a closer look fuel gas line

8/8/2019 A Closer Look Fuel Gas Line

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Table of ContentsAbstract ................................................................................................................................................... 3

Compressor Discharge pressure calculation based on Gas Pressure Required at FG1........................... 3

The Operating Temperature Range of Gas Fuel for GT........................................................................... 4

Minimum Start-up Temperature of Fuel Gas...................................................................................... 4

Analysis of the Plant Performance.......................................................................................................... 5

Theoretical Background ...................................................................................................................... 5

The effect of Fuel Gas Temperature on plant performance ........................................................... 5

Fuel Gas Suction Temperature........................................................................................................ 5

Compression Ratio .......................................................................................................................... 6

Case High (Performance Heater and Scrubber is Present on the Gas Line but by-passed) .................... 6

Discussion of Results for Case High................................................................................................... 10

Case Low (Performance Heater and Scrubber is not present).............................................................. 10

Discussion of Results for Case Low.................................................................................................... 13

Conclusion ............................................................................................................................................. 14


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Abstract

The intent of this study is to investigate whether using performance heater and scrubber on the fuel

gas line commercially beneficial for Gama or not. The results show that not employing performance

heater and scrubber is more commercially attractive for Gama. This result is based on a model

created for estimating the performance of combined cycle for varying fuel gas temperature and

pressure at suction. Several cases are found to be detrimental to the stable operation of Gas Turbinehence the entire plant. Measures to overcome these short comings should also be investigated yet

this is beyond the scope of this study.

Compressor Discharge pressure calculation based on Gas Pressure

Required at FG1

The most important design parameter for fuel gas is its pressure. Fuel gas should be brought to a

pressure that matches to that of GT air compressor. If the fuel gas pressure is lower than that of GT

air discharge pressure; mixing of fuel and air is not possible. Therefore the fuel gas pressure should

be equal to or above that of required by GE at FG1. Fuel Gas Compressor, however; being the most

demanding auxiliary power consumer in the plant; pressures excessively above that of required ispenalizing both in terms of higher auxiliary electric consumption and due to Joule-Thompson effect.

Gas Compressor discharge pressure is calculated based on considering the pressure losses over the

equipment located on fuel gas line and the pipe line itself. Table 1 below; summarizes the

equipment located on the fuel gas line and the pressure losses taken into account for these

equipment. The lines highlighted with light green shows the equipment supplied by GE and the

pressure losses for these equipment is set by GE. The pressure loss for performance heater and

scrubber is taken from Empresarious Aguparos. The compressor discharge pressure is calculated by

aggregating all of the pressure losses over the minimum required pressure at FG1.

Table 1: Pressure losses over various fuel gas equipment

Min Pressure at FG Module bar(g) 31,73

Duplex coalescing Filter 1,03

Electrical heater for Start up (by-passed) 0,07

Performance Heater 1,03

Scrubber 0,21

Orifice Plate flow meter 0,34

Piping (Customer scope To be defined) 0,69

SSOV 0,34

Compressor Discharge Pressure bar(g) 35,44

In this report; 2 distinct cases with 20 subcategories will be considered. The two main cases is based

on a classification of gas compressor discharge pressure. For the first case named as Case High the

compressor discharge pressure is set to 36.3 bar(g) with an additional pressure loss margin of 0.8 bar.

The second case discussed in this report is named as Case Low and based on a gas compressor

discharge pressure of 35 bar(g). The 1.3 bar differences comes from the pressure losses over the

performance heater and scrubber of which the report discusses whether the installation of these

equipment is commercially beneficial for GAMA or not.


4/14

The Operating Temperature Range of Gas Fuel for GT

The other important consideration for the fuel gas is the temperature. In 9FB technology, the

protection set up is based on Modified Wobbe Index, and consequently linked to temperature.

Based on a nominal gas temperature of 110 C for the current design of GT:

T min Start-up = 10 C at start up and until 45% of gas mass flow and approximately 30% load

T min Operation =80 C from 45% to 100% of gas mass flow

In case the gas temperature does not reach the acceptable limit (in our case 80 C) the premixed

mode is not allowed and the GT cannot go above approximately 30% of load, corresponding to

roughly 45% of gas mass flow.

In case the flow is higher than 45% and the gas temperature decrease below 80 C, the GT will

initiate a run back decreasing until 45% of gas mass flow (~30% of load). Therefore; the range of

operating gas temperature for GT is 80 C 150 C from 45% to 100% gas mass flow.

Minimum Start-up Temperature of Fuel Gas

From firing to 45% gas mass flow, minimum required operating gas temperature is calculated as 10

C. This calculation is based on the gas moisture dew point temperature which is -2.5 C at maximum

gas pressure 37.9 bar (g) for the specified composition of the gas.

Electrical heaters sole purpose is to bring the fuel gas to the required minimum start-up

temperature and therefore it is designed to bring the fuel gas to a minimum temperature of 10 C

until 45% of gas mass flow that means electrical heater is designed for an increase of 10 C with a

maximum flow of 7.5 kg /s.

Moisture superheat is calculated with the formula mentioned in GEI-41040 chapter III.C. (Page 14

GEI-41040.PDF) and the formula yields 12.3 C. (See attached for easy reference)

Considering the above data, the minimum operating gas temperature at FG1 is calculated as 9.8 C.

Therefore; we have taken into account a temperature increase of 10 C for the electrical heater

(considering minimum gas temperature is 0 C)


5/14

Analysis of the Plant Performance

Fuel gas temperature has an effect on the plant output and heat rate. At different compression ratios

and suction temperatures of the fuel gas, gas compressor power consumption also varies along with

the heat energy required to bring the gas to nominal temperature of 110 C.

An excel model is created to model the cases. Please note that the model presented in the excelsheet takes the variations in the exhaust energy into account for different fuel gas temperatures and

hence; gives an indicative value for the overall plant performance considering Steam turbine output.

Yet the response of the system to variations in the fuel temperature is complex and the performance

figures presented for combined cycle should be treated as only indicative which may be subjected to

change.

Theoretical Background

The effect of Fuel Gas Temperature on plant performanceThe effect of fuel gas temperature on the Gas Turbine is plotted in figure 1:

Figure 1: Effect of Fuel Gas Temperature

As seen Fuel Gas temperature has an effect on thermodynamic efficiency of GT and hence in

conjunction it has an effect on the overall plant thermodynamic performance. This study strives to

model this effect. The differences on the slopes of heat consumption curve and heat rate curve willcreate the different response of the overall plant response than that of GT to variation in the fuel gas

temperature.

Fuel Gas Suction Temperature

As the fuel gas suction temperature increases Gas compressor power consumption and discharge

temperature increase; heat load decreases.

0.993

0.994

0.995

0.996

0.997

0.998

0.999

1

1.001

1.002

-50 0 50 100 150 200

Ratiotonominal

Fuel Gas Temperature

Output

Heat Rate

Heat Consumption


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Compression Ratio

As the compression ratio increases power consumption and discharge temperature increase; heat

load decreases.

These intricate correlations complicate the system response and put the fuel gas line design at the

apex of plant engineering.

Case High (Performance Heater and Scrubber is Present on the Gas

Line but by-passed)

The discharge pressure of the gas compressor is fixed to a set point of 37.3 bar(a). This set point is

calculated considering the losses over the pipe line and the gas conditioning equipment including

performance heater and scrubber. If the performance heater and the consequent scrubber are by-

passed the pressure losses over the equipment will not be present and the stop-speed ratio valve of

the GT will have to throttle to bring the gas to the correct pressure. This adiabatic throttling will

induce Joule Thompson effect which will cause a further reduction in the temperature of the fuel gas.

Whats more, suction temperature and the compression ratio also effect the gas compressor

consumption. Basically; as the suction temperature and/or compression ratio increase the power

consumption of the compressor increases. Since the gas compressor procurement is still on-going the

power consumption of the compressor can only be estimated for the moment. Likewise the fuel gas

line is still under development and depending on the pressure losses over the fuel gas line

compression ratio can be decreased or increased which further complicates the estimation of the

performance figures.

Due to all of the above stated reasons; the overall response of the combined cycle is complex and

the figures presented are only estimat ions which should be treated as indicative estimated values

that are subjected to change.

The main concern for this study is the identification of cases at which the GT will not be operational

at full speed 100% load.

During the analysis; only one case is found to be critical for the operation of the GT. This case is

presented with the reference Case 1a. For Case 1a;the temperature and pressure of the gas at the

customer terminal point considered as 0 C and 15.5 bar (g) respectively. The discharge temperature

of the fuel gas compressor for this case is estimated to be 77.5 C and the temperature at FG1 is

considered to be 76.5 C due to Joule Thompson Effect. For this condition the GT will initiate a run

back decreasing until 45% of gas mass flow (~30% of load).

Currently; we are not in a position to confirm the operation of the GT below 80 C for loads higher

than approximately 30%. From a controls philosophy stand point; the transient response of the GT at

case 1a will be HUNTING. If the fuel gas temperature is 76.5 C GT will initiate a run-back and reject

fuel supply. At 7.5 kg/s flow Gas compressor will be running in recirculation mode and the

consequent result is temperature increase thus GT will take load again yet the gas compressor will

get out of re-circulation mode thus the fuel gas temperature will decrease again. This loop is named

as hunting from a controls stand-point and the operation of GT will be extremely unstable.


7/14

For all of the other cases GT will be operational at Full Speed, 100% load stable with the estimated

performance figures for combined cycle.

Temperature at

Customer

Terminal

Point C

Pressure atCustomer

Terminal

Point

bar(a)

Discharge

Pressure

bar(a)

Compressi

on Ratio

Discharge

Temperatu

re C

Temperatu

re

decrease

C

Temperatu

re at FG1

Case 1a 0 16,5 37,3

2,4064516

13 77,5 1 76,5

Case 2a 5 16,5 37,3

2,4064516

13 83,9 1 82,9

Case 3a 10 16,5 37,3

2,4064516

13 90,3 1 89,3

Case 4a 15 16,5 37,3

2,4064516

13 96,7 1 95,7

Case 5a 20 16,5 37,3

2,4064516

13 103,1 1 102,1

Case 1b 0 16 37,3

2,4866666

67 81 1 80

Case 2b 5 16 37,3

2,4866666

67 87,2 1 86,2

Case 3b 10 16 37,3

2,4866666

67 93,7 1 92,7

Case 4b 15 16 37,3

2,4866666

67 100,2 1 99,2

Case 5b 20 16 37,3

2,4866666

67 106,7 1 105,7

Case 1c 0 15,5 37,3

2,5724137

93 84 1 83

Case 2c 5 15,5 37,3

2,5724137

93 90,7 1 89,7

Case 3c 10 15,5 37,3

2,5724137

93 97,3 1 96,3

Case 4c 15 15,5 37,3

2,5724137

93 103,8 1 102,8

Case 5c 20 15,5 37,3

2,5724137

93 110,4 1 109,4

Case 1d 0 15 37,3

2,6642857

14 87,8 1 86,8

Case 2d 5 15 37,3

2,6642857

14 94,4 1 93,4

Case 3d 10 15 37,3

2,6642857

14 101,1 1 100,1

Case 4d 15 15 37,3

2,6642857

14 107,6 1 106,6

Case 5d 20 15 37,3

2,6642857

14 114,2 1 113,2


8/14

Table 2: Cases considered for Case High

Figure 2: Effect of Fuel Temperature on Heat Rate of Overall plant for Case High

Figure 3: Effect of Fuel Temperature on Efficiency of Overall plant for Case High

6472

6473

6474

6475

6476

6477

6478

6479

6480

6481

6482

0 5 10 15 20 25HeatRatekJ/kWh(Withoutth

e

effectofGasComp.Consumption)

Temperature deg C

Pressure 15.5 bar(g)

Pressure 15 bar(g)


Pressure 14 bar(g)

0.5554

0.5555

0.5556

0.5557

0.5558

0.5559

0.556

0.5561

0.5562

0 5 10 15 20 25

Efficiency(Withoutthe

effectofGas

Comp.Consum

ption)

Temperature Deg C


Pressure 15 bar(g)


Pressure 14 bar(g)


9/14

Figure 4: Effect of Fuel Temperature on Output of Overall plant for Case High

Figure 5: LD for different conditions

427000

427200

427400

427600

427800

428000

428200

428400

428600

0 5 10 15 20 25

PlantNetOutputMW(

Withoutthe


Temperature Deg C


Pressure 15 bar(g)


Pressure 14 bar(g)

0

100

200

300

400

500

600

700

800

0 5 10 15 20 25

LDinEuros(Thou

sands)

Temperature Deg C


Pressure 15 bar(g)


Pressure 14 bar(g)


10/14

Discussion of Results for Case High

The effect of Gas Compressor electric consumption is found to be dominating over all other effects of

fuel gas. This deduction is based on the steep increase observed in the LD rate curve slopes with

increasing suction temperature and increasing compression ratio which aggravates gas compressorconsumption. Similar effect is expected for Case low that the effect of gas compressor auxiliary

consumption is dominant over all other effects.

Case Low (Performance Heater and Scrubber is not present)

The discharge pressure of the gas compressor is fixed to a set point of 36 bar(a). This set point is

calculated considering the losses over the pipe line and the gas conditioning equipment excluding.

Whats more, suction temperature and the compression ratio also effect the gas compressor

consumption. Basically; as the suction temperature and/or compression ratio increase the power

consumption of the compressor increases. Since the gas compressor procurement is still on-going the

power consumption of the compressor can only be estimated for the moment. Likewise the fuel gas

line is still under development and depending on the pressure losses over the fuel gas line

compression ratio can be decreased or increased which further complicates the estimation of the

performance figures.

Due to all of the above stated reasons; the overall response of the combined cycle is complex and

the figures presented are only estimat ions which should be treated as indicative estimated values

that are subjected to change.

The main concern for this study is the identification of cases at which the GT will not be operational

at full speed 100% load.

During the analysis; two cases are found to be critical for the operation of the GT. These cases are

presented with the reference Case 1a and Case 1b. For Case 1a;the temperature and pressure of

the gas at the customer terminal point considered as 0 C and 15.5 bar (g) respectively. For Case 1b;

the temperature and pressure of the gas at the customer terminal point considered as 0 C and 15

bar (g) respectively. The discharge temperature of the fuel gas compressor for case 1a is estimated to

be 74 C. The discharge temperature of the fuel gas compressor for case 1a is estimated to be 77 C.

For these conditions the GT will initiate a run back decreasing until 45% of gas mass flow (~30% of

load). Currently; we are not in a position to confirm the operation of the GT below 80 C for loads

higher than approximately 30%.

From a controls philosophy stand point; the transient response of the GT at case 1a will be HUNTING.

If the fuel gas temperature is below 80 C GT will initiate a run-back and reject fuel supply. At 7.5 kg/s

flow Gas compressor will be running in re-circulation mode and the consequent result is temperature

increase thus GT will take load again yet the gas compressor will get out of re-circulation mode thus

the fuel gas temperature will decrease again. This loop is named as hunting from a controls stand-

point and the operation of GT will be extremely unstable.

For all of the other cases GT will be operational at Full Speed, 100% load stable with the estimated

performance figures for combined cycle.


11/14

Table 3: Different Cases for Case Low

Temperatureat Customer

Terminal Point

C

Pressure atCustomer

Terminal Point

bar(a)

Discharge

Pressure bar(a)

Compression

Ratio

Temperature

at FG1

Case 1a 0 16,5 36 2,322580645 74

Case 2a 5 16,5 36 2,322580645 80

Case 3a 10 16,5 36 2,322580645 87

Case 4a 15 16,5 36 2,322580645 93

Case 5a 20 16,5 36 2,322580645 99

Case 1b 0 16 36 2,4 77

Case 2b 5 16 36 2,4 84

Case 3b 10 16 36 2,4 90Case 4b 15 16 36 2,4 96,5

Case 5b 20 16 36 2,4 103

Case 1c 0 15,5 36 2,482758621 80,6

Case 2c 5 15,5 36 2,482758621 87

Case 3c 10 15,5 36 2,482758621 94

Case 4c 15 15,5 36 2,482758621 100

Case 5c 20 15,5 36 2,482758621 106,5

Case 1d 0 15 36 2,571428571 84

Case 2d 5 15 36 2,571428571 91

Case 3d 10 15 36 2,571428571 97

Case 4d 15 15 36 2,571428571 104

Case 5d 20 15 36 2,571428571 110


12/14

Figure 6: Effect of Fuel Temperature on Heat Rate of Overall plant for Case Low

Figure 7: Effect of Fuel Temperature on Efficiency of Overall plant for Case Low

6469

6470

6471

6472

6473

6474

6475

6476

6477

6478

6479

0 5 10 15 20 25HeatRatekJ/kWh(Witho

utthe


Temperature deg C


Pressure 15 bar(g)


Pressure 14 bar(g)

0.5556

0.5557

0.5558

0.5559

0.556

0.5561

0.5562

0.5563

0.5564

0.5565

0 5 10 15 20 25

Efficiency(Witho

uttheeffectof

GasComp.Co

nsumption)

Temperature Deg C


Pressure 15 bar(g)


Pressure 14 bar(g)


13/14

Figure 8: Effect of Fuel Temperature on Output of Overall plant for Case High

Figure 9: LD for different conditions

Discussion of Results for Case Low

The effect of Gas Compressor electric consumption is found to be dominating over all other effects of

fuel gas. This deduction is based on the steep increase observed in the LD rate curve slopes with

increasing suction temperature and increasing compression ratio which aggravates gas compressor

consumption. Similar effect is also observed for Case high that the effect of gas compressor auxiliary

consumption is dominant over all other effects.

427200

427400

427600

427800

428000

428200

428400

428600

428800

0 5 10 15 20 25PlantNetOutputMW(Without

theeffectofGasComp.

Consumption)

Temperature Deg C


Pressure 15 bar(g)


Pressure 14 bar(g)

-50

0

50

100

150

200

250

300

350

400

450

0 5 10 15 20 25

LDinEuros(Thousands)

Thousands

Temperature Deg C


Pressure 15 bar(g)


Pressure 14 bar(g)


14/14

Conclusion

It is found that the benefit of increasing the fuel gas temperature does not offset the auxiliary

consumption of the gas compressor. The penalty of higher pressure requirement of performance

heater and scrubber assembly is well above the gains associated with increased gas temperature. For

the commercially most attractive solution GAMA needs to lower the consumption of the gas

compressor. This is achievable by decreasing the temperature at suction and reducing thecompression ratio as much as possible. The gains associated with these measures are also limited to

a certain extent; yet these marginal cases are beyond the scope of this study.

a closer look fuel gas line

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