[Gasification] [WoodGas] Re: Perfect Hearths

[jim mason]

2008/3/31 Greg Manning <a31ford at inetlink.ca>:
>
>
>  I'm currently experimenting with some really "radical" thinking (that is my
>  "MO" lol), in that making a hearth from 314ss tubing, and drawing the inlet
>  air through that tubing, gas evolution happens just above the hearth, and
>  the heat is being drawn down to the hearth, therefore my thoughts are two
>  fold.
>
>  1) With downdraft units, pre-heating of inlet air does one thing, assists
>  in the water/gas phase shift, by keeping all things in the carbonization
>  area hotter than with no preheat (shift is dependent on reaction temp.)
>
>  2) Because gas evolution happens just above, and heat rises / cool falls,
>  the tubing would act as a "pre-cooler" to gas "cooling" for non-dioxin/furan
>  formation, and I believe the CO to CO2/soot shift would also be much harder
>  to have happen.
>


while preheating air (and char), as well as fast gas cooling after
reduction, is certainly desireable for the above reasons, we need to
be careful where that heating and cooling is happening.

the design you describe able seems to be a tube around the reduction
tube scenario whereby air comes up from the bottom the in annular
space around the reduction tube, then likely enters some nozzle
coaming and to the combustion area.  then back down the tube towards
the grate.  is this accurate?

my speculation is that it is likely unhelpful to be doing any
preheating of air with heat derived from the combustion or reduction
zone.  yes, it is somewhat of a zero sum question, as the air has to
get heated anyway in combustion, but it does matter for maintenace of
even temps across a combustion or reduction area.  heat transfer walls
will always be colder.  you will be creating low temp paths for
passage of undesireable.

it seems to me that air preheating should only mine heat from the
moment after reduction starts.  not the combustion or reduction zone.
pyrolysis zone we can debate depending on your architecture.  it seems
the best point to start mining is pretty much from the grate onwards
in the gas flow train

there is a ton of heat to be used after the reduction tube/area.  this
heat is wasted energy unless you are direct combusting, and it is a
large problem that usually requires more elaborate downstream cooling.
 thus doing something which mines this not only improves the
performance of your gasificaiton reactions, it also simplifies your
cooling problem, as well as the volume of cyclone needed.

in the downdraft world, the best scenario i have to date to achieve
the above is the J tube preheating scenario here:
http://theshipyard.org/gek/large-16.html.  air intake goes counter
current to gas exit, all the way to the bottom of the reduction zone.
from here it penetrates the bottom of the vessel and passes upwards
only through insulation zones before it emerges at the nozzles.  there
is NO heat mining from the reduction and combustion areas.  mining is
only from the produced gas stream.   this relates to the MEN J tube
design, but seems to improve on it by following all the way around the
grate, not penetrating the sides of the cowling at air nozzle level.

i have yet to measure the temp of air at the nozzles in this design.
but i do know my product gas by the time it exists the gasifier is 150
- 180C at mid flow levels.  this was with the current vertical only
heat exchange tubes.  the better way to do this would be with flexible
tubing that spirals around the upward gas flow cowl  such would keep
gas flow direction near perpendicular to the heat exchange tubes.

this also appears to better quench the gas temp immedately after reduction.

j




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jim mason
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