Optimizing Pinch and Approach Points in HRSGs

Chapter 5 explains the importance of pinch and approach points and how they are selected. Procedure to estimate the steam generation and gas–steam temperature profiles is also discussed. When trying to maximize steam generation, HRSG suppliers should use the lowest pinch and approach points avoiding the possibility of steaming at low loads; cost is also a factor. Plant engineers should also be able to evaluate proposals with different pinch and approach points and choose the best offering. Presented in the following text is an example of HRSG proposal from two different vendors.

Example 4.1

A gas turbine of capacity 7.5 MW has an exhaust gas flow of 100,000 kg/h at 500°C. Gas analysis is % volume CO 2 = 3, H 2 O = 7, N 2 = 75, and O 2 = 15. In the unfired mode,

the HRSG will generate about 15,000 kg/h, while 40,000 kg/h of saturated steam at

40 kg/cm 2 g is required by the plant, which will be achieved by supplementary firing;

the HRSG is likely to operate 50% of the time in unfired mode and 50% of the time in fired mode. The boiler performance in various cases is shown later. Vendor A has

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

TABLE 4.6

HRSG Designs with Low and High Pinch Points

Vendor B Data Case

Vendor A

Unfired Mode

Fired Mode

Unfired Mode Fired Mode

Gas flow, kg/h

100,000 100,000 Gas temp. in, °C

500 Firing temperature, °C

952 Gas temp. to eco, °C

302 Stack gas temp., °C

168 Gas press. drop, mm wc

121 Feed water in, °C

105 Water to eco, °C

192 Sat temperature, °C

251 Burner duty, MM kcal/h

0 13.0 0 14.0 Steam flow, kg/h

Evaporator surface, m 2 2,951

Economizer surface, m 2 2,129

Boiler duty, MM kcal/h 8.75 22.51 8.21 22.51 Pinch point, °C

5 13 20 51 Approach point, °C

9 62 10 59 No of rows deep, evap.

No of rows deep, econ.

Efficiency, % 68.8 87.5 64.6 84.2 Note:

50.8 × 44 mm tubes, 24 tubes/row, length of evaporator = 4 m, length of economizer tubes = 3.5 m. staggered arrangement, 101.8 mm pitch, 197 × 19 × 1.5 solid fins. Burner duty is on fuel lower heating value basis.

provided a bigger boiler with a lower pinch point, while vendor B has offered a smaller boiler that requires more fuel than vendor A to generate the same amount of steam. The plant engineer has to evaluate which offering is better (Table 4.6).

Let us use the following cost factors for evaluating the better option; cost of steam: $10/1000 kg; cost of electricity = 11 cents/kWh; cost of fuel = $1/MM kcal/h.

Vendor A has used a lower pinch point and has a bigger evaporator and economizer.

1. Advantage of A over B in steam generation in unfired mode = (15,565 − 14,587) × 4,000 × 10/1,000 = $39,120. 2. Advantage of A over B in fired mode: (14 − 13) × 1 × 4000 = $16,000. 3. Typically, 100 mm wc back pressure increase in the HRSG is equivalent to 1%

loss in gas turbine power output. Hence, advantage of B over A is (−107 − 121 + 159 + 176) = 107 or an average of 53.5 mm wc throughout the year = 7,500 × 0.01 ×

0.11 × 8,000 × 53.5/100 = $35,310. Hence, annual savings of design A over design B = 39,120 + 16,000 − 35,310 = $19,810.

One may check the difference in price and compute the payback period for vendor A. It is likely that vendor A is a better option though the initial cost may be slightly more. The payback may not be that long, considering that the cost of instruments, controls, casing, duct, and supporting steel may not be that much different and only the HRSG per se may differ in cost. The purpose of the exercise is to show how to evaluate the variables affecting the operating costs and see if a higher investment in lower pinch point is worth the money.

Waste Heat Boilers 199

Optimizing Pinch and Approach Points in HRSGs