Hi Crispin,
Replies under your questions.
On 29/11/2011 06:00, Crispin Pemberton-Pigott wrote:
Dear Peter
Good to hear from you, I was thinking of you last week testing a
downdraft stove using chunks of wood the same size you use on your DD
BBQ (barbie). The chunks of wood provide quite a bit of excess air so
the way the heat transfer takes place it affected a lot.
>As far as I know the difference between kerosene (paraffin) and higher
hydrocarbon fractions, including vegetable oils is the fact that they
are not distillable under atmospheric pressure.
The stove must work then by boiling something in the veggie oil that
boils at a temperature achievable in the evaporator tube. Perhaps that
is why there is so much gunk left behind. A completely different
approach (and an old one) is dripping oil onto a flat plate that is
completely contained inside the combustion chamber. People burn old
engine oil that way, and for similar reasons.
Yes, as the veggie oil decomposes liquids, vapours and solids are
produced, some of the latter may be sticky and decompose further into char.
Dripping oil on a flat plate is a good idea except that we need access
of air to the resulting char, the reason I thought of a perforated plate.
>Undistillable hydrocarbons are fed into the fire as a spray of very
fine droplets that burn completely.
With the residence time in the flame matching the burnabilty of the
droplet, right? When a droplet of diesel burns, has it been evaporated
by radiant heat under that increasing pressure of the pressure wave?
I don't think so, being a droplet part of it will produce gaseous
decomposition products and the tiny char skeleton should burn given
enough residence time.
The same could be done with vegetable oils at the combustor end.
However, most vegetable oils tend to react slowly with oxygen to form
gunk which eventually blocks the passages it has to flow through.
Is this process dramatically accelerated by heating the oil? In other
words, is the depositing cause by O2 already in the fuel? That bodes
badly for the future of burning raw oil.
Yes, the polymerisation would speed up at higher temperatures. At still
higher temperatures the tendency is to break up into smaller molecules,
which certainly happens in a flame.
So for a stove that burns vegetable oil, the piping from the storage
to the burner should be of very simple shape and easy to clean.
Daily, as I understand it.
Realising that producing a spray of fine droplets is out of the
question for domestic stoves, we have to find something that feeds the
oil to the combustion zone where the carbon, resulting from the
decomposition of the oil is burnt as well.
How about using one of those spinning disks with a spiky periphery
that are used in greenhouses to make as fine a mist as possible? They
are very small (50-75mm) and use only a small amount of power. It is
conceivable they could be driven by electricity, heat or draft.
Yes, that should work.
Possibly something like a perforated disk where the oil burns in
updraft mode and where the holes occupy a sufficient part of the disk
area that all the char comes in contact with air.
Good idea. A variation on the drop-onto-plate idea might do.
Always interested in your comments in the List.
Best regards,
Peter
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