[Gasification] the most important thing (quite possibly) i'velearned to date

jim mason jim at allpowerlabs.org
Mon Oct 18 02:51:56 CDT 2010 

On Sun, Oct 17, 2010 at 10:43 PM, Kevin <kchisholm at ca.inter.net> wrote:

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?
>
>


there isn't really any maximum limit on h2o until you are referencing to a
particular reactor, and the degree or lack thereof of external heat
sources.  in principle if you have unlimited external heat, you do not need
any combustion internally, and can just crack tars with heat and reduce the
chars with steam.  this is otherwise called "perfect indirect gasification".

on a more typical downdraft fixed bed air blown gasifier, the optimum
moisture content is still going to relate to the degree of external heat
being added through the air and fuel, the insulation of whole system, and
other particualrs of the reactor.   if just a straight through no preheating
design, we usually say 15% is ideal (or top, depending on who is
resporting).   but this assumes a good pull rate.  if you are trying to
maximize the range of flow, then much less moisture is ideal.

the ideal in all cases, would be to be able to vary the water content in
real time, in relation to real temps measured in the reactor.  being stuck
with one moisture content of the fuel is always non-optimal except for one
thin slice area of operation.

i've been surprised how much higher the moisture content can go with a full
heat recycling rig like we and others are doing.  these are closed top
designs, not open top where higher moisture is much easier.   we're
regularly seeing good operation in the 20-30% moisture range.

here's some numbers from our recent workshop, with caveats for our still
figuring out the testing process.   the main point here was to get better
numbers on the kg/kwh.  this required accurate moisture measurements using
the oven method (not a spike meter), so the moisture numbers at least are
accurate.  all are dry basis.

Current results (subject to modification as we better refine the process):

10/13 (Weds)-
   Dry Mass Consumed: 31.4 kg
   kWh Produced: 24.75
   kg/kWh: 1.3
   Average MC: ~24% (mass weighted average)

10/14 (Thurs)-
   Dry Mass Consumed: 26.6 kg
   kWh Produced: 21.75
   kg/kWh: 1.2
   Average MC: ~35%


we still need to get the gas analyzer inline during these runs.  we did
reach the point where the gas quality was decreasing as fuel moisture
increased.  this surprised me.  the temps in the reactor were still above
tar making temps.  there wasn't goo in the cyclone collection glass.  but
the gas was clearly reducing in quality as the amount of work lights we
could support was reduced.   the why for this we need to explore a bit more.

jim








Thanks!
>
> Kevin
>
> ----- Original Message -----
> *From:* jim mason <jim at allpowerlabs.org>
> *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
>
>
>
>
>
>
>
> ------------------------------------------------------------------------------
> Jim Mason
> Website: http://www.whatiamupto.com
> Current Projects:
>    - Gasifier Experimenters Kit (the GEK): http://www.gekgasifier.com
>    - Escape from Berkeley alt fuels vehicle race:
> www.escapefromberkeley.com
>    - ALL Power Labs on Twitter: http://twitter.com/allpowerlabs
>    - Shipyard Announce list:
> http://lists.spaceship.com/listinfo.cgi/icp-spaceship.com
>
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-- 
------------------------------------------------------------------------------
Jim Mason
Website: http://www.whatiamupto.com
Current Projects:
   - Gasifier Experimenters Kit (the GEK): http://www.gekgasifier.com
   - Escape from Berkeley alt fuels vehicle race: www.escapefromberkeley.com
   - ALL Power Labs on Twitter: http://twitter.com/allpowerlabs
   - Shipyard Announce list:
http://lists.spaceship.com/listinfo.cgi/icp-spaceship.com
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