Doug Williams, Fluidyne, New Zealand, August 13, 2007
Hi Gasification Colleagues,
This posting is about operating the charcoal gasifier, and how to start the process. It will be important to remember, that even charcoal gasifiers are fuel specific, and as a rule of thumb, the smaller the gasifier, the smaller the fuel (within reason). The charcoal should be sieved, and I suggest that nothing under 20mm be used until you establish the optimum size for the type of wood the charcoal is made from.
Your gasifier should have a single air nozzle with a screw cap for shut-down on the outside end, a gas outlet, and a lid with a heat proof seal, which is a gasifier in the most basic format. If you decide to make one with a grate, then you will also have to include a gas clean-out port in the bottom.
We are going to talk about cooling and cleaning after the Q & A following this posting.
1. In all types, you must fill the gasifier carefully, and the first char to be put in, should be the size of your thumb nail, filling to about 100mm over the air nozzle. then fill up the rest of the hopper with larger charcoal, but leaving the gas outlet free from blockage. You might consider a mesh grill over the outlet to prevent char being sucked out.
2. Not everyone wants to cool the gas, as it might be used in a furnace, so the gasifier might be blown for a furnace, and sucked for an engine, In each case, there is only one way to light the gasifier easily, and that is by suction. You cannot suck hot gas successfully without a special fan, so any suction without a gas cooling system, should only be sucked enough to establish the ignition of the charcoal, then the fan reconnected to blow the air directly into the air nozzle.
3. Suction or blower fans need to be a good size, and there has been discussion about these on this Forum over the past week. Get a big one if you can, and just use a valve to control the flow, as you will want to experiment while you are learning to optimize the gasifier size to suit your needs.
4. Assuming that you have a cooling system of some sort, if you are going to run an engine, the fan should be attached to the final outlet that supplies the engine. Switch the fan on, and feel the air being drawn in at the air inlet. It should make a clear sucking sound, and will pull in a flaming from any burning wand or LPG torch. We use a piece of woven furnace seal dipped into kerosene for the best results. This is wired to a short length of steel rod for a handle.
5. Hold the flame to the air inlet for a few seconds, then either use a mirror, or have a quick peep to see if the charcoal is alight. Do not look at the char without safety glasses, as sometimes a pop-back may occur, and char might eject out the air nozzle. The charcoal should be burning very red hot, but not white hot, as this only creates excessive heat, unless steam is to be added.
Steam will cool the oxidation lobe, so the blast rate (velocity) needs to be higher than that needed for just plain charcoal gas with minimum H2.
6. Within about 2-3 minutes you should get a combustible gas at the outlet, which will improve as the gasifier comes up to operating temperature. The gas flare pipe only needs a 25 mm larger pipe about 150mm long to go around and extend the flare pipe. This is a shield, and the gas burns inside it as the it mixes with the air.
7. You may wish to use a U-tube manometer to measure the suction or pressure. Usually the pressures are about 2"-10" WG on suction, and upto 15" WG if blown. This is only a rough guide, as pressure drops can differ for many reasons.
Sorry I have not yet organized sketches and more photos, but have a look at the Fluidyne Archive http://www.fluidynenz.gwprojects.org/ at the Australian Kent gasifier, and the other Australian charcoal gasifier. Now I will answer any questions on the subject so far, but stick to just charcoal gasification.