[Stoves] Charcoal burning, secondary flame vs no flame

Tue Aug 4 09:15:37 CDT 2015

THANK YOU, ROBERTO!  THANK YOU, CRISPIN!  THANK YOU, PAUL! - for discussing a topic that is of great concern to me.  
Not being able to afford a combustion analyser, I have always been worried about the CO level that is produced in wood burning stoves (3-stones, traditional and improved) that become charcoal burning at the end stage of cooking.  I am specifically referring to the traditional way rice is cooked in the Philippines to prevent it from burning at the bottom of the pot.  First, the water is brought to boil at high power; then rice is added and stirred until most of the water is absorbed; and then any remaining unburned wood is removed and the rice, in the covered pot above the stove, continues to cook to perfection the "palangay" way -- i.e., with the heat from the glowing embers of the charcoal remains.  Alternatively, rice and water are brought to boil together and then cooked the "palangay" way. 

Have you or anyone else taken CO/CO2 measurements under the above described cooking conditions, both indoor and outdoor? 

Warm regards, 
Rebecca A. Vermeer 
Eco-Kalan Project in the Philippines 

----- Original Message -----

From: "Paul Anderson" <psanders at ilstu.edu> 
To: "Discussion of biomass cooking stoves" <stoves at lists.bioenergylists.org> 
Sent: Monday, August 3, 2015 4:59:20 PM 
Subject: Re: [Stoves] Charcoal burning, secondary flame vs no flame 

Crispin, 

I have no basis for doubting nor the time or talent for reviewing your calculations.   If someone does have revisions or corrections, I hope they speak up right away. 

Otherwise, here is my comment: 

Concerning the issue of what is the heat lost (not liberated) if CO from charcoal escapes instead of being burned, you have illuminated my (our) understanding. 

THANK YOU, Crispin, for once again providing technical and quantitatively supported excellent information!!!!   You are (IMO) a major asset to this Stoves Listserv and to our entire community of Stovers.   

And a further question:   In a "typical" charcoal stove that is generating dangerous amounts of CO, what percentage of the consumed carbon is "well burned" (and becomes CO2) versus the carbon that is only transformed into CO?   The reply might need to give some range of values or qualify the responses relating to a "high heat" charcoal fire versus a "low heat" charcoal fire, probably with the latter being proportionately much more of the carbon ending up as CO, with a corresponding 75% loss of the heat value for that amount of charcoal. 

There is a big difference between (hypothetical numbers follow) 50% of carbon going to CO instead of CO2 (with the loss of 75% of the heat value of half of the carbon [75% of 50% = 38% lost heat) versus only 5% of carbon going to CO (75% of 5 % = 4% lost heat).   Of course, it is the CO that can kill you that is more important than some amount of lost heat. 

Comment:  We are discussing "char-gasification", the oxidation of solid carbon (but as charcoal it is not necessarily pure) into either CO or CO2.   The O2 that enters as primary air can result in either CO or CO2.   The role of secondary air is a very different story for char-gasification than it is for the combustion of pyrolytic gases as in the context of TLUD micro-gasifiers. 

Answers / comments from anyone would be appreciated.   

Paul 

Doc  /  Dr TLUD  /  Prof. Paul S. Anderson, PhD  
Email: psanders at ilstu.edu Skype: paultlud      Phone: +1-309-452-7072
Website: www.drtlud.com 
On 8/3/2015 2:02 PM, Crispin Pemberton-Pigott wrote: 

Dear Roberto 

It is unlikely that you will be able to burn charcoal without any CO2 at all. 

<blockquote>

In the centre of a charcoal flame, under the middle of the pot, there is often a very high CO2 level, but it is so hot, excess C is available and it is  absent of free oxygen so it breaks apart to make 2 CO. 

If you can estimate the mass of CO2 and the mass of CO you can calculate the difference. C to CO2 is 33 MJ/kg (close enough) and C to CO is a bit more than 8. So you lose ¾ of the available heat energy if you fail to burn the CO to CO2. 

For 600m of char, (assuming 85% carbon) the loss would be 32.8 x 0.85 x 0.75 x 0.600 = 12.6 MJ loss 

Or 29.5 MJ/kg for the char x 0.6 kg to get the available heat  = 17.7 MJ 

And 

Subtract the CO loss: 0.600 kg x 0.85 carbon x 75% x 32.9 carbon heat = 12.6MJ loss, the same thing. Heat released by burning C to CO = 5.1 MJ. 

If you have a portion of the gas as CO2 and another as CO, you can pro-rate the heat available as ‘yielded’ or ‘lost’ according to CO/(CO2+CO) and CO2/(CO+CO2) times the carbon mass times 32.9 MJ/kg. 

The CO loss is closer to 74.5 % of the CO2 energy if you really want to be picky. 

Regards 

Crispin 

Hi, 

I want to know what is the difference in combustion efficiency between burning charcoal with secondary flame (burning CO) vs burning without secondary flame (CO escape to the chimney). 

For example, burning 3 kg in a TLUD produce 600 grs of charcoal. Then if we decide to burn this charcoal, burning with secondary flame is more efficient than burning charcoal with no secondary flame. 

How can I measure this difference? 

Greetings 

Roberto Poehlmann 

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