Natural Gas Quality Consideration

[mocan]

Hi everyone,

I wonder why the quality of natural gas is based on its methane composition? while we know that pentane,hexane,etc has more carbon and hydrogen..
What is the consideration for the issue?

I really appreciate any help.
Thanks before

 

[zdas04]

Is this a serious question? Commodity (i.e., post processing) natural gas is primarily methane for two reasons: (1) heavier hydrocarbons have greater value if separated into pure components; and (2) end users need a well-defined heating content of the gas they are burning to allow for burner design (i.e., a process designed for 950 BTU/SCF gas will burn far too hot with more than incidental heavier hydrocarbons).

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[dcasto]

The biggest concern is liquid drop out of components sending liquid to a gas burner is not a pretty site and incomplete combustion of gases because not enough air when gases "heavier" than methane are allowed into the stream.

Look up WOBBIE INDEX and the relationship of gas gravity to orifice flow.

 

[mocan]

Thanks for the replies

zdas04, isn't it good to have a greater value? thus, the energy produced will be higher. And also, if we have almost the same amount of methane and pentane, then we aren't able to define the heating content, are we?

dcasto, what about if we supply more air for the gases?

Sorry for my silly respond, I'm stil a beginner though

 

[zdas04]

There was a plant upset in central Colorado in the U.S. in the 1980's that sent 1600 BTU/SCF into the homes near the plant. The company that owned the plant ended up replacing every HVAC heater, every water heater, and a couple of dozen houses. The hot flames just destroyed the aged internals of the equipment in people's homes. That was a pretty small change (no pentane at all) from spec. A gas stream with 50% methane and 50% pentane (which I think is what you are suggesting) would be 2300 BTU/SCF which would destroy industrial burners that were set up for natural gas in addition to destroying all homeowner equipment.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[LittleInch]

What all industries really like is consistency of supply or requirements. When electricity first started to become available, there was a plethora of voltages, frequency and types of plus. Only when a national grid spec was initiated did suppliers start to produce one device much cheaper.

Gas is the same. You need a standard or closely controlled spec for heat and for suppliers to produce efficient low cost burners and equipment. The cost impact of changing burners or offering different versions is very costly. When countries changed from coal gas to natural gas, there was a huge cost in changing burners on boilers and cookers due to the different properties. Not all gas fields have other components available, I've seem C1 components of >95% coming out of the ground and ail the other components can be removed relatively easily by cooling of the flow.

So basically there is now simply far too much installed equipment to start messing around with the input gas for little or no overall benefit.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[mocan]

waoow, thanks for the responds everyone. really help me a lot.

 

[dcasto]

If you really want to see how gas specs Google up gas interchangeability.

http://www.aga.org/our-issues/issuesummaries/Pages/NaturalGasQualityandGasInterchangeability.aspx

The state of Wyoming puc came out and said that pipelines could not change their wobbe number more than 7% which created nightmares because they didn't take into account appliances that were installed. They made it illegal to supply gas that matches what all gas appliance companies built. Cute