[Stoves] Excess air

Wed Aug 21 16:32:10 CDT 2013

Philip: 

You fail to make your case, in my opinion, when you call 90 kg fuel per second a "stove". With a typical real stove doing maybe 0.9 kg per hour, we are talking a ratio of 3600*100 - the stove population of a small city - with a great variation, but none operating like the uniform fuel and air flows of any coal -fired operation. I compare your analogy of "operating" a kayak or a battleship. We don't apply anywhere near the same rules with 5 and 6 orers of magnitude. 

Maybe you can convince me with something one or two orders of magnitude apart. 

Ron 

----- Original Message -----
From: "Philip Lloyd" <plloyd at mweb.co.za> 
To: "Ronal W. Larson" <rongretlarson at comcast.net>, "Discussion of biomass cooking stoves" <stoves at lists.bioenergylists.org> 
Sent: Wednesday, August 21, 2013 2:04:02 PM 
Subject: RE: [Stoves] Excess air 

Thanks, Ron 

In essence I think it comes down to what you are trying to do. Do you want a crude measure of stove performance or an accurate one? In running a really big stove (and I’m thinking of one that burns around 90kg of fuel per second) it is critically important to get the combustion optimal, with a balance between CO going up the stack, C leaving in the ash and minimal N 2 /Ar needing unnecessary heating. You have to get the excess air just right. You play around, altering it about 0.1% each time, until you get it right for the stove and the fuel. Near the optimum, 0.1% shift in the excess air can cost you 100g of carbon per second, or 0.3% in efficiency as you get excess nitrogen – that’s how closely you have to monitor the excess air. So if the big stovers can use such a measure, why can’t we? 

[RWL: Hopefully the answer is above.]. 

Kind regards 

Philip 

From: Ronal W. Larson [mailto:rongretlarson at comcast.net] 
Sent: 21 August 2013 06:16 
To: Discussion of biomass cooking stoves; Philip Lloyd 
Subject: Re: [Stoves] Excess air 

Prof.Lloyd: 

I agree that combustion efficiency is hugely important. I would love to see it reported separately (and could be theoretically, I think as CO is already measured and reported). 

I would love to see excess air reported (and I think that possible also). 

I would love to see more on the oxygen content of various woods (air to fuel ratios for combustion are given as 4-7. If you are making char, the range is probably wider, as lignin, cellulose and hemicellulose all have rather different O2 content, and contribute differently to char. 

Why not separately also report on the H2 content of all the fuels, and it also has a small content in chars. Then we can move to sulfur. 

Why not report the lost radiant energy? 

If you had all this in an expanded WBT procedure (with all Crispin and you want about each and every fuel), what would the average user of Jim Jetter's test do differently than they are now doing? I just continue to see these as useful academic exercises that don't advance stove development. 

I nit pick below a bit more. 

On Aug 21, 2013, at 12:06 AM, "Philip Lloyd" < plloyd at mweb.co.za > wrote: 

Dear List 

In one of his responses to Crispin, Ron said "RWL: In summary, I think you 
are raising issues that are hopelessly complicated for the world of stove 
testing and comparisons. I see insufficient reason so far to explore your 
metric words "possible" and "to check" and "Obviously". I hope you will 
try again to convince this list (with citations), if you disagree." 

I think stove testing needs to be comparable between both stoves and fuels. 

[RWL: I think this is now being done. Each fuel seems to have enough known about it and it is reported. The unused CO is reported. I don't know this for sure, but think that the same stove with different fuels gives very comparable results - and especially in a tier-ranking sense. (Anyone have data on this last point?) 

What Crispin has done is bring standard combustion theory to bear on the 
question so as to allow this. 

[RWL: Jetter's work is about improving the performance via measurement. Details about each fuel doesn't seem as important as reporting the fuel combustion efficiency - which is (sort of) in the data. (meaning high CO means low combustion efficiency). 

I am not sure that any important aspect of combustion theory is missing. If so - exactly what? 

I need, for instance, to be able to compare a 
single stove burning either wood or charcoal. 

[RWL: I hope not too often. Rarely will the same stove be the right thing to use for both fuels. 

I have to be able to take the 
oxygen present in the cellulose and other constituents of the wood into 
account in calculating the excess air, because it contributes to the 
combustion, whereas with charcoal there is essentially no oxygen present in 
the fuel. 

[RWL: Maybe this is important for something you are doing, but why impose this on all stove testing? I think we should concentrate first on getting excess air measured accurately, not on the percentage coming from the fuel (which can mostly be determined from the literature - no need I see to encumber each test with that level of detail). The operation and performance of charcoal-making and charcoal-using stoves is so different (even ignoring how the char in a char-using stove was produced), that I think the O2 content of the fuel is in the noise. Can you give a counter example? 

The difference is real, measurable, and has an impact on the 
efficiency of combustion. 

[RWL: Yes to all - but I don't think it needed as a new adjunct to the WBT. On the best stoves (tiny CO emitted) , the information will change the second or third significant efficiency digit, and I am still worried about the first digit. 

I, for one, am convinced. 

[RWL: I will be when I see a written justification for holding up progress on getting an agreed ISO standard. How about helping me get char production ( a first digit issue) as an accepted part of the standard? 

Need to repeat, some of the above might have been covered in yesterday's webinar which had to be cancelled due to an equipment glitch. Ron 

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