The treatment of wood chips with steam at elevated pressures and temperatures for short time periods followed by rapid decompression changes the physical state of the woody structure by defibration. Although some chemical changes occur with the hemicelluloses and lignins in this process, the particle sizes are reduced and surface areas and pore volumes are increased, so some discus­sion of the technique is warranted. The process was originally developed in 1925 and has been extensively used in the manufacture of hardboard (Spalt, 1977). The commercial process involves pressurization with saturated steam at pressures up to about 7 MPa. The process has also been proposed for the pretreatment of lignocellulosic feedstocks in the production of fermentation ethanol (Chapter 11) because of the large increase in accessibility of the cellulosic fraction to enzymatic hydrolysis (cf. Schultz, Biermann, and McGin­nis, 1983; Mes-Hartree, Hogan, and Saddler, 1987; Foody and Foody, 1991).

In a series of steam-explosion experiments with different wood chips, sugar­cane bagasse, ground corn stover, and ground rice hulls at selected tempera­tures between 190 and 250°C and treatment times of 1 or 2 min, the results were as follows (Schultz and McGinnis, 1984): All material was defibrated; the hemicelluloses were at least partially degraded, while the remaining hemi­celluloses were extractable with hot water; the lignins were depolymerized by cleavage of the ether linkages, while at the higher temperatures, they became moderately condensed; and the enzymatic hydrolysis rates of the steam — exploded material increased dramatically for all materials except corn stover. Corn stover hydrolyzed at high rates without treatment. This study and others on steam explosion suggest that the technique can be used for several different biomass applications ranging from modifying the fibrous structure and particle sizes alone at the lower temperatures to a combination of physical and chemical changes at the higher temperatures.