[Stoves] Vegetable oils
Peter Verhaart
pietverhaart at bigpond.com
Tue Dec 6 18:58:03 CST 2011
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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