We have 2 Frame 9E Gas turbines operating on duel fuel. It is observed that Heat rate is coming more when operating on Gas fuel than Naphtha. We are not able to find the reason.

can any one help?

 

If you have dueling fuels, that's a problem all by itself.

Heat rate is a "funny" calculation. I believe it's expressed in Btu/kw-h, so a lower number would mean that more energy is produced for the same fuel flow-rate, or less fuel is required for the same energy production rate. It's an expression of efficiency, with a lower number being more efficient. (It's been many years since my university thermodynamics class.)

Therefore, if you have two heat rates, the one which is the lower magnitude is the "better" heat rate.

Are you referring to simple-cycle efficiency or combined cycle efficiency?

Is this something that just started, or has it been this way since commissioning? Is the difference significant?

I believe that machines are typically *not* rated identically for multiple fuels, and I would not necessarily expect that the heat rates for two different fuels would necessarily be the same.

If there's been a change in heat rate, though, over time, then there should be a way to track down the cause. Different fuel supplier? Different fuel characteristics (gas and/or naptha)? Different fuel nozzles?

Are you comparing the two fuels with the same compressor cleanliness and at the same atmospheric conditions? Or, are you using correction tables for ambient conditions?

Lastly, when taking any kind of performance data on a heavy duty gas turbine it's always best to run the unit up to load and then let it sit there to allow internal machine temperatures to stabilize.

The normal indicator used to determine when the machine is at stable operating temperatures is when no single wheelspace temperature reading changes by more than 5 deg F in a five minute time period. As a machine is "warming-up" after reaching a particular load, the internal temperatures, as indicated by the wheelspace temperature T/Cs, will increase until they "level off", indicating that the machine is at "steady state" operating condition and performance data-taking can begin. Form most GE-design heavy duty gas turbines, this can take as much as four hours if the unit is started from a "cold" condition to reach steady-state operating conditions.

So, if you start a machine from a "cold" condition and load it to Base Load and start taking data 15 minutes after it reaches Base Load, then operate it for a few hours or days or weeks, and transfer fuels and take data, the data is not considered to be "comparable". It would be necessary to load a machine to Base Load, ensure the internal temperatures are stable (as indicated by the leveling off of the wheelspace temperatures), take the performance data, then transfer fuels, ensure the internal machine temperatures are again stabilized, and then take the performance data. Hopefully, the ambient conditions haven't changed by much during the testing, but there are correction tables for that.

Also, it's not a representative test to take data at one load for one fuel, then take data at another load for the other fuel, either. One needs to try to take data for the two fuels at the same operating conditions as much as possible. You can take operating data at different times of the year (seasons) but you need to use correction tables to correct the data back to isobaric conditions to be making a good comparison.

 

Hai,

I read your problem but it is very good observation.

Heat rate is the ratio of Amount of heat energy supplied to a Turbine per unit power generation.

Higher heat rate is not commercially profitable, So, Lower heat rate is prefarble.

Coming to point sir, While GT on fuel oil operation Power generation is more as compare to the fuel gas operation for same SRC(Site reference conditions like-Ambient pressure, temperature, RH) and Grid frequency.

Based on the available fuels on India, I am explaining this one that the caloric value of the fuel gas is 9000 to 9500 Kcal/Sm3 and caloric value of the fuel oil is 10500 Kcal/Kg,

So, amount of heat energy supplied to the turbine= product of fuel consumption*caloric value.

For the same generation the above product will be high for fuel gas compare to the fuel oil.

by..

 

Hai,

Heat rate for the Fuel oil operated gas turbines are low due to the high quantity of mass flow handle by the gas turbine

Normally while fuel oil operation, Water injection or Steam injection will be enabled to maintain the NOx levels and also to maintain the firing temperature with in the limits after reaches the gas turbine>15% of the rated load.

Due to water or steam injection, amount of mass flow from the combustion chamber(Air+fuel+Water) will be high. But combustion chamber quantity at fuel gas=fuel gas+air. So, this additional mass flow due to water injection at fuel oil operation useful increase the GT load. Due to this load Heat rate will be less(If the water injection not enabled it will leads to increase the NOx levels and also control the firing temperature at high load operation).

Bi

 

Please send me your contact details and our Technical Support Centre will be able to assist you.

With best regards
Maurice Rutten
Wood Group Gas Turbine Services
maurice.rutten [at] woodgroup.com

 

Thanks to you all for sharing your knowledge.

Out put on Gas is about 2 MW more than distillate fuel at ISO conditions.we noticed the same phenomenon even at ambient conditions. More water injection is required for fuel oil operation but more water means more heat loss and high heat rate.

I was referring to combined cycle heat rate only. Because we don't have precision measurement at GT inlet. we are not applying fuel oil calorific value and density at the ambient conditions. we apply the 15 degree C GCV. It may give some error in calculations.

my mail ID is ankarao.ch [at] gentingenergy.com

 

Hi,
Mr.Anka Rao Garu,

I read your message but your question is not clear.
In the first post, you asked Heat rate with fuel oil will be less as compare to the the Fuel gas operation which is correct.

But, now you are asking in the different way,

GT output will be high in distillate fuel as compare to the Fuel gas operation due to water injection.

Water injection will increase the mass flow to the gas turbine.
Normally water injection enable to reduce the firing temperature to avoid the thermal Nox (More than 2800Deg.F will leads to generate thermal NOx in the combustion chamber)

Work ouput = m*Cp*DT

DT is fixed for both fuel operation. So, work out is directly proportional to the product of mass flow and Cp.

Mass flow will be higher for the distillate fuel as compare to the flue gas even without water injection also.If you add water injection to control the Nox will further increase mass flow.

So, Net work output with fuel oil high as compare to the Fuel gas.

So, heat rate will less for fuel oil compare to fuel gas.
By..
Anil

 

MR Anil,

Thanks
Mr dev mentioned that mass flow is more in distillate fuel due to more WI flow. I was referring to his post.

I am not convinced that mass flow is more in distillate fuel. can u elaborate

As per GE at ISO conditions out put on NG is 123 MW and out put on distillate fuel is 121 MW. This is due to high specific heat of flue gas in NG combustion. so heat rate is supposed to be less in gas firing.

regards
ankarao

 

Hai to all,

Thanks to all,

I am revising my earlier post,

At ISO conditions, GE frame 7FA rated capacity is 171.7 MW for Fuel gas and 183.8 MW.

Recently I observed fuel change over from Fuel gas to fuel oil
Before change over, GT output will be the 174MW (Base load) at 13.8Deg.C and 44% RH and 60 HZ frequency. After fuel change over GT load raise to 191MW at same ambient conditions. So GT load selected to pre-select to maintain the 170MW with fuel oil.
“Heat rate will be the high with fuel oil compare to the fuel gas though the power generation will be high for fuel oil”
Above explanation may not be the same for all type of GE machines.

For GE frame-5 or 6 machines ( I am not comparing with frame-5 to 6, Comparing the same frame machine with different fuels) the output will be the high with fuel gas operation compare to the fuel oil. But Heat rate will be the high for Fuel oil operation compare to the fuel gas for the same frame machine.

For frame-5: ISO Power output with fuel gas- 26.07MW
ISO Power output with fuel oil- 25.57 MW

For the same exhaust flue gas quantity, Exhaust temperature and compressor pressure ratio for the different type of fuels with Machines like frame-5 or 6.
But heat rate for the Frame-5 will be the high with fuel oil as compare to the fuel gas.

So, “IRRESPECTIVE OF FRAME, HEAT RATE WILL BE THE HIGH WITH FUEL OIL COMPARE TO THE FUEL GAS”

I am coming to the point, the power output for the frame-5 machine will be high with fuel gas compare to the fuel oil. This is due to difference of specific heats.

Prime mover work ratio is basically depends on Amount of mass flow handled by the turbine and specific heat of the flue gas.

Amount of mass flow generated with fuel oil or fuel gas will be the same for frame-5 or 6 machines. So, Power output is based on the specific heat of the flue gas.

Specific heat at constant pressure of flue gas with fuel gas will be high as compare to the fuel oil. So, Prime mover work output will be the high with fuel gas.

May be in the next post, I will receive a reply from you that”what could be the reason for same quantity of exhaust flue gas for the frame-5 or 6 machines with different types of fuels also”.

Mass of flue gas= (Amount of fuel+ Compressed air+ water vapor generated in CC)

Water vapor generation in the combustion chamber is based on the two reasons
1. Fuel External moisture content

2. Internal moisture generation due to high H/C ratio.
Amount of fuel is based on the caloric value, amount of air flow is based on the type of fuel, water injection is based on the fuel flame temperature to be reduced to avoid the Nox.

Firing temperature will not be the same for all frame machines. Based on the high firing temperatures, capacity of the machine will increase.

Work ratio= (Wt-Wc)/Wt

Here Wt- Prime mover work done
Wc- Compressor work done
Net work output= (Wt-Wc)

Work ratio= 1- (T1/T3)* (Rp)^(r-1)/r

Here T1- Ambient temperature
T3- Turbine inlet temperature
Rp- Pressure ratio
r- Ratio of specific heats (Cp/Cv)

So, Higher the turbine inlet temperature leads to increase the work ratio of the prime mover.

To increase the turbine inlet temperature, basic hurdle is metallic constraint and NOx generation.

At high firing temperatures hot spots will generate and cause for thermal Nox generation. Normally more than the 2800Deg.F firing temperatures rate of NOx generation id drastic. So to reduce the Nox levels, there are 2 methods,

1. Internal method of NOx reduction.
a. NOx water/Steam injection introducing to the combustion chamber
b. DLN(Dry low NOX design- Premix combustion methods)
2. External method: SCR (Selective catalytic reduction)

Due to the water injection in frame-7 and 9 machines, amount of exhaust flue gases will be high with fuel oil operation as compare to the fuel gas operation.

Even though the Specific heat of exhaust flue gas with fuel gas is high compare to the fuel oil, the product of the Specific heat and the exhaust flue gas quantity will dominate.

i.e M1*Cp1 > M2Cp2

Here M1- Exhaust flue gas quantity with fuel oil
Cp1-Specific heat of the Exhaust flue gas with fuel oil
M2- Exhaust flue gas quantity with fuel gas
Cp2-Specific heat of the Exhaust flue gas with fuel gas

So, power output will be the high with fuel oil compare to the fuel gas. But Heat rate will be the high with fuel oil compare to the fuel gas.

So, For the frame-5 &6 machines Power output with fuel gas will be high compare to the fuel oil. But heat rate will be high with fuel oil compare to the fuel gas(Specific heat domination due to same quantity of mass flow)

For Frame-7&9 Machines, Power output with fuel gas will be low compare to the fuel oil. But heat rate will be high with fuel oil compare to the fuel gas(here product of the Specific heat and mass flow of the flue gas with fuel oil will dominate the product of the Specific heat and mass flow of the flue gas with fuel gas, though the specific heat is less)

So, “HEAT RATE WITH FUEL OIL WILL BE HIGH WITH FUEL OIL COMPARE TO THE FUEL OIL”

 

Good after noon to all,

sorry for the delay

Mr Dev has shared valuable information.

I do come across the 9FA document saying the output on Gas is 256 MW and naphtha is 272 MW. Here the machine adopts DLN combustion technology and there is no need of water/steam injection. So exhaust flow increase due to water injection is ruled out. still the out put on fuel oil is high.

In your machine what kind of oil are you burning? what is the calorific value?

In our machine we are maintaining almost same water injection flow for both fuels as the emission limits are different (more for Naphtha fuel). The output on gas fuel (NG with GCV of 9400 kcla.sm3) is seen more than naphta (with gcv of 11300 kcal/kg). consequently the heat rate is good less in fuel oil operation.

online exhaust flow measurement would be more useful to under stand this phenomenon

Thanks

 

When an F-class unit with a DLN combustion system is running on fuel gas, it's operating in Premix Steady State (at or near Base Load) and does not require water- or steam injection for emissions reduction.

When an F-class unit with a DLN combustion system is running on liquid fuel, it generally must have water- or steam injection running to meet emissions requirements because, with but a couple of units in the world, there is no such thing as "dry oil" DLN combustion. It's been tried, but it didn't work as well as expected.

So, a 9FA running on liquid fuel will usually also have water- or steam injection running, which will add to the mass flow through the turbine, which affects its power output and heat rate.

It's not common that a GE-design heavy duty gas turbine will be rated at the same output at the same ISO conditions for multiple fuels. Close, but not the same. It seems that someone is trying to say the output should be the same for any fuel, and that's generally not the case. It can be made to be the case, but it's generally not the case. (When I say it can be made to be the case, I'm referring to situations where the output must be limited to some specific value and in that case the exhaust temperature control curves are adjusted to reduce the output of one or both fuels to limit the power, but that's not typical of most installations.)

 

According to GER4172 - Gas Turbine NOx Emissions Approaching Zero - Is it Worth the Price?, Water/Steam Injection:

"However, higher firing temperatures mean higher combustion temperatures, which produce more NOx, resulting in the need for more diluent injection to achieve the same emission levels of NOx. There has also been a reduction of allowable NOx emissions and lower NOx levels re-quire even more injection. The increased injection rate lowers the thermodynamic efficiency, seen as an in-crease in heat rate (fuel use), due to taking some of the energy from combustion gases to heat the water or steam. "

Maurick

 

Good evening everybody

I read your post and useful for me

Based on your old post that your machine capacity is nearly matching to the Frame-9E. As per GE manuals, Frame -9E machines heat rate & output is high for Fuel oil (Water injection) operation compare to the fuel gas (No water injection) operation at ISO conditions.

What is the purpose enabling the water injection to the gas turbine while fuel gas operation in your existing machine?

For New machines like Frame 9FA DLN combustion were designed for fuel gas operation only. I don’t know regarding your latest frame-9 DLN combustors for dual fuel.

Normally at ISO conditions, for a particular frame machine—configuration (Output, Heat rate, firing temperatures, DLN burners, Speed of the machine (frequency) will be the same., But output and heat rate is depends on the respective SRC(site reference conditions like ambient temperature, RH etc...) and also depends on available fuel sources.

Frame will change to change the configuration of machine.

Now, Total topic is on caloric value affects the power output of the Gas turbine.

Actually says here is no direct relation between the LHV and output but make a note on this for assumption.

Gas turbine output is based on the Mass flow of the flue gas, specific heat of the flue gas and DT across the gas turbine. To maintain the DT across the gas turbine, maintain the firing temperature constant irrespective of the type of fuel for a particular configuration to maintain the NOx levels.
Example: NOx levels in the ABB or Alstom duel fuel machines, Turbine inlet temperature is lower in the fuel oil operation as compare to the fuel gas operation by injecting the more water injection for the same machine.

Consider DT across the gas turbine fixed, So the out put of the gas turbine is based on the specific heat of the flue gas and mass of the flue gas.

Flue gas is depends on the ingredients of fuels, quantity of fuel flow,quantity of air flow, amount of water vapor generation due to High H/C ratio or Water/Steam injection.

If the fuel contains only hydrocarbons, with out inert gases and oxygen—Output of the gas turbine will increase to increase the Hydrocarbons (LHV). Here output is high due o the high specific heat of the flue gas.

In this case, water vapor also will generate in the combustion chamber due to high hydrocarbons. While combustion 9H( H2+ 0.5*O2-9H20) times of water vapor will generate due to hydrogen.

If the amount of inert gases increased, LHV decreases which leads to increase the output due to high mass flow generation.

In this case, mass flow participation to the combustion chamber from the fuel gas will increase due high quantity of inert gases and high quantity of fuel flow due to lower heating value by lower hydro carbons. These excessive inert gases through the fuel gas to the combustion chamber are not compressed by the gas turbine compressor. So, compressor work done will unchange. But quantity of flue gas flow will increase.
Due to low hydro carbons, water vapor generation also less. But total mass flow generated in the combustion chamber with LHV will high as compare to the higher heating value.

As per the above explanation, If LHV decreases, gas turbine output will increase. But lot of drawbacks while burning the lower calorific value.

As per my observations, for GE machines, irrespective of the frame, Heat rate will be high for fuel oil compare to the fuel gas.

As per your information gas GCV of 9400 kcal/sm3 and Naphtha GCV of 11300 kcal/kg, what is the relative density of your fuel gas?

My plant, calorific value of the gas> calorific value of the distillate fuel but it is very close (all units on K.cal/Kg only), As per online exhaust flow transmitter, flue gas flow with fuel oil is 4% higher than the fuel gas.

waiting here for your reply...