[Stoves] The upside of Down feed

Sun Jan 22 21:36:24 CST 2012

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.

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. 

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?

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?

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.

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.

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.

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.

Crispin

From: stoves-bounces at lists.bioenergylists.org
[mailto:stoves-bounces at lists.bioenergylists.org] 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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