Charcoal Kilns - what to buy?

[the village idiot]

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

 

If I'd known it was going to be that simple I would have answered it myself!

 

Great work Alec and Openspaceman, you are true BBQ Boffins.:thumbup:

[agg221]

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...

 

This is the one bit I really don't know how to handle in the calculations. I think I could do a reasonable job of retort vs. traditional as the paper I referenced earlier gave some good data on the mix of emissions. However, what I can't find is data for pit burning. This is less efficient on yield, but it does flare the offgas well as the flame front is always at the interface with the air. In the end, I decided the best option was to assume that, since they are both flared, emissions would be a similar in composition between a pit and a retort but allow for it through dropping presumed efficiency.

 

Alec

[openspaceman]

 

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

 

cheers, steve

 

Who said anything about banning them? In fact I think charcoal making still enjoys some exemptions from the various regulations which other industries are not granted.

[SteveA]

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

 

The Village Idiot mentioned the above earlier in the thread.... so it must be true.

cheers, steve

 

Who said anything about banning them? In fact I think charcoal making still enjoys some exemptions from the various regulations which other industries are not granted.

[richy_B]

I believe that because rings kilns are portable they benefit from several exemptions. If that was a fixed light industrial items you'd run into issues.

[SteveA]

This is basically what I am doing - pit burning, adding layer by layer when the surface just starts to ash and quenching at the end. I am burning dry brash, up to 3" diameter, plus some bigger leylandii. Hard to say on yield as you just keep throwing it in, but it seems OK. I am riddling out at 1" and above as charcoal (mine is looking nice and black and shiny rather than ashy), below 3/8" as biochar. In between gets thrown in a barrel of water to separate out stones and wet it down, then thrown through the shredder. Surprisingly little dust and a good even material as the output.

 

Roughly, 1.5hrs to burn, then quench, stamp down well and take out the unburnt ends. Digging out and riddling takes a long time as my riddles are too small, but if I make them wheelbarrow-sized I reckon I could process a burn in about half an hour, total yield ~250l of char.

 

So far I have found I need a supply of short logs to fill in the ends. Getting the timing right on adding more is fairly easy to judge but I haven't found a good way to finish a burn - the last layer tends to end up ashing on the outside before the middle is charred. The best option I have found so far is to cover the pit over with a metal sheet once the last layer starts to ash, then wait 15mins before quenching.

 

Very low impact, zero investment (beyond a pick and shovel) and whilst I am using a fixed location, it would be very easy to dig pits where you want them and fill them in again afterwards. The only thing you need is some water - I am finding about 20l from a watering can is enough if I take out the unburned ends and stamp it down well, then riddle quickly afterwards (the lumps going in the barrel deals with anything still 'live' and the fines don't seem to keep burning, and get mixed with grass cuttings soon after anyway).

 

Alec

 

Hi Alec, what you've described above sounds like my best option, at least for now.

Cheers, Steve

[Woodgirl]

I'm very keen to meet Richy_B and The Village Idiot, but neither have replied to my request. If anyone knows how to nudge them into looking at their personal messages, I'd be very grateful. Cheers.

[HertsWood]

Hi Woodgirl,

 

TVI lives and works deep in rural suffolk - phone signal is poor and I think broadband is still of the windup type.....

 

I've just taken delivery of a Pressvess retort - single barrel to a Specification drawn up and agreed with the designer of all Pressvess Retorts - the great Nick Harris.

 

It's being commissioned next week by Nick, so once up and running if you'd like to come and look, please let me know. Matt