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.



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