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Thermogravimetric curves have been measured at a heating rate of 5 K/min for several hardwoods (beech, alder, birch, and oak) and softwoods (Douglas fir, two pine species, redwood, and spruce), whose chemical composition varies within the usual standards. The analysis of the devolatilization characteristics is based on the introduction of several reaction temperatures. A comparison between hardwoods and softwoods shows that, in the latter case, the decomposition starts at lower temperatures, the hemicellulose shoulder is more delayed, and both the hemicellulose and cellulose zones are wider. Furthermore, the yields of char are higher. However, a devolatilization mechanism, consisting of three parallel reactions and the same set of activation energies for hemicellulose, cellulose, and lignin (100, 236, and 46 kJ/mol), can describe the high-temperature (>553 K) degradation behavior of all of the woods with a good accuracy. Modifications for the extension of the mechanism at lower temperatures are required only for species with significant extractive contents and consist of two further reactions (activation energies of 105 and 127 kJ/mol, respectively).

Conventional pyrolysis of beech wood has been carried out for heating temperatures in the range 600−900 K, reproducing conditions of interest in countercurrent fixed-bed gasification. The yields of liquids (water and tars) increase with the heating temperature from about 40 to 55% of dry wood mass, confirming results previously obtained with a laboratory-scale gasifier. Apart from qualitative identification of ∼90 species, GC/MS techniques have been applied to quantify 40−43% of tars (40 species, with major contributions from acetic acid, hydroxypropanone, hydroxyacetaldehyde, levoglucosan, formic acid, syringol, and 2-furaldehyde). Decomposition of holocellulose leads to the formation of furan derivatives and carbohydrates, with a temperature-dominated selectivity toward hydroxyacetaldehyde against levoglucosan. Syringols and guaiacols, originating from primary degradation of lignin, present a maximum for heating temperatures of about 750−800 K whereas, owing to secondary degradation, phenols continuously increase. A comparison is also provided with fast pyrolysis liquids obtained from four commercial plants.

The pyrolysis characteristics of agricultural residues (wheat straw, olive husks, grape residues, and rice husks) and wood chips have been investigated on a bench scale. The experimental system establishes the conditions encountered by a thin (4 × 10 -2 m diameter) packed bed of biomass particles suddenly exposed in a high-temperature environment, simulated by a radiant furnace. Product yields (gases, liquids, and char) and gas composition, measured for surface bed temperatures in the range 650-1000 K, reproduce trends already observed for wood. However, differences are quantitatively large. Pyrolysis of agricultural residues is always associated with much higher solid yields (up to a factor of 2) and lower liquid yields. Differences are lower for the total gas, and approximate relationships exist among the ratios of the main gas species yields, indicating comparable activation energies for the corresponding apparent kinetics of formation. However, while the ratios are about the same for wood chips, rice husks, and straw, much lower values are shown by olive and grape residues. Large differences have also been found in the average values of the specific devolatilization rates. The fastest (up to factors of about 1.5 with respect to wood) have been observed for wheat straw and the slowest (up to factors of 2) for grape residues.

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