| OCR Text |
Show In the last three years, a large data base has been generated for various operating conditions that include large variations in operating parameters such as furnace temperature, air temperature, fuel compositions, fuel splits, etc. With this data base available, initially, correction curves that were similar to those developed for the conventional burner and staged fuel burner were developed. It soon became obvious that the correction factors were different than those for the conventional and staged fuel burners. For example, a change in furnace temperature has a greater impact on the N O x formed in the internal recirculation burners than on the N O x formed in conventional and staged burners. With a little consideration of the phenomena, it becomes obvious as to why this is the case. For example, furnace temperature controls the rate of heat transfer from the flame to the surroundings. For high furnace temperatures, the heat transfer rate is low; therefore, the flame temperature is high. This phenomena occurs for both the older burners and the internal furnace gas recirculation burner. However, because the furnace temperature increases, the temperature of the recirculated furnace gases also increases. Because of this second factor, the increase in N O x levels with increasing furnace temperature is greater for internal furnace gas recirculation burners than conventional and staged combustion burners. Now consider the change in NOx emission levels with air preheat for the different burner types. Preheated air or combustion air temperature has a lesser effect on the quantity of N O x formed by the internal flue gas recirculation burner than on the N O x levels produced by conventional and staged fuel burners. Again, with some consideration of the phenomena that are occurring, it is easy to understand why. N O x increases with an increase in combustion air temperature because the flame temperature increases. However, for a given heat release, because of the recirculated furnace gases, the mass in the flame zone is greater for the internal recirculation burner. The increased mass tempers the effect of air preheat on flame temperature. Therefore, air preheated to the same temperature will have a lesser effect on the flame temperature for the internal recirculation burners than for the older style burners resulting in lesser increase in N O x levels as the air temperature is increased. A third example is the effect of hydrogen content in the fuel. As the hydrogen content in the fuel increases, the N O x levels generated by conventional, staged fuel and staged air burners rises rapidly. The same increase in N O x level is not experienced when the fuel hydrogen content is increased for internal flue gas recirculation burners. In fact, there is essentially no increase. Again, an explanation can be offered based on the N O x reduction method of the internal recirculation burner. The increased mass in the primary combustion zone that results from the induced flue gases reduces the combustion temperature. Recall Figure I which provides N O x equilibrium concentration for various temperatures. The temperature in the primary combustion zone is reduced to the point where the curve in Figure I that represents the equilibrium concentrations has a relatively "low" slope. In addition, because of the |
