CHARCOAL

Charcoal is perhaps the oldest known fuel, having been found in archeological sites dating as far back as the Pleistocene era. Charcoal is a relatively smokeless and odorless fuel, and thus ideal for cooking and heating.

HISTORY

As humans entered the Bronze Age, charcoal was the only material that could simultaneously heat and reduce metallic ores. Later, the addition of an air blower made it possible to achieve temperatures high enough to soften or melt iron. During the Industrial Revolution, charcoal was largely displaced in most ironworks by coke derived from coal. However in Brazil, which lacks adequate coking coal resources, most of the charcoal produced is still used to reduce iron ore.

Charcoal was produced in pits, and later in kilns, by burning wood with air insufficient for complete combustion. The heat generated drives off the volatile materials in the wood, leaving a char that contains 60 to 90 percent carbon. In the nineteenth and early twentieth centuries, these volatiles were a major source of raw materials, chiefly acetic acid, acetone, methanol and creosote, for the burgeoning organic chemical industry. However charcoal's utility as a source of starting compounds was short-lived because petroleum-derived feedstocks proved to be cleaner and cheaper sources of these chemicals. By World War II, U.S. charcoal production declined by two-thirds, but since then, the popularity of backyard cooking raised charcoal production to an all-time high of about 1.5 million tons in 1996. As of 1999,

worldwide production is roughly 30 million tons, with about half of this in Asia and India, and one-third in South America. Precise data are not available from developing countries, where production is fragmented and manufacturing information is not systematically monitored.

USE

Charcoal is used in electrically heated furnaces to smelt specialty metals such as ferrosilicon. It is a preferred household fuel in developing countries with adequate forest resources. In the United States 95 percent of charcoal use is for barbecuing, while in Japan and Europe charcoal use is split evenly between cooking and industrial needs.

Much has been said about the backyard barbecue as a major air pollutant. However, most cooking smoke comes from food, not from the fuel used. Charcoal contains almost no sulfur, the major pollutant from burning coal. Of the total U.S. 1996 energy consumption of 93.8 quadrillion Btus, only 3.2 percent came from burning biomass. Most of this was derived from direct combustion of fireplace wood and industrial wastes like sawdust, bark and bagasse. In developing countries charcoal may constitute up to 40 percent of energy use. This energy source may not be sustainable because of conversion of forests to farmland. In Brazil, most of the charcoal destined for metallurgical use is made from fast-growing wood species raised on plantations.

Other markets for charcoal are as a filtration medium, a horticultural soil improver, and an adsorbent. Its large surface area of hundreds of square meters per gram enables it to adsorb large organic molecules and thus improve color, taste, and/or odor in liquids and gases. Its adsorptivity can be improved by steaming in the presence of certain chemicals. The resulting "activated carbon" is used for a wide variety of applications ranging from water purification to poison-gas masks.

PROCESS

Modern charcoal retorts are charged with wood, biowaste (bark, sawdust, etc.), peat, and sometimes low-rank coals. Yield and properties (hardness, density, surface area, etc.) can vary widely so the desired end use must be considered. Charcoal from coniferous trees is soft and porous, while that from hardwoods is dense and strong. For barbecuing, charcoal is usually compressed into briquettes, with binders and additives chosen to improve handling and ease of ignition.

The manufacturing process usually involves slow heating to about 275°C in a kiln or retort. The reaction is exothermic, and about 10 percent of the heat of combustion of the original woody feed is lost. The gases and volatile liquids generated are usually burned to supply process heat. Then the wood is further carbonized by heating to about 500°C without air. Modern plants generally employ closed retorts where dried wood is heated by external means. This allows for better process control and enhanced pollution abatement. Both batch and continuous methods are used.

There are many process variations. A typical batch process from a DeGussa patent is shown in Figure 1. The reactor is 100 cubic meters and wood pieces are fed by a conveyer belt. Hot, 550°C, wood gas is fed concurrently. A carbonization zone (where the char, mostly carbon, is being formed) travels downward as the wood is pyrolyzed (heated intensely in the absense of air), and the off gases are burned to generate all of the process heat plus some of the energy needed to dry the raw wood. Energy requirements for producing 1 kg. of charcoal, including the drying operation, are 2.5 MJ of heat and 0.25 MJ of electrical energy. The process also requires 0.05 cubic meters of cooling water. A retort of this size and design will produce about 300 tons of charcoal a month.

Continuous production of charcoal is typically performed in multiple hearth furnaces, as illustrated in the Herreshoff patent shown in Figure 2. Raw material is carried by a screw conveyor to the uppermost of a series of hearths. Air is supplied counter-currently and burns some of the wood to supply process heat. As the layers of wood carbonize, they are transported to the lower (hotter) hearths by rakes. The hot charcoal product is discharged onto a conveyor belt and cooled with a water spray.

Fresh charcoal is a strong absorbent for gases, and this is an exothermic process. The heat generated can be enough to cause spontaneous ignition in some cases. Hence it is customary to age charcoal by exposure to air and thus cover the absorption sites with a layer of nitrogen gas. Larger molecules will desorb and replace smaller molecules, so the charcoal will still be effective as a decolorant or deodorizer.

Herman Bieber

See also: Explosives and Propellants.

BIBLIOGRAPHY

Bansal, R. C. (1998). Active Carbon. New York: M. Dekker. DeGussa, German Patent 744,135 (1937).

Johansson, T. B.; Kelly, H.; Reddy, A. K.; and Williams, R. H., eds. (1993). Renewable Energy: Sources for Fuels and Electricity. Washington, DC: Island Press.

Nichols, Belgian Patent 309,763 (1968).

Proceedings of the ESCAPE/FAO/UNEP Expert Group Meeting on fuelwood and Charcoal. (1982). Energy Resources Development Series #24. New York: United Nations.

U.S. Bureau of the Census. (1998). Statistical Abstract of the United States. Washington, DC: U.S. Government Printing Office.

char·coal / ˈchärˌkōl/ • n. a porous black solid, consisting of an amorphous form of carbon, obtained as a residue when wood, bone, or other organic matter is heated in the absence of air. ∎  briquettes of charcoal used for barbecueing: lamb grilled on charcoal. ∎  a crayon made of charcoal and used for drawing. ∎  a drawing made using charcoal. ∎  a dark gray color: his charcoal sweater | [as adj.] charcoal gray. • v. [usu. as adj.] (charcoaled) cook over charcoal: charcoaled lobster.

charcoal XIV. The second el., COAL, orig. meant ‘charcoal’; the first el. is obscure, but has been referred to CHARE, as if the comp. meant ‘turn-coal’. Cf. CHAR 2.

charcoal Porous form of carbon, made traditionally by heating wood in the absence of air, and used in western Europe until late medieval times for smelting iron ore. Today, charcoal is chiefly used for its absorptive properties, to decolourize food liquids such as syrups, and to separate chemicals. Artists use charcoal sticks for sketching. The porous activated charcoal is used in gas masks.

charcoal Finely divided carbon, obtained by heating bones or wood in a closed retort to carbonize the organic matter. Used to purify solutions because it will absorb colouring matter and other impurities; wood charcoal is commonly used as a fuel for barbecues.