Naked Science Forum
Non Life Sciences => Technology => Topic started by: peppercorn on 24/09/2008 18:20:51
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My dad -living in the sticks (No pun intended)- has an open fire & hence a wood shed.
In it wood is air dried over many months - making it good for burning.
Imagine this wood shed was not in inclement Suffolk, but a nice hot clime with a big solar collector on it.
Inside the atmosphere is inert say mostly CO2 gas & the wood is pummelled & rotated mechanically.
The ambient temperature is above 100degC, rising to 150degC when exposed to the collector's direct gaze at the top of the 'shed'.
Chunks of wood would crack in the heat & generally decompose whilst additional disassociation through mechanical action would cause the wood to degrade to something approaching powder.
The end product would be similar to charcoal dust only (hopefully) without the normal loss of other volatiles- ie the temperature is high enough to drive off the water content (which would be scrubbed from the inert atmosphere), but not so high as to boil off useful hydrocarbons.
So, am I crazy???
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The point of using charcoal as a heat source is (afaik) that it is a very clean, odourless, flame with very little residue. If you produced your 'partially distilled' charcoal then you would get a good, dry fuel but would it be much better than the original well-dried wood? After all, you would have lost /wasted some of the chemical energy.
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I suppose the thought-experiment is really aimed at asking: Can the the wood be completely (or near) dried without some of the chemical energy escaping? And would all this heating/cooling/bashing-about cause the cellulose/lignin bonds to fracture -so making the end result more processable.
For example, if this were the case, the super-charcoal could be compressed into dense briquettes with the minimum of additional energy.
Unlike standard charcoal it should not have lost a significant proportion of the energy in the original wood.
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Yes but I think we are into diminishing returns. Once the wood is 'dry enough' to get a fire going, it will dry itself out whilst burning and the effective heat output need be very little affected if condensing boiler techniques are used. Wood burning stoves are not very efficient but that is true for solid fuel heaters in general - unless you use pulverisers and fluidised beds etc which I believe are only suited to large scale systems.
I guess a good compromise would be to store the wood in the roof space of the house where it would benefit from being warmer and would dry quicker.
A special kiln could consume a lot of energy which must be included in a total cost / total benefit assessment. Latent heat of vaporisation has to come from somewhere and at sometime.
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Although I gave the example of my father's wood shed I was really thinking of something on a more industrial scale.
Although the drier the wood the hotter it will burn (which offers advantages), the reduction is water content also lends itself to making transport & storage more efficient. Though, unlike standard charcoal all the energy potential is retained.
There are a number of systems around the world which combine solar heating with a furnace for heat applications (municipal hot-water or power generation) - In these solar collectors pre-heat the inlet air prior to combustion of a fuel (eg. Natural gas, biomass) but I question whether, in cases where plant material is burnt, the solar heating would be better used to 'condition' the fuel - pre-combustion; hence the 'wood-shed'.