To address the expense of the AFEX method, the su­percritical carbon dioxide explosion method was devel­oped. Compared to steam explosion, the supercritical CO2 explosion method produces fewer inhibitory compounds. Additionally, CO2 is much more environ­mental friendly than organic solvents used in the orga — nosolv method and the ammonia used in the AFEX method. Because carbon dioxide is nontoxic, physiolog­ically safe and inexpensive it is used in a variety of in­dustries, for example, in food and pharmaceutical production. The critical temperature of CO2 is 31.1 °C and its critical pressure is 73 atm. The term supercritical refers to a fluid that at standard temperature and pres­sure would exist in its gaseous state. However, when compressed using high pressures and at temperatures above the critical point, the gas condenses into a liquid­like density. In this state it retains the characteristics of mass transfer that are "gaslike" but with the solvating power that is "liquidlike" (Kim and Hong, 2001). Carbon dioxide molecules are small, like water and ammonia, and thus it penetrates the small pores of LB. It is believed that CO2 forms carbonic acid and thus it should increase the hydrolysis rate. Furthermore, at low temperatures it is thought to prevent significant decomposition of the monosaccharides by the weak acid. However, the pri­mary effect of supercritical carbon dioxide explosion is from the explosion whereby it disrupts the biomass structure and increases the surface area and improves its vulnerability to enzymatic attack (Conner and Lorenz, 1986; Zheng et al., 1998).

Despite these advantages, the operating and capital costs of the supercritical carbon dioxide explosion pre­treatment option remain prohibitive.