Combining Solar Energy with Heat Recovery Systems

There are a few plants around the world using solar energy effectively. Solar energy may

be used to generate low- to medium-pressure steam. This may be superheated in waste heat boilers or steam generators fired by biomass or flue gas or gaseous fuels. When the solar energy is unavailable, the boiler has to generate additional steam. Boilers can be designed to handle such options. Figure 4.18 shows a scheme of a solar plant operating in conjunction with a waste heat recovery system that can be from biomass or gas turbine HRSG exhaust or any waste heat source.

While traditional solar power facilities need to employ expensive energy storage tech- niques to ensure continuous operation in all weather conditions, the heat recovery system or steam generator can eliminate that need. Whenever there is not enough direct sunlight to generate solar power, the facility’s heat recovery system can be brought on line enabling the continuous production of electricity.

Gas Turbine HRSGs

Gas turbine–based combined cycle and cogeneration plants (Figure 4.19a and b) are found in refineries, chemical plants, power plants, process industry, and cogeneration plants,

Waste Heat Boilers 185

Steam turbine

Steam—350°C

Economizer

Solar panels

Solar

Condenser

steam generator

Solar energy and heat recovery system (typical). generating steam for power and process or for both. The exhaust energy may also be

used for heating industrial heat transfer fluids such as therminol, glycol, and fuel oils. The unfired combined cycle plant with a gas turbine exhausting into a multiple-pressure reheat HRSG supplying steam to the steam turbine is the most efficient electric power generating system today, approaching 60% on lower heating value (LHV) basis. The cost of a combined cycle power plant in the range of 800 MW and above is $550–650/kW, while that of a coal-fired plant is $1200–1400/kW. Several improvements have been made in gas turbine technology such as developments in material technology to handle the high firing temperatures in the range of 1500°C. The HRSGs in combined cycle plants differ from those in cogeneration plants in the following ways:

1. The main steam pressure, reheat steam pressure, medium and low pressure, and temperatures are all optimized in combined cycle plants. In cogeneration plants, it varies from plant to plant depending on process steam demand and plant conditions.

2. Supplementary firing temperatures can vary depending on process steam needs

in cogeneration systems, while in combined cycles plants, HRSGs are generally unfired.

3. Steam may be imported into the HRSG superheater or saturated steam may be

exported out of the HRSG in cogeneration plants; in cogeneration plants, there can

be several heat recovery systems generating medium- to low-pressure saturated steam that has to be superheated, while some application may require saturated steam, which may be taken off the HRSG.

4. Each HRSG design in cogeneration plant is dependent on the needs of the particu- lar plant and steam parameters, and application can vary. The author is aware of a plant where a portion of turbine exhaust gas is used for drying wood chips.

Typical gas turbine combustor operates in the range of 1350°C–1400°C for metallurgical reasons. Hence, a large amount of compressed air is used to cool the flame, which in turn increases the exhaust gas flow. After expansion in the gas turbine, the exhaust gas at about 500°C–550°C and at 150–250 mm wc pressures enters the HRSG. It contains about 6–8% volume of water vapor, but if steam injection is used for gas turbine NO x control, the water

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

Fuel Steam turbine

12 bar/237°C steam Air

Gas turbine

100 bar/450°C

Steam 56 bar/380°C

HRSG

Water 70°C

Pump Deaerator

Steam to process Drum

Exhaust heat recovery

Engine cooling

(a)

Fuel Steam turbine

Gas turbine

Hot water

Chiller

Pump

Chilled water

HRSG To distillation unit

(b)

FIGURE 4.19

(a) Gas turbine and engine heat recovery system. (b) Cogeneration systems.

Waste Heat Boilers 187

6 R1, supp. firing with steam injection R2, derating with supp. firing

, MW R1

R3, derating with steam injection

Steam

To stack d

3.5 c to process

R3 R2

To gas turbine

ctrical power Ele

Steam output, kg/h Economizer

Burner

Evaporator

Gas turbine

Superheater

FIGURE 4.20

Cheng cycle system. vapor can go up to 10%–11%. In systems like the Cheng cycle (Figure 4.20), where signifi-

cant amount of superheated steam is injected in the gas turbine, the % volume of water vapor can go to about 22%–23%. This naturally impacts the gas specific heat and thermal conductivity and the gas–steam temperature profiles in the HRSG. The oxygen content of typical gas turbine exhaust is 14%–15% volume, and one can use this to add additional fuel in the duct burner or register burner as discussed in Chapter 1. Due to the large ratio of exhaust gas flow to steam in HRSGs, the HRSG cross section will be large though the steam generation is small. The cross section of an HRSG generating the same amount of steam as a steam generator will be about six times larger.

Another aspect of gas turbines is that the exhaust gas flow remains nearly constant with load variations, while the exhaust gas temperature changes. As the exhaust contains 14%– 15% volume of oxygen, supplementary firing can be carried to generate large amount of steam without the addition of air. Hence, this is an efficient means of generating additional steam as shown later. A comparison of various types of HRSGs is shown in Table 4.5.

Due to low inlet gas temperature and pinch point limitation as discussed in detail in Chapter 5, the exit gas temperature in an HRSG is a strong function of inlet gas temperature. Lower the inlet gas temperature, higher the exit gas temperature and vice versa in a

TABLE 4.5

Comparison of Unfired and Fired HRSGs

Item

Unfired

Supplementary Fired Furnace Fired

Gas inlet temp to HRSG, °C

900–1400 Gas to steam ratio

1.2–2.5 Burner type

Duct register Fuels fired

No burner

Duct burner

Oil–gas–solid Casing type

None

Gas distillate

Internal insulation

Internal insulation or Membrane wall

membrane wall

Circulation Natural, forced, once-through Natural, forced, once-through Natural Back pressure, mm wc

250–350 Configuration, pressure levels

Single Design

Single or multiple

Convective

Radiant furnace

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

single-pressure system. Hence, multipressure HRSGs are often required to cool the exhaust gas temperature. Supplementary firing increases the HRSG efficiency as discussed later. Also due to the low LMTD in unfired units, extended surfaces are required to make the HRSG compact in unfired and supplementary-fired units. In furnace-fired units due to the large amount of steam generated in the furnace and evaporator sections, the economizer acts as a large heat sink, and hence, single-pressure steam generation is adequate to cool the exhaust gases to 120°C–140°C levels. Fire tube boilers are rare in gas turbine but may be justified in very small gas turbine applications with exhaust gas flow less than 30,000 kg/h.

Natural, Forced Circulation, Once-Through Designs, and Special Applications

HRSGs are generally categorized according to the type of circulation system, which could

be natural, forced, or once-through as shown in Figure 4.21 a through d. Natural circulation units have vertical tubes and horizontal gas flow orientation, whereas forced circulation units have horizontal tubes and vertical gas flow orientation. Once-through can have either horizontal or vertical gas flow orientation. In natural circulation units, the difference in density between the colder, denser water in the downcomer pipes and the hotter, less dense steam–water mixture in the evaporator tubes enables the circulation process. CR can range from 10 to 30 depending on location of drum, system resistance, and steam pressure. Since finned tubes are used in HRSGs, the heat flux inside the tubes will be much higher than in plain tubes. For example, in 50.8 × 44 tubes with157 fins/in.,

15 mm high, 1.5 mm thick solid fins, the ratio of external to internal surface area is 9.64. If U = 50 kcal/m 2 h °C, the heat flux with 500°C gas temperature and 240°C fluid temperature will be 50 × (500 − 240) × 9.64 = 125,230 kcal/m 2 h (46,000 Btu/ft 2 h). In fired units, the heat flux could up to nearly double this value, and hence, one has to be concerned about DNB, particularly with horizontal tubes as discussed in Chapter 2 on furnaces and circulation. If the firing temperature exceeds 850°C, typically, a membrane wall furnace is used. Membrane wall furnace is perfect for any firing temperature. The problem of laying insulation, liners, and their maintenance is avoided. Then a few rows of plain tubes are used followed by the superheater if used. Then we have finned tubes with varying fin density to ensure that the heat flux gradually increases along the gas flow direction. A once-through steam generator looks like an economizer coil (Figure 4.21d). As water is converted into steam inside the tubes, the water should have zero solids; else, deposition of salts can occur inside tubes causing overheating of tubes. A once-through HRSG does not have a defined economizer, evaporator, or superheater regions. Slight variations can occur depending on gas inlet conditions. The single-point control for the OTSG is the feed water control valve. Based on inlet gas conditions and exit steam temperature, the water flow (and hence steam flow) is varied. As there is no drum, the water holdup is small compared to drum-type units, and hence, response time to load fluctuations is quick. The OTSG is designed to run dry, and hence, alloy 800 or 825 tubes are used. The high-grade tube material minimizes exfoliation concerns, which are likely with carbon steel and low-grade alloy tubes. (One may fill up the evaporator and economizer of natural circulation units with water and then start the unit. By ensuring a low gas inlet temperature to the superheater, the superheater is also protected well till steam starts generating in the evaporator; hence, carbon steel tubes are adequate for natural circulation unit evaporators and economizer.) When boiler tubes are heated, they form an oxide layer inside the tubes, and when cooler steam flows inside, the oxide particles are dislodged and carried away by steam and deposited in the steam turbine, a process called exfoliation. If this heating takes place for a long time and frequently, alonized or chromized tubes may be used for the superheater.

Waste Heat Boilers 189

(a)

To stack

Drum

Water in

SH steam Superheater

GT exhaust in

(b)

FIGURE 4.21

(a) Natural circulation gas turbine HRSG. (b) Forced circulation gas turbine HRSG. (Continued)

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

Evaporator 2 (c)

Water in

Steam out

(d)

FIGURE 4.21 (Continued)

OTSGs can be started up and shut down quickly due to the low mass of water and steel (water equivalent). On the flip side, the steam pressure decay is faster when the gas turbine trips than in designs having a steam drum and large water inventory. Absence of downcomer and riser piping makes the layout easier and lowers their cost of fabrication and erection. The two-phase pressure drop inside the tubes could be large and could be in the range of 8–12 bar depending on steam pressure and a significant operating cost.

The once-through steam generator (see Chapter 3) used in steam flooding applications generates 80% quality steam at high steam pressures using the energy from gas turbine exhaust as well. These units generate steam at 100–200 barg depending on the depth of oil

Waste Heat Boilers 191

field. Multiple streams are used in large HRSGs with flow control valve at the water inlet of each stream to ensure that two-phase instability problems with water–steam mixture do not arise. Eighty percent quality steam may also be generated using exhaust gases from gas turbines. The HRSG is also designed to ensure low heat flux inside the tubes by manipulating the fin geometry. Since many HRSGs use horizontal tubes, the allowable

heat flux is limited to 270,000 kcal/m 2 h (100,000 Btu/ft 2 h or 314 kW/m 2 ). The wet steam (80% quality) ensures that the salts in feed water such as sodium are soluble in the steam– water mixture and are not deposited inside the tubes and carried away with the steam– water mixture. To minimize acid or water dew point concerns, a heat exchanger to preheat the cold water is used as shown.

Figure 4.22a shows a plant with furnace-fired HRSGs, while Figure 4.22b shows the details of its construction. As discussed earlier, the convection sections follow the furnace, and finned tubes of varying density are used in the convection section to keep the tube wall and fin tip temperatures low. Downcomers are located at the rear as shown. The convection bank is followed by an economizer. Figure 4.22c shows a dual-pressure natural circulation HRSG in the field.

Another interesting application in gas turbine plants is the intercooler for compressed air used for combustion (Figure 4.23). Intercooling improves the efficiency of compression process in gas turbine compressors. Hot compressed air from the first stage of the compressor is cooled by feed water (typically high-pressure economizer water) before it is again sent back to the next stage of the gas turbine compressor and combustor. Since the air is at high pressure and the water also is at high pressures, say, 100–150 barg, the heat transfer coil should be placed inside a pressure vessel with high-pressure–high-temperature air, typically at 15–25 barg and at 400°C–450°C outside the tubes and the colder water inside the tubes in counter-flow direction. Finned tubes are used for the coil. Baffles are used to divert the pressurized air over the finned tube bundle and then exit the pressure vessel.

Fluid heaters are another important application of turbine exhaust. Industrial heat transfer fluids such as therminol, glycol, or even hot water are heated by turbine exhaust.

A finned coil is used in these applications. The coil looks like an economizer. As many of these fluids have limitation on heat flux and have a low tube-side heat transfer coefficient (compared to water), low fin density not exceeding 98–117 fins/m should be used on the heating surfaces.

Combining Solar Energy with Heat Recovery Systems