3_Off-Design_Off-Design Performance of Integrated Waste-To-Energy, CCPP

7/31/2019 3_Off-Design_Off-Design Performance of Integrated Waste-To-Energy, CCPP

1/10


2/10

(1) Eliminating the superheater from the WTE boilerincreases reliability and decreases costs. In fact, thecorrosion of superheater tubes is typically a majorsource of forced outages and expensive maintenanceoperations.

(2) By generating most of the steam in the WTE section,lower mean temperature differences can be achievedin the HSRG. This implies lower irreversibilities

and thus higher efficiencies, although the correspond-ing increase of heat transfer areas gives somewhathigher capital costs.

(3) The single, relatively large steam turbine serving boththe WTE section and the CC can achieve efficienciesmuch higher than those typically achieved by thesteam turbines of WTE plants, which rarely gobeyond 5060 MW.

(4) Sharing a large fraction of the steam cycle (steam tur-bine, condenser, feedwater pumps, etc.), as well ascontrols, auxiliary systems, electric equipment andcivil works allows significant reductions of capital

and O&M costs.(5) Discharging the flue gases of the WTE plant and theCC through the same stack can significantly reducelocal environmental impact.

The optimal configuration of the integrated plant varieswith the operating parameters (evaporation pressure, max-imum steam temperature, minimum flue gas temperature,etc.) and the ratio between the combustion power suppliedby waste and that supplied by natural gas. In this paper weconsider two situations relevant to the new WTE plantbeing considered for the city of Milano, which will com-prise three air-cooled grate combustors fed with residual

MSW1 for a total LHV combustion power of 180 MW.For the CC section we have considered two different classesof heavy-duty gas turbines:

medium-scale, 70 MW class; large scale, 250 MW class.

As representative of the medium scale class we havereferred to the General Electric 6FA; very similar resultswould be obtained for the Siemens V64.3a. For the large

scale machine we have considered cycle parameters andperformances close to those of the General Electric 9FAand the Siemens V94.3a2.

2. On-design conditions

The new WTE plant being considered for the city ofMilano comprises a rather advanced steam cycle with evap-

oration pressure at 85 bar, dry flue gas clean-up with limeand active carbon and selective catalytic reduction forNO

xcontrol. Table 1 summarizes the main on-design fea-

tures of the stand-alone arrangement.

Nomenclature

A heat transfer areaCC combined cycleG reduced mass flow

GT gas turbineHP, IP, LP high, intermediate, low pressureHRSG heat recovery steam generator

LHV lower heating valueMSW municipal solid wasteRH reheat

U overall heat transfer coefficientWTE waste-to-energyw load coefficient

Table 2Main assumptions adopted for the on-design conditions of integratedWTE-CC plants

Gas turbine

Medium Large

Pressures (bar)Evaporator(s) 65 85/23/

4.3a

Bleed for air heater, de-aerator and district heating 2.4 2.4Deaerator 1.4 1.4Bleed for feedwater heater 1.1 1.1b

Bleed for air heater 0.6 0.6Condenser 0.07 0.07

Temperatures and DT (C)Max steam temperature (SH/RH) 550 535/560

DT at pinch point 10 10/10/15a

Minimum approach DT 25 20/15/10a

Subcooling at economizer outlet 25 2/2/10a

a Respectively for HP/IP/LP level.

b Only at off-design conditions, when the gas turbine is down.

1 Residual means that the waste fed to the grate combustor is what is

left after selective garbage collection and material recycling.

Table 1Basic on-design features of the stand-alone WTE plant

Thermal power input (MWLHV) 180.8LHVMSW (MJ/kg) 10Un-burnt fraction (% of thermal input) 1Thermal losses (% of thermal input) 1

Air pre-heat final temperature (C) 115O2 flue gases vol. content (%) 5

Exhaust gas recirculation (%) 15Gross electric power output (MW) 58.91Net electric power output (MW) 51.99Net electric efficiency (%) 28.76

S. Consonni, P. Silva / Applied Thermal Engineering 27 (2007) 712721 713


3/10

The main assumptions adopted to estimate the on-design heat and mass balances and the performances ofthe integrated plants are summarized in Table 2. The rela-tive high subcooling DT of 25 C for the medium-size GThas been chosen to reduce the chances of steaming at off-design conditions. In all cases the condenser is cooled with

water at 22 C made available by wet cooling towers.In the plant based on the medium scale gas turbine, thethermal capacity of the gases in the HRSG is insufficient toheat the whole flow of water; in addition to the de-aerator,we have considered therefore a feedwater heater fed withsteam bled from the steam turbine at 1.1 bar, placed in par-allel to the economizer of the HRSG. A more sophisticatedfeedwater heating line with additional regenerators andmore steam bleedings would increase electric power outputby 11.5 MW; its convenience should be verified by weigh-ing such added power output against added cost and com-plexity. In the plant based on the large scale gas turbine, alow-temperature regenerator has been considered only for

the off-design condition where the gas turbine is downand no heat source is available to heat the water aheadof the de-aerator; at on-design conditions, the regenerator

is excluded by shutting off the bleed that provides its steamsupply.

Fig. 1 illustrates the configuration and the on-designoperating conditions assumed for the integrated plantbased on the medium scale gas turbine. Almost all the heatrecoverable from the gas turbine exhausts is needed to

superheat the steam generated in the WTE section andpre-heat the water. As a consequence, multiple evaporationpressures or reheat are useless and we have assumed a sin-gle level at 65 bar to optimize the temperature profile in theHRSG and maximize power output. At the on-design con-ditions reported in Fig. 1 the production of steam in theevaporator of the HRSG is nearly zero. Nonetheless, anevaporator tube bank with adequate heat transfer area isnecessary to operate the plant properly when the WTEplant is down. As illustrated further, an acceptable off-design situation can be obtained by imposing an on-designpinch point DT of 10 C.

The configuration and the on-design operating condi-

tions of the integrated plant based on the large scale tur-bine are reported in Fig. 2. In this case the heatrecoverable from the gas turbine exhausts is much larger

Fig. 1. Configuration and on-design operating parameters for the plant based on the medium-scale GT.

714 S. Consonni, P. Silva / Applied Thermal Engineering 27 (2007) 712721


4/10


5/10


6/10

refrigerant flow rate. This strategy gives the lowestcondensation pressure and maximizes gross poweroutput. Alternatively, one could reduce the flow of

refrigerated water, thereby reducing auxiliary powerconsumption at the expense of higher condensationpressures. Which strategy gives the highest net power

Table 3Performance characteristics of integrated plants based on medium and large scale GT, at on-design and off-design conditions

Medium scale GT Large scale GTOn-design GT off WTE off On-design GT off GT off

Grate combustors in operation 3 3 0 3 3 1Wwaste MWLHV 180.80 180.80 180.80 180.8 60.27

Wnat gas MWLHV 216.76 216.76 680.93 msteam,WTE kg/s 92.17 61.75 98.71 61.37 21.59

msteam,HRSG kg/s 0.01 31.78 32.91, 6.46, 4.47 msteam,tot kg/s 92.18 61.75 31.78 131.62, 6.46, 4.47 61.37 21.59Psteam,EVA bar 65 65 25 85 85 65Tsteam,SH/RH C 550 433 550 535/560 430 429

Texhaust,WTE C 130 114 130 Texhaust,HRSG C 90 136 82 Wel,GT MWel 73.96 73.96 253.54 Wel,ST MWel 106.34 57.06 29.73 201.45 57.62 14.39

Waux MWel 7.21 6.22 1.93 8.71 6.30 2.53Wel,net MWel 173.09 50.84 101.76 446.28 51.32 11.86gnat gas % 55.86 46.95 57.90gMSW % 29.80 28.12 39.69 28.39 19.68

Fig. 3. Configuration and operating parameters of the medium-scale GT plant when the GT is down.



7/10

output depends on the off-design behaviour of thecooling towers, which however have been excludedfrom our analysis.

(10) The auxiliary power consumption of the waste treat-ment section (waste feeding lines, combustion airand exhaust gas recirculation fans, flue gas cleaning

system,. . .

) is proportional to the number of gratecombustors in operation. The one of the power sectionis the sum of four terms: (i) a constant for the GT (oilpumps, cabinet fans, control system, etc.); (ii) a con-stant for the steam cycle (lubrication systems, controls,etc.); (iii) feedwater pump consumption, calculated byassuming constant head and efficiency variations typ-ical of multistage centrifugal pumps; (iv) consumptionof the refrigeration system, assumed proportional tothe power discharged by the condenser.

These assumptions have been embodied in a computercode developed to estimate the off-design conditions of

the steam cycle starting from the on-design features esti-mated by GS. Off-design calculations can be numericallyinvolved because the solution is often very far from theon-design conditions.

5. Results

The results for the three off-design cases selected here arereported in Table 3 and Figs. 35. The table reports twomarginal efficiencies that are helpful in appreciating theactual benefits brought about by integrating the WTE sec-

tion and the CC:

natural gas efficiency gnat gas, i.e. the ratio between (i)extra power generated by the integrated plant withrespect to the power of the conventional WTE plant(51.99 MW, see Table 1) and (ii) combustion power ofnatural gas;

MSW efficiency gMSW, i.e. the ratio between (i) extrapower generated by the integrated plant with respectto a CC fed by the same amount of natural gas andnet efficiency 55% and (ii) combustion power of MSW.

To appreciate the variations occurring in the boilers and

the steam turbine, Figs. 6 and 7 compare the on-design andoff-design temperature profiles in the HRSG and the boilerof the WTE section, while Fig. 8 compares the steam tur-bine expansion lines.

Fig. 4. Configuration and operating parameters of the medium-scale GT plant when the WTE is down.



8/10

When the gas turbine is down, the system behaves andreaches performances very close to the ones of the stand-

alone WTE plant. In the extreme situation where both

the large gas turbine and two grate combustors are down,net electric efficiency is close to 20%, about 8% points

below the one of the fully-fired, stand-alone WTE plant.

Fig. 5. Configuration and operating parameters for the large-scale GT plant when the GT is down and only one (out of three) grate combustor isoperated.

1125

130

297

283

110

270

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Thermal Power [%]

Tem

perature[C]

flue gas

water / steam

1119

300

114

470

650

281 281

433

110

216

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Thermal Power [%]

Tem

perature[C]

flue gas

water / steam

Fig. 6. Temperature profiles of water/steam and flue gas in the WTE boiler for on-design (a) and off-design (b) operation of the plant based on themedium-scale GT.



9/10

The temperature of the flue gases at the outlet of the boileris significantly lower than that on-design ($115 C vs.130 C). Should this be incompatible with the flue gas treat-ment system, the water temperature at the inlet of the econ-omizer should be increased, e.g. by recycling warm watertaken further downstream.

When the WTE is down, a desuperheater must be placedacross superheater tube banks to limit the steam tempera-ture to the maximum value of 550 C; as shown in Fig. 7,

the temperature drop across this superheater is very sub-

stantial. The significant decrease of evaporation pressures

would cause steaming at the end of the economizers; toavoid this, the water pressure must be kept much abovethe drum pressure by valves3 placed at the economizer out-let. Also, for the economizer ahead of the de-aerator a by-pass is needed to keep the water temperature low enough toinsure proper de-aeration.

6. Conclusions

The integration of a WTE plant and a natural gas firedCC generates relevant technological, performance, envi-ronmental and economic advantages but poses significant

off-design issues. The analysis presented in this paper illus-trates suitable technical solutions and control strategies forplants where a 180 MWLHV WTE section is integrated witheither a medium scale or a large scale CC.

Results give a sense of the behaviour of the system underthe most severe conditions, i.e. when either the gas turbineor the WTE sections are down. When the gas turbine isdown, the net electric efficiency of the WTE section is lessthan one percentage point lower than the efficiency of thestand-alone WTE plant. When the WTE section is down,the efficiency of the CC is much below that of a stand aloneCC: about 8% points for the plant based on the medium-scale gas turbine. These performances appear most conge-

nial to what is likely to be the operational strategy of theseplants, i.e. paramount priority to waste treatment and CCdispatched according to the requirements of the nationalgrid.

Acknowledgements

This work has been carried out within a research pro-gram sponsored by CESI SpA. The support of CESI, in

90

128119

291

318

611

281

550

109

256

60

99

0

100

200

300

400

500

600

700

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Thermal Power [%]

Temp

erature[C]

546

611

154

192

287

461

136

550

321

219

109

60

0

100

200

300

400

500

600

700

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Thermal Power [%]

Temp

erature[C]

water / steam

flue gas

water / steam

flue gas

desuperheating

Fig. 7. Temperature profiles of water/steam and flue gas in the HRSG for on-design (a) and off-design (b) operation of the plant based on the medium-scale GT.

2200

2400

2600

2800

3000

3200

3400

3600

3800

6.4 6.6 6.8 7 7.2 7.4 7.6 7.8 8 8.2

Entropy [kJ/kgK]

Enthalpy[kJ/kg] on-design

off-design

P=61.1 bar

P=0.07 bar

P=0.76 bar

P=1.6 bar

P=2.4 bar

P=0.05 bar

2200

2400

2600

2800

3000

3200

3400

3600

3800

6.8 7 7.2 7.4 7.6 7.8 8 8.2

Entropy [kJ/kgK]

Enthalpy[kJ/kg] P=61.1 bar

P= 21.49 bar

P=2.4 bar

P=0.69 bar

P=1.6 bar

P=0.042 bar

P=0.07 bar

off-design

on-design

Fig. 8. Comparison between the design and off-design expansion lines of

the steam turbine when the GT is down (a) and when the WTE section isdown (b). Both figures refer to the plant based on the medium-scale GT.

3 Stellite valves with multiple flashes are often installed downstream ofeconomizers and can accept some steaming. Economizer banks are

designed for the shut-off pressure of the feedwater pump.



10/10

particular the constructive discussions with Dr. M. DeCarli, is gratefully acknowledged.

References

[1] M. De Carli, S. Consonni, G. Lozza, E. Macchi, D. Salimbeni,Evaluation of an integrated power plant for the city of Milan

consisting of a combined cycle and a waste-to-energy system, paperID-92, PowerGen Europe 2005, Milan, 2830 June 2005.

[2] S. Consonni, P. Silva, S. Alquati, A. Mugnaini, F. Begnis, Cicli ibriditurbina a gas inceneritori di rifiuti per generazione di energiaelettrica. Relazione 2: varianti per sistemi basati su turbina a gas ditaglia intermedia e prestazioni fuori progetto, Final report of researchcontract commissioned by CESI to the Department of EnergyEngineering of Politecnico di Milano, Apr. 30, 2005 (in Italian).

[3] T. Wiekmeijer, Improvements in Incinerators by Means of GasTurbine Based Cogen Systems, ASME paper 90-GT-180, 1990.

[4] H. Haneda, Efficiency improvement options for municipal waste-fired power generation recent development activities in Japan: a

review, Proc. Inst. Mech. Eng. Part A: J. Power Energy 209 (1995)81100.

[5] M.A. Korobitsyn, P. Jellema, G. Hirs, Possibilities for gas turbineand waste incinerator integration, Energy 24 (1999) 783793.

[6] S. Consonni, Combined Cycles for High Performance, Low Cost,Low Environmental Impact, Waste-to-Energy Systems, ASME paper2000-GT-24, 2000.

[7] S. Consonni, M. Ferrari, T. Greco Impianti ibridi RSU/combustibilefossile. Configurazioni e prestazioni, La Termotecnica, December2000, pp. 7785 (in Italian).

[8] P. Chiesa, S. Consonni, G. Lozza, E. Macchi, Predicting the UltimatePerformance of Advanced Power Cycles Based on Very High Tem-perature Gas Turbine Engines, ASME Paper 93-GT-223, 1993.

[9] E. Macchi, S. Consonni, G. Lozza, P. Chiesa, An assessment of thethermodynamic performance of mixed gassteam cycles, J. Eng. GasTurbines Power 117 (1995) 489508.

[10] S. Consonni, G. Lozza, E. Macchi, Turbomachinery and Off-DesignAspects in Steam-Injected Gas Cycles, in: Proc. 23rd IECEC (Denver,CO, August 1988), ASME, NY, pp. 99108.


Documents

3_Off-Design_Off-Design Performance of Integrated Waste-To-Energy, CCPP