Hi Brian, You ask:
>Got a question about Imbert style downdrafts…. One of the obvious >characteristics of an Imbert style gasifier is the hearth restriction. Consider the restriction as a scavenging diameter that pulls all the uncracked pyrolysis gas back into the high temperature core, not as a way to alter velocity. > The combustion/oxidation zone is physically larger than the subsequent > charcoal/reduction zone. It does represent stuffing a tree through a funnel, but the reduction zone volume can vary considerably depending on the overall design parameters you are invoking. > As best I can find in the literature, the size change is worked out so that > there is an approximate four-fold increase in superficial gas velocity > through the reduction zone versus the oxidation >zone. Because for the last 8 months the team that I work with have been studying this almost 7 days a week, the literature has been put to some pretty critical physical testing, and while not incorrect, have been compiled in, shall we say, specifically applied circumstances. You get what you measure, but are you measuring a correctly functioning process. It becomes very confusing when you try to transfer this information to your own project, when you are not even aware what constitutes a working process. It is one of the pitfalls of learning from a book. > The actual velocity increase is even higher due to the higher temp of the > reduction zone over the oxidation zone and also to an increase of total mass > as the gasification of the solid fuel >adds its molecular load to the gas > stream. As Greg has pointed out, the oxidation zone is the hottest producing all the heat (exothermic) that the reduction zone consumes heat (endothermic). So my question is, why? Why did the engineers of the Imbert decide that they needed a higher gas velocity through the reduction zone versus the oxidation zone? As you can see, you actually have a number of questions requiring answers in order to explain this final question, but as all those who contributed to the principles of the Imbert design are long gone, only interpretations are left. However, see if this can help pull your thinking in a way that helps explain some of the things you are investigating. The nozzle velocity or blast rate, must be such that it acts like a forge and creates the high temperature in the charcoal that should be in front of the nozzles. The number of nozzles are generally fewer in small diameters, as are their bore size, plus the fuel size, which is the determining factor of setting all the sizing parameters. The resulting oxidation lobe will be of some length, resulting from the surface area of the charcoal relating to the fuel size, and temperatures of the oxidation lobe. The length and velocity is also affected if you use preheated air or just ambient, so this alone can add problems while explaining at the same time, why some work, and others don't. I prefer cold air myself, but either way, the design has to be able to match the exothermic heat generation behaviour of the lobes. If the blast rate and temperature are correct, then velocity is just describing the gas speed transferring the heat through the throat and into the reduction char. It is easier to watch the temperature of the blast and it's colour, rather than try to match some velocity specification that may not create the right bed phenomena in your own creation. The reduction zone starts from the throat, and that should be the peak of the oxidation temperature, so reduction must not start above the restriction. If it does, the temperature drops very quickly, and the char in the reduction bed stops making gas. How the reduction bed behaves relates to it's depth and diameter of the bell shape if you use one, then how the gas clears the bed on it's way to the outlet. I like to see 12-1500C oxidation temperatures, and expect to see gas exit temperatures of around 800-850C for a reasonable quality producer gas using wood as fuel. There is plenty of general information available relating to Imbert sizing, but no matter what you build, it will need some minor adjustments to function correctly. Hope this helps fill in the blank spots. Doug Williams, Fluidyne Gasification. Thanks! -brian _______________________________________________ Gasification mailing list [email protected] http://listserv.repp.org/mailman/listinfo/gasification_listserv.repp.org http://gasifiers.bioenergylists.org http://info.bioenergylists.org _______________________________________________ Gasification mailing list [email protected] http://listserv.repp.org/mailman/listinfo/gasification_listserv.repp.org http://gasifiers.bioenergylists.org http://info.bioenergylists.org
