Nitrogen injection in natural gas for Wobbe Index control.

[I2P]

Friends,
Similar to the issue of injecting N2 in LNG, is one of maintaining Wobbe Index of natural gas. I am working in a natural gas processing facility and currently we are having a higher Wobbe Index tyhan allowed by the specs. I have read of injecting Nitrogen in LNG regassing terminals but I would like to know whether this is practised in natural gas processing plants or not? We do not produce LNG but only compressed natural gas.
If at all one does inject nitrogen, how does it affect the calorifc value? How to best estimate the requirement of amount of Nitrogen which needs to be injected per metre cube of natural gas?
I have read in a few reports that increasing the N2 by 1% decreaes the Wobbe index by 1.6%. How do we calculate this.
Our facility is seriously considering injection of inert since we stand to gain revenue due to offspec gas. Also since there is no market for extracted NGLs or LPG that route is also not economically viable.
Thanks to all in advance

 

[MortenA]

The formula for calculating Wobbe indix is quite simple. Its the gross heating value (in MJ/Sm3) divided by the square root of the specific density of the gas at standard conditions (15 deg C and 1 atm abs).

Since N2 has a faily high molecular value and 0 in heating value adding N2 will both decrease the heating value (pr Sm3) and increaes the specific density.

This is fairly basic knowledge and i would recommend that you seek in-house assistnace with this project - or contact a consultant.

Extracted NGLs could e.g. be used for fuel gas or power generation if tehre is a power grid nearby? This is a larger exersize that you should go through before determining the amouth of N2 required.

Best regards

Morten

 

[Flareman]

I2P

Not to sound like a grinch, but the wobbe Index is just a measure of how much heat you get through a hole at constant (and low) pressure.

The volume flow through an orifice is basically related to the formula for velocity pressure in the hole and the heat release of the eventual flame is the calorific value x the volumetric flow.

To display the numbers a little more clearly, you can say that
Wobbe # = [ Calorific value / sqrt(SG) ]
= [ Heat Release of burner / flow area / sqrt(Pressure drop) / constant ]
SG = [ Mol wt / mol wt of air ]

The problem with Wobbe number is that it doesn't entirely take account of the reactive nature of the component gases in the mixture and so the flame speeds of supposedly similar mixtures can be somewhat different. This isn't a problem when burning as a non-premixed, diffusion flame but, in burners designed with venturi mixers it has the potential to change the stability of the flame as a function of the burner head design.

You mention LNG. Do you mean LPG? (Liquid Natural Gas and pressurized Natural gas sound the same to me and I've not heard of injecting nitrogen into LNG - but then again, I might just be un-informed.)

Most often, people change LPG to make it more like natural gas and it's not usually a problem because although the LPG flame speed slows down, natural gas is slow anyway.
If you change natural gas and slow it down more it might lead to flame lift on some burners.

It seems to me that, for a small correction, nitrogen addition isn't going to be a big deal, but if you have a large correction to make you should consider the bigger picture.
At this point, I'm reaching the limit of my specific knowledge but if this is a contractual issue perhaps you should talk to your buyers/users and ask their technical people for advice. If you are in the USA I would suggest asking the AGA.

Good luck
David

 

[Monagas]

You should consider all the recommendations of Mortena & Flareman.
Also you can try to simulate your process and compression facilities, so it will be easy to determine how much nitrogen you can inject in order to low the HHV.

Regards from Venezuela !!