Post by rectifier on Nov 26, 2015 20:57:03 GMT -8
I now live on the prairies and have access to large amounts of a new fuel - flax straw. No more wood, it's too rare to be considered a serious heat source.
Flax straw has tremendous energy density by mass but not much by volume. It likes to burn incredibly fast, where available stoves that burn flax straw bales often cannot idle below 1 million BTU (!)
So to make it more useful, I've been able to throttle the burn rate back by smoldering it in gasifiers. My biggest test gasifier to date has an 8" fuel chamber with secondary air inlets in an 8" ring, choked down to a 4"x18" ceramic heat riser. This throws a hot clean flame for about 10 minutes, then consumes the charred residue for a minute or two of flameless carbon burning. This is great so far as the straw contained in it would burn for under 1 minute if unconfined. Flax is a great fuel for gasifiers, as it leaves no char behind and burns to a small quantity of fine white ash.
Now I need more volume at the same power. The small chamber only holds about a pound of straw. I want to be able to burn for at least an hour to heat a boiler. Preferably, I would like to gasify an entire 30lb square bale (18" x 14" x 30"). I've never seen a TLUD in these large sizes or any shape other than round. Has anyone done anything like this? I'm planning to weld up a box to drop the bale into or build it out of brick. The gasifier chamber runs fairly cool so it doesn't need to be any exotic material. A few issues I forsee:
- relationship between BTU and gasifier surface area. There must be one. Once the pyrolysis front has spread to the full width of the chamber, it will require enough oxygen to keep it hot enough to continue to advance through the bale. Turning primary air too low will probably result in the entire front starving out and dying, so this stove may end up being too powerful to be useful. I also suspect the 4" riser will start to choke with this wider gasifier.
- how to build the flameholder and secondary air inlets. Traditional TLUD introduces the secondary air at the top of the chamber, where it mixes as it scrapes across the choke point and enters the heat riser. There is no real flameholder and if it flames out, it will usually reignite from the gasifier heat. With such a wide chamber, should it be done this way or should it have a cylindrical flameholder welded to the top of the chamber, where the mixing occurs?
- the corners of the rectangular chamber. Will the pyrolysis front move smoothly or will something strange happen?
Any input from TLUD builders would be appreciated!
Flax straw has tremendous energy density by mass but not much by volume. It likes to burn incredibly fast, where available stoves that burn flax straw bales often cannot idle below 1 million BTU (!)
So to make it more useful, I've been able to throttle the burn rate back by smoldering it in gasifiers. My biggest test gasifier to date has an 8" fuel chamber with secondary air inlets in an 8" ring, choked down to a 4"x18" ceramic heat riser. This throws a hot clean flame for about 10 minutes, then consumes the charred residue for a minute or two of flameless carbon burning. This is great so far as the straw contained in it would burn for under 1 minute if unconfined. Flax is a great fuel for gasifiers, as it leaves no char behind and burns to a small quantity of fine white ash.
Now I need more volume at the same power. The small chamber only holds about a pound of straw. I want to be able to burn for at least an hour to heat a boiler. Preferably, I would like to gasify an entire 30lb square bale (18" x 14" x 30"). I've never seen a TLUD in these large sizes or any shape other than round. Has anyone done anything like this? I'm planning to weld up a box to drop the bale into or build it out of brick. The gasifier chamber runs fairly cool so it doesn't need to be any exotic material. A few issues I forsee:
- relationship between BTU and gasifier surface area. There must be one. Once the pyrolysis front has spread to the full width of the chamber, it will require enough oxygen to keep it hot enough to continue to advance through the bale. Turning primary air too low will probably result in the entire front starving out and dying, so this stove may end up being too powerful to be useful. I also suspect the 4" riser will start to choke with this wider gasifier.
- how to build the flameholder and secondary air inlets. Traditional TLUD introduces the secondary air at the top of the chamber, where it mixes as it scrapes across the choke point and enters the heat riser. There is no real flameholder and if it flames out, it will usually reignite from the gasifier heat. With such a wide chamber, should it be done this way or should it have a cylindrical flameholder welded to the top of the chamber, where the mixing occurs?
- the corners of the rectangular chamber. Will the pyrolysis front move smoothly or will something strange happen?
Any input from TLUD builders would be appreciated!
