A typical autohydrolysis process [48] uses compressed liquid hot water at a temperature of about 200°C under a pressure that is higher than the saturation pressure, thus keeping the hot water in liquid phase, to hydro­lyze hemicellulose in minutes. Hemicellulose recovery is usually high, and unlike the acid-catalyzed process, no catalyst is needed. The process is represented as shown in Figure 4.8. Very high temperature processes may lead to significant pyrolysis, which produces inhibitory compounds. The ratio of the rate of hemicellulose hydrolysis to that of sugar degra­dation (more pyrolytic in nature) is greater at higher temperatures. Low-temperature processes have lower xylose yields and produce more degradation products than well-controlled, high-temperature processes that use small particles.

According to a study by Dekker and Wallis [49], pretreatment of bagasse by autohydrolysis at 200°C for 4 min and explosive defibration resulted in a 90% solubilization of the hemicellulose (a heteroxylan) and in the pro­duction of a pulp that was highly susceptible to hydrolysis by cellulases from Trichoderma reesei. Saccharification yields were 50% after 24 hours at 50°C (pH 5.0) in enzymatic digests containing 10% (w/v) bagasse pulps and 20 filter paper cellulase units (FPU), and their saccharification yield could be increased to 80% at 24 hours by the addition of exogenous p-glucosidase from Aspergillus niger.

In general, xylose yields in autohydrolysis are low (30-50%). An autohy­drolysis system is used as the pretreatment in separate hydrolysis and fer­mentation (SHF). The reaction conditions are 200°C for 10 minutes, with a xylose yield of 35%.

Steam consumption in autohydrolysis is strongly dependent upon the moisture content of the starting material. Wet feedstock requires consid­erably more energy because of the high heat capacity of retained water. An important advantage of autohydrolysis is that it breaks the lignin into relatively small fragments that can be easily solubilized in either base or organic solvents.

The steam explosion process [50] was first developed in 1925 for hardboard production and more recently was applied on aspen wood in the early 1980s. In a typical steam explosion process, cellulosic material is heated using high — pressure steam (20-50 atm, 210-290°C) for a short period (seconds to min­utes). At the increased pretreatment pressure, water molecules diffuse into the inner microporous structure of the lignocellulose [47]. In this process, some steam condenses under high pressure, thereby wetting the material. The wetted material is then driven out of a reactor (i. e., ejected from a reac­tor) through a small nozzle by a pressure difference. Due to a rapid decrease in the pressure, the material is ejected through the discharge valve. The term "explosion" is used due to the process characteristics of ejection driven by a sudden large pressure drop of steam.