The Gassifier- Initial Build Notes
This is the initial build for the wood gassification unit and the filter/blower. The unit is a down draft gassifier, meaning air is pulled in through the top of the fire tube, down through the charcoal at the bottom of the pyre, and the resulting gasses, mainly carbon monoxide and hydrogen, are cooled and filtered to be burned off at the outlet. This is my take on an old WWII design offered up by FEMA for times of oil rationing. These units were running a lot of tractors during the war, mostly in Europe. I’m hoping to burn wood chips and woody garden debris in the gassifier, then use to flammable gasses to fire an airtight kiln chamber for the production of biochar. Charcoal has been theorized as the driving force behind the legendary “Terra Pretta” soils of the Amazon. These “dark soils” are anthropogenic soils of ancient civilizations, and have a high carbon content from the disposal of cooking fires and daily refuse. Despite hundreds of years of non intervention, these soils continue to exhibit high biological activity and excellent nutrient value. Needless to say, this also has potential for carbon sequestration in light of climate change. OK, let’s get to it:
Here’s a pile of useful materials. I’m basing the design on discarded 5 gallon drums from work. Make sure you’ve cleared out any flammables before you get started.
Take the top off of drum #1. This is going to be the fuel hopper that sits on top.
Take off the bottom of drum #1. I used a key hole saw with a metal cutting blade- crude but effective. Watch out for sharp edges throughout.
Shop around for a stainless steel bowl that will fit inside the drum. This one press fit perfectly.
I’m using 6″ ducting for my fire tube, so here I’m using a dremmel tool to cut a 6″ hole in the bowl.
This shows the modified bowl fitting into the bottom of the fuel hopper.
I used a 6″ end cap here. This is going to go through the hole in the bowl, then compression fit into the 6″ fire tube. The flanged edge on the end cap makes for a snug, airtight fit.
Cut a hole in the end cap leaving enough material on the outside to maintain its structural intergrity.
The cap goes through the bowl….
Then into the fire tube (6″ galvanized ducting- this should be a much heavier material, but this is a proof of concept prototype).
Then push the bowl down into the bottom of the fuel hopper. The fire tube will extend down into drum #2 which will form the airtight fire chamber.
At the bottom of the fire tube, we need a fire grate. Another thrift store bowl goes into service.
Drill a whole bunch of 1/2″ inch holes. Note the grid drawn on the bowl. I was trying to keep about 1/4″ inch of metal between the holes
Finished grate. Now we have to attach it to the bottom of the fire tube.
Using a couple small bolts and a bunch of nuts and washers, attach the bowl to the fire tube so it swings freely. Drill through the edge of the bowl and the fire tube at 180 degrees, install the bolts and lock into place with the extra nuts. The bottom of the bowl is about 1.25″ from the bottom of the fire tube.
Remove the top of drum #2- this will be the fire drum that houses the fire tube. This needs to be air tight except for the air coming through the fire tube.
I really lucked out here, these two drums fit very tightly. They have a slight taper, so the bottom of the fuel hopper fits over the top of the fire chamber. They also can vary in size, the uncut drum in the left of the picture was a little smaller, so the fuel hopper slid almost half way down before achieving a tight fit. If I had welding capabilities, I wouldn’t have to fuss with all these perfect press fits. The advantage here, though, is that everything comes apart while I dial in the prototype.
Determining length of the fire tube, position of the fire grate, etc. There will be .5″ of concrete/vermiculite in the bottom of the fire drum, and you need some room for the ashes to shake down. Prop the grate up on some wood, place the fuel hopper on the fire chamber, measure the fire tube and trim it down so that everything hangs right in the drum. Then attach the grate to the tube. (see above). Keep in mind that we’ll be cinching the fuel hopper down later, so give it a little extra room. I trimmed that fire tube about four times during construction.
Looking down through the fuel hopper, fire tube and grate.
Oh yeah, you need some way of agitating the charcoal bed and jostling the fuel down. The WWII design had a fancy little crank mechanism. Here, I get primitive with a simple wire that feeds up through the fuel hopper. Install the wire at 90 degrees from the two mounting bolts.(Careful, that wire will get HOT when this thing is burning)
Here we have the juncture of the two drums, and this has to be an air tight seal. If air is drawn in from anywhere other than the fire tube, then the wood gas gets diluted (bad) or could start combustion inside the fire chamber (very bad.explosive?) Drill four holes through the bottom of the fuel hopper at equal intervals around the drum, and drill matching holes in the top of the fire chamber drum. Small eye bolts are installed in these holes, then .25″ hex bolts are installed through the eye bolts and tightened. Mark a line on the fire chamber around the bottom of the fuel hopper. Remove the fuel hopper and put a HUGE bead of high temperature RTV silicone along the drawn line, and let the gasket dry overnight (do not put fuel hopper back on at this point- you are making a gasket, NOT gluing the two drums together) Taper the bead so that the fuel hopper makes a tight fit when you cinch it down. I ended up making one gasket as shown, then coming back and improving on it with another shot of silicone.
Cut a hole in the fire chamber and install a 1.75″ steel nibble with end cap. This is the ignition port for initial start up. I used conduit lock nuts inside and out, then silicone. I’m using bentonite clay to seal the threads of the end cap.
The design calls for hydraulic cement lining the bottom of the fire chamber and the filter unit. Here I mixed equal parts portland cement, vermiculite, and perlite. This lightens the mix and gives a good thermal insulation. Put .5″ of the mix into the bottom of the fire chamber and filter unit and let it cure for 24 hours.
Here is the fire chamber with the tuff mix.
You need to drill/ cut a hole at the top of the fire chamber for the exhaust. Initially, I used .75″ EMT conduit with a compression fitting. I have since added a second exhaust of the same size on the opposite side of the chamber. I felt I wasn’t getting enough draw during the first test burn.
Another thrift store purchase forms the lid for the fuel hopper. Notice the tangs installed on the rim of the drum. You want clear air flow into the fuel hopper as this is the intake for the fire below. The lid is just to keep the fuel dry on raining days.
This photo also shows the filter unit and blower on the left. You need to cool the wood gas down for proper combustion. As the gas expands with higher temperature, it becomes less concentrated. We want the gas to exit the filter unit around 80 degrees F. The blower mounted on top is what provides the draw for the fire. The WWII design was hooked up to tractors, so after start up, the compression from the engine took over and provided the draw. In my design, the blower is on all the time. So let’s get into the filter unit.
The filter is pretty basic. We just need an airtight tube to cool the smoke. The picture shows the basic set up with an 8″ duct pipe, a crimp end cap at the base, and the exhaust from the fire chamber leading into the bottom of the filter unit. Again, a compression fitting is used.
Don’t be shy with the gasket maker/silicone. The filter unit needs to be air tight.
An end cap is used at the top of the filter unit. Here I’ve cut a hole in the end cap, and mounted a small squirrel cage fan/blower. Use some silicone between the fan and the end cap to get a tight seal.
The final exhaust pipe will be another .75″ conduit pipe, so the fan needs a reducer. I used a scrap from the previous construction, and drill holes to match the mounting holes on the blower. Then cut a hole to accommodate the the .75″ fitting.
Here’s the fitting in place before mounting. Again, plenty of silicone.
Then mount the plate to the exhaust using more silicone for a tight fit.
Get everything put together and check the draw! Eureka! As I said, I added another exhaust from the fire chamber to the filter unit to get better air flow.
This completes the initial build section. Stay tuned for notes on test burns and improvements!
Update: The First Failure 3/31/11
Sorry about the lack of updates on this project. Spring always takes my head out of the shop and into the garden, and it’s just been too rainy. However, I have gotten a couple of burns under my belt and this is what I’ve found. First off, with the second exhaust port the thing burns much better and hotter. I am still getting an awful lot of moisture coming off the exhaust stream though. The gas won’t ignite and hold a flame, but if I hold the plumber’s torch in the plume the smoke and vapor almost disappear, so I think there is still potential for this to work. I’ve been working on a carburetor/grill to slow the exhaust stream down and mix it with some air, and I’ve been chasing down any air leaks behind the final output. I must have had a pretty good fire going though:
I managed to melt the EMT fitting for one of the exhaust tubes right off of the barrel. I guess I should have known the stuff wouldn’t hold up to the temperatures.
The barrel seems fine, even the RV silicone gasket seems all right, although it came off when the paint started to burn off. Now that I’m down to bare metal, I think I’ll just glue it back on with a thin layer of silicone.
So I got some actual steel pipe and fittings from the plumbing section to replace the exhaust ports. Much beefier (and spendy). I’ll try and get some pictures up in the next week or so. I am hoping to get a test burn going this week end.