HRSG SIMULATION
What is HRSG Simulation Knowing gas flow,temperature & analysis and steam parameters,establish HRSG temperature profiles,duty and steam flow. This is the design case,where for each surface we solve for:
UA=Q/ T In the off-design case,we know the new gas flow & temperature.It is desired to obtain the temperature profiles and steam flows. Calculation is tedious as surface area is indirectly known. Correct (UA) for effects of gas flow,temperature and analysis and solve for: Q=(UA)c T In typical design,we compute U and then A for each surface.In sim imulation ulation we we comput compute e (U (UA A) and hence hence there there is no need to to phys physical ically ly the HRSG in terms of tube size,fin density etc.Hence anyone familiar with heat balances can perform these studies. Consultants,plant engineers,project planners can use this method to evaluate HRSG performance without even knowing its size!They need not also contact a HRSG supplier!
Applications of HRSG Simulation •Obtain design temperature profiles •Obt btain ain off-design HR HRSG performance(unfired/fired) at different gas/steam conditions •Evaluate different gas turbines during initial project planning stages •Maximize energy recovery by modify odifying ing HRS HR SG config configurat uration ion •maximize energy recovery by adding secondary heat recovery such as deaerator,condensate heater
Ideal tool for cogen/combined cycle plant evaluation
•Evaluate field performance and relate it with HRSG performance guarantees
Saves time for consultants
•Write better HRSG specifications by knowing HRSG’s capabilities
No need to physically design the HRSG!
Limitation:Convective type surfaces with wit h no external external radiation
Pinch and Approach Points Note: This is the Design mode ..We cannot preselect pinch and approach points in offdesign mode!
pinch point point,F ,F bare finned
a. evap ty type pe
approach point point,F ,F
b.inlet gas temp,F 1200-1800
130-150
700-1200
80-130
30-60 10-30
40-70 10-40
Facts about Pinch and Approach Points •P inch and Approach points are selec selectted in unfired mode mode at “Design” gas flow,exhaust gas temperature.These are called “design” pinch and approach points •Once selected,they fall in place in other cases of gas flow/inlet gas temperature/steam conditions,whether unfired or fired. •P inch/ inch/approach approach points increa increas se with with inlet gas gas tem temperature perature •They cannot be arbitrarily selected -temperature cross can occur -low pinch point may not be physically feasible unless extended surfaces are used -affected by inlet gas temperature -economizer steaming is a concern ;suggest minimum approach at coldest ambient HRSG conditions -steam temperature can be achieved in fired conditions if it is achieved in unfired conditions • HR HRS SG surfaces are det determ ermined ined once once design pinch/ap pinch/appro proach ach point points are selected
Why HRSG exit gas temperatures cannot be assumed Exit gas temperature cannot be assumed as in conventional fired steam generators as temperature cross can occur.Looking at the superheater and evaporator,we have: WgxCpgx(tg1-tg3)=Ws(hso-hw2)
(1) Lookin ooking g at the ent entire ire HR HRS SG,
WgxCpgx(tg1-tg4)=Ws(hso-hw1)
(2) [blow down and heat los loss s neglected]
Dividing (1) by (3) and neglecting effect of variations in Cpg with temperature,we have: (tg1-tg3)/ (tg1-tg4)=(hso-hw2)/ (hso-hw1)=K For steam generation to occur and for a thermodynamically feasible temperature profile,two conditions must be met: If pinch and approach points are arbitrarily selected,one of these may not be met.
t g3>t s and t g4>t w1 . P inc inch= h=20F,a 20F,approa pproach= ch=15 F,g F,gas as inlet=900 F,feed water=230 F
(3)
Ps i g
s t m t em p ,F
s at t em p ,F sa
K
ex i t g as ,F
100
sat
338
.904
300
150
sat
366
.8704
313
250
sat
406
.8337
332
400
sat
448
.7895
353
400
600
450
.8063
367
600
sat
490
.7400
373
600
750
492
.7728
398
Temperature calculations Example 1: Determine HRSG exit gas temperature when inlet gas temperature=900 F,steam press=100 psig.use 20 F pinch and 15 F approach points.
Solution:K=0.904 sat temp=338 F.hence tg3=358F.tw2=323 F.(900-358)/(900-tg4)=0.904 or tg4=300 F Example Exampl e 2: 2: what is tg4 when steam press=600 psig and temp=750 F?
Solution:K=0.7728.sat temp=492 F.tw2=477F.tg3=512 F.(900-512)/(900-tg4)=0.7728 or tg4=398 F.So 300 F stack temperature is not feasible! Example 3:Why can’t we obtain 300 F at 600 psig,750 F steam?
Solution:K=0.7728 Let us compute tg3 from:(900-tg3)/(900-300)=0.7728 or tg3=436 F.This is called tem temperat perature ure cross cross!! Example Exampl e 4: 4: What should be done to get 300 F stack temperature?
Solution:Increase tg1 by firing.say tg1=1600 F.(1600-tg3)/(1600-300)=0.7728 or tg3=595 F and pinch=103 F. Example Exampl e 5: 5: If tg1=800F,what is tg4 at 100 psig sat?
Solution:(800-358)/(800-tg4)=0.904 or tg4=312 vs 300 F when tg1 was 900F. Example Exampl e 6: 6: With 1600 F gas inlet,can we use 20 F pinch?
Solution:(1600-512)/(1600-tg4)=0.7728 or tg4=192 F,which is below 230 F.Not feasible! That’s why pinch & approach points sh Tha sho ould not be se sellected in the fired mode!We have no idea in what range they can fall.
Temperature Profile Calculations The following procedure desc The scrribes “De Desi sig gn” temperature profile calculations for HRSGs. Assume pinch and approach points. Saturation temperature ts is known by assuming a pressure drop through the superheater. tg3=ts+pp and tw2=ts-ap Energy absorbed by sh +evap = Q12=WgxCpgx(tg1-tg3)xhl =Ws[(hso-hw2)+bd(hf -hw2)] .Ws is then computed. Q1=Superheater duty=Ws(hso-hv)=WgxCpgx(tg1-tg2) .Q1 and tg2 are thus obtained From above,Q2=(Q12-Q1)=evaporator duty is obtained. Economizer duty=Q3=Ws(1+bd)(hw2-hw1)=WgxCpg(tg3-tg4)xhl ; from this both Q3 and tg4 are obtained. hl=heat loss is on the order of 0.5 to 1 % bd=blow down varies from 1 to 7 % depending on feed water quality and boiler water conditions.
Simplified HRSG Performance Using the concept that firing in a HRSG is 100 % efficient,we can evaluate the performance in fired case for estimation purposes. Example:160,000 lb/h of exhaust at 950 F enters a HRSG to generate 600 psig steam at 750 F from 230 F water.Determine unfired steam production and also burner duty,firing temperature and exit gas temperature when generating 35,000 lb/h of steam at 600 psig,750 F. Solution :Using 25 F pinch and 20 F approach,compute energy absorbed by SH+evap=160,000x0.27x(950-517)x0.98=18.33 MM Btu/h=Ws(1378.9-455.4) or Ws=19,850 lb/h. Energy absorbed by HRSG=19,850x(1378.9-199.7)=23.4 MM Btu/h=160,000x0.98x0.268x(950-tg4) or tg4=393 F.
Fired case: Energy absorbed by steam=35000x(1378.9-199.7)=41.27 MM Btu/h. Additional fuel energy required=(41.27-23.4)=17.87 MM Btu/h. Oxygen consumed=17.87x106/(160000x58.4)=1.91 % So there is plenty of oxygen left. Firing temperature=17.87x106=160000x0.3x(T-950) or T=1322 F Exit gas temperature=1322-41.27x106 /(160000x.275x.98)=364 F
Design & Off-design calculations DESIGN
•unfired •establishes configuration •establishes surface areas indirectly •only one case •zero desuperheater spray •pinch and approach points selected OFF-DESIGN
•unfired/fired/fan mode/combination
•zero economizer steaming WHATIF STUDIES
•several cases possible
•steam pressure variations
•computes desuperheater spray
•firing temperature restrictions
•pinch and approach points computed
•effect of fuels
•economizer steaming possible
•performance testing •effect of gas turbine load •variations in ambient temperature
A simple example of simulation The Th e energy transf sfe erred to the evaporator is given by: Q=WgCp(T1-T2)=USΔ T=US (T (T1-T2)/ln[(T1-ts)/(T2-ts)] ; simplifying, ln[(T1-ts)/(T2-ts)]=US/WgC /WgCp p . In In a fire tube tube boiler,U boiler,U ∞ Wg0.8. For a water tube boiler,U ∞ Wg0.6 ,neglecting the effects of temperature. The Th en, Wg0.2ln[(T1-ts)/(T2-ts)]=K 1 for a fire tube boiler and Wg0.4ln[(T1-ts)/(T2-ts)]=K 2 for a water tube boiler
Example: A water tube boiler is designed to generate steam at 250 psig with 100,000 lb/h of flue gas at 1000 F.Exit gas temperature is 500 F.What is the exit gas temperature when 90,000 lb/h of flue gas enters the boiler at 970 F and steam pressure is 200 psig?
Solution: First compute K 2 using design conditions... 1000000.4ln[(1000-406)/(500-406)]=184.4=K 2 In the off-design case,900000.4ln[(970-388)/(T2388)]=184.4 or T 2=473 F.Duty and steam generation may be computed from this. [406 and 388 F are saturation temperatures corresponding to 250 and 200 psig respectively.]
Example of a HRSG simulation Example:140,000 lb/h of turbine exhaust gases at 980 F enter a HRSG generating sat steam at 200 psig.Determine the steam generation and temperature profiles if feed water temperature is 230 Fand blow down=5%. Solution : Let us choose a pinch point of 20F and approach of 15 F.Sat temperature=388F. Gas temperature leaving evaporator=408 F and water temperature entering it is 373 F.Evaporator duty=140000x.99x.27x(980-408)=21.4 Mm Btu/h. [ 1% heat loss and average specific heat of 0.27 Btu/lbF is assumed]
Enthalpy absorbed in evaporator=1199.3-345+.05x(362.2-345)=855.2 Btu/lb [1199.3,345 and 362.2 are enthalpies of sat steam,water entering evaporator and saturated water respectively]. Hence steam generation=21.4x106/855. /855.2= 2=25,000 lb/ lb/h h Economizer duty=25000x1.05x(345-198.5)=3.84 Mm Btu/h .gas temperature drop=3840000/(140000x.253x.99)=109 F.Hence exit gas temperature=408-109=299 F
Off-design Performance Simulate the HRSG performance with a 165,000 lb/h of gas flow at 880 F.Steam pressure =150 psig. Using the model for evaporators discussed elsewhere,ln[(980-388)/(408388)]=Kx140000-0.4 or K=387.6 Under new condi conditions tions:: ln[(880-366)/(T ln[(880-366)/(Tgg366)]=387x165000-0.4 =3. 3.1724 1724 or Tg= Tg=388 F.Evapora F.E vaporator tor duty= duty=165000x.99x. 165000x.99x.27x(88027x(880388)=21.7 MM Btu/h In order to determine the steam flow,the feed water temperature to evaporator must be known.Try 360 F.Then steam flow=21.7x106/[1195.7-332)+.05x(338.5332)]=25,110 lb/h. Economizer duty(assumed) Qa=25110x1.05x(332-198.5)=3.52MM Btu/h.Compute (US)d=Q/° T for economize zerr base sed d on desi sig gn conditions. Q=3.8 .84 4x106 ° T =[(408-3 -37 73)-( -(2 299-2 -23 30)]/ln[(69/35)]=50 F.( .(US) US)d d=3840000/50=76800. Co Corrrect this for off-design case. (US)p=(US)dx(165000/140000).65=85200.The effect of variations in gas temperature is minor and not considered. The energy transferred =(US)p x° T T.. Based on 360F water exit temperature,the economizer duty=3.52MM Btu/h and gas temperature drop=3520000/(165000x.99x.253)=85 F or exit gas =388-85=303 F.° T=[(303-2 -23 30)-( -(3 388-3 -35 50)]/ln[(73/28)]=47 F or or transf sfe erred duty=85200x47=4.0 .00 0 Mm Btu/h.As this does not match the assumed value of 360F and duty ,another iteration is required. It can be shown at 366 F,the balance is obtained.
Superheater performance P erform erformance ance of of a superh superheater eater is obt obtained ained from: from: Q=(US)p T ° T=[(Tg Tg1 1-t -ts2 s2))-( -(Tg Tg2 2ts2)] 2)]/ln[ /ln[(T (Tg1 g1-ts2)/(T -ts2)/(Tg2 g2-ts2)] -ts2)] (U (US S )p is obtained obtained fro from m design (US) (US ) values values as follow follows: s: (US)p=Wg 0.65FgK1(Ws/Wsd)0.15 where K 1=a constant obtained from design case = T Wg 0.65Fg Fg)) where Fg = (Cp 0.33k 0.67/
0.32
Basically we are correcting for the effects effect s of: 1.Gas 1.Gas flow 2. gas analysis analysis, gas temperat temperature ure and hence gas propert properties ies,, which is significant if the superheater operates say in unfired and fired modes. Similar constants K2,K3 may be evaluated for evaporator and economizer. Q1/(
.
Example:In design mode, gas flow=150,000 lb/h.Gas in=900F and leaving SH=842F. steam flow=18510 lb/h.steam pressure=450 psig. steam in=460F,out=650F.duty=2.34 MM Btu/h Cp=.273, =.0826 ,k=.029. F g=.2730.33x.0290.67/.08260.32=0.135.° T=[(842m
460)-(900-650)]/ln[(842-460)/ 460)-(900-650)]/ln[(84 2-460)/(900-650)] (900-650)] =31 311F.H 1F.Hence ence 6 0.65 K1=2.34x10 /(311x.135x150000 ) =24.1 Off-design :steam flow=18050 lb/h,gas flow=165000,gas in=840F.steam pressure=450 psig.Let exit steam temp=640F.Duty=18050x(1325-1204.4)=2.177MM Btu/h. Exit gas=840-2177000/165000/.99/.271=791F. Since gas temperatures are close, Use same Fg=0.135. (US)p=1650000.65x0.135x24.1x(18050/18510) 0.15=7974. ° T=[(840-6 -64 40)(791-460)]/ln[(840-640)-(791-460)]=260F. Hence Qt=7974x260=2.074MM Btu/h. This is close to the assumed value, else another iteration would be required. The NTU method may also be used here by using the new US term.
HRSG Performance Calculations P erform erformance ance may be obtained even if HRSG geometry is unknown using simulation concept.
Why are HRSGS inefficient? •Low steam/gas ratios •Low inlet gas temperatures(900 F vs 3300 F) •Temperature profiles depend on steam pressure and temperature •Higher the pressure,lower the steam generation •Higher the steam temperature,lower the steam generation (and higher the exit gas temperature)
Impro Imp rovin ving g HR HRSG SG Effi Effici cien ency cy approach point points s •Design with lower pinch and approach
•Use of secondary surf s urfa aces such suc h as conde cond ensate heater,hea heater ,heatt exch exchanger, anger,deae deaerr ato ator r onsid ide er mult mu ltip iple le pr pre ess ssur ure e HRSG •Cons
•Use sup suppleme plementary ntary firi fi ring ng •Opti ptimize mize tempe temperature rature profi pro files les by rea rearrangin rranging g surfaces
Improving HRSG performance
Bottom line li ne is to lowe low er the t he exi exitt ga g as temperature!
RESULTS OF A SIMPLE STUDY Dat a
b as e
975
Co n d h t r Heat exch 975 975
LP evap 975
Gas inlet temp,F Stack gas temp,F
374
310
323
297
Steam to turbine,Klb/h
80
80
80
80
Steam to deaerator
10250
1730
3400
0
F eed water temp,F
240
240
151
240
Electric power,kw
6528
6830
6770
6890
Gas flow=550,000 lb/h pinch=20 F approach=20F,make up=60 F,cond pr=2.5 in hg,steam at 620 psig,650F
HRSG simulation Knowing gas flow,temperature,analysis flow,temperature,analysis and steam ste am parameters,establish HRSG temperature profiles,duty and steam flows.In the design case,solve for: UA=Q/ T.In the off-design case knowing the new gas parameters,use the NTU method to establish performance using Q=(UA) T.Correct for UA using new gas parameters. parameters . We We do not have to compute U. Hence there the re is no need to know the tube size,fin details,HRSG mechanical data;anyone can perform such calculations and evaluate HRSG performance in unfired,fired modes,evaluate burner burner duty,optimize duty,optimize temperature profiles,predict part load performance,review performance different gas turbines...
HRSG Temperature profile
HP stage is followed followed by LP section. Not a very efficient efficient design
HRSG Temperature profile
Using common Economizer concept,we improve energy recovery
Why Steaming occurs in HRSG 100 % load Economizers
HRSG performance at Low Load
HRSG performance at at 40 % load. Note steaming in economizer and also the high exit gas temperature. temperature.
HRSG Simulation-unfired case
HRSG simulation-fired case
Effect of ambient temperature on HRSG performance
Multiplication factor on steam flow is 0.1
Evaluating HRSG performance
HRSG performance is evaluated at different different gas flow,exhaust flow,exhaust temperature conditions to see if the performance is reasonable. reasonable .
Evaluating HRSG performance
Design basis
Evaluating Operating Data
Note:Actual steam flow is 68,700 lb/h and exit gas temperature is 380 F,while it should have been about 364 F.Hence further evaluations are necessary to check if HRSG design is adequate. The gas flow was estimated based on steam duty and inlet/exit gas temperatures.
TWO OR SINGLE PRESSURE HRSG-case 1
We are trying to see if a 2 pressure HRSG is required. Customer wants about 40,000 kg/h,30 kg/cm2 steam and 3000 kg/h steam at 6 kg/cm2 in fired mode and about 3500 kg/h LP steam in unfired mode,which is taken off the drum and pressure reduced..
Two or Single pressure HRSG-case 2
HRSG makes 40,000 kg/h HP steam at 400 C and 30 kg/cm2 and 3000 kg/h steam is taken off the drum for process and pressure reduced to 6 kg/cm2
Two or Single pressure HRSG-case 3
Here we have a dual pressure HRSG.
Two or Single Pressure HRSG-case 4
Two or Single Pressure HRSG?case 5
Here we see what happens if LP steam pressure were 3 instead of 6 kg/cm2
Two or Single Pressure HRSG?-case 6
Note the stack gas temperature with lower LP steam pressure.
Summary -two or single pressure? OPTION
DESIGN A
DESIGN A
DESIGN B
DESIGN B
Case HP steam steam flo w,kg/h HP steam temp,C Process steam,kg/h Stack temp,C Firing t emp, emp,C C HP/LP HP/ LP st eam press Fuel,MW
Unfired 23434 370 3500 177 500 30/6 0
F ired 40000 400 3000 161 681 30/6 12.6
Unfired 24800 370 3500 164 500 30/6 0
F ired 40000 400 3000 163 681 30/6 12.6
DESIGN C Unfred 26340 370 3900 139 500 30/3 0
DESIGN C F ired 40000 400 3000 137 655 30/3 10.64
It may be seen that as long as the HRSG operates in the fired mode,the single pressure system has the same performance as the dual pressure unit when process LP steam is at 6 kg/cm2a,thus saving lot of expenditure, field costs,operational costs etc. A pressure reducing station replaces a complete LP evaporator,which could cost several hundred thousand dollars.If the LP steam pressure were different,then the outcome will be different.When process steam is at 3 kg/cm2a,then dual pressure looks attractive as seen in columns 5 and 6.HP steam is at 30 kg/cm2a.So there is an optimum LP pressure below which multiple pressure is justified. We cannot simply go for dual pressure without doing this analysis. If the HRSG runs more often in unfired mode,then a dual pressure may be warranted even at 6 kg/cm2
Effect of Exhaust gas analysis
A B OVE: TYPICAL AB TYPICA L EXHAUST EXHA UST GAS. BELOW: B ELOW: WITH STEAM INJ ECTION. Note the dif ference in steam generatio generation n
Multiple Pressure Level HRSG
GT exhaust vs Fan Operation
HRSG temperature profiles Q 1: Exhaust gas flow is 100,000 kg/h at 550 C. % volume co2=3,h2o=7,h2o=75,o2=15. co2=3,h2o=7 ,h2o=75,o2=15. Steam at 60 kg/cm2a and 450 C is required. Feed water is at 110 C and blow down=1 %. Using say 10 C pinch and approach, arrive at the HRSG temperature profiles and steam generation.
Q 2:Repeat the calculation at 10 kg/cm2a for saturated as well as superheated steam and discuss the findings. If pinch and approach increase by 5 C, 10 C, how much duty we lose and also the steam generation for Q 1 above? Table shows enthalpy in btu/lb vs temperature.
Temp,F
200
400
600
8 00
1000
enthalpy
34.98
86.19
138.7
192.48
247.56
If gas flow changes to 80,000 kg/h and steam pressure to 45 kg/cm2a in operation, what is the HRSG performance, duty, duty, steam temperature and ASME efficiency? ASME efficiency=energy absorbed by steam/water/fluids or duty/(gas flow x enthalpy+fuel input on LHV basis) basis )
EFFECT OF PART LOAD AND HIGH LOAD
What are the effects of part load operation of gas turbine on HRSG and effect of supplementary firing? Discuss.
MULTIPLE PRESSURE HRSG
HRSG performance at part loads
HRSG WITH REHEATER
Module Mo dule 1
2
3
4
5
6
7
8
9
10
SMAL SM ALLER LER HRSG HRSG – UN UNFI FIRE RED D CASE
UNFIRED AND FIRED HRSG PERFORMANCE-ALSTOM
SMAL SM ALLER LER HRSG HRSG – FI FIRE RED D CASE
ACTUA A CTUAL L DESIGN
RESULTS FOR MODULE 1
MULTIPL MULTIP L E PRESSURE HRSG
Mod 7 feeds 4. mod 8 feeds 7. mod 10 feeds 8 and 9 mod 5 fed by mod 9. mod 13 feeds 11 and 12
HRSG PERFORMANCE SUMMARY
