Charcoal Kilns - what to buy?

[Woodworks]

 

Traditional ring kilns are reckoned to have an input to output ratio of around 7:1. My exeter retort is thought to have a ratio of around 4:1. It is hugely dependent on the moisture content of the wood, unfortunately I have never weighed the charge wood so can't give my opinion on this.

In terms of volume, if I stack the charge chamber (1.7 cbmtrs) with hazel in the round, I reliably get about 0.9 cbmtrs of charcoal out. Giving a volume ratio of around 2:1.

From memory there was never a day when I opened the ring kiln to find it 50-60% full of premium charcoal like I get with the retort.

 

As you suggest, an insulated retort is certain to be dramatically more efficient in terms of heat retention and, due to the burning off of the wood gas, will be massively less polluting.

Some people believe that charcoal burning in ring kilns and pits may well be outlawed before too much longer.

 

Yes we get around 50% by volume in our retort. Also as you say must be a lot cleaner with all the gas being burnt off. Ours runs pretty much spotless after the first half hour.

[the village idiot]

Cor blimey guv, I dint know Hazel could return at 2:1 ....we have a few acres of unmanaged Hazel coppice (overstood) that needs a lot of stool hacking so turning it into charcoal/ biochar at 2:1 sounds like a super dooper option.

 

You kind of nearly answered my question.... what I'm saying is.... eg; would a Hazel burn at 2:1 ratio have 50% less emissions compared with burning a different type of wood at 4:1 ratio.

 

And how are emissions tested on a device that gets so hot and leaky to the outdoor environment?.... especially at the stage where the kiln lid is off/ or retort door is open, or earth clamp smouldering?

 

Another question I have is on embodied energy, and probably much harder to answer:.... Are the emissions from manufacturing & repairing metal retorts as bad/ or worse as burning charcoal in an earthy pit (that uses no metal)?

cheers, steve

 

2:1 ratio on a hazel burn is a volume measurement. You would get very much the same with any species of wood seasoned to the same level. If I put freshly felled hazel (or any other species) into the retort the volume ratio would be closer to 3:1 and I would be emitting more steam than Skyhuck after a taxing day!

 

I sell by volume now, but if I sold by weight and had all species available at the same cost, I would choose the densest species to convert to charcoal to maximise my weight ratio. I suppose it is likely that the 'heavier' woods would produce more emissions as there is more 'stuff' to burn off. This is fairly irrelevant with a retort as the vast majority of gasses are burnt off, whatever the species, making a retort much cleaner than other production methods given the same charge wood.

 

Accurate emission measurements would be incredibly difficult to obtain, so I can't help you with that. I would imagine that they are comparable across all species. Embodied energy is a good point and well beyond my intellect. I would be interested to hear people's thoughts on this as it is something I hadn't considered. Agg221, Openspaceman, can you help?

[jaymo]

Hi jaymo,

 

Are you Tregothnan? I would love to come and see your setup at some point. Do you do visitors? (I will only ever be very small scale, so not a competitor)

 

Hi.

Yes I work for Tregothnan, have done for a little over 4 years now so relatively new to the charcoal world. Personally I would be more than happy to show you around, it's great to have other burners opinions and I feel there is a lot we can learn from eachother experiences. Let me clear it with management first just to be on the safe side. what system are you using at the moment?

[SteveA]

Then again I don't know why I question such things as embodied energy or a bit of pollution when we have monstrous pollutive things such as this: The world's largest cruise ship and its supersized pollution problem | Environment | The Guardian

cheers, steve

[the village idiot]

Hi.

Yes I work for Tregothnan, have done for a little over 4 years now so relatively new to the charcoal world. Personally I would be more than happy to show you around, it's great to have other burners opinions and I feel there is a lot we can learn from eachother experiences. Let me clear it with management first just to be on the safe side. what system are you using at the moment?

 

Just an Exeter retort, a shovel and a masochistic disposition..

Look forward to meeting you if the higher powers give you the nod.

[the village idiot]

Then again I don't know why I question such things as embodied energy or a bit of pollution when we have monstrous pollutive things such as this: The world's largest cruise ship and its supersized pollution problem | Environment | The Guardian

cheers, steve

 

Just do what you can Steve, and feel good about it!

[SteveA]

Just do what you can Steve, and feel good about it!

 

Yeah ~ I should read more of that Positive Thinking thread.

cheers, steve

[agg221]

Embodied energy is a good point and well beyond my intellect. I would be interested to hear people's thoughts on this as it is something I hadn't considered. Agg221, Openspaceman, can you help?

 

In theory, yes. I do have access to some life cycle analysis (LCA) software (GaBi) which does this if you have the right input data. However in practice the input data is probably not good enough to achieve anything more than a crude estimate. With that in mind, I get the following.

 

Working purely on an emissions basis (ie this is not an economic or energy-source based calculation), some quick figures I can find from a Google search suggest:

 

Steel production: 1.37 tons CO2/tonne

( G.P.Hammond and C.I.Jones (2006) Embodied energy and carbon footprint database, Department of Mechanical Engineering, University of Bath, United Kingdom)

 

Kiln manufacturing: 33 tons CO2/tonne

(assuming forging, cutting and machining energy values from N. Duque Ciceri et. al, 'A Tool to Estimate Materials and Manufacturing Energy for a Product' - http://web.mit.edu/ebm/www/Publications/9_Paper.pdf and converting MJ to tonnes CO2 using the Carbon Trust calculator - http://www.knowlton.org.uk/wp-content/files/Energy%20carbon%20conversions.pdf)

 

Shipping (assuming an Exeter retort from their site to Hadleigh, towed behind a diesel vehicle averaging 35mpg unladen, 25mpg laden): 0.25tonnes CO2

(Diesel emissions figure from Calculation of CO2 emissions)

 

Weight of Exeter retort: 1.25tonnes

(The Exeter Charcoal Retort - The cleaner and more efficient method of producing barbeque charcoal and biochar.)

 

Operational outputs

For a retort kiln: 1.95±0.21 tons CO2/ton of charcoal

For a standard kiln: 2.38±0.97 tons CO2/ton of charcoal

(Sparrevik, M. et. al. 'Emissions of gases and particles from charcoal/biochar production in rural areas using medium-sized traditional and improved “retort” kilns', Biomass and Bioenergy, Volume 72, January 2015, Pages 65–73)

 

Based on the above (very) crude assumptions, working with mid-point values throughout. If TVI has an Exeter kiln, the embodied CO2 at point of use is 43 tonnes.

 

TVI's kiln will produce 0.43 tonnes CO2 less per ton of production than my pit.

The embodied CO2 will therefore be offset after 100 tonnes of production. This would be longer if the retort has to have work done on it.

 

The biggest error in this calculation is that I have only included CO2. Other emissions from the burn have greater greenhouse gas (GHG) potential and the levels will be higher from the pit burn, so in practice the offset will be significantly faster.

 

Not a great calculation, but shows the general principle.

 

Alec

Edited May 21, 2016 by agg221

[openspaceman]

I

Not a great calculation, but shows the general principle.

 

Alec

 

Much better than I could offer as I have never considered the subject other than in terms of conversion efficiency and pollution.

 

Traditional kilns are highly polluting because they do not flare the pyrolyisis offgas, it is generally possible to flare the stacks toward the end of the burn when a lot of CO is being produced but early on when a lot of steam is vented with polycyclic aromatic compounds and methane in the mix the calorific value is too low to support an open flame.

 

Methane is actually about 25 times more effective at capturing re emitted infra red than CO2 and although its life in the atmosphere is short it will become more significant as more is emitted from soil carbon stores.

 

So a good clean flare flame is the best way to make charcoal burning acceptable, we actually flared ours inside a gas turbine for a brief but successful experiment.

 

An earlier question about yield needs a bit of consideration. You can make a charcoal stick from oven dry pine with a yield of 45% of the dry weight but whilst it is undeniably charcoal it is not pure carbon. Heat the same stick to 900C instead of 400C and the yeild drops to 15% plus the carbon content is in the high 90s.

 

So apart from the obvious point that you have to boil off any water before pyrolysis starts, and this will be around 50% of the weight in fresh wood, the yeild is highly temperature dependant. A retort run at the same temperature each time will produce the same yeild no matter what the initial moisture content but more support fuel is needed, a kiln will consume much of the charcoal produced in vaporising water so the yield plummets with high moisture content.

[SteveA]

Awesome answer! Very impressive mathematical stuff.

 

If traditional kilns & pits are banned does that also mean that open fires & inefficient wood burners should also be banned?

 

cheers, steve