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</o:shapelayout></xml><![endif]--></head><body lang=EN-CA link=blue vlink=purple><div class=WordSection1><p class=MsoPlainText>Dear Jaakko<o:p></o:p></p><p class=MsoPlainText><o:p> </o:p></p><p class=MsoPlainText>This is very helpful and I am happy to share in this conversation!<o:p></o:p></p><p class=MsoPlainText><o:p> </o:p></p><p class=MsoPlainText>>the energy from char combustion goes partly to evaporate the moisture by radiation down and partly to heat the gas and it is obtained as usable energy for cooking. <o:p></o:p></p><p class=MsoPlainText><o:p> </o:p></p><p class=MsoPlainText>Understood.<o:p></o:p></p><p class=MsoPlainText><o:p> </o:p></p><p class=MsoPlainText>>The velocity of the propagation of the ignition is slower with high moisture and the available time for cooking becomes then longer. <o:p></o:p></p><p class=MsoPlainText><o:p> </o:p></p><p class=MsoPlainText>Rather obvious, yes.<o:p></o:p></p><p class=MsoPlainText><o:p> </o:p></p><p class=MsoPlainText>>However, above some limiting moisture, the ignition front does not propagate at all. <o:p></o:p></p><p class=MsoPlainText><o:p> </o:p></p><p class=MsoPlainText>Exactly when will be fuel dependent and would change if there were primary air preheating, however I agree. I have tried it both ways. I found that the surface to volume ratio of the fuel is a factor. Hard wood needs to be split smaller (etc).<o:p></o:p></p><p class=MsoPlainText><o:p> </o:p></p><p class=MsoPlainText>>So there seems to be some optimum moisture giving maximum total amount energy for cooking, if no value is given for the residual char (and one is not interested in the heating power but only in total energy). <o:p></o:p></p><p class=MsoPlainText><o:p> </o:p></p><p class=MsoPlainText><span style='color:black'>I was not specifically looking for the maximum total energy though of course I have an interest in it. I am trying to track down the 'extra' energy that lies between the missing char (burning wet fuel) and the energy needed to dry the fuel. I was going to make a chart to post here showing the amount of extra energy involved but have been travelling.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText>>Water gas shift reaction takes palace in gas CO+H2O = CO2+H2 and it does not have effect on char. H2 is oxidized to H2O rapidly if O2 is present.<o:p></o:p></p><p class=MsoPlainText><o:p> </o:p></p><p class=MsoPlainText><span style='color:black'>This is what several people have been saying and I can report, with some mysteriousness, that in the coal fires ignited with wood that we tested so many of in Mongolia, there is something taking place that produces 'far too much' H2 during ignition. The simple explanation is contradicted by the test measurements.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>To put numbers on it, we often reach 2,000 ppm H2 in the stack when there is plenty of O2 available, a healthy fire burning (wood under coal) and the coal is igniting just fine. The H2(EF) which means the undiluted hydrogen concentration reaches >15,000 ppm. It is very strange to watch a burning coal+wood fire inside an enclosed stove that cannot ignite the hydrogen emerging from that same fire. <o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>One of the metrics we watch is the ratio of CO:CO2 as you would expect, however we also watch the CO:H2 ratio. Both gases indicate poor combustion. There is nearly always some H2 in the stack and when it goes high, it becomes interesting. Even very late in a coke/charcoal fire there is some H2 present. Ditto for H2S and SO2 which move opposite, in concert<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>Every conventional explanation is that there is no water gas shift reaction taking place, even though the conditions for it are present. This does not add up. The reaction was deliberately created in gas plants, why should it not take place when conditions are right in a stove?<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>Most interesting that so many people are sure the reaction is not taking place. Is the H2 coming from direct outgassing from the non-carbon portion of the wood and the coal volatiles?<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText>>However char is oxidised by gasification reactions H2O + C = H2 + CO and CO2 +C = CO. The rate of these is much lower than reaction with oxygen, but however at about temperatures [above?] 850°C they become influential. <o:p></o:p></p><p class=MsoPlainText><o:p> </o:p></p><p class=MsoPlainText>That also happens to be the temperature at which we can find a consistent ignition of CO (in free space, not on the surface of the char. Also interesting...<o:p></o:p></p><p class=MsoPlainText><o:p> </o:p></p><p class=MsoPlainText>>The reactions are endothermic, which adjusts the extent of gasification (high gasification rate results in low temperature decreasing the gasification rate). There is always some H2O present in the gas from ignition front since wood contains hydrogen and combustion air also contains some H2O. Char is also consumed by CO2 produced in the ignition front. So gasification of char takes place also for combustion products from pyrolysis of dry wood. <o:p></o:p></p><p class=MsoPlainText><o:p> </o:p></p><p class=MsoPlainText><span style='color:black'>Understood.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText>>So the main reason for "missing char" for moist wood as Tom Reed and AJH have explained is that in addition to the part going down to dry the fuel, a considerable part is going up (and obtained as usable energy) and the cooking time is longer. <o:p></o:p></p><p class=MsoPlainText><o:p> </o:p></p><p class=MsoPlainText><span style='color:black'>I like the explanation of the particle size being a factor. It means that by carefully choosing a particle size one can manipulate the </span>burn so as to reduce or increase the amount of char created.<o:p></o:p></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>It is going to emerge soon as a major stove testing issue that the char remaining cannot be considered 'unburned fuel' in a char making stove. This greatly affects the stoves rating on fuel consumption which at present is quite misleading. The importance is that someone who is making a stove that maximises char production will find the penalty on the fuel consumption number to be so great in certain markets (where there is no interest in the char) they should change the combustion parameters to reduce the char to nearly zero. If this can be done without adding moisture, that is a huge bonus. If the moisture is itself a contributor to the removal of the char (being charred on the outside and damp on the inside as you suggest) then there is a more difficult problem to solve.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>My attempts to burn all the char while creating gas have been reasonably successful so I called the device a semi-gasifier because it is able to switch back and forth between the char making and char-burning modes. That name may not be technically correct but names are not as important as function.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>In very brief, the fuel consumed by a stove is the amount of raw fuel it takes to perform some task assuming that the task is repeated, i.e. one of a series of identical burns. The thermal efficiency overall is the task (work done) divided by the heat potential of that quantity of raw fuel. When applied to ordinary stoves the answer is pretty much the same as any water boiling test, for example. However for char producing stoves where the char mass is a significant fraction of the initial fuel, the fuel consumption and thermal efficiency are significantly changed, often dramatically so. They more or less match the real numbers observed in the field during, say, an uncontrolled cooking test (UCT). The problem is at present the calculation methods do not consider that stove would every produce large amounts of charcoal so the calculated results are (erroneously) far from the real values.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>Being able to switch a char making gasifier into a whole fuel consuming one will be an important feature for the different markets.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>Thanks for your observations<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'>Crispin<o:p></o:p></span></p></div></body></html>