Optimizing HRSG Arrangement

Optimizing HRSG Arrangement

Often, dual-pressure steam is required, and the question arises, how should the HP and LP sections be arranged to maximize energy recovery? One option is HP section followed by the LP section as shown in Figure 5.18a. In this example (in British units), about 39,500 lb/h of HP steam at 800 psig is required, and the plant wants the maximum amount of LP steam at 100 psig. Exhaust gas flow is 300,000 lb/h at 1000°F. The pinch and approach points for the HP section were manipulated to give the required 39,500 lb/h of HP steam and low pinch and approach points were used to maximize the LP steam as required by the plant. Figure 5.18a shows that this arrangement gives only about 6200 lb/h of LP steam. The HP and LP sections have their own economizers in this option.

However, another option to maximize the LP steam is to use what is called the com- mon economizer concept (Figure 5.18b). That is, by increasing the water flow of the common

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

HRSG performance—Design case Project—opti

Units—British case—B Remarks - Amb. temp., °F= 60 Heat loss, % = 1 Gas temp. to HRSG C = 1,000 Gas flow, Lb/h = 300,000 % vol CO 2 = 3. H 2 O = 7. N 2 = 75. O 2 = 15. SO 2 =. ASME eff., % = 70.69 tot duty, MM Btu/h = 52.5 Surf. Gas temp. Wat./Stm. Duty

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

Pres.

°F Btu/h °F Sh.

in/out °F MMB/h Psia

750 Sh. Evap. Eco.

Evap. Eco.

(a) 230 HRSG performance—Design case

Project—opti Units—British case—common eco Remarks - Amb. temp., °F= 60 Heat loss, % = 1 Gas temp. to HRSG F = 1,000 Gas flow, kg/h = 300,000 % vol CO 2 = 3. H 2 O = 7. N 2 = 75. O 2 = 15. SO 2 =. ASME eff., % = 76.77 tot duty, MM Btu/h= 57.

Surf. Gas temp. Wat./Stm. Duty Pres. Flow Pstm. Pinch. Apprch. US Module no. in/out °F in/out °F MMB/h

Btu/h °F Sh.

Sh. Evap. Eco. Evap. Eco.

(a) HP section followed by LP section. (b) Use of common economizer concept can improve energy recovery.

HRSG Simulation 305

economizer, which feeds both the HP and LP sections, one increases the heat sink capacity at the exit of LP evaporator. This arrangement increases the surface area of the LP section and common economizer. However, considering the additional amount of LP steam or the 60°F lower stack gas temperature or the 4.5 MM Btu/h additional energy recovery, one can make a quick economic evaluation and determine if this option is worth it. Typically considering the life of the HRSG (30–40 years), this option will pay off in a short period as the cost of the HRSG will be only slightly more. The purpose of this example is to let process and plant engineers know that one can recover additional energy from turbine exhaust gases by rearranging heating surfaces (particularly in a multimodule configura- tion HRSG), and simulation is a good tool for this exercise. Note that HRSG suppliers may not have time to do such studies to optimize the HRSG configuration.