GAS-L: Re: Anaerobic digestion - Energy Yields

["MINI WELL SYSTEMS [PVT] LTD" <mwsys@slt.lk>]

At 07:57 AM 9/26/02 +1200, you wrote:
>WK,
>
>I was curious about your rule of thumb for anaerobic digestion, which was
posted on the Gasification list (GAS-L).  So I have run a few calculations.
Do these rough assumptions and estimates make sense to other GAS-L participants?
> 
>The rough theoretical yield from anaerobic digestion can be determined by a
carbon balance.  Lets start with biomass with 40% moisture, because
digesting biomass makes best sense when the feed is moist and we want to
avoid the energy penalty of drying it.  Biomass with 40% moisture has a net
calorific value (CV) of about 10 MJ/kg (million Joules per kilogram).
> 
>The dry material in biomass typically comprises 50% carbon with the rest
being hydrogen and oxygen in various forms of carbohydrates, such as cellulose.
>
>In wet anaerobic conditions bugs convert carbon in cellulose into equal
molecular proportions of methane (CH4) and carbon dioxide (CO2). So half of
the decomposed carbon ends up in methane.
>
>So if we start with 1 kg of moist boimass with a net calorific content of
10 MJ.  Then we have 600 grams (g) of dry biomass of which half is carbon.
So we have 300 g of potentially decomposable carbon.  If it all decomposes,
half the carbon would become methane.  So perfect anaerobic digestion would
yield methane containing 150 g of carbon, that is 200 g of methane.  The net
CV of methane is 50 MJ/kg.  So the energy content of our resulting
theoretical yield of methane in biogas is 10 MJ.  This is the perfect limit.
>
>However, the second law of thermodynamics tells as that when you change
energy from one form to another you can't do it without losses. For example,
our bugs would want their small share of the energy to pay them for their
hard work.  So the conversion of carbon in moist biomass to equal
proportions of CH4 and CO2 in biogas must be less than 100% efficient. 
> 
>In a landfill, a good rule of thumb is that 50% of the decomposable organic
carbon eventually decomposes to landfill gas, typically over a few decades.
A landfill is a essentially a crude biodigester with 50% overall energy
conversion efficiency.  In a well-engineered biodigester a higher energy
efficiency should be achievable over a much shorter time scale. If an
engineered biodigester could achieve 80% energy conversion efficiency, then
our one kg of moist biomass would yield 8 MJ of net energy.  This yield
corresponds to 1008 Watt-hours per pound of moist biomass (40% moisture) on
a net heating value basis.
> 
>I would like to compare this estimate with your suggested yield of "1 watt
per pound"  However, that rule of thumb is hopelessly ill-defined, not least
because it mixes power and energy units with no indication of the time
basis. It provides an illustration of the message from my previous posting
on the Gas-L that good definition of units is crucial to the communication
of technical information.
> 
>I suspect that the original intent of that rule-of-thumb might have been to
indicate that anaerobic digestion might yield about "one kilowatt-hour of
energy per pound of harvested biomass"
>
>Of course, if one wanted to convert that energy resource into electricity
then a further very large energy loss would be involved.  If a small
gas-engine generator can achieve, say, 20% thermal efficiency (net basis),
then a digester/engine set-up might yield about 200 watt-hours of
electricity from each pound of moist biomass, that is 200 watts of
electricity from one pound per hour of moist biomass (at 40% moisture).  
> 
>I hope these comments help to clarify the matter.
> 
>Regards
> 
>Steve
> 
>Steve Goldthorpe Energy Analyst Ltd
>PO Box 68, Greenhithe
>Auckland, New Zealand
>Phone 09 413 9696
>Fax 09 413 9642
>Mobile 0274 849 764
>Email gldthrp@nznet.gen.nz
>          ----- Original Message ----- 
>  From: wskletzker@aep.com 
>  To: Jeanne B. Koger 
>  Cc: gasification@crest.org ; MINI WELL SYSTEMS [PVT] LTD 
>  Sent: Thursday, September 26, 2002 2:19 AM
>  Subject: Re: GAS-L: Energy Yields and costs


Steve,

Thank you for an excellent scientific response.

In most third world countries the bio fertiliser 
often determines the economics of anaerobic digestion. 
Is there any means of bringing the energy and the fertiliser to a common unit.
Perhaps even in terms of the energy requirement for the equivalent
manufactured fertiliser.


In gasification as far as I am aware the ash has no practical value

In wind and solar power these are often justified by the net energy input
for their hardware

Should not gsifiers have similar justification for the different routes to
gaification.


Regards 


Tharu


V.Tharumaratnam
Mini Well Systems (Pvt) Ltd
161/1 Colombo Rd, Hewagama,
Kaduwela, Sri Lanka
e-mail: mwsys@slt.lk


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>