If the desired CO and NOx levels are very low, on the order of single digits, a selective catalytic reduction (SCR) system may have to be used in boilers and HRSGs, Because most catalysts operate efficiently within a temperature window, generally 650-780°F, the boiler should have a gas bypass system to accommodate the gas temperature […]

In selective noncatalytic reduction a NOx reduction agent such as ammonia or urea is injected into the boiler exhaust gases at a temperature of approximately 1400-1650°F. The ammonia or urea breaks down the NOx in the exhaust gases into water and atmospheric nitrogen, plus CO2 if urea is injected. This reaction takes place in a […]

Methods for NOx control can be classified into two broad categories: 1. Postcombustion methods: methods that are deployed after flue gases are generated. 2. Combustion control methods: methods that are deployed during the combustion process. Postcombustion Methods As the name implies, postcombustion methods deal with the flue gases obtained after combustion. They are more expensive […]

Nitrogen oxides are of environmental concern because they initiate reactions that result in the formation of ozone and acid rain, which can cause health problems, damage buildings, and reduce visibility. The allowable NOx emissions from boilers and HRSGs vary depending on local regulations but are gradually edging toward single-digit values in parts per million volume […]

Before going into further details of how the boiler or HRSG is impacted by emission regulations, one should first understand what the various pollutants are Figure 4.3 Options for NOx removal in boilers and HRSGs. And how they are formed. In the process of combustion of fossil fuels, be it in steam generators, gas turbines, […]

Boiler and HRSG designs have undergone significant changes during the last few decades with the enforcement of emission regulations in various parts of the world. Decades ago boiler and HRSG users were concerned about two issues only: the initial cost of the boiler or HRSG and the cost of operation. Low boiler efficiency, for example, […]

The following process data should be specified as a minimum. 1. Steam parameters such as flow, pressure, temperature, and feedwater temperature. If saturated steam is taken from the boiler for deaeration or for NOx control, fuel oil heating, etc., it should be so stated. If the makeup water flow is 100%, the deaeration steam could […]

Borosilicate glass (Pyrex) tubing has been used in heat recovery applications because it is most resistant to chemical attack and presents no corrosion problems. Fouling is minimal due to the smoothness of the surfaces. These tubes also have a low coefficient of expansion and are resistant to thermal shock, which makes them suitable as heat […]

The conventional design of economizers and air heaters ensures that cold end corrosion due to condensing sulfuric acid or water vapor does not occur because the minimum tube wall temperature is maintained above the dew points. However, owing to this design philosophy, a significant amount of energy is lost or not recovered in boilers and […]

Heat pipes (Fig. 3.28) were introduced into the heat recovery market about 40 years ago. A heat pipe consists of a bundle of pipes filled with a working fluid such as toluene, naphthalene, or water and sealed. Heat from the flue gas evaporates the working fluid collected in the lower end of the slightly inclined […]