[Digestion] compressing biogas

[Duncan Martin]

Michael

Your proposal begins to make more sense if you are using wind power to 
compress your biogas. In effect, that gives you a way of storing wind power.

Once you have compressed the biogas, removal of most of the carbon dioxide 
becomes easy, since it is far more soluble in water than methane is.  All 
you need is a simple packed column in which the gas flows countercurrent to 
a downflow of water and a pump to pressurise the water to the same pressure 
as the gas.

I suspect this process would also remove most of any hydrogen sulphide that 
happens to be present.

There would not usually be much nitrogen in biogas and it would have little 
effect in use.

However, the capital cost of your wind turbine, compressor, carbon dioxide 
absorption column and high-pressure storage tank will be pretty high. 
Plenty of other people have investigated the same idea and come to the 
conclusion that using the biogas as it is produced, at low pressure and 
without much purification makes most sense. More than 24 hours storage 
capacity is rarely economic.

Note, however, that this capacity is enough to give you the possibility of 
generating electricity when you need it, rather than continuously. Also, if 
you can export to the grid and if your utility company will pay more for 
power at peak periods (unlikely, for a small player), this gives you the 
potential to generate only when it will maximise your income.

However, you must add the cost of your generator and a large storage tank to 
your capital budget.  A low pressure store for 24 hours gas production will 
be comparable in size with your digester, so a significant cost item in its 
own right.

So do your sums carefully!

Incidentally, you can still store energy even if you do not store biogas. 
You mention using the biogas for electricity generation, domestic hot water 
and greenhouse heating  It is much easier to store hot water than gas, so 
heat a well-insulated tank whenever you have gas and use this for the latter 
two purposes.  If you plan to run a gas engine to generate electricity, use 
the waste heat from the engine.  Otherwise burn the biogas directly in a 
boiler.

You also mentioned using grass cuttings as a major feedstock. You might 
check out any Austrian websites you can find that give information on AD, 
because quite a lot of farmers there grow grass as a biomass crop and feed 
it into digesters. You should find some very relevant experience there.

Best of luck!

Duncan J Martin

Chair
Republic of Ireland Centre
Chartered Institution of Wastes Management

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Duncan J Martin, PhD, CEng, CSci, MIEI, MCIWM, MIChemE
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Duncan Martin

----- Original Message ----- 
From: "Michael Smith" <michael_1234 at msn.com>
To: "digestion" <digestion at listserv.repp.org>
Sent: Sunday, September 02, 2007 9:31 PM
Subject: [Digestion] compressing biogas


> If the methane in pressurization is liquefied @ approx. 1100 psi what are 
> the consequences of the CO2 and Nitrogen at these pressures? I expect more 
> than enough electricity from wind power at various times to pressurize...2 
> to 4 cu. ft. per minute.
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> Beginner's Guide to Biogas
> http://www.adelaide.edu.au/biogas/
> http://info.bioenergylists.org
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