[Gasification] thermoacoutics, rijke tubes, sondhauss tubes, pulse jets-- how do we make them not work?

jim mason jim at allpowerlabs.org
Mon May 16 13:00:44 CDT 2011


On Mon, May 16, 2011 at 2:32 AM,  <sabbadess at aol.com> wrote:
> Jim,
>
> Why did you stick with 5" tube"?  I would try using a 6" or 8" chimney and
> just set the swirl burner under or slightly in it.  The swirl burner will
> act as a heat source to provide natural convection like a wood stove if you
> just want to get the smoke away from the operator.
>


the swirl burner is now 6" in diameter.  we've tried a whole variety
of induced draft devices, but have been underimpressed.  it is hard to
induce an air flow equal to your gas flow right at your burner.  you
don't have the fuel pressure on a wood gas start up system that you
have with propane or nat gas systems.  yes, a tall stack can do it,
but our 3' stack won't do much draft inducing.

moreover, as soon as you put any heat exchange device in the stack you
start to get some flow resistance, and your induced draft performance
changes.  it is easy for things to go out of correct mixture, and on
shut downs also have your fuel come back out the passive intake.
we've gone through many rounds of these types and ultimately decided
we want to drive both the fuel and air, and have a fully sealed
system.

we want to make it impossible for co to come out of the system on
operator error.  this is much easier to achieve with a fully closed,
driven "draft" system.  also, we're trying to get to an extremely
clean burn, with the ability to go through the entire start up, run
and shut down cycle without smoke or significant co emissions.  the
driven systems make this easier as you can actively control your
fuel/air ratios.

yes, the usual method of a gas pipe with atm mixing will burn fine,
but if you put a meter on it you'll find the burn is rather
incomplete, and your co emissions are high.  to get the combustion to
complete you need to finish your mixing and combustion while the temps
are still high.  you cannot do this in open air.  you need to do it in
some sort of combustion chamber.

so we're currently doing this with separate fuel and air blowers, into
a swirl burner.  we separated the fuel and air blowers to solve the
touchy flame back up problem, as well as to gain a little more
capacity on the gas vac pull potential.  i'd rather get rid of the
separate blower and go back to a premixed in one gas drive solution.
thus the flame holding question.

we're trying for a very fast start up that gets temps up quickly, thus
through the tar problem areas quickly.  running on wood chips, this
means we need to pull about 10"wc at flow.  you need about 20"wc at
stall to do this.  this is a lot of blower(s).  much easier with an
ejector.  unfortuantely an ejector is too inefficient to do this with
a 12vdc compressor without a power consumption that will overwhelm
common batteries.  we ran all the options and found power consumption
of over 1kw needed to make it work.  we can't run this regularly on a
12vdc battery and expect it to survive.

a really good flare solution has proven an unexpectedly hard problem.
its much easier with shore power and lots of compressed air.  for a
12vdc self-contained solution that meets all the rest of the needs,
we're still refining the options.

j






> Once you try to fire a boiler/heat exchanger with the flare you are looking
> at a different animal.  I bet the multiple narrow firetubes will move any
> resonance out of of your operating range.
>
> Stephen
> -----Original Message-----
> From: jim mason <jim at allpowerlabs.org>
> To: Discussion of biomass pyrolysis and gasification
> <gasification at lists.bioenergylists.org>
> Sent: Mon, May 16, 2011 1:51 am
> Subject: Re: [Gasification] thermoacoutics, rijke tubes, sondhauss tubes,
> pulse jets-- how do we make them not work?
>
>> we're having lots of problems with thermoacoustics now that we've put
>> a tall stack on our gasifier swirl burner.  we're using a tall stack
>> now to keep startup gasses away from the operator.  the swirl burner
>> is 5" in diameter, 10" tall. we've added a 4 foot or so stack of the
>> same 5" diameter. it looks like this
>> http://www.gekgasifier.com/forums/showthread.php?t=501
>  (though this
>> one without the joint at 10")
>
>
> completing the circle on this question from a few months ago, here's
> what we finally found solved the problem- insulation.  we found that
> if we simply insulated the stack from bottom to top, eliminating the
> hot/cold delta, the thermoacoustic effect went away.   more reading in
> the rilke / sondhaus tube effect clarified that it needs this temp
> differential, and if you eliminate the temp differential, the effect
> stops (which it did).
>
> this was one of the many good suggestions offered here.  we didn't try
> all of them.  but some of note.
>
> 1.  cut a V in the top of the stack like done to quiet some pulse jets
> 2.  drill holes towards the top, or other shapes to soften the
> transition at the end to reduce the pulse wave
> 3.  put a floating "deresonator" plate in the tube
> 4.  angle the tube, or otherwise put angle plates in it to break up
> the standing wave.
> 5.  drill a hole in the side down low and put a rod across the flame path.
> 6.  experiment with expansion chambers like a two stroke exhaust,
> except with the intent of "detuning".
>
>
>
> i don't know which of the suggested will work.  we ultimately decided
> the easiest one was the insulation and tried that.  it worked, so we
> stopped there for now.
>
> as usual, thank you for the input.
>
> and btw, what problem of physics do gasifiers not touch?
>
> j
>
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