Hydrogen in Fuel Gas 3

[sheiko]

Hi,

Maybe the answer is obvious but I have read contradictory opinions on this subject.

Does H2 increase or decrease the lower heating value of a Fuel Gas composed
Of C1/C2? In other words, is it a good thing to have H2 in a FG?

I ask the question because, when expressed in kcal/kg, H2 has a high LHV compared to the C1/C2, but the reverse is true when the LHV of H2 is expressed in kcal/Nm3...

Thanks.

"We don't believe things because they are true, things are true because we believe them."

 

[25362]

As you rightly say the volumetric heating value drops with increased hydrogen content. Burners may have to be modified to cope with pressure increases, or to deal with the flash-back possibility when the hydrogen mol% increases beyond a certain value. The present tendency in refineries is to recover the hydrogen for further hydro-processing even from gases vented to the flare system.

 

[poli60]

Adding something to 25362 post:
- with increased H2 you will burn less fuel for the same duty
- with increased H2 your fuel cost will increase, unless you cannot economically recover (and reuse) H2 from it
- PSA (or membranes) are used to recover H2 from (fuel) gas. The gas has to be compressed to 15-30 barg, depending on the requested recovery performance and the H2 utilization

 

[sheiko]

Thank you.

FYI below is a quote from "Process Equipment Malfunctions" written by Norman Lieberman:
"One of the most common malfunctions that reduce burner capacity is lean gas. This means that the heating value of the fuel gas is lower than design. In a refinery, the main culprit is H2. It's caused by excessive venting from our hydrotreating unit's recycle loop.
Projects that recover C3s and C4s from refinery fuel gas will also make the gas leaner and will lower its heating value"

So, in light of the previous posts, I believe that statement is not correct. Right?

"We don't believe things because they are true, things are true because we believe them."

 

[TD2K]

All depends on the heating value you are discussing, BTU/scf or BTU/lb. The fuel gas compositions for burners are typically given on a BTU/scf in my experience.

As the H2 content increases, the BTU/lb increases but the BTU/scf will decrease.

As you remove C3 and C4s, the BTU/scf will decrease as Norm suggests.

You can look in the GPSA data book for various gas's BTU/scf heating value.

What will happen to the BTU/lb as you remove C3 and C4 is harder to say. Once you get past methane, ethane and propane, the BTU/lb heating value for a hydrocarbon doesn't vary much.

 

[sheiko]

Thanks TD2K,

The problem is that, in my refinery, everybody believe that increasing the H2 content (by lowering the catalytic reformer operating pressure BTW) is good to reduce the consumption of combustibles.

So, is that true?

"We don't believe things because they are true, things are true because we believe them."

 

[poli60]

sheiko,
please carefully read the Lieberman statement: it is referred to the burner capacity, reduced by the increased volume. I repeat: with increased H2 you will burn less fuel for the same duty.
As a consequence of that, I basically agree with the idea that by increasing H2 content in the fuel gas you'll obtain a saving in fuel consumption... but maybe the burners won't be able to manage the increased volume.