How does the gas management work in a CNG-tanking station for vehicles?

[MartinLe]

For a project I need a better understanding of the gas management of gas stations. I understand that (in Europe) usually the gas is taken off grid, compressed (250-300bar) and stored in racks of 80 l bottles, usually a low, mid and high pressure bank, with the low-pressure used first to tank the vehicle. If that is empty, the mid and lastly the high pressure bank are used to fill the vehicle.

Now, my question - I understand the three banks are used so that I don't need to compress all the gas to high pressure, correct?

Why are 80l bottels common?
I would think that larger bottles are cheaper, though by the costing info I have seen so far this may not be the case.

I would also appreciate links to in depth articles about this.

 

[BigInch]

Less heat is generated in compression when done across multiple stages, rather than by one high differential pressure stage. Thus less unrecoverable heat generated in a multistage process makes that more efficient than a one-stage compression to maximum pressure.

Independent events are seldomly independent.

 

[MartinLe]

Right now I only have a german explanation handy, this seems typical for a tanking station as built in germany:

http://www.gibgas.de/Tankstellen/Technik/Funktion EGT

slide two shows the compression and storage, with the racks of 80l bottles and the compressor.

The compressor is multistage, the vessels on the left are blowdown tanks into which the compressor is emptied. So I I think the rationale behind the rack is not what you say (multistage compression), though of course you raise a valid concern.

I understand that changes of load/pressure wear out the tanks and that after a few to ten years a check is mandatory*. What I don't understand is why many 80l bottles are used, instead of less larger bottles - surely there should be an economy of scale here, so why use small bottles?

 

[BigInch]

Compressing a lot of gas to high pressure in one shot really can cause an inconvenient amount of heat to be generated and require overpressure at that high temperature to result in a required volume at a lower temperature, later after it cools off. If done in stages, where the gas is allowed to cool off to ambient temperature before being raised to the next level would be more economical.

The 80 l limit I would suppose, should one small cylinder become damaged and leak, it places a fixed limit on the amount of gas that can accumulate or be released. I think I've heard that there is a limit to the amount of gas that can be lugged around in the trunk in the US as well, but I'm not in that business and can't tell you definitely if it is true.

Similar reasoning might allow storage of more gas on site, if there were limitations of how much gas could be stored, or on various quantities stored at various pressures, but I admit I'm guessing there too. Sorry I don't know the specifics.



Independent events are seldomly independent.

 

[zdas04]

80 L is 2.83 ft³ which is awfully close to the volume that we put in the trucks in our CNG fleet in the 1990's. The size bottle in the trucks was just under 3 CF and was set by law. I wouldn't be surprised if the DOT regulation started with 80 L and converted it to an odd number of CF for domestic consumption.

Filling strategy from a bank of tanks is an interesting juggling act. A full tank has 248 barg (3600 psig) of pressure. This number is quite arbitrary, but it is the number that I most often see for in-vehicle use. If I take an 80 L tank at 300 barg and connect it to a depleted 80 L tank that is at 50 barg and wait long enough, then the result will be both tanks at 180 barg and you've basically wasted the energy needed to get the fill tank above 180 barg. The scenario that the OP suggests, lets you use the tank three times and not waste so much energy. Say the LP tank is at 160 barg before and 105 barg after, then it goes to the fill rack to be topped up to 300 barg. The MP tank starts at 230 barg and is depleted to 160 barg. The HP step tops off the bottle from 160 barg to 248 barg. This scenario seems to minimize the wasted hp.

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"

 

[MartinLe]

zdas04, this explanation sounds very plausible and is much appreciated.

 

[MortenA]

Could temperature in the "empty" tank?

The tank that receives gas is undergoin isentrop compression whereas the gas thats being moved is isenthalpic throttle (JT effect). So the gas get hotter and as the filling gas upstream P gets closer to the downstream P the JT cooling becomes less. Hmm havnt thought this quite through if doing it in steps will reduce this effect.

 

[zdas04]

When you do the J-T calcs you get really small temperature changes for this scenario. The rate of introduction of new gas is never approaching a significant portion of the speed of sound (when I did this arithmetic in the 1980's I don't think I ever got above 0.3 Mach) so the compression component is tempered by a mixing component and the adiabatic heat of compression is distributed among the gas that is cooling from it's pressure drop and the gas that is experiencing the heat of compression. The net result is very little to no heating.

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"

 

[MartinLe]

Can anyone add how the gas storage and management is done when there is no grid?
This, for example suggests that the gas is delivered by trailer:

http://www.chesterfieldbiogas.co.uk/vehicle_refueling/

Apparently they talk about compressed and upgraded biogas that is not fed into the grid.

But that would mean that either the transporting tank is only emptied to 200 bar (or whatever the low-pressure rack on site is) and ferried back half full or even three quarters full, or that there is a second compressor at the site of the gas station and the energy for compressing the gas is expanded twice. Two bad solutions in my view, but the only good solution may be to find a sweet spot inbetween - or did I miss something?


The situation I'm looking at is one with compressed Biogas, where we have to match a constant production to a non-constant demand (no fueling on weekends), large-scale high-pressure storage is expensive, and I need to understand the people who supply the gas upgrading and other fueling technology (and so far did not come out with great ideas for storage). So I'm asking here how a storage solution (with no need for transport) that's economically and technologically feasable might look like?

 

[BigInch]

They are probably not compressing the gas again, once it has been delivered to site. Burning gas at the point of use generally does not require such high pressures, in fact, still lower pressures. The transport can be by rail, ship, barge, or truck at relatively high pressures, which when delivered to site is delivered into spherical, or bullet tanks at very much larger volume and at much lower pressures.

You can get a rough idea of the storage volume required at site, by multiplying the volume contained in the truck tanks X its pressure and dividing that result by the site's storage tank pressure, then adding around 25% more. It is very similar to how butane-propane mix, "LPG", and anhydrous ammonia is trucked all over the world from city centers and railroad yards out to the farms, where no pipelines exist to do so. You will notice that these sites will contain 5 to 10, or more very large, "bullet" tanks, typically constructed to around 250 psig pressure ratings. The trucks will discharge their contents into all tanks, using a truck mounted compressor to evacuate the last of the gas contained in the truck tanks. That requires only compressing that small residual volume left in the truck tank to a maximum of 250 psig to get the last of it into the site's tanks.

Independent events are seldomly independent.

 

[zdas04]

BigInch,
The BIG difference between CNG and LPG is that the LPG is stored as a liquid and the CNG is not liquified. When we were doing research for converting a fleet in the mid-1990s no one was storing CNG outside of vehicles. You brought in a natural gas line at industrial pressures (usually around 10 barg) and compressed it up to your defined "full" pressure into the vehicle's tank. Gas storage is just so much less efficient than liquid storage and vaporization that it doesn't often make economic sense for applications using commodity natural gas.

The OP's problem is a bit different. He's producing a version of "stranded gas" that either does not have access to the grid or doesn't meet pipeline specs. If his biogas does not meet pipeline specs then he has some serious choices to make (if it is just an access thing then he'd probably be money ahead to be doing an LNG project instead of a CNG project). Most of the successful biogas operations I've looked at use the biogas at pretty low pressures to run on-site gensets and either offset their electric bill or send power to the electrical grid. Using it as a motor fuel is new to me, and presents a number of interesting challenges.

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"

 

[BigInch]

Yes zdas, thanks. I get the difference. I was just describing what the typical process was behind his questions. In any case he still he does not appear to have any real alternatives, other than to 1) store at high pressure or 2) store at low pressure. Regardless of it if is LPG, or CNG, propane, butane, CO2, He, ... he can always make a trade off between low pressure-large volume-lo/no compression cost, or small volume, thick walled tanks and recompression cost. The product only changes the economics of the compression and storage tank cost. He could do either method. LPG is less costly, because of the lower vapor pressure amd higher liquid densities available at relatively low pressures and ambient temperatures. With the third method, trade volume, pressure for temperature as well. He could consider the lower pressure NG to LNG storage technique, but he needs really cold temperatures and regasification equipment, but that is an option none the less.

I'd imagine that, where he is, land is probably cheaper than heavy walled, high pressure, or cold tanks and regassification equipment, so I suggested that he investigate dumping the cng to a lot of lower pressure bullet tanks, with cheaper, thin-walled tanks and no to little recompression costs to empty the CNG tanker. Seems like the most practical way he can go at the moment.

There are additional problems with biogas as well as it usually comes along with CO2, nitrates, high water content and sulfides. Some pipelines won't take it until it's cleaned up and, if you try to generate electricity yourself, it requires special NG engines to run on the stuff. In some cases there is nothing you can do with it at all, ... too far to truck, too far to pipe for the sales value ... so you ... flare it to keep the methane from going directly to greenhouse. The heat isn't so bad as the gas itself, because we've found out, it's not really "global warming" after all, it's "climate change".

Independent events are seldomly independent.

 

[zdas04]

I was talking to a dairy farmer last month who has started routing all the cattle waste to a digester. The biogas off the top is used to run a genset, heat from the digester is used to heat buildings, and the sterilized waste is sold (he said that he routed the biogas to his house but had to back off due to the smell from the stove). He says that he is net positive into the electric grid and is looking for reasonable electric vehicle technology to make himself totally self sufficient on energy. It was a conversation on a plane, so I don't have any confirmation of any of this, but everything he said sounded plausible as hell. That seemed like a viable (and pretty impressive) biogas project. CNG doesn't.

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"

 

[MartinLe]

zdas04, what you describe is somewhat typical (over here BG plants normally don't have to sterilize their wastes). But I wonder who he sells the rest to - while the digestate is a good fertilizer, it's also high in moisture and thus dead weight to ferry around. I know of deals where the digestate is given away for free to the farmers who supply the fodder, in extreme cases the plant operator even applies it to their land himself (and at his cost!).
Electrical run vehicles sound very interesting to me, though then there would again be the headache of storing the energy (either in batteries or as gas) if he did not have the grid.
Did his project work out without any subsidies?


Back to the discussion, I will now look at two options:
- "low"-pressure storage - there's a gas upgrading step to remove CO2 and other impurities between the digester and the fueling station, the upgraded gas will be delivered at ~15 bar
- finding better uses for the gas than vehicle fueling

 

[zdas04]

It was a conversation on an airplane. I have no idea how he is selling the solids, he just said he was selling the solids. He also didn't mention his tax situation to a stranger on an airplane. I would have liked to have done a detailed case study (I was interested), but we didn't even exchange names and I don't know where he lives. He did say that without significant energy charges his dairy farm was showing more profit than it ever had. And no, I don't know if he is including depreciation of capital equipment in that assessment.

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"

 

[BigInch]

Its been a common practice all over the US, Australia, Europe, China and Vietnam since the early 2000s. There are many private farm installations and numerous larger central collection power stations that compost the waste from small farms in the area into gas in many agricultural centers. The waste that is not used on site is sold as fertilizer to nearby farms. Search the USDOE website to get plugged in on that program. They are money makers for the most part.

Independent events are seldomly independent.

 

[zdas04]

Thanks. I'll take a look. I'm not in that business, but I'm always looking examples of people behaving in their own actual best interest. This idea definitely qualifies.

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"