(see discussion on this topic in GEKforum here )

Wood pellets are non-ideal downdraft gasifier fuel.  However, their ubiquity and relatively consistency make them an obvious contender for a gasifier fuel standard.  They are the closest thing we have in contemporary times to the standardized chunkette production and delivery infrastructure that supported the Imbert in WWII. 

The main positive feature of wood pellets is they are consistent dry chunky pieces of wood.  They're also nice and smooth, so their mechanical flow characteristics are good.  Most of the bridging problems others are reporting on wood chips go away on wood pellets.  Bridgine equally goes away on shell type fuels like walnut shells-- thus why we perfer to run walnut shells at the APL shop when we can get them.  (However, we admit that such is a bit unfair, as walnut shells are not typical "biomass waste"  . . .)

The main problems with wood pellets in a gasifier are the following:

1. They fall apart when exposed to moisture and make a giant sawdust mess, which plugs the reactor.

2. They are very dense and small, thus have a higher "fuel to open space" ratio than desired.  They create a heavy bed that is much more difficult to penetrate with air, thus combustion and tar cracking is challenged.

3. They appear to pyrolyize slower than natural biomass chunks.  This seems to follow both from their high density as well as the lack of the usual grain pathways for gas flow.  Pellets are pressed "dusts", thus usual internal pathways not present.  (this is conjecture at this point, i haven't seen any real data on this yet)

These problems seem very solvable.  The solutions are largely just dimension adjustments.  But some new EQ will also help significantly.  Here's what i propose

1.  Decomposition with moisture: 

The main fix here is running technique.  Never shut the unit down with raw pellets inside or the steam will make them fall apart.  Once the pellets are charcoal, they seem to stay together better and will be ok for the next start.  Always use NEW pellets, not ones that have been sitting around in an open bag for awhile.  I've found that pellets really soak up the water.  One or two nights out in an open bag and they're very ready to go to mush.  Get new, dry, good quality pellets.

The horizontal fuel feed auger on the way from APL will also help considerably in keeping the pellets dry and only in the reactor when wanted.  With a feed auger, you can stop the feed and run the reactor out to just charcoal before shut off, but still with fuel immediately ready for your next start up in the auger. 

The double jacketed auger housing can circulate either heat for drying/preaheating fuels, or cool air for faster shut down or steam prevention.  Being able to stop the rapid drying process, and thus steam after shut down, will help greatly to prevent decomposition after a run.

2.  Density and small size:

We need higher nozzle blast rates to penetrate the heavier pellet bed.  We get this by reducing nozzle size, while keeping gas flow rates constant (thu the air has to come out the nozzles faster).  You can change nozzle size on the gek by drilling holes of your size choice in 3/8" black plumbing caps, then screwing them on over the stock nozzles street 90 elbows.  Very simple.  I suggest going down to about 1/4".  Stock size is 3/8"

Most of us are currently seeing pressure drops across the reactor of about 0.25" running the stock GEK nozzles.  This is actually too low i believe.  Higher pressure drops are needed to faster blast rates, and thus good be penetration, and thus complete tar cracking.  I believe we want to see about 1-2" h2o drop across the bed, as measured by the gek manometer, at the bung in the gas cowling at grate level.

I've never heard it said directly, but it seems we can measure and tune blast rate indirectly by measuring pressure drop across the reactor.  Instead of just saying, "use x size nozzle for x gas output rate", it seems we should more accurately determine nozzle size by deciding the range of pressure drop we want to see, then adjusting nozzle sizes until we achieve this, while running our real fuel.  Once you are working with a specific fuel, the main variation in pressure drop across the reactor comes from the nozzle constriction.  For pellets, we're going to want to run at the higher end of acceptable pressure drops to create the higher blast velocities. 

It would very useful data for all of us if someone could run some tests with pellets at a constant gas output rate (say, medium on the fan), but with a variety of nozzle sizes, with resulting pressure drops across the reator recorded.  Observe changes in gas condition.  Better yet, directly measure tar production or lack there of on some filter paper with a pump and filter rig.  (pssst: bear has one.  bear?)

3. Slower pyrolyiss due to density and lack of wood grain:

We aren't going to change the character of the pellet, so we need to change the character of the pyrolysis regime.  Namely, we need to make pyrolysis longer, and ideally at a lower temp.  The proposed ejector venturi nozzle add on, with the vertical internal tar recirculating convection currently, is one way to do this.  See here, especially the added note at bottom:fhttp://gekgasifier.pbwiki.com/Internal-Tar-Recirculating-Ejector-Nozzles

Making the nozzle ring more open to the upper reactor will also likely better direct heat upward, and therefore increase pyrolysis activity.  The auger fuel preheating will similarly get pyrolysis going faster.  Also, good insulation upper reactor will retain more heat to do useful pyrolysis work. 

So there's a few ideas for configuring the GEK to more successfully run wood pellets.  Now we need to run the tests and permutations to see what works.

 (see discussion on this topic in GEKforum here )