[Stoves] Traditonal Charcola Making Process / retort

Tue Jul 3 17:22:06 EDT 2007

Crispin (cc Tom, etal):

    I haven't been following as well as I might, but I like that there is a 
terra preta (charcoal) aspect to this discussion.

    I think you may find that a suitable kiln system  for your stove firing 
can be built around the downdraft charcoaling method developed by Elsen 
Karstad.

    I visited his site (www.chardust.com ) a month ago, but it would not pop 
up today.  Accordingly,  I am copying Elsen as well.  For others new to 
this, there are lots of references to his down-draft design in old archives 
of this list - and maybe Tom can cite the best.

    Briefly, Elsen developed a system where bagasse or wood chips are spread 
out on many grates, and workers keeps enough un-charred material in place so 
that one never sees a red glow.  The downward flowing air is kept small so 
only pyrolysis occurs.  Then below the grate, I think after multiple grate 
pathways merge, there is a mixing with (controlled) secondary air, and 
(presumably) efficient combustion of mixed gases (not the usual diffusion 
flame seen with stoves?) and the resultant hot gases exiting through a tall 
stack that drives the entirely natural draft system.  There may be a fan or 
blower in the system - but I do not think so.  I presume that the initial 
downdraft is augmented/started by some start-up flame.

    I think one "only" need put a pottery kiln chamber in the chimney path 
and the power level (rate of kiln temperature rise) would be controlled by 
the number of different grates being utilized (and somewhat by the depth of 
the fuel pile, which controls the individual grate power levels).  Perhaps 
Elsen has done something along these lines, as he is producing a lot of 
charcoal per day and there is a lot of potentially useful energy going up 
that (40-gallon drum stack?) chimney stack.  Elsen?

    I believe Elsen is selling charcoal briquettes at under $100/tonne - and 
so the raw char cost should be appreciably less.  There are a lot of people 
employed in his approach, and low hardware costs, and still attractive 
prices (below that of other char in the market, I have read at Elsens site.)

    On the conversion back and forth with different power and energy units, 
I have not heard anyone repeat recently that one kg of biomass is often 
thought of as 18 MJ.  So in a combustion process using 1 kg per hour (3600 
seconds), and with a watt = 1 joule/sec), we have 18 E6/3.6E3 = 5 kW at this 
1 kg/hr combustion rate.   If you are pyrolyzing, not combusting, you will 
leave about half the energy in the char, or be operating at about a 2.5 kW 
rate if your biomass input stays at 1 kg/hr.  With your postulated 3.5 
kWh/stove, you will need either .7 kg (combustion) or 1.4 kg (pyrolysis) of 
wood per stove (round numbers) - producing about 30%* 1.4 kg = .4 kg per 
stove (2.5 stoves per kg).

  Economics:   I am hoping that your future sale of biochar at $100/tonne or 
$100/2500 stoves = $.04/stove, may help pay for the wood input.  As an 
example:

Combustion:  Wood @$50/tonne = $.05/kg;  $.05/kg*.7kg/stove = $.035/stove 
(fuel cost)

Pyrolysis: ($.05/kg- .333*$.10/kg)*1.4kg/stove = $.01667*1.4 = $.023/stove 
(net fuel cost).

So you save $.01 per stove, if everything is 100% efficient and these prices 
prevail. My guess is that it will be twice that, because of losses.  But 
more important probably are the labor charges - which might work for you as 
the pyrolysis gas combustion might have more control over the excess air and 
(once set) won't take much labor to maintain at a desired kW or MW setting. 
I am referring here to Tom's observation that it will be tough to get to the 
desired temperatures with wood-firing.  But the pottery web literature (my 
wife is a potter) is full of talk of wood firing getting to cone 10 - and 
you are operating at less than that (but not by much).  The key will be to 
keeping the excess air down - and combusting the pyrolysis gases gives you 
some good control over that.   I also hope you can drop the max temperature 
some.  Few commercial clay products are fired at these high temperatures.

I really hope you can prove it pays to pyrolyze!

I hope that Elsen will chime in on the advantages of his approach over 
standard charcoal-making kilns. Especially needed is his estimate of power 
levels achievable per square meter of grate (and the turn down ratio).  My 
guess is that he uses a lot of excess secondary air to keep the exhaust gas 
temperatures lower.  Is anyone else working with this nice, novel method of 
char production besides Elsen?

Ron

Ron

----- Original Message ----- 
From: "Crispin Pemberton-Pigott" <crispinpigott at gmail.com>
To: "'Discussion of biomass cooking stoves'" <stoves at listserv.repp.org>
Sent: Saturday, June 30, 2007 2:27 AM
Subject: Re: [Stoves] Traditonal Charcola Making Process / retort

> Dear Jeff
>
> More explanation needed...
>
> You are on the right track too, but remember that I did not introduce a
> 'heat per hour'. The total quantum of heat required to fire a stove is 3.5
> KWH so it never gets to a kg of wood per hour figure - it is just a total 
> of
> wood per stove fired. The 18 hours is not relevant actually.  That is the
> kiln cycle time.
>
> The heat applied is very inconstant.  In the beginning the formed stoves 
> are
> heated to 120 over a period of 1 hr and then dried for at 120 for 1.5 
> hours.
> They are then heated to 600 at 135 deg per hour and held there for 3 hours
> with air entering and leaving the kiln to burn out the charcoal to make a
> porus structure (remember the discussion about porosity).  The air is
> closed, then they are heated at 100 deg per hour to 950, then at 65 per 
> hour
> to 1170.  It is then held for 3 hours at 1170 and allowed to cool 
> naturally
> which takes about 4-6 hours.  When it is 200 the door can be opened 
> because
> the stoves can take 200 degrees of thermal shock (being transformed during
> firing into a low thermal expansion material).
>
> Thus the amount of gas burned in the first few hours would be quite small
> and the exit temperature very low, barely above boiling.  As the firing
> progresses, the temperature and exit temp rise.  What I did not see was a
> decent heat capture device to preheat the air going into the gas furnace 
> to
> as high a temp as possible.  The materials exist to preheat the air to at
> least 1000 so a great deal could be recovered. Above a certain 
> temperature,
> no ignition source would be required!
>
> Back to the calc: a total of 3.5 KHW of electrical power is needed to fire 
> a
> stove, using the pattern above.  It works well with the material we have
> mixed.  Very predictable.  Gas furnaces need about twice as much heat to
> fire something (presuming they are not recovering much).  OK.  Then I need
> to solve two problems: first to size the unit for the number of stoves in
> the kiln (suppose it is 250), and to control the gas production rate from
> the initial very low heat demand to the very high one at the end.  The
> _power required_ (a ref to Peter's 'current') is 200 KW.  By that I mean a
> kiln with a 200 KW rating could follow that firing sequence successfully.
> The total heat energy required to fire a stove would not change from 3.5
> KWH.  With gas it is likely to be 7 KHW (3600*7000 Joules) or 25.2 MJ.  I 
> am
> not asking questions (yet) about how to get 200 KW, just how much wood 
> would
> be needed to fire one stove.
>
> The charcoal making device type is apparently important.  Let's say I want
> to get 30% charcoal yield, with that charcoal having a volatiles content 
> of
> perhaps 5% (of the 30%). The heat energy of the gas from the volatiles and
> the charcoal turned into CO or burned is _____% of the total available 
> from
> the wood.  There have been some different suggestions.
>
> So the answer seems to be best fit into the following format:
>
> Pyroliser A:   anKg to give 7KWH and axKg of charcoal yielded from that
> process
> Pyroliser B:   bnKg to give 7KWH and bxKg of charcoal yielded from that
> process
> Charcoal kiln C:   cnKg to give 7KWH and cxKg of charcoal yielded from 
> that
> process
> Charcoal kiln D:   dnKg to give 7KWH and dxKg of charcoal yielded from 
> that
> process
>
> We can deal with the kiln efficiency separately.
>
> So there will be two numerical answers: the amount of wood needed to make
> the gas in a certain device, and then a charcoal yield.  The economics can
> then be optimised: so much investment required for the gasifier, so much 
> for
> the kiln, so many $ for the stove, so much for the charcoal.
>
> As the stove has a certain working life, and therefore a certain total 
> wood
> saving can be calculated.  If the electricity used was coal generated, 
> there
> is a major CO2 offset available comparing the present devices with the
> woodgas-fired charcoal-saving stove.
>
> In many places it is easier to get wood than 200 KW of electricity.  From 
> an
> energy point of view the whole process can run on the biomass saved, and
> have net wood saving as well.
>
> Regards
> Crispin
>
>
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