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
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