OCR Text |
Show 5 If you consider a mixture of fuel and air burning in some controlled manner the temperature of the flame will be dependent on a number of parameters. These will include: The temperature of the combustion air supply. The heat content of the fuel, the rate at which the heat is being released. The specific heat capacity of the mixture of gases in the flame. The radiation heat exchange between the environment and the flame. The rate at which the flame dissipates heat in the environment. From this list of parameters it is possible to infer that to simply arrive at a design for a low NOx burner or a low NOx control system may not necessarily have the desired affect. NOx formation has to be considered in concert with parameters such as furnace geometry and load geometry, furnace temperature profiles, the operating partial pressure of oxygen inside the furnace, the isothermal temperature of the furnace and there are certain situations where the load could be a catalyst to the combustion reactions. Chemical composition and load morphology have to be considered together and this is unfortunately where the combustion engineer arrives at the sticky ground between the furnace technologist and the fuel technologist. Here again we should attack the simple things first. For instance, controlling the static pressure inside the furnace will help to reduce fuel costs but can also lead to NOx reduction. The substitution of conventional burners with high velocity burners can also lead to a marked decrease in the amount of NOx being produced. This is because the flame envelope is generally more defined with less mixing between the envelope and the furnace atmosphere. But also the expansion and the subsequent dilution of the flame envelope are much more rapid in a high velocity flame than they are in a conventional flame which in turn, means less time at temperature for the reactions that produce NOx to occur. This fits the obselVations that we have made of a number of pulse fired systems where the burners are only fired at their high fire settings or a low fire setting with no intermediate settings available to the system. Heat input being controlled by the time cycle of the high temperature firing. We have obselVed an overall net decrease in the amount of NOx being generated from pulse fired systems as compared with their conventional systems prior to modification. This effect being particularly pronounced on heat treatment furnaces operating in excess of 2000 degrees F. It is possible to negate these beneficial effects by ill placement of the load. For instance if the load was placed directly in front of the burner then the natural expansion of the flame envelope would be disrupted. The flame envelope would be relatively ill defined and well mixed with the furnace atmosphere. This would tend to suggest that the type of temperature criteria we have applied to NOx formation would show no overall net benefit. |