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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 I have not found empirically based figures for anaerobic digestion yet, but, was given a rule of 1 watt per pound of solids from anaerobic digestion. Perhaps someone can direct you to a good resource for this. The AgStar data is very general. Regards, WK
This is a topic of interest to me as well. I hope folks will reply to the entire list, not just to Tharu. Thanks, Jeanne "MINI WELL SYSTEMS [PVT] LTD" wrote: > Hi, > > Will anybody tell me the comparative energy yields and > capital costs for > > 1. Gasification > 2. Anaerobic digestion > 3. Aerobic digestion. > > On gasifiers I have been getting costs varying from$150 to $600 per KW > On Anaerobic digesion costs seem to be in the reange of $250 > > Regards > > Tharu > > V.Tharumaratnam > Mini Well Systems (Pvt) Ltd > 161/1 Colombo Rd, Hewagama, > Kaduwela, Sri Lanka > e-mail: mwsys@slt.lk > > Gasification List Moderator: > Tom Reed, Biomass Energy Foundation, tombreed@attbi.com Biomass = > Energy Foundation, www.woodgas.com > List-Post: <mailto:gasification@crest.org> > List-Help: <mailto:gasification-help@crest.org> > List-Unsubscribe: <mailto:gasification-unsubscribe@crest.org> > List-Subscribe: <mailto:gasification-subscribe@crest.org> > - > Gasification List Archives http://www.crest.org/discussion/gasification/200202/ > Bioenergy 2002 http://www.bioenergy2002.org/ > 200 kWe CHP Discussion > http://crest.org/discussiongroups/resources/gasification/200kWCHP.html > Gasification Reference http://www.crest.org/articles/static/1/1011975339_7.html > > > Gasification List Moderator: Tom Reed, Biomass Energy Foundation, tombreed@attbi.com Biomass = Energy Foundation, www.woodgas.com List-Post: <mailto:gasification@crest.org> List-Help: <mailto:gasification-help@crest.org> List-Unsubscribe: <mailto:gasification-unsubscribe@crest.org> List-Subscribe: <mailto:gasification-subscribe@crest.org> - Gasification List Archives http://www.crest.org/discussion/gasification/200202/ Bioenergy 2002 http://www.bioenergy2002.org/ 200 kWe CHP Discussion http://crest.org/discussiongroups/resources/gasification/200kWCHP.html Gasification Reference http://www.crest.org/articles/static/1/1011975339_7.html >
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