Rapid Small-Scale Biochar Production Method
By John Briggs
ENC Consultancy, Mount Pleasant, Menith Wood, UK-Worcester WR6 6UB
john.briggs8@btopenworld.com, +44 (0) 1584881216
The options for disposal of waste from small scale agriculture,
gardens or hedges are: ploughing in, burning, composting,
mulching or shredding, or landfill. Each method has its drawbacks:
some are energy or labour intensive; all produce greenhouse
gases. Bonfires produce greenhouse gases and smoke.
The ENC batch process consumes around 6 cubic feet of
dry/semidry wood and gardening waste. The output is one cubic
foot of char in less than 30 minutes. The more awkward materials
(brambles, hedge or shrub trimmings, etc) are packed into an
open-ended steel cylinder measuring 40 cm in diameter which sits
above a hearth made of bricks. Initially, a fire is lit in the hearth
with loose twigs laying down a layer of coarse pyrolising material.
The pre-packed cylinder is then lowered onto the hearth. Steam is
produced and when a lid is placed on the cylinder the back
pressure of steam slows down combustion in the hearth. It is
important to push down the hot material periodically to maximise
packing and therefore charring. Steam evolution is eventually
replaced by smoke plus combustible gases. After a few more
minutes the process is terminated by quenching with water. The
total cycle time is under half an hour and preparation for the next
batch can be carried out in parallel with the current batch.
The simple quick method uses inexpensive and portable
equipment. It allows different types of combustible materials to be
charred for comparison. As an alternative to eliminating waste in
small bonfires, the technique is much more acceptable
environmentally.
The emphasis in recent research and development on biochar
has been on large scale (1-100 ton/day) processing of agricultural,
forestry or municipal waste. This scale of operations presents
logistics problems such as collecting the feedstock, the distribution
of the char and incorporating it into the soil after production.
Large-scale processing may be appropriate in the richer countries
(USA, Europe and Australasia) but most farmers and growers work
on a much smaller scale (most of Asia, Latin America and Africa).
Small inexpensive equipment for processing batches of waste
within the range 5-100 kg/batch could be very valuable to such
farmers. Demonstrations of improved crop yields should rapidly
lead to widespread adoption of small biochar production units in
villages or on individual plots. Gradual incorporation of biochar
into the soil by hand or with simple tools is an option in such cases.
EQUIPMENT: Bricks and tiles, quenching media, steel cylinder and
lid. (Non-metallic equivalents, such as clay-lined baskets, could be
an alternative.)
ENC BIOCHAR METHOD
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Select and cut feedstock
Dry for at least one month depending on type and weather
Stuff metal cylinder with dry material as densely as possible
Start fire in hearth with coarser material
Add to fire for 5-10 minutes to store heat in bricks
Place filled, open-ended cylinder on the fire
Steam evaporation will build
Steam turns yellowish as more volatiles evolve
Material in cylinder should be pushed down towards the
hearth
Seal off air ingress hole partially (>80%)
Place metal lid on cylinder as steaming decreases
Remove lid and wait for volatiles to catch fire as steaming
slows
Replace lid after a few minutes to quench flames
Remove lid and quickly push down pyrolysing material to
hearth
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Replace lid and seal air ingress hole more completely
Leave for approximately 10 minutes
Remove lid, inspect
As flames start again quench completely with water
Remove char from hearth and dry
Crush, grind and sieve
General rules:
The drier the starting material the higher the yield
It is beneficial to aim for a size/diameter gradient of
feedstock, with coarser materials in the initial fire and finer
thinner material at the top of the cylinder
Avoid large leaves and unshredded paper to ensure good
air/vapour circulation
Comparative Speed & Capacity of Production
Group
Drum
Diameter
(cm)
Feed
Volume
(litres)
Cycle
Time
(hours)
F Gunther
Holon (Sweden)*
30
40
~3.0
ENC J Briggs
40
150
0.5
GBD P Wright
52
>200
24.0
5446
(2 t dry
wood)
72.0
Lakeland Coppice
213
* Inner drum diameter of two-drum method-claims minimum GHG/smoke
production
Summaries of other groups’ activities given below:
www.holon.se/folke/carbon/simplechar/simplechar.shtml
www.gbd.freeuk.com/charcoal.htm
www.lakelandcoppiceproducts.co.uk