Dear Frank and Stovers,
I changed the subject line so that this conversation about igniting
TLUDs does not mix with the small stoves discussion that I hope will happen.
Igniting a TLUD stove can be accomplished in several ways, but the
bottom line is that a "sufficient" (not yet quantified) layer of hot
charcoal is across the entire top layer of the TLUD fuel so that the
pyrolysis front can progress downward.
At the first moments of ignition, the oxygen for the combustion of the
ignition material comes from the immediately surrounding air. I am
sure that as the initial combustion sends flames upward, the initial air
(which serves both the primary and secondary combustion) is pulled in
from the sides of the top of the TLUD. But soon it is also being drawn
from below, that is, upward through the pile of fuel.
It follows that there needs to be sufficient ignition materials to
establish the up draft and also create the initial layer of hot charcoal
on the top.
Let's say that there are three scenario: Too little ignition material;
the right amount; and too much.
1. Too little is quickly observed with self-extinquishing of the
ignition material, or maybe some fire in only one sector of the surface
of the pile of fuel.
2. Too much has two variations:
a. The excess starter material could actually being burning like the
main fuel. And since in general the starter is more costly (in money
or preparation time or scarcity) , excess starter is wasteful. So in
general we do not want to overload with starter material, but if it is
plentiful, be sure to use a sufficient amount.
b. The excess starter material could have an undesirable
characteristic. For example, this occurs when the starter material is
fuel pieces that have been "enhanced" with a small amount of more
volatile material such as kerosene, alcohol, wax, or if it is natural
biomass such as "fat wood" (resin-rich wood) or birch bark. The correct
amount of starter is great, but excess will result in too many volatiles
so that the secondary air is insufficient, resulting in a taller sooty
flame until the extra volatiles are gone. Therefore, do not overdo the
amount of these "enhanced" starters. (and that also represents waste
of the more valuable starter material).
3. The correct amount of starter is the "Goldilocks' just right"
situation. And there are several observations, and I hope that others
will add even more.
First a NOTE: From the Boy Scouts. Fires are made with tinder,
kindling, and main fuel. Tinder (such as wood shavings) can be ignited
with a match. Kindling (small twigs, etc) would need too many matches,
so it is ignited by the burning tinder. And the main fuel (sticks,
pellets, wood chips, etc) is ignited by the burning kindling. Tinder
in general cannot ignite the main fuel unless you used quite a bit of
this thin and dry biomass.
A. In Uganda, the common way that I have observed is for the user to
first fill the TLUD with the main fuel, and then in the top 2 to 4 cm of
the fuel chamber to make a small "teepee" of thin tinder and then some
kindling above the tinder. This would generally have a diameter and
height of about half the diameter of the fuel chamber. This little
material is ignited and carefully tended, for perhaps 1 to 4 minutes,
without any concentrator lid being used. It is a very clean burn.
When it is clearly burning well, the starter fire is spread with a stick
or rod to cover the top layer of the main fuel. There is not rush, but
the concentrator lid is soon put into place with an appropriate riser to
enhance the updraft.
(Personally, I lack the patience and skill to do that type of ignition,
but my Ugandan associates do it all the time, with great ease. Their
success negates any criticism that extremely poor people cannot afford
or have access to even a small amount of kerosene to start the TLUD
cookstoves.)
B. The other way is to use some "enhanced starter material" that serves
the function of the tinder and kindling. In the past (in the early days
of TLUD stoves as recently as 2006), the critics of TLUDs at stove
testing events smiled only slightly when they referred to the use of
enhanced starter material as "cheating." Thankfully such comments seem
to have disappeared. But it is appropriate to note whenever they are
used. And also note that if too much starter materials are used, they
can give off excessive emissions at the beginning that unfavorably
obscure the otherwise extremely low emissions of TLUD stoves.
C. Starter materials are commonly liquids or wax-type substances that
catch fire easily when the match is struck. Those can be either
(1) added onto the main fuel shortly before ignition. This is not the
best way because drips can run down the sides of the fuel or fuel
chamber. or
(2) can be added to (coated or soaked into) some kindling-type biomass
that is then placed on top of the pile of fuel. How much to use is
learned by experience because each starter material has different
characteristics. NOTE: If you are tending to the ignition stage for a
minute or two, you can watch for the three scenarios of too much, too
little and just right.
Too much will be obvious, but be aware that the initial high and sooty
flame might not be leaving behind sufficient charcoal over the whole
upper surface of the column of fuel. And it is unwise to attempt to
remove ignited starter material.
Too little can be overcome IF you have some additional starter material
prepared to add carefully. This is NOT a time to be spraying volatile
liquids into a burning area. Better to have some pre-soaked pieces of
fuel that can be dropped in as needed.
Goldilocks' Just Right is by far the best scenario. And it does take a
little practice. But when it goes well, you will know it and you will
replicate that situation easily the next time.
4. About starter materials: There is a wide range. Here are a few:
a. Liquids:
Kerosene
Charcoal lighter: very good, if available
Lamp oil
Diesel fuel (not great)
Alcohol (91% or higher): VERY clean burning, but is quite volatile,
can go "poof", and can be burning but not visible especially in sunlight.
Turpentine
NOTE: Gasoline is NOT acceptable. Simply too volatile.
b. Solids:
Candle wax
Fat-wood
Commercially prepared fire starters (sold in hardware stores).
c. Preparation of appropriate fire starters:
Whatever fire starter you prepare needs to be saved in an appropriate
seal-able container. I like to use clear-plastic jars such as those
for peanut butter or mayonnaise. (Glass can break, so I do not use
glass.) It is generally convenient to prepare the fire starter in the
container that will store it. The amount in a standard peanut butter
size jar (about 500 ml.) should be sufficient for starting 5 to 10 fires.
With moderate care, select pieces of fuel that are kindling size
appropriate for your size of TLUD and place them into the jar. Add as
SMALL amount of the liquid, close the lid and shake so that the liquid
comes into contact with most of the pieces. The liquid will be
absorbed, so there should be no liquid visible in the container.
The intention is to NOT saturate the fuel pieces. Very little of the
liquid is needed to get the kindling pieces ignited.
Note: Pellets are highly densified and do NOT absorb much of the
liquid (or they would start to break up). The liquid on them will
vaporize quickly and will generally not ignite the pellets. It is far
better to have some wood chips with starter fluid to ignite the layer of
pellets below. Note that pellets can be difficult to ignite initially,
but once they are pyrolyzing, they are a fantastic fuel in TLUD cookstoves.
5. Another trick (logical but often forgotten): When the fire started
is ignited, most of the heat goes upward. Therefore having "some"
(undefined quantity) of regular kindling-type fuel spread thinly ABOVE
the enhanced starter material greatly assists to create the needed layer
of charcoal. This is especially important if the main fuel below is
pellets.
Summary: Provided that the main fuel is dry and ready for pyrolysis,
and provided the stove user has some minimal experience with TLUDs with
that fuel and an appropriate fire starter, the ignition should be quite
easy and virtually without smoke.
Further comments are welcome from everyone. We will try to get them
incorporated into some useful document to post at www.drtlud.com and at
other sites.
Paul
Paul S. Anderson, PhD aka "Dr TLUD"
Email: [email protected] Skype: paultlud Phone: +1-309-452-7072
Website: www.drtlud.com
On 10/20/2012 1:15 AM, Frank Shields wrote:
Dear Paul,
I am interested in knowing what is the common method used to ignite the fuel
in your small stoves? I'm thinking steps to making igniting the fuel easier
is perhaps the next limiting factor to making these stoves more useable-
agree?
Thanks
Frank
.
-----Original Message-----
From: Stoves [mailto:[email protected]] On Behalf Of
Paul Anderson
Sent: Friday, October 19, 2012 10:38 PM
To: Discussion of biomass cooking stoves; Hugh McLaughlin; James S. Schoner;
Noeli Anderson; Steve Sewell; John M Bare; Doc Anderson
Subject: [Stoves] Very small stoves and reheating food
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