[Gasification] the most important thing (quitepossibly) i'velearned to date
Kevin
kchisholm at ca.inter.net
Tue Oct 19 08:51:42 CDT 2010
Dear Mark
----- Original Message -----
From: Mark Ludlow
To: 'Kevin'
Cc: gasification at lists.bioenergylists.org
Sent: Tuesday, October 19, 2010 5:14 AM
Subject: RE: [Gasification] the most important thing (quitepossibly) i'velearned to date
Dear Kevin,
It is the Second Law that I refer to. In simple terms it states that entropy will never decrease (spontaneously) within a given system. So while there may be a way of recovering, say, "waste heat" from a process, no re-formulation of the process will make the end products more energetic than they were in their native, unreacted state. I am quite certain that you will not find this simplified expression worthy of argument (or will you?).
# The way Jim posed the original problem was that his fundamental "system" (Gasifier, Engine, Generator) was wasting low and medium grade thermal energy in the form of sensible and latent heat in the engine exhaust, and he was wondering if there is a good way to use this thermal waste productively. Clearly, evaporating water contained within fuel is an endothermic heat load which the energy in the exhaust could meet. His overall "energy objective" is to reduce the biomass energy input per unit of electrical energy output, by using an available energy resource that would otherwise be wasted, rather than reducing entropy.
# If you feel this is an argument, then I am guilty. :-) However, I would prefer to think of it as "polarizing the issue clearly" so that the results of his tests will be clear
Best wishes,
Kevin
Very best regards,
Mark
From: Kevin [mailto:kchisholm at ca.inter.net]
Sent: Monday, October 18, 2010 9:08 PM
To: mark at ludlow.com; Discussion of biomass pyrolysis and gasification
Subject: Re: [Gasification] the most important thing (quitepossibly) i'velearned to date
Dear Mark
----- Original Message -----
From: Mark Ludlow
To: 'Discussion of biomass pyrolysis and gasification'
Sent: Monday, October 18, 2010 4:48 AM
Subject: Re: [Gasification] the most important thing (quitepossibly) i'velearned to date
Hi Kevin!
Second Law! Energy is not "created" by the reaction. It already exists in the individual components of the reaction.
# Very interesting comment! However, I think you are inferring that my questions are an attempt to violate the First Law, and not the Second. The First Law relates to the balance between mass and energy in a reaction. It can be simply stated as:
"First Law: You can't get something for nothing."
The Second Law deals with randomness and disorder, and the impossibility of getting a perfect reaction. It can be stated equally simply as:
"Second Law: As a matter of fact, you can't break even."
# My question deals with how to set up the reaction so that with a given amount of chemical energy in the feed input, the use of energy content of the output gas is maximized.
Kevin, you can answer your own question.
# I think you might have missed the significance of my question. What do you think my question was?
The only excess "free" energy is that which is expressed as sensible and latent thermal energy of the gaseous output (however: we cool gases for greater volumetric efficiency in UC engines, etc.).
# No. The excess "fee energy" that would otherwise be wasted includes:
1: The sensible and latent energy in teh gasifier output product gas.
2: The sensible and latent heat in teh engine exhaust after teh fuel gas passed through teh engine.
# I am not at all talking about "free energy." What I am talking about is, as stated above, the potential for maximizing energy content of output gas from a given energy content of biomass feed. More specifically, if there is more water in the feedstock, its endothermic heat burden on the reaction is increased, but if there is "waste heat" available, such as hot engine exhaust, it may be able to recover this waste thermal heat, and feed it into the reaction, to end up with a greater energy content in the output gas than is presently attainable. There is no "Something for Nothing" scam here... it is a question being able to recover waste exhaust heat as chemical energy in the output gas.
Even tars require energy for "cracking" (that's why catalysts are important). Look at the Gibbs Free Energy of the fuel infeed and you can compute the theoretical, 100% efficient (read:impossible!) output.
# Using catalysts to crack tars for gasification is totally unnecessary. Simply set fire to them.
One may wish to alter the composition of the output gas, but it ain't free, baby!
# I would strongly disagree with you. If the excess waste energy in the hot engine exhaust can be used to counteract the endothermic heat load of extra water in wet fuel, then the extra recovered energy is indeed free, from a cost standpoint, but in no way does it violate the First Law.
Otherwise I would be filling the petrol tank on my double-engined Hummer with quadruple A/C with my garden hose!
# No comment.
Best wishes,
Kevin
Yah!
Mark
From: gasification-bounces at lists.bioenergylists.org [mailto:gasification-bounces at lists.bioenergylists.org] On Behalf Of Kevin
Sent: Sunday, October 17, 2010 10:43 PM
To: Discussion of biomass pyrolysis and gasification; gasification at bioenergylists.org
Subject: Re: [Gasification] the most important thing (quite possibly) i'velearned to date
Dear Jim
Interesting thoughts!
What are your thoughts on the optimum moisture content of biomass feed to a gasifier? On the one hand, with low fuel moisture content, there is a low endothermic load on teh reaction. On the other hand, a high moisture content could yield greater hydrogen content in the output gas.
To take things to the limit, what are your thoughts on feeding a gasifier with torrified wood?
Thanks!
Kevin
----- Original Message -----
From: jim mason
To: gasification at bioenergylists.org
Sent: Monday, October 18, 2010 2:08 AM
Subject: [Gasification] the most important thing (quite possibly) i'velearned to date
there are many variables to move around in a gasifier, and all of them are
important. but once the basics are in order, i'm coming to the conclusion that
the most impactful thing one can do/add/fix to the basic imbert design is to use
the ic engine exhaust to heat the incoming fuel.
no, i don't have any numbers on this yet (we hope to get these soon), but the
anecdotal experiences keep piling up. the biggest flexibility gains i'm finding
both in poor fuel shape/size and moisture tolerance, as well as gas turndown ratio,
are from the ic exhaust heat exchanger. on our rig, this is called the pyrocoil. other
rigs call this something else.
this is not terribly surprising. the ic exhaust is the biggest waste heat
source we have around a gasifier. in principle, the ic exhaust has about about
3x or 4x the heat available as the outgoing syngas. the incoming fuel similarly
has much more heating capacity than the incoming air (the multiple i forget at
the moment, but i do know the incoming air can only take up about 1/2 of the
heat available in the outgoing syngas).
also, the ic exhaust is much hotter than the outgoing syngas after preheating
the incoming air. thus you can use the ic exhaust to do much more than dry the
fuel. you can it to drive the fuel through pyrolysis, and really, get it up to
about 4-500c before it falls into the hearth proper. this is a big difference
from the typical situation of still moist fuel falling into the hearth.
not only can you use the ic exhaust to add a very significant amount of heat to
the system, you can also use it to change the character of pyrolysis in the
reactor. a typical downdraft has very high temp short residence time pyrolysis
right on top of the nozzles. this high temp pyrolysis optimizes the creation of
teritary tars, or refractory tars, which are difficult to crack again (given
lots of double carbon bonds). if you externally drive pyrolysis at lower temp
over longer time, you get more primary and secondary tars, which are easier to
crack downstream (fewer double carbon bonds). this seems to allow hearth
conditions to be less perfect and still get good gas out.
using ic exhaust to heat incoming fuel is not a complete get out of jail free
card. but to me it seems the most impactful new thing one can do on these rigs.
it seems to have more of an impact than any other single thing we've done to
date on the gek.
yes, all of it is important, and all of it should be tended to, but the above is
my current vote for the biggest bang for the effort. hopefully we can get some
proper numbers on this soon (and prove or disprove the above conjecture).
jim
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Jim Mason
Website: http://www.whatiamupto.com
Current Projects:
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