[Gasification] tar and the mass balance problem (was Re: W-Gas and P-Gas defined)

jim mason jim at allpowerlabs.org
Sun Dec 11 22:34:22 CST 2011


the char to tar gas ratio out of pyrolysis can be "unbalanced" in the
manner i describe, and we can still get clean gas out of the gasifier.
 lots of things happen between the input of biomass and the output of
gas.  i was trying to clarify something i find importnat about what
the challenges are in the middle.

any "excess tar gas" that follows from the fixed carbon to volatiles
ratio of biomass as described, is handled by cracking.  you can make
co and h2 equally well by cracking (high temps and residence times) as
you can by combustion and reduction.   this is the point i'm trying to
clarify- there are two equally important routes to the end co and h2
(as well as some from the original pyrolysis).

my mass balance calcs suggest we actually make more co and h2 via the
cracking route than the combustion and reduction route.  both are
happening, though we seldom call them out clearly.  we call out the
combustion and reduction zones clearly in our explanatory graphics,
but don't so clearly call out the cracking zone, and its majority
contribution to co and h2 formation.  at some point i want to make a
sankey diagram that shows these flows, and provide an alternative to
the current understanding biases that follow from our current graphic
representations of gasification.

i find this useful to better define the problem we are trying to
solve, as well as understand why the window where tar conversion is
reasonable is so narrow.


On Sun, Dec 11, 2011 at 8:08 PM, Kevin <kchisholm at ca.inter.net> wrote:
> Dear Jim
> ----- Original Message ----- From: "jim mason" jim at allpowerlabs.org
> ...del...
>> Some gasifiers, such as the GEK produced and sold by Allpower Labs, run on
>> wood, and can produce an "Engine Grade Gas" with no provisions for tar
>> removal. Their kind of W-Gas has virtually no tar, while te stratified
>> downdraft gasifiers that I sell produce W-Gas with significant tar
>> quantities.
> kevin, thank you for the plug, but remember that nearly any gasifier
> can be made "tar free" or "tar laden" depending on how it is operated,
> and that goes for ours too.
> #  I'd have to respectfully disagree. I don't think it is possible to make a
> TLUD that gives tar-free gas, nor is it possible to make a top fed updraft
> gasifier. Imberts can be operated in a tar-free manner, but with a given
> Imbert geometry, an individual gasifier is limited to a range of fuel
> properties and hearth rates.
> sadly there is no single on/off switch that fixes the tar problem.  i
> think it is more helpful considered a "chronic health issue" that
> requires active management.  the goal is to design a system that has
> the widest window for good tar conversion, over the widest fuel
> variation as possible.  then add in controls or operator expertise to
> keep it within those windows.
> # As far as I can see, the Imbert design concept seems to offer the greatest
> potential at teh present for producing a reasonably clean gas from wood.
> There are about 4,500 intelligent and motivated people on the Wood Gas List,
> and only about 1/2 of one percent have been able to get gasifier systems
> operating to Engine Grade Gas standards. That tells me that the Imbert
> Design has some very serious limits.
> we cannot have a single done solution to the tar problem until we get
> rid of the volatiles from biomass.  if biomass was only fixed carbon,
> or rather, charcoal/coal already, then we'd have no heavy hydrocarbon
> stream to deal with.
> # Very true.
> the huge insight i had into this which doesn't seem to be shared
> widely by others is the excess tar problem that is revealed with you
> look at the mass balance through the stages of gasification.  biomass
> is approximately 20% fixed carbon and 80% volatiles.   what we like to
> tell ourselves i happening in a gasifier is the following:
> 1. pyrolysis to split the biomass into char and tar gas
> 2. burn the tar gas to co2 and h20
> 3. reduce the co2 and h2o over the char from step 1.
> this is a great story, but it assumes the outputs of pyrolysis (char
> and tar gas) are in some reasonable equilibrium so that we can burn
> all of the tar gas and have adequate char to reduce the results.  when
> you run the mass balance, you learn that we do not.  my calcs suggest
> we have less than half the needed char to reduce the amount of co2 and
> h2o that would result if we oxidized all the tar gas.
> # That can't be true. If it was true, then it would not be possible for you
> to produce tar free gas. The fact that you make tar free gas with consistent
> regularity proves you are wrong. :-) You are one of teh Gasifier Savants
> made the Bumble Bee fly. :-)
> this for me was the aha!  biomass produces an excess of tar gas vs the
> amount of char available.  this follows directly from the fixed carbon
> to volatile ratio common to most biomass.
> # How come you can make consistently good quality gas? :-)
> you have two basic choices to deal with this problem.
> 1.  you run your gasifier with the amount of oxygen that burns the
> portion of tar gas to the scaled amount of co2 and h2o to reduce the
> available char.  the excess tar gas that is not directly burned has to
> be cracked to co and h2 by adequate heat and residence time somewhere
> else in the process.
> 2. you pull the gasifier to a point where you do burn all the tar gas,
> but overwhelm the available char and send the excess out at co2 and
> h2o in the gas stream.
> mostly we try to regulate a gasifer to condition 1, which is the usual
> 25% equivalence ratio or so.  at somewhere around 50% equivalence
> ratio we'd get condition 2.
> # Given that your gasifiers make Engine Grade Gas (EGG) when operated as
> recommended, you have to be doing something right. :-)
> interestingly, the more you insulate a gasifier, or the more external
> heat you add to it, the less combustion you need, and the bigger the
> tar cracking challenges become (as you are converting less of it
> through burning and more through cracking).
> # While the mechanisms are interesting, the bottom line is the fact that you
> make Engine Grade Gas quality.
> this is the irony of all
> the heat recovery and return work we do on the gek totti.  the closer
> we get to a fully indirect gasification system, the closer we get to
> all heat for the processes being externally contributed, the closer we
> get to zero internal combustion, and thus all tar gas conversion
> needing to happen via cracking.  "fortunately", we are a long ways
> from this, but do see the problem.
> # Your TOTTI is indeed an elegant solution, but at the other extreme is the
> elegant simplicity of Mike LaRosa's "Larosifier" that also appears to make a
> reasonably good gas.
> the other solution route of course is to change the fuel type.  this
> is why charcoal or coal gasifiers are so much easier.  they do not
> have the excess volatile (or any volatile at times) problem to deal
> with.  to the degree there is volatiles they are a relatively small
> proportion, and much more easily burned to completion given the
> altered mass balance.
> # Sure, but the problem there is to find a sensible use for the energy in
> the volatile fraction of the wood.
> but as long as we want to work with biomass, we have the fundamental
> problem of about 80% volatiles and 20% fixed carbon.  the ratios are
> highly skewed and not well designed for what we want to do in a
> gasifier.  and thus the problems begin.
> # You seem to have one of the best gasifiers on teh market now, well able to
> produce Engine Grade Gas, when operated as per your recommendations. I must
> be missing something, because I can't see a problem. There is a famous
> saying "It doesn't matter if it is right or wrong. If it works, it is
> right." :-) The GEK works.
> this for me is the tar problem, and why its solution can at best be
> like the maintenance of a chronic health issue.
> # I can see that one would have to keep fuel properties and hearth rates
> within certain limits for a given design, but you seem to be able to do this
> very well.
> Best wishes,
> Kevin
> jim
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Jim Mason
Website: http://www.whatiamupto.com
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