[Stoves] Tar characterization

[Paul Medwell]

Hi Tami,

Thanks for the detailed response.

The answer to your question is "the more we can have the better". 
Realistically, it would be too easy to end up with too much information 
and just become overwhelmed. Even if it were possible to obtain full 
speciation measurements of the pyrolysis/gasification products, it 
probably isn't practicable to use. For example, in the case of gaseous 
turbulent combustion there is already too much of a compromise needed 
for methane combustion: detailed kinetic mechanisms are often too large 
for CFD...obviously the problem becomes much worse for other fuels. 
There is a huge amount of work in the area of kinetic mechanism 
reductions (mainly for engine fuels), but detailed kinetics of tars 
seems out of the question any time soon.

In the first instance, it would be interesting to see a plot of the mass 
spectrum of the pyrolysis/gasification products in a TLUD (before the 
secondary air inlets) for different operating conditions. It would also 
be nice to know the global C/H ratio. We are in the process of getting a 
new GC/MS, but only to 300 amu. We'll wait and see how that goes. The 
long-term plan is to include this in a model of the system, as well as 
subsequent experiments of the specific details of soot production in the 
combustion zone. However, this is all work to be done in the future.

Cheers,
Paul


On 13/11/2016 6:52 AM, Bond, Tami C wrote:
> Hi Paul, and all,
>
> Sorry that I have not been keeping up with Stoves discussions….
>
> We have fussed a bit with trying to characterize the organic matter 
> emitted from wood combustion.
>
> What kind of characterization do you want? Full speciation is 
> well-nigh impossible. But one needs to start with the question of what 
> you want to know and why?
> The problem is that this stuff is complex chemically and doesn’t take 
> well to matching with standards.
>
> Brief summary of what I know:
> - GC/MS is really hard for characterization because it is poor at 
> identifying compounds that are large (molecular weight) and polar. 
> Wood organics are both. You’ll get the light organic material, but 
> that’s a very small fraction of the emissions. You can get into 
> derivatization of the emissions before GC/MS and that may get you a 
> little more. See all Jamie Schauer’s, Wolfgang Rogge’s work. Most of 
> it ends up being classified as “UCM” (unresolved complex mixture).
> - FTIR— have tried a little. Although it nominally gives you 
> functional groups, and occasionally you can see differences between 
> one kind of wood or burning condition and another, it’s not very 
> quantitative. Plus there are a lot of overlaps between the interesting 
> functional groups, so you can’t distinguish some of them.
> - 1NMR— kind of like FTIR— you can sort of see functional groups and 
> that’s about it.
> - UV-Vis— used for humic matter (and some “tar” isn’t far off that)— 
> Again not quantitative, rather just gives a sense of how strong an 
> absorber it is. You won’t see any structure as you do in UV-Vis of 
> individual compounds. There are so many compounds that the entire 
> spectrum smooths out.
>     —> From our UV-Vis work we found that most of the stuff dissolves 
> in methanol, not hexane not water. (Chen & Bond, Atmos Chem Physics 
> 2010, open access)
> - 13CNMR— which can get at the carbon skeleton rather than functional 
> groups… tough. You need HUGE sample sizes and they don’t stay 
> suspended because anything concentrated enough to measure wants to 
> precipitate. We ended up using DCM, I think— I’d have to check. And 
> then, the result was not all that interesting: we again couldn’t 
> distinguish the unsaturated bonds, e.g. aromatic vs conjugated but 
> linear bonds.
> - LC/MS— expensive and full of artifacts— I’ve never gotten into it. 
> Too expensive and haven’t seen the benefit, although I’d be interested 
> if someone tried.
>
> The question, again, is what you want to know and why. If it’s some 
> kind of physical property of the emissions, you’d be better off 
> measuring that.
>
> Tami
>
>
>
>> On Sep 2, 2016, at 12:22 PM, Paul Medwell 
>> <paul.medwell at adelaide.edu.au <mailto:paul.medwell at adelaide.edu.au>> 
>> wrote:
>>
>> Dear Philip,
>>
>> Thanks for another great suggestion.
>>
>> This is also something that is planned. As part of an unrelated 
>> project we have been looking at this: we are still in the process of 
>> finding a solvent that has a collection yield, that doesn't interfere 
>> with (or gives an interference profile that we can correct for) and 
>> that OH&S will let us have. Fortunately, for the cookstove work there 
>> are fewer constraints than the other project (which is limited to 
>> sampling extremely low flowrates). We'll find a solution and share it.
>>
>> Cheers,
>> Paul
>>
>
>
>
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