Friday, August 17, 2007

Tom Miles comments on Biochar production costs

This is from the Terra preta site.

Agreed. Production and use of the charcoal on the farm is
not trivial. It's at a different scale than commercial
charcoal production but it is done with a purpose. That
purpose is clearly defined in your case. It is not yet
clear in many cases.
The actual cost may exceed the current returns on the
investment of labor and capital but the value
(cost/benefit) may not be calculated in strictly current
economic terms. That's not uncommon when developing new
technologies or applications, so I jokingly say that it
must be amortized on its entertainment value. The point
is that there must be a purpose, a product and a value.
Serious farm production of biochar in our area will be
regulated in a similar manner as outdoor wood boilers:
systems will have to comply with air, soil and water
quality regulations. The amount of regulation will
depend on the scale of the charcoal production. Let's
look at scale.
In your plots you have used 30 gallons (4 ft3) or 60 lbs
(4 ft3 x 15 lb/ft3) of charcoal in 85 ft2 plots
(5 x 17ft= 4). 60 lb/85 ft2 = 0.7 lbs/ft2 equal to about
14 tons of charcoal per acre. If your planted area is
50% of the total area you would use 7 tons of charcoal
per acre.
If you used a kiln the size of Robert Flanagan's (1.5
tonnes [1.65 t] biomass per charge) you would produce
about 0.66 tons per day (at two charges/day) or 20 tons
of charcoal in 30 days

So if you ran Flanagan's kiln for 30 days at two charges
per day you could treat about 3 acres per year (20
tons/7 tons per acre). In 15 years you'll cover the
whole 45 acre nursery. 1.65 tons/8 hours with wood
vinegar recovery would exhaust about 3 MMBtuh which is
large enough to be regulated in some states.
If you treat 5 acres per year that's 35 tons of
charcoal per year representing 175 tons of biomass (35
tons charcoal/20%) per year. If you make your own
charcoal at 5 tons of charcoal per day (175 tons/30
days = 5.8t/day) each kiln charge would be about 25
tons of biomass/24 hours or 1 ton per hour (2 big bales).

Your kiln will be rated at about 12 million Btuh(80%
biomass x 15 MM Btu/ton x 1 ton/hour) if no oil is
recovered, or 5 million Btuh if just the offgas is
burned to drive the process of making oil and char.

Either way you have an system is large enough that it
will be regulated for particulate, CO and NOx emissions.
A system of this size is likely to be operated as a
stationary production facility operating 250 days per
year (6250 tons biomass or 1250 tpy charcoal). Large
bale combustors of the 1980s (Agrifurnaces, IA) were
rarely moved. Most systems included debalers like the
farm scale straw burning gasifiers and boilers or today.
A farm scale charcoal system might include the same
amount of equipment as Vidir's Greenhouse Gas
Displacement system which gasifies straw to replace
natural gas for heating heat poultry houses.

Vidir's smallest system consumes 500lb/hr (3 Million
Btuh) of wheat straw. If built as a pyrolyzer it would
produce 100 lb charcoal and 1-2 million Btuh heat. The
system cost is $200,000. Annual operating cost with straw
at $10/bale is estimated at $16,000. Labor is figured at
3 hours per day $15/hr. Economics are based on 6 months
operating time (in Manitoba) or 375 tonnes (752 x 500
kg bales/year).

At 20% yield that would produce 82 tons (75 tonnes) of
charcoal which could treat about 12 acres (at 7 tons/
acre). In four years you would produce enough charcoal
for a 40 acre farm. At $200/ton the charcoal would be
worth about $16,400/year whch would just offset the
operating costs but not capital. If you had a use for
the heat (2 million btuh x 70% to hot water = 1.4
MMBtuh, 33.6 MMBtu/day) in 30 days you would recover
more than $12,800 additional revenue to help pay for
the plant. In six months you would recover $16,400 in
charcoal value and $76,800 in heat savings. So the
payback could be 4 years with heat recovery. To a see
a system like that in operation would be entertaining.

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