Methane Vs Ethane 1

[Falcon03]

Hello everyone,


Some time next year we are planning to shift our fuel gas firing from Ethane to Methane. We are planning to have a look at our fired heater system to see whether we need to change some equipment such as control valves, burner tips. Does any one has such experience in this matter? thanks

 

[TD2K]

I don't have any direct experience with this. But the major areas I would look at are:

1. Header sizes, you'll need to move a lot more C1 rather than C2. You may be able to partially compensate for the lower density by raising system pressure depending on your relief valve(s) settings

2. Control valves will have to move more gas, you need to look at their Cvs. Trim and body materials won't be an issue.

3. Burner tips. You'll need to run quite a bit higher pressures with the existing burners to get the same heat release, new tips with larger orifices are likely a must.

Take a quick look at the total flue gas flow rate in the furnace, it shouldn't be an issue but you might want to confirm it with the vendor for heat transfer within the radiation and convective section.

There may also be emission issues (eg. NOx) that might or might not be another issue.

 

[Jonkatz]

Falcon,

Since Methane is about half as dense as Ethane, the gas pressure would theoretically need to be Doubled to keep the same SCF through the burner piping. BUt you should check what the max pressure the burner system can handle. Then, the control valves should be re-sized and an appropriate Cv calculated. You should be able to change the trims only.
Burner tip sizing should be done by the vendor. They will advise you what the minimum pressure required AT THE Burner is with the new fuel. In fact, it is likely that the burner documentation came with curves that predict the heat release vs. gas pressure, for different gas LHV's.

As for the furnace itself, Methane firing will produce slightly more flue gas (1.3% more) than Ethane firing. This is only a small increase, and the only problem presented is the stack draft. Check that the stack has been designed to handle the increased flue gas flow. (Normally stacks have some overdesign built in).
J. Katz

 

[GULF]

daer
my experience in the above change over from methane to ethane was as follow.
1-we changed all the burnners tips for all the three furnacess
2-inelt air duct modified
3-one of furnacess stack hight modifed ( expaned by 5 meters0

 

[25362]

Ethane has a calorific value 75% higher than methane on a volume basis. Thus to get the same heat output from combustion the volume of methane should have to increase by 75% (about a reduction in 7% by wt).
Thus overall gas piping and instrumentation should be checked and modified accordingly. The air-to-gas volume ratio is much smaller for methane, a fact that may need burner adjustments. However, combustion volumes, for equal heat release, are expected to rise by only 1%.

1 mole ethane + 3.5 moles oxygen ---> 2 moles CO2 + 3 moles H2O, and
1.75 mole methane + 3.5 moles oxygen ---> 1.75 moles CO2 + 3.5 moles H2O

Stoichiometric combustion products increase from (2+3) = 5 to (1.75+3.5) = 5.25 moles. If we add these to 13.2 moles of inerts (in air) for both cases, and we add 20% xs air (3.3 moles) to the combustion, we get an increase of about 1% in total exit gas volume. Combustion gases for both cases would have MW's near that of nitrogen ie, 28. Thus updraft effects would be very similar at equal stack temperatures.

Although the overall air demand, for equal heat release, doesn't change when switching from one gas to the other, the air-to-gas volume ratio (including 20% excess) does change from 11.4:1 for methane to 20:1 for ethane.

If it were a change in reverse, as Daer says, from methane to ethane, changes to the burners would be sine qua non, because an adequate air supply to the higher calorific value gas must be ensured, especially on 100% pre-mix types.

I'd ask Daer whether the increase in chimney height that he mentions would have been dictated by stack emmission dispersion rules rather than by updraft enhancement.

 

[25362]

Falcon 03: Although my previous estimates were nice and clean, there were based on HIGH calorific values (a 75% drop), not on LOW Cal (a 78% drop) values. Therefore I must upgrade my 1% results to almost 3%. Still a small deviation between methane and ethane, but I felt the correction had to be done.

 

[Flareman]

For the Burners, the conversion factor usually used to evaluate the operating conditions is the Wobbe Number.
Wobbe Nr = [Calorific Value (volumetric)]/[sq rt(Specific gravity)]
To maintain the same furnace heat release which you have now, adjust the operating pressure according to the square of ratio of the Wobbe Nrs of the gas
For Ethane W = 1641/sqrt(1.018) = 1627
For Methane W = 913 /sqrt(0.553) = 1228 (using Btu/scf)
Methane pressure = (1627/1228)^2 x ethane pressure = 1.755x

Alternatively, to keep the same pressures, change the burner tip flow area in the ratio of the Wobbe Nrs = (1627/1228) = 1.325x.
For example 6 x 2mm holes becomes 6 x 2.3mm. If you are changing tips, call the burner manufacturer rather than drill them out yourself. He may know of a reason (or previous bad experience) why you should not exceed a particular hole size or number of holes per tip.

If you fire the same heat release you should almost expect the same furnace results. Just two things change the balance a little. Jonkatz already mentioned that the stoichiometric flue gas goes up by 30%. That's not really entirely true for your case because the air you use is more closely related to the total heat release in the furnace than it is to the fuel being used. Stoichiometry works out at (roughly) 1 cu ft of air per 100 Btu/h (+/-) regardless of the fuel. The excess air (ratio) which you use can swamp the figures so you may not even notice the difference.

Ethane, being a little heavier than methane with more carbon, can produce slightly more luminous flames than methane. You may not notice this in the convective section of the furnace but, if the furnace is natural draft, you may have a less radiant flame with methane and have to adjust the air registers to change the flame shape a little and force some heat transfer back down into the radiant section, (if you need it there). Overall, with a little juggling, you should find something which works.

If you are sooting in the stack or coking in the tubes, both of those might reduce.

Good luck

David

 

[pmover]

all previous posts are valid; however, i would also consult with the fired heater and burner mfg. as they would be able to provide the best response. use the previous posts as an aid in consulting with the mfg's. they would need gas composition, along with P & T, at original design conditions and the proposed gas composition. they should be able to conduct thermal and performance analysis for you.
-pmover