[Gasification] questions for Tom Reed on stratified downdraft parameters

[jim mason]

in playing with our new woodgas truck (which i realize i am late posting
pictures, but hope to do so shortly), some new questions and confusions have
arisen about the nature of stratified downdrafts.  tom and others who have
worked with these much likely have easy answers to the below, which would
greatly help out us newbies.  here is my list of questions:

1.  turndown ratio:
what is the likely turndown ratio for a well insulated, "properly" sized
stratified downdraft unit?  so far i think mine is about 3:1 or 4:1, which
makes driving a vehicle difficult.  i find it very easy to either over pull
the gasifier, and thus have a poor gas, or under pull it, and thus have the
fire quickly top out in the fire tube, which leaves nonfully charcoalized
fuel below the flame front, and thus a giant tar problem.  also, it just
gets too cool for a good reduction zone.

2. vacuum readings:
what is the range of tolerable vacuum readings on a stratified downdraft?
on acceleration i am pulling 20 inches of water.  any sort of power seems to
be 5-10 inches of water.  i know i am running a too small firetube for full
power.  my guess is that good operation is somewhere around 3-5 inches.
below that seems to indicate too low of flow and thus too cool and thus
tar.  i do not remember any general notes on what gasifier vacuum readings
should be in the books, but i'm probably forgetting.  so in general, what is
the rule of thumb for minimum acceptable, as well as maximum acceptable,
assuming a non-impacted bed.

3. fire tube dimensions:
how were the fire tube dimensions determined which are in the lafontaine
fema book on "building a simplified downdraft gasifier"?  in general it
seems to me that the firetube length suggested is longer than necessary.  or
rather, if the flow rate is correct, it is going  to be using less than half
the length of the tube.  if the flow is slow, it travels upward.  so i guess
yes, the length is good for flow "error" tolerance.  but how did this
"tolerance" length get decided?

4.  bed depths:
what is the ideal depth of char bed and combustion zone in a stratified
downdraft.  for an 8" firetube,. the optimal charbed depth seems to be about
4".  i can't see what the combustion zone depth is, a it is gone by the time
i dig out the bed.  but you likely saw this when you made the clear ceramic
glass version at nrel.

5.  combustion zone temp:
i am making a thermocoupler "wand" that i can insert down into my various
gasifiers.  the plan is multiple TC's, one about every 2", over about a 20"
length, built into a stainless steel square tube.  therefore the temp
gradient across all the zones can be monitored, without hard mounting it
into any specific gasifier.  i know what the temp should be for all zones
except the combustion zone.  what temp should i be seeing in the combustion
zone?  is there a chart in the "simplified method for tar measurement" book
that gives data on the temp needed to crack primary, secondary and teritary
tars?  the book is elsewhere from where i am writing, so i can't look
myself.  on a related note, what is the relevance of combustion zone size vs
temp in burning tars?  is size (residence time) largely irrelevant and only
temp is important?

6. filters:
i am finding dry filters for secondary particulate filtering to not stay
very dry, and generally become rather useless very quickly.  or rather, they
get wet and then require huge vacuum to pull through them.  i am using shop
vac paper/synthetic filters.  i am using them after a giant cooler/radiator
and the gas going into the filter is very cool.  i think about 80-90deg.  i
realize i could reheat partially before the filter to prevent this problem,
but such isn't very attractive.  so from the experience of others out there,
are dry "paper" filters generally unreasonable on gasifiers for condensate
reasons?  do dry filters usually end up as some sort of sawdust, coffee or
other granular filter media?  are others finding the "wisdom" of wet filters
after the failure of dry ones?

any input greatly appreciated.  i continue to be hugely in debt to so many
here for all the great experience and knowledge that is commonly passed back
and forth.

thank you.

jim