[Default] On Sun, 25 Aug 2013 19:30:45 -0400,[email protected] 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. 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