Emissions Affect Steam Generator Designs

Present-day package boilers operate at high excess air (15%–20%) with FGR rates rang- ing from 0% to 30% to limit CO and NO x . Higher the combustion air temperature, higher the FGR rate to achieve the same level of NO x . If fuels containing hydrogen are fired, the flame temperature is higher compared to natural gas, and hence, more NO x is generated and more FGR is required. Hence, air heating is avoided in oil- and gas-fired boilers. The reason for the use of high excess air can be seen from Figure 3.7, which shows that as the excess air increases, the NO x level increases and then drops off. When the excess air is

a little above stoichiometric, the flame temperature increases up to a certain level due to good mixing of fuel and air, and hence, the NO x increases; as the excess air increases further, the combustion temperature decreases due to the increased mass flow of air and consequent cooling effect; also at low excess air, the CO formation is high due to poor mixing between the fuel and the air. Hence, 15% excess air and 15–25% FGR are widely used in all gas- and oil-fired package boilers instead of 5% excess air a few decades ago. This increases the boiler size by 15%–25% if gas pressure drop has to be limited. A larger economizer is also required to maintain the same stack flue gas temperature and efficiency as the design without FGR.

If SCR is added to lower NO x (Figure 3.8), the evaporator section may have to use a gas bypass system to ensure that the catalyst operates in the optimum range of gas temperatures (350°C–420°C) to guarantee NO x levels adding to the complexity of the boiler con- struction. Catalysts are available for low gas temperature operation also, which can be located ahead of the economizer or even behind it. The location of the catalyst, whether between the evaporator and the economizer or beyond the economizer, also affects the boiler layout. More information on SCR catalysts and ammonia grid is provided in Chapter 4. Lo-NO x burners also have been developed by boiler suppliers to handle the low emission levels of NO x and CO (Figure 3.9). These may require a minimum furnace cross section to avoid flame impingement and a furnace length different from a standard furnace. Thus, standardization is possible only to a limited extent in large water tube boiler designs. While standard boiler applications find a place in the industry, there are numerous applications where a custom-designed boiler would fit the needs of the end

CO NO x

0.9 1.0 1.1 1.2 1.3 1.4 Excess air factor

FIGURE 3.7

Typical NO x and CO levels versus excess air.

Steam Generators

Convection bank

Ammonia grid

(b)

SCR — Economizer

FIGURE 3.8

Arrangement of (a) FGR and (b) SCR system in package boiler for NO x control.

Dual burner, 600 MMBtu/h

FIGURE 3.9

Low NO x burner. (Courtesy of Cleaver Brooks Inc., Natcom Burners, Thomasville, GA.) user better and lower the user’s cost of owning the boiler. Plant engineers prefer custom-

designed boilers as it lowers the operating costs and saves them money in the long run. Operators must also consider the risk of operating a boiler near the limits of inflammability when using high FGR rate. Figure 3.10 shows the narrowing between the upper flammability limit and lower ignition limit as FGR increases. Integrating control systems to maintain fuel/air ratios at high FGR rates is difficult because FGR dampens the combustion process to the ragged edges of flammability-flameouts and flame instability. Full metering combustion control systems with good safety measures are necessary in such cases (Figure 3.11a and b).

94 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

10 Upper ﬂammability limit

8 el–air ratio 6

Fu 4

Lower ignition limit

% Flue gas recirculation

FIGURE 3.10

Flue gas recirculation and limits of inflammability.

Figure 3.8 shows the scheme for a typical FGR system in a package boiler that can achieve NO x in the range of 9 ppmv; the use of the SCR can achieve 90%–95% NO x removal or NO x under 3–5 ppmv. With a typical FGR system, 15%–30% of flue gas quantity from the economizer exit as suggested by the burner supplier is recirculated to the suction of the forced draft fan. This lowers the combustion temperature of the flame, and hence, NO x is reduced and NO x levels of 9–30 ppmv are achieved for natural gas firing. FGR also increases the combustion air temperature entering the forced draft fan. With 26°C combustion air (80°F) and 15% FGR at 160°C (320°F) flue gas temperature, the mixed air temperature is about 44°C (112°F). The air density at the fan also decreases adding to the volume of air to be handled by the fan. FGR also affects the gas temperature profile through the boiler and affects the furnace heat absorption and heat flux.

The superheater performance is also impacted by FGR depending on where it is located, whether radiant or convective. This is discussed later. Custom designing of boiler evaporator section, which is the source of major gas-side pressure drop, can help lower the gas pressure drop arising out of higher mass flow through the boiler due to FGR. Tube length can be increased or tube spacing increased to reduce the gas velocity and pressure drop.

Catalyst operates efficiently in a range of gas temperatures, typically between 340°C and 420°C. If one looks at the gas temperature profile along the flue gas path, one may note that the gas temperature decreases as the load decreases (Figure 3.12). Hence, a gas bypass system is often used to maintain the gas temperature at the catalyst at low loads. The flue gas should be taken from a reasonably high gas temperature zone and mixed with the flue gas at the evaporator exit as shown; the ammonia injection system should be located upstream of the SCR and should have sufficient mixing length so that the flue gas can react with ammonia. The SCR catalyst pressure drop is typically in the range of 50–75 mm wc. One can thus see how complex a steam generator has become due to NO x and CO emission regulations, not to speak of the operating and installation costs! Hence, a standard off- the-shelf boiler cannot be used for applications involving emission controls, particularly those using the SCR system. Figure 3.13 shows the scheme of ammonia injection system and the SCR. The ammonia grid is installed ahead of the catalyst box, and one should ensure good mixing of ammonia with the flue gas before it passes through the catalyst.

Steam Generators

(a) Scheme of boiler controls—gas side. (b) Scheme of controls—steam side. (Courtesy of Cleaver Brooks Inc., Engineered Boiler Systems, Thomasville, GA.)

96 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

Scrn. Sh.

Econ.

erature, °F Steam pressure = 475 psig mp

Steam temp. = 665 F 1000

Gas temperatures at exit Load

100% 75% 50% 25% Furnace 2296 2167 1949 1666

Screen 1696 1549 1332 1072 Sh

Flue gas temperature profile in a steam generator versus load. Reduction agent

Catalyst bed

H O H NN

Exhaust gas H

H O H Treated gas

Typical SCR system with ammonia injection grid. (See Chapter 4 for more information on ammonia grid and catalyst.) Hence, boiler layout

also becomes more complex. Chemical reactions for NO x reduction briefly are as follows:

4NO + 4NH 3 +O 2 ⇒ 4N 2 + 6H 2 O + NH 3 slip

NO + NO 2 + 2NH 3 ⇒ 2N 2 + 3H 2 O + NH 3 slip 2NO 2 + 4NH 3 +O 2 ⇒ 3N 2 + 6H 2 O + NH 3 slip

6NO + 4NH 3 ⇒ 5N 2 + 6H 2 O + NH 3 slip 6NO + 4NH 3 ⇒ 5N 2 + 6H 2 O + NH 3 slip

Steam Generators

The idea is to convert the NO x to N 2 and H 2 O using the ammonia. Excess ammonia called ammonia slip should be as low as possible as it is also considered a pollutant. Controls and instrumentation are used to ensure that the slip is within levels permitted by local regulations.

Sulfur-containing fuels present problems for the catalyst. Vanadium present in the catalyst convert SO 2 to SO 3 , which can react with excess ammonia to form ammonium sulfate or with water vapor to form sulfuric acid causing problems such as fouling and plugging of tubes downstream of the gas path. One method is to limit the number of hours of operation on fuels containing sulfur. Lowering ammonia slip helps, but this can affect the NO x reduction efficiency. Environmentally, ammonium sulfate and bisulfate are particulates that contribute to visible haze and acidify lakes and ground areas where they settle out of the air. Sulfates are formed according to the following equations:

SO 3 + NH 3 +H 2 O → NH 4 HSO 4 SO 2 + 2NH 3 +H 2 O → (NH 4 ) 2 SO 4

Ammonium sulfate is a sticky substance that can be deposited on heat transfer surfaces such as finned economizer. If the ammonia slip is less than 10 ppm and SO 3 is less than

5 ppm, experts say that the probability of ammonium sulfate formation is nil unless the gas temperature is low on the order of 200°C at the catalyst. One may keep the boiler warm during shutdown or in standby conditions to prevent condensation of acid vapors.

Emissions Affect Steam Generator Designs