One of the best features of the GEK is that it's the only gasifier I'm aware of that has highly effective air preheating. This allows for lower tar production, and a somewhat higher tolerance for fuel moisture. It's not nearly "fuel agnostic" as Jim would like it to be, but the high air preheat temperature is a good contribution toward this goal.
I am presently trying to figure out how to incorporate a tar recirculating and reburning mod to the GEK. I didn't come up with this idea, it was invented _years_ ago, and it is called a deLacott gasifier. It's basically a conventional fixed-bed downdraft gasifier, with the addition of a suction unit that also draws some very tar-rich gas off the top of the fuel bed. This portion of the gas is essentially produced in updraft mode, which is thermally much more effective than downdraft, and which can therefore tolerate much higher moisture content in the fuel feed. The downside is that this gas is extremely high in tar and water vapour, and is also quite cool. This gas (which is sucked by a steam-jet ejector pump) is led into a refractory-lined burner chamber, where it is burnt with a sub-stoichiometric quantity of air. The tar-rich gas is of course an excellent fuel for this. The amount of air is controlled so that a suitably high (1000-1200C) temperature is achieved, thus destroying virtually all the tars. This very hot gas is then directed to the gasification zone. Some of it goes down through the char, gasiftying the char until the Boudard reaction stops (i.e. down to a gas temp of about 800C). This produces good low-tar "downdraft gas". The other portion travels upwards, and produces "updraft gas", which is what feeds the tar burner.
I'll see if I can find an online description, but until then anybody who has the Biomass Energy Foundation "Handbook of Biomass Downdraft Gasifier Engine Systems" can see a diagram and description of it on pg. 44, and similarly-inspired one by Susanto on pg. 45. It seems to be viewable at Google Books.
I am thinking that the GEK's highly preheated air should work extremely well as a motive fluid for an ejector pump; a given mass of air at 600C has three time the volume as it does at 30C. A certain amount of active power will be needed; the air would have to be compressed to the neighbourhood of 1-10psi, before beinf fed into the GEK's stainless preheater tubes. From there it would be rerouted to an ejector unit, which would use the pressurized 600C air to induce the flow of gas drawn off the top of the GEK's fuel bed. It's an open question at this point as to whether we would desire for combustion to take place inside the ejector (in which case it chould probably be refractory-lined), or not. In any case, the tar-rich gas and hot air mixture (or the gas from its combustion) would then be directed to the gasification zone.
Comments (2)
jim mason said
at 5:30 pm on Feb 8, 2009
good idea daniel. yes, we do need this sort of system.
remember that the early geks were set up with a tee at the air inlets which allowed you to scavenge tar from inside the reactor right about at pyrolysis height, and introduce it into the ss air lines. this was done by an ejector made out of a reducing bushing that is screwed into the top of the tee. see here: http://www.allpowerlabs.org/gasification/gek/gekreports/report1/geknohopper.jpg
the problem with this system is you are putting tar gas in the air lines at a temp where you will likely get deposits and in time, a mess. also, i've become as interested in this air preheating system as an output gas cooling system, as an air preheating system, thus putting the hot tar gas in the air lines works against the goal.
the new method that will soon be included with the kit, is an internal tar recycling ejector that fits right on top of the nozzles, with a pipe extended upwards to pyrolysis level inside the reactor. imagine a 1" tube over the nozzle forming the ejector. somewhat of a nozzle cowling. then add another 1" tube welded on with a 45deg bevel joint so there is a gas through space, extending straight upwards to mine the tar and pull it downward to premix it with the air. put some sort of hat on teh top to keep the solid fuel out of the pipe.
this will tend to augment the "up in the center, down on the sides" passive convection currents in the reactor, keeping tar away from the center of the combustion heart, directing it outwards towards the walls, and back into the combustion area via the nozzle cowling route.
such a system also preserves the heat in the tar gas, whereas most external tar recycling schemes lose it to external eq. i have not been able to find an internal tar recycling system like this in the historic literature. it seems a much easier and better performing way to do a de lacotte like system, with significantly less equipment.
jim
gasman said
at 3:08 pm on Feb 12, 2009
Jim, so far so good!
The injector-established gas-mix stream now needs a cavity to burn without to “drown” in the charcoal.
If one blasts this gas-mix directly into the white-glowing charcoal inferno, the charcoal will “rob” most of the available free oxygen. This will of course be advantageous what comes to achieving a high gas temperature in advance of successful reduction, but then again the burden of cracking will not be enlightened by the potential heating value of the tar gas…
It is possible to let the blast jets first go trough a (horizontal) zone of white-glowing char to create a higher temperature, and then let them come in over the char-bottom of a small-diameter cavity in the char. The cavity stretches upward some 3—4” under a ceramic bell with up-split sides, trough which the glowing flue gas spreads out to the sides, and return to the nozzles. The incoming jets sweep in just under the bell’s low edge.
This configuration resembles more of the ideal heat-globe, concentrating the ultimate heat in the absolute centre.
The blasting toward the centre at a small angle to the radius starts a swirl in the cavity, which is very beneficial.
This would even make the peripherical down channel obsolete at “birth”… less condensation trouble—more bell-problems…
Comments?
Max
You don't have permission to comment on this page.