| OCR Text |
Show Effect of Water Injection on Other Emissions Figure 13 depicts the influence of water injection on CO and UHC emissions. Both pollutant levels showed an increasing trend with water injection but the effect on UHC was much smaller. Other emissions changed insignificantly, probably due to the long residence time offered in the large combustion chamber which is favorable to achieve high carbon burn-out. Effect of Water Injection on Power Output and Efficiency In this concluding section, information is provided on the economic aspects of NO control. Thermodynamically, the addition of steam and water increases the turbine mass flow, hence the power output of the unit. Steam or water injection should have identical effect on the power output. The top line in Figure 14 shows these changes in power output as a function of the water to air mass ratio. The introduction of steam or water into the gas turbine for NO control, however, has a negative effect on the machine efficiency. Liquid water has to be vaporized first, then the steam superheated to reach the turbine inlet temperature. Both steps require heat and lead to an overall reduction in efficiency. The bottom line in Figure 14 illustrates this loss as a function of water to air mass ratios, covering the case of injecting water in liquid phase. A similar but less pronounced drop is associated with steam injection, which per unit of mass basis, is obviously a less effective medium for flame temperature reduction and NO control. A different picture emerges, however, for a gas turbine plant operation, where the heat content of the exhaust is released unutilized. The potential recovery of heat in this plant by steam raising and its use for NO control - if credited to the operation - could enhance the effici-x v ency, as depicted by the mid-line of Figure 14. This line illustrates the return of recovered heat through injecting of steam at 300 C. In Figure 15, an example is given on the relationship found in a combined cycle operation between steam injection, power output and efficiency. The results presented in Figure 15 indicate, as expected, that the total power output increases with steam injection, but the efficiency of the combined cycle is reduced. This drop in efficiency is one of the main cost elements in the state-of-the-art NO control. 1.4.11 |
