[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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