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This paper investigates the effects of site based parameters such as ambient temperature, humidity, altitude and heat transfer characteristic of a dual pressure heat recovery system on the performance of the combined cycle power plant within an equatorial environment. The bulk heat utilization and configuration of a dual pressure heat recovery system are investigated. It is observed that the heat system configuration play a vital role in optimizing the combined cycle overall performance, which has proportionality relationship with the operating ambient temperature and relative humidity of the gas turbine. The investigation is carried out within the ambient temperature range of 24 ℃ to 35 ℃, relative humidity of 60% to 80%, and a high level steam pressure of 60 bar to 110 bar. The results show that at 24 ℃ ambient temperature, the heat recovery system has the highest duty of 239.4 MW, the optimum combined cycle power output of 205.52 MW, and overall efficiency of 47.46%. It further indicates that as the ambient temperature increases at an average exhaust gas temperature of 530 ℃ and mass flow of 470 kg/s, the combined cycle power output and efficiency decrease by 15.5% and 13.7% respectively under the various considerations. This results from a drop in the air and exhaust mass flow as the values of the site parameters increase. The overall results indicate that decreasing the ambient temperature at optimum exhaust gas flow and temperature increases the heat recovery system heat duty performance, the steam generation, overall combined cycle power output and efficiency, which satisfies the research objective.

The growing need of energy in modern civilization has prompted the need to optimize energy sources globally. This can be enhanced through an effective parametric evaluation of exiting plant to minimize losses, and to understand the performance rate of such system. Although gas turbine is a very satisfactory means of producing mechanical power [

Heat recovery steam generator (HRSG) is the standard term used for a steam generator producing steam by cooling hot gases. Heat recovery system is obviously a very desirable energy source, since the product is available almost operating cost―free and increase the efficiency of the cycle in which it is placed, either for steam generation or for incremental power generation. So its performance and configuration have a great impact on the overall plant performance. We have two typical HRSG types, which are unﬁred and supplementary-ﬁred. For the unﬁred HRSGs, we have less recovery efficiency and steam temperatures than supplementary-ﬁred con- ﬁgurated HRSG [

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The objective of this investigation is to evaluate the effects of site based parameters and heat system characteristic on the overall combined cycle power plant efficiency and power output within the equatorial environment. The parameters used in the analysis include the ambient temperature, humidity, heat duty, log mean temperature difference (LMTD), heat transfer coefficient per unit area, steam pressure, exhaust gas temperature, and mass flow. The study area is the South-South zone of Nigeria which lies between latitudes 4˚N and 6˚N, and longitude 5˚E and 8˚E. The vegetation of the area is equatorial rain forest. There are basically two seasons―the wet (April to September) and the dry (October to March). However, rain fall throughout the year. The mean annual rainfall in the area is between 200 mm in the North and 400 mm in the South of the region. The mean daily temperature of the region varies slightly from 27˚C to 30˚C all the year round. The maximum and minimum temperatures are 40˚C and 20˚C respectively. The relative humidity varies between a minimum of 50% and a maximum of 90% [

The research methods adopted involved the collection of data from the plant station for 12 months as follows:

1) Direct monitoring and measurement from the control room through the human machine interface (HMI)

2) Direct reading of design parameter from engine manual.

3) Field study of the overall plant in wet and dry season.

4) Modeling energy based (thermodynamic) relations and equation for parameters that could not be directly measured.

5) Results from the modeling equations were analysis, discussed and conclusion were made.

The methodology adopted was designed to produce facts about the behavior of the plant, and to determine those factors that influence the combined cycle thermodynamic process within the equatorial environment. Hysys^{®} V8.7 software (Aspentech, USA) was used to process the temperature gradient across the various heat exchanging units of the HRSG at various ambient temperatures of the gas turbine. MATLAB 7.3^{®} (Mathworks USA) was used to evaluate the equations. Thermodynamics properties such as temperature, pressure, mass flow, heat flow, compression ratio and turbine inlet temperatures (TIT) are crucial in this research because their behavior affect every other parameter in the analysis.

In the data collection and treatment, the mean values of daily parameters were considered by the use of statistical method.

Modeling Relations and EquationsThe net power output (W_{net}) is the power generated by the generator and is given as

where W_{T} is the shaft work of the turbine and is given as

and W_{C} is the compressor work given as

where m_{g} is the mass of the product of combustion (kg/s), c_{pg} is the specific heat capacity of the product of combustion, T_{3} is the TIT, m_{a} is the mass of air (kg/s), c_{pa} is the specific heat capacity of air,

The humidity relation gives

where f is the relative humidity

The overall heat transfer coefficient U can be determined by the equation

where

f = Correction factor obtained from charts.

The Log Mean Temperature Difference (LMTD)

where T_{gi} is the exhaust gas temperature into the HRSG elements (˚C).

T_{ge} = Exhaust gas temperature out of the HRSG elements (˚C).

T_{wi} = Temperature of feed water/steam into the HRSG elements (˚C).

T_{we} = Temperature of feed water/steam out of the HRSG elements (˚C).

The Heat Duty of the HRSG heating surfaces are evaluated using the equations

where K signifies

where X signifies

The steam generation can be evaluated using the equations

The steam turbine work can be evaluated using the equation

The condensate from the condenser is extracted by the pump and is raised to the economizer pressure. The corresponding work is given by

Therefore the net power output for the steam turbine is given by

The efficiency for the steam turbine is evaluated using the equation

The overall thermal efficiency for the combined power plant is

The parameters in

S/NO | Parameter | Units | Design Data |
---|---|---|---|

1 | Gas Turbine Power Output | MW | 160 |

2 | Thermal Efficiency | % | 35.7 |

3 | Heat Rate (HR) | kJ/kWh | 10,084 |

4 | Compressor Ratio | 14:1 | |

5 | HRSG MCR (LP) | kg/s | 80 |

6 | HRSG MCR (HP) | kg/s | 220 |

7 | Steam Turbine Power Output | MW | 100 |

8 | Isentropic Efficiency | % | 89.06 |

9 | Thermal Efficiency | % | 30.25 |

10 | Condenser Pressure | bar | 0.35 |

Ambient Temp. ˚C | Compressor Exit Temp. ˚C | Exhaust Temp ˚C | Actual Power Output MW | Thermal Efficiency % | Power Drop (%) | Thermal Efficiency Drop (%) |
---|---|---|---|---|---|---|

24 | 319.2 | 547.62 | 148.4 | 30.55 | 7.25 | 14.43 |

25 | 332.5 | 554.62 | 148.0 | 29.78 | 7.50 | 16.58 |

26 | 345.8 | 561.62 | 147.2 | 29.01 | 8.00 | 18.74 |

27 | 359.1 | 568.62 | 145.1 | 28.24 | 9.31 | 20.89 |

28 | 372.4 | 575.62 | 143.2 | 27.51 | 10.50 | 22.94 |

29 | 385.7 | 582.62 | 140.3 | 26.70 | 12.31 | 25.21 |

30 | 399 | 589.62 | 133.1 | 25.93 | 16.81 | 27.37 |

33 | 412.3 | 596.62 | 132.3 | 25.16 | 17.31 | 29.52 |

35 | 425.6 | 603.62 | 131.5 | 24.39 | 17.81 | 31.68 |

HRSG Elements | Amb. Temp 24˚C, Humidity 60%, Exhaust Temp 545˚C | Amb. Temp 30˚C, Humidity 65%, Exhaust Temp 535˚C | Amb. Temp 33˚C, Humidity 70%, Exhaust Temp 500˚C | Amb. Temp 35˚C, Humidity 80%, Exhaust Temp 480˚C | ||||
---|---|---|---|---|---|---|---|---|

Heat Duty (kW) | LMTD | Heat Duty (kW) | LMTD | Heat Duty (kW) | LMTD | Heat Duty (kW) | LMTD | |

HPSH | 30,839 | 82.49 | 29,157 | 106.60 | 16,821 | 90.98 | 17,382 | 60.35 |

HPEVA | 57,192 | 112.48 | 64,014 | 118.53 | 75,135 | 108.43 | 49,903 | 75.1 |

HPECO | 57,753 | 112.48 | 43,735 | 62.07 | 40,372 | 72.63 | 34,764 | 69.92 |

LPSH | 25,232 | 87.81 | 22,428 | 60.41 | 15,700 | 67.98 | 33,082 | 64.28 |

LPEVA | 41,493 | 51.35 | 52,146 | 21.05 | 27,186 | 73.98 | 30,839 | 63.87 |

LPECO | 26,914 | 33.84 | 19,064 | 18.57 | 24,857 | 38.50 | 31,400 | 60.65 |

pressure effects on steam generation is presented in

Figures 1-3 present the effects of the ambient conditions on the turbine system key parameters such as the compressor exit temperature, turbine inlet temperature, power out and thermal efficiency of the plant.

Steam Pressure (Bar) | Amb. Temp 24˚C, Humidity 60%, Exhaust Temp 545˚C | Amb. Temp 30˚C, Humidity 65%, Exhaust Temp 535˚C | Amb. Temp 33˚C, Humidity 70%, Exhaust Temp 500˚C | Amb. Temp. 35˚C, Humidity 80%, Exhaust Temp 480˚C | |||||
---|---|---|---|---|---|---|---|---|---|

Steam Generation (kg/s) | Steam Generation (kg/s) | Steam Generation (kg/s) | Steam Generation (kg/s) | ||||||

LP | HP | LP | HP | LP | HP | LP | HP | LP | HP |

4 | 60 | 63.15 | 163.35 | 58.09 | 160.71 | 53.89 | 159.84 | 45.02 | 1499.82 |

6 | 70 | 64.33 | 175.82 | 58.79 | 163.76 | 54.11 | 160.71 | 46.11 | 150.87 |

8 | 80 | 65.51 | 180.52 | 59.70 | 166.68 | 54.33 | 162.87 | 47.98 | 153.72 |

10 | 90 | 67.77 | 186.14 | 60.11 | 169.92 | 54.98 | 167.33 | 48.00 | 155.14 |

12 | 100 | 69.93 | 194.02 | 61.34 | 173.38 | 55.82 | 168.14 | 48.83 | 159.17 |

14 | 110 | 70.85 | 198.06 | 63.57 | 177.04 | 56.08 | 169.33 | 49.90 | 162.92 |

Steam Pressure (Bar) | Amb. Temp 24˚C, Humidity 60%, Exhaust Temp 545˚C | Amb. Temp 30˚C, Humidity 65%, Exhaust Temp 535˚C | Amb. Temp 35˚C, Humidity 80%, Exhaust Temp 480˚C | ||||||
---|---|---|---|---|---|---|---|---|---|

CC Power Output (MW) | Total Heat rate (kJ/kWh) | CC Overall Efficiency (%) | CC Power Output (MW) | Total Heat rate (kJ/kWh) | CC Overall Efficiency (%) | CC Power Output (MW) | Total Heat rate (kJ/kWh) | CC Overall Efficiency (%) | |

60 | 200.734 | 8.47 | 46.58 | 183.984 | 9.39 | 44.65 | 173.494 | 9.46 | 40.91 |

70 | 204.443 | 8.34 | 47.15 | 184.603 | 9.45 | 44.84 | 173.388 | 9.65 | 40.86 |

80 | 205.526 | 8.39 | 47.46 | 185.106 | 9.55 | 44.99 | 173.846 | 9.77 | 41.04 |

100 | 206.959 | 8.39 | 47.88 | 185.749 | 9.62 | 45.2 | 174.869 | 9.92 | 41.06 |

110 | 209.061 | 8.38 | 48.48 | 186.411 | 9.71 | 45.4 | 174.641 | 10.20 | 41.35 |

120 | 206.401 | 8.42 | 47.71 | 185.130 | 9.79 | 44.39 | 172.591 | 10.10 | 40.55 |

temperature, the steam generation increases by 0.33% and 0.55% for the HP and LP levels for every 1 bar rise in the steam pressure. At 33˚C and 35˚C ambient temperatures, the steam generation increase by 0.19%, 0.22% and 0.26%, 0.49% for the HP and LP respectively, for every 1 bar increase in the steam pressure.

The results from

Figures 6-8 present the effects of the site parameters on the total heat rate, steam turbine efficiency and

combined cycle efficiency at different steam pressure.

It may be pertinent to state here that though much of the data collected are obtained from the performance records of the GT13E2 Gas Turbine located within the tropical environment, the results of this investigation can be applied on similar power plant elsewhere. From the data analysis, it can be generally said that the climatic

condition is peculiar to the tropical zone. This particular area of the study has a mean daily value of about 30˚C which varies only slightly on both sides of this value as against the general design ambient temperature of 15˚C. The investigation reveals the percentage influence of the various gas turbine operating parameters and heat recovery property on the overall plant output. This assessment will actually aid concerned engineers, operators and product developers on understanding key parameters that influence the combined cycle performance with regard to geographical location. It provides technical and operational guide on the need to modified designs or augment the plant for optimum performance.

Sidum Adumene,Samson Nitonye, (2016) Assessment of Site Parameters and Heat Recovery Characteristics on Combined Cycle Performance in an Equatorial Environment. World Journal of Engineering and Technology,04,313-324. doi: 10.4236/wjet.2016.42032

_{ }Heat Duty of the Superheater High Pressure Section kW

_{ }Heat Duty of the Evaporator in High Pressure Section kW

_{ }Heat Duty of the Economizer in High Pressure Section kW

_{ }Heat Duty of the Superheater Low Pressure Section kW

_{ }Heat Duty of the Evaporator in Low Pressure Section kW

_{ }Heat Duty of the Economizer in Low Pressure Section kW

UA Heat Transfer coefficient per unit Area kW/K

H P High Pressure

L P Low Pressure

S T Steam Turbine

G T Gas Turbine

C C Combined Cycle

LMTD Log Mean Temperature Difference

MCR Maximum Steam Circulation Rate kg/s

HRSG Heat Recovery Steam Generator

HMI Human Machine Interface