[Stoves] Excess air

[ajheggie at gmail.com]

[Default] On Sun, 25 Aug 2013 19:30:45 -0400,crispinpigott at gmail.com
wrote:

>There is almost enough O2 in biomass to burn all the Hydrogen but not quite. If there is no carbon burning a great deal of the combustion can take place with no outside air at all. 

I think you are looking at this the wrong way round, the wood (which
Tom Reed suggested a surrogate formula for of CH1.4O0.6) is a mixture
of cellulose, lignin and hemicellulose, these are derived from the
simple sugars that are made during photosynthesis, the oxygen is
always bonded to hydrogen or carbon atoms , i.e the fuel is already
partially oxidised. To burn wood first it gets hot enough to pyrolyse
and offgas is driven off leaving the  fresh char,  an O2 molecule,
which has a double bond weak enough to be broken by the energy of a
spark or flame, dissociates into two oxygen radicals which are highly
reactive and attach to the carbon. The heat from this reaction builds
up until the bonds in the offgas are broken to allow the fuel gases
and oxygen radicals to combine in a flame.

Working out the bond energies involved needs a younger more agile
brain than mine but I somehow doubt that any of the oxygen originally
bonded to the wood survives as free oxygen in the flue gas.

What we can easily show is that the energy avail bale in wood is
almost exactly the same as the energy available in burning the same
mass of carbon as is in the wood, which again suggests that it is
carbon oxidation that is providing the energy for the combustion.

Derived from the above formula for wood we can say the mole weight  of
C5H7O3 is 12*5+1*7+16*3=115

The carbon content 12*5=60

So the carbon content is 52% and the cv of carbon is about 31MJ/kg.
The energy from oxidising 0.52kg of carbon is thus 0.52*31=16MJ

Wood cv is about 20MJ/kg

>If there is no carbon burning a great deal of the combustion can take place with no outside air at all. 

Well the case in a retort is that no carbon burns, can free oxygen
other than that which occupies interstitial spaces in the wood be
detected in the offgas of a retort?



>The term Lambda is used loosely. Testo uses it incorrectly in their computer outputs to describe Excess Air. Lambda is strictly the total air demand, not the excess air only. Suppose the EA is 250%. Then Lambda is 350%. 


A number of terms are used too loosely but this is much as I see it.
Lambda is the stoichiometric amount of air, in a petrol engine we do
indeed get a near stoichiometric burn. 

Again this ties in with the equivalence ratio and Lambda being the
same, so lambda=er=stoichiometric air plus excess air.


>I agree re the less than 0% EA conditions. The right way to talk about it is Lambda, and that could be less than 100%. 

The point I was trying to make is that a lambda sensor as used in
stoves and boilers never detects lambda of less than 1 because there
is no oxygen to exchange electros with in the detector, all the oxygen
has been consumed in the primary reaction.

Most of the big boilers I dealt with burning woodchip with a moisture
content of more than 30% seemed to still showing 5-10% oxygen in the
flue gas, so an excess air of 50-100% and lambda of 1.5-2.

We know from gasifiiers that primary air only needs to be 0.2 lambda
in order to drive off the pyrolysis offgas and turn the char to CO
plus then we have the special case of TLUD where the heat for the
pyrolysis front is released by a lower still amount of primary air.

S Varunkumar in his thesis discussed here recently further breaks down
this small amount of primary air and its behaviour into separate modes
of pyrolysis and offgas evolution depending on its superficial
velocity.
 
>?The manufacture of O2 in the fire is the Water Gas Shift Reaction. It would happen using combustion moisture I suppose but I have not seen that definitively.    Have definitive measurements though from wet fuel fires. I have not worried about how it happens - I was occupied trying to understand how to get a true calculation of the EA equivalent. The EA value is really a re-expression of the O2 available, not the actual amount of air it is consuming. It is just a way to talk about the chemistry.

OF course unless one could use some sort of isotope marker for the
oxygen in the wood the detection instrument cannot show where the
oxygen originated, the trouble with wet fuels is that they need so
much primary air just to burn them that lots of this subsequently
travels through the bed and if it doesn't get consumed in a secondary
flame will appear in the flue gases.

I thought the water shift reaction was an equilibrium reaction and
needed high temperature, I think water vapour only dissociates above
1500C in the absence of a catalyst, either way it is highly
endothermic and I doubt a high moisture content fuel can either reach
the required temperature of supply the energy.


> As I said, ALL the O2 might be coming from the fuel. 

I hope I have given reason why I doubt this.


AJH