[Stoves] Role of secondary air on wood burning and charcoal stoves

[Crispin Pemberton-Pigott]

Dear Sarbagya

 

Interesting questions as always.

 

>However, one conclusion I obtained was that secondary air if not pre-heated
would have the tendency to put off the fire by blowing in cold air. 

 

Quite true especially of the environment has high excess air (EA).

 

>What is the role of secondary air for wood burning stoves ? 

 

Secondary combustion always takes place, it is just that by staging it
carefully (in two or three sections) the fuel burn rate can be controlled
better and a wider range of moisture and fuel sources can be used. 

 

It is not that difficult to burn any particular fuel cleanly under constant
conditions. But real stoves have varying conditions.

 

>Is there an optimum gap for the secondary air to travel between the jackets
of the combustion chamber before ejecting out into the combustion chamber ? 

 

Definitely not. The flow should be 'adequate' and the capacity to flow
should be slightly higher than the actual flow, that's all. Remember that
flowing air, even if rising while being heated, requires energy. That energy
can be concentrated on the inlet to produce a high velocity jet of air
rather than a continuous dribble. The higher velocity jet can penetrate into
O2-starved gases (list from a TLUD) thus mixing and burning everything well.
Things that do not work well with small stoves over a range of conditions
include large diameter round gas paths (like 250mm dia) and wide, thin gas
paths like 300 wide, 40mm deep. It is difficult to get a clean burn at
different burn rates.  As cooking stoves pretty much all require power
control it means these two approaches are problematic.

 

>How does this effect the performance of the cookstove ? 

 

What effect?

 

>I know a few cookstoves which have secondary air concept included and which
seemed to decrease the CO and PM up to some extent. 

 

Even if it is not apparently there, there is secondary combustion (and
tertiary combustion) taking place in all flames.

>What is the role of secondary air for charcoal burning stoves ? 

 

It is used to burn the evaporated volatiles (charcoal has lots) and to burn
the CO to CO2. It is not uncommon in a charcoal fire (a very hot one) to
have CO2 turn back into CO. That CO needs to burned without cooling the
gases. If it is cooled to much (which is common with charcoal stoves that
have large air holes and no preheating to speak of) the CO remains unburned
and is lost. 

 

Many people, writing in many articles, think that charcoal fires 'make a lot
of CO' - that charcoal is inherently 'CO-making'. There is nothing special
about charcoal. It is just that the stove burning it was not well designed.
Most charcoal stoves are not well designed and do not burn the carbon
effectively. It is as simple as that. People attribute to the fuel a problem
with the stove design. It happens all the time.

 

>"Secondary air is necessary to burn charcoal in a low O2 environment at a
high temperature." How does this affect the performance of the charcoal
cookstove?

 

First make the CO and then burn it. It is a good fuel with high energy. Even
at a CO/CO2 ratio of 2%, there is a great deal of heat lost to the CO
leaving unburned.

 

Regards

Crispin

 

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