Several pretreatment processes combine physical and chemical elements. The most common of physicochemical pretreatments used are steam explosion: SO2-steam explosion, Liquid hot water, ammonia fiber explosion (AFEX), Microwave pretreat­ment, Ultrasound pretreatment, and carbon dioxide (CO2) explosion.

Steam explosion is the most widely employed physicochemical pretreatment for lignocellulosic biomass. It is a hydrothermal pretreatment in which the biomass is subjected to pressurized steam for a period of time ranging from seconds to several minutes, and then suddenly depressurized. This pretreatment combines mechanical forces and chemical effects due to the hydrolysis (autohydrolysis) of acetyl groups present in hemicellulose. Autohydrolysis takes place when high temperatures promote the formation of acetic acid from acetyl groups; furthermore, water can also act as an acid at high temperatures.

Addition of dilute acid in steam explosion can effectively improve enzymatic hydrolysis, decrease the production of inhibitory compounds, and lead to more complete removal of hemicellulose.

The most important factors affecting the effectiveness of steam explosion are particle size, temperature, residence time, and the combined effect of both tempera­ture and time (Alfani et al. 2000). Higher temperatures result in an increased removal of hemicelluloses from the solid fraction and an enhanced cellulose digestibility; they also promote higher sugar degradation.

AFEX involves liquid ammonia and steam explosion. A typical AFEX process is carried out with 1-2 kg ammonia/kg dry biomass at 90 °C pH values (<12.0) during 30 min. It reduces the lignin content and removes some hemicellulose while decrys — tallizing cellulose. The important advantages of AFEX include: (1) producing neg­ligible inhibitors for the downstream biological processes, so water wash is not necessary (Mes-Hartree et al. 1988); and (2) requiring no particle size reduction. However, ammonia must be recycled after the AFEX pretreatment based on the considerations of both the ammonia cost and environmental protection. Therefore, both ammonia cost and the cost of recovery processes drive up the cost of the AFEX pretreatment (Holtzapple et al. 1992).

Carbon dioxide (CO2) explosion acts similar to steam and ammonia explosion: high-pressure CO2 is injected into the batch reactor and then liberated by an explo­sive decompression. It is believed that CO2 reacts to carbonic acid (carbon dioxide in water), thereby improving the hydrolysis rate. The glucose yields in the later enzymatic hydrolysis are low (75 %) compared to steam and ammonia explosion. Overall however (CO2 ) explosion is more cost-effective than ammonia explosion and does not cause the formation of inhibitors as in steam explosion.