Flue Gas Composition and Gas Pressure

Flue gas analysis is important to the design of the boiler. A large amount of water vapor or hydrogen increases the gas specific heat and thermal conductivity and hence the gas

and overall heat transfer coefficient, boiler duty, and heat flux. Presence of SO 2 lowers the gas specific heat and hence the duty. For example, in hydrogen plant reformed gas boilers where we have a large % volume of hydrogen and water vapor in the flue gas at high gas pressure, the heat transfer coefficient can be five to eight times that of typical flue gas from the combustion of natural gas. Heat flux is a concern in these applications.

The presence of chlorine and hydrogen chloride vapor in flue gas is indicative of corrosion potential particularly if a superheater is used and the tube wall temperature is above 475°C. Chlorine attacks carbon steel even at low temperatures as shown in Figure 4.8. Hence, low-pressure saturated steam is generated with flue gas containing chlorine and economizers are avoided. Presence of hydrogen chloride gas (HCl) also indicates high- temperature corrosion. There are instances of high-temperature stainless steel superheaters in several municipal solid waste incineration plants having been destroyed within a few months of operation due to HCl. Hence, modular municipal solid waste incineration plants generate steam at medium pressure and low temperature (about 40 barg, 375°C) enabling longer life of superheater tubes.

Waste Heat Boilers 171

Dry chlorine

Dry HCl

Corrosion rate

Metal temperature, °F

FIGURE 4.8

Chlorine and hydrogen chloride in flue gas cause corrosion problems.

The presence of SO 3 and HCl in the flue gas also suggests low-temperature corrosion problems due to the acid vapor condensation. To minimize low-temperature corrosion concerns, there are several methods that are discussed in Chapter 3. Major decisions such as using membrane wall construction or refractory-lined casing depend on the flue gas analysis and dew point of the flue gas and cost considerations.

More care should be taken when the flue gas contains ash, which has slagging potential. This is common in steel plant effluents or municipal solid waste incineration systems.

A furnace to cool the flue gas to below the ash melting temperature as shown in Figure 4.5 or Figure 4.6 is one solution. Some plants recirculate flue gas from the boiler rear and lower the gas inlet temperature to the boiler below the ash melting temperatures. However, this increases the flue gas quantity through the boiler and increases the gas pressure drop, or the boiler cross section may have to be increased. Double-spaced screen section, which prevents molten ash from sticking to the boiler tubes, is recommended, if slagging is envis- aged. Tube spacing of 150–200 mm is used at the convective bank inlet as shown for a few rows deep. One can gradually reduce the tube spacing as the gas cools. The superheater, which is more prone to corrosion when deposits stick on to them, is located in a much cooler zone by using adequate number of screen tubes ahead of it.

Flue gas pressure is generally atmospheric in many applications; in pressurized systems (such as gas turbine HRSGs or incineration plants), the flue gas is well contained in the boiler if a membrane wall construction is used. If a suction system with an induced draft fan is used as in some biomass heat recovery projects, then there is possibility of leakage of cold air from the atmosphere into the boiler through man-way doors or hopper openings. This is likely to affect local gas temperatures, and hence, performance prediction may not

be accurate. Superheated steam temperature may also be impacted by ingress of cold air into the system. In reformed gas boilers in hydrogen plants or nitric acid plants, the gas pressure can

be very high and hence fire tube boilers are preferred, though special water tube boilers have been built. Table 4.1 shows the flue gas analysis and pressure for different waste gas [1] streams. Alloy steel tubes are recommended when hydrogen is present in flue gas; hence, reformed gas boilers use T11 or T22 tubes for the evaporator to minimize high-temperature concerns. Nelsons chart is often referred to for material selection (Figure 4.9).

In incineration systems involving halogenated compounds, one has to ensure that the combustion temperature of 850°C–900°C is maintained for at least 2 s in the incinerator combustion chamber to destroy the formation of dioxins. Following combustion, the flue

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

TABLE 4.1

Analysis of Waste Gases from Process Plants, % Volume

NO 9 Press., atm

1 1 1.5 3–10 1 1 Temp., °C

250 150 100 Notes: (1) Gas turbine exhaust, (2) raw sulfur gases, (3) SO 3 gases after converter, (4) reformer flue gas,

(5) reformed gas, (6) MSW incinerator exhaust, (7) chlorinated plastics incineration, (8) sulfur condenser effluent, (9) nitrous gases, (10) preheater effluent—cement plant, (11) clinker cooler effluent—cement plant.

Surface decarburization

6% Cr 0.5% Mo

10 3% Cr 0.5% Mo

1.25% Cr 0.5% Mo

900 erature, °F

2% Cr 0% Mo

mp Te 700

Not welded

0.5% Mo 500

0.25% Mo

Welded or hot formed

Carbon steel

Hydrogen partial pressure, psia

FIGURE 4.9

Nelson’s chart for the selection of materials. gas should be cooled very quickly in the waste heat boiler, typically in less than 0.5 s

to ensure dioxins are not formed. Finned water tube boilers have shorter gas path and hence shorter residence time. With fire tube boilers, the residence time may be longer and has to be checked. One option as we shall see later is to use smaller-diameter tubes in fire tube boilers to reduce their length.

Waste Heat Boilers 173

Water Tube versus Fire Tube Boilers

A common classification of boilers is whether the gas flows inside the tubes as in fire tube boilers (Figure 4.10) or outside the tubes as in water tube boilers (Figure 4.5). The features of each type are discussed in Table 4.2 [2].

Generally, water tube boilers are suitable for large gas flows in the range of millions of kg/h and can handle high steam pressures and temperatures. Fire tube boilers are suitable for low steam pressures generally below 3500 kPa (35 bar) and low gas flows. Table 4.3 shows the effect of pressure on tube thickness for both types. A given tube thickness can with- stand about twice the pressure when it is inside than when outside. Hence, fire tube boilers require large thickness as the steam pressure increases and thus become uneconomical;

Fire tube boiler with superheater and economizer. (Courtesy of ABCO Industries, Abilene, TX.)

TABLE 4.2

Comparison of Fire Tube and Water Tube Boilers

Item

Fire Tube

Water Tube

Gas flow

Large—30,000–1,000,000 kg/h Gas inlet temperature

Small—less than 30,000 kg/h

Low to adiabatic combustion Gas pressure

Low to adiabatic combustion

500–1000 mm wc Firing

As high as 150 barg

Possible

Plain and finned tubes Superheater location

Type of heating surface

Plain tubes

Anywhere in the gas path Water inventory

At inlet or exit

High

Low

Can be high with finned tubes Multiple pressure

Heat flux on steam side

Generally low

Yes

Multiple modules

Yes

Soot blower location

Inlet or exit

Anywhere in gas path

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

TABLE 4.3

Tube Thickness versus Steam Pressure

Tube Thickness, mm

External Pressure, barg

Internal Pressure, barg

50.8 mm SA 192, SA 178a carbon steel tubes at 371°C.

they also get huge and heavy as plain tubes are used unlike water tube boilers where finned tubes are used to make the boilers compact.

Sometimes, a combination of fire tube and water tube boilers may be required. Figure 4.11 shows a waste heat boiler for a small hydrogen plant cooling both the furnace flue gas and the reformed gas. The reformed gas is at a high gas pressure and hence a fire tube boiler is appropriate, while the clean flue gas is at atmospheric pressure and a water tube boiler with extended surface is ideal. Since the gas flows and steam generation are small, a compact package unit as shown was designed and built.

Water tube boilers weigh less as they can be made compact using finned tubes (see Appendix E on finned tubes). One may install superheaters even in fire tube boilers, but the location is either at the inlet or exit (Figure 4.10). It may be installed at the turnaround section between the second and third gas passes also as discussed in Chapter 3. In any case compared to water tubes, these locations may not be the optimum. If the flue gas is slagging in nature, a water tube boiler with cleaning devices is a better choice compared to a fire tube boiler as the tube inlet will be plugged up with slag in a fire tube unit and cleaning will be required at a greater frequency.

Multiple-pressure steam generation is required to lower the exit gas temperature if the inlet gas temperature is low. This point will be illustrated with calculations in Chapter 5 on HRSG simulation. While multiple-pressure modules are easy to build with water tube boilers, they are rarely seen with fire tubes as it is uneconomical and space requirement will be huge.

Flue Gas Composition and Gas Pressure