Split Superheater Design

HRSG simulation may be used to evaluate the best option for a particular steam plant, and then based on the studies, one can suggest to the HRSG supplier or to the consultant what the HRSG configuration should look like. If simulation analysis is not done, it is doubtful if HRSG suppliers or consultant will come up with such solutions.

A plant had a peculiar problem. It did not have demineralized water for desuperheating steam, and it did not want to spend money on a more expensive sweet water condens- ing system (see Chapter 3). It wanted an HRSG that could meet its steam temperature

HRSG performance—Design case

Sh. Evap. Eco. Evap.

Eco.

Project—HG 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., % = 75.54 tot duty, MW = 55. Surf. Gas temp. Wat./Stm. Duty Pres.

Flow Pstm. Pinch Apprch. US Module no.

kcal/h °C Sh.

in/out °C in/out °C MW kg/cm 2 a kg/h

3 Gas–steam temperature profiles

(a) Dual-pressure HRSG in unfired mode with 15k kg/h process steam. (Continued)

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

HRSG performance—Off—Design case

Sh. Evap. Eco. Evap.

Eco.

Project—HG 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., % = 82.35 tot duty, MW = 83.5 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

Desh 694 694 323 296 0 41.6 2,975 Evap. 653 308 219 252 50.01 42.2 96,949

196,247 3 Stack gas flow = 455,069 % CO 2 = 3.78 H 2 O = 8.53 N 2 = 74.4 O 2 = 13.28 SO 2 =.

Fuel gas: vol % Methane = 97 Ethane = 3

LHV - kcal/cv m = 105 LHV - kcal/kg = 11,922 aug air - kg/h = 0 721

Econ 3 (b)

FIGURE 5.13 (Continued)

(b) Fired dual-pressure HRSG with 15k kg/h process steam.

HRSG Simulation 297

requirements in both unfired and fired modes without much deviation so that desuperheater spray could be avoided. When we have an HRSG that fires into a two-stage superheater with a desuperheater in between, the steam temperature will go up in the fired mode, and hence, desuperheater spray would be required to control the steam temperature. If demineralized water is unavailable, a sweet water condenser system, which is more expensive, will be required. The tube wall temperature will also be higher in this system as direct radiation from the flame will be significant in the first few rows of the superheater tubes as discussed in Appendix D.

By locating part of the superheater upstream of the burner, one can minimize or even avoid the need for desuperheating. The cross section of the final superheater and the one downstream of the burner may be different, and more ductwork and piping will be required. If we look at the surface areas between the two options (Figures 5.14 and 5.15),

Gas in

Sh.

Sh. Evap. Eco.

Burner

Project—ef Units—Metric case—B Remarks - Unﬁred case Amb. temp., °C = 30 Heat loss, % = 1 Gas temp. to HRSG C = 500 Gas ﬂow, kg/h = 400,000

% vol CO 2 = 3. H 2 O = 7. N 2 = 75.5 O 2 = 15.1 SO 2 =. ASME eﬀ., % = 60.2 tot duty, MW = 35.6 Surf.

Gas temp. Wat./Stm. Duty

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

Pres.

Flow

°C kcal/h °C Sh.

in/out °C

MW kg/cm 2 a kg/h

0 136,873 2 Project—ef Units—Metric case—B Remarks - Fired case

Amb. temp., °C = 30 Heat loss, % = 1 Gas temp. to HRSG C = 500 Gas ﬂow, kg/h = 400,000 % vol CO 2 = 2.4 H 2 O = 7. N 2 = 75.5 O 2 = 15.1 SO 2 =. ASME eﬀ., % = 76.26 tot duty, MW = 73.1

Surf. Gas temp. Wat./stm.

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

°C kcal/h °C Sh.

in/out °C

MW kg/cm 2 a kg/h

Stack gas flow = 402,652 % CO 2 = 3.53 H 2 O = 9.2 N 2 = 74.63 O 2 = 12.62 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 5.14

HRSG with split superheaters.

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

HRSG performance—Off—Design case Project–v g1 Units–METRIC Case–B Remarks Amb. temp., °C = 20 heat loss, % = 1 gas temp. to HRSG C = 500 gas flow, kg/h = 400,000 % vol CO 2 =3. H 2 O = 7. N 2 = 75. O 2 = 15. SO 2 =. ASME eff, % = 60.53 tot. duty-MW = 35.8

Apprch US Module Surf.

Gas Temp. Wat./Stm.

°C kcal/h °C No. Sh.

In/Out, °C In/Out, °C

kg/cm 2 a kg/h

9.46 55.0 48,770 0 141,415 1 HRSG performance—Off—Design case

PROJECT–v g1 Units–METRIC Case–B Remarks Amb temp., °C = 20 heat loss, % = 1 gas temp. to HRSG C = 500 gas flow, kg/h = 400,000 % vol CO 2 = 3. H 2 O = 7. N 2 = 75. O 2 = 15. SO 2 =. ASME eff, % = 76.51 Tot. duty, MW = 73.4

Apprch US Module Surf.

Gas Temp. Wat./Stm.

C C kcal/h °C No. Burn

In/Out, °C In/Out, °C

kg/cm 2 a kg/h

Stack gas flow = 402,653% CO 2 = 4.13, H 2 O = 9.21, N 2 = 74.13, O 2 = 12.51 SO 2 = 0.

Fuel gas: vol% Methane = 97, ethane = 2, propane = 1 LHV. kcal/cu m = 105 LHV. Kcal/kg = 11,910 aug air, kg/h = 0

FIGURE 5.15

Two-stage superheater downstream of burner.

the difference is not much. The (US) values of both options are close to each other. In order to see how much the steam temperature varies between the fired and unfired modes,

a simulation was performed.

Example 5.12

Exhaust gas flow from a gas turbine is 400,000 kg/h at 500°C. % Volume CO 2 = 2.4, H 2 O = 7, N 2 = 75.5, O 2 = 15.1. About 48,500 kg/h of steam at 54 kg/cm 2 a and at 355°C

± 5°C is required in the unfired mode and 100,000 kg/h of steam at the same temperature in the fired mode. Natural gas is the burner fuel, and feed water is at 109°C. See if the steam temperatures can be met in both modes of operation without control.

Solution

Using the simulation program, the arrangement shown in Figure 5.14 was simulated. The final superheater stands aloof and is located ahead of the burner. A module consisting of superheater, evaporator, and economizer generates superheated steam and feeds the final superheater. In the unfired mode, 312°C steam temperature was selected as the primary superheater exit temperature and the final superheater was simulated to give 359°C. In the fired mode, which is the off-design case, the steam temperature is 333°C and the final steam temperature is 358°C. Hence, this scheme is feasible. Chapter 4 on waste heat boilers shows the physical design of this HRSG. One can see that there is a good agreement between the simulation results and the actual

HRSG Simulation 299

results from the physical design. Hence, simulation offers an easy method to conceive complex HRSG arrangements without physically designing them. Once a concept or arrangement is arrived at through simulation, one can design the HRSG with the help of an HRSG supplier. It is doubtful if HRSG suppliers will be able to arrive at these options straightaway without results from such simulation studies unless they have had experience with similar units before.

When the burner is located ahead of a two-stage superheater with desuperheater in- between, the performance is shown in Figure 5.15. It may be seen that the firing temperature is higher as the entire boiler duty is handled downstream of the burner. This results in higher steam temperature, which is controlled by the desuperheater. Due to the higher firing temperature and direct radiation from the flame, it is likely that the tube wall temperature of the superheater is higher than in the previous case of split superheaters. The overall performance is nearly the same. The burner duty has also not changed. If desuperheater is not used in the case of downstream superheaters, the steam temperature will increase by about 50oC and so will the tube wall temperature.

Split Superheater Design