OCR Text |
Show parameters are held constant. The dilemma is that if any of the constant parameters have a different value than the value for which the curve was constructed, then the correction curve will no longer accurately predict the change in N O x for the new conditions. To illustrate the above and to illustrate how the impact of various parameters is different for the recirculation burners, when compared to old style burners, two operating parameters have been selected for investigation. The first is furnace temperature and the second is combustion air temperature. Process Insights was used to determine the impact of each of these parameters on N O x emissions by varying only one parameter and holding all others constant. To illustrate how these correlation curves change when the value of one the parameters that is held constant is changed, the curves were generated for primary fuel rates of 2 5 % , 3 0 % and 3 5% of the total fuel. Values for other parameters are: Heat Release: 5.7 MMBtu/hr Oxygen Concentration: 3.2% Fuel L H V : 1054 Btu/Ft3 Fuel Composition: CH4 6 0% H 2 20% C 3 H 8 2 0% Figure VII illustrates the effect of changing furnace temperature. Note that for the furnace temperature range used, the N O x levels almost double with high primary fuel rates, while for lower primary fuel rates, the N O x levels increase by only approximately 50%. As noted in the discussion on burner design, the internal flue gas recirculation burner uses the primary fuel to induce furnace gases into the primary combustion zone. The temperature of the furnace gases introduced into the primary combustion zone impact the N O x level. Just as is the case for external flue gas recirculation, higher temperature recirculated gases do not reduce the N O x levels as m u c h as lower temperature recirculated gases, provided the mass flow rate is the same. Also, with higher temperature furnace gases, the primary fuel actually induces a lower mass flow rate of furnace gases. It should also be noted that as a greater portion of fuel gas is injected into the primary combustion zone, a proportionally greater amount of N O x is formed in the primary combustion zone. Therefore, it is to be expected that the increase in furnace temperature will produce a larger increase in N O x levels for high primary fuel rates. NOx vs FURNACE TEMPERATURE INTERNAL RECIRCULATION BURNER RELATIVE NOx LEVEL fj- PRIMARY FUEL ^ <s? \s V *& / /i / / / [// J 1000 1100 1200 1300 1400 1600 MOO 1700 MOO 1900 2000 FURNACE TEMPERATURE (DEG F) ! Figure VII Now compare the Figure VIII2- to Figure VII. Note that the older curve that was developed for conventional burners (Figure VIE) actually predicts an increase in N O x rv-21 |