[Stoves] Coal burning in downdraft gasifiers.

[Crispin Pemberton-Pigott]

Dear Paul

I will send along some photos when I get more to choose from. Mark Hopson is
here with Bernice Chan working on their 4th year project (U of Waterloo). He
has been diligent in getting pictures.

Your questions:

>> The key to getting the CO level down that low is to get the combustion
>> chamber temperature up and the excess air down.

>AND to have a good burner of those gases.

Well, it is not really a gasifier in any way - it starts the flames in the
lower portion of the fuel and they continue through the grate.  The flame
temp is about 1200 max, just below the grate.  I am concerned the grate
material will not take that for long, though it is a demo not a product yet.
The outside temperature of the hottest portion (the conical section under
the fuel bin which you refer to as a concentrator) reaches 600 C though a
couple of times we got it red hot. That was done by using a higher surface
to volume ratio fuel size. It works fine with fist sized lumps.  The
diameter of the fuel bin is 8 inches. Height about 12.

>Quite possibly, and certainly worth additional efforts.  Downdraft 
>gasifiers are known for having cleaner created-gases

Again, it is not generating gases to burn elsewhere, it is directly burning
the coal in a downdraft fashion. It is the same as the units I made for
testing around Johannesburg a few years ago, except those had 3CR12 drop-in
burners with preheated secondary air.

As for power, that has not been established (no scale) because I was not
worried about the rating - just a clarification as to how cleanly the coals
could be burned (there are about 4 or 5 mines). It seems to be in the 6-12
KW range, but there is a pretty good degree of control over the power
because the air is easily choked to control the burn rate. A yurt ('ger')
needs about 5 KW so it is a little overpowered. Without any heat exchanger
it is about 70% efficient (heat coming off the whole body) so it should be
possible to get the thermal efficiency up to 85-90% without much trouble.

The coal is low in sulphur and high in volatiles, the moisture a huge
30-40%.  This is a good fuel for lowering the temperature even at very low
(under 70%) excess air. No matter how you cut it is amazingly clean with
that device+fuel combination.  I did not get such a good result with a coal
of unknown origin that had lower volatiles.  The next step is to establish
which fuels and stove work best, possibly working the fuel a little to
optimize it to an as yet un-designed production heating/cooking stove.

>Maybe others can tell us how small in size downdraft gasifiers 
>have been made in the past, their difficulties, and issues of coal vs 
>biomass in them.

I found that ones this simple work well with charcoal and coal but not all
that well with biomass unless you have taken the trouble to get the
secondary air right.  Wood burns but it tends to be smoky. It doesn't seem
to burn well confined in a metal cylinder. Perhaps it needs to spin.

The lighting technique is important:

Put a 20-25mm thick layer of coal as small as will sit on the grate.
Put wood on top, about thumb diameter.
Put really small wood on top of that.
Put a few balled strips of newspaper on top.
Put half a sheet into the draft inducer under the burner
Light it
Light the paper balls
When the chimney roars, close the draft inducer hole
The paper balls then have their flames pulled down.  
Put in coal - perhaps 1/2 to 1 kg in pieces about the size of an egg
After 3 minutes, add wood on top and toss on a lighted paper ball if
necessary.  This will induce a higher draft briefly.
After about 6 minutes the smoke turns clear and remains clear thereafter.

>Will you have one at ETHOS?

No, but I can draw it. It only took 2 hrs to make it from scratch.

See you!
Crispin in Ulaanbaatar on a particularly crisp -30 night