Burner Lines Ducting

(ebullient-cooled)

- 1,614,000 Btu/h

(1,703 MJ/h)

Exhaust

- 859,000 Btu/h

(906 MJ/h)

Total

- 2,473,000 Btu/h

(2,609 MJ/h)

As indicated in Figure 8-19, firetube units may also be equipped with supplementary firing. The separated chambers allow for firing the forced draft burner, without running the heat recovery induced draft fan, or for the

Fig. 8-18 Packaged Exhaust and Coolant System Recovery Silencers, Producing 15-psig Steam from Ebullient-Cooled Reciprocating Engines. Source: Vaporphase by Engineering Controls. Inc.

Fig. 8-17 Packaged Heat Recovery Low-Pressure Steam Generator. Source: Vaporphase by Engineering Controls, Inc.

Fig. 8-18 Packaged Exhaust and Coolant System Recovery Silencers, Producing 15-psig Steam from Ebullient-Cooled Reciprocating Engines. Source: Vaporphase by Engineering Controls. Inc.

simultaneous firing of exhaust gas and the supplementary forced draft burner.

Figure 8-20 shows a reciprocating engine cogeneration system featuring two gas-fired heat recovery silencers (fire-tube HRSGs). Each HRSG can produce up to 10,000 lbm (4,500 kg) per hour of steam at 140 psig (10.7 bar), using heat recovered from the each of the two 1,928 kW dual-fuel reciprocating engine-generator sets.

system featuring two gas-fired heat recovery silencers (fire-tube HRSGs). Each HRSG can produce up to 10,000 lbm (4,500 kg) per hour of steam at 140 psig (10.7 bar), using heat recovered from the each of the two 1,928 kW dual-fuel reciprocating engine-generator sets.

Lbm Molding Machine

Source

■21 Sectional View of Large HRSG Arrangement. Babcock & Wilcox

Fig. 8-19 Dual-Chamber Firetube Heat Recovery Boiler Featuring Supplementary Firing Capability. Source: Superior Boiler Works

Source

Fig. 8-20 Supplementary Gas-Fired Firetube-Type HRSG Producing 140-psig Steam with Heat from Reciprocating Engine. Source: Vaporphase by Engineering Controls, Inc.

■21 Sectional View of Large HRSG Arrangement. Babcock & Wilcox

Watertube Boilers

As discussed in Chapter 7, watertube boilers are primarily employed in very large applications. Figure 8-21 shows a sectional view of a large HRSG arrangement, and Figure 8-22 shows a large modular HRSG featuring natural circulation design. Fins (Figure 8-23) are often used to increase heat transfer rates and reduce the total amount of surface area required.

Fig. 8-22 Large Modular Natural Circulation HRSG. Source: Babcock & Wilcox

Fig. 8-20 Supplementary Gas-Fired Firetube-Type HRSG Producing 140-psig Steam with Heat from Reciprocating Engine. Source: Vaporphase by Engineering Controls, Inc.

Fig. 8-22 Large Modular Natural Circulation HRSG. Source: Babcock & Wilcox

Figure 8-24 is an HRSG flow diagram showing counter-flow through smooth monotube coil design. The coil tube is wound in a spiral pattern arranged to control velocities of the boiler gases. Figure 8-25 shows a five-section unit featuring this design, with a side inlet, top outlet configuration.

Fig. 8-23 Fin Tubes Used in Waterlube Boiler. Source: Nooter/Eriksen
Fig. 8-24 HRSG Flow Diagram of Monotube Coil Design. Source: Clayton Industries

As in conventional-fired boilers, HRSGs can include economizers downstream of the boiler section to preheat feedwater to near saturation temperature. Superheaters may also be used, performing much the same function in conventional-fired or unfired heat recovery boilers.

Figure 8-26 shows four HRSGs applied with twin-pack gas turbine systems. The twin-pack systems each feature two FT8 aeroderivative gas turbines, with a combined power output of about 50,000 kW The HRSGs are highly effective triple-pressure units that supply high-pressure steam at

Fig. 8-25 Five Section HRSG Featuring Monotube Coil Design. Source: Clayton Industries

1,005 psig/805°F (70 bar/429°C), intermediate-pressure steam at 190 psig/550°F (14 bar/288°C), and low-pressure steam at 24 psig/266°F (2.7 bar/130°C). Figure 8-27 shows a labeled cutaway illustration of one of these units. Features of this unit include carbon monoxide (CO) and selective catalytic reduction (SCR) catalysts, integral deaerator, induced draft fan, and stack damper.

The combined exhaust of the two gas turbines manifolded into a single HRSG can generate from 100,000 to 190,000 lbm (45,000 to 86,000 kg) per hour of steam, depending on ambient conditions. Supplementary firing can increase steam production by as much as 50%. Figure 8-28 shows expected steam generation performance curves for the twin-pack system based on a simple single-pressure level HRSG.

Once-Through hrsg

The once-through HRSG is an emerging technology that consists of one or more serpentine circuits encompassing the economizer, boiler, and superheater sections. This eliminates the need for steam drums, level controls, blow-down, bypass controls, recirculation systems, and active water treatment. Also, the HRSG can be operated dry. These HRSGs can be constructed of a corrosion resistant material that permits lower stack temperatures.

Dry operation of the heat recovery boiler at turbine exhaust temperature is necessary for systems without a gas bypass to permit gas turbine operation whenever the boiler is inoperative. Some designs use carbon steel tubes and are rated at up to 6,000 hours dry operation at 900°F (482°C). Dry operation is possible for longer periods at reduced temperature.

Figure 8-29 is a flow diagram of a once-through steam generator showing the temperatures of the exhaust gas and

Fig. 8-26 Four Triple-Pressure HRSG Applied with Twin-Pack Gas Turbines. Source: Nooter/Eriksen

the water/steam as they progress through the unit. Water and steam flow through continuous serpentine tube circuits that are countercurrent to the hot gas flow.

Figure 8-30 depicts a representative boiler performance profile for a large gas turbine application featuring steam injection. The plot shows gas and waterside temperatures, relative heat duties, and selective catalytic reduction (SCR) operating windows. Boiler duty sections are shown with areas in proportion to their heat duty.

Supplementary-Fired Systems

Supplementary firing is the use of a burner in the sides of the duct upstream of

Hrsg Burner

Fig. 8-28 Expected Steam Generation Performance Curves for FT 8 Twin Pack System with Single-Pressure HRSG. Source: United Technologies Turbo Power Division

Fig. 8-27 Labeled Cutaway Illustration of Triple-Pressure HRSG with Integral Deaerator and CO and SCR Catalysts. Source: Nooter/Eriksen

Fig. 8-28 Expected Steam Generation Performance Curves for FT 8 Twin Pack System with Single-Pressure HRSG. Source: United Technologies Turbo Power Division

Super- Dry-Out Heater Zone

Pre-Heater

Steam Header

Evaporator Finned Tubes

Evaporator Finned Tubes

Hot Gas

Feedwater Header

Exhaust

982C 815C 607C 413C 121C (1800 F) (1500°F) (1125°F) (775 F) (250F)

Fig. 8-30 HRSG Temperature Profile for Gas Turbine STIG-Cycle Application. Source: Cogen Designs, Inc.

Fig. 8-29 Once-Through Steam Generator Flow Diagram. Source: Solar Turbines

Fig. 8-30 HRSG Temperature Profile for Gas Turbine STIG-Cycle Application. Source: Cogen Designs, Inc.

Fig. 8-31 225 MMBtu/h In-Line Duct Burner Applied for Supplementary Firing of Gas Turbine Exhaust. Source: Coen Company

the HRSG to raise the temperature of the entering gas stream. This is most commonly applied in gas turbine applications where the oxygen-rich (15 to 18%) exhaust can provide efficient combustion. Supplementary firing can also be applied with certain reciprocating engines, though on a more limited basis due to a lower oxygen content in the exhaust. The typical exhaust temperature from a turbine is in the range of 875 to 1,000°F (470 to 540°C). Duct firing can increase the temperature significantly, allowing increased steam production and higher temperatures and pressures. Figure 8-31 shows a 225 million Btu (237,375 MJ) per hour in-line natural gas duct burner. This unit has been applied for supplementary firing of exhaust from a 40 MW gas turbine into an HRSG.

Figure 8-32 shows the placement of the duct burner between the gas turbine and the HRSG. Efficiency is increased with supplemental firing because almost every Btu (kJ) of burner fuel is converted to useful thermal energy. This is because the mass flow and final temperature of the exhaust remain almost constant during supplementary firing. The increased temperature difference across the HRSG results in more heat recovered per lbm (kg) of exhaust gas.

With duct firing, the exhaust gas entering the HRSG can be elevated to as high as 1,800°F (980°C), given the typical operating limits of the ducting materials. Increased gas stream temperatures ranging up to nearly 3,000°F (1,650°C) can be achieved by adding a radiant heat section to the system. This section transfers radiant energy from the burner flame to water contained in a membrane on the outer wall of the gas duct. Elevated steam temperatures increase the efficiency of the steam cycle. However, steam temperatures exceeding 1,000 or 1,100°F (540 to 590°C) may require an upgrade in material quality, which will increase the cost of the equipment considerably.

Fig. 8-32 Illustration Showing Placement of Duct Burner Between Gas Turbine and HRSG. Source: Coen Company

Fig 8-33 shows a large horizontal-gas-flow HRSG with supplementary firing. Units with horizontal gas flow use vertical tubes connected to headers at the top and bottom, with natural circulation.

Hrsg Units
Fig. 8-33 Horizontal-Gas-Flow HRSG with Supplementary Duct Firing. Source: Combustion Engineering, Inc., Reprinted with permission from Combustion Fossil Power, 4th Ed., 1991
Hrsg Steam Flow

Fig. 8-34 Boiler Designed for Supplementary Firing in Conjunction with Gas-Turbine Combined Cycle. Source: Combustion Engineering, Inc., Reprinted with permission from Combustion Fossil Power, 4th Ed., 1991

Assuming a high oxygen content, in excess of 15%, a portion of the exhaust gas can alternatively be used for combustion air in a conventional-fired boiler. Exhaust gas temperatures can thereby be raised to about 3,000°F (1,650°C), greatly increasing steam production and allowing higher temperature and pressure. As the rate of supplementary firing increases above the level of available

Fig. 8-34 Boiler Designed for Supplementary Firing in Conjunction with Gas-Turbine Combined Cycle. Source: Combustion Engineering, Inc., Reprinted with permission from Combustion Fossil Power, 4th Ed., 1991

Assuming a high oxygen content, in excess of 15%, a portion of the exhaust gas can alternatively be used for combustion air in a conventional-fired boiler. Exhaust gas temperatures can thereby be raised to about 3,000°F (1,650°C), greatly increasing steam production and allowing higher temperature and pressure. As the rate of supplementary firing increases above the level of available

Fig. 8-35 Temperature Impact of Supplementary Firing Reduced Gas Turbine Load on HRSG. Source: Cogen Designs, Inc.

Temperature Distribution

Steam/Water

Temperature Distribution

Steam/Water

Fig. 8-36 Sectional Schematic of Unfired HRSG with Temperature Distribution Profile. Source: Nooter/Eriksen

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