acrolein

[Ivan]

How many people have heard of acrolein? It's a very nasty chemical produced if you try to burn the glycerine by-product from biodiesel production (if combustion temperature falls below 300C).

I've always wondered if the same product is produced by vegetable oil burning at low temperatures, but can't find any references - any ideas?

[PEMTEK]

If you are worried about it in your exhaust it might be eaten fairly well with a small automotive catalyst connected as close to the exhaust port of your engine as possible. Theoretically It would have the added benefit of producing more exhaust heat as it burnt up any combustion left overs too.

Phil

[Eleanor]

Hi Ivan,

It seems the answer is yes  Hope this helps. The link is www.inchem.org/documents/hsg/hsg/hsg067.htm

2.1  Exposure to Acrolein

    The general population will normally be exposed to acrolein through
    the air, particularly through exposure to smoke.  Only persons who are
    present when acrolein is being applied as a biocide, or who are near
    an accident during the transport of acrolein, are likely to have
    contact with the liquid or its solution in water.  Some alcoholic
    drinks contain a little acrolein, and it is also produced during the
    heating of foodstuffs.

    Average levels of up to approximately 15 µg/m3 and maximum levels of
    up to 32 µg/m3 were measured in urban air, away from industrial
    sources.

    Acrolein is principally produced by fire, especially during the
    partial burning of organic material.  Thus, atmospheric levels ten or
    one hundred times higher than those mentioned above may be found near
    exhaust pipes, near hot smoking oil, or in the smoke of wood or
    garbage fires.

    In indoor air, smoking one cigarette per m3 room in 10-13 min was
    found to lead to acrolein vapour concentrations of 450-840 µg/m3.

    Thus, humans may be exposed in the home through cigarette smoking or
    through the smoke from cooking; they may also be exposed in a street
    polluted by exhaust gases, or near a fire.  In the workplace, exposure
    may occur through the heating of organic material during the welding,
    cutting, or sealing of plastic sheeting, or near hot smoking oil in
    oil-seed mills, where levels as high as 10 000 µg/m3 have been
    measured.

    2.2  Fate of Acrolein

    Acrolein is degraded in the atmosphere.  In natural water, it
    dissipates fairly rapidly, though the toxicity of the compound for
    microorganisms may limit biodegradation.  Bioaccumulation of acrolein
    is not expected, and it is not likely to persist in the environment.

    2.3  Uptake, Metabolism, and Excretion

    Acrolein is a very reactive substance and reacts with substances
    present in the tissue at the site of contact.  As a result, irritation
    and damage occur at these contact sites and transport through the body
    is only slight.  Thus, metabolism is expected to occur only on a minor
    scale.

It is published by the WHO  so it should be right!

Eleanor

[Ivan]

Thank you - useful information.

It's good news as well as bad:

If acrolein is produced in significant concentrations from wood, cigarette smoke, smokey fryers etc then it suggests that burning glycerin or veg-oil soaked sawdust in woodstoves or veg oil in engines may not be any more harmful. It's very difficult these days to assess whether processes forming harmful materials pose a real risk, or whether they are no worse than the alternatives (eg perhaps smouldering wood fires produce just as much acrolein?)


Ivan

[Eleanor]

This is quite a good source of info if you have the time!
http://www.lib.ncsu.edu/theses/available/etd-07312007-153859/unrestricted/etd.pdf.
He has been researching into a burner for glycerin and it seems there's a bit more to it than first meets the eye.
The problem is that glycerin undergoes thermal decomposition to acrolein at a lower temperature (>280C) than it undergoes combustion.

OH-CH2-CH(OH)-CH2-OH  + HEAT  -->  O=CH-CH=CH2 + 2H2O

It seems that there isn't enough energy produced by the process of burning glycerin alone to keep it going and it requires the use of another fuel eg kerosene to maintain a high enough temperature for the glycerin to ignite and keep the flame alight. At < 600C the flame was going out and they concluded that the temperature needed to be around 1000C to be sure! At this temperature acrolein formation was negligible.
So although acrolein can be produced from incomplete combustion of many organic compounds it would seem that there is a far greater potential for it to be formed in larger quantities by trying to burn pure glycerin or mixtures containing it.

I don't think I will be trying it!

If anyone has any further insight please squeak up! This is just my interpretation 

[renewablejohn]

eleanor

mix 10% glycerin with woodchip by weight and burn at minimum of 850C and you should not be left with your nasty acrolein

[Eleanor]

Hi John,

Thanks but I don't have any glycerin to burn (just time to waste!). You make it sound so easy! Do you think it's possible for your average bod (Ivan excluded!) in his/her garage producing biodiesel to guarantee to burn woodchips and glycerin at > 850C?

I would have thought that mixing it with a solid material would make it more difficult to regulate the local temperature in the mix and prevent decomposition of glycerin at lower temperatures? Maybe you are using (or suggesting) some sort of woodchip burning stove or boiler which most people don't have. It would be interesting to have an analysis of the gases to see if adding the glycerin makes any difference. If it is all completely burnt then perhaps you have a business opportunity removing the glycerin lake! Hopefully this is the case.

You have confirmed that high temps are required (you say 850C, I say 1000C, hey 925C who cares?! What's a few degrees between friends  ). This is a completely different scenario to people burning it on open fires or in woodstoves at unregulated temperatures.

As I said I don't have any glycerin to burn but if I did I would probably do something else with it!

[Paulh_Boats]

This issues goes back to another thread where I suggested modern diesels with fuel injection computers and all the emission gizmos would be better than a clunky old Lister invented long before EEC emission limits were established.

I know for a fact the older British Seagull 2-stroke outboards despite being very chunky and simple were made obsolete because they did not meet modern emissions requirements.

A typical engine computer will monitor the exhaust temperature and oxygen content and adjust the input mixture to ensure that the catalytic converter has enough oxygen to do its job properly (in a petrol engine oxidise the poisonous CO, NO to CO2 and NO2....not sure about diesels though).

Perhaps as Pemtek suggested a catalytic converter would help, but probably not without modern fuel injection tackle.

[renewablejohn]

Eleanor

It does scare me at times when a bio diesel producer is sat in front of a wood burning stove bragging about how his waste glycerin makes his woodstove burn so much better. Thankfully he is no longer a bio diesel producer.

The problem I have is not the burning but the definition of glycerin as a waste product rather than a by product which means if I burn it I would no longer be entitled to the green energy ROC's. Therefore there is no incentive to reduce the glycerin lake.

[Eleanor]

Hi John

Thanks for the reply - seems like we are in agreement that the wanton burning of glycerin in any old woodstove is not to be encouraged! Very interesting what you say about the ROCs. Does the glycerin have to be accounted for in some way and are there some acceptable things you can do with it and still get the ROCs?

Thanks

Eleanor

[renewablejohn]

Eleanor

If the glycerin is clean then there is a market for soap production however most glycerin from bio diesel production is to dirty and not economic to clean up. As for accounting at present the market seems to be unregulated and hence the cases of illegal dumping.

ROC's are only available for virgin sources so I can get ROC's for renewable energy derived from woodchip so long as it is straight from the forest. If the wood was recycled from the tip then I could not claim the ROC's. The same applies with Bio Diesel from waste vegetable oil compared with virgin crushed oil seed rape. Pressure is being applied to get the definitions changed but in the meantime we have to stick with virgin sources.

[Ivan]

I've got a woodstove with a flue temperature thermometer. The temperature registers between 200C and 300C. It very rarely goes over 300C even when burning hot. It's a waterstove, so temperatures are probably a bit lower than one without a boiler, but even so, they sound rather too low to combust glycerine safely.

The glycerine makes great soap, but it's very difficult to remove all of the methanol...and methanol is toxic, so that's not a great idea. The soap works very well, and is moisturising, unlike most soap....but brown, which puts people off. I think the best use is an industrial degreaser or engine part cleaner - it has a remarkable ability to clean up sticky oily deposits (anyone need any - free if you bring your own container!). In the spring, I'll have a go at making a biogas digester for the glycerine - it seems a shame to waste all that energy that it contains.


Ivan

[Bob]

I'm told that it is possible to "compost" the glycerin?  Anybody tried this?  What would you mix it with to keep the Carbon to Nitrogen ratio?

Another issue is what to do with the wash water?  I've read that if you use KOH instead or NaOH you can use it as a fertilizer - probably invalidates any organic certificate you may have!

[Eleanor]

Hi everyone, yes I had heard that composting or a biodigester as Ivan says is a way of disposing of it as it is pretty harmless. As Bob says using KOH instead of NaOH would be better for the soil.

How about reacting it with nitric & sulphuric acids and making nitroglycerine  Don't suppose it's good for your ROCs  but it would be more interesting  Please don't do this at home children!

[Bob]

You a bit of a dodgey chemist are you Eleanor?