Ben Peterson of Victory Gasworks has made a series of videos outlining the problems he sees with the current GEK downdraft design.  These claims are interesting and insightful, despite the loose play with the science and engineering.   Reviewing the particulars might help clarify some design issues and fabrication trade offs-one always makes when developing a gasifier.  Hopefully the discussion will help all of us improve our gasifiers, whatever type they may be.

You can find the Victory Gasworks videos here:  http://victorygasworks.ning.com/profiles/blogs/12-flaws-with-the-gek-design#comments

Claim #1.  GEK has no condensate solution.  The insulation ring fills with water, which leads to a steam bloom on start up.

This is surprising to see as the top problem, as we have never experienced this problem.  Both walls of the insulation annular space in the GEK run above 100C, from top to bottom, so there cannot be water condensating on or in them.  All surfaces are above water condensation temperature.  The problem Ben found might follow from increasing the reactor height and insulation sleeve on the Woody- the same with the industrial size GEK Ben fabricated previously.  If the reactor gets too tall, the top areas will drop below 100C, and condensation will start to happen.  The annular insulation ring will then operate somewhat like a monorator hopper, with water collecting as a byproduct in the insulation ring.

Remember that the GEK reactor is relatively short as it is only intended for short testing runs.  It is not tall as it is not intended to store fuel.  I'm not sure why there is a comparison of reactor fuel storage capacity, as that is not their purpose.   For any real running you want to use the hopper.   For a monorator type hopper to work properly, heat must get up inside it.  Too tall of reactor will prevent adequate heat rise to make your monorator work, and thus reduce your ability to extract water from moist fuel. 

Our goal for reactor height was to use the minimum height necessary to allow for hopperless tests run, as well as fully insulate the pyrolysis zone, and allow for reduction bell size and nozzle position change outs over the full range possible for this basic hearth vessel size.  We wanted no more height than this, or other variables are compromised.  It is possible the extra height Ben has been working with added this issue.  I'm not sure.  Either way, he has well solved it with a condensate take off.  So far we have not had a need for one.

Claim #2.  The bottom up J tubes create cold spots and tar passes.  J tubes also make it difficult to adjust nozzle height as nozzles are down in the char/ash.

The data published on the main GEK site shows this air intake riser from the reactor bottom to nozzles has air temps between about 500 and 600C.  As reduction ends around 625C at the bottom of the bell, this is not an icy breeze blowing through the bottom of the reactor.  In fact, the air intakes rising through here are making the passive char ash insulation more efficient, as they are recovering some heat that is being lost from the center, and returning it back to the center.  Either way, this is a minor point, as tar passing or not passing is more an issue of how the hearth is set up, and the internal convection currents that happen (or not) as a result.

The point of the J tube design is that it creates a very robust air preheating and syngas cooling system.  It starts cooling the outbound gas about 1" from the bottom of reduction.  If you are concerned about reversion (which I am not), this gives you immediate gas cooling.   It gets the air up to 600C by the time it reaches the nozzles.  This reduces the load on the combustion zone, which would otherwise be having to heat the air up to the auto-ignition point of tar gas, where it then ignites.  All reductions on the thermal load of the combustion zone result in higher temp and better tar handling, or the ability to run wetter fuels and still reach adequate temps for tar cracking.

See here for GEK temperature profile data: http://allpowerlabs.org/gasification/gek/gekreports/report3/report3.html

Ben accurately points out that this J tube design creates tighter tolerances inside the vessel, and results in the nozzles being further away from the hearth wall.  These are the main liabilities of the J tube design.  We have found the benefits of an aggressive air preheating and syngas cooling system to greatly outweigh these liabilities.  Especially since these liabilities are easily solved in other parts of the downdraft reactor design.  This robust air preheating system also creates the potential for a steam addition system given the newly available heat, as well reduces the need for a total loss downstream cooling system.  Less bandaids are needed downstream for not having well tended the heat transfer potentials inside the reactor.

But again, all of these are trade offs.  We are clearly trying to get beyond the basic Imbert gasifier.  The limitations of the basic Imbert are well known, and its viability for real use contemporary situations we find very limited.   This is what we have set out to solve with the GEK collaboration.  A foundation for these later systems is a very efficient air preheating and syngas cooling system.  Nonetheless, if you want to set up a traditional Imbert type hearth in a J tube reactor architecture, here's how we do it.

Claim # 3. The GEK Perlite insulation is unimpressive.

What you put in the annular ring space for insulation is flexible.  It is made flexible so a developing world builder could use ash, char or other onsite obtainium.  Someone who has access to kaowool should of course use that.  As you see, some do in the GEK.  If someone has access to aerogel, they should use that.  Even better.  At some point we'll likely seal it up completely top and bottom and pull a full vacuum in it.  That would be even better.  I'm not sure why such an issue is being made out of this.  The issue is whether the architecture allows for an insulation insert wrap from the top of the pyrolysis zone, to the bottom of reduction.  The Woody has been smart to incorporate this feature from the GEK.  Many different insulative materials can go in the open ring.

The main GEK add in this realm was introducing this notion of an open top annular ring space to use for insulation of the hearth area, from top to bottom.  This is not done in the traditional Imbert, nor in the Fluidyne or MEN gasifiers.  In the WWI Imbert, the outside of the hearth is exposed to the uprising gas, which ultimately is a thermal drag on hearth temp.  It was done in this manner back in the day so the hearth material would survive, as well as provide modest insulation as the delta T across the hearth wall would be less than if exposed directly to atmosphere.

We've made a big point of the benefits of doing this double jacketing with the GEK.  It is part of the general tube-in-tube with flange and end plate architecture that we formalized in concept, method and detail for building the GEK. 

The L tube architecture of the Woody likely requires nozzle tubes passing through this insulation ring.  I'm not fully sure of this as I haven't seen any detailed pictures of the inside yet.  If they do, this will complicate the insulation fill, but it is certainly not a show stopper.  The L tube architecture is a very reasonable way to set up an air intake.  I find the increase in nozzle adjustability, and much more robust heat exchange potentials with the J tube, to be much superior, but the L tube will work fine.  The L tube or straight-in nozzles like the Fluidyne are easier to get to, but have much less adjustability.   The Woody seems to have 2 positions.  The GEK J tube nozzle risers can adjust to any position from the bottom to the top of the reactor.  This is important to support different size and length reduction bells, for different gas flow rates.   One size will not fit all.  Adjustability is not an issue for experimentation only.  It is critical to support the range of configurations needed in real world use installations as fuel type and gas production needs change.

Claim # 4.  The Woody has more fuel capacity in reactor than the GEK.

This is another odd variable for comparison.   The point of the reactor is not to be the fuel storage area.  The fuel storage area is the hopper.  If this was truly a consequential variable for comparison, we should be comparing hopper size.  But such is still not terribly consequential.    Do we compare the performance of a vehicle by the size of its gas tank?

The point of the reactor is to well set up the various thermal relationships, including transfering an adequate amount of heat to the hopper above the reactor.  You need to get some heat up into the hopper so you can run a monorator type condensing cycle.  A too tall or otherwise voluminous reactor will prevent adequate heat rising to the hopper above for this important moisture extraction process to happen.

The whole notion of making a reactor slightly taller so it can run on its own without the hopper, as well as continue the double jacketing upwards to fully insulate the pyrolysis zone, was a new introduction with the GEK.  Previously gasfiers that broke between the reactor and hopper always did the break right above the nozzles.  See the Fluidyne design.  This is fine of course, but the results are that you cannot run the reactor without the hopper.

Our goal was to add the minimum extra height so there was enough fuel to support test runs with reactor only, but also so that when the hopper is installed, you can still have a proper monorator cycle.  The extra volume/height added with the Woody will challenge a good monorator type hopper, and is potentially what introduced the condensation problem noted above.

(i've only gotten through 4 of the 12 tonight.  I'll get to the others tomorrow.  These are the main ones Ben is concerned about anyways.)