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Woodworks

Any new small charcoal retorts out there?

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At these higher temperatures radiation becomes important, thermocouples read low because they are constantly radiating heat away unless in equilibrium with the walls of the surroundings. This doesn't matter if you are just comparing burns.

 

The obvious one is to have a smaller fire underneath and a more conventional design with a grate. The power is then controlled by the primary air and the secondary air is entrained by the primary combustion.

 

 

As I keep saying you need more time to dry wood than to pyrolyse it, once pyrolysis starts its self sustaining as long as the temperature doesn't drop. At this stage lots of offgas is evolved and most of these simple retorts pipe the gas into the firebox where it adds to the heating just when you don't really need it or you flare it off wastefully. When I built one of these after seeing them at the merriworth estate and being developed by Robbie Webster I had two pipes out of the retort with one cap. The idea being to take the offgas under the retort when needed and flare it when not. I intended to use a bimetallic strip to open and close the flues but we moved back toward kilns (having moved away because the Viper produced poor charcoal for barbecues).

 

Thanks again for contributing to my folly :001_smile:

 

So whats the problem if the gases start igniting before the wood is completely dry? I know you often mention it but I don't understand the reasoning other than less other fuel required. I can pre heat subsequent barrels and this took ignition times down from 30 mins to 20 mins.

 

Cant redirect the gases with mine as it has to be simple to keep price down.

Edited by Woodworks

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Best way round this is to fix the thermocouple against the relevant wall. If you want to measure burn temperature (most useful for controlling product) it goes inside the vessel but if you want to understand what is going on you need it on the outside wall at the hottest part. The easiest way to fix it is to weld a tag on to slip the thermocouple behind but you could bolt it on instead, so long as you either drill and tap blind or you seal the holes over. For your set-up it may be easier to fix it to the SS support bar instead.

 

 

 

Alec

 

Ordered it with a 300mm probe. Was just going to drill the right sized hole through the lining. I might be able to change the order. Do you think this will be useless?

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Is using lovely fine ash logs a bit of a waste of firewood or does the type of wood make a big difference n the end product?

 

Yes but not really for barbecue charcoal.

 

Aldershot is a place where gunpowder was made for the military, alder could be ground to the right size to vary how quickly it reacted, alder buckthorn was favourite for fuses. Later I think they dried the granules of black powder into different sizes to vary the speed of detonation.

 

Beech and Hornbeam were favourite for iron smelting, hence the copses at Leith hill, where you could see the charcoal hearths in the hillside prior to 1987 storm. The sandstone yielded the iron ore and the lime came from across the valley, with the Tillingbourne providing the power for the hammer at Abinger.

 

Pine was used to co produce stockholm tar.

 

I suspect the pyrolysis was at much higher temperatures for these processes to have a much higher proportion of fixed carbon. Barbecue charcoal tends to retain many less volatile compounds which is why it lights easier and smokes, Charcoal made at 900C is nearly pure carbon, doesn't smoke and the yield is only 15% of the original dry weight of wood.

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My very limited knowledge of practical retort operation relates to the Exeter, from TVI (inc. observing a burn) and from the manufacturers who also operate one. The heat-up phase to the point where the off-gas is combustible appears to be a couple of hours and the retorting phase appears to be around 6hrs, during which there is still drying going on (water is still coming off) but the reaction is self-sustaining.

 

My point being that drying in the retort is very inefficient, especially as it slows the pyrolysis down, if you put warm oven dry wood in the retort how would the times compare?

 

 

You therefore do want the heating, but you do have an enormous amount of surplus gas and it would be handy if you could have had it earlier!

 

Which is why a continuous or batch sequential system is preferable (a bit like the way crematoria now work), The first retorts that Peter?? brought to Merryworth had two chambers, they appeared to be designed to hot swap cassettes so the offgas from one supplemented the heating of others. Pictures of the original Lurgi coal retorts seem to be this way.

 

 

The more interesting thing for us is looking at using the surplus gas productively in a cost-effective manner.

 

 

Yes too right on both counts, up to 70% of the heat in the wood is evolved in the offgas, the flame temperature of which is many times what is needed to pyrolyse the wood or dry it. Keeping the capital cost down is the difficult part, once the university got involved in ours the costs skyrocketed and the complexity made commercialising the concept to expensive.

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Beech and Hornbeam were favourite for iron smelting, hence the copses at Leith hill, where you could see the charcoal hearths in the hillside prior to 1987 storm. The sandstone yielded the iron ore and the lime came from across the valley, with the Tillingbourne providing the power for the hammer.

 

Slightly off topic, but one of my ancestors was one of the original iron masters in Surrey (Warren furnace at Hedgecourt).

 

Alec

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Thanks again for contributing to my folly :001_smile:

 

Blaming me now :001_tongue:, I'd have dropped across to discuss things

 

So whats the problem if the gases start igniting before the wood is completely dry? I know you often mention it but I don't understand the reasoning other than less other fuel required. I can pre heat subsequent barrels and this took ignition times down from 30 mins to 20 mins.

 

Too many points but the major one is that the drying needs heat, the means to transfer the heat into the barrel is restricted to the barrel walls and convection in the retort.

 

Cant redirect the gases with mine as it has to be simple to keep price down.

 

Simple is good, so let's find a simple and cheap way

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My point being that drying in the retort is very inefficient, especially as it slows the pyrolysis down, if you put warm oven dry wood in the retort how would the times compare?

 

I'm not quite sure on this one.

 

Certainly if you measure efficiency in burn time then warm, oven-dry wood would take less time to process. However, if it doesn't reduce the total cycle time (inc. heat up and cool down) by enough to get a second cycle in a working day, assuming you want to stick with single shift working it won't increase productivity.

 

However, drying the wood to this level will take longer or require more energy input.

 

In an ideal world, where the off-gas was being productively extracted you it would pay to run with the driest wood possible, however in current systems where there is a surplus of gas, it could be argued to be more productive overall to burn the gas to dry the wood, ie to start with the wettest wood in the retort that you can still process in a day.

 

The measurements we took suggest that at standard retorting temperatures you don't get the water-gas shift reaction so you don't reduce yield by doing the above.

 

Which is why a continuous or batch sequential system is preferable (a bit like the way crematoria now work), The first retorts that Peter?? brought to Merryworth had two chambers, they appeared to be designed to hot swap cassettes so the offgas from one supplemented the heating of others. Pictures of the original Lurgi coal retorts seem to be this way.

 

Batch-sequential definitely has advantages as you are using the waste heat so don't suffer time penalties for drying. Continuous brings different issues - it is typically used at much higher temperatures for gasification, so is deliberately inducing water-gas shift to achieve H2 and CO output rather than charcoal.

 

Yes too right on both counts, up to 70% of the heat in the wood is evolved in the offgas, the flame temperature of which is many times what is needed to pyrolyse the wood or dry it. Keeping the capital cost down is the difficult part, once the university got involved in ours the costs skyrocketed and the complexity made commercialising the concept to expensive.

 

Universities are good at understanding fundamental concepts, ie working out why something happens. They are generally not so good at the commercial side. The consortium we are working with is purely industrial (apart from us - we are a commercial research and technology organisation) and everyone just wants to make it work, technically and financially. Everyone accepts that the markets will be niche, but niche commercial is still commercial.

 

If you're interested, the public summary of the project is here:

 

http://gtr.rcuk.ac.uk/projects?ref=132408

 

Note, in the course of the project we have established that internal combustion is more cost-effective than a Stirling engine and within the relatively small project we have focussed more on charcoal and firewood drying than on agri-waste to biochar and grain drying, but the principles are transferrable.

 

Alec

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