| OCR Text |
Show toroidal ring set up by the momentum of the first stage combustion air entering the combustor along the walls. This first stage is significantly substoichiometric, and the oxygen deficiency and relatively low temperature conditions are ideal to minimize nitrogen fixation and break down fuel bound nitrogen The second stage provides the balance of air, usually in the 5 to 15 percent excess range, specified for the process. This air, after passing over the outlet refractories, mixes with the "gasified" fuel which is exiting the combustor as a high velocity jet and promotes a vigorous recirculation within the firebox. The entrainment of these cooler combustion products into the flame, and the substantial radiant heat flux from the flame, lowers the temperature in the second stage, minimizing NOx reformation, assisting heat transfer, and yet retaining a good flame shape. COMBUSTION AIR PREHEAT/VITIATION In many cases where it is necessary to use air preheat for optimum fuel efficiency, it is possible to negate the detrimental effect on NOx formation by using flue gases from the stack to dilute down the oxygen content of the air, increase the mass flow of inerts, lower flame temperature and thus reduce NOx. The same NOx reduction technique can be applied without air preheat to any burner system that can accept vitiated air without detrimental effects on combustion. FIELD RESULTS NOx AND PARTICULATES During the last few years, NOx readings have been taken using Draeger tube method, in a number of large refinery crude heaters. These furnaces are usually fired by several upshot burners, with individual capacities ranging from 12 MM BTU/hr. (natural draft) to 70 MM BTU/hr. (forced draft - high velocity) at 10 to 30% excess ambient 13/11 |
