Compressing Methane

Far from being an expert on this, let me still mention that efficient compression of gases requires an integral cooling system to operate in conjunction with the compressor, in a unique alternating interval mode.

There is dedicated industrial equipment for this highly specialised process.

from artbyjoe

this sounds very expensive. i can not afford much cost.

Not necessarily.

Industrial facilities which usually compress (and liquefy) gasses, are already equipped and optimised for such processes. They liquefy anything from air to butane, they do it on large scale, in quantities for industrial use, and this means that you can probably find one, which already does it anyway, on regular basis.

The question is, do you need it ready-compressed for further use, or, do you need to compress it, from raw gas you collected with your own means and facilities.

from artbyjoe

i just need to compress it a little. propane liquifies at 160 psi. methan liquifies at 6000 psi. i just want to do up to 50 psi.

I would try an automotive air conditioner compressor. I don't know how you would lubricate it, but I would guess that it wouldn't leak and you wouldn't have any troubles with air getting into the system. You could run it with a windmill. 12 DC could operate the EM clutch so it only compresses when you need it to. Then you wouldn't pull a vacuum on the system.

from artbyjoe

this is very confusing to me. i can see where an automotive air conditioner compressor COULD work.

i don't see how the system would know when to pump and when not to. i guess i could put it on a timer. are you suggesting a windmill producing 12v power, or with an excentric wheel?

would pulling a vacuum be such a bad thing?

I'm assuming the digester would produce some pressure. A pressure switch would activate the clutch on the compressor. The compressor could be turned directly by a wind turbine.

I would assume you wouldn't want to pull a vacuum on the digester. Depending on the design, it might suck air into it.

Are you sure there's not a reasonable minimum inlet pressure for that regulator? I'd talk to the vendor with your requirements or find something to suit. Unless you're compressing it cause you need storage space you're just pissing away all of your compression work.

from artbyjoe

no, i am not sure about minimum inlet pressure for the regulator, haven't gotten that far yet. won't be producing gas until next spring. i will talk to the vendor. i do know that propane regulators working at different pressures, work until the bottle is completly empty.

compressing it because of space is exactly why i want to compress. if i use an open system, ie: keep my storage vessel, lines and digester at .25 psi, then i would have to use a large propane storage cylinder, (500 gallon size), or multiple large cylinders to get good storage. this is because the digester produces methane continuously, but the demand is intermitant. so, by the numbers, 1 psi normal output compressed to 10 psi, would be the equivelant of 40 times larger storage vessel. 1psi x 10 x .25psi = 40x, compared to an open system.

hello moon161

yes, i finally did find a pressure regulator that worked from 50 psi down to 0 psi. it is relatively expensive to my world. about $450 the diaphragm is about 20 inches across.

and yes, i want to compress for storage. i am not sure that the output, would be sufficient for use. besides if i don't compress it, i either have to vent it (which is not very green, because methane is a terrible greenhouse gas), or flare it. i would rather store it for future needs.

By the way, Methane is the most powerful of all Alkanes, with 1 to 4 (structure to energy content) ratio, and with the least initiative energy required to burn. Alas, this makes it hardest to compress safely without cooling.

It may easily explode while trying to compress it as-is, that is without cooling.

This may sound silly. But I cannot resist expressing my concept.

Why not get an inverted drum in a water (may be a small pond) and store gas in it? You can add weights on the drum so that you can chose the pressure you want to operate. May be if you have enough space you can have a bigger system so that you just compress gas only up to the operating pressure of your burners.

Do I sound silly?

from artbyjoe

no it is not silly, just clunky. the variation of this theme are endless, but all are clunky in that they require you to do something manually to get pressure.

water pressure is the easiest by far, by still is clunky. the best water system is to put a barrel, with a water inlet and water drain, with two shut off valves. a gas inlet and a gas outlet with two shut off valves. you would then close the gas inlet and water outlet, open the gas outlet and water inlet and water pressure would then compress the gas. but doing this all the time would get tedious. also, there is the possibility of disolved air in the water coming out of solution, and into the gas. this is not a good thing to do.

other variations are to fill truck inner tubes, plumbed together, with inlet and outlet shutoff valves. manipulate the valves, and put a weight on the truck tires. here again you could use water for the weight, safer than the above method, but still clunky in that you have to set 4 valves, and after compression, reset all 4 valves.

i just want to do it electricly, with a timer.

the really elegant way is with a pressure sensor, but i have not found any sensors that work down at the range i need. ie: for example, 3 psi to turn on compressor, 0 psi to turn off compressor. plus sensors and controllers are not cheap. after all alternative energy needs to be cost effective, or no one would do it. a system that is passive and uses no energy is the goal, but not easy to achieve, except around the tropics where the digester need not be heated, to work properly.

artbyjoe+

my suggestion to compress the methane is find an old refrigerator the type that had the motor separate there base is oil filled . Place the compressor in a water bath of running water to keep the compressor cool add a condenser in running water as for a switch POTTER makes one 0-5 lbs adjustable you will find it at a alarm equipment supply. Input the inner tube or drum in water is a good suggestion. And for the regulator propane regs. input is 250lbs to 0 with an adjustable out , and add a water flash back for safety

nick

The inverted drum idea is still the best bet. If the drum can be completely submerged in a bigger drum (open at the top) the pressure in the inverted drum can be controlled adding or removing water. This could be done automatically with solenoid valves.

Exposing the bio-gas to water may have the added effect of removing some of the ammonia and sulphur dioxide from the gas. Don't underestimate the corrosive potential of these byproducts. The ammonia reacts with copper alloys and the SO2 forms sulfuric acid when burned. I'm speculating on the amount of ammonia that may be present, but the SO2 will be present in concentrations high enough to cause problems. The generation and use of bio-gas has been done at the University of Nebraska since the 1970's, and as I understand it one of the biggest problems they faced was with corrosion in the IC engines where the gas was used.

from artbyjoe

yes, any time someone tries to run an ic engine on methane, it is a lot of trouble. but the real culprit, by my reading, is the HS. hydrogen sulfide. it is the rotten egg smell. over 300 ppm you stop smelling it because it is already so strong that it is killing your brain cells. smell is the first thing to go. 800 ppm is certain death. inbetween, brain damage gets progressivly worse. in ambient air it is less than 1 ppm, and in a digester it is about 10 ppm.

yes when you burn HS in an internal combustion engine, it is highly corrosive. engine won't last long unless you strip out the HS, which is a nasty business.

but i have learned that burning low ppm amounts in an open flame is very safe.

If you live in a place that does not freeze solid, than 5 plus recycled plastic barrels full of water can be connected in series with very simple plumbing. As the methane pushes the water through the barrels, it is stored for use. The trick is the last barrel, which uses a "head" of water from a large source, say a reservoir or an active stream.

recycled barrels are cheap. last forever. easy to use. infinitely expandable. Don't stop at five, go to fifty! They don't all have to be stored in one place...hoses and pipes can snake around to join these barrels all over the barn!

It wouldn't take much hydraulic head to give you a decent PSI rating either.

hi,

water pressure just doesn't work without valves. reason is that the input gas pressure of 1 to 3 psi, doesn't overcome the pressure exerted by the water, therefore raising the pressure in the digester, to the point where it stops working.

if this is the only way i can do it, then that is what i will do. my maxium head right now is very low. only about 3 pounds. if i move my tanks to the highest ground, i might get 9 psi. if i do that and move my digester (which i eventually plan to do) i might get up to 15 psi water pressure. so that would do a lot of compressing.

i am wondering if there is a really low pressure regulator, that could work with 1 to 3 psi on one side and .25 psi on the other. the advantage of that is i would not need to compress methane, and could just put in lots of storage.

Storing the methane in the barrels sounds do-able but dangerous. Why not use an old belt driven refrigeration compressor which could be obtained from a junk yard or from a refrigeration supply house. Any old cold storage plant would probably have one laying around. (I use one for an air compressor.) Use an adjustable pressure control for the propane tank.

By the way, there are pressure switches that you could obtain for free from an air-conditioning contractor's scrap yard that should work to start the compressor. The pressure switches commonly used on 90+ furnaces operate in the pascal range and are made to work with corrosive gases.... I think you'd have very little invested in this!

I realize this is an old post but if you are still working on this then it occurs to me that you could take a small capacity compressor like an airbrush compressor and hook it to a small tank from another small compressor ( unless your Paasche or whatever has one already) and have a valve for the input to the tank that closes when the pressure drops from the compressor. The pressure from the compressor would shut off when the supply of methane was dropping significantly from the digester. Once that valve into the holding tank was shut off then you could have it so that it triggered a switch to let pressure out to the carburetor(?) and of course pass through the appropriate type of regulator. I can see time has passed since you posted this originally. Have you already solved all of this?

I beleive that what you are attempting is dangerous. If not done properly, it may not be worth the effort.

1 - The higher the pressure, the more energy you need to compress it.

2 - The gas is highly corrosive, so you will probably have to replace the compressor often.

3 - The compressors I have seen are specialized, though I am not an expert.

4 - If you will be using the in an engine (generator) you will need a special oil, due to the corrosive nature

5 - When sizing a pressure regulator, you need to know the maximum flow

Good Luck!

Have you considered using scrubbers to remove the corrossives?

I am facing the same compression issue. The thoughts in this thread are being very helpful, thank you!

As for the corrosive nature of the gas, I have solved some of that by taking a piece of 2"PVC tube and filling it with industrial steel wool. By passing the gas through that, most of the elements corrosive to steel and iron attach to the steel wool. In addition, we pass the gas through a similar filter of aluminum filings. Thirdly, it passes through a solution of Limewater to remove the CO2. We will be changing this to (i think?) MEA as it is easier to remove the CO2 from MEA than limewater. We may put a further filter to remove the moisture from the gas as well.

This should make it 'sweetened' as the gas people call it, and greatly improve the caloric output of the gas.

Technicavolous

..

Can you use a Stainless steel Bimba air cylinder? A one way cylinder with a check valve. They are used in very corrosive environments and rated for around 130 psi. They are affordable and it would be easy to replace. Use a geared motor on a timer to compress the gas. Put a wheel on the motor to recipricate the cylinder like that of an old steam train. You could also submerge the cylinder in water for cooling. A You Tube video shows how to make a scrubber using steel wool and then running the gas through a barrel of lime water. That is suppose to rid the methane of sulfer and something else I can't remember.

Try an old refrigerant compressor from an old fridge (after initial startup, they are reasonably effiecient). My 1000w inverter squealed a bit on startup but ran nicely after that off a small car battery. It is a completely sealed unit and can easily compress up past 100psi. You can store the compressed methane in an out of date but in good shape propane tank 20# bottle (they are rated over 150 psi). The propane valve going into the bottle is a standard 3/4 inch npt thread and can be a bugger to get out but once it is removed, you can put in a npt fitting and adapt to whatever you wish. A standard shut off valve from an air compressor plus safety relief valve would be wise but make sure they are used in an open area as they will release methane when they relieve the overpressure. Bubble check your fittings upon assembly and during operation periodically after to be sure there are no leaks. I have not compressed methane, just air. Please be careful. email:jamesedwardstork@gmail.com if you have questions

I just posted a few minutes ago re: using a refrigerator compressor to compress your methane. You might also want to put cooling fins or run your compression tube through water to help cool it. Pure methane compressed and hot should not combust in the tube. I am not an expert but combustion requires oxygen and if there is no oxygen in the mix, then you should be good. I am also not sure if the methane will mix with the oil in the compressor and if you will have any oil come out of the compressor with the methane. It should be okay, but I am not a compressor mechanic or engineer. You could run a cheap in line separator after compression to make sure there is no oil passing through. The unit should be as level as possible. just like a fridge.

Email: jamesedwardstork@gmail.com