From Figs. 4.10 And 4.11 It can be seen that any effort to reduce NOx such as reducing flame temperature or water/steam injection results in an increase in CO; therefore a balance must be struck between the efforts to reduce NOx and CO. In packaged boilers, in addition to using proper excess air and FGR, ensuring that the combustion products do not leak to the convection pass from the furnace helps to lower CO. Some boilers that use the tangent tube construction instead of the membrane wall design for the partition between the furnace and the convection section have experienced leakage of hot furnace gases from the furnace side to the convection section; the tangent tubes are likely to warp due to thermal expansion during operation and allow gas to leak. The difference in gas pressure between the furnace and the convection section can be on the order of 10-30 in. WC depending on the boiler design, so the leakage could be significant. In that case the flue gases do not have the residence time needed to complete the combustion process in the furnace, which can result in higher CO formation. The presence of water vapor also increases CO. Increasing the boiler size reduces both CO and NOx because the furnace temperatures and heat release rates are reduced and the residence time for CO conversion to CO2 is increased; however, this adds to the boiler cost.

Generally 30-100 ppmv of CO can be achieved with most packaged boiler burners in operation today and about 25-50 ppmv in gas turbines. If single-digit CO emissions are required, an oxidation catalyst is suggested in packaged boilers and HRSGs, which can add to their cost and operating gas-side pressure drop.

CO + 1O2 ! CO2

HxCy + O2 ! CO2 +H2O

An oxidation catalyst increases the conversion of SO2 to SO3, which can react with ammonia to form ammonium sulfate. However, with natural gas fuel with a low sulfur content, this is not a serious concern. This conversion is higher at higher temperatures, say at 1100°F, and decreases to about 10% at 600°F without significantly affecting the efficiency of CO or formaldehyde removal. Good combustion controls can also help reduce CO formation. VOCs are also some­what reduced by oxidation catalysts.

The dry low-NOx (DLN) combustors used in gas turbines have demon­strated CO levels of less than 5 ppm.

Figure 4.7 Shows the use of a CO catalyst in an HRSG. Generally, higher temperatures on the order of 600-1000°F are acceptable for CO catalysts, so the catalyst can be placed at the inlet of the unfired gas turbine HRSG. However, when a burner is used in the HRSG, it is advisable to have another heat transfer surface precede it so that the burner flame does not impinge on the catalyst. The CO catalyst should also precede the NOx catalyst to keep it away from ammonia. Typical CO conversion efficiency can range from 60% to 85%, though higher values may be obtained. Depending on its size, the gas pressure drop across the CO catalyst can range from 2 to 3 in. WC. The cost of a typical CO catalyst is about 50% of that of a SCR catalyst.