[Digestion] Nitrogen Emissions from Biogas

[Bjorn Dahlroth]

Hi everybody
When you try to reduce Nox emmissions from powerstations or district heating plants by for instance changing the air supply in such a way that you reduce the combustion temperture it is not uncommon that you get more formation of N20 instead and the amount you get varies from plant to plant depending on the design and how you balance the air supply. However you can expect that the power station will produce more than the digestion plant. Whenever you burn something at lower temperatures you get some N20 - be it power stations or camp fires. Nitrogen that forms N2O comes more from the fuel than the combustion air. You will for instance get more N20 from burning biomass as this fuel is more rich in nitrogene than coal or oil. 

I believe the emission of N2O from digestion plants has not been studied but I would expect - without being an expert - that if you get N2O it would more likely come from any composting of the solid digestion residues if not handled properly and if the moisture level is on the high side. However one can not be certain. How much ammonia would there be in the biogas that is not uppgraded and going directly from the plant to a spark plug gas engine? I have not seen such figures. If you know that you can say more about if there is risk for formation of N2O and then you can decide whether to measure or not.

Anyways I would not be surprised if there is more N20 produced by nitrogen fertilizer of any kind that the farmer has spread on the fields and especially in wet weather, but this also depends on how it is done, whether the fields have something growing or not and how far down from the soil surface that most of the fertiliser is staying.  These questions are rather interesting and important when we have to hunt all kinds of green house gases. N20 is also suspected to be not so good for the ozon layer. One must also remember that all the ammonium that is lost from the handling and spreading of the residues will end up somwhere else where we don't want it and some of it will cause acidification, some will cause eutrofication and some will be converted to N20 - all dependeing on where it falls and when.

Regards
Bjorn Dahlroth
Stockholm County Association of Local Authorities



-----Ursprungligt meddelande-----
Från: digestion-bounces at listserv.repp.org [mailto:digestion-bounces at listserv.repp.org] För Paul Harris
Skickat: Thursday, March 08, 2007 11:45 PM
Till: Tom Miles
Kopia: 'The Digestion Discussion List'
Ämne: Re: [Digestion] Nitrogen Emissions from Biogas

G'day Tom et al,

If biogas is 60% methane and about 40% CO2, with traces of water,
ammonia, H2S etc there will not be much N20. I understand (from my
simple engineering knowledge, which does not really extend to
combustion!) that NOX (N2O?) emissions are a result of combustion, as
they require elevated temperatures, so there should be very little from
a digester compared to a power station.

Since a 2000 cow dairy herd will only generate about 140 kW of
electricity anyway it should be WWAAYY in front of a 10MW power station
anyway.

Happy digesting,
HOOROO

Tom Miles wrote:
> 
> We have had an inquiry from a concerned farmer in Washington state (US)
> about the promotion of biogas digesters for dairy use.  His impression is
> that "Such a  dairy waste water and solids treatment system is a prolific
> emitter of N20."
> 
> His question:"Does anyone have any citations for N20 emissions from a 2000
> cow dairy manure digester? How does such a digester compare to say a 100 mw
> coal plant?"
> 
> How can nitrogen emissions from digestors be captures for use in
> agriculture?
> 
> Where can we find answers to these questions?
> 
> Thanks
> 
> Tom Miles
> 
> _______________________________________________
> Digestion mailing list
> Digestion at listserv.repp.org
> http://listserv.repp.org/mailman/listinfo/digestion_listserv.repp.org

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