Chapter 12 - Recovery of by-products from hardwood carbonization
in FAO Simple Technologies for Charcoal Making, FAO FORESTRY PAPER 41 FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS Rome, 1987
Recovery of chemicals from the vapours given off when hardwood is converted to charcoal was once a flourishing industry. However, as soon as petrochemicals appeared on the scene, wood as a source of methanol, acetic acid, speciality tars and preservatives became uneconomic. Wherever charcoal is made the possibility of recovering by-products is discussed. Present high costs of petroleum are advanced as an argument. Unfortunately the price of wood rises correspondingly removing most of the price advantage. Although the outlook for recovery of by-product chemicals from wood distillation does not appear promising, there are possibilities of recovering tars and using the wood gas as fuel to assist in making the carbonization process more efficient. The economics, however, appear to be rather marginal but, since recovery of by-products does reduce atmospheric pollution from wood carbonization, the combined benefit makes it worthwhile having a close look at the possibilities in this direction.
When wood is heated above 270° C it begins a process of decomposition called carbonization. If air is absent the final product, since there is no oxygen present to react with the wood, is charcoal. If air, which contains oxygen, is present, the wood will catch fire and burn when it reaches a temperature of about 400-500°C and the fuel product is wood ash.
If wood is heated away from air, first the moisture is driven off and until this is complete, the wood temperature remains at about 100-110°C. When the wood is dry its temperature rises and at about 270°C it begins to spontaneously decompose and, at the same time, heat is evolved. This is the well known enothermic reaction which takes place in charcoal burning. At this stage evolution of the by-products of wood carbonization starts. These substances are given off gradually as the temperature rises and at about 450°C the evolution is complete. The solid residue, charcoal, is mainly carbon (about 70%) and small amounts of tarry substances which can be driven off or decomposed completely only by raising the temperature to above about 600°C.
In the common practice of charcoal burning using internal heating of the charged wood by burning a part of it, all the by-product vapours and gas escapes into the atmosphere as smoke. The by-products can be recovered by passing the off-gases through a series of water to yield so-called pyroligneous acid and the non-condensible wood gas passes on through the condenser and may be burned to provide heat. The wood gas is only useable as fuel and consists typically of 17% methane; 2% hydrogen; 23% carbon monoxide; 38% carbon dioxide; 2% oxygen and 18% nitrogen. It has a gas calorific value of about 10.8 MJoules per m³ (290 BTU/cu.ft.) i.e. about one third the value of natural gas.
Proligneous acid
Small scale recovery of tars