Doug Williams, Fluidyne, New Zealand, July 28, 2007
Hi Gasification Colleagues,
It is clear from the private mail coming to me, that charcoal gasification has created an interest that allows many to now enter these discussions. While my emphasis is to try and make it understandable for beginners, it should also stimulate those more advanced to test what I say against their own experiences,( such as Ken Calvert has provided). His real DIY experiences show that you can get going if you have some help to start. I really do not like to ignore or delete mail asking questions, but please wait until the end of these informal lessons, to initiate questions and answers. If we do it that way, anyone in future can find all the discussion under the common Subject title, so please do not contaminate the title by using it to discuss other topics.
No 4 nozzle mounting is located in the centre of the top pointing down, and the gas exits a grate about 6-8" (150-200mm) below the nozzle, which can slide up or down to adjust the correct position from the grate.
Because the air enters from the top, the nozzle tube has to be long to descend right through the fuel hopper, and this can make it difficult to ignite. Ignition issues will be separated as part of the operating instruction.
Of all the designs, it is my opinion that these centrally mounted air nozzles are more demanding for their design in the bottom end. Because of the central positioning of the grate, and the gas outlet on the side wall under the grate, the suction pulls the gas flow from the nozzle to that side of the grate.
In this situation, the shape of the oxidation lobe narrows, and the reduction char in front of the lobe is consumed faster, and a channel will begin to open up in the bed towards the grate. The problem is further enhanced as charcoal has to actually flow around the nozzle tube to feed the oxidation lobe, which in turn feeds the reduction char replacement. Charcoal beds do not move side ways, except as it descends and is directed by the shape of the sloping cone that forms over the oxidation lobe.
It is important to understand, that the charcoal that forms the gas making process of first oxidation, then reduction, is in a state of evolution, or transition, from the moment the input charcoal meets the air. Oxidation begins the segmentation of the original char size, increasing it's surface area, and speed of reactiveness. It also reduces the interstitial space between the char particles needed to allow the evolving gas to pass through the bed. If the charcoal is too small or contains too many fines, the bed will plug quickly a short time after start-up.
Because the reduction charcoal is being consumed by the incandescent CO2 passing through it, this must be replaced by small char coming from the oxidation lobe. If large char of the original fuel size is put into this space, say on start-up, it will stay big if it is on the grate. This is called frozen reduction, because there is not enough temperature or surface area to make it reactive. This will be covered in operating instructions.
Before you inundate me with how successful the WW2 Kalle charcoal gasifier worked, it is not a simple gasifier, and is a project for the more skilled and experienced. The central nozzle has the oxidation lobe pulling the high temperature back up and around the nozzle assembly, and the char does not flow into this space under the nozzle, without some vibrational movement. The plunging motion of the nozzle for increased gas demand, is to increase the amount of reduction char made by the oxidation lobe, and because it's role changes in an eye blink, from oxidation to reduction char, it carries the exothermic heat needed for instant reduction activity. This make the gasifier very responsive to load changes.
The Kalle also adds CO2 from the engine exhaust, and yes this does reheat as it passes through the oxidation lobe, not to increase the gas flow, but to replace an incremental amount of char consumption.. It also lowers the oxidation temperature, and was really to control the temperature to stop the air nozzle melting. As this improves the fuel economy, it is worthy of experimentation to add this feature with all the nozzle positions (220.127.116.11.) suggested, because these oxidation lobes are moving away from the nozzle assembly. Again, as it controls the oxidation temperature, we can take advantage of being able to increasing the nozzle velocity more than we need, to get the extra heat to recycle possibly more of the CO2.
Charcoal gasification No 4 will discuss the operation of the gasifier, then we will have a Q+A session, before talking about the cooling and cleaning system options. I should also have some sketch drawings and a few photos posted by then, and it will be easier to see what I'm trying to explain.