[Stoves] Saving the WBT

[Ronal W. Larson]

http://www.et.byu.edu/~tom/classes/733/ReadingMaterial/Jenkins-Baxter.pdf

"Stoichiometric air fuel ratios …………..for biomass they are 4 to 7,"

I have seen "6" a lot, and the inverse (fuel to air weights) would be 17%


On Aug 17, 2013, at 5:49 AM, Alex English <english at kingston.net> wrote:


> Ron, Paul,
> Below; Paul refers to 'equivalency ratio'. This would be the amount of primary (under fuel air)
         [RWL:  Alex,  thanks  _ I wasn't thinking this way.  For your moving grate design, this term  "under fuel air" makes sense.  But for TLUDs,   I believe the term "under" makes less sense, as all the O2 is used up at the pyrolysis front, regardless of its magnitude in volume per unit time.  Since it would seem that CO needs about half the oxygen as CO2  (except some O2 is coming from the biomass and we have to account for H2 going to H2O), maybe a number near half  (meaning the 30% and 60% numbers below) makes sense.  Or, maybe Paul's definition of equivalency ratio includes excess air - not stoichiometric air.  Paul - do you have a cite we can go to?

> divided by the theoretical amount of air (stoichiometric) for complete combustion of that fuel. Then he speaks of CO2, CO and H2 production and syngas quality and variable fuel moisture contents.  It would be nice to see data that would correlate to his instance #2. I have yet to see "Syn" gas composition measurements from a TLUD.  "process temperature might be below 500C" Where does this number come from?
> 
      [RWL:  I am going to stay away from this, due to press of other business.  The above cite with Tom Miles as co-author might have some of this.   I think the 500 C term means at the pyrolysis front.  Would you go higher?

> "A lot of CO is emitted by the stove" 
> Here he refers to CO that fails to be combusted in the burner portion of a stove making it sound like it is a consequence of conditions that occur in the fuel bed. "Syn"gas quality does affect burner performance but burner parameters also affect stack CO emissions.
>      [RWL:  Maybe,  but I think Paul is repeating what I heard often at the Stove Camp.    All the stoves burning char (not done in TLUDs usually) suffer from very high CO production.  (emphasis added below in Paul's comment).


> Instance #3 seems plausible.
>        [RWL:  Agreed.  but there should be a paper to see the details and definitions.]      Whew -  this is a good topic - but I need something more to read.   Thanks to both Paul and Alex.      Ron
> 
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> Alex
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> Paul writes;
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> Ron,
> 
> One should look at a stove according to what it is designed to use as fuel. Let us look, for example, at stoves that process rice hulls. 
> 
> In a first instance, the stove might simply burn rice hulls. Here we are talking about direct combustion where an air equivalency ratio situates close to 1. Such a stove will produce a lot of CO2 and H2O as well as relatively high levels of CO. The fuel for such a stove is rice hulls.
> 
> In a second instance, the air equivalency ratio might be 0.6, the process temperature might be below 500 C, the moisture of the biomass might be 20% or more, and too much secondary air might be applied to the combustion of a dirty syngas containing a lot of CO2 and H2O. Since the production of CO and H2 is suboptimal, it might make sense in this instance to burn the char in order to maximize the production of energy. But unfortunately burning the char has serious problems: a lot of CO is emitted by the stove, and heat is generated far below the pot. If the char is burned within this second stove, the fuel for such a stove is rice hulls.
> 
> In a third instance, the air equivalency ratio situates close to 0.3, the process temperature rises above 800 C, the moisture content of the biomass situates at 10%, and the supply of secondary air is kept low, but still adequate, to achieve total combustion of the syngas. Here the production of CO and H2 is optimized, the temperature of the syngas prior to combustion at the burner reaches as high as 500 C, and not too much secondary air is mixed in with the syngas. In this instance, up to 30% of the weight of the rice hulls would still remain as biochar. But it would make no sense to burn this biochar, since the production and combustion of the syngas were optimized.
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>> 
> 
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