recirculation I was reminded recently that if a furnace can be operated with coke oven gas which is essentially a mixture of carbon monoxide, carbon dioxide and nitrogen usually with a calorific value of 3 - 400 BTU's per cubic foot their gas recirculation is already implicit in that the fuel in that the fuel is carrying a significant proportion of inerts as part of the flow therefore one can expect to see much lower NOx levels from burners operating with coke oven gas and this would probably be true of those using digester gas in the refuse and water treatment industries. Some of our largest burners, those used for aggregate drying and asphalt preparation give us some of our biggest headaches in respect of the amounts of pollutant gases they produce in the exhaust. A number of areas of the U.S. are now designated non attainment areas which means they are legislating lower and lower NOx levels. We expect to see a trend to total air burners in this type of application with flue gas recirculation to keep NOx levels down. There is however a Safety Code issue involved with the use of flue gas recirculation which has yet to be addressed. The first is that potentially the flue gas can contain a portion of carbon monoxide. It is also possible under certain conditions that the flue gas itself can become flammable, and finally if the flue gas is switched on at too Iowa temperature then this could severely quench the flame and result in the productions of noxious gases, aldehydes etc., plus an incomplete combustion of the methane gas or whatever fuel is being used. The most convenient way of controlling the amount of flue gas recirculation occurring, particularly for large burners, may be through the use of a pIc controller which would itself be linked to a safe temperature controller plus any other safety systems required for the operation of the flue gas recirculation. I have no need to remind you of the speed which CGA can make changes to the gas code particularly in light of delays in preparing a suitable pIc operating code, one which would be acceptable across all the provinces of Canada. |
