Industrial Boilers and Heat Recovery Steam

CARBON MONOXIDE REDUCTION

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, […]

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Gas Reburn

One of the methods to reduce NOx in large industrial boilers is natural gas reburning, which is capable of providing a 50-70% reduction in NOx. In this method, natural gas is injected into the upper furnace region to convert the NOx formed in the primary fuel’s combustion gases to molecular nitrogen. The overall process occurs […]

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Flue Gas Recirculation and Excess Air

Present-day packaged steam generators operate at high excess air (15-20%) with flue gas recirculation (FGR) rates ranging from 0% to 30% to limit CO and NOx. Flue gas recirculation refers to the admission of flue gases from the boiler exit back into the burner region in order to lower the combustion temperature, as shown in […]

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Burner Emissions

Duct burners used in HRSGs also generate NOx and CO, adding to the emissions from the turbine exhaust gases. The calculation procedure for estimating the NOx and CO in ppmv after combustion is shown in Q6.26e. It may be noted that the values of NOx and CO in lb/h are always higher after combustion; however, […]

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Furnace Modifications

The completely water-cooled furnace (Fig. 4.2) provides a cooler envelope for the flame than a refractory-lined front wall or floor and hence produces less NOx. Most NOx is generated at the flame front when combustion is initiated, and a water-cooled furnace absorbs some of the radiation from the flame, which helps cool it, whereas a […]

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Burner Modifications

Staged combustion is widely used by burner suppliers to reduce NOx. In this method, the fuel or air is added in increments (Fig. 4.12) so that at no point in the flame is an exceptionally high temperature obtained. In air staging, a fuel-rich mixture is initially created, followed by the addition of air at the […]

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Steam-Water Injection

Boiler and burner suppliers sometimes use steam injection to reduce the flame temperature and thus decrease NOx. Steam generators as well as gas turbines use this method. In boilers the steam consumption could vary by 1-3% of the total steam generated, thus reducing the boiler output; however, the significant reduction in NOx may offset the […]

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Figure 4.10 Typical NOx and CO levels versus excess air. 0.9 1.0 1.1 1.2 1.3 1.4 Excess air factor vs CO and NOx Oxygen Control

In steam generators, oxygen trim can be added to control the excess oxygen levels. Too little oxygen increases CO formation, and too much can increase the NOx. Also, the boiler efficiency is impacted by the excess air levels as discussed in Chapter 3. The higher mass flow also affects the gas temperature distribution throughout the […]

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Combustion Control Methods

The formation of NOx has been well understood by burner manufacturers, who are able to offer several methods to reduce the formation of NOx in steam generators. Gas turbine manufacturers also have come up with design improve­ments to lower NOx emissions. During the combustion process, several complex reactions occur within the flame, and NOx formation […]

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SCR

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 […]

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