2 Lists, Tom, Paul, etal
1. This is to throw in three other ideas that relate to " nested barrel"-scale
production of chars. I talk below 1) of the possibility of using bottom lit
down-draft (BLDD) designs with nested barrels, 2) other non-barrel approaches
that appear able to produce char more efficiently (and the importance of trying
to avoid barrel designs that do not utilize the pyrolysis gas energy, and that
don't create the maximum amount of char possible and c) a bit of analysis to
relate power levels and fuel consumption with pyrolysis vs gasification and
combustion approaches (per Tom Miles question).
1. Possibly replacing TLUD designs designs for large nested barrels with bottom
lit down-draft (BLDD)
a. Back in 1995, when I first wrote about charcoal-making stoves (now mostly
called TLUDs, replacing the term "inverted down draft"), Tom Miles asked me to
moderate the stoves list. The dominant alternative approach then was down
draft. The early arguments were whether the TLUD could even work but that of
course soon passed. But the BLDD approach itself never made it with cook
stoves. But I think BLDD could be easily used with the double or nested barrel
approach we are now discussing. It would require a third small BLDD unit off to
the side, with its bottom pipe feeding the (now non-fuel) space between the two
nested barrels. With this approach, one would get the heat directly and quickly
at the bottom of the inner barrel, and a large sufficient draft could be
generated naturally with the geometry. The BLDD design naturally allows
continued feeding of fuel and (most importantly in this dialog) full
preservation of all produced char. I don't have the time to experimentally
checking this, but hope someone else will, as it solves the several problems
that Paul has correctly described below. Crispin has described doing something
like this with BLDD in Mongolia - for coal and space heating, not char-making.
b. The nearest BLDD char-making technology that I know of like this is (I
think) still in operation in Kenya. Elsen Karstad was the developer, and its
operation can presumably be found in the stove list archives. Briefly, the raw
biomass resource is continually placed where the upward moving pyrolysis front
shows up on a circle of ground-level grates. The downward moving pyrolysis
gases then travel radially inward through "tunnels" to a central tall chimney,
with combustion beginning there. No use of the waste heat - but I think there
are possibly some uses such as brick making that would make sense. The
important point here is that all of the biomass can be turned to char; none is
totally combusted. Elsen briefly served as a coordinator of the stoves list -
and also developed a very nice TLUD. It is a shame that we don't hear more now
from Elsen. Any stove list members working in Kenya should talk with Elsen on
his highly successful production of char - especially from bagasse. An
advantage in Nairobi is the heavy use of labor rather than capital equipment. I
guess that his overall conversion rate to char is probably about 25%.
2. Some other competitors to the nested barrel approach and the importance of
trying to avoid barrel designs that do not utilize the pyrolysis gas energy,
and that don't create the maximum amount of char possible
a) Another alternative to the nested barrel approach has been described many
times on this list by Dr. Yuri Yudkevich. He is using multiple barrels inside a
very large container, with staged exchange of the "barrels". No biomass is
combusted - only pyrolyzed.
b. The main company supporting the Sonoma Biochar conference was "Biochar Now".
I have not completely understood their approach, but they also are producing
char with multiple "barrels" and no combustion - only pyrolysis. The flared
pyrolysis gases from one retort fire many others in a "row".
c. Lastly, I think that a potential competitor to the nested barrels (with
mostly pyrolysis and minimum combustion) is being done in Hawaii by Joshua Hunt
(See
https://groups.google.com/forum/?fromgroups=#!topic/biochar-hawaii/ohoqDcuHMiQ
).
This differs from traditional highly polluting pits by having most of the
conversion with open flames and covering the produced char only at the right
time. Again, I think there is larger total conversion efficiency than with the
nested barrel approach with either top or bottom lighting of the biomass placed
between the barrels.
d. All this being said in favor of something other than the nested barrel
approach, I think that it is important that the pyrolysis gases be put to
productive use as in charcoal-making stoves or residential heating (maybe soon
with Whitfield Biochar designs - or similar). In sum, this is to support Paul
Anderson's comments below on nested barrels and the desirability of not losing
the char if the "between barrels" space is filled with (non-char-making) fuel.
3. Relating power levels and fuel consumption with pyrolysis vs gasification
and combustion approaches.
a. There is nothing new here - but this following may help with those who know
biomass flow data, but not power levels, as asked in Tom's initial question
below. Paul below mentions char production of 20%-30%, but I think it has been
quite a while since we talked about the kW ratings of different rates of char
production.
b. Fortunately, there are some easy computations that go with the common
assumption of 18 MJ/kg = 18,000 kJ/kg for any biomass feedstock. Knowing that
there are 3600 seconds in an hour, and that 1 watt = 1 Joule per second, then
if we take 1 hour to consume that 1 kg, we are at exactly a 5 kW rate (that is
18,000 kJ/3600sec = 5 kW). The computations are linear in time and amounts, so
10 tonnes/2 hours would be 5kW*(10,000/2) = 25 MWth (as might be used in a
small electric power plant producing about 5 MWe.).
c. If we want to talk in terms of more familiar energy (kWh) terms, we must
convert 18 MJ = 18,000 kW-sec, by dividing by 3600 sec/hour to get 5 kWh (th)
in the 1 kg. Consumption of 1 kg per hour obviously is then at a 5 kW rate. If
the 1 kg is consumed in 2 hours, then the power level is halved to 2.5 kW.. I
emphasize the big difference between kW (power; Tom's question) and kWh
(energy).
d. The same computations are valid for most gasifiers - which typically end up
with only a percent or two of char. Note that Tom Mile's gasification list
(sister to both "stoves" and "biochar") never talks about char production.
There are gasifiers (all downdraft?), of course, that have been modified to
produce char - but typically "gasification" implies avoidance of char. I think
the world of char-making is better off avoiding the term "gasifier".
e. But for pyrolyzers with a substantial amount of char production, how do we
calculate power levels? The key new variable is the energy content of char,
which often is given as 30 MJ/kg. Again conveniently, with Paul's upper value
of 30% creation of char, the char produced from 1 kg of biomass has an energy
level of 0.3*30,000 kJ = 9 MJ - exactly half of the combustion/gasification
computation value of 18 MJ above for 1 kg. After again dividing by 3600 and
subtracting from the combustion power value of 5 kW, means that this
hypothetical (1 kg/hr) char-making stove would have a power level of half- or
2.5 kW. The respective values for 25% and 20% char production are 2.92 and 3.33
kW in the pyrolysis gases (and 2.08 and 1.67 kWh in the cases of 0.25 and 0.20
kg of char). These last numbers are needed to allot carbon credits, should we
ever get to that point. Note again that I have used both kW (in stove
performance) and kWh (in the produced char). I am not attributing a (kW) power
number at all to the char - it just looks that way because this example uses a
1 hour time period.
f. I have run out of time to impute separate dollar values to the gases and
char. But there are reasons to think that the dollar value of the char can
exceed the dollar value of the gases (especially with the 30% char by weight
computation that leaves about 50% of the initial carbon in the char). Let me
fudge the starting value to be 4 tonnes of biomass, which will yield 1.2 tones
of char, which itself contains about 1 tonne of carbon. Because of the ratio of
atomic weights (44/12), that tonne of carbon has sequestered 3.67 tonnes of
CO2. With a dollar value for CO2 about $30 per tonne CO2, the four tonnes of
input biomass (worth maybe $200) can have a carbon credit value of about $100
(roughly, after subtracting transactional costs). Char is now selling for as
much as $2/kg ($1.00 / lb) - but if we take 10% that level, the char for ag
purposes might eventually sell for as little $200 per ton. That could look like
$100 for the farmer buying the char, depending on to whom the carbon credit
accrues.
g. But how much are the pyrolysis gases worth? Natural gas is selling for about
$5 per GJ (has been much higher) - and the input 4 tonnes of biomass will have
about 4 tonnes*18 GJ/tonne = 72 GJ, of which half (or 36 GJ) I have shown is
available as a gas. Assuming $5/GJ gives $180 for the value of the gas in the
initial $200 worth of biomass - together the total gross income per tonne input
is ($180+$300)/4 = $120 on an assumed material cost of $50/tonne Net profit
will have to include equipment and labor - and is beyond this computation and
is certainly a lot less than the $70 difference here - but still potentially
attractive. The point only is that we can postulate different price and credit
levels that favor char production over gas production (in this case by a ratio
of $75/$45). The economics become less favorable for the char when the output
is a liquid fuel, but this is a totally different question.. Favorable
economics with combined liquids and char are now coming into prominence with
the ideas expressed at:
www.coolplanetbiofuels.com
I have tried to look carefully for typos, but sure don't guarantee there are
none.
Ron
----- Original Message -----
From: "Paul Anderson" <[email protected]>
To: "Kobus Venter" <[email protected]>, "Discussion of biomass cooking stoves"
<[email protected]>, [email protected], "Hugh McLaughlin"
<[email protected]>
Sent: Sunday, September 30, 2012 8:54:33 AM
Subject: [biochar] Re: [Stoves] Fabricated Burn Barrel TLUDS
Dear Kobus and all,
Retort: When 50/50 inner and outer amounts of fuel as Kobus has mentioned: the
Outer burning XX kg yields zero biochar; Inside retort yields 20 to 30& of XX
kg, net of 10 to 15% of total fuel used.
(Note: charcoal yields from inside a retort of greater than 25 or 30% indicate
considerable volatile matter is in the charcoal, which is not generally
considered good for the plants/soils. Beware of charcoal that is 40+% of
original fuel weight. It will burn in a charcoal stove probably with some
limited smoke, but is not good as biochar.)
TLUD: One batch, expect about 20% yield by weight. Nearly double the net weight
output, and not needing double barrels, etc.
Fines are a result of size of feedstock. If making biochar, fines are fine (pun
intended). Or fines can be briquetted quite easily if sold to the charcoal
market.
Biggest problem I see with many attempts to make TLUD-style barrel-size
charcoal makers is the poor quality of the lid or top to allow in secondary air
that goes to the concentrator hole. If this sentence does not make sense to
someone, then that person has not studied the basics of TLUD operation.
Users should start with small TLUDs (like McLaughlin's "1-G Toucan" out of
tincans - plans are on the web) so that they know that the TLUD fires CAN and
should be very clean burning. There is no reason to have smoky TLUDs when using
200 liter barrels unless using wet fuel or not yet adjusting the TLUD to the
fuel at hand.
Paul
Paul S. Anderson, PhD aka "Dr TLUD"
Email: [email protected] Skype: paultlud Phone: +1-309-452-7072
Website: www.drtlud.com On 9/30/2012 8:54 AM, Kobus Venter wrote:
Tom, Paul, Dan and others,
I have gone away a bit from the TLUD principle as I started from a very
polluting open top burn approach and using feedstock that is not uniform in
particle size. Very wasteful, and as in Dan's case I ended up with a lot of
fines ( http://vuthisa.files.wordpress.com/2012/09/img00177.jpg ). The only
cost effective way that I could improve emissions of the Transportable Metal
Kiln was to convert it into a retort, by using the heat from the conventional
burn to heat the inner retorts. The flow rate of the air through the outer burn
will be high, probably around 1.5 m3/sec and the quantity of feedstock
necessary to provide the heat to the inner retorts before the reaction becomes
exothermic will probably match the closely packed feedstock volumetrically
inside the 3 x 55 gal drums, which will ultimately yield the biochar. The burn
is seen as successful only if ash remains in the outer vessel and all the
biomass inside the 55 gal drums is pyrolysed. I don't need a TLUD type flow
rate to ensure complete carbonisation, but the consumed wood in the outer drum
has to be included in the overall yield percentage calculation. The addition of
a conical lid and chimney (much akin to the New Hampshire metal kiln design)
has increased the draft needed to vent emissions. In the conventional open drum
burn I would place ±550 kg feedstock and end up with 120 kg charcoal, but 50%
will be fines. In the 3-drum retort I would probably also use 550 kg but end up
with ???? I would not like to venture an answer at this early stage, but
hopefully end up with more than just 60 kg of biochar. The principal advantages
of my three drum retort should be the 25% yield of char from the retort
contents, coupled with the ability to use lower quality fuel as the starter
fuel (outside the drum) and the self-stopping of the retort design (better
safety, no need fro water), and the self-running aspect (light it and step
back).
http://vuthisa.files.wordpress.com/2012/09/slits_drum.jpg
http://vuthisa.files.wordpress.com/2012/09/dscf335.jpg
http://vuthisa.files.wordpress.com/2012/09/branches_lid-removed2.png
http://vuthisa.files.wordpress.com/2012/09/bottom.jpg
Regards
Kobus
********************************************************************************
AD, Paul, Kobus and others. Many thanks for the suggestions.
What is the largest practical size (kg fuel/hr, kW) for a single TLUD with a
clean stack for heat recovery? There must be a limit to the air penetration
to get a clean gas burn form a natural draft stack or even a fan driven
TLUD.
Tom
http://www.vuthisa.com Twitter: @vuthisa https://twitter.com/vuthisa Facebook:
www.facebook.com/Vuthisa LinkedIn: http://za.linkedin.com/in/vuthisa
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