[Digestion] Digestion Digest, Vol 15, Issue 3

[Carolyn Henri]

RE: To scrub or Not to scrub?  That is the question.  (Scrubbing biogas for 
use as fuel in internal combustion engine).  I have observed on several 
small farm digester systems that are using biogas to produce electricity 
with an I-C engine that there is not one right answer to this question.

It appears that operationally, the decision to scrub or not to scrub comes 
down to a COST TRADE-OFF between a capital investment in scrubbing 
equipment, and an ongoing maintenance/lubrication cost for the internal 
combustion engine.   The scrub/not scrub decision must be made based on how 
much it will cost to install and operate the scrubbing equipment vs. what it 
will cost to maintain the engine in the presence of higher H2S 
concentrations in the (unscrubbed) gas.    Cost analysis should be 
undertaken only after there is a clear understanding of: 1) the chemical 
components of the gas coming from the digester in question and 2) the fuel 
quality requirements of the specific generator/engine that will be used.

Capital cost of scrubbing systems for on-farm digesters (using primarily 
cow-manure feed stock) run the gamut from low-tech, in-expensive, to highly 
complex, enormously expensive.  I've not found any technology consistency AT 
ALL in this aspect of on-farm biogas clean up (in the US).

The volume of gas available from the digester to run the generator, the 
energy output of the generator, and the price received for the electricity 
sold or consumed will also be factors in this decision.

That's the economic view from here.
Cheers,
Carolyn Henri
**************************************************
Carolyn J. Henri, Ph.D.
Resource Consulting Service, LLC
4805 Belvedere Ave.
Everett, WA 98203

Phone: (425) 290-3181
Fax: (425) 290-7586
Cell: (425) 308-1634
E-Mail: Carolyn at ResourceConsulting.us
Website:  www.ResourceConsulting.us

**************************************************

Regarding ----- Original Message ----- 
From: <digestion-request at listserv.repp.org>
To: <digestion at listserv.repp.org>
Sent: Sunday, September 02, 2007 9:00 AM
Subject: Digestion Digest, Vol 15, Issue 3


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> Today's Topics:
>
>   1. Re: Scrubbing biogas (Paul Harris)
>   2. Re: Scrubbing biogas (Paul Harris)
>   3. Re: compressing biogas (Paul Harris)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Sun, 02 Sep 2007 06:20:07 +0930
> From: Paul Harris <paul.harris at adelaide.edu.au>
> Subject: Re: [Digestion] Scrubbing biogas
> To: adkarve <adkarve at pn2.vsnl.net.in>
> Cc: digestion at listserv.repp.org
> Message-ID: <46D9D07F.AEE7FBA9 at adelaide.edu.au>
> Content-Type: text/plain; charset=us-ascii
>
> G'day All,
>
> As I said recently in relation to CO2, you need to look at the whole
> system when considering H2S scrubbing. If the H2S level is such that it
> halves engine life or causes other major problems it is probably worth
> the effort of scrubbing, but if it just means a slight reduction in
> engine life I would suggest leaving it in the biogas (it s a fuel, after
> all) and doing away with all the messing around. This "engineering"
> approach amy not satisfy the purists, but most of us want minimum work.
>
> Happy digesting,
> HOOROO
>
> adkarve wrote:
>>
>> Dear Biogas Workers,
>> All over the world, efforts are on to use biogas as fuel in an internal
>> combustion engine. We were told by an expert that there was no need to
>> remove CO2 from the biogas. I would like to know, how important it was to
>> remove H2S from biogas. In my system, where we produce biogas from food
>> waste, we have only traces of H2S in our biogas. Due to oxidation, it is
>> supposed to get converted into S02, and by combining with water vapour,
>> perhaps into H2SO3. This is not as strong an acid as H2SO4. The cylinder,
>> piston, piston rings and valves of an internal combustion engine are
>> supposed to be coated by a film of lubricating oil. So how much is the
>> danger of corrosion of the engine due to sulphurous acid? There is also
>> nitrogen in the air, which goes into the engine, and a part of it gets
>> converted into NOx.  Does this compound get converted into nitrous and
>> perhaps also into nitric acid? Don't these acids harm the engine?  I 
>> shall
>> be grateful to get answers to my questions.
>> Yours
>> A.D.Karve
>>
>> ----- Original Message -----
>> From: Duncan Martin <duncanjmartin at eircom.net>
>> To: Warren Weisman <weiswar at yahoo.com>; <digestion at listserv.repp.org>
>> Sent: Friday, August 31, 2007 3:39 AM
>> Subject: Re: [Digestion] Scrubbing biogas into biomethane article
>>
>> > Whatever method you think of using, do reflect first on the advice 
>> > given
>> by
>> > Paul Harris the few hours ago to consider very carefully why you should
>> want
>> > to produce pure methane.
>> >
>> > Unless you want it for some kind of chemical feedstock, you presumably
>> want
>> > it as a fuel.
>> >
>> > If you want it as a transport fuel, it is true that storage capacity 
>> > would
>> > be enhanced by the use of pure methane.  However, it would not even be
>> > doubled, so the cost would have to be very low to make this worthwhile.
>> >
>> > Pure methane would be more energy efficient in any combustion device 
>> > BUT
>> the
>> > energy required to purify it might well exceed any efficiency gain --  
>> > and
>> > that is before you even consider the cost.
>> >
>> > Bear in mind too that the diluent effect of carbon dioxide in 
>> > combustion
>> is
>> > vastly exceeded by the diluent effect of the nitrogen in the combustion
>> air.
>> >
>> > Consider the stoichiometry of combustion.
>> >
>> > With pure gases:
>> > CH4 + 2O2 = CO2 + 2H2O
>> >
>> > If the CH4 comes with some unwanted CO2 (lets say 2:1 for simplicity), 
>> > the
>> > eqn becomes
>> > (CO2 + 2CH4) + 4O2 = 3CO2 + 4H2O
>> >
>> > Now the extra CO2 is indeed a 'passenger' in any boiler or gas engine,
>> > adding to the volume of the  hot gases that escape via the flue or 
>> > exhaust
>> > pipe -and thus to the energy lost in them.
>> >
>> > BUT remember that the oxygen comes with a lot of unwanted gas too -
>> > nitrogen, in a 4:1 ratio, so the true equation for the combustion 
>> > process
>> > is:
>> > (C02 + 2CH4) + (4O2 + 16N2) = 3CO2 + 4H2O + 16N2
>> >
>> > The nitrogen is just as much a passenger as the carbon dioxide is --  
>> > and
>> > there is 16 times more of it (by volume). We all know that it is very
>> rarely
>> > economic to supply an engine or any boiler with pure oxygen instead of 
>> > air
>> > (unless you work for NASA!), so this should be a warning that the 
>> > removal
>> of
>> > carbon dioxide is unlikely to make sense either. Of course, it might be
>> > economic if the removal of carbon dioxide was very much cheaper than 
>> > the
>> > removal of nitrogen. However, there is little point in considering one
>> > without considering the other, since for all you know you might be
>> > considering the less economic option.
>> >
>> > Food for thought?
>> >
>> >
>> > Best regards
>> >
>> > Duncan J Martin
>> >
>> > Chair
>> > Republic of Ireland Centre
>> > Chartered Institution of Wastes Management
>> >
>> > ================================
>> > CONTACT DETAILS
>> > Duncan J Martin, PhD, CEng, CSci, MIEI, MCIWM, MIChemE
>> > 24 Townsfield, Cloughjordan, N Tipperary, Ireland
>> > Mobile: +353 86 8377 906
>> > Home: +353 505 42087
>> > Email: duncanjmartin at eircom.net
>> > ================================
>>
>> _______________________________________________
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>> Digestion at listserv.repp.org
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>> Beginner's Guide to Biogas
>> http://www.adelaide.edu.au/biogas/
>> http://info.bioenergylists.org
>
> -- 
> Mr. Paul Harris
> Room G8, Leske Building
> Faculty of Sciences,
> The  University of Adelaide, Roseworthy Campus, AUSTRALIA 5371
> Ph    : +61 8 8303 7880
> Fax   : +61 8 8303 7979
> mailto:paul.harris at adelaide.edu.au
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> ------------------------------
>
> Message: 2
> Date: Sun, 02 Sep 2007 06:23:26 +0930
> From: Paul Harris <paul.harris at adelaide.edu.au>
> Subject: Re: [Digestion] Scrubbing biogas
> To: "alastair ward (IGER-NW)" <alastair.ward at bbsrc.ac.uk>
> Cc: digestion at listserv.repp.org
> Message-ID: <46D9D146.92ED6310 at adelaide.edu.au>
> Content-Type: text/plain; charset=us-ascii
>
> G'day Again,
>
> Yes, and one landfill site I just visited (with Caterpillar engines) did
> not worry about H2S and got reasonable performance but another serage
> treatment site with different engines had real trouble (and both had
> quite similar H2S readings). You need to get all the details!
>
> Happy Digesting,
> HOOROO
>
> "alastair ward (IGER-NW)" wrote:
>>
>> I read somewhere that the H2S is a problem for copper based components in 
>> an engine, including gaskets and some bearing surfaces so I would think 
>> that for this use scrubbing H2S is very important. I believe the waste 
>> water treatment industry has most experience with biogas engines?
>>
>> Alastair Ward
> -- 
> Mr. Paul Harris
> Room G8, Leske Building
> Faculty of Sciences,
> The  University of Adelaide, Roseworthy Campus, AUSTRALIA 5371
> Ph    : +61 8 8303 7880
> Fax   : +61 8 8303 7979
> mailto:paul.harris at adelaide.edu.au
> I now use "MailGuard" - if you do not get a reply please make contact
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> http://www.adelaide.edu.au/directory/paul.harris
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> ------------------------------
>
> Message: 3
> Date: Sun, 02 Sep 2007 06:40:39 +0930
> From: Paul Harris <paul.harris at adelaide.edu.au>
> Subject: Re: [Digestion] compressing biogas
> To: Michael Smith <michael_1234 at msn.com>
> Cc: digestion <digestion at listserv.repp.org>
> Message-ID: <46D9D54F.FF28A1E6 at adelaide.edu.au>
> Content-Type: text/plain; charset=us-ascii
>
> G'day Michael,
>
> One of my favourite quotes comes from Lord Thomas Dewar - "Minds are
> like parachutes, they only function when open!" to which I add that both
> are othen used only in emergencies.
>
> You need to look at each of your energy NEEDS (not wants!) and see what
> best meets that need. By the time you cook, heat water (which can
> possily be mainly solar hot water anyway) and a greenhouse I doubt you
> will have too much biogas left, even using waste heat from the
> generator.
>
> Liquid fuels are far better for transport use than compressing biogas
> (but for short hauls you could use a flexible bag).
>
> All the best,
> HOOROO
>
> Michael Smith wrote:
>>
>>  What a GREAT discussion. If someone will, please respond to my scenario.
>>    A 10 acre plot with a home and one or more greenhouses....10x100 feet 
>> or so. I will have access to grass clippings from commercial and 
>> multi-family area contractors 5 days a week for 8-9 months of the year. 
>> Irrigation from all is from well water and the fertilizer is the 
>> available commercial brands used in the Midwest. I would like to digest 
>> this for the fertilizer affluent and solid matter most of all. I also 
>> would like to compress the gas for intermittent use for electrical, water 
>> heating, or heating for the greenhouses. If feasible to power a vehicle, 
>> it would be nice to have the option of driving to market, to the store, 
>> or social events in a bio vehicle.
>>   Power for the homestead is wind, battery backup and generator backup as 
>> well.
>> I'm sure there are others in the U.S. with the same idea. However, the 
>> standard answer is to use it at the time it is made. Is there an 
>> economical way for this scenario to come alive?
>>  By the way, orchard waste, as well as garden wastes would contribute 
>> also. All this organic material is FREE. In fact, it is savings to the 
>> contractors to give it to me instead of paying to dump it in a landfill.
>> _______________________________________________
>> Digestion mailing list
>> Digestion at listserv.repp.org
>> http://listserv.repp.org/mailman/listinfo/digestion_listserv.repp.org
>> Beginner's Guide to Biogas
>> http://www.adelaide.edu.au/biogas/
>> http://info.bioenergylists.org
>
> -- 
> Mr. Paul Harris
> Room G8, Leske Building
> Faculty of Sciences,
> The  University of Adelaide, Roseworthy Campus, AUSTRALIA 5371
> Ph    : +61 8 8303 7880
> Fax   : +61 8 8303 7979
> mailto:paul.harris at adelaide.edu.au
> I now use "MailGuard" - if you do not get a reply please make contact
> again (by fax?)
> http://www.adelaide.edu.au/directory/paul.harris
> Member IOBB http://www.iobbnet.org/drupal/
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> End of Digestion Digest, Vol 15, Issue 3
> ****************************************
>
>