It is easier to design for a given NOx or CO level in a new boiler or HRSG than in an older one, because we can design around the various options and size the boiler or HRSG accordingly. Modifying an existing boiler or HRSG to meet new emission levels presents more challenges. For example, the existing boiler furnace dimensions may not be adequate if a low-NOx burner is retrofitted, owing to possible flame impingement concerns. The existing fan may not be able to handle the increase in pressure drop if FGR is used. If an air heater is used it must be replaced by an economizer. If a catalyst is required, an existing HRSG may have to operate in a gas temperature regime that may not be optimum for it unless the heating surfaces are split. A different catalyst material capable of operation at the gas temperature window available between the evaporator and economizer or capable of operating ahead of the evaporator may have to be used. If there are space limitations, the designer may even have to reduce the boiler capacity. Steam injection in the burner may be examined.

It is possible to improve the emissions of existing boilers through options such as replacing the refractory-lined boilers with water-cooled furnaces, using membrane walls where possible to minimize flue gas bypassing between the furnace and convection bank, and using a low-NOx burner. With HRSGs, if steam injection is introduced to minimize NOx, the effects of gas flow and temperature have to be reviewed because they may affect the HRSG performance. In a new boiler or HRSG project, there are fewer constraints.

There are several ways to control NOx and CO in packaged boilers and HRSGs, some of which affect the quantity of flue gases flowing through the boiler, thus affecting the temperature profile, efficiency, and gas pressure drop. Catalysts require a specific gas temperature window for efficient operation, which is achieved by modifying the boiler or HRSG design as discussed above. These factors must be evaluated on a case-by-case basis, because no two boilers are identical. In the case of gas turbine HRSGs, optimum locations must be found for the SCR and the CO catalyst by considering the various loads and gas temperature profiles. The cost of meeting the emission limits is quite large, because boiler and HRSG designs have to be modified to incorporate catalysts, dampers, and low-NOx burners. Operating costs are also increased due to the higher gas pressure drop across the heating surfaces and ducts. The fan may have to be replaced.

New plants evaluate the best available control technology (BACT) for emissions on the basis of cost and environmental conditions. The cost per ton of pollutant removed is estimated, and the best technology to achieve this within the maximum cost allowable is chosen. Emission limits vary depending on location. Typical limits for a combined cycle plant in California that were both gas turbines and auxiliary boilers are listed in Table 4.4.

As the technology improves, it is hoped that the cost of emission control will also be reduced. For example, research work is going on to lower NOx and CO to single-digit percentages in gas-fired burners by using internal recirculation of partial combustion products without the use of flue gas recirculation and while using low excess air. This will lower operating costs and also improve the boiler efficiency.