[Gasification] reducing temp. of gasification

[jim mason]

On Sun, Sep 11, 2011 at 8:08 PM, Anand Karve <adkarve at gmail.com> wrote:
> Dear Henri,
> yes it is too complicated. I give below my basic reason for using this process.
> India generates

> When we heat the biomass in a closed vessel, can we add a catalyst to
> the biomass so that the producer gas is generated at a lower
> temperature? In this way, we would require much less material for
> burning outside the reactor vessel. If anybody has already done this
> kind of work, is the catalyst known? Once we get producer gas without
> nitrogen, we can look at filling it into cylinders and supplying it to
> rural people as high grade fuel for using in internal combustion
> engines.


a.d.

it would be wonderful to have a conversion route like this.  sadly i
think the thermo-chemistry is against us here.  but you should prove
us wrong ;-)

however even if solved, the result would still be marginally able to
be compressed economically in low tech situations.  think of the
difficulty in even ch4 compression for fuel volumes.  now, 2-3x that
difficulty with pure h2 and co? (btw, what is the actual energy
density difference here?)


here's some challenges for the "magic retort" that i see.

first, you have both a volatile gas/liquid and char conversion
problem.  pyrolysis is going to give you both.  yes, we can easily
complete pyrolysis at 300c or so with common ic exhaust heat exchange,
or other relatively low grade heats.  don't need catalysts to do this.

by well controlling the temp, and having lots of time, you can make
the tars less nasty to deal with later.  however, they will still be
tars needing cracking and/or combustion and reduction.  we usually use
the rule of thumb of 1000c to crack them.  if you keep their
generation to only primary tars via low temp pyrolysis, it will take
somewhat less temp to do this conversion.  how much less i don't have
good numbers on yet.  wish i did.

however, it is very unlikely that could get down to 300c.  most of the
catalytic tar cracking assists i've read about are in the 700c and up
range.  mostly really 900c and up.  here's a page from reed's handbook
that give results for some common routes, including dolomite:
http://wiki.gekgasifier.com/w/page/6123685/Catayltic%20Tar%20Cracking%20Methods
 (thank you tom)

it is also known that putting dolomite in the biomass during pyrolysis
will make the tars produced a bit more benign and more easy to deal
with downstream.  this is a method used in pyrolysis to oil situations
where control of the liquid / gas output is desired.  its still a
mess, but a bit less so.  under highly controlled lab conditions,
everyone gets excited.  DOE soon writes big checks.  tragedy usually
follows.

assuming you do somehow solve the tar cracking problem at low temp,
either through pyrolysis control or tar cracking, now you have to get
rid of the char.  well, you can just agree to use that for biochar too
of course.  but if you want that to be fuel you need to get up into
temps where reduction happens at useful rates.

in a typical reactor situation the usable bottom end temp for
reduction ends around 650-700c.  it is usually much higher with
smaller reduction volume reactors.  in our units we're usually still
800c at the bottom of reduction.  this is non-ideal, as you are
wasting reduction potential.  however you are gaining fines purging
out the bell, which is needed to prevent bell packing.

as you approach infinite reduction times you can get reduction down
into the 500c range.  that is where it stops chemically under atm
conditions.  you can use vacuum to assist reduction, but that is hard
to gain under temp.  though with batch load the reducing temps will
reduce volume and increase vacuum which will help drive more
reduction.

here's the gas equilibrium charts that reveal some of this.
http://wiki.gekgasifier.com/w/page/6123723/Gas%20Equilibrium%20Charts

i don't know of any catalysts that alter reduction temp curves.  there
must some.  i'd be very interested to know too.

but really, you are trying to do this all in one step in a barrel, so
separating them like the above isn't fully helpful.  what you are
describing here is really a pyrolysis retort, not a gasifier.  it is a
magic retort that goes "biomass to gas" in a manner that many have
wanted a magic retort that goes "biomass to oil".  the gas in this
case being the controlled output, not the oil.  relatively high temps
are needed to increase the gas fraction vs liquid fraction, and you do
this at the cost of them going progressively nasty and difficult to
post process.  this also works against the original desire for low
temps.

so there's some problems.  you knew all this.  not sure that helped
any.  unfortunately, i don't have this magic catalyst in my
possession.  if you find it, please do send me a can!


jim