Heating organic materials in the absence of air/oxygen gives rise to chemical and physical changes. The oldest of chemical engineers, the charcoal maker, piled his wood in mounds covered with mud and heated ("burned") it in the relative absence of air. The great clearance of forests in north-western Europe and Britain, which reached its climax during the population expansion of the 11-12th centuries, is thought to have been accomplished by the hand of the charcoal-maker, himself. The grave effects of similar deforestation are today being felt in parts of Africa and Asia. Wood pyrolysis releases light gases like Carbon Monoxide and Carbon Dioxide, light alcohols, aldehydes, ketones and organic acids. Tars are the larger molecular mass volatile products of wood pyrolysis, which readily condense at ambient temperature. With a far higher carbon content than the parent wood (40-50% carbon vs. 75-90% carbon), and nearly negligible sulfur content, the solid residue ("char", "charcoal") has traditionally been the prized fuel of agrarian communities. The early steel manufacture of the 15-17th centuries in Europe, including the eventually successful attempts at casting steel guns directed personally by Henry VIII, were based on reducing iron oxides by carbon in charcoal; steel production using charcoal currently based on farmed eucalyptus wood-charcoal in modern Brazil appears to survive economically due to low wage structures.
To retain good ignition properties as a household fuel, the charcoal must retain some (ca. 10%) of its original volatile content; hence temperatures in this type of pyrolysis rarely exceed 400-450°C. As in all pyrolytic processes the primary products of the thermal breakdown of wood are reactive: the thermal cleavage of bonds gives rise to the formation of a wide array of free radicals. Smaller free radical are highly reactive, with nano- or microsecond half-lives whilst larger free radicals, such as the triphenyl methyl free radical, are chemically stable in solution. Final product distributions of pyrolytic processes are therefore critically dependent on such reaction parameters as temperature, pressure, residence times of volatiles in the heated zone and degree of contact of tar vapors with heated solid surfaces. Without access to our terminology, the traditional charcoal-maker nevertheless piled his wood high, so as to extend the path of his evolving tars, to ensure maximum tar recondensation and secondary char formation. The expected product distributions in slow pyrolysis would involve 15-30% char and 35-45% tars and liquids (including an aqueous phase) with the remainder evolving as gases.
No longer considered economical or environmentally acceptable, the "destructive distillation of wood" was, until the last 60 years, a principal source of chemical feedstocks such as Methanol (wood alcohol), acetone, acetic acid and tars. The latter has historically been much used as isolation against penetration of water or humidity on structures as different as roofs and exterior walls of houses and wooden hulls of ships and boats. "Stockholm tar" is still widely marketed in the UK for similar routine household applications and for antiseptic use in horsecare. More recently, the pyrolysis of agricultural and forestry wastes and municipal solid waste has been studied as a likely process route for waste disposal and energy and chemical feedstock production. While some of these thermochemical processes are actively being researched, no industrial scale applications have as yet emerged. For a recent overview of the thermochemical processing of biomass see Bridgewater (1994).
The widest current direct industrial application of pyrolysis is in coke-oven operation. This consists in heating coal, in cellular ovens which may be 0.60-1 m in width, up to 15 m in length and several meters high. Coke oven design and reaction conditions (e.g., temperatures) may be changed to modify product distributions, according to demand and price structures. In the Europe of the 1840s coal "carbonization" was used to produce gas for home heating and illumination; initially only Royal Palaces and comparable homes could take advantage of this new product of "science". Strengthening demand for aromatic chemicals (e.g., toluene for explosives, naphthalene for moth balls) in the early decades of the 20th century transformed the tars into the principal product. Domestic use of coal-gas in the UK ceased with the generalized distribution of natural gas from the North Sea in the early 1970s. In the latter part of the century coke for metallurgical applications, notably steel production, has come to the fore. Lack of markets and current legislation on pollution have led to the use of the gas and by-products as coke-oven fuel.
When city-gas was the desired product, oven temperatures would reach a maximum of 850°C, with the resultant coke considered a desirable household fuel; reaction times ranged between 7-8.5 hours. Up to 45% gas could be expected from a good gas-coal, the combustible components of the gas consisting of methane, Carbon Monoxide and Hydrogen. Whilst anywhere from 22-30% tar can be released from a middle rank bituminous coal during heating, intense extraparticle secondary reactions in coke ovens usually reduced tar yields to between 5-10% by weight of the original coal. Where metallurgical grade coke is required, oven temperatures may be raised to between 1100-1200°C, with reaction times extending to between 10-14 hours. The hardness and fracture resistance of the resulting coke are critical properties in blast furnace operation. Greater tar yields, closer to amounts originally released from coal particles, can be obtained by lowering temperatures and residence times of volatiles in the reaction zone. In the 1930s "low temperature carbonization" was attempted as a process route to produce higher yields of liquid products from the pyrolysis of coal: many reactor configurations ranging from coke-oven lookalikes to fluidized beds have been used. Before the start of World War II, these attempts were economic failures, mainly due to the cheaper price of petroleum derived fuel oil. The oil shortage of the war years gave a new impetus to a number of coal based industries, particularly in Germany and Japan, where low temperature carbonization technologies were retained and expanded to include tar refining and secondary processing. Available literature on the subject of coking/carbonization is vast and good reviews are available [Lowry (1963) and Elliott (1980)].
The term pyrolysis also covers the thermochemical processing of liquid and gaseous species, usually for making smaller molecules by cracking. Large tonnages of Ethylene are produced from such widely differing feedstocks as methane, ethane, petroleum naphtha and light gas and fuel oils. These processes are normally carried out at pressures between 1-30 bar and temperatures ranging from 700 to 1200°C in externally heated long (20-30 meters) thin (1-2" id) reactor tubes made of refractory alloys. Initial reactions in these processes involve covalent bond cleavage, releasing very reactive free radicals. Reaction schemes involving primary, secondary etc. products are thought to be very complicated and a wide spectrum of products (light gas to tar and coke) may result from the cracking of a gas as simple as pure Ethane. Industrially, product distributions are controlled by manipulating process variables including residence time in the heated zone, and by introducing marginally reactive diluents such as steam, or inert diluents like nitrogen. Desirable contact times can be as short as a few milliseconds and industrial scale rapid quenching devices are often elegantly designed. Many industrially important reactions are performed in such pyrolysis reactors (with length/diameter ratios which could be as low as 10-12), leading to the production of bulk chemicals like VCM (vinyl chloride monomer) and specialty chemicals such as tetrafluoroethylene [Albright et al. (1983)].
Pyrolytic processes involving the release of volatiles (gases and tars) and the formation of chars constitute the initial (or an intermediate) step in many common industrial applications, e.g., pulverized coal combustion. In the pyrolysis of coal, the temperature, pressure and heating rate of the pyrolytic step would be expected to affect
the product (gas-tar-char) distribution,
structural features of product tars,
the reactivity of chars produced during the pyrolysis.
The latter reactivity is thought to be relevant in attempting to solve problems as wide ranging as suppression of carbon content in fly ash (in pf-combustors) and estimation of the performance of fluidized bed coal gasifiers and combustors in new generation combined cycle power generation systems.
Investigation of the pyrolytic behavior of solids requires careful experimental design. For example due to the reactive nature of the products, a shallow and a deep fixed bed reactor would not give the same product distribution: comparison of yields obtained from bench scale experimental reactors of different configuration have shown that tar yields could be improved by almost 50% if the tars are recovered in the relative absence of extraparticle secondary reactions. The types of apparatus used for these purposes range from Fixed and Fluidized Bed reactors, where volatiies could spend relatively long times in the heated zone, to entrained flow ("drop-tube") reactors, where product volatiles cross the whole length of the heated reactor tube and to wire-mesh reactors, where a monolayer of sample is held between the folded layers of mesh stretched between a pair of electrodes, the mesh also acting as the resistance heater. In the wire-mesh reactor application, if volatiles are swept from the vicinity of the mesh into a quench zone, reactor configuration allows volatiles to clear the shallow heated reaction section (less than 1 mm) relatively rapidly [Gonenc et al. (1990)]. Tar samples recovered in this type of apparatus would be small (1-2 mg) but would be expected to show structural features relatively little changed from tar vapors released by the pyrolysing coal particles. Analytical techniques requiring relatively small sample sizes have been used in the structural evaluation of these materials, including FT-infrared and UV-fluorescence spectroscopies, size exclusion chromatography and various mass spectroscopic techniques [Li et al. (1994), Li et al. (1995), and Herod et al. (1994)].
Bridgewater, A. V. (Ed.) (1994) Advances in Thermochemical Biomass Conversion (2 vols), Blackie, London.
Lowry, H. H. (Ed.) (1963) Chemistiy of Coal Utilisation, Supplementary Volume I, Wiley, New York.
Elliott, M. A. (Ed.) (1980) Chemistry of Coal Utilisation, Supplementary Volume II, Wiley, New York.
Albright, L. F., Crynes, B. L. and Corcoran, W. H. (1983) Pyrolysis, Academic Press, New York.
Gonenc, Z. S., Gibbins, J. R., Katheklakis, I. E. and Kandiyoti, R. (1990) Comparison of coal pyrolysis product distributions from three captive sample techniques, Fuel, 69, 383-390.
Li, C-Z., Madrali, E. S., Wu, F., Xu, B., Cai, H-Y., Güell, A. J. and Kandiyoti, R. (1994) Comparison of thermal breakdown in coal pyrolysis and liquefaction, Fuel, 73, 851-865. DOI: 10.1016/0016-2361(90)90104-X
Li, C-Z., Wu, F., Xu, B. and Kandiyoti, R. (1995) Characterisation of successive time/temperature-resolved liquefaction extract fractions released from coal in a flowing-solvent reactor, Fuel, 74, 37-45. DOI: 10.1016/0016-2361(94)P4328-Y
Herod, A. A., Li, C-Z., Parker, J. E., John, P., Johnson, C. A. E, Smith, G. P., Humphrey, P., Chapman, J. R., and Kandiyoti, R. (1994) Characterisation of coal by matrix assisted laser desorption ionization (MALDI) mass spectrometry I: the Argonne coal samples,Rapid Comm. Mass Spec., 8, 808-814.