[Stoves] re-kindling stoves

Wed Dec 29 16:01:24 CST 2010

Tom,
Joel Chaney did an interesting study of the progression of the combustion of the briquette as part of his PhD engineering studies  at University of Nottingham.  Well before that Kobus Venter, John Davies and I studied the burn a rate per various configurations of the briquette and altered the shape of the briquette accordingly, in South Africa, between 2004-6. More recently, Rok Oblak studied it with help of Approvecho...in the development of his side-fed briquette stove.

Many others may well have been looking into it as well including the KIRDI institute in Nairobi and the TEMDO organisation in Arusha Tanzania. 

Kobus, John, Rok & Joel at least have published through  the fuel briquette site as well as I think the stoves news groups... 

Heres what I know,  more generally: 

> Has anyone dampered the inlet air to control the air flow through the
> briquette?  
>> For any one blend and shape (in this case, a briquette of 100 mm OD, 35m ID x 75 mm height, with a density of  of 0.3 to 0.4g/cc),  and assuming a side fed configuration in which the annular space between the briquette and the feed chute in which it is placed, is not more than say 10mm(5mm per side) ,  dampening by resticting the airflow to the  hole-- to an extent, tends to creates a faster air flow and  more of a surface-only combustion. Opening the hole sees more 'combustion creep' back into the hole. There are differences too, with changes in the amount of annular space between the briquette and the feed tube...but Rok and Joel can best articulate that.
> 
> Or, has anyone used a ceramic insert to partially block the air flow and
> still provide a radiating surface?
>> Not to my knowledge but again please see the others mentioned first...
> 
> Has anyone put a fan on the air supply and given the air a spin to scrub the
> exposed 
> fuel on the inside of the briquette?
>> Sanu Kaji of the FoST organisaton in Nepal uses a fan in his tlud-style stove regularly. See Fost-nepal.org (may be without hyphen..).
This would assure a very much faster burn but its usually accomplished by kicking the side of the stove to help the spalding off of the ash layer and rekindle the glowing embers, at least for those blends which tend to build up such...
> 
> Or, damper the inflow to promote gasification and induce a draft with an
> outflow eductor to provide secondary air mixing? 
>> Kobus Venter, John Davies and I tried this in south africa. The issue was a I recall, having sufficient carbon to work with in the blend. With Kobus's special insulation and fine-tuned secondary air feed, we managed to gassify almost  all blends nicely but where we blended in 40-50% charcoal fines (dust and crumbs), we managed a sort of tertiary combustion (my term) ..

>> We had first created a domed top to the combusiton chamber such that the outlet was only about 80mm dia.
>> This produced all the now well documented white to blue gassifier flames one associates with gassification. Kobus had the numbers. 

>> What happened next took it to a whole new level ad one that has not to my knowledge, been since repeated.

>> On noticing how similar that briquette stove flame was to an ordinary gas cooker flame--without the gas cookers typical cast metal burner top, we decided to simulte that  top on Kobus' briquette stove. We inserted a snugly fitted tin can end with holes only around the side wall (to simulate an ordinary gas cooker stove burner). The tin was about the size of an ordinary beer can, cut cross section to leave a side wall of only about 30 mm height. With some fiddling, we actually managed to see blue gas jets emerging out this burner...The steaks tasted like something off a wood fire. It was a once in a lifetime experience but almost impossible to repeat and totally impractical of the intended mass briquette user...but memories of that flame-- and the taste still linger.  
>> I have a photo of the flames--and the steaks being cooked if you are curious but again Kobus and/or John are perhaps the best ones to articulate what was actually going on...

Cheers for the new year all, 

Richard Stanley
www.legacyfound.org

>> 
> Tom
> Musing
> 
> -----Original Message-----
> From: stoves-bounces at lists.bioenergylists.org
> [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Crispin
> Pemberton-Pigott
> Sent: Tuesday, December 28, 2010 8:19 AM
> To: 'Discussion of biomass cooking stoves'
> Subject: Re: [Stoves] re-kindling stoves
> 
> Dear Richard and Andrew
> 
> I know we all quote the 6 kg of air for burning biomass please keep in mind
> that to get an excess air ratio of about 100% it takes twice as much as 'it
> seems'. 
> 
> With the gasifiers, the ratio between primary and secondary might be a 1:6
> split, but the secondary needs to provide all the excess air as well so it
> is more like 1:10 than 1:6, yes?
> 
> The primary side for the gasifiers needs to remain the same (which is based
> on experience, really) but the theoretical need for air on the secondary
> side is surely less than the real need?
> 
> So when it comes to the air moving through the hollow briquette, can we
> treat the whole needed air supply as being present 100% (instead of a
> separate secondary supply) and concentrate only on mixing and combustion
> chamber temperature? It looks as if the hole is a means to sneak the whole
> air supply past the light biomass fuel without increasing the burn rate
> which is what happens in most ordinary fires. I think Paul made this point
> clear when we were talking about getting secondary air through the coal bed
> earlier in '10.
> 
> So, one big advantage of using hollow briquettes is that it is probably
> possible to get the whole air supply through in one go and avoid having the
> complexity of secondary admission.
> 
> This being the case, it should be possible to create a perfect hole size for
> each briquette mix and compression/forming method. This means in practice
> that for any consistent product, one should vary the hole diameter and test
> them in a stove, measuring the excess air. A simple method to do that is to
> look for visible smoke most of the time. That means the air supply is
> inadequate.
> 
> On the briquette press side, it means having replaceable cores with many
> diameters. Fully implemented, one could select briquettes with the
> appropriate burn rate: denser probably needing smaller cores to have the
> right air supply to burn slower and longer.
> 
> Got thoughts on this?
> 
> Regards
> Crispin NOT in the snowstorm
> 
> 
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