[Stoves] Broken Brick Mixed with Charcoal

[John Davies]

Paul and All,

Andrew has answered Paul's question better than I could have. I agree 100% 
with his description.

Now the 15  particle diameters is true for a coal bed which is continually 
being topped up from above, with a forced updraft.
In the case of the "gas producer" fuel bed in a railway locomotive, the 
divisions have been described as the bottom 3 layers producing CO2. at 
1200C.
the next nine layers reducing it to CO with the temp dropping to about 800C, 
and the top 3 layers as driving off volatiles. The gas velocity is many 
times that which would be encountered in a small stove. ( here we are 
talking about furnace type conditions where all the metal surrounding the 
fuel bed is water cooled, and the fuel burned is about 20 times that of a 
stove per unit of grate area per unit of time)

I suspect that it is more a case of contact time with carbon, i.e., a slower 
gas velocity should allow the same effect, with fewer layers of hot carbon.

Our energy scientists should be able to enlighten us more accurately, I am 
but a student of combustion, with much to learn.

Hoping that we together, can learn to understand combustion much better.

John Davies.
*********************
The concept is that you need a sufficient height of hot char for the
rising CO2 to have the chance to react with and be reduced back to
CO. You mention a dimensionless 15 pieces and Tom Reed has
previously posted a 20 particle diameter figure. You will see from
this that if the particles are 1mm in diameter the depth would be
20mm but if 1cm diameter it rises to 200mm. So intuitively it
relates to the size:space ratio of the column and is a measure of
the probability of a molecule of CO encountering a hot carbon atom.
Adding inert brick to this equation will have exactly the opposite
effect, it will both decrease the possibility of the  CO meeting a
carbon atom but also enable the primary airflow to keep the area
oxygen rich, favouring the production of CO2.


AJH

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