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.
12.1.1. The yield of pyroligneous acid
12.1.2. Refining pyroligneous acid
Proligneous acid is the name of the crude condensate and consists mainly of water. It is a highly polluting noxious corrosive liquid which must be either worked up properly to produce by-products for sale, or burned with the help of other fuel such as wood or wood gas to dispose of it.
The non-water component consists of wood tars, both water soluble and insoluble, acetic acid, methanol, acetone and other complex chemicals in small amounts. If left to stand, the proligneous acid separates into two layers comprising the water insoluble tar and a watery layer containing the remaining chemicals. Recovery of the water insoluble tar, often called wood or Stockholm tar, is simple - it is merely decanted from the water phase. This wood tar has uses as a veterinary antiseptic, a preservative for wood, a caulking agent, and as a substitute for road tar. Generally the quantity available and its price and physical properties make it a poor substitute for tar derived from the oil and coal industry for use in road-making. It does, however, have limited markets as a speciality industrial chemical. If it cannot be sold it can be burned as liquid fuel. One ton of dry wood, however, only produces about 40 kg of tar, i.e. about a 4% yield.
The water layer contains water soluble tars which are complex tarry chemicals, acetic acid, methanol, acetone and methyl acetone and small amounts of more complex acids and other substances.
The economics of by-product recovery depend on the yield of the more valuable components, especially the acetic acid, but also the mixture of methanol and acetone. Yield varies greatly with the kind of wood distilled. Wood with a high pentosan content such as European beech (Fagus spp.) gives a high yield of acid, eucalyptus wood gives low to intermediate yields. The yields from wood distillation quoted by various authors vary widely. Not only the kind of wood but the type of plant, its condensing efficiency, efficiency of the by-product refinery and so on all effect yields. Therefore it is of the utmost importance before investing in by-product recovery to be quite sure what sort of yields can be expected. For example, a plant in Europe working with beech and close to good markets for pure acetic acid may be economic. But a plant working with eucalyptus or mixed tropical hardwood far from markets for its products and obtaining only about half the yield of acid may be quite uneconomic. Therefore, proper full scale tests are necessary to find out what the yields are likely to be from the actual wood which will be carbonised. Careful market and plant design studies are essential. For guidance, the following yields can be taken as typical of northern hemisphere deciduous hardwoods:
Yield per ton (1 000 kg) of air dry wood
|
Acetic acid |
50 kg |
|
Methanol |
16 kg |
|
Acetone and methyl acetone |
8 kg |
|
Soluble tars |
190 kg |
|
Insoluble tars |
50 kg |
To recover saleable by-products from the pyroligneous acid a refinery somewhat similar to a small oil refinery but built of stainless steel or copper is required. The cost would nowadays be of the order of U.S.$ 5-10 million but it is rather difficult to give a precise figure since such a refinery must be specially designed and built. They are not available as a stock item.
The whole process resembles quite closely in plant and technology to an oil refinery but on a very small scale. Unlike oil refining, however, which uses a feedstock which is theoretically 100% saleable, the refining of pyroligneous acid involves throwing away about 90-95% of the feedstock as contaminated, unsaleable water. The whole of the pyroligneous acid, less the insoluble tar, must be evaporated to separate the methanol and acetic acid from the soluble tars. Evaporation of water is costly as it requires a large fuel input. Furthermore, the acid products are very corrosive and the plant must be built from copper or preferably stainless steel, adding greatly to its cost. The products are sold in competition with products of the hugh petrochemical industry and competition is therefore difficult. On the credit side, the quality of the acetic acid is high and it usually can be sold easily. But the distance from major markets reduces profitability. Although the continued operation of these existing wood distillation by-products recovery plants may be marginally profitably, the construction of new by-product recovery plants seems unlikely. The future will probably see some increased recovery of tar for sale and the use of the off gases and vapours from carbonization plants for heating of retorts and boilers. How to do this effectively without investment in costly plant, however, still remains largely an unsolved problem.
The crude condensed liquid is decanted to separate insoluble tar which is sold usually without further processing. The watery phase must now be processed to recover three saleable products: methanol acetone, acetic acid and soluble tar. The acetic acid is the most valuable. The liquor is distilled in a primary steam heated still to separate the methanol acetone and acetic acid from the soluble tars. The soluble tars remain in the bottom of the still and the vapours consisting mainly of methanol acetone, acetic acid and waste pass to a distillation column which separates crude 85% methanol containing acetone from the mixture of acetic acid and water. The crude methanol can be sold as a solvent.
The acetic acid is nowadays solvent extracted from the liquid phase using a solvent, usually ethyl acetate or ether. These solvents do not mix with water and dissolve or strip the acetic acid from the water phase, leaving only a trace of acetic acid in the water phase. After the recovery of any ethyl acetate or ether dissolved in the water phase, it is run to waste. It may still contain about 0.1% acetic acid. The ethyl acetate or ether solution of acetic acid (about 3%) must now be processed to recover the solvent for recycling and the acetic acid for sale. The solvent is distilled out in a fractionating column, the crude acetic acid (70%), freed of its solvent, is run from the base of the column and is purified by fractional distillation to 90% or more concentration, depending on market requirements. The solvent is recycled to extract more acetic acid from fresh feedstock. There is a small loss of solvent, which is topped up as needed.
The recovery for sale of some of the tars produced in carbonization is possible on a small scale and is now being carried out by some producers of charcoal.
The insoluble tars are so called because they separate as a distinct black tarry phase when the vapours evolved from the retort or kiln are condensed. The other phase in the condensate is mainly waste containing acetic acid, methanol, acetone and the so-called soluble tars which are complex tar-like compounds, which will mix with water and do not separate as a distinct phase. The insoluble tar is the product known commercially as Stockholm or wood tar. These tars are chemically complex but do contain definite phenolic compounds, particularly guiacol, which are useful antiseptics and preserving agents. Stockholm tar has uses in veterinary medicine, as a caulking agent in boat-building and as a wood preserving paint or mastic. Today in the developed world a variety of other substances are substituted for Stockholm tar. Nevertheless, in the developing world there can be markets for wood tar as a wood preserving paint and caulking compound. Some use as an antiseptic is also possible. Though this tar can be used as a road binder the small and sporadic amounts available and the low price and hugh quantities of road tar produced by the pit industry make this outlet unattractive. The price of wood tar at the point of charcoal production should be somewhat higher than road tar. Tar can be burned as a fuel but it is generally more reasonable to use wood wherever possible and collecting tar merely to burn it is hardly worth the effort required. Wood tar is more valuable for other uses.
Tar can be condensed usually wherever the vapours from the kiln pass through a metal flue. The heat is lost to the air through the metal wall of the flue and tar condenses on the inside surface. The flue must be inclined or preferably vertical to allow the tar to drain into a receiver, otherwise the build up of tar on the walls of the flue acts as an insulator and condensation practically ceases. A little acid water may condense at the same time but this is easily separated from the collected tar.
It is not feasible to condense tar from brick flues since their conductivity is too low to permit the tar to condense in any significant amount.
Metal (steel) flues are required and this calls for metal working skills and availability of suitable steel. The two types of kiln most suited for tar collection are the portable metal type and the Casamance kiln, or any other type fitted with a steel chimney. In all cases the chimneys have to be modified to allow condensed tar to drain into a receiver of some kind. Other types of carboniser are not generally modified to collect tar, either because the smoke does not exit through a flue, e.g. the pit system, or because the cost and trouble of modification is too great to make recovery of tar worthwhile.
The amount of tar which can be collected in practice is not large. In practice about 25-35 kg of tar can be collected from each ton of air dry wood. It is difficult to set a value but about US$ 0.50 per kg is a reasonable assumption.
