Hi James, I have seen these designs presented in a number of versions and at best are not as simple as one might expect. As a concept design it will fail in the way it is perceived to work, as the oxidation zone shown at the bottom cannot be made to stay in place. I will list issues and you can then see how the bed behaves.
1. Ignition of the char at the bottom would for a very short time make gas, but not from over the full diameter of the bed. 2. The air entering the top hatch will take the shortest route through the bed towards the gas outlet on the bottom, effectively cutting your hopper diameter in half. All the oxidation activity will take place within that diagonal air flow. 3. Once ignition is initiated, the top of the combusting char known as the char/air interface,(for it will not be incandescent oxidation/reduction temperatures), will move upwards towards the incoming air following that diagonal line of air flow. 4. As the combustion zone moves upwards, the char under the very narrow fire band becomes finer and begins to collect ash and fines that will increase the bed resistance to gas flow. 5. A certain amount of positional movement will take place of the rising combustion zone until some sort of equilibrium of the pressure drop is reached, and the combustion bed will be then burning on the top of the fuel pile. 6. The combusting char exposed to the incoming air will then oxidise completely to ash on the air side of the char and then begin to smother the combustion at the air/char interface. 7. The combustion temperature will drop and the exothermic heat generation is lost, thus preventing any reduction of the combustion CO2>CO. 8. The only reliable way to hold the oxidation zone in place at the bottom as shown in your design would be to add air at an appropriate height from the bottom. 9. Suction fans are not reliable when flow resistance is a bed issue, causing both a drop in gas flow and temperature of the char/air interface. A Roots blower with constant displacement is the better choice for suction. 9. Alternately fit a sealed lid and blow the air in the bottom and make a conventional up draft charcoal gasifier out of your design. That way you will also get better heating of your water jacket, and it will assist to cool the gas. You would have to ensure your hopper fuel is kept at a fairly high level to ensure you maintain the bed stability. 10. You will need steam to enter with the air if you go up draft, as char moisture will be blown away with the gas stream. There would not have been much H2 in the down draft concept, in fact the damp char will have further inhibit exothermic heat generation as the moisture was pulled through the combustion zone.. Hope this might help before you get to caught up in the simplicity(:-) Regards, Doug Williams, Fluidyne. > Hello list. I have been asked to design a 500 kW heat gasifier, for operation on damp nut shell charcoal. They are using the heat from the producer gas burner to heat water, so I have the luxury of using a water jacket to keep the barrel cool. > > Presuming the attachment gets through, any comments on the viability of this > rather simple downdraft design ? Has anyone down a similar unit ? > > Given the down-draft design it can be run as a batch fuelled unit or > semi-continuous fulling. The end user wants to be able to run 24 hours/day 6 > days a week at rates from 50 to 500 kW heat output. They will be drawing the > gas out to a burner tube using a suction fan on the cool side of a fin tube > heat exchanger. > > If it works well for heat, then further down the track they will look to pull > some of the producer gas off to cooler/filter and into a small diesel engine > generator as a co-fuel. > > > Regards, > > James _______________________________________________ Gasification mailing list to Send a Message to the list, use the email address [email protected] to UNSUBSCRIBE or Change your List Settings use the web page http://lists.bioenergylists.org/mailman/listinfo/gasification_lists.bioenergylists.org for more Gasifiers, News and Information see our web site: http://gasifiers.bioenergylists.org/
