a new AD forum guy

Digester design and construction info

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Billtu
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a new AD forum guy

Postby Billtu » Tue Aug 05, 2008 8:55 am

Greetings from Central United States.
Great to find an AD forum. Here's some pix of a system built this spring:

http://elwood.longlines.com/~jonz/AD/FULL_SYS_k.jpg

any .jpg with an underscore_k is new.

http://elwood.longlines.com/~jonz/AD/

Producing half a cubic foot a day at slightly over 3" WC. Or 1.5 cubic feet @ 1 atmosphere. I feed it a handfull of fermented grass clippings every time i pump off every day. I started it with 3 spades of grass clippings and 1 spade of fresh cow manure. Haven't added any more manure.

Instead of building a 30 gallon digestor, I’ve been building the equipment to harvest and store the gas. I've got a York/Tecumseh auto AC compressor rigged up so i can evacuate and fill tanks.
I'm not into liquification as that gets expensive. Presently i'm compressing to 145 psi, but may go up to 250 psi.

Why? To run small engines. I've successfully run my 50 cc Yamaha on this stuff, right out of the barrel. Just for test purposes only. I've set about removing the acids by bubbling the gas through filters for long term use and storage.

Cheers,

Bill

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Bob
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Re: a new AD forum guy

Postby Bob » Wed Aug 06, 2008 9:01 am

Hi Billtu,

Thanks for sharing! Great pix. I'd like to hear more about your experience with scrubbing and compressing the gas. I had no idea you could get those kinds of pressures from auto AC compressors.

Billtu
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Re: a new AD forum guy

Postby Billtu » Thu Aug 07, 2008 6:45 am

Thanks for reading,
I've found a York brand compressor made from aluminum dissipates heat better than the cast iron Tecumseh counterpart. See by the pix i've got a muffin fan blowing on the compressor head while it's running.
Here's a good page:
http://www.meicorporation.com/pdfs/Comp07.pdf

Come to find out, these little compressors are really large in displacement, from 163 to 262cc. So it's really a matter of what you belt up to it. I use a fully enclosed .75hp motor. Can't recall the rpms.

You can see my filtration is a gallon glass jar. They are getting harder to find. The port leading to the compressor suction side has a block of polyurethane foam stuck on it. Also there's an intensional low swale in the clear hose going to the compressor suction side. If there were any moisture getting past the foam, it would settle in the bottom of the swale. I change the water by purging it out, and sucking new in. The top/lid is permanently sealed with RTV silicone. I use fresh water every two or three batches, of .5 cubic feet @ 3"WC.
There's a valve at the top of the AD and the storage tank has both a valve and a female Milton quick coupler. I first evacuate the line going up to the AD. I can tell by the lack of bubbles in the jar. Then i connect the storage tank and open the hand wheel valve. Last thing, i crack the needle valve on the AD letting the product slowly into the jar, compressor, to tank. BTW i first evacuated the storage tank. Who know what kind of dillution you'd get if you started filling a tank from 1 atmosphere?
This is the point where i urge safety and safe practices. If you haven't already, talk to a propane or natural gas service person and learn safe practices and methods from those that work with flamable gasses.

Thanks again,

Bill

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Bob
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Re: a new AD forum guy

Postby Bob » Tue Aug 12, 2008 3:19 pm

Very interesting information, Billtu. Thanks.

I don't have any sense of how much gas you can practically store in, say, a 100 lb propane bottle using an automotive compressor. Or about how much energy it would take to compress it. Have you worked any of these numbers out?

(I'm also interested in biomass gasification, and am in the process of building a woodstove / gasifier, a modification of the old-fashioned barrel stove -- aka "Yukon stove" ~ still strictly experimental...)

Billtu
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Re: a new AD forum guy

Postby Billtu » Wed Aug 13, 2008 6:56 am

When i calculate tank capacity, i typically measure the tank and calculate the area in cubic feet and round down. The best way would be to know the liquid capacity by a label or by filling the tank with water and somehow measuring that amount of water and converting. This gives you the capacity at 1 atmosphere. Whatever the current barometric pressure is. Standard is 29.92 inches of mercury. Anything less, is a vacuum, anything more is compression.
I measured the 100# tank to be slightly more than 3 cubic feet. I hope somebody will correct me if i am wrong, but if i compress that to 100 psi, i will have 300 cubic feet of gas. So compressing as high as 250psi, i'd have 750 cubic feet.

If you wish to run a spark ignition engine on the gas, you need to know the engine displacement. Divide by two if it's a four cycle engine. Factor in your running rpm or average rpm, and know that it take approximately 10% biogas (the rest being air) to run an engine at a given rpm. For examples i've calculated for a 125cc = .44 cubic feet/minute @ 2000 rpm. Double that for a 250cc engine.

Back to our tank @ 250psi. You could run for slightly over 1700 minutes or around 28 hours.

Like i said i'm using a 3/4hp motor. 1hp is just under 750 watts.

The more i explore this, the more i think a person needs to find what he/she needs to power, say an engine on a generator. Then calculate the consumption as above. Then if a 5 gallon digester makes .25 cubic feet @ 4"WC daily, what's it take to run a given engine, either continuosly or for a given daily period.

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Bob
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Re: a new AD forum guy

Postby Bob » Wed Aug 13, 2008 9:49 am

I hope somebody will correct me if i am wrong, but if i compress that to 100 psi, i will have 300 cubic feet of gas. So compressing as high as 250psi, i'd have 750 cubic feet.

According to Boyle's Law, the relationship between pressure and volume is not linear, but in actual practice, I don't know exactly how to apply it. As you compress it further, there are diminishing returns. For example, it will take a lot less compressor energy to take it from 0 to 100 psi than it will to take it from 100 to 200 psi. And as it is compressed, the temperature of the tank will rise, further complicating it. Especially given the relatively low energy density of the biogas to start with, I'd like to figure out an optimum between size of storage tank and degree of compression, but am not sure how to go about it.

Boyles_Law_animated.gif
Boyles_Law_animated.gif (263.5 KiB) Viewed 17847 times

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Bob
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Re: a new AD forum guy

Postby Bob » Thu Aug 14, 2008 12:55 pm

After thinking about it further -- and asking around and getting some "how to think it" help from a friend, I think I have an answer for you on how to calculate the compression factor correctly. You need to use absolute pressure (psia) rather than gauge pressure (psig). Atmospheric pressure at sealevel is about 14.7 psig. So if a reading of 0 psi on your gauge = 14.7 psi absolute (i.e. 1 atmosphere), a reading of 100 psi would be equivalent to 100/14.7 = 6.8 atmospheres.

At 100 psi compression, you will have compressed the original volume of gas by 6.8 times. So, for a 3 cu ft tank, it would work out to about 3*6.8 = 20 cu ft -- if it were released back to atmosphere. For 250 psig it would be 3 * 250/14.7 = 51 cu ft.

And I don't know at which point the curves cross, but at some point, the energy required to compress it further will exceed the heat value (energy content) of the gas itself. -- An interesting problem, to be sure.

Billtu
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Re: a new AD forum guy

Postby Billtu » Thu Aug 14, 2008 2:24 pm

Excellent information! Thank you very much!
That's good stuff to know.
I was aware that methane is a chriogenic gas but as i was not going to compress it much, i didn't factor that in.

Re-figuring very soon.

Another question: is Water Collumn figure from absolute psi? In other words 27.68"WC = 1psi or 1 psia?

Thanks again,

Bill

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Bob
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Re: a new AD forum guy

Postby Bob » Thu Aug 14, 2008 2:34 pm

Another question: is Water Column figure from absolute psi? In other words 27.68"WC = 1psi or 1 psia?

It's psi.

Billtu
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Re: a new AD forum guy

Postby Billtu » Mon Aug 18, 2008 5:45 am

Sorry, of course water collumn would be from gage zero, as one end is open.

My uncle worked for a compressed gas supplier so i discussed it with him, as well. Although there is the term vacuum, the way to think of it is an extreme reduction in air pressure. If i evacuate a tank, although not perfect, there is close to 14.7 psi that can't be used out of the tank because it's equal to the exterior pressure. Drawing a tank down to "best" vacuum, i have to put roughly 14.7 psi into the tank before the gage needle will come off it's low-stop. It would be interesting to find a gage calibrated for absolute.

To bring it to a practical reference, I've pumped batch after batch over to the storage tank, but as yet the needle on the gage has not come off the stop. Now i know why.

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Re: a new AD forum guy

Postby Bob » Mon Aug 18, 2008 9:49 am

So you have to pump a lot more gas into it to get it to zero psig. (Sounds like you need to get going on your production scale digester... :) )

I guess the only way you could have a gage directly read absolute pressure would be if it were in orbit, not at the bottom of this ocean of air.

davidwillis
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Re: a new AD forum guy

Postby davidwillis » Thu Jun 17, 2010 8:21 am

Does anyone know a way to estimate how much energy it takes to compress the biogas?


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