Hi Pete, You need a reply to this, and your post about cheap gas analyzers.
I probably have no business asking entering into this discussion since I am neither a chemist nor a physicist, but--- This is an open forum, not an exclusive "club" for only academics, who need us as much as we need them (:-) Over the last several months I have been trying to learn the "truth" about the dissociation of the water in my wood chips into Hydrogen and oxygen. I had previously read someplace that it was a pretty simple process. You just heat water to somewhere above 350 degrees C and there you go. Gasification does hide a complex set of temperature related phenomena, but because we have to have packed incandescing carbon beds to make it all work, a minimum temperature to create the exothermic heat requirement to create permanent gases begins at "about" 850C. Water literally gulps up this heat, so if you don't have plenty of high temperature heat generation in the oxidation zone, a proportion of the tar and water laden pyrolysis gases pass through uncracked. Well, lately I have become quite disconcerted as I have read that there are so many other factors that can be involved, that it's anybody's guess as to whether it will happen at all. When you understand the order of created thermal phenomena within packed carbon beds, you begin to identify if any system you build or buy, will be best suited to your end need. Some of the variables: Pressure Only if you are talking about Syngas, not producer gas made with air. Temperature (obviously) Yes. Residence time Only means deeper beds after the very hot oxidation zone, which can be mechanically dropped to shed char, but, possibly at cost to the CO %. Presence or absence of carbon and form of that carbon. And, apparently, the availability of carbon from other molecules. All carbon like coal, coke, or charcoal from any biomass will become incandescent if you pass air through it, but you get mainly CO as the combustible gas, and it also brings in the nitrogen dilution factor. Early updraft gasifiers had a boiling water bath under the grate, so wet steam both cooled the grate bars, and turned to H2 as it passed through the incandescent oxidation zone, then had a long contact/dwell time passing through the upper bed. And apparently, the list goes on and on. So what is an ordinary human with a stratified downdraft gasifier to do to reliably pry enough hydrogen out of the process to make it worth doing? DIY gasification is never less than a steep learning curve, which unfortunately can leave you disappointed with the results. That you target or identify H2 as your desired gas, should have in the first instance, told you that your gasifier isn't made to maximize this specific gas. You will get some, but probably, <12% at a guess. Your second posting regarding low cost gas sensors is likely to also create some grief if all you do is hook them up and expect them to work as specified. You may or not know that gas analyzers need to be calibrated against a cylinder of prepared gases, and this is very expensive. Add to this the special regulator for the cylinder, and you can start thinking a small truck would be of better value(:-) Playing with dirty gases can also throw out calibrations, so be really careful if you go down that track. Hope this may be of some assistance. Doug Williams.
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