A bit more on VG and acrolein; and formaldehyde.
"dehydration gives acrolein (acrylaldehyde) by tautomerization, and the oxidation product of that would be acrylic acid.
Without dehydration there would be so many hydrogen bonding groups available that the boiling point would probably exceed decomposition temperature."
Not sure the point in italic. but looks interesting. Can avoid decomp. some by maintaining water presence perhaps (but this cannot be near 100% in heat of atty coil); but having some small % of water in the mix might be helpful?
VG + heat > acrolein + 2 water
acrolein + oxygen > acrylic acid
So what we see deposit-wise with VG will likely be via acrylic acid?
"The primary use of acrylic acid is in the production of acrylic esters and resins, which are used primarily in coatings and adhesives. It is also used in oil treatment chemicals, detergent intermediates, water treatment chemicals, and water absorbent polyacrylic acid polymers. Acrylic acid is used widely for polymerization, including production of polyacrylates. It is a monomer for polyacrylic and polymethacrylic acids and other acrylic polymers. It is used in the manufacture of plastics, as a tackifier, as a flocculant, in the production of water-soluble resins and
salts, as a comonomer in acrylic emulsion and solution polymers and in molding powder for signs, construction units, decorative emblems and insignias. It is used in polymer solutions for coatings applications, in paint formulations, in leather finishings, in paper coatings, in polishes and adhesives and in general finishes and binders."
"Acrylic acid undergoes the typical reactions of a carboxylic acid and, when reacted with an alcohol, it will form the corresponding ester. The esters and
salts of acrylic acid are collectively known as acrylates (or propenoates). ...
Acrylic acid and its esters readily combine with themselves or other monomers (e.g amides, acrylonitrile, vinyl, styrene, and butadiene) by reacting at their double bond, forming homopolymers or copolymers which are used in the manufacture of various plastics, coatings, adhesives, elastomers, as well as floor polishes and paints. ... Acrylic acid is severely irritating and corrosive to the skin. Eye contact can result in severe corneal burns and may result in irreversible injury. Inhalation of vapors may cause irritation to the respiratory tract."
Polymers of both acrolein and acrylic acid (and its esters) would be degraded on the heater coil. As increasingly ashed, small light parts may flake off into the air-stream; this may be what SurbitonPete has reported finding. These could potentially be so small as to be invisible to the eye.
Polymerisation - in random lengths - plus degradation might partly account for the wide spread of compounds seen in analysis of the vapor.
Next, potential for formaldehyde:
"On the glycerol molecule, you cannot completely oxidize all 3 -OH groups (to carboxyl), simply because the middle -OH group is attached to a carbon which itself is again attached to two carbons, and can only form a double bond at most with the oxygen (as in acetone).
The most oxidized product of glycerol would be something like dicarboxy-formaldehyde, but I can't find anything about it, and I wouldn't be surprised if it is not stable and will instead form
formaldehyde and 2 CO2.
My first guess here is that glycerol doesn't want to oxidize any further without falling apart."
Sources:
Glycerol-Ethanol fuel gel - Science Forums, The Original
http://www.npi.gov.au/database/substance-info/profiles/6.html
http://en.wikipedia.org/wiki/Acrylic_acid