Charcoal is a first-class fuel for heating and cooking. For household use it should be burnt in a properly constructed stove with an outside chimney. The heating power of charcoal is greater than that of the same kind of wood, weight for weight, and a good kiln-burnt charcoal has about the same calorific value as medium-quality domestic coal. This makes it a very suitable fuel for curingtobacco.
- CHARACTERISTICS OF CHARCOAL
Charcoal which has been properly carbonised has a jet black colour with a steel-blue metallic sheen and oily lustre. A dull colour or loss of colour in storage indicates that the wood has been over-burnt, or that unsound wood has been used, while a reddish-brown colour is a sign that carbonisation was incomplete.
The texture is distinct the grain of the wood being plainly visible. Charcoal made from unsound wood will lose its texture.
HARDNESS AND DENSITY
A good charcoal should be hard and firm and not easily crushed. It does not splinter or crumble when broken but breaks with a clean fracture. Two pieces when struck together should emit a metallic clink. The heavier the charcoal the higher its heatingpower.
Charcoal which is contaminated with loose ash, grit, earth and other mineral matter forms an excess of clinker when burnt. The amount of dirt and impurities should not exceed 0, 3% of the moisture- free weight of the charcoal. A good charcoal does not soil the hands when touched, is tasteless and odourless.
Commercial charcoal contains impurities in the form of gases and undecomposed carbon compounds which form tarry substances. The impurities known as volatiles, should not exceed 25% of the total weight of the charcoal excluding moisture. A rough test to indicate the amount of tarry substances in charcoal is to heat about 4kgs of the material in a metalcontaineroverafire,thecontainerbeingcoveredwitha
tin plate. If after 10 minutes, no blackening of the plate occurs, the charcoal is low in tar content. A satisfactory charcoal burns with a short bluish flame free from smoke or odour and without crackling or emitting sparks.
The ash left after burning charcoal is due to the mineral matter forming part of the wood from which the charcoal was made. All charcoals produce ash, but the percentage varies widely depending on the timber used. Too much ash may result in clinker formation in the fire zone at the base of the generator.
Charcoal is hygroscopic and rapidly absorbs moisture from the air until it is in equilibrium with the atmosphere. If charcoal has been quenched with water after its removal from the kiln it must be spread out to dry before use or storage. The moisture content of charcoal in use should not rise above 10% of its weight and in the dry season should be about 4% or less.
The size of charcoal fragments depend on its planned usage, but generally a uniform mixture of pieces varying in size from 2 – 4cm is standard.
3. YIELD AND QUALITY OFCHARCOAL
Freshly felled wood should be allowed to dry out for about 3 months before being put into the kiln or charcoal is produced which will not retain its texture. Although some of the thin-barked indigenous timbers and eucalypts are difficult to debark, all or as much as possible of the bark should be removed from the timber to obtain cleancharcoal.
Unbarked wood leaves a high ash residue when burnt. Furthermore, because of the insulating property of bark, unbarked wood may be unevenly carbonised.
Figure 1: A stock pile of charcoal
USE ONE SPECIES PER CHARGE
Different woods have different rates of combustion. Mixing different species in the same kiln may result in parts of the charge being over-burnt while other parts only partly carbonised. The heavier timbers produce hard, heavy charcoal which stands up better to handling and transporting than the softer charcoals.
USE BILLETS OF UNIFORM SHAPE AND SIZE
The wood must be cross-cut to suit the dimensions of the kiln. Crooked pieces should be reduced in length to facilitate the packing of the kiln as tightly as possible and to ensure the maximum yield of charcoal. Unless this is done, the smaller pieces will have carbonised before the larger pieces.
Clean, dry and grade the charcoal. Grit and dirt must be removed from charcoal by washing it in water for about 5 minutes in a shallow trough. Charcoal must never be used or bagged in a wet state so must be spread out on a clean concrete floor to dry before being bagged.
Dry charcoal should be screened and graded into recommended sizes, since graded charcoal will sell better than ungraded. Bagged charcoal should be stored under cover and off the ground. To prevent the charcoal from being crushed, the bags should be stacked in not more than two vertical layers.
Timbers suitable for charcoal are shown in the table on the next page:
Table 1: Suitable timber for the use of charcoal
|Botanical name||Common Name|
|Acacia mollissima||Black wattle|
|Acacia karoo||Sweet thorn|
|Brachystegiaboehmii||Prince of Wales Feathers|
4. SIMPLE METHODS OF CHARCOAL MANUFACTURE
The methods of charcoal production described here are considered to be suitable for use where economy is the prime consideration, and where it is more convenient to take the kiln to the timber than the timber to the kiln, and where a charcoal of a high quality is not of very great importance.
The simplest method of manufacturing charcoal is to light a fire in a pit, pile timber on the fire and when it has been reduced to embers, repeat the process until the pit is full of glowing embers, cover it with soil and leave it to cool down. This method is however extremely wasteful and inefficient. It produces a poor quality charcoal with high ash content and is contaminated with soil and mineral matter.
BEEHIVE EARTH KILNS
Charcoal burning in earthen kilns, while still a primitive method can produce suitable charcoal for a number of uses and be of reasonable quality. It is relatively simple and economical and can be carried out by farm labour once the operations have been demonstrated. Capital outlay is practically nil. The implements required are picks, shovels, axes, sickles, hoes and screens or sieves, which are the usual farm tools. The kiln is paraboloid in vertical cross-section. Its size varies and it is
advisable to start with a small kiln, about 12 feet in diameter at the base and from 5 – 8 feet high at the centre. A kiln of this size holds about 2.5 – 5 cords of hardwood which will produce from 500 – 1000kgs of charcoal. However, a small kiln takes more labour in terms of charcoal produced. As the beehive kiln is a temporary structure it is best suited on, or near to, a tree felling site.
Figure 2: A beehive kiln:
When the flue and firing chamber have been constructed, the charge (i.e. the wood to be converted to charcoal), can be placed in the kiln. This is done by stacking the billets in three tiers. The smaller sizes are stacked nearest the flue. The largest billets are stacked halfway between the flue and the circumference of the kiln, where the heat is greatest. The billets must be stacked as close together as possible and all interstices filled with dry kindling wood. If branch wood is used the bark must first be removed. The charge is covered with green grass or any green vegetation which is suitable for thatching. A layer of grass, 3 – 4 inches thick, will usually be sufficient. Thatching should start from the bottom of the charge, leaving the top of the flue and the entrance to the firing chamber uncovered.
If the fire has caught properly, a dense smoke, mixed with steam emerges from the flue. A yellowish smoke indicates that the wood is burning too rapidly with an excess of air. When the wood begins to carbonise the smoke thins and takes on a bluish tinge. A clear bluish vapour indicates that
carbonisation is complete and the charcoal is being consumed. The kiln must be watched carefully throughout both the burning and cooling stages to see that no cracks or hollows occur in the outer covering, causing the kiln to collapse and burst into flames.
To test for complete carbonisation of the charge, vent holes are opened at the base of the kiln. If flames emerge from these vents the burning is complete and preparations may now be made to cool down the kiln.
The outer cover can be watered to accelerate the cooling process or it may be removed in strips and fresh earth applied to the openings and beaten down. The kiln can be opened when it has cooled down sufficiently. Opening commences from the leeward side around the kiln and the charcoal which has been removed must be sieved to remove earth andashes.
It will be noted that a feature of the simple earthen kiln is its partial or complete destruction during the process of removing the charcoal.
These disadvantages are obviated when permanent kilns are used and the initial greater cost of such kilns may be compensated for by their permanency. Furthermore, the control of the carbonisation process is easier and more complete in permanent kilns and the product is free from contamination with earth.
TYPES OF PERMANENT KILNS
These may be classified into two types:
- Fixed kilns constructed of masonry (cast concrete, cinder block, and brick);and
- Portable kilns of steelconstruction.
Modern kilns incorporate the reverse draught principle of burning, in which the exhaust flues are so arranged that hot gases and vapours are led back from the zone of carbonisation over the uncarbonised wood. The principle is one of heat economy, as the charge is thus heated and its moisture content is reduced before the wood starts carbonising. A quicker burn and higher yield of charcoal are obtained through this pre-heating process.
It is advisable that if charcoal is manufactured for sale, the manufacturer should bear in mind the specifications and requirements of prospective customers. Charcoal should be offered for sale in bags labelled with the suppliers name and address, the net weight and grade (size) and the common name of the wood from which the charcoal wasmade.
Figure 3: Large Charcoal Kiln