Mobile Retorts

[steve collins]

It was down to the diameter and length of timber as 16" was the maximum diameter and 2m lengths but the retort was loaded with the larger timber in the 'hot spot' to gain best results, only one probe was used

[the village idiot]

It was down to the diameter and length of timber as 16" was the maximum diameter and 2m lengths but the retort was loaded with the larger timber in the 'hot spot' to gain best results, only one probe was used

 

Crikey! That's some mighty big twigs for charcoal production. I presume it must have been viable converting them to charcoal rather than firewood?

[steve collins]

It was viable doing it but however you look at it firewood will always claim a price of 35- 50 a ton. its down to how quickly you can convert it and at what volume is weather you can make it pay is key

hope that makes sense?

[agg221]

I hadn't viewed this thread since TVI's original comments on his kiln. There are some materials questions which have come up where I can offer some thoughts.

 

There are several different failure mechanisms here, the key ones being creep, corrosion and differential thermal stress. Ideally you would use a thin section so the more corrosion and creep resistant it is, the better as you can then afford to go thinner. Thermal conductivity is also important since it will lead to better efficiency.

 

Creep is the simplest. This is a temperature/service stress related mechanism. Stress here is minimal (it isn't a pressure vessel) so the ultimate service temperature probably applies. For continuous service, mild steel only takes 425degC (540degC excursions) so is not strong enough, whereas 316 stainless steel takes 815degC. Even this isn't high enough for the hot side, so higher strength materials such as 9% chromium steels with molybdenum and other alloying additions, as used in state of the art high pressure steam piping at 650degC might be better.

 

Corrosion here is not straightforward. The inside of the retort is a reducing atmosphere. The outside could be anywhere from moderately oxidising to moderately reducing, depending on the fuel/air mix. This is fairly benign, so a highly oxidation resistant alloy would not be needed. The outer wall of the firebox is the most oxidising part of the system. Loss of section would therefore probably be fairly low even in an unalloyed steel (as evidenced by the reasonable service life in mild steel ring kilns) but a higher alloy would increase durability. However, there will be some acidic corrosion between firings, due to byproducts from the wood. This suggests a corrosion resistant surface would be useful, potentially coated. The environment is fairly similar to a bio-fired power station so thermally sprayed corrosion resistant coatings over a high alloy steel may be suitable, or alternatively the use of an iron/chromium/aluminium alloy may be an option as it also gives very good high temperature strength.

 

Differential thermal expansion is the hardest to overcome. As the material heats up, it expands. If one part is heated more than another, the expansion causes stress which usually results in distortion of both the heated and unheated part. When it cools down, it doesn't go back to the original shape. These stresses can be enormous - we have looked at heat exchanger designs which can literally blow themselves apart with an internal temperature differential as low as 100degC. How much is determined by its coefficient of thermal expansion (CTE). Different materials will expand to different extents; the higher the CTE, the greater the expansion. This depends on the internal microstructure of the material - mild steel and other alloy steels are ferritic with a CTE of 12, whereas ordinary stainless steels are austenitic with a CTE of 17, which makes the problem worse. Coincidentally, most ceramics, including brick and concrete, have a much lower CTE (between 4 and 10) which is why they don't blow themselves apart in kilns.

 

Overcoming differential thermal expansion is a combination of material selection and design. If you heat a kiln such as the Exeter from below you will get significant temperature differences between the top and bottom of the retort. More even heating, and design of supports to enable movement could help.

 

The Hookway retort has a lot going for it in terms of minimising the above effects, although I am not sure whether this is by luck or design. The upright design makes heating around the structure more uniform; heating through the centre means the hoop stresses are minimised as the hottest 'tube' is the smallest diameter, and the use of a single heated tube means only one part needs to be made of a material with good high temperature strength - the outer wall can probably be mild steel as it won't typically exceed its stress limits, potentially with a high temperature coating to limit corrosion. I'm not sure how well it would scale up on diameter, but some Masters students at Sheffield did some thermal modelling on a horizontal version to increase efficiency. This should also make it modular, to allow heat from one unit to initiate the next. I have spotted some suitable tube which I am trying to scrounge and I then intend to give it a go.

 

Alec

Edited May 4, 2015 by agg221

[the village idiot]

I hadn't viewed this thread since TVI's original comments on his kiln. There are some materials questions which have come up where I can offer some thoughts.

 

There are several different failure mechanisms here, the key ones being creep, corrosion and differential thermal stress. Ideally you would use a thin section so the more corrosion and creep resistant it is, the better as you can then afford to go thinner. Thermal conductivity is also important since it will lead to better efficiency.

 

Creep is the simplest. This is a temperature/service stress related mechanism. Stress here is minimal (it isn't a pressure vessel) so the ultimate service temperature probably applies. For continuous service, mild steel only takes 425degC (540degC excursions) so is not strong enough, whereas 316 stainless steel takes 815degC. Even this isn't high enough for the hot side, so higher strength materials such as 9% chromium steels with molybdenum and other alloying additions, as used in state of the art high pressure steam piping at 650degC might be better.

 

Corrosion here is not straightforward. The inside of the retort is a reducing atmosphere. The outside could be anywhere from moderately oxidising to moderately reducing, depending on the fuel/air mix. This is fairly benign, so a highly oxidation resistant alloy would not be needed. The outer wall of the firebox is the most oxidising part of the system. Loss of section would therefore probably be fairly low even in an unalloyed steel (as evidenced by the reasonable service life in mild steel ring kilns) but a higher alloy would increase durability. However, there will be some acidic corrosion between firings, due to byproducts from the wood. This suggests a corrosion resistant surface would be useful, potentially coated. The environment is fairly similar to a bio-fired power station so thermally sprayed corrosion resistant coatings over a high alloy steel may be suitable, or alternatively the use of an iron/chromium/aluminium alloy may be an option as it also gives very good high temperature strength.

 

Differential thermal expansion is the hardest to overcome. As the material heats up, it expands. If one part is heated more than another, the expansion causes stress which usually results in distortion of both the heated and unheated part. When it cools down, it doesn't go back to the original shape. These stresses can be enormous - we have looked at heat exchanger designs which can literally blow themselves apart with an internal temperature differential as low as 100degC. How much is determined by its coefficient of thermal expansion (CTE). Different materials will expand to different extents; the higher the CTE, the greater the expansion. This depends on the internal microstructure of the material - mild steel and other alloy steels are ferritic with a CTE of 12, whereas ordinary stainless steels are austenitic with a CTE of 17, which makes the problem worse. Coincidentally, most ceramics, including brick and concrete, have a much lower CTE (between 4 and 10) which is why they don't blow themselves apart in kilns.

 

Overcoming differential thermal expansion is a combination of material selection and design. If you heat a kiln such as the Exeter from below you will get significant temperature differences between the top and bottom of the retort. More even heating, and design of supports to enable movement could help.

 

The Hookway retort has a lot going for it in terms of minimising the above effects, although I am not sure whether this is by luck or design. The upright design makes heating around the structure more uniform; heating through the centre means the hoop stresses are minimised as the hottest 'tube' is the smallest diameter, and the use of a single heated tube means only one part needs to be made of a material with good high temperature strength - the outer wall can probably be mild steel as it won't typically exceed its stress limits, potentially with a high temperature coating to limit corrosion. I'm not sure how well it would scale up on diameter, but some Masters students at Sheffield did some thermal modelling on a horizontal version to increase efficiency. This should also make it modular, to allow heat from one unit to initiate the next. I have spotted some suitable tube which I am trying to scrounge and I then intend to give it a go.

 

Alec

 

Fantastic!!!

What I understood of it at any rate!

Would you consider doing a half day's consultancy at the wood? You sound like the man who can advise me on the best way forward to keep my retort going as long as possible.

[MartinN]

There is no problem producing a durable, relatively low maintenance retort for producing charcoal.

 

It is all down to the economics. 316H stainless is indeed a good material to use. It is more resistant to the acids produced in pyrolysis although any chlorides present can make for an agressive corrosive environment. I looked into getting a retort made last year and the cost were prohibitive let alone the problems for getting welding done. In fact I could not get a quote. It is a specialized area for thinplate welding in stainless. Most high end welders will only touch thick materials. Apparently welding 1" or 2" thick material is easier then 2mm thick. If it is not done correctly then the weld will fail relatively quickly especially at the higher temperatures of the retort. One major advantage of mild steel.

 

With regard to warping the usual practice is to have things like dished ends on you retort tube. In stainless this is upwards £400 each for small and if you want it in heat resisting stainless £1000 + each and that's small thin section. Think of a thin relatively thin walled hollow sphere . Heating it up will cause less distortion and stress than a flat plate like the baking tray twisting in the oven.

 

I'm in the process of a new design on a cheaper basis ignoring all the above! So far estimated costs are in excess of £3,500, though that does include some tooling to manufacture. Time permitting I'll make a 1/5 scale model to clarify everything goes together. If it does I may bite the bullet, cash in the pension and have a go, but I've got to be pretty sure it will work. Still looking for a way to test it in Solihull. Hint hint.

 

Initial full scale design, on paper, will give half the yield of the Exeter, but will be scalable. If it works I think it could be scaled up to 4 cubic metres for about £1,500 more and still transportable on a trailer pulled by a small car. I have been reading the posts on this site regarding the Stein Arb truck and think this would be a good way to move it around a wood.

Weight will not be a problem for the retort.

 

The logic for operation would be for coppicing around centre of 2 acres and pull this wood in for processing using Stein truck. I guess this wood be about 6 tons per annum for good coppice? And then move on to the next 2 acres. By the way I' m not a wood folk person so I'm going on info on this forum regarding yield. So coppice 2 acres, move and store under temp cover, then move onto next. By the time you've done 100 acres back to the start ready for retort after some drying has been achieved.

 

Capital investment on this would be small, trailer, saw, Stein truck, training in coppicing chainsaw ins etc but not a lot else apart from a spanking new retort. Not a huge amount for someone to nick. If I don't cut it grows a little more so could fit in with my existing non woody work.

 

Please find holes in my logic so I can patch them up. All thoughts positive and negative welcome! Especially if it stops me wasting money.

[Johny Walker]

had to read that twice :001_rolleyes:1,st glance thought it read mobile retards :laugh1:

[agg221]

There is no problem producing a durable, relatively low maintenance retort for producing charcoal.

 

It is all down to the economics. 316H stainless is indeed a good material to use. It is more resistant to the acids produced in pyrolysis although any chlorides present can make for an agressive corrosive environment. I looked into getting a retort made last year and the cost were prohibitive let alone the problems for getting welding done. In fact I could not get a quote. It is a specialized area for thinplate welding in stainless. Most high end welders will only touch thick materials. Apparently welding 1" or 2" thick material is easier then 2mm thick. If it is not done correctly then the weld will fail relatively quickly especially at the higher temperatures of the retort. One major advantage of mild steel.

 

With regard to warping the usual practice is to have things like dished ends on you retort tube. In stainless this is upwards £400 each for small and if you want it in heat resisting stainless £1000 + each and that's small thin section. Think of a thin relatively thin walled hollow sphere . Heating it up will cause less distortion and stress than a flat plate like the baking tray twisting in the oven.

 

I'm in the process of a new design on a cheaper basis ignoring all the above! So far estimated costs are in excess of £3,500, though that does include some tooling to manufacture. Time permitting I'll make a 1/5 scale model to clarify everything goes together. If it does I may bite the bullet, cash in the pension and have a go, but I've got to be pretty sure it will work. Still looking for a way to test it in Solihull. Hint hint.

 

Initial full scale design, on paper, will give half the yield of the Exeter, but will be scalable. If it works I think it could be scaled up to 4 cubic metres for about £1,500 more and still transportable on a trailer pulled by a small car. I have been reading the posts on this site regarding the Stein Arb truck and think this would be a good way to move it around a wood.

Weight will not be a problem for the retort.

 

Please find holes in my logic so I can patch them up. All thoughts positive and negative welcome! Especially if it stops me wasting money.

 

I will try to find some holes

 

Welding thin 316 is not difficult. I am not the world's best welder, but I am pretty good at it - I have run a whole load of seams in 1mm 316, both in butt and corner lap configuration. Some of the butt welds were unsupported freehand across about a foot and I managed not to burn holes and where I did, to fill them in again. Finding a nuclear coded welder is harder, but you don't need one for this. If you just want a fabricator I can find you one who is very reasonable if you want me to drop you a number (they're near Newmarket but if this is to be trailable it shouldn't be a problem).

 

Dished ends are good, but the fundamental problem here is that the heating in a horizontal barrel design is non-uniform. This creates differential stresses, it has to go somewhere and so it warps it. Extra strength does not help you enough as the material is too thin to resist the stresses.

 

Graham, I would be very happy to come over and see your set-up. I can even try to explain what I am on about in English!

 

Alec

[Woodworks]

Do you think there is any mileage in just using disposable old oil barrels for the inner vessels and line the outer one with refractory brick? Old barrels are as cheap as chips and last a good few burns.

Edited May 5, 2015 by Woodworks

[MartinN]

Yes indeed.

 

Using brick means a stationary retort. These exist far and wide but the economics of moving wood to the retort kills limits, dramatically, the profitability.

 

I have just had 3 BBQs this last week using " oxford charcoal co" charcoal. One box weighing 3 kg. I always water down charcoals after I'm finished leaving a bed for the next time. Before Using English charcoal I used Argentinian not through lack of patriotism but lack of availability. One 3 kg bag would do just one BBQ so at £1 per kg that £3 per burn. English works at at £3 per burn, is much easier to light and is ready much much quicker. It's a no brainier.

 

Unfortunately customers are generally too thick to do the maths and buy on weight. Getting the cost of production down is the key to transforming this business in the UK. I think the figures are about 94% imported. Once you've used English charcoal you'd never go back to cheap imports, though the American is quite good too and longer lasting.

 

Oil drums in a mobile application are ideal due to low cost £8 to £10 each but as in the Adam retort, see earlier link you will only get 60 kg per burn. At £1.25 wholesale that is not a lot. In my business I charge out £400 plus per full day (nowt to do with wood) and pro rata. I don't do much per day but what I do is profitable. I'd like to do this as a side line but I'd need to produce 1 to 1.5 tons per week finished charcoal to be justifiable. About 4 to 6m3 per week processed. Moving that amount of wood a large distance to a static kiln or one in a remote location I think kills the business model.