10.1. The quality of charcoal.
10.2. Burning charcoal efficiently
Charcoal will go further if it is used efficiently and if its quality is optimum for the particular end use. Charcoal quality can be specified and measured in various ways which are usually derived from the various end use requirements. Efficiency in use normally means transferring the maximum amount of the heat content of the charcoal to the object to be heated, be it water for cooking, the air of a room, or the charge in a blast furnace. Efficiency depends on the use of properly designed charcoal burning equipment.
10.1.1. Moisture content
10.1.2. Volatile matter other than water
10.1.3. Fixed carbon content
10.1.4. Ash content
10.1.5 Typical charcoal analyses
10.1.6. Physical properties
10.1.7. Adsorption capacity
The least demanding market for charcoal, quality-wise, is the domestic one. The reasons are that performance cannot be measured easily, the power of consumers as individuals to specify and obtain good quality charcoal is minimal and there is a certain trade-off possible between price and quality which the household consumer uses to obtain satisfactory results. However, this does not mean that quality control is not worthwhile. Provided it does not become unwieldy and bureaucratically counterproductive, a system of quality guidelines for household charcoal is a worthwhile step in ensuring maximum yield from the wood resource, nevertheless giving adequate household performance. On the other hand, large users such as the charcoal iron industry know from their operating experience and research, the properties they seek in charcoal and have the means in the form of concentrated buying power and control over at least a part of their own charcoal production, to ensure that the charcoal they use conforms to their specification and produces pig iron at minimum overall cost.
Most of the specifications used to control charcoal quality have originated in the steel or chemical industry. When charcoal is exported, buyers tend to make use of these industrial quality specifications even though the main outlet of the imported charcoal may well be the domestic cooking or barbecue market. This factor should be borne in mind since industrial and domestic requirements are not always the same and an intelligent appraisal of actual market quality requirements may allow supply of suitable charcoal at a lower price or in greater quantities beneficial to both buyer and seller.
The quality of charcoal is defined by various properties and though all are interrelated to a certain extent, they are measured and appraised separately. These various quality factors are discussed below.
Charcoal fresh from an opened kiln contains very little moisture, usually less than 1%. Absorption of moisture from the humidity of the air itself is rapid and there is, with time, a gain of moisture which even without any rain wetting can bring the moisture content to about 5 to 10%, even in well-burned charcoal. When the charcoal is not properly burned or where pyroligneous acids and soluble tars have been washed back onto the charcoal by rain, as can happen in pit and mound burning, the hygroscopitity of the charcoal is increased and the natural or equilibrium moisture content of the charcoal can rise to 15% or even more.
Moisture is an adulterant which lowers the calorific or heating value of the charcoal. Where charcoal is sold by weight, keeping the moisture content high by wetting with water is often practised by dishonest dealers. The volume and appearance of charcoal is hardly changed by addition of water. For this reason bulk buyers of charcoal prefer to buy either by gross volume, e.g. in cubic metres, or to buy by weight and determine by laboratory test the moisture content and adjust the price to compensate. In small markets sale is often by the piece.
It is virtually impossible to prevent some accidental rain wetting of charcoal during transport to the market but good practice is to store charcoal under cover even if it has been bought on a volume basis, since the water it contains must be evaporated on burning and represents a direct loss of heating power. This occurs because the evaporated water passes off into the flue and is rarely condensed to give up the heat it contains on the object being heated in the stove.
Quality specifications for charcoal usually limit the moisture content to around 5-15% of the gross weight of the charcoal. Moisture content is determined by oven drying a weighed sample of the charcoal. It is expressed as a percentage of the initial wet weight.
There is evidence that charcoal with a high moisture content (10% or more) tends to shatter and produce fines when heated in the blast furnace, making it undesirable in the production of pig iron.
The volatile matter other than water in charcoal comprises all those liquid and tarry residues not fully driven off in the process of carbonization. If the carbonization is prolonged and at a high temperature, then the content of volatiles is low. When the carbonization temperature is low and time in the kiln is short, then the volatile matter content increases.
These effects are reflected in the yield of charcoal produced from a given weight of wood. At low temperatures (300°C) a charcoal yield of nearly 50% is possible. At carbonization temperatures of 500-600°C volatiles are lower and retort yields of 30% are typical. At very high temperatures (around 1000°C) the volatile content is almost zero and yields fall to near 25%. As stated earlier, charcoal can reabsorb tars and pyroligneous acids from rain wash in pit burning and similar processes. Thus the charcoal might be well burned but have a high volatile matter content due to this factor. This causes an additional variation in pit burned charcoal in wet climates. The resorbed acids make the charcoal corrosive and lead to rotting of jute bags - a problem during transport. Also it does not burn cleanly.
The volatile matter in charcoal can vary from a high of 40% or more down to 5% or less. It is measured by heating away from air, a weighed sample of dry charcoal at 900°C to constant weight. The weight loss is the volatile matter. Volatile matter is usually specified free of the moisture content, i.e. volatile matter - moisture or (V.M. - moisture).
High volatile charcoal is easy to ignite but may burn with a smoke flame. Low volatile charcoal is difficult to light and burns very cleanly. A good commercial charcoal can have a net volatile matter content - (moisture free) of about 30%. High volatile matter charcoal is less friable than ordinary hard burned low volatile charcoal and so produces less fines during transport and handling. It is also more hygroscopic and thus has a higher natural moisture content.
The fixed carbon content of charcoal ranges from a low of about 50% to a high or around 95%. Thus charcoal consists mainly of carbon. The carbon content is usually estimated as a "difference"; that is to say, all the other constituents are deducted from 100 as percentages and the remainder is assumed to be the % of "pure" or "fixed" carbon. The fixed carbon content is the most important constituent in metallurgy since it is the fixed carbon which is responsible for reducing the iron oxides of the iron ore to produce metal. But the industrial user must strike a balance between the friable nature of high fixed carbon charcoal and the greater strength of charcoal with a lower fixed carbon and higher volatile matter content to obtain optimum blast furnace operation.
Ash is determined by heating a weighed sample to red heat with access of air to burn away all combustible matter. This residue is the ash. It is mineral matter, such as clay, silica and calcium and magnesium oxides, etc., present in the original wood and picked up as contamination from the earth during processing.
The ash content of charcoal varies from about 0.5% to more than 5% depending on the species of wood, the amount of bark included with the wood in the kiln and the amount of earth and sand contamination. Good quality lump charcoal typically has ash content of about 3%. Fine charcoal may have a very high ash content but if material less than 4 mm is screened out the plus 4 mm residue may have an ash content of about 5-10%. Buyers naturally suspect fine charcoal and it is difficult to sell (and use, unfortunately).
To illustrate the range of composition found in commercial charcoal Table 7 lists the composition of random samples of charcoal from various kinds of woods and various kinds of carbonization systems. In general, all woods and all systems of carbonization can produce charcoals falling within the commercial limits.
Table 8 records the variations in charcoal composition as found in the blast furnace charge at a large charcoal iron works at Minas Gerais in Brazil. All of this charcoal was made using beehive type brick kilns. The wood used was either mixed species from the natural forests of the region or eucalypt wood from plantations.
Table 7. Some Typical Charcoal Analyses
|
Wood species Production Method |
Moisture content % |
Ash % |
Volatile matter - m.c./% |
Fixed carbon % |
Bulk density raw kg/m³ |
Bulk density pulverised kg/m³ |
Gross calorific value KJ/kg |
Remarks | |
|
Dakama |
Earth pit |
7.5 |
1.4 |
16.9 |
74.2 |
314 |
708 |
32410 |
Pulverised fuel for rotary kilns 1/ |
|
Wallaba |
Earth pit |
6.9 |
1.3 |
14.7 |
77.1 |
261 |
563 |
35580 |
Pulverised fuel for rotary kilns 1/ |
|
Kautaballi |
Earth pit |
6.6 |
3.0 |
24.8 |
65.6 |
290 |
596 |
29990 |
Pulverised fuel for rotary kilns 1/ |
|
Mixed Tropical Hardwood |
Earth pit |
5.4 |
8.9 |
17.1 |
68.6 |
|
|
|
Low grade charcoal fines 1/ |
|
Mixed Tropical Hardwood |
Earth pit |
5.4 |
1.2 |
23.6 |
69.8 |
|
|
|
Domestic charcoal 1/ |
|
Wallaba |
Earth mound |
5.9 |
1.3 |
8.5 |
84.2 |
|
|
|
Well burned sample 1/ |
|
Wallaba |
Earth mound |
5.8 |
0.7 |
46.0 |
47.6 |
|
|
|
Soft burned sample |
|
Oak |
Portable steel kiln |
3.5 |
2.1 |
13.3 |
81.1 |
|
|
32500 |
2/ |
|
Coconut shells |
Portable steel kiln |
4.0 |
1.5 |
13.5 |
83.0 |
|
|
30140 |
4/ |
|
Eucalyptus Saligna |
Retort |
5.1 |
2.6 |
25.8 |
66.8 |
|
|
|
3/ |
1/= Guyana.
2/= U.K.
3/= Brazil.
4/= Fiji.
Table 8. Characteristics of Charcoal for Blast Furnaces in Brazil
|
Chemical and Physical Composition of Charcoal Dry Bass - by weight |
Range |
Yearly Average |
Charcoal considered good to excellent |
|
|
Max. |
Min. |
|||
|
Carbon |
80% |
60% |
70% |
75-80% |
|
Ash |
10% |
3% |
5% |
3-4% |
|
Volatile matter |
26% |
15% |
25% |
20-25% |
|
Bulk density - as received (kgs/m³) |
330 |
200 |
260 |
250-300 |
|
Bulk density - dry |
270 |
180 |
235 |
230-270 |
|
Average Size (mm) - as received |
60 |
10 |
35 |
20-50 |
|
Pines content - as received (-6.35 mm) |
22% |
10% |
15% |
10% max. |
|
Moisture content - as received |
25% |
5% |
10% |
10% max. |
The ranges and yearly averages refer to charcoal used by Belgo Mineira. This is a mixture of 40% eucalyptus charcoal produced in company operated kilns and 60% heterogenous natural wood charcoal manufactured by privately operated kilns. "Good to excellent" charcoal refers to that produced from eucalyptus wood in company kilns.
The properties described so far are referred to as chemical properties but physical properties, especially for industrial charcoal, are no less important. It is in the charcoal iron industry that physical properties have great importance. The charcoal is the most expensive raw material in the blast furnace charge. Charcoal's physical properties influence the output of the blast furnace whereas chemical properties are more related to the amount of charcoal needed per ton of iron and the composition of the finished iron or steel. (1).
Blast furnace charcoal must be strong in compression to withstand the crushing load of the blast furnace charge or "burden". This compression strength, always less than charcoal's rival, metallurgical coke made from coal, determines the practical height and hence efficiency and output of the blast furnace. The ability to resist fracturing when handled is important to maintain constant permeability of the furnace charge to the air blast which is vital in maintaining furnace productivity and uniformity of operations.
Various tests have been developed to measure fracture resistance; a rather difficult property to define in objective terms. These tests rely on measuring the resistance of the charcoal to shattering or breakdown by allowing a sample to fall from a height onto a solid steel floor or by rumbling a sample in a drum to determine size breakdown after a specified time. The result is expressed as the percentage passing and retained on various sized screens. Charcoal with poor shatter resistance will produce a larger percentage of fines when a sample is tested. Fine charcoal is undesirable in the blast furnace since it blocks the flow of air blast up the furnace. Fragile charcoal may also be crushed by the weight of the charge and cause blockages.
Wood charcoal is an important raw material for activated charcoal. This product is beyond the scope of this manual but some data could be useful where charcoal producers are selling charcoal to be turned into activated charcoal by specialist factories.
As produced, normal wood charcoal is not a very active adsorption material for either liquids or vapours because its fine structure is blocked by tarry residues. To convert the charcoal to "activated" this structure must be opened up by removing the tarry residues. The most widely used method today consists in heating the pulverised raw charcoal in a furnace to low red heat in an atmosphere of superheated steam. The steam prevents the charcoal from burning away by excluding oxygen. Meanwhile the volatile tars can be distilled away and are carried off with the steam, leaving the pore structure open. The treated charcoal is run off into closed containers and allowed to cool. Activation furnaces are usually continuous, i.e. the powdered charcoal passes continuously cascade fashion through the hot furnace in the steam atmosphere.
After activation the charcoal is tested to quality specifications to determine its power to decolorise, by adsorption, watery solutions such as raw sugar juice, rum wine, and so on; oils such as vegetable oil and to adsorb solvents such as ethyl acetate in air. Adsorbtive power tends to be specific. Grades are made for aqueous solutions, others for oils and others for vapours. The tests measure the adsorptive power. There are small differences in the finished product made from raw charcoals of different origin but generally all are useable if properly burned. A good basic charcoal for making activated charcoal can be made from the wood of Eucalyptus grandis in brick type kilns.
Charcoal for adsorption of gases and vapours is usually made from coconut shell charcoal. This charcoal has high adsorptive power and resists powdering in the adsorption equipment - a very important factor.
Given good quality charcoal it must still be burned efficiently to produce the best results. This is specially true in domestic use where most charcoal is burned. Industrial furnaces for burning charcoal such as blast furnaces, cupolas, sintering furnaces and so on, are usually efficiently designed and operated. They will not be discussed here. The main use of charcoal in the households of the developing world is to heat water either to cook food or provide hot water for washing, etc. Some food is cooked by direct heating without immersion in water, such as when corn or meat is roasted. A cooking system would be 100% efficient if all the heat released by burning the fuel were taken up by food being cooked. In practice, this is far from the case, A typical result for well designed and operated equipment is an efficiency of around 30%, that is 70% of the heat escapes uselessly. In a cold climate, some of this waste heat may be captured and used to heat up the air of the room, thereby performing a useful function which raises overall efficiency.
In theory, it is possible to increase the efficiency of transfer of heat from the burning charcoal to the food being cooked by increasing the cost and complication of the stove. This is rarely practical. Those who could afford such complication would usually not be found burning charcoal but some other fuel of higher social prestige or convenience. A compromise is necessary to achieve the best possible efficiency, consistent with reasonably simple, low cost stove equipment which can be used by the bulk of charcoal users. Charcoal, unlike fuelwood, transfers a good deal of its heat to the cooking vessel by radiation from the glowing fuel bed. Burning fuelwood, where the hot gases are produced by long lazy flames, must transfer a good deal of the heat to cooking vessels by convection. For heat transfer by convection, the hot gas must actually contact the pot but radiant heat is transferred by infrared radiation emitted directly from the fuel bed and absorbed by the surface of the pot or other object. Thus the pot must be able to "see" the fuel bed to be able to collect and absorb the radiant heat energy. The surface of the pot plays an important part. It must be preferably dull black. The pot itself should be a good conductor of heat as well. Thin fire blackened aluminium is probably ideal. Thick low density earthenware is probably the worst. Fire blackened pots should not be polished outside, but surface layers of loose soot and soft tar should be removed.
Charcoal reacts with oxygen of the air at a glowing red heat to form colourless carbon monoxide gas, which then burns with a blue flame with more oxygen from the air to produce carbon dioxide gas. Due to the heat liberated by both of these reactions, the charcoal reaches a glowing red and radiates heat energy and the hot carbon dioxide gas leaves the combustion zone, hopefully giving up by convection most of its heat by direct physical contact with the cooking pot. The gas temperature falls as it transfers heat and it passes off into the room. Flues are not usually used with charcoal, since its combustion is relatively odourless and smoke-free compared to wood or coal. Unburned carbon monoxide gas can be given off by burning charcoal. It is very poisonous and ventilation of rooms where charcoal is burning is essential.
The fact that charcoal can be burned in a compact portable stove not requiring a flue, is one of its most important attributes and explains its widespread popularity, especially in cities and built up areas. Even though it is more efficient in overall energy terms for a country to endeavour to use actual wood burned efficiently for cooking rather than convert it first to charcoal, such a policy is difficult to implement. For most people at present who burn charcoal, changing to wood is difficult. A wood burning stove with a flue is costly. The stove itself may be of rammed earth and may cost nothing, but a metal flue may cost $10 or more. For those living in cramped city housing, installing flues may be impossible and the pollution free features of charcoal fuel are compelling in these cases.
The important factors noted in efficient well designed, domestic, charcoal-burning units can be summarised as follows:
(i) The charcoal fuel bed must "see" the pot it is heating and should be as close to it as possible. The walls of the fuel bed chamber should not "look" directly at the fuel bed. This implies a combustion space shaped somewhat like an inverted cone of 80-90% included angle with the fuel bed located at its vertex.(ii) The stove body should be made of refectory material, not metal, unaffected by temperatures around 1 000°C and should be a good thermal insulator, in order not to conduct heat away from the fuel bed. A good material is porous earthenware made from a white burning clay to reflect heat better onto the pot. The stove body should be replaceable in the support frame of the stove to reduce maintenance costs. A cheaper and more or less satisfactory stove body can be made by ramming a damp plastic mixture consisting of 60% clay, 20% sand and 20% charcoal fines approximately, into a wooden mould and allowing it to dry. Though not as permanent as burned earthenware, it is cheap.
(iii) The conical fire hole of the stove should have about four flue gas channels in its surface, about 30 mm wide and 4 mm deep to permit the hot gas to pass out, even though the cooking pot may be a close fit in the cone.
(iv) The grate should be of steel sheet with 3 mm nail holes spaced approximately 1 per cm².
(v) The frame of the stove which can be made of recycled steel sheet should have legs giving a clearance of 4-5 cm between the bottom of the clay stove block and the floor. A recycled steel sheet tray is placed under to collect hot ash, so that the stove may be placed on any surface without creating a fire hazard.
The design shown in fig. 12 is only one of many. But all good designs adhere to the principles enumerated in this section. It is worth emphasizing that the objective is maximum efficiency at minimum cost, otherwise the equipment will not be used.
Fig. 12. Charcoal Cook Stove of Good Design
1. Round cookpot
2. Channels for flue gas in stove body
3. Recycled steel shell
4. Recycled steel ash tray
5. Recycled steel perforated grate
6. White-burning earthenware stove body or clay-sand-charcoal fines mixture
7. Burning charcoal
