[Digestion] Digestion Digest, Vol 5, Issue 21

[Art Krenzel]

Stan,

I had picked up a thread where Rex had described a great, simple idea for 
the removal of CO2 from a contaminated biogas stream by compressing the gas 
and bubbling it through water.  That is where the 5 atmosphere pressure 
figure came from.  Here is a copy of the original message:

Jason, Bjorn & Listers,
One of the best comments made on Digestion Digest, Vol 5, Issue 15 is
that the Internet provides so much help to one another.
I am looking to build a digestor that will treat cow & pig manure with
the gas driving an engine to produce power. As we will have a small
dairy, we need cold. Instead of an absorption chiller, the engine
supplier I was in contact with via the Internet suggested that I bolt on
a car's A/C compressor and use that to generate cold. Not only is the
equipment proven, it is easy to maintain (second hand units are freely
available and relatively cheap). If you need more cooling than a car's
A/C compressor, get a bigger compressor as you will have a bigger engine
because you will have more cows generating more biogas!
Another lister, Steve from New Zealand, sent me an article on using
water to remove CO2. Essentially, if you have a multi-stage compressor
to get up the pressures required for gas powered vehicles, you pull the
gas off after the first stage say at 5 bar, pass it through a column
irrigated by water and the water will absorb the CO2 just as in a fizzy
cooldrink. The CO2 loaded water is pumped to an open tank and irrigated
onto the surface. As the water depressurises, the CO2 escapes to
atmosphere. The water is then recycled to the scrubbing tower. The gas
in the meanwhile leaves the top of the tower and goes into the second
stage of the compressor. Just remember that there should be water traps
after each stage (anyone working with air compressors will also tell you
that). Finally, the beauty of the water scrubber is that if you add a
small about of alkali (lime, caustic, etc), you will also capture H2S!
This is a double win.
Cheers
Rex

I had added my view that if the biogas had been compressed to 5 atmospheres 
and bubbled through room temperature water, the biogas, after the pressure 
had dropped to atmospheric pressure, would be at a lower relative humidity. 
The condensed water from the biogas would dissipate in the scrubber water 
and disposal would be no problem.

I thought it was a great idea and wanted to clarify a needless concern about 
carbureter icing when using the improved biogas which had been cleaned by 
water scrubbing.  I also suggested using a gasifier as a fuel source for the 
IC engine as well.

Northern based IC engines operating under -50 deg F conditions are normally 
provided some protection against the elements.  This would be especially 
true if there was a biogas plant setting next to it that had to run at 70 - 
120 deg F to produce the fuel gas.

I support using waste heat of any sort and the IC engine produces a great 
deal of low AND high quality waste heat for a variety of uses.  Keep waving 
the flag!

Art Krenzel, P.E.
PHOENIX TECHNOLOGIES
10505 NE 285th Street
Battle Ground, WA 98604
360-666-1883 phone
phoenix98604 at msn.com


----- Original Message ----- 
From: "stan simon" <slsimon at tds.net>
To: <digestion at listserv.repp.org>
Sent: Friday, November 24, 2006 10:17 AM
Subject: Re: [Digestion] Digestion Digest, Vol 5, Issue 21


> If you are talking about relative humidity there may be a change in that
> parameter through  compression and expansion , however absolute humidity
> will not change without some separation of liquefied  water out of the
> process.
>
> The post that I was responding to did not mention anything about
> compressing, refrigerating and expanding the methane.
>
> What is the gasifier that you are referring to?  I thought we were looking
> at a digester producing impure methane with 40 % CO2, H2S, and saturated
> with water vapor.
>
> Northern Climate ground based IC engines operate at well below 60F intake
> air temperature, this is a variable that can range down to -50F depending 
> on
> where the system is .
>
> Without some specific datapoints at each of the stages in this process,
> (temperature, pressure, mole fractions of each gas)  it is very difficult 
> to
> make precise predictions.
>
> As for the low COP of absorption this is a well known fact, the double or
> triple stage units are better, the idea is to utilize waste heat or some
> other inexpensive source of energy.  To simply throw away the 65% engine
> heat of rejection has been a common practice.

>>
>> Today's Topics:
>>
>>   1. Re: condensation in gas lines (Art Krenzel)
>>   2. Re: Digestion Digest, Vol 5, Issue 20 (stan simon)
>>   3. Re: Digestion Digest, Vol 5, Issue 20 (stan simon)
>>
>>
>> ----------------------------------------------------------------------
>>
>> Message: 1
>> Date: Thu, 23 Nov 2006 10:52:20 -0800
>> From: "Art Krenzel" <phoenix98604 at msn.com>
>> Subject: Re: [Digestion] condensation in gas lines
>> To: <DIGESTION at LISTSERV.REPP.ORG>
>> Message-ID: <BAY108-DAV1ECEE69252B835D81FC609BE20 at phx.gbl>
>> Content-Type: text/plain; format=flowed; charset="iso-8859-1";
>> reply-type=original
>>
>> Stan,
>>
>> I think there is a fundamental error in this thinking about air saturated
>> with water vapor after a compression process.
>>
>> If you have a water saturated gas stream at 5 atmospheres at room
>> temperature and just allow the gas to drop to atmospheric pressure as you
>> would to feed it into the intake of an IC engine, the natural humidity
>> content of the gas DROPS.
>>
>> This is the basic process for removing water from instrument air.  You
>> take
>> a airstream which is saturated with water at room temperature and 
>> compress
>> it to a higher pressure.  Then cool it (using refrigeration) and the 
>> water
>> precipitates out, release the pressure again and now you have operating
>> dew
>> points in the -40 deg F range.
>>
>> You may not have a saturated air problem after all.
>>
>> As a pilot, carburetor icing can occur at a wide range of temperatures 
>> but
>> are more common with water saturated air streams below 40-50 degree F.
>> Most
>> ground based IC engines operate in an environment of 60 - 100 deg F range
>> using 50% relative humidity air and carburetor icing is not considered a
>> problem.
>>
>> I like the idea of using waste heat and the romance of absorption
>> refrigeration but the Coefficient of Performance of the absorption 
>> process
>> is quite low.  The gases exiting the gasifier come out at 1200 - 1500
>> degrees F.  Why don't you recover that heat and use it to heat the
>> gasifier
>> intake air.  You also gain by cooling the product gases so you can feed
>> them
>> into an IC engine at normal temperatures.  There should be a favorable
>> balance of energy to heat the gasifier feed air to a high temperature
>> easily.  The location of the hot and cool airstreams are adjacent so the
>> insulated piping of the air becomes much easier than ducting from an IC
>> engine located some distance from the gasifier.
>>
>> Art Krenzel, P.E.
>>
>>
>>>
>>>> Gentlemen,
>>>> Regarding the water based CO2 scrubbing, wouldn't this give you methane
>>>> that was saturated with water vapor?  I'm not sure what this would do
>>>> inan engine.  Maybe cause icing in your carburetor?
>>>> The beauty of absorption cooling is that it is driven primarily by low
>>>> quality (thermal) energy rather than expensive shaft power or
>>>> electricity. Hence you could make electricity with your methane and 
>>>> then
>>>> use the waste engine jacket and exhaust pipe heat to create cooling.
>>>> Have your cake and eat it too.
>>>> Stan L Simon, P.E.