Previous Page Table of Contents Next Page


Chapter 6. Charcoal utilisation and marketing


6.1 Charcoal as household fuel
6.2 Charcoal as fuel for industry
6.3 Charcoal in metal extraction
6.4 Activated charcoal
6.5 Speciality markets
6.6 Charcoal for producer gas
6.7 Summary of industrial markets
6.8 Pyroligneous acid as a fuel
6.9 Utilization of by-products from hardwood carbonisation
6.10 Using retort or converter gas
6.11 Synopsis of major usages for charcoal and by-products


The earliest industrial, use of charcoal, more than four thousand years ago, was as a reductant for iron smelting to change iron oxide into metallic iron. But charcoal was already well known as a high grade smokeless fuel for cooking and domestic heating. With the emergence of industrial society as we know it today in the middle of the nineteenth centry new and expanding uses for charcoal in industry opened up to the charcoal maker at the same time as traditional industrial uses such as iron smelting began to decline with the widespread use of coke from coal as the principal reductant of the iron and steel industry.

Today two distinct markets for charcoal are recognised, the industrial and the domestic ones.

6.1 Charcoal as household fuel


6.1.1 Lump charcoal
6.1.2 Charcoal briquettes


Charcoal in some countries is the principal fuel for preparing food.

In some others the price has climbed so high that only the wealthy can afford to use it everyday. It is this growing shortage and the desire to help alleviate it that is the motive behind the preparation of this manual. (5, 15).

In industrial countries charcoal is no longer the main domestic cooking fuel but has become a symbol of an affluent lifestyle through its use in leisure activities as a fuel in open-air barbecues for cooking meat.

6.1.1 Lump charcoal

Ordinary lump charcoal is still the consumer's preferred form of charcoal since it is easy to handle and to ignite. It is sold by volume or by weight. Price is by far the most important reason for choosing one brand over another but users are becoming aware that there are differences in calorific value and ease of combustion between various types of lump charcoal.

The future trend in charcoal used domestically is difficult to forecast with precision due to inadequate statistics, especially in developing countries. (7). But the steady growth of population and the rising price of oil and gas based energy causes the demand for charcoal in the developing world to expand constantly.

6.1.2 Charcoal briquettes


6.1.2.1 Making charcoal briquettes


Charcoal briquettes appeared on the markets of developed countries as a serious alternative to lump charcoal in the early fifties with the development of methods for producing fine charcoal from sawdust and bark on a large scale (22).

Charcoal briquettes are composed of three main components - charcoal, binder and filler or burn-rate controller. This latter lowers production cost and enables the burning rate of the fuel to be slowed down compared to lump charcoal which is supposed to be advantageous for some kinds of cooking. But the net heat output depends on the amount of charcoal the briquette contains.

The most important advantages of the briquette are its relative freedom from dust and its ease of handling. Briquettes appear on the market in a wide range of shapes and sizes: oblong, egg-shaped, hexagonal and pillow shaped. The latter seems to be the most accepted with an edge length of about. 50 mm and thickness of 25 mm approximately. The standard properties of briquettes are listed below.

Charcoal Briquette Specifications

 

Weight Percentage

Lump Charcoal

Briquettes

Without Filler

With Filler

Ash

%

3-4

8

25

Moisture

%

5

5

5

Carbon

%

Balance

Balance

Balance

Volatiles

%

10-15

10-15

10-15

Binder

%

-

10

10

Calorific

%

Value

kJ/Kg

28 000

25 000

22 000

6.1.2.1 Making charcoal briquettes

The raw materials for charcoal briquettes are charcoal in the form of fines, binder and filler.

Fine charcoal is used as it is unsaleable in this form and has a low price. The binder is the costly item. Filler is added to reduce costs especially as briquettes are sold by weight rather than volume. Claims are made that filler also helps to control the burning rate of the briquette.

Photo. 8. Wood residues transformed into charcoal briquettes

Photo. 9. Charcoal briquettes automatic packing device. (Sweden)

Photo. 10. Simple technology for charcoal briquettes production. (Thailand)

Photo. 11. Charcoal briquettes produced using starch as binder

The fine charcoal may be specially made from feedstock which can only produce powdered charcoal (22) e.g. bark or it can be residual fines from conventional wood charcoal making operations (3).

The usual binder is starch from any source, whichever is the cheapest. It doesn't have to be an edible grade. Although other binders have been used such as china clay and molasses these do not produce as satisfactory a briquette as starch. For special briquetting operation where the binder is to be carbonised later as in electrode manufacture pitch and tar can be used. Binders which produce an objectionable odour or smoke on burning are not acceptable for the domestic market. The binding agent also needs to be fairly resistant to fermentation and bacterial attack during storage to meet market fluctuations. All these considerations tend to favour starch as the best all-round binder.

The filler if used is selected on price - it has to cost less by weight than charcoal, be not abrasive to the machinery and free of objectionable odour on burning. Calcium carbonate in powder form fits the requirements. Sources are: ground limestone, chalk and ground shells.

Flame colourants and odour producing material such as hardwood sawdust have been used in order to obtain a distinctive product for high priced markets.

The manufacturing process is as follows:

First the charcoal must be pulverised and screened to reduce it to a uniform size and remove gross impurities. Hammer mills are used fed from hoppers via automatic feeding devices. The screen analysis of the ground charcoal is the coarsest which will give a satisfactory briquette with minimum binder usage.

The ground charcoal is next mixed with the binder and filler, if used, plus any other additives. If starch is the binder it must first be cooked (gelatinised) with hot water before adding to the mixer. A ribbon type mixer is normal and proper mixing is essential to reduce binder consumption to the minimum.

The mixture then passes to the briquetting press. Photo 7 shows a typical press installation. The accepted type of press is the roller type adjustable for speed and pressure. The press illustrated has moulds for making pillow shaped briquettes.

Photo. 7. Charcoal Briquetting Press

The wet pressed briquettes are next dried to complete the binding operation. When the water is dried out from the gelatinised starch it sticks the charcoal particles together to form a stable briquette which will retain its form right up to the point of combustion. The briquettes usually contain about 10% of binder and about 30% of water before drying down to about 5% moisture content. The normal dryer is a continuous tunnel dryer which carries the briquettes through without allowing them to tumble. Heat for drying out the water is a significant operating cost. (Fig. 12)

When the briquettes are cool they can be packed for the market in paper or plastic sacks of about 5 kg. to take account of the seasonal nature of the briquette market in industrial countries there must be provision for storing the briquettes in hoppers awaiting packaging. This adds considerably to operating costs (3) and is one of the principal obstacles to success in briquetting operations aimed at using up waste charcoal fines.

When considering the profitability of charcoal briquetting the retailing component of the marketing chain must not be overlooked. It can easily amount to 30% of total costs as a minimum and in the developed world will be the final determinant of commercial success or failure. Charcoal briquettes in this market are not a need but a luxury.

6.2 Charcoal as fuel for industry


6.2.1 Specifications


In the developed world charcoal is an almost indispensable industrial commodity, especially in metallurgy and as an adsorbent.

With the development of the chemical industry and the increasing legislation concerned with the control of the environment, the application of charcoal for purification of industrial wastes has increased markedly.

Whereas in the barbecue fuel market charcoal has little competition, in almost all other applications charcoal could be substituted by coal, coke, petroleum coke or lignite. The advantages or charcoal depend on six significant properties which account for its continued use in industry.

- low sulphur content
- high ration of carbon to ash
- relatively few and unreactive inorganic impurities
- stable pore structure with high surface area
- good reduction ability
- almost smokeless

Fig. 12 Continuous Carbonisation with Hot Gases Recovery and Briquettes Manufacture - Schematic Outline

Up to 1960, charcoal was widely used by the chemical industry for the production of carbon disulphide and sodium cyanide.

Although these markets have declined, they were off-set by the enlarging demand for reducing and adsorbent agents.

Some applications of charcoal in various industries are as follows:

Chemical industry

- manufacture of carbon disulphide, sodium cyanide and carbides.

Metallurgy

- smelting and sintering iron ores, production of ferro-silicon and pure silicon, case hardening of steel, purification agent in smelting non-ferrous metals, fuel in foundry cupolas, electrodes.

Cement industry

- as a fuel, (34).

Activated carbon and filter industry

- water purification, dechlorination, gas purification, solvent recovery;
- waste water treatment, cigarette filters.

Gas generator

- producer gas for vehicles and carbonation of soft drinks.

The chemical and activated carbon industries prefer lump charcoal. This is partly due to their process requirements. Fine charcoal particles behave more reactively, but airborne losses in processing make fines an undesirable raw material. Fines usually have a higher ash content than lump charcoal and may be more contaminated generally.

Therefore, the market for charcoal fines and powder are restricted in industry to processes where cells for powdered material such as in sintering processes and case hardening of steel.

6.2.1 Specifications

Each application has its own charcoal specification and must be studied to determine both present process requirements and future modifications due to possible alterations in the process technology.

Such alterations have frequently had effects on the charcoal industry in the past and led to changes in production technique or to shut-down of plants. Charcoal quotations usually specify the content of fixed carbon, ash, volatiles, density, bulk density, moisture and the sieve analysis.

It is not possible to list here all specifications and variations. They can be determined by a careful market study, which should be carried out before engaging in industrial charcoal production.

6.3 Charcoal in metal extraction

Since iron was first made by man, charcoal has been used widely as a reductant.

Today the charcoal based iron industry still exists in a number of countries and continues its expansion and modernization (15, 28, 29).

The focus, however, has moved away from developed countries. Today, Brazil's charcoal based pig iron industry is considered by far the biggest in the world. Malaysia and Argentina are other significant producers.

Charcoal has strong reducing properties. When heated with metallic ores containing oxides and sulphides, the carbon combines readily with oxygen and sulphur, thus, facilitating metal extraction.

Most of the charcoal used in blast furnaces is made from hardwood species (29). Although charcoal is generally acknowledged to be a better reductant than coke, there are practical difficulties in operating very large blast furnaces with charcoal in the large iron and steel mills.

It is only in countries with extensive forests and deficient in coking coal that the use of charcoal for iron-smelting is likely to be profitable.

6.4 Activated charcoal


6.4.1 Introduction
6.4.2 Production of activated charcoal
6.4.3 Applications
6.4.4 Manufacturing processes
6.4.5 Specifications


6.4.1 Introduction

The use of charcoal for producing activated carbon is fairly new when compared to its utilisation in metallurgy or the chemical industry. The first markets started to develop in Europe around the beginning of this century.

Activated carbons are carbons which have undergone an intricate treatment to increase their adsorption properties. (17, 18, 26, 30, 32).

Activated carbons are available in powdered, granular and pelletised form and are used in liquid and gas phase adsorption processes.

More than seventy types of activated carbons are currently marketed.

Although the surface area of the pore structure and the adsorption capacity of all activated carbons are interrelated, the size of the surface area is not the only determinant on the adsorption capacity of a given carbon for a specific purpose.

In other words, activated carbons with large total surface areas but with a microporous structure may be effective in removing slight odour causing impurities from gases, but ineffective in adsorbing large colour-forming compounds from solutions.

This may explain the great number of types, grades, and shapes of activated carbon available.

6.4.2 Production of activated charcoal

Estimated production capacities for activated charcoal 1979

North America

160 000 t

western Europe

105 000 t

Eastern Europe

20 000 t

U.S.S.R.

70 000 t

Japan

80 000 t

Total 435 000 t

Charcoal was initially the only raw material for producing activated carbon, but it has been partly replaced due to price considerations and the limited availability of charcoal, by other carbon materials such as coals, lignite, petroleum coke, peat and moss.

Experience has shown that there are no basic differences in the quality of activated carbons made from other raw materials, except that, in the gas/vapour applications, charcoal-based activated carbon is superior. Activated carbon production is a low yield process in relationship to the raw material input, whether or not charcoal is the base.

6.4.3 Applications

- Treatment of liquids

Drinking water purification, municipal waste water and industrial waste, water treatment plants, swimming pools and acquaria are examples.

Purification of fats, oils, beverages, water purification in breweries, cleaning of bottles and tanks in wineries, cleaning of tanks for insecticides and pesticides, spraying, cleaning of electroplating baths, dry cleaning. Decolourization of cane and beet sugar solutions, vitamin solutions and pharmaceuticals and high fructose syrup are also important uses.

- Treatment of Gases and Vapours

Important applications are:-

Purification of exhaust emissions of recirculated air purification. Recovery of solvent from printing machines and processes where highly volatile matter is continuously being released. Reduction of toxic and harmful vapour levels and objectionable odours in air. Air purifiers for commercial and domestic kitchens. Gas masks for military and civilian purposes.

- Miscellaneous Uses

Some of these myriaduses include pharmaceutical, cigarette filters, catalysts for chemical processes, support for platinum and palladium catalysts, food additives, depolariser in electric batteries, additives in rubber tyres, evaporation control systems and evaporative air coolers.

Legislation on water and air pollution control in industrialised countries, particularly since 1977, has been a great stimulant to the activated carbon market and this trend is expected to continue.

6.4.4 Manufacturing processes

Ordinary commercial charcoal has very limited ability to adsorb substances in the liquid or gas phase. To give charcoal this property it must first be activated by removing the tarry materials which block the structure of the pure carbon skeleton of the charcoal. When this is done the surface area of the porous carbon skeleton is increased literally millions of times providing equally large numbers of sites where molecules of other substances can be 'held' or adsorbed and thus removed from gases or from liquids in which the treated charcoal is placed. Charcoal processed in this way is called activated charcoal. Charcoal is not the only type of carbon used for activation but it is an important raw material for activated carbons.

The activated carbon industry uses many variations in basic processing methods to achieve activated carbons having optimum properties for the various end uses. These variations mainly relate to the final stages of processing rather than the basic activation process which is usually nowadays carried out by heating the charcoal to a temperature of about 800° C in an atmosphere of superheated steam which permits the breakdown and removal of the tars blocking the microfine structure of the charcoal. Fig. 13 shows the general features of the activation process.

There are several kinds of equipment used which depend mainly on the volume of charcoal to be processed. For large throughputs the multiple hearth roasting furnace as used for producing charcoal from bark and sawdust on a large scale is often used. Smaller volumes are often processed in a vertical furnace in which the charcoal cascades over refractory baffles which allow the charcoal to be fully exposed to the atmosphere of the activating furnace. The principle, whatever system is used is the same: the charcoal is heated and stirred in an atmosphere of superheated steam to burn out the tars. Although in principle other gases can be used steam is the most widely used.

The hot charcoal leaving the furnace is allowed to cool in steel drums or containers until it reaches room temperatures. The charcoal which is now about the same size as sand grains is finely ground to reveal the active structure to the maximum extent. At the same time specialised treatment is given to the activated carbon to adapt it for its particular use. For example activated charcoal intended for purifying vegetable oils is treated differently to charcoal to be used for decolourising wines. All these processes are kept as confidential as possible by the factory to improve its competitive position in the market.

Exact details of these processes are not relevant to the charcoal producer who is looking to the activated carbon producer as a buyer for his charcoal. Experience has shown over many years that charcoal is a good raw material for activation. Providing it has a low ash and a low volatile content almost any charcoal is suitable providing it is available in dependable quantity and quality. There is one notable exception and that is the charcoal used to make activated carbon for purifying gases as for solvent recovery in printing and related processes and in gas masks for military and civilian use.

Experience has shown that for gaseous adsorbtion processes the most suitable charcoal is that from coconut shells since the high strength combined with a fine porous structure of this charcoal allows it to be recycled many times in the equipment without losing its granular structure and impeding the gas flow.

Fig. 13 Manufacture of Activated Charcoal - Schematic Outline

6.4.5 Specifications

The charcoal which the activated carbon producer buys will be to a specification derived from tests in the plant to determine limits of acceptability.

The useability of wood charcoal depends on its low ash content and availability in consistent quality.

Exceptionally good activated carbons can be produced with charcoal made from coconut shells, hardwood and even, sawdust and wood waste. However, bark is of no use.

Each activated carbon producer will set his individual standard determined by his own production process.

However, the requirements will not vary much whether the finished activated carbon is made for liquid, gases and vapours or other applications.

Although the charcoal from a new supplier offered to the activated carbon producer may meet all criteria specified, the purchaser is not likely to accept it before testing its behaviour in a pilot plant and testing of the final product.

These tests are complex and aimed at determining the adsorption capacity of the finished product on model substances. The industry has developed such indicators as the 'molasses figure', 'methylene-blue value', "benezene, isotherms', etc.

The charcoal supplier does not need to become involved in these tests nor does he usually have the facilities and skilled staff to do so.

Listed below are average requirements for charcoal for activated carbon production.

Fixed carbon

82% minimum

Ash

4%

Volatiles

10%

Moisture

4%

pH 1/

4-10

1/- pH refers to an acidity test of a water extract of the charcoal.

6.5 Speciality markets

Horticulture

Charcoal is used in different grades as a top dressing for the improvement of lawns and bowling greens. These top dressings act as mulch and also provide valuable trace elements and sweeten the soil.

Pottery mixtures used in nurseries often contain fine charcoal.

Poultry and Animal Feeds

These are sometimes supplemented with charcoal fines to control certain diseases.

Pharmaceuticals

Charcoal is used for controlling infections of the digestive tract.

Pigments

Vegetable (charcoal) blacks are dead black and of great strength.

6.6 Charcoal for producer gas

A promising means to improve the fuel energy situation in many developing countries is to use wood or charcoal to produce gas as fuel for diesel and gasoline engines.

Producer gas as it is called is made by passing air through a bed of granular carbonaceous fuel contained in an air tight shell. Most producer gas is made from coal or coke but wood and charcoal can be used. Charcoal in fact is an optimum fuel as it is low in ash and produces a clean gas free of tars. As well its energy content is comparable with good quality coal.

The air passed through the bed first contacts the burning layer of charcoal in the bottom of the producer and the charcoal burns to produce carbon dioxide. The carbon dioxide is reduced by contact with hot charcoal next to the combustion zone and is reduced to carbon monoxide. If water vapour is present it is also decomposed by the hot charcoal to form hydrogen and carbon monoxide. The hot gas is cooled as it passes further up the fuel bed and the gas emerging is a combustible mixture of carbon monoxide, carbon dioxide, some hydrogen and the residual nitrogen from the air. The gas burns readily but has only about one fifth the calorific value of natural gas. Despite this it is a useful fuel for gasoline and diesel engines especially the latter.

To be economic as a replacement for oil fuel the following cost relationships must apply. If diesel or gasoline costs about US $0.50 per litre of US $500 per tonne and taking into account the relative efficiency of use of the fuels charcoal must cost at the side of the gas producer about US $300 per tonne. In most developing countries charcoal costs less than this but as there are some disadvantages in use of charcoal compared to the convenience of a liquid fuel there needs to be a fair margin to make it worthwhile to install the equipment to make use of charcoal. Amortization costs for the producer equipment are not serious if the equipment is in constant use but can be uneconomic if the fuel is used sporadically. For best results the engines used should be specially designed to be used with producer gas. This is impractical because of the limited market.

Producer gas is probably most successful as a partial replacement for diesel fuel. The producer gas is drawn in with the air and it is possible to get good running with a replacement up to 70% of the diesel fuel, a worthwhile saving and especially suited for stationary engines.

Use in vehicles is practical but more complicated. The extra weight of the producer must be carried and refuelling points with charcoal must be available. Acceleration compared with liquid fuels is sluggish and this limits the use of charcoal to public transport and delivery vehicles where slow acceleration can be accepted without loss of face.

Although the advantage of replacing some imported petroleum with producer gas is fairly easy to see, in practice it is very difficult for users to give up the convenience of liquid fuels. There are some notable examples to be found of successful local replacement of some liquid fuels by producer gas. However one must still wait and see how far this development will be able to spread.

6.7 Summary of industrial markets

Chemical Industry

- carbon disulphide
- sodium cyanide
- metallic carbides
- silicon carbide

Iron and Steel

- iron smelting
- high purity irons
- ferro silicon
- silicon
- sintering and ore benefication

Metallurgy

- foundry operations
- copper smelting
- tin smelting
- specialised metal smelting and casting
- electric furnace electrodes
- sintering operations

Activated Carbons

- water purification
- pollution control
- gas purification
- solvent recovery
- distillation columns
- ageing of distilled spirits
- pharmaceuticals
- food industry
- softdrinks
- catalyst
- wine processing
- electric batteries
- cigarette filters
- sugar industry

Gas Generators

- gas for motor vehicles
- gas for stationary engines
- gas for carbonated drinks

Miscellaneous

- animal feed additives
- soil conditioners
- tobacco curing
- fruit drying
- arts and printing industry
- fireworks
- black powder explosives

6.8 Pyroligneous acid as a fuel

The condensate produced from distillation of wood is called pyroligneous acid. Water is the main constituent ranging from 20 to 80% depending on the moisture content of the wood being carbonised and the stage in carbonisation at which the sample is collected. The other components comprise water insoluble and water soluble tar, acetic and related acids, methanol, acetone and small quantities of complex esters and similar compounds which may have uses in flavouring.

If the condensate is heated to near the boiling point of water it is possible to ignite it and it can be burned as a fuel in a similar way to oil fuel. However its net calorific value is rather low since most of the heat energy it contains is used to evaporate the contained water. The heat in this evaporated water can be recovered by allowing the vapour in the flue gases to condense but only at a temperature below 100°C. Consequently the use of the condensate as liquid fuel is hardly practical even if the insoluble tar is burned as well.

A proven process for recovering the heat content of the condensables is to burn them as they emerge as hot vapour from the retort. Burning the material this way eliminates the polluting effects which accompany recovery of liquid pyroligneous acid since all the non-water components are converted to water and carbon dioxide and pass harmlessly into the atmosphere as ordinary flue gas. Pyroligneous acid is a highly corrosive substance and by avoiding handling it in the liquid state this problem is also eliminated.

The rise in oil prices throughout the world in the seventies encouraged interest in the possibility of using pyroligneous acid in liquid form as a substitute for fuel oil. To aid promotion of various schemes of dubious economic worth there was a tendency to rename 'pyroligneous acid to 'pyrolysis oil' in order to make this noxious liquid seem more palatable and useful.

The correct way to take advantage of the heat content captured in wood as a result of photosynthesis is to burn it as such in properly designed furnaces. Alternatively where a technically complex system of carbonising wood has been installed all the condensable and non-condensable off-gases from the retorts should be burned in a boiler or similar industrial furnace as described above.

6.9 Utilization of by-products from hardwood carbonisation


6.9.1 Introduction
6.9.2 Pyroligneous acid
6.9.3 yield of Pyroligneous acid
6.9.4 Refining pyroligneous acid


6.9.1 Introduction

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. Whereever 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 carbonisation process more efficient. The economics, however, appear to be rather marginal but since recovery of by-products does reduce atmospheric pollution from wood carbonisation, the combined benefit makes it worthwhile having a close look at the possibilities. (2), (21).

When wood is heated above 270 C it begins a process of decomposition called carbonisation. 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 final 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 heat is evolved. This is the well known exothermic reaction which takes place during charcoal burning. At this stage evolution of the by-products of wood carbonisation 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 cooled condensers to yield pyroligneous acid. The non-condensable wood gas passes on through the condensers 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 gross calorific value of about 10.8 MJoules per m³ (290 BTU/cu.ft.) i.e. about one third the value of natural gas.

6.9.2 Pyroligneous acid

Pyroligneous 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 pyroligneous 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.

6.9.3 yield of Pyroligneous acid

The economies 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

"

Acetone and methyl acetone

8

"

Soluble tars

190

"

Insoluble tars

50

"

6.9.4 Refining pyroligneous acid

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 US $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 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 50% 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 huge petro-chemical 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 existing wood distillation by-products recovery plants may be marginally profitable, the construction of new by-product recovery plants seems unlikely. The future will probably see some increased recovery of tar and the use of the off-gases and vapours from carbonisation 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 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 water 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.

6.10 Using retort or converter gas

This gas is given off by the decomposing wood in the retort or other conversion equipment. It is called non-condensable gas since it passes, without condensing, the water cooled condensers which collect the tar and pyroligneous acid.

The composition of this gas varies within wide limits. The gas given off from conventional kilns changes composition as carbonisation progresses and is diluted by the products of combustion of the wood inside the kiln which is being burned to dry, heat up and carbonise the remainder. On the other hand the gas from a continuous retort system is more uniform in composition and heat content. It changes as carbonisation proceeds and with operating temperature. The off-gas from rinsing gas retorts will also be diluted by the inert rinsing gas which lowers the total calorific value of the gas.

Thus it is difficult to give exact figures for gas composition and heating value. A typical retort gas could have the following composition by volume:- methane 17%, hydrogen 2%, carbon monoxide 23%, carbon dioxide 38%, oxygen 2% and nitrogen 18%. Offgas from rinsing gas retorts could be diluted more than its own volume with inert rinsing gas.

The calorific value varies widely. Undiluted retort gas has a calorific value of about 11 MJ/m³ but would be less than half this when diluted with inert rinsing gas being approximately equal to producer gas. The gas will burn easily but the heat output is limited. For every tonne of charcoal produced about 0.25 tonnes of non-condensable gas is produced having a heat content about 10% of that of the charcoal produced.

Recovery of the heat is worthwhile if it can be carried out simply and if the pyroligneous acid and tars are burned at the same time the total recoverable heat is about 20% of the heat content of the charcoal produced. (4)

The simplest way of using the heat in the gas is to divert it back and burn it under the retorts themselves. (15) Alternatively it can be burned under a steam boiler and the steam used as such or in a large installation used to generate power. The gas can also be burned in a wood predryer in much the same way as in a steam boiler. Best results are obtained when the tar and pyroligneous acid are burned at the same time with the gas. It is essential to burn the gas close to the retort system because if the gas cools too much the flues become blocked with condensed tar requiring costly shutdown to clean the gas mains.

6.11 Synopsis of major usages for charcoal and by-products

Product

Raw Material

Application

Charcoal, lump

hard wood, soft wood

activated carbon, ferro-silicon, cooking, metal working, sodium cyanide, carbon disulfide, iron and steel, silicon.

Charcoal, granular

charcoal. lump

activated carbon, additive to animal food, filling compound for bottled gas, steel hardening compound.

Charcoal dust

charcoal, lump

activated carbon, lining of moulds in metal foundries, production of briquettes, cementation granulate, pyrotechnics, explosives, electrodes, batteries.

Soluble or pyrolytic tars

hard wood, soft wood, agricultural wastes

fuel for steam boiler, furnaces, metallurgy, fire brick making, raw material for chemical industry, electrodes.

Wood gas

hard wood, soft wood, agricultural wastes

heating gas for all types of operations, gas engines.

Wood vinegar

hard wood

food preservation and flavouring of meat and smoked fish, perfume and aroma industry.

Wood tar

hard wood

rope industry, veterinary medicine, pitch, creosote.

Crude methanol

wood alcohol

methyl acetate, solvent, denaturant.

Solvent

wood alcohol

cellulose esters and agglutinants, synthetics, lacquers.

Methyl formate

crude wood vinegar and crude methanol

cellulose esters and agglutinants, synthetics, lacquers.

Methyl acetate

crude wood vinegar and crude methanol

cellulose esters and agglutinants, synthetics, lacquers.

Acetic acid

crude wood acid

chemical, pharmaceutical, food, rayon, textile and film industries, vinegar.

Propionic acid

crude wood acid

pharmaceuticals, flavour and fragrances.

Butyric acid

crude wood acid

pharmaceutical and perfume industries.


Previous Page Top of Page Next Page