HCV Charcoal Making Guide
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Scientific Background
Wood Composition
Wood in addition to its moisture content is composed of
cellulose, hemicellulose, and lignin, plus small amounts of extractives, i.e.
resin, and minerals (which produce ash when the wood is burnt). Cellulose and
hemicellulose are carbohydrates similar to sugars because they are effectively
compounds of carbon (C) and water (H2O). The chemical composition
and percentages in wood of the first three are given in Table 2 below:
Table 2 - Wood Composition
|
Chemical formula |
Hardwood
mass % |
Softwood
mass % |
Cellulose |
(C6H10O5)n |
43% |
43% |
Hemicellulose |
(C5H8O4)n |
34% |
28% |
Lignin |
[(C9H10O3)(CH3O)0.9-1.7]n |
23% |
29% |
The atomic masses can be taken to be: carbon, C=12; hydrogen,
H=1; and oxygen, O=16. From the chemical formulae of the compounds and the
atomic masses the percentage mass of each element in the wood can be calculated
as approximately:
C≈50%, H≈6%, O≈44%.
Moisture Content
All wood for practical purposes contains some moisture
content. The majority is so called "free water" contained in the cell cavities
which can evaporate fairly easily. The rest called "bound water" is much more
tightly bound within cell walls and takes much longer to evaporate. The point during
drying at which all free water has evaporated but the bound water has yet to do
so is called the fibre saturation point. The fibre saturation point is the
point at which shrinkage of the wood starts to occur. The moisture content is
still chemically water, i.e. H2O; it is distinct from any hydrogen
and oxygen chemically bound with carbon in the wood compounds.
Moisture content can be quoted on a wet or dry basis, i.e.
the mass of water as a percentage of the mass of wet wood or the mass of the
wood when fully dry, i.e. all moisture removed, respectively. Moisture content
is usually quoted on a dry basis, so it can be more than 100%.
To convert between moisture content on dry basis, MC, and
moisture content on wet basis, MCw, use
the following formulae:
MC = MCw/(1−MCw)
MCw=MC/(1+MC).
Removal of Moisture Content
During carbonization the free water is driven off once the
temperature reaches ≈110°C,
while the bound water requires a temperature of ≈150°C.
The heat energy required to remove moisture from wood is the
latent heat of evaporation, hfg, for the
relevant ambient temperature. At a typical outdoor temperature of 15°C
this is 2613kJ/kg(water).
The energy used for moisture removal per unit dry mass of
the wood is given by MC×hfg.
Burning wood involves chemical reactions producing carbon
dioxide and water, for example for cellulose:
C6H10O5 + 6O2 → 6CO2
+ 5H20.
The calorific value of wood, Zd,
is the heat energy released by burning unit mass of dry (0%MC) wood. This is
typically in the region 20000-22000kJ/kg.
The fraction of combustion energy used to evaporate the
moisture content of wood when it is burnt, which is the same as the fraction of
the wood charge which needs to be burnt to dry out the charge during
carbonization, is given by:
MC×hfg/Zd
Practical calorific values for wood may be quoted on a wet
basis where:
Zwet = (Zd−MC×hfg)/(1+MC),
Zd = Zwet(1+MC)+MC×hfg.
Typical moisture contents and corresponding fractions of
wood charge burnt are set out in Table 3 below:
Table 3 - Wood moisture content &
evaporation energy
|
MC (dry basis) |
MC wet basis |
fraction of wood charge to dry1 |
Energy required (per kg of dry wood) |
green softwood (max) |
200% |
67% |
26% |
5226kJ/kg |
green hardwood (max) |
100% |
50% |
13% |
2613kJ/kg |
green wood (min) |
45% |
31% |
5.9% |
1176kJ/kg |
fibre saturation point (typ.) |
28% |
22% |
3.7% |
732kJ/kg |
air-dried wood (typical outdoor seasoned) |
20% |
17% |
2.6% |
522kJ/kg |
1. Calorific value 0%MC wood,
20000kJ/kg. Ambient temperature, 15°C.
Pyrolysis
Once the temperature of the wood reaches about 250°C the
pyrolysis reactions start, first with the hemicellulose breaking down, followed
by the cellulose above 300°C, and then lignin above 320°C. The volatile products
of pyrolysis include combustible gases carbon monoxide (CO), methane (CH4),
and hydrogen (H2), plus carbon dioxide (CO2) and steam (H2O),
together with smaller quantities of other hydrocarbons, oils, tars, acetic
acid, and methanol.
At the lower at of the temperature range for pyrolysis,
around 300°C,
the pyrolysis tends to be incomplete, resulting in a higher yield of low
quality impure charcoal. The proportion of carbon in the wood retained is
higher, but only a fraction of this is pure carbon with the majority still in
un-decomposed wood compounds or in partial breakdown products with low volatility
such as oils and tars.
As the temperature is increased towards 400°C
more complete pyrolysis occurs resulting in a lower charcoal yield and
proportion of wood carbon retained, but the charcoal becomes mostly pure
carbon. This is probably because the lower volatility breakdown products can
evaporate at the higher temperature or possibly breakdown further.
Yield and Conversion Efficiency
The proportion of the carbon content of the wood which is
converted to charcoal can be termed the carbon conversion efficiency. The yield
is defined as the ratio of the mass of charcoal produced to the dry mass of the
wood charge. If the carbon conversion efficiency is 100%, the yield equals the
proportion of carbon in dry wood, i.e. ≈50%.
The main chemical reaction and energy release in burning
charcoal is:
C + O2 → CO2 +
393500kJ/kmol.
The atomic mass of carbon is 12kg/kmol so the energy
released = 32792kJ/kg.
This is very close to measured calorific values of charcoal
which are in the range 29770-33200kJ/kg. It is also similar to fossil coal at
30MJ/kg.
The ratio of the energy available from burning the charcoal
produced to the energy from the wood charge if it had been burnt directly is
termed the energy yield.
If 100% of the carbon content of the wood could be converted
to charcoal (e.g. for the cellulose component: C6H10O5
→ 6C + 5H2O), the energy available from burning the charcoal produced
per unit dry mass of original wood would be given by:
32792kJ/kg × fraction of carbon in original wood
= 32792kJ/kg×50% = 16396kJ/kg.
For a calorific value of the original wood of 20000kJ/kg,
the theoretical maximum efficiency of energy conversion is 16396/20000 = 82%.
In this case the energy difference of 20000−16396 = 3604kJ/kg is most likely
released in the form of heat, making the reaction exothermic once the moisture
has evaporated.
In practice charcoal production never reaches 100% carbon
conversion and consequently yield and energy conversion figures are
proportionately less as shown in Table 4 below.
Table 4 - Carbonization Efficiency
|
Carbon conversion efficiency |
Yield |
Energy yield |
theoretical |
100% |
50% |
82% |
retorts with waste recovery |
73-85% |
37-43% |
60-70% |
high efficiency kiln |
66% |
33% |
54% |
metal kiln |
27% |
13% |
22% |
The additional energy difference is accounted for by
practical radiation and convection heat losses from the kiln and the calorific value
of the combustible volatile products of carbonization vented from the kiln.
Torrefaction
If the temperature of the wood only reaches about 250°C,
only partial carbonization will occur. The moisture content will be removed but
pyrolysis will be quite limited. This process is termed torrefaction, and the
result is called torrefied wood. The mass yield is considerably higher than
when making charcoal, as is the energy yield. However, the calorific value of
torrefied wood is much lower than that of charcoal, being only slightly higher
than that of 0%MC wood.
Bibliography
Charcoal Production, A Handbook,
A.C. Hollingdale, R. Krishnan, and A.P. Robinson of Natural Resources Institute,
2nd Ed, ©1999, Eco-logic books, ISBN 1-899233-05-9.
British Woodland Produce, J.R. Aaron and E.G. Richards,
©1990, Stobart Davies, ISBN 0-85442-047-9.
Woodlands, a practical handbook, Elizabeth Agate,
3rd Ed, ©1980-2002, BTCV, ISBN 0-946752-33-8.
Understanding Wood, a craftsman’s guide to wood technology,
R. Bruce Hoadley,
2nd Ed, ©2000, The Taunton Press, ISBN 1-56158-358-8.