[Stoves] The upside of Down feed

Mon Jan 23 19:45:36 CST 2012

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:*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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