11.1. Properties of charcoal fines
11.2. The techniques of briquetting
11.3. Economics of briquetting
11.4. Briquetting as a cottage industry
11.5. Using fine charcoal without briquetting
Transport and handling charcoal to point of use produces fines which may amount to about 10% by weight in the most favourable cases and 20% or more in the worst ones. The more the charcoal is handled and the more transport stages, the more fines are produced.
Charcoal fines have a much lower purity than lump charcoal. The fines contain, in addition to charcoal, fragments, mineral sand and clay picked up from the earth and the surface of the fuelwood and its bark. The fine powdered charcoal produced from bark, twigs and leaves has a higher ash content than normal wood charcoal. Most of this undesired high ash material can be separated by screening the fines and rejecting undersize material passing, say, a 2 to 4 mm screen. This fine material may still contain more than 50% charcoal depending on the level of contamination but, nevertheless, it is difficult to find uses for it. Material retained on the screen will be mostly fragments of good charcoal and, after hammer-milling is suitable for briquetting. Fines cannot be burned by the usual simple charcoal burning methods and hence are more or less unsaleable. But if fines could be fully used, overall charcoal production would rise by 10 to 20%. Briquetting - turning fines into lumps of charcoal - seems an obvious answer. Unfortunately, experience has shown up to now that, though it is technically possible to briquette charcoal fines, the economics are not usually favourable, except where the price of lump charcoal is very high and the fines are available at a very low or zero cost.
Briquetting requires a binder to be mixed with the charcoal fines, a press to form the mixture into a cake or briquette which is then passed through a drying oven to cure or set it by drying out the water so that the briquette is strong enough to be used in the same burning apparatus as normal lump charcoal.
Charcoal is a material totally lacking plasticity and hence needs addition of a sticking or agglomerating material to enable a briquette to be formed. The binder should preferably be combustible, though a non-combustible binder effective at low concentrations can be suitable. Starch is preferred as a binder though it is usually expensive. Highly plastic clays are suitable providing not more than about 15% is used. Tar and pitch from coal distillation or from charcoal retorts have been used for special purpose briquettes but they must be carbonised again before use to form a properly bonded briquette. They are of good quality but costly to produce.
The press for briquetting must be well designed, strongly built and capable of agglomerating the mixture of charcoal and binder sufficiently for it to be handled through the curing or drying process. The output of briquettes must justify the capital and running costs of the machine. Briquetting machines for charcoal are usually costly precision machines capable of a high output. Brick making presses have been used but there do not appear to be any commercially effective, really low cost machines for this purpose. Charcoal is quite abrasive so that equipment for screening fines, grinding, mixing them with binder, Briquetting and so on must be abrasive-resistant and well designed.
The binders which have been tried are many but, as stated, the most common effective binder is starch. About 4-8% of starch made into paste with hot water is adequate. First, the fines are dried and screened. Undersized fines are rejected and oversized hammermilled. This powder is blended with the starch paste and fed to the briquetting press. The briquettes are dried in a continuous oven at about 80°C. The starch sets through loss of water, binding the charcoal into a briquette which can be handled and burned like ordinary lump charcoal in domestic stoves and grates. Generally briquettes are not suitable for use as industrial charcoal in blast furnaces and foundry cupolas, since the bond disintegrates on slight heating. For this briquettes bonded with tar or pitch and subsequently carbonised in charcoal furnaces to produce a metallurgical charcoal briquette of adequate crushing strength are needed. The cost is too high for the process to find industrial application in most countries.
It is possible to add material to aid combustion of briquettes such as waxes, sodium nitrate, and so on, during manufacture to give a more acceptable product. Also clay as a binder, silica, and so on, can be mixed with the fines to reduce the cost of the briquette. This, of course, lowers the calorific value and is merely a form of adulteration for which the user pays, though claims may be made that burning is improved. But well made briquettes are an acceptable, convenient product. The virtual absence of fines and dust and their uniformity are attractive for barbecue purposes. Generally they sell at around the same cost per kg as lump charcoal in high price markets and have more or less the same calorific value as commercial charcoal of 10-15% moisture content.
Successful briquette operations are found mostly in developed countries. An example is the industry based on carbonization of sawdust and bark in the southern U.S.A. using rotary multiple hearth furnaces which produce perhaps 25 to 50 tons of fine charcoal per day. When briquetted, this charcoal, intended for barbecues, can be sold in retail outlets. The furnace gases are burned to produce steam of electric power, thus transforming the waste sawdust and bark into two useful products, electric power and charcoal briquettes. Air pollution and waste disposal problems are minimised at the same time.
The cost of briquetting mainly depends on three factors. The cost of the fine charcoal delivered at the plant ready for processing, the cost of the binder and the incidence of capital costs. Fines which normally have little value, are usually reckoned to have zero cost to justify the briquette plant investment. However, this is not true because to supply the fines to the plant from sources, even close by, costs money. If the fines are not all derived from captive operations of the briquetting organization, it will be found that the price of purchased fines moves steadily upward as soon as the briquettes appear on the market. The binder of preference is starch which is a food material and costs about ten or more times the cost of raw lump charcoal at the side of the kiln. Hence, as a 4-8% addition to charcoal fines is needed to make the briquettes, it is a very significant cost item. Successful briquetting operations such as those operating in the U.S.A. and other developed countries depend on a favourable conjunction of factors which are usually absent in developing countries. They are:
- An established high priced household market for barbecue fuel.
- Ability to produce fine charcoal for briquettes at very low cost, close to major markets, and in steady volume through the year.
- A high volume of sales adequate to absorb the potential production of the plant.
- Adequate capital for good equipment and skilled labour for operations and maintenance.
- A proper marketing, packaging and distribution system to enable the product to achieve adequate market penetration at a rewarding price.
Generally speaking, and this is supported by the lack of successful briquetting operations in the developing world, it is better to concentrate attention on producing charcoal efficiently from fuelwood, striving for maximum conversion yield and minimum production of fines by careful handling of the product. Furthermore, such production using simple brick kilns requires a low level of imported components, whereas briquetting machinery is normally a costly imported item. Unless a briquette plant can be kept operating over the year at near full production, the capital charges become a drain on profitability.
There is no doubt that unused charcoal fines are a waste of resources and alert management will always be seeking an economic outlet for them. Unfortunately, there is no simple, really satisfactory means of briquetting them on a small scale industry level. It is possible by primitive means to press charcoal fines mixed with starch paste or clay in a mould and dry them, each cakes of charcoal are made in a number of countries but success depends on providing charcoal fines at a very low price to households. Normally this is not possible since fine charcoal in quantities is mainly to be found near the production centres and not in the cities where unpaid household labour is available to produce the product. But despite the difficulty, opportunities of this type should be closely studied in the interests of overall national fuel economy.
Charcoal fines when available in large quantities do have industrial uses. Unfortunately, the customary lack of a developed industrial infrastructure where charcoal is usually made in the developing world precludes using charcoal fines this way. The main industrial uses of fine charcoal are as fuel in metallurgical and calcinning operations. For example, in charcoal iron making fine charcoal can be injected at the base of the blast furnace with the air blast. About 5% of the total charcoal can be injected this way. The charcoal iron works in Wundowie, Australia, was able to use up all its fine charcoal this way. Fine charcoal is excellent for producing sinter, partially reduced iron ore, to provide a high grade feed to the blast furnace. This is one of the best ways to use charcoal fines as the amount which can be used is not limited to a percentage of the total as is the case of injection into the base of the blast furnace. (1, 22). Sintering with fine charcoal is used in Argentina and Brazil. Pulverised fine and lump charcoal can be burned in rotary furnaces producing cement clinker and calcium bauxite. Such cement plants operating in Kenya and Guyana are experimenting with pulverised charcoal for bauxite calcination.
Despite these possibilities, the fact remains that, for the typical charcoal producer, it is better to reduce fines production to the minimum by good charcoal making techniques, rather than invest money in a marginally economic production of wood from plantations or natural forests and to use this wood to expand profitable production of charcoal.