Doug Williams, Fluidyne, July 24, 2007
Hi Gasification Colleagues,
While charcoal gasifiers are as simple as desperation determines, the choices as to how they are designed is very much a factor of applied knowledge, using the materials that might be available. In the main, we will only be using components made of steel, and using basic welding equipment for assembly.
What is not discussed in literature on the subject of gasifier design, is the philosophy behind the design, and the phenomena created by that design. It is very clear that far to many people are locked into a mind set of chemical equations, and mathematical calculations before first being able to creating the correct phenomena to achieve the desired results.
From a beginners perspective, it is important to understand that nothing about gasification is as simple as it appears, and nothing within most industrial suppliers understanding of gas, allows them to offer you advise about producer gas. The following information therefore, should be read carefully, and if not understood, noted for the question and answer session that we will have at the end of these lessons. I will also by that time, be able to post some basic drawings to assist you understand the design concepts.
A charcoal gasifier has a fuel hopper that can be almost any shape, and made out of a drum, box, rectangular container, that then has a filler lid that has a heat proof sealing. It does not have to be thicker than most sheet metal used for containers, remembering, that this is only your experimental prototype. What you have available in size, then allows you to decide the best way to position the air nozzle mounting, and the size of nozzle to do your first tests.
Air nozzles can be made of any steel pipe, but you can expect to see the tips burn away slowly in the intense heat. In operation they are open to the air, and when you want to stop the gasifier, you need to blank off the air inlet of the nozzle. It is best if you can use a standard pipe thread on the end of the nozzle, and screw a cap on to shut it down. It can be quite hot, so do not use inflammable soft plugs.
AIR NOZZLE MOUNTING.
There are four ways of mounting the air nozzle.
1. In the centre of the bottom pointing upwards.
2, In the side at the bottom pointing to the centre, and ending just short of centre.
These are updraft in principle.
3. In the side pointing diagonally down, about 1/3rd up from the bottom.
4. In the centre of the top pointing down.
These are downdraft in principle, and require a grate.
Because you will want to try all sorts of gas outputs as you progress in confidence, the nozzle mounting is a piece of pipe about 2" (50mm) inside diameter that you can slide another pipe through and be clamped by the mounting welded into the case. This allows you to try air nozzles of various sizes from small 1/2" (12.5mm) to large 1.5" (37mm), and you might even try for larger for really big engines.
Now is the point where you need to decide the best option for your circumstances.
If you are wanting the most basic and easiest to build No.1 is your option, and the most important factor is how high your casing is from the bottom. You must have a height of nothing less than 24" (600mm)and a minimum diameter of 12" (300mm) to encase the oxidation lobe. The gas outlet pipe should be nothing less than say 2" (50mm) and be located at the top with a side entry. Cut the inside end at 45 angle, and centre this end into the casing, so that the suction pulls up to the top centrally. The height of the case is only related to how much fuel to holds, and how long it will last at the output set by the nozzle size.
In operation, some char and ash will drop out the bottom as suction and load changes take place, but it prevents ash build-up around the internal part of the nozzle, which should stick up through the bottom about 2" (50mm). These bottom entry nozzles need to be kept as short as possible as long nozzles are difficult to ignite the char.
If you have a need to run a mobile gasifier, then there becomes a need to enrich the gas with hydrogen, so that you get a faster response to load change. This requires an air nozzle that has steam generating capability, so No.2,3,or 4 are the options best suited for this design of nozzle position.
No2 has the nozzle inserted from the centre of the casing at the side, about 4" (100mm) up from the bottom. The nozzle must protrude inside almost to the centre, but must still have at least 8" (200mm) in front of the nozzle to the wall of the casing. Depending on how deep the casing extends, you might find it necessary to mount a clean-out port right under the nozzle outlet, to enable the removal of ash that will accumulate in the bottom under the oxidation lobe.. If ash builds up, the oxidation lobe will move forward seeking charcoal, and the high temperature will burn through the outer casing. When you clean out, the ash and char will drop out leaving a cone of char in place. Do not disturbed this char, as it is an inert part of the bed, and does nothing except funnel the char down to the oxidation lobe.
No 3 This is a cross downdraft design that should clear the ash away from the oxidation lobe, and drop it into a cleanout space under the char bed, and cleaned out via a port in the side or bottom. A vertical grate is used, so that char slides down the grate to reduce the potential to block.
The casing which might be a cylinder in shape, has an internal plate on an angle to match the diagonal of the air nozzle. At its lowest point, you need about 6" (150mm) from the bottom, where a section is cut out to allow a vertical chamber to be mounted with the grate.
The grate will only be small rather than large, say 6"x 6" (150 x150mm) so that the heat from the oxidation lobe is concentrated into a relatively small area of reduction charcoal to drive the exothermic heat consumption. The gas outlet is on the under side at the high end of the plate, so that ash and char do not entrain with the hot gas.
With an air nozzle that is mounted diagonally pointing down, the ash that forms around the oxidation lobe slides away on the sloping char face towards the grate. The end of the air nozzle should be about 6-8" (150-200mm) from the grate bars, but be able to slide in and out to adjust this distance once tested. The air nozzle mounting should be 4" (100mm) up from the high end of the plate.
I am stopping at this point as it is taking longer than I thought to cover all the options. This will get you thinking as to what materials you can find, and give you a chance to begin making a plan. Please do not cut any metal until you have the whole story, and I will continue discussing options in my next posting.