Single- or Multiple-Pressure HRSG

Single- or Multiple-Pressure HRSG

When steam at dual pressure is required in cogeneration projects, often consultants assume that a dual-pressure HRSG is the way to go without doing any analysis; they even develop specifications around this multipressure design, which is more expensive than

HRSG Simulation 287

TABLE 5.9

Physical Design of HRSG at 550°C General data

Exhaust gas flow

40.0 kg/cm 2 g Exhaust gas temp.

kg/h

Steam pressure

463 ±5°C Exhaust gas pressure

±5°C

Steam temp.

15,367 kg/h Heat loss

1.00 kg/cm 2 a Steam flow

Feed water temp.

Process data Gas temp. in, ±5°C

Gas temp. out, ±5°C

Gas spht., kcal/kg °C

Duty, MM kcal/h

Surface area, m 2 268

Gas press. drop, mm wc

Foul factor, gas

Steam side

Steam press., kg/cm 2 g 41 42 42

Steam flow, kg/h

Fluid temp. in, °C

Fluid temp. out, ±5°C

246 Press. drop, kg/cm 2 1.53 0.00 0.52

Foul factor, fluid

a single-pressure HRSG [3]. Sometimes it is possible to meet the specific steam needs using

a single-pressure HRSG. The controls and instrumentation are also simpler than for a multiple-pressure unit. Here is an example of how simulation process helps one arrive at the HRSG configuration when there is a demand for dual-pressure steam.

Example 5.10

A cogeneration project requires 100,000 kg/h of steam at 350°C and at 40 kg/cm 2 g. Gas turbine exhaust = 453,000 kg/h at 540°C with an analysis of % volume CO 2 = 3, H 2 O = 7, N 2 = 75, O 2 = 15. It also needs 7000 kg/h of saturated steam at 10 kg/cm 2 g for process. The

consultant is suggesting a multiple-pressure HRSG with a high-pressure module consist- ing of a superheater, evaporator, and economizer followed by a low-pressure evaporator and a common economizer that feeds both the modules to optimize energy recovery.

The HRSG is first simulated in the dual-pressure unfired mode to generate about 8000 kg/h LP steam and in the fired mode generates close to 7000 kg/h LP steam. In the unfired mode, the exit gas temperature is 161°C. In the fired mode, the burner fuel input is 25.9 MW and exit gas temperature is 155°C. The plant is going to operate most of the time in the fired mode only. Results are shown in Figure 5.10a and b.

An HRSG supplier who is more resourceful comes up with the suggestion of a single- pressure HRSG as shown later. He wants the plant to take off steam from the drum and pressure-reduce it. The degree of superheat is about 5°C, and hence, this is a less expen- sive option. His simulation results are shown in Figure 5.11a and b.

The exit gas temperature is about 149°C in the fired mode. The high pressure (HP) steam generated is 105,072 kg/h, and of this, 98,000 kg/h is saturated steam is taken to the superheater, and the desuperheater spray used compensates for the total steam

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

TABLE 5.10

Off-Design Performance Matching Field Data General Data

Exhaust gas flow

35.00 kg/cm 2 g Exhaust gas temp.

kg/h

Steam pressure

442 ±5°C Exhaust gas pressure

±5°C

Steam temp.

11,038 kg/h Heat loss

1.00 kg/cm 2 a Steam flow

Feed water temp. 120 ±5°C

Process Data Gas temp. in, ±5°C

Gas temp. out, ±5°C

Gas spht., kcal/kg °C

Duty, MM kcal/h

Surface area, m 2 268

Gas press. drop, mm wc

Foul factor, gas

Steam side

Steam press., kg/cm 2 g 36 36 37

Steam flow, kg/h

Fluid temp. in, °C

Fluid temp. out, ±5°C

244 Press. drop, kg/cm 2 0.9 0.0 0.3

Foul factor, fluid

Spray, kg/h Geometry

Tube OD

Tube ID

Fins/in. or fins/m

Fin height

Fin thickness

Fin width

Fin conductivity

Tubes/row

Number of rows deep

Transverse pitch

Longitudinal pitch

Parl = 0. countr = 1

of 100,000 kg/h. (105,072 − 98,000) = 7,072 kg/h of steam is taken from the drum and pressure-reduced for the LP steam process.

The fuel input is even lower, 25 MW. In the fired mode, the exit gas temperature is lower. Hence, the plant may review this option seriously. If we add up the total surface areas (US values), the single-pressure HRSG has a total US value of 775,855 in the unfired mode (for comparison, unfired mode US is used), while the US value in multiple-pressure design is 604,573. With finned heating surfaces, this difference will not result in a more expensive design. A few more rows of finned tubes may

HRSG Simulation 289

be required in the economizer and the evaporator. However, ductwork, piping, con- trols, valves, and instruments will be less expensive; the LP steam drum is also eliminated.

The purpose of this exercise is to show that plants should not jump to the conclusion that if steam at dual pressure is required, then dual-pressure HRSG is the only option. Now it is likely that there are some situations where multiple-pressure steam generation is the right thing to do. For example, if the plant wanted 100,000 kg/h HP steam and maximum amount of LP steam, then dual-pressure HRSG is the way to go as it is more efficient than a single-pressure HRSG.

HRSG performance —Design case

Sh. Evap. Eco. Evap.

Eco.

Project—MM1 Units—Metric case—B Remarks - Amb. temp., °C = 30 Heat loss, % = 1 Gas temp. to HRSG C = 540 Gas flow, kg/h = 453,000

% vol CO 2 = 3. H 2 O = 7. N 2 = 75. O 2 = 15. SO 2 =. ASME eff., % = 72.54 tot duty, MW= 52.8 Surf. Gas temp. Wat./Stm. Duty Pres.

Flow Pstm. Pinch Apprch. US Module no. in/out °C

in/out °C MW kg/cm 2 a kg/h

°C

°C

kcal/h °C

3 Gas–steam temperature profiles 540

Econ 3 (a)

(a) Dual-pressure HRSG with process steam. (Continued)

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

HRSG performance—Off—Design case

Sh. Evap. Evap. Eco.

Eco.

Project—MM1 Units—Metric case—B Remarks -

Amb. temp., °C = 20 Heat loss, % = 1 Gas temp. to HRSG C = 540 Gas flow, kg/h = 453,000

% vol CO 2 = 3. H 2 O = 7. N 2 = 75. O 2 = 15. SO 2 =. ASME eff., % = 79.77 tot duty, MW = 78.7 Surf. Gas temp. Wat./Stm. Duty Pres.

Flow Pstm. Pinch Apprch. US Module no. in/out °C

°C kcal/h °C Burn 540 705

in/out °C MW kg/cm 2 a kg/h

119,936 3 Stack gas flow = 454,873 % CO 2 = 3.7 H 2 O = 8.38 N 2 = 74.46 O 2 = 13.44 SO 2 =.

Fuel gas: vol % Methane = 97 Ethane = 2 Propane = 1

LHV - kcal/cv m = 105 LHV - kcal/kg = 11,910 aug air - kg/h = 0 705

Econ 3 (b)

FIGURE 5.10 (Continued)

(b) Dual-pressure HRSG with process steam in fired mode.

HRSG Simulation 291

Example 5.11

If LP steam required is 15,000 kg/h and HP steam is still 100,000 kg/h at 350°C, which option is better, the dual pressure or single pressure?

Analysis was performed using the simulation program, and results for single- pressure option are presented in Figure 5.12a and b. The single-pressure option is not efficient now. If we take off 15,000 kg/h of steam from the drum as before, 57,535 kg/h of steam at 350°C is generated, and the exit gas temperature is 166°C. In the fired mode, when 15,000 kg/h is taken off the drum, (112,564 – 15,000) of saturated steam is taken to the superheater, and if we add the 2,334 kg/h of spray, we obtain the desired final steam

HRSG performance—Design case

Sh. Evap. Eco. Project—MMG Units—Metric case—B Remarks -

Amb. temp., °C = 30 Heat loss, % = 1 Gas temp. to HRSG C = 540 Gas flow, kg/h = 453,000 % vol CO 2 = 3. H 2 O = 7. N 2 = 75. O 2 = 15. SO 2 =. ASME eff., % = 71.31 tot duty, MW= 51.9

Surf. Gas temp.

Flow Pstm. Pinch Apprch. US Module no. in/out °C

Wat./Stm. Duty Pres.

kcal/h °C Sh.

in/out °C MW kg/cm 2 a kg/h

168 105 247 12.53 43.2 71,412 0 338,983 1 Gas steam temperature profiles 540

Econ 1 (a)

(a) Unfired single-pressure HRSG with process steam. (Continued)

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

HRSG performance—Off—Design case

Sh. Evap. Eco. Project—MMG Units—Metric case—B Remarks -

Amb. temp, °C = 30 Heat loss, % = 1 Gas temp. to HRSG C = 540 Gas flow, kg/h = 453,000 % vol CO 2 = 3. H 2 O = 7. N 2 = 75. O 2 = 15. SO 2 =. ASME eff., % = 80.23 tot duty, MW= 78.7

Surf. Gas temp.

Flow Pstm. Pinch Apprch. US Module no. in/out °C

Wat./Stm. Duty Pres.

kcal/h °C Burn 540 700

in/out °C MW kg/cm 2 a kg/h

Stack gas flow = 454,826 % CO 2 = 3.68 H 2 O = 8.35 N 2 = 74.47 O 2 = 13.48 SO 2 =.

Fuel gas: vol % Methane = 97 Ethane = 3

LHV - kcal/cv m = 105 LHV - kcal/kg = 11,922 aug air - kg/h = 0 700

FIGURE 5.11 (Continued)

(b) Single-pressure fired performance with process steam.

HRSG Simulation 293

of 100,000 kg/h. The fuel required for this is 29.67 MW. The multiple-pressure option as seen in Figure 5.13b requires 28.7 MW and exit gas temperature is 138°C. In the unfired mode also, we generate 61,800 kg/h compared to 57,535 kg/h steam at 350°C. Hence, as the ratio of LP steam flow to HP steam flow increases, dual-pressure HRSG will be more attractive. Simulation helps to see which option is better suited for the particular case. The surface area equivalent of the single-pressure HRSG is 785,988, while that of the dual-pressure HRSG is 741,258. One can convert the fuel cost differential to dollars and see the payback period. The exercise shows that sometimes a single-pressure HRSG can meet the steam needs of a plant and sometimes cannot, and one has to evaluate these options before suggesting a dual-pressure design.

HRSG performance—Design case

Sh. Evap. Eco. Project—GG Units—Metric case—B Remarks -

Amb. temp., °C = 30 Heat loss, % = 1 Gas temp. to HRSG C = 540 Gas flow, kg/h = 453,000 % vol CO 2 = 3. H 2 O = 7. N 2 = 75. O 2 = 15. SO 2 =. ASME eff., % = 71.67 tot duty, MW= 52.1

Surf. Gas temp.

Flow Pstm. Pinch Apprch. US Module no. in/out °C

Wat./Stm. Duty Pres.

kcal/h °C Sh.

in/out °C MW kg/cm 2 a kg/h

166 105 246 12.59 42.2 72,535 0 347,168 1 Gas–steam temperature profiles 540

(a) Single-pressure HRSG in unfired mode with 15k kg/hLP steam. (Continued)

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

HRSG performance—Off—Design case

Sh. Evap. Eco. Project—GF Units—Metric case—B Remarks -

Amb. temp., °C = 30 Heat loss, % = 1 Gas temp. to HRSG C = 540 Gas flow, kg/h = 453,000 % vol CO 2 = 3. H 2 O = 7. N 2 = 75. O 2 = 15. SO 2 =. ASME eff., % = 81.63 tot duty, MW= 83.6

Surf. Gas temp.

Flow Pstm. Pinch Apprch. US Module no. in/out °C

Wat./Stm. Duty Pres.

kcal/h °C Burn 540 727

in/out °C MW kg/cm 2 a kg/h

Stack gas flow = 455,141 % CO 2 = 3.8 H 2 O = 8.58 N 2 = 74.38 O 2 = 13.22 SO 2 =.

Fuel gas: vol % Methane = 97 Ethane = 3 LHV - kcal/cv m = 105 LHV - kcal/kg = 11,922 aug air - kg/h = 0

FIGURE 5.12 (Continued)

(b) Single-pressure fired with 15k kg/h process steam.

HRSG Simulation 295