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

jim mason jim at allpowerlabs.org
Sun Dec 11 18:37:17 CST 2011

On Sun, Dec 11, 2011 at 2:41 PM, Kevin <kchisholm at ca.inter.net> wrote:
> Dear Dr. Reed
> You raise some very interesting points and issues!
>> Wood is composed of ~20% lignin and ~80% cellulose (cellulose plus
>> hemicellulose). On heating in the absence of air, the cellulose produces
>> mostly the Woodgas, while the lignin is converted to charcoal.  Since the
>> lignin is the principal source of tars in Woodgas (up to 2% by weight of the
>> wood) tar removal is a very big problem for using W-gas in engInes.
> 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.

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.

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.

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.

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.

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.

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).  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.

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.

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.

this for me is the tar problem, and why its solution can at best be
like the maintenance of a chronic health issue.


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