[q]Here are a couple of ideas concerning the mechanical processing of wood waste or biomass into a finely divided powder form suitable for an efficient gasifier. Many of you will be familiar with the cyclone type vacuum cleaners which separate out the various sizes of dust into separate containers using a pair of concentrically arranged cyclones (I’m talking Dyson Dual Cyclones here). The large particles are separated out first and dumped in the outer container according to their mass, then the very fine dust travels with the column of air into the much smaller diameter inner cyclone which spins out the very fine stuff and collects it in the inner container. Well, it occurred to me that this is the very thing that we want to happen in a gasifier, sorting out the various charred particles according to size and then delivering only the very fine stuff into the reaction zone – Kalle’s gasifier achieved this by using a mechanical grinding “poker” to ensure a constant supply of the finely divided charcoal dust to the gasification zone. The cyclone could be made from heat resistant stainless steel or even cast or moulded from fire clay. It could be preceded by a drying chamber which not only dries the fuel stock in a stream of hot air (or more likely hot nitrogen), but chars it to the point of it becoming friable and easily broken up into smaller particles. I imagine a revolving drum – not unlike that of a washing machine, where chipped wood enters at one end and is tumbled between an inner and outer drum, drying and then charring. If the inner drum is perforated like a cheese grater it can be used to grind down the wood chips to the point where they pass through a perforated screen or mesh and into the cyclone system. If such a system was required to power a domestic scale combined heat and power system, then something as simple as a 2hp garden shredder could be used to first chip the firewood up into manageable chips prior to the drying drum. [/q]
[a]
I’m thinking about it. But I am packing to be away the whole of November, so I won’t have much time. What are the options before us?
1) Burn untreated wood and plant waste. This would harness ALL the energy in the material, and it is the obvious thing to do, but historically this has not succeeded. Why not? Have advances in technology changed the verdict of history?
2) Moderate degree of treatment, ie, Cartridges. Fairly simple treatment, little loss of energy, probably adaptable to a wide (but not infinitely wide) range of input materials, overcoming many of the drawbacks of 1) above.
3) Severe treatment, eg charcoal, Rather more complex treatment, but still reasonable, not sure how much loss of energy, but it might be considerable, can take a complete range of input materials to produce a very convenient and high calorific value fuel. To produce GAS is only an option for fixed installations, but for fixed installations, certainly large ones, why not burn the untreated materials? Actually, I very much like the Nye way of side-stepping all these problems: http://www.gas-turbines.com/nt6/nt6.htm by loading a day’s worth of fuel by hand into a pot, lighting up, screwing down the lid, running up the turbine and off you go. Not really “portable”, more “TRANSPORTable”, but in many applications that’s not a problem. It’s WORKABLE, and SIMPLE and CHEAP. Those are the things that really matter. For a really large fixed installation running continuously, you would want mechanized feeding and ash removal, but that’s simply surplus to requirements for small applications.
As to this specific proposal of yours: I very much liked the originality of using ceramic materials, but that is normal for large fixed installations. Why did this seem original? Energy is always required to break down solid lumps into smaller lumps. One way may be more energy-efficient than another, but (like the Carnot cycle) there is a limit.
What we are really after is to produce lumps which we can handle. We could go 2 ways:
a) Make the lumps small (relative to the size of the machine) so that they can be handle IN BULK – screw feeds etc.
b) Make the lumps up to a standard size and shape (ie, cartridges) so that they can be handled on a one-by-one basis as machine-gun bullets are. I like this proposal. Everything depends on the practicalities of MAKING IT WORK.
There is nothing wrong with cyclones, and they do avoid the clogging problems of seives, but the gas will have to be inert (eg flue gas, with all the oxygen used up) which must involve fairly leak-proof construction and maintenance and the fan energy needed to create gas speeds at which this can be made to work is not trivial – a Dyson has a 1 KW motor, though I grant that much of this power is not used for the cyclones. But I don’t see the point of going to all this trouble to produce fine particles which are then going to be gasified. [/a]
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