---------- Forwarded message ----------
From: "P.M.Lawrence" <pml540114@gmail.com>
Date: Jun 10, 2010 10:57 AM
Subject: Re: [Open Manufacturing] Re: An open source gasifier
To: "Open Manufacturing" <openmanufacturing@googlegroups.com>
Leo Dearden wrote:
> On 9 June 2010 15:12, P.M.Lawrence <pml540114@gmail.com> wrote:
>
> > Bryan Bis...
OK. It's a modification of the Imbert gasifier, using the principles
of the reverberatory furnace and counterflow heat exchange, so I'll
start by recapping the first of those.
An Imbert gasifier has a vertical, cylindrical fuel hopper of thin
metal, with a lid held down with a spring or weights rather than being
secured so that if pressure builds up it can be relieved without
damage. Air enters through "tuyères", downward slanting tubes a few
inches long entering the cylinder and ending a few inches above the
grate, with burning going on below them (in fact, kindling is lit
through them); there are an odd number of them so that you don't get
any opposite pairs burning a through channel and blowing flame
through. There is a flat grate above a deep ash pan, which is kept
closed up tight in use; gas is drawn off by suction from below the
grate and passed through a filter (of various possible types, e.g. a
bed of fresh fuel that gets put in the hopper later), to cool it and
clear out particles and corrosive gasses that can form during heating
the fuel. Fuel can be anything dry enough and carbon containing enough
to burn, that is in small pieces and neither gunks up the works nor
has a sulphur content that makes gas too corrosive even after
filtering (coal and rubber fail on both counts, and turning wood into
charcoal first both eliminates corrosive gasses and improves the gas
to fuel weight ratio, at the cost of losing some of the initial fuel
energy content). The gas contacts the gasifier before filtering,
though, so that will tend to corrode; it should be made either of
stainless steel or of materials cheap enough to be replaced regularly.
The improvement starts by making the grate conical, with an angle of
about ninety degrees, with the point upwards and capped. Below it,
with the same rim, is a downward conical metal heat reflector baffle
ending with a hole rather than a point (this means that the ash pan
has to be even deeper). Gas is no longer drawn off directly from the
ash pan zone, but through an L shaped, thermally insulated draw pipe
that runs horizontally above the ash pan, then vertically up through
the hole in the baffle to terminate just under the cap at the top of
the grate. Inside the draw pipe is a water feed pipe that runs along
it horizontally, then spirals up and passes through the cap, where it
turns down so fuel can't fall in; it needs to be descaled with mild
acid occasionally. A simple system maintains a head of water part way
up it; this has a reservoir pan outside at that level feeding it,
staying topped up from a main reservoir bottle resting in it cap
downwards with two short tubes going through its cap, one sticking out
further than the other, so whenever the level drops enough to uncover
the other tube it admits air to the bottle and releases enough water
to restore the level in the reservoir pan (to replace the bottle after
filling it, cover the tubes until it is in place to stop the water
pouring out). It is also possible to adapt the Archimedes screw
principle to feed fuel in or remove ash without letting air in or gas
out.
The operating principle is that producer gas (carbon monoxide, some
hydrogen, flammable vapours, diluting atmospheric nitrogen and trace
gasses) is generated near the rim of the grate in the usual way, then
steam is formed with the heat of the gas drawn out and passes into the
zone near the cap of the grate; this forms water gas (carbon monoxide,
hydrogen, flammable vapours and trace gasses) as the suction pulls it
through the fuel, which has to be kept at least as high as the cap, as
that region is kept hot by the combustion further off because the
grate and baffle work as a reverberatory furnace. Together, these
yield semi-water gas; the higher the proportion of air used, the more
heat and therefore steam gets formed, which reduces the air intake and
restores an equilibrium (any excess steam is caught in the filter).
Obviously this information is now freely available, but I would
appreciate it if anybody who finds this helpful would contact me to
arrange whatever donation they think fit; my email address is
pml540114@gmail.com. P.M.Lawrence.
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