Dear Crispin,
On 22/01/2012 10:36 PM, Crispin Pemberton-Pigott wrote:
Dear Alex
This is interesting news. The grate looks great, and if there is a
pile of char that is maintained in front, we must conspire to burn it.
Not so fast. I rather like the idea of a naturally formed biochar venturi:)
To review, it is a crossdraft fire with all the advantages inherent in
it: refuellable and lots of room for flames.
If the char builds up in front of the hopper but self-limits (i.e.
burns away) then it has to be eaten by the passing flame and available
O2. That being so, I see a couple of options. One is to admit air
through some small holes (1.6, 2.0mm) drilled under the early part of
the pile (on the grate side of the pile). That will bleed air under
the char which is a good way to burn it. Another is to change the
shape of the pipe in that area to allow ash to drop. I presume at the
moment the ash is blown into the larger chamber of the stove.
I see the odd spark or glowing particle fly into the stove. This is what
you see in most pellet stoves which do tend to have relatively low PM
emissions. In part that has to do with the way the size of the particles
in the pellet ('sawdust' is really tiny wood chips) and how they tend to
hold together. When the hopper does runs out of fuel the added air flow
literally blows whole charred pellets into the stove. A chimney draft is
nothing to sneeze at, so to speak.
It is highly likely you have a fairly large PM10 number compared with
the same fuel burned in a pile in that larger chamber because of
lofted ash.
I am really pleased to hear that the flow is so reliable. I have some
really short fat pellets here which are probably going to feed well
because they are nearly marbles. Probably made with one of those
trochoid, concentric-ring pellet formers.
As for the fire rising into the hopper, that is not going to happen if
the air velocity is high enough. Conditions we have observed it is
when the velocity is quite low. If the heat is enough, a rising
current of heated air and gas circulates in the fuel immediately above
the burning layer and the fire works its way up. That can only happen
if a) there is some air (especially from above) or b) the fuel is
volatile enough to run an air-free charring burn in the present of
enough heat.
The advantage of coal, even with a highly volatiles one like the
lignite from Nalaikh mine, it is still less volatile than wood. The
talk of torrefied pellets intrigues me for that reason. It is more
likely to behave like slow roasting coal. Very controllable. There are
small coal pellets, say 16mm diameter which might feed well too.
Is there any reason you can think of that the hopper, feed tube and
burning chamber should be round?
No.
If you pass by the house I can show you a stove body that your burner
will directly attach to with several novel features. It won't be shown
until March and sort of solves the 'rest of a stove' for you. I am
pretty sure you will like it. It /might/ address the PM10 issue, or
not, and solves the heat extraction issues for cooking and heating.
I have some Kanthal wire here which we are recommending for grate
material. It will work well for you and will hardly get hot at all. If
some small bits of fuel drop through (being ahead of the grate) a
pocket can be provided underneath to let them smoulder, feeding CO and
C and H2 into the beginning of the fire. No problem.
The arrangement is made similarly on the SeTAR BLDD5 with the
smoulderings fed into the flaming portion of the pipe. The reason I
mention a wire grate is it is cheap and easy to make. But like the
layered flats too. It should be possible to punch that in a single
stroke with the right tool.
How deep is the fuel in the hopper?
40 cm. I have materials to go up to 100cm, and I could take it up to
near the ceiling with some stove pipe, though I might have to counter
balance the stove:)
I tried a number of things with hopper shape and decided there is a
general rule about bridging which is the following: if the hopper
tapers inwards the point of burning, there is an upper limit to the
height of the fuel for each degree of narrowing. I tried shoulders too
and they are OK further up (for example, within 1 hopper width of the
top of the fuel pile). On all cases, if the fuel is compressed by
pushing down on it or by stacking lots of fuel up, bridging occurs
just above the beginning of the taper.
These pellets are highly resistant to bridging. On the other hand I have
seen chips defy gravity:)
The solution is agitation or a non-tapering hopper bottom.
This is bound to be affected by the surface smoothness of the fuel
particle. If the pellets are shiny and hard that has to be a help
avoid bridging, right? I am impressed that you are able to keep it
feeding so well with such a significant narrowing.
It is difficult to measure but the pellet flow is in fact air (wind)
assisted. However they are more bullet than sail. Small dry wood chips
are sails and if they make it down through the hopper, they blow right
out of the tube.
To you think it is necessary to tilt the flame tube downwards at all?
If the char pushed ahead it would be burned by the continuous heat of
the flame. Using wood, I can report that I have seen flames reaching
more than 24 inches along the tube so you may get better combustion
efficiency by lengthening the one you have.
Yes, a view of the flame is useful for understanding the process but is
not ideal for combustion.
Thanks for an interesting (tiny) burner idea. If you bring it here I
have some 5mm switchgrass pellets I have had difficulty burning it
anything yet. Maybe...
I suspect the ash would bung it up, but maybe...not.
Alex
Crispin
*From:*[email protected]
[mailto:[email protected]] *On Behalf Of *Alex
English
*Sent:* Sunday, January 22, 2012 7:02 PM
*To:* Discussion of biomass cooking stoves
*Subject:* Re: [Stoves] The upside of Down feed
Dear Crispin,
The pellets are 6mm dia. 1-2.5 cm long. The hole is 5cm in diameter. I
tried a few chips to block the hole. It usually took more than one to
stop it, and put out the fire. There isn't much evidence of the fire
mving up into the hopper. The attached picture shows the position of
grate in relation to the hopper. It is adjustable. A mount of glowing
charred pellets form in the bottom half of the tube (6.2 cm ID pipe)
out in front of the grate. This seems to be self limiting, at least
over the five hours it ran today.
I ran it with a second hopper tube such that the air flowed outside
the inner 14cm diameter hopper (with lid) and inside the outer15.5cm
diameter hopper. The air has to pas through pellets in the cone
portion only.
Shutting off the grate air does reduce the burn rate by as much as
half, but that is a guess and I am unsure if it works for hours.
but wood pellets are cheating...
next, a complete redesign for chips...
courting failure....
Alex
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