My New TLUD Burner
"TLUD" is a "Top-Lit Up-Draft" stove that burns biomass by gasification to create charcoal.
In the last year, I built several TLUDs out of 55-gallon barrels wrapped in insulation. The barrel bottom has 1-inch holes to admit "under-air" that rises up through the biomass in the barrel. Wood chips, straw, broken brush, hay, cornstalks, or other biomass in the barrel is lit at the top, and—like a cigar—burns slowly from top down to bottom, drawn to fresh air entering the barrel bottom.
As flames descend, intense heat "gasifies" the biomass, releasing volatile gases, vapors and smoke. All the oxygen in the upward-moving air is consumed by this "gasification" of the biomass. Carbon burns hottest, and thus, burns last. So, the flip side of biomass gasification is "carbonization"—wood converted to black carbon char.
In a TLUD, all the oxygen is used before the carbon can ignite, so the descending, smoldering flame front leaves behind the carbon as "char." In a highly efficient burner, up to half the carbon can be retained in the barrel as this "biochar." In my crude and leaky TLUD, I do well to get 30% yields—often less.
Volatile gases are released as fire smolders downward through the biomass. Being super-heated, gas and smoke rise up, and exit out the top of the barrel through a 6-inch chimney hole. Without oxygen, this discharge is dense, smelly smoke. A TLUD without a secondary gas flare is a smoldering smoke pot.
Stovepipe Adapter Ring
As gas and smoke exit the barrel top, air is added to this upward-moving stream through holes around the base of the chimney. With added oxygen in this secondary "over-air", the super-hot gases ignite and flame off up the chimney, burning up all the carbon, smoke and soot.
This gas flare expanding inside the chimney further boosts the up-draft, and creates suction to pull even more air up through the biomass in the barrel below. Most of the energy released in a burn will be in this chimney gas flare.
Three lengths of stovepipe above this "over-air" intake further assures strong up-draft to suck fresh air up through the densely packed biomass. One-stovepipe chimney provides inadequate up-draft. Two-stovepipe chimneys work much better. But a third stovepipe significantly improves updraft further.
|Venturi Vortex Generator
slanted slots encourage air-gas rotation
Not-Quite Rocket Science
My latest TLUD upgrade is a simple chimney adapter to connect the 6-inch lid hole with an 8-inch stovepipe. This small, modest-looking device seems simple, but uses rocket science to rotate and turbocharge the gas flare. This modified adapter has huge effects on "over-air" intake, burner performance, combustion speed, gas flare geometry, and thermal efficiency.
A side view above shows the 6-to-8-inch stovepipe adapter attached to the burn barrel lid. In the top-down view at right, slots sliced in the flared funnel face are visible. These slots admit air on a tangent, rather than a line radial to the center. This device at the chimney bottom induces sideways flow to "over-air" entering the chimney.
Taylor Stetlor, a KU industrial design student, nicely sliced seven slots into the bevel of the adapter. Slots are slanted to encourage a sideways slipstream of air into the chimney from bottom to top. Slots are pried and bent wider apart to open the air gap, and encourage thin, wide sheets of air to be drawn into the chimney.
slips the lid on a new burn
This lateral air influx initiates counter-clockwise rotation of incoming air, and generates rotation to cause gas & smoke exiting the burn barrel below to spin and roll in a vortex, rather than streaming straight up the chimney.
This "venturi"-style adapter creates an organized, circular air-gas flow to cause rapid, efficient mixing and combustion of any volatile gases. First, a vortex spins and folds fresh air with hot gas to swirl them for thorough, rapid mixing and quick, complete combustion.
Second, this rotation folds the air to the inside and gas to the outside. Gas at the center of the rising stream spins to the edge where fresh air is entering. Turning inside to the outer edge assures oxygen mixes intimately with all the gas.
Third, this rotation slows the ascent of the gas flare up the chimney, and increases the time the flaring gases are held in the chimney. Instead of rushing straight up the chimney, flames take the longer, circular path to the top. More flames occur in a shorter space. This helps contain the gas flare in the lower chimney, where temperature and combustion conditions are optimized.
|Gas Flare Combustion Chamber
Gas & smoke exiting the barrel are super-heated, and immediately ignite in contact with oxygen in the inrushing air. This gas ignition and explosion generates powerful expansion right at the bevel where the chimney opens from 6 to 8 inches. This increasing diameter also induces rotation in the gas-air mixing to encourage vortex flow.
Proper proportion and shape of the venturi adapter, chimney and lid can optimize this gas-air mixing, and assure complete, quick combustion of all carbon in the smoke. This will yield a higher operating temperature temperature for smokeless burns.
Gas Flare Combustion Chamber
The 6-to-8 expansion adapter is topped by one length of 8-inch stovepipe, then an 8-to-6 adapter, then two lengths of 6-inch stovepipe. A tall chimney creates strong up-draft to suck air up through the biomass packed in the barrel. The enlarged zone in the lower chimney is a chamber to contain the gas flare and concentrate heat release in that space.
One benefit of this chimney chamber is to reduce effects of wind shear. Kansas has lively winds, which play havoc with burner updraft, sharply curtail air intake pressure, and can even create downdrafts that snuff out the flare. The enlarged combustion chamber provides a buffer space to depressurize downdrafts and reduce effects on gas flare and burn barrel updraft.
guy wires, eyebolts & turnbuckles
Purpose is of this 8-inch diameter lower chimney combustion chamber is to contain the gas flare, and maximize the energy released inside that space. This assures high efficiency, smokeless combustion, and allows optimum use of heat released by the gas flare. Increasing chimney diameter at the gas flare ignition and expansion point encourages lateral rotation to form a vortex and optimize gas-air mixing.
Dancing Donut of Fire
When our first burn with this new adapter reached bottom, we disassembled the chimney until only the 6-to-8 adapter was left. When we lifted the 8-inch stovepipe out of the adapter, we were enchanted to see a donut ring of pale red fire dancing inside the adapter. So, our first attempt to use venturi to generate vortex gas flare flow was a success.
sifts cedar chips through 1/4-inch screen
We'll do a night burn sometime to capture a clear video of this dancing red donut.
Performance of this first venturi adapter suggests that more "over-air" is needed. Our second venturi design will have ten or twelve slots. We may experiment with fabrication techniques to widen the slots. We may also add a few curved vanes inside the chimney to enhance the gas flare's folding and spin geometry.
A different design path is to enclose outside the venturi air intakes in a circular manifold, and use an electric blower to force air into this manifold. Properly done, this can sharply increase rotation of the venturi vortex, and boost the temperature and geometry of the gas flare. This "turbocharge" booster could be an exciting way to control and harness all those BTUs.
One immediate step is to install water pipes in and around the chamber as heat exchangers to harness TLUD process heat as hot water. The TLUD can be a burner at the heart of a system to heat a greenhouse.
Screening Cedar Chips
We again loaded the burn barrel with cedar tree chips. This is an ongoing, long-term investigation of one feedstock we began at Hoyland Farm.
Bob & Joy Lominska have cedar weeds (juniperus virginiana) over-running their pastures, and cedar is a weed tree in many American pastures and prairies. Bob & Joy wondered if pesty cedars might make good char. They cut a few, rented a chipper, reduced entire trees—leaves, stems, limbs, trunk, and all—to chips. But the chipper reduced trees to such fine chips, the smallest particles clog up-draft "under-air" when packed in a barrel, and choke a burn.
sifting biochar through 1/4-inch screen
So, we sift chips through 1/4-inch metal screen to remove dust and smaller chaff. Coarse materials left in the screen load tightly in a barrel, yet still burn nicely, slowly, smoothly, often for over two hours.
Cedar dust & chaff, in general, are excellent soil conditioner. They may be a suitable substitute for peat moss in potting soil. I intend to blend dust & chaff with biochar and compost before spreading on soil. The woody dust & chaff is an easy fungal food to nurse the mycorrhizae and other microbes as they start to colonize char and soil.
Cedar chips burn into char quite well, with consistently high yield. The fire from cedar chips seems more intense than for hardwoods, perhaps due to the flammable cedar oils and resins, and perhaps cedar will prove to be a high-yielding biofuel source.
Biochar Screening & Sizing
|Cedar Tree Biochar
crushed through 1/4-inch screen
We use the same 1/4-inch screen to crush and sift raw char into small particles (photo above). This increases char surface area for greater rates of ion adsorption and microbial colonization. Smaller particles also assure rapid, widespread, effective dispersal in soil for optimum effect and rapid response.
Raw, fresh char is brittle, and shatters easily. Most chunks crunch quickly when abraided by a flat surface, or with the edge of a shovel, rake or hoe. Thus far, over 80% of the char passes through the screen. Also, sifting char separates out most unburned, and partially burned, wood chips and sticks, which are added to the next burn.
Seedling Tests with Potting Mix
The next step in our cedar char experiment is a test for effects—positive and negative—on seed germination and seedling growth. Potting soil mixes were prepared with 0%, 5%, 10% and 20% char added to a base blend of topsoil plus 20% compost. As of today (2/21/13), four kinds of char were mixed: cedar, cow manure, bamboo, and Char-King wood powder. Two further types of char will be included in this trial: pellet char by the Whitman process, and mixed hardwood. A 7th tray was prepared to test effects of cedar dust & chaff.
These different blends are loaded in seedling trays, watered and allowed to rest for one week while the minerals react and microbes grow. Then the trays are seeded with lettuce, a sensitive leafy crop useful for these toxicity indicator tests..
The first week will reveal any effects on seed germination. Another three weeks of growth in the trays will show any effets on seedling growth.