Lowering Emissions in a Short, Natural Draft TLUD

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Lowering Emissions in a Short, Natural Draft TLUD

The great thing about 12” high tincanium natural draft TLUDs is that they are cheap, short and powerful. Dr. Tom Reed had a dream that a billion TLUDs would protect health, save wood, etc. We wondered if an all Tier 4, affordable, stainless steel ND-TLUD might help?

The first step was to test the baseline ND-TLUD. It’s great for camping outdoors but not close to Tier 4 for PM2.5, CO and Thermal Efficiency. We are now trying three ways to achieve all Tier 4:

  1. A nothing fancy prototype. Control primary and secondary air and don’t let flames touch the bottom of the pot.
  2. A static mixer prototype. 
  3. A better concentrator ring prototype.

Usually when we make one change a day in a prototype and test the effects under the emission hood it takes about a month to get close to an “optimal” solution. Doing one iteration a day leaves us time to do all the other work here at the lab. 

Looking forward to sharing test results! 

Concentrator Disk/Chimney in a TLUD

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Concentrator disk/chimney in a TLUD

When I asked Google AI, “Is flow more laminar in a smaller diameter tube?”  it immediately responded:

“Yes, thanks for asking! A smaller diameter tube can promote more laminar flow for a given flow rate. This is because reducing the diameter of a tube increases the Reynolds number, which is a dimensionless quantity that helps determine whether flow is laminar or turbulent.”

I had been noticing that the chimney also increased draft in a TLUD we have been developing, lengthening the rising column of flame.

As a rule of thumb, Dr. Winiarski advised that flame should burn out before it touches the bottom of the pot. Yellow flame is very hot (around 1,100C) which is great, but sufficient mixing and residence time are also needed to burn up smoke.

Have you also experienced that lifting the pot off of the top of a TLUD can reduce the emissions of PM2.5?

Reducing draft by removing the concentrator disk/chimney this week has allowed flame enough time to burn up more smoke in a relatively short TLUD. At the same time, thermal efficiency has been reduced when short flames dance playfully on top of the larger diameter combustion zone (with a larger Reynolds number).

Dr. Paul Anderson shows a concentrator ring on a TLUD

The Concentrator Ring in a TLUD

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Dr. Paul Anderson shows a concentrator ring on a TLUD

Image from Dr. Paul Anderson’s Introduction to TChar (TLUD) Stoves for Haiti

The size of the hole in the middle of the flat plate, usually round, that seals the top of the combustor in a TLUD stove has important functions. The flat plate forces air jets/flame to travel horizontally in an attempt to completely cover the fuel bed. The diameter of the round hole has a dramatic effect on firepower. When it is too small, the fire can even be extinguished.

As with other variables, iterating changes in a prototype under the emissions hood can determine the compromise that best meets project goals. Larger holes in the concentrator result in higher firepower but can also increase emissions. In a 5” in diameter stove, a 3” hole in the middle of the concentrator is commonplace.

A StoveTec Fire Fly Lantern burns with a single column of flame

In the StoveTec FireFly lantern, developed at ARC, a small hole in the concentrator ring (1.5”) forces the flame into a narrow, vertical cylinder used for illumination. While the firepower is very low, so are the health-affecting emissions per minute.

Applications of Pulling Woodgas Through Burning Charcoal

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We experimented at SSM with a “Horizontal TLUD”

As seen above, the draft of a Rocket stove can pull woodgas horizontally through made charcoal. The tips of the sticks turn into hot charcoal.

Secondary air is pre-heated in the circular annulus before entering the fire in front (or above) of the made charcoal, as in a TLUD.

Primary air is adjusted by covering the fuel entrance. More air makes a bigger fire.

PM2.5 is dramatically reduced when conditions are well adjusted under the emissions hood: (850C for 0.2 seconds, etc.)

Recently, we have been trying the same approach in pellet and log burning biomass heating stoves.

It is so interesting to switch back and forth between heating and cooking stoves.

Solvable problems make life great!

An Easier Institutional Stove?

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Libhubesi stove (photo: New Dawn Engineering)

Institutional-size stoves like this Lihubesi stove frequently use a sunken pot or pot skirt to increase heat transfer efficiency.

While testing the institutional-size Alpha Limited TLUD, ARC staff conducted an experiment to see if a skirt is strictly necessary with a very large pot. A 58cm in diameter pot was heated by the six-inch in diameter Tom Reed Alpha Limited forced draft pellet stove with an added 0.75 constant cross sectional area Winiarski stovetop.  

A complete stovetop was also made that increased heat transfer efficiency to the entire bottom of the pot. As-hot-as-possible gases are directed to flow as closely as possible to the surface without reducing their velocity.

The bottom of the 60 liter, 58cm in diameter pot (used in institutional stoves in Africa) had an external surface area of 2,640 square cm. The slanted Winiarski stovetop created a 5mm gap at the outer edges of the pot (See above).

The seven inch deep, Alpha Limited FD-TLUD stove ran for 82 minutes using 2.03 kg Douglas fir pellets. 20 liters of water boiled in ~60 minutes when a lid was placed on top of the pot. (A higher firepower stove is needed to boil 60 liters in a reasonable period of time).

The single test results were:

efficiency_with_char_                          57%          

firepower_with_char_high power        4.80 kW

CO_useful_energy_delivered_            1 g/MJd       

PM_useful_energy_delivered_            15 mg/MJd     

Summary

When pots have sufficient bottom surface area, using a Winiarski stovetop can result in high thermal efficiency. After one hour, the highest temperature of gases in the 5mm channel gap under the outer edges of the pot was 111C. Adding a skirt to the sides of the pot would not be help very much when gas temperatures are this low. 

Perhaps cooks would appreciate institutional stoves without sunken pots? 

Let’s see what happens?

Secondary Air Injection

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S

Dr. Reed’s Alpha Limited stoves

Lefebvre, Vanormelingen, and Udesen examined secondary air jets air in cylindrical TLUD combustion chambers and described the most successful patterns and penetration depths. They reported that air jet penetration lengths approaching the middle of flame in a cylindrical combustion chamber resulted in a maximum reduction of PM2.5. An increase in the number of jets also created more thorough mixing. They advised that it was important to have the jets meet in the middle of the flame, but with minimal necessary force, to ensure highest temperatures and highest velocity of hot gases to the pot. (Lefebvre, 2010) (Vanormelingen, 1999) (Udesen, 2019)

It has been interesting to test Tom Reed’s larger Alpha Limited stove. More powerful secondary air jets forcefully meet in the middle, forcing the flame to create a cyclone that plunges down into the cylindrical combustion chamber. As reported in last week’s newsletter, in one 94-minute cold start high power test with Doug fir pellets, the larger WoodGas stove and insulated SSM SuperPot, achieved 58% thermal efficiency with a white filter and very low PM2.5.

Perhaps downward cyclonic mixing, caused by more powerful secondary air injection, might be another clean burning technique when combustion temperatures stay high enough? 

The Winiarski stove top that was added seemed to reinforce the downward cyclone. Larry always pushed better heat transfer efficiency in his pot supports, etc.

Tom probably invented using very little primary air and a lot more secondary air in forced draft TLUDs. So much easier to improve performance since Tom and Larry did a lot of the work!

Wow! Tom Reed Forced Draft TLUDs are great!

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The Tom Reed Alpha Limited Forced Draft TLUD stoves (India)

The six-inch and four-inch in diameter FD-TLUD stoves are powered by two AA batteries and are well known to be inexpensive and clean burning. The smaller stove was the “high combustion efficiency” stove used in a 2015 Round Robin test series at Regional Testing and Knowledge Centers. As shown, the Tom Reed stove uses two crossed pieces of metal as pot supports.

The four inch in diameter ARC Round Robin test results were:

As a part of a recent Osprey stove improvement project, ARC added a stainless steel Winiarski stovetop to the smaller and larger Alpha Limited stoves that increased thermal efficiency and resulted in reductions of PM2.5 in the larger stove. The added stovetop seemed to encourage the injected, horizontal secondary air jets to more powerfully cover the top of the fuel? 

Winiarski stove top

With the addition of the SSM Winiarski stovetop, (6mm pot supports and flat perimeter to accommodate a pot skirt) the larger Alpha Limited stove became a very clean burning TLUD. The ISO PM2.5 Tier 4 is less than 62 mg/MJd. The Tier 5 (an inspirational goal for PM2.5) is less than 5 mg/MJd. In a single test, the improved larger Alpha stove achieved 6mg/MJd for PM2.5 after burning for 94 minutes at 4.6 kW.

Thermal efficiency_w_char_ 58% 
firepower_w_char_high power 4.6 kW
CO_useful_energy_delivered_  1 g/MJd      
PM_useful_energy_delivered 6 mg/MJd     
PM mass time  1 mg/min 
time_to_boil_high power  9.4  min (5 liters in SSM SuperPot) 
ISO Tiers     
Tier_efficiency_w_char Tier 5     
Tier_CO_useful_energy_delivered Tier 5     
Tier_PM_useful_energy_delivered Tier 4

The thermal efficiency in the smaller diameter Alpha Limited stove was improved but the PM2.5 was not reduced when adding the Winiarski stovetop. The smaller stove ran for 26 minutes on 0.4 kg of Douglas fir pellets.

Thermal efficiency_w_char_ 56% 
firepower_w_char_high power 2.6 kW
CO_useful_energy_delivered_  1 g/MJd      
PM_useful_energy_delivered 30 mg/MJd     
PM mass time  3 mg/min 
time_to_boil_high power  25.6  min (5 liters in SSM SuperPot) 
ISO Tiers     
Tier_efficiency_w_char Tier 5     
Tier_CO_useful_energy_delivered Tier 5     
Tier_PM_useful_energy_delivered Tier 4

The following chart describes the features in the larger Tom Reed Alpha Limited FD-TLUD. Perhaps, adapting these hole sizes and air pressure, etc. to other stoves might result in reductions of emissions while increasing thermal efficiency?

Diameter of 13 Primary Air Holes (mm)2.5
# Secondary Air Holes36
Diameter Secondary Air Holes (mm)4.7
Chamber Diameter (mm)155
Chamber Area (mm^2)487
Distance between Secondary Air Holes (mm)13.52
Secondary Air Pressure (in H2O)0.095
Secondary Air Pressure w/ Blocked Primary Air Holes (in H2O)0.11
SSM Stovetop Hole Diameter (mm)105
SSM Stovetop Hole Diameter / Cross Sectional Area0.677

Give it a try? 

Tell us what happens?

Dr. Tom Reed: “Smoking Backwards”

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Dr. Tom Reed: “Smoking Backwards”

Our beautiful friend Tom Reed (1926-2018) at a Winter Stove Camp

ONE: Starting in 1985, Tom invented the natural and forced draft Top Lit Up Draft stoves (TLUD). He would demonstrate one of the clean burning principles at Stove Camps by lighting the tip of a cigarette and then sucking on the burning end in his mouth. With practice, he did not burn his tongue.  Remarkable!

Sucking the woodgas through the burning end of the cigarette cleaned up a lot of the harmful gases and protected him from most of the smoke. Similarly, lighting the top of the fuel bed in a vertical cylinder pulled the made woodgas up through the charcoal bed and flame in a TLUD. (UP DRAFT)

TWO: In the same way, sticks of wood can be placed in a horizontal cylinder, lit under the short chimney of a Rocket stove, where the made woodgas is pulled through the burning combustion zone resulting in cleaner combustion. (HORIZONTAL DRAFT)

THREE: We tune stoves under the emissions hood to reduce emissions by combining the proper 1.) amount of metered fuel turned into woodgas, 2.) temperature, 3.) air/fuel ratio, 4.) mixing, 5.) 100% of woodgas pulled into the flame, and 6.) long enough residence time of woodgas in the combustion zone.

FOUR: Before iterating changes in the stove to reduce emissions, we try to optimize heat transfer efficiency so the least amount of biomass completes the task.

FIVE: Tom, we remember you so fondly! Thanks for the help!

Metering!

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Watching a Rocket stove or a pellet stove (as above), it becomes obvious that metering the fuel is a primary factor in achieving close to complete combustion. When too much fuel is introduced into the combustion chamber, the emissions of smoke increase almost immediately.

For the clean burning of biomass, the controlled metering of fuel seems to be as necessary as it is in the engine of an automobile. The rate of reactions (how fast the solid biomass is being converted into wood gas) is then matched with the corresponding amounts of Time, Temperature, and Turbulence required to minimize CO and PM2.5.

ARC has added Metering to Time, Temperature, and Turbulence while unsuccessfully searching the thesaurus for a synonym that starts with the letter T. Maybe someone can succeed where we have failed?

Secondary Air in TLUDs and Rocket Stoves

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Forced draft mixing with 2nd air jets in Dr. Tom Reed’s WoodGas Stove at around 1,000C

Forced draft mixing with preheated jets of primary air reduced emissions of PM 2.5 by around 90% in our stove tests with the Jet-Flame. Would adding secondary air jets further decrease emissions?

Secondary Air Works in TLUDs

Lefebvre, Vanormelingen, and Udesen examined secondary air jets air in cylindrical combustion chambers and describe most successful patterns of penetration depth. Jet penetration lengths approaching the middle of a cylindrical combustion chamber resulted in a maximum reduction of PM2.5 emissions. An increase in the number of jets created more thorough mixing. It was important to have the jets meet in the middle, but with minimal necessary force, to ensure highest temperatures and highest velocity of hot gases to the pot.

Forced draft secondary air jets can decrease the upward draft in a cylinder. Jets of air aimed horizontally into the flame most efficiently create mixing. But even when aimed upwards toward the pot they create a ‘roof of air’ that slows the draft by creating a high-pressure front.

Regardless of the velocity of secondary air flow rates, or the angle at which air is injected into the combustion chamber, supplying secondary air also tends to significantly lower the temperature. For this reason, using a minimal amount of air was found to be best. There is a reported balance resulting in optimized mixing, draft, residence time, and temperature. (Lefebvre, 2010) (Vanormelingen, 1999) (Udesen, 2019)

How Do We Add Secondary Air Successfully to Rocket Stoves?

One obvious difference between TLUDs and Rocket stoves is the large fuel door in the side of the Rocket stove. A TLUD is an open topped cylinder with a small amount of primary air entering the batch of fuel from below the packed fuel bed. In the TLUD, the fuel is initially dropped into the cylinder, while in a Rocket stove horizontal sticks are pushed into the combustion zone through a fuel door. The pressure/volume of secondary air jets introduced into a Rocket stove may be limited because the high-pressure front can create a backdraft that sends smoke out of the fuel door.

Supported by funding from The Osprey Foundation, ARC is currently experimenting to determine: 1.) How much pre-warming can be achieved and 2.) What is the most effective pressure/volume for secondary air jets in a forced draft Rocket stove.