[Stoves] Tar characterization

Sun Nov 13 01:38:55 CST 2016

All,

This is a good discussion, thank you.  

What might help me is an analysis of the post secondary particulates, after combustion is finished.  If these are ash, then they cannot be burned more completely.  If they are tars and/or carbon then it would tell me what is getting through the burner.  This might help direct me how to improve the design such that it would better break them down and burn them.  At least it would tell me that the stove design can be improved.  This might also be simpler then characterizing the more complex pre-secondary particulates since most of these would be burned into CO2 and H2O.

I am looking forward to seeing the results from your work.

Kirk H.
  ----- Original Message ----- 
  From: Frank Shields 
  To: Discussion of biomass cooking stoves 
  Sent: Saturday, November 12, 2016 9:37 PM
  Subject: Re: [Stoves] Tar characterization

  Paul,
  This more detailed analysis may be useful when fine tuning or research for a specific stove and its fuel. Tweet this and that and see if a more combustable gas is released for a longer period of time or something. Thats work for a specific stove because those ’tars’ are developed in the stove based on its design (cold spots or faster air flow etc.). 

  One group of fuels (based on my suggested table) will produce all sorts of different volatiles when tested in different stoves. But they all start with using the same fuel. So all this is before testing and has nothing to do with the stoves. Like testing road surface. Some cars designed for bumpy roads and some do very poorly on bumpy roads. But they all are tested on the same surface. 

  Regards

  Frank

    On Nov 12, 2016, at 9:21 PM, Paul Medwell <paul.medwell at adelaide.edu.au> wrote:

    Hi Frank,

    Personally, I'd like a little more fidelity than a TGA and ultimate/proximate analysis (but without necessarily going too detailed). I think some interesting models of a TLUD could be developed with a bit better understanding of the incoming flow composition to the combustion zone. The gasification process is not my area of expertise: I'd need to speak with my colleagues about what they can measure, beyond the "normal" stuff. Because a TLUD is more more uncontrolled than an industrial gasifier, I just don't know what sort of starting range of species we expect to see.

    Cheers,
    Paul

    On 13/11/2016 3:26 PM, Frank Shields wrote:

      Stoves,

      All we really need is to characterize the fuel. The moisture is an easy one. Ash is an easy one. Particle size and shape is an easy two. The organic fraction is what we need to work on. I’m thinking we need to develop a Table of the flammable volatiles being released from the Standard Combustion Chamber (SCC) using a specific biomass at different temperatures when operated to simulate for a specific stove type. Flammability measured per the O2 feed into that heated tube (mentioned before) and measuring temperature increase or CO2 exiting.  
      So we have a specific biomass (or a representative biomass) and heat in the combustion chamber using controlled heating coils and controlled air flow and measure the flammability of the gasses leaving. Perhaps end up with a table like this:
      Moisture %
      Size distribution range and uniformity coefficient
      Size shape 
      Ash %
      Total volatile matter % dw
      Flammability at 450 deg. C
      Flammability at 550 deg. C
      Flammability at 650 deg. C

      Once we start testing and with suggestions from others I am sure we will change the above as we learn more as what tests are such they are shown to effect the combustion process.  Any specific biomass will have their own reading from the above tests. When testing stoves the above results of the fuel used is reported along with the results. 

      Readings of a specific nature like measuring the chemicals released that change all the time means nothing. It is only the flammability of the released gases thats important. And, perhaps, the pattern of this flammability as temperatures in the combustion chamber or air flow change. 

      Each different design of a stove tested using a single fuel (single mapping of flammability and characteristics) will result in different degrees of success based on cooking. So it is the job of the stove designer to optimize success using a fuel he/she specifics based on the above tests. The fuels stay the same and the stoves modified to work with a fuel. That stove can be marketed in places where that fuel is available. 

      I don’t see this being that hard. A local fuel that is close to the established mapping that works best for Pauls TLUD (for example) can be modified to fit using equipment sent to them to do the job. Dry, chip, split or clean - whatever is needed. Low temperature may just be interested in temperatures to 550c while Rockets need the mapping up to higher temperatures. 

      So no expensive or special equipment needed for testing. Only a lot of research using a Standard Chamber and operated to simulate a stove type. Someone needs to design and build the combustion chamber. 

      Regards

      Frank

        On Nov 12, 2016, at 7:58 PM, Paul Medwell <paul.medwell at adelaide.edu.au> wrote:

        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> 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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