Cogeneration Plant Application
Cogeneration Plant Application
The steam parameters of combined cycle and cogeneration plants differ significantly: Steam parameters of combined cycle plant HRSGs have been standardized. HP, MP, and LP parameters are more or less universal for a given plant power output or gas turbine capacity. These HRSGs are generally unfired as discussed in Chapter 4. Fired HRSGs are more common in cogeneration plants. Steam demand is quite high in refineries, chemical plants, or fertilizer plants. Due to the high firing temperatures, single-pressure HRSGs are adequate to cool the exhaust gases to a reasonably low temperature. Condensate heater or
a deaerator may be added to maximize energy recovery. In a chemical or fertilizer plant, saturated steam is generated is generated in numerous
boilers and, to superheat the steam, the HRSG is often used. Similarly, saturated steam is taken from the drum for process heating purposes. In such plants, one also has to evaluate the HRSG performance when the export/ import steam is absent. This affects the steam temperature, and the steam temperature may be reduced significantly or large amount of desuperheating may be required as the steam flow is now reduced. HRSG simulation may
be used to determine the HRSG performance upfront.
Example 5.14
Exhaust gas flow from a gas turbine is 250,000 lb/h at 1000°F. Gas analysis % volume
is CO 2 = 3, H 2 O = 7, N 2 = 75, O 2 = 15. Superheated steam at 600 psia at 875°F and about
20,000 lb/h of saturated steam are required for process, which is taken off the drum. Predict the performance using 20°F pinch and approach points, 230°F feed water, and 1% blowdown and heat loss. Check what happens when the process steam is not used but sent through the superheater.
It is seen that for the normal performance with export steam (Figure 5.17a), the steam temperature is 875°F. However, when the export steam is not used (Figure 5.17b), the entire steam flow goes through the superheater. Hence, the steam temperature drops to 749°F. It is possible to oversize the superheater and ensure that the desired steam temperature is obtained when all the steam is passing through the superheater and then control the steam temperature when process steam is taken off the drum. This may require even a three-stage desuperheater depending on steam temperature requirements.
Example 5.15
Here is another simple example on simulation. A water tube boiler has inlet and exit gas temperatures of 700°C and 250°C with saturation temperature at 220°C and a gas flow of 100,000 kg/h. If gas flow increases by 20% to 120,000 kg/h at the same temperature of 700°C, how much will the duty increase? Assume gas specific heat is
0.27 kcal/kg °C.
302 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers
Solution
Use the following equation:
ln ( 700 − 220 )( / 250 − 220 ) = UA/ 100 000 0 27 0 99 , / . / . or UA = , 74 111
With 20% increase in gas flow, UA is 1.20 .65 × 74,111 = 83,435
ln ( 700 220 − )( /T − 220 ) = 83435 120000 27 99 2 6 / / . / . = .
or T = 256°C. Hence, ratio of new duty to earlier duty = 120,000 × 0.27 × 0.99 × (700 − 256)/ [100,000 × 0.27 × 0.99 × (700 − 250)] = 1.184 or 18.4%, slightly less than the 20% increase in mass flow.
HRSG performance—Design case
Sh. Evap. Eco. Project—study1 Units—British case—B Remarks
Amb. temp., °F = 60 Heat loss, % = 1 Gas temp. to HRSG F = 1,000 Gas flow, Lb/h = 250,000 % vol CO 2 = 3. H 2 O = 7. N 2 = 75. O 2 = 15. SO 2 =. ASME eff., % = 68.69 tot duty, MW Btu/h = 42.5
Surf. Gas temp. Wat./Stm. Duty
Flow Pstm. Pinch Apprch. US Module no. in/out °F in/out °F MMB/h
Pres.
°F Btu/h °F Sh.
506 359 230 458 8.55 625. 38,262 0 126,636 1 Gas–steam temperature profiles
(a) Design with process steam.
HRSG Simulation 303
HRSG performance—Off—Design case
Sh. Evap. Eco. Project—study1 Units—British case—B Remarks -
Amb. temp., °F= 60 Heat loss, % = 1 Gas temp. to HRSG F = 1,000 Gas flow, Lb/h = 250,000
% vol CO 2 = 3. H 2 O = 7. N 2 = 75. O 2 = 15. SO 2 =. ASME eff., % = 67.22 tot duty, MW Btu/h = 41.6 Surf. Gas temp. Wat./Stm. Duty
Pres. Flow Pstm. Pinch Apprch. US Module no. in/out °F in/out °F MMB/h Psia
°F Btu/h °F Sh. 1,000 909 503 749
FIGURE 5.17 (Continued)
(b) Performance without process steam.