Make your own free website on Tripod.com
HOW SIMULATION CAN IMPROVE HRSG EFFICIENCYby V.Ganapathy

The example shows how one can use the HRSG simulation methods to improve the efficiency of a system.
problem: A gas turbine HRSG is required to generate a desired quantity of High Pressure steam and maximize the Low pressure steam. The question is what should be the HRSG configuration.
data:
exhaust  flow=500,000 lb/h
exhaust temperature=900 F
gas analysis:% vol CO2=3,h2O=7,N2=75,O2=15
HP steam: 31,500 lb/h at 600 psig,600 F
LP steam: maximum in the unfired mode at 150 psig sat.
feed water=230 F
assume heat loss=1 %
Solution:
The HRSGS program was used to simulate different configurations. Two are presented below. In Option a,there are three modules. HP superheater with evaporator,LP evaporator,follwed by the common economizer.The HP pinch point was selected to give the desired steam flow,while low pinch and approach points were used for the LP section to maximize the LP steam flow. In option b,the HP section has its own superheater,evaporator and economizer,followed by the LP section with its own evaporator and economizer. Tables a and b show the results from the program.
 

Table showing results of Option a
surface gas in gas out wat in stm out duty press flow % stm pinch approch
suph 900 879 491 600 2.72 615 31759 100 - -
evap 1 879 671 351 491 28.00 630 31759 100 180 140
evap 2 671 381 351 366 37.93 165 43453 100 15 15
econ 381 306 230 351 9.51 650 76716 0 - -
[units: temperatures-F,duty-MM Btu/h,pressure-psia,flow-lb/h]

Table showing results of Option b
surface gas in gas out wat in stm out duty press flow % stm pinch approch
suph 900 879 491 600 2.70 615 31468 100 - -
evap 1 879 706 476 491 23.36 630 31468 100 215 15
eco 1 706 642 230 476 8.38 640 32097 0 - -
evap 2 642 381 351 366 34.09 165 39050 100 15 15
eco 2 381 342 230 351 4.95 175 39831 0 - -

ANALYSIS
It can be seen that in option a,we have a larger heat sink in the economizer feeding the two modules. This economizer handles the water flow equivalent of the two evaporators and hence is able to pull down the gas temperature from LP evaporator exit to a much lower value.In option b,the heat sink is not as large,as the economizer handles only the LP flow.Hence the exit gas temperature in option a is lower and thus more energy is recovered. Thus using the HRSGS program,one can simulate different configurations,modules and rearrange their locations to maximize energy recovery.

Click here for more information on HRSGS program
V.Ganapathy's Home page
Generating steam efficiently in cogeneration plants
Understanding HRSG temperature Profiles