[Stoves] briquette air feed

ajheggie at gmail.com ajheggie at gmail.com
Tue Dec 28 12:01:30 CST 2010


On Tuesday 28 December 2010 16:18:54 Crispin Pemberton-Pigott wrote:

> 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'.

Yes Crispin but it makes my case for using a tube to separate the primary 
and secondary supply more intuitive. Generally I guess for wood we are 
looking at about suppling 50% more total air, which makes it about 9kg of 
air needs to pass by or through the briquette per kg of dry briquette. 

With a traditional burner all the primary air passes through the bed and 
if the bed is thick and hot enough no primary oxygen survives into the 
secondary combustion area.

With passing all the air through the hole it's only the configuration of 
the hole that decides what gets used to supply heat to pyrolyse the 
briquette and burn out the char. 

I'm simply surmising that if the amount necessary for primary combustion 
passes around the tube or along side the briquette and the remaining 8kg 
is induced up the middle of the tube then we can have primary air control 
for power. The secondary air will be proportional to the power and 
largely self regulated by draught.

I see Tom has chimed in with similar thoughts.

> 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?

This all gets a bit debatable because the primary to secondary ratio for a 
gasifier assumes no oxygen surviving the primary firebed and the offgas 
being a mixture of pyrolysis gases , CO, H2 and N2 all above 850C. A 
small stove will not reach these ideals. Indeed it's likely the fact that 
full combustion products, N2,H20 and CO2, dilute the gases in the 
secondary combustion zone and inhibit the complete burning of PICs which 
go on to be emitted as PMs. The very fact that tlud limits this primary 
combustion and delivers a fuel gas rich in pyrolysis products with little 
dilution by N2, H2O or CO2 which means it can burn cleanly in the 
secondary combustion area without reaching the high temperatures needed 
for complete gasification.
>
> 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.

As I said before, if the geometry of the fire and hole can be made to do 
theis then great. My experience of providing all the air via the same 
jets is it leads to high excess air, yet it does look like moder pellet 
burners, using a blast tube approach are tending that way.
>
>
> Got thoughts on this?

Well just a couple more: some small 3mm telltale holes could be drilled in 
the feed tube, they would supply some primary air but also indicate to 
the cook when the fire was burning back and a new briquette needs 
screwing in.

Also if the secondary air tube protruded into the secondary combustion 
zone before turning back to form Tom's "outflow eductor" it would gain 
some preheat prior to mixing.

AJH
No snow left, 6C and raining




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