Thanks Jon, This is a good question and you're right. Biochar represents untapped carbon energy... this could be further reduced to achieve maximum energy output... however, if we at all care about carbon it'd be better to bury the biochar (See Gaunt and Lehmann, 2007 "Energy Balance and Emissions Associated with Biochar" http://www.css.cornell.edu/faculty/lehmann/publ/ES&T%2042,%204152-4158,%202008%20Gaunt.pdf) I remember seeing the energy density of biochar... I might be able to find it, but don't remember off the top of my head.
Gasification is essentially pyrolysis without the biochar left over which accomplishes this more complete oxidation of the biomass carbon content. The reason I said it was "more efficient under certain circumstances" (and I might be wrong here...) is three fold: 1) The advantage of pyrolysis or gasification is that syngas can be combusted at greater temperatures than can the biomass directly... thus the efficiencies of Carnot's limit are extended and efficiencies increased. (Carnot's limit says something like the maximum efficiency of an internal combustion engine cannot be greater than the hypothetical "Carnot engine" and that this is a function of the absolute heat of heated reserve vs. the absolute heat of the colder reserve... the greater these differences the greater potential efficiency... any physicists out there should correct me if I'm wrong... ). 2) The syngas stream can be transformed into electricity in a gas generator thereby permitting both electricity and heat from a single process (coming at the cost of greater sophistication and complexity, of course). I don't think this dual energy generation capacity necessarily provides more energy in terms of net energy out, but it could, perhaps, offer more useful net energy out 3) Syngas could potentially be put through a fuel cell thereby bypassing the Carnot limit altogether.... Lastly (and again this isn't an efficiency question... but kinda is) pyrolysis or gasification can utilize a lot of materials which are not easily combusted. Can't throw grasses into the wood burner, but you can pyrolyze them fairly efficiently. This is fairly important when it comes to developing countries where people are having to resort to cutting down forested land to burn biomass or to convert into charcoal to be burned (see Haiti, for instance). They could instead be growing perennial low-input grasses which need no fertilizer and no irrigation and sequester carbon in their root structures and can be harvested 2 or 3 times a year in tropical countries. Again, I might be off on one or more of these things and Jon you certainly may be right about efficiencies of pyrolysis vs. direct combustion... Gaunt and Lehmann, in the aforementioned paper, suggest slow pyrolysis is between 2-9 in terms of net energy (EROEI: Energy Returned On Energy Invested) which is pretty darn good compared to the best numbers for corn ethanol 1.8 and significantly better than the worse number for corn ethanol 0.8. It's also important to bear in mind that there are a lot of different pyrolysis and gasification technologies... some of which are capable of producing greater energy out of a given input, but usually at the cost of producing less biochar. To conclude, I think perhaps the most significant efficiency of pyrolysis is the energy produced vis-a-vis the carbon sequestered... it is the only proven carbon negative energy generation system I've seen. And it creates some really nice carbon input to help crops... particularly in low fertility and highly degraded soils which are common among the weathered tropics and semi-arid drylands where the majority of the developing world live. But to reemphasize what Karl said... there's no simple, one-off solution out there. Holistic thinking is the only way that we're going to get to where we need to be... By way of footnote, I recently found this "development" organization which I think may be doing among the most important work in Africa using synergistic sustainable solutions that use this holistic thinking Karl suggests: http://excellentdevelopment.com/ (watch the 30 min film "Walking on Water") Best, Ryan On Fri, Aug 7, 2009 at 4:58 PM, Jon Bosak <[email protected]> wrote: > Ryan Hottle wrote: > >> 2) Pyrolyzation of biomass is under many circumstance more >> efficient than direct combustion and can additionally produce >> electricity and biochar. >> > > I'm not clear on how this works. (I'm not quarreling with the > statement; I just don't understand it.) > > Basically the energy is coming from the oxidation of carbon, > right? So if some of the carbon stays carbon, wouldn't that mean > less net energy out? > > Jon > > > _______________________________________________ > For more information about sustainability in the Tompkins County area, > please visit: http://www.sustainabletompkins.org/ > > RSS, archives, subscription & listserv information for: > [email protected] > http://lists.mutualaid.org/mailman/listinfo/sustainabletompkins > Questions about the list? ask > [email protected] > free hosting by http://www.mutualaid.org > _______________________________________________ For more information about sustainability in the Tompkins County area, please visit: http://www.sustainabletompkins.org/ RSS, archives, subscription & listserv information for: [email protected] http://lists.mutualaid.org/mailman/listinfo/sustainabletompkins Questions about the list? ask [email protected] free hosting by http://www.mutualaid.org
