The main advantage of the acid hydrolysis is that acids can penetrate lignin without any preliminary pretreatment of biomass, thus breaking down the cellulose and hemicellulose polymers to form individual sugar molecules. Several types of acids, concentrated or diluted, can be used, such as sulphurous, sulphuric, hydrocloric, hydrofluoric, phosphoric, nitric and formic acid (Galbe & Zacchi, 2002). Sulphuric and hydrochloric acids are the most commonly used catalysts for hydrolysis of lignocellulosic biomass (Lenihan et al., 2010).

The acid concentration used in the concentrated acid hydrolysis process is in the range of 10-30%. The process occurs at low temperatures, producing high hydrolysis yields of cellulose (i. e. 90% of theoretical glucose yield) (Iranmahboob et al., 2002).

However, this process requires large amounts of acids causing corrosion problems to the equipments. The main advantage of the dilute hydrolysis process is the low amount of acid required (2-5%). However this process is carried out at high temperatures to achieve acceptable rates of cellulose conversion. The high temperature increases the rates of hemicellulose sugars decomposition thus causing the formation of toxic compounds such as furfural and 5-hydroxymethyl-furfural (HMF). These compounds inhibit yeast cells and the subsequent fermentation stage, causing a lower ethanol production rate (Larsson et al., 1999; kootstra et al., 2009). In addition, these compounds lead to reduction of fermentable sugars (Kootstra et al., 2009). In addition, high temperatures increase the equipment corrosion (Jones & Semrau, 1984).

In 1999, the BC International (BCI) of United States has marketed a technology based on two-step dilute acid hydrolysis: the first hydrolysis stage at mild conditions (170-190°C) to hydrolyze hemicellulose; the second step at more severe conditions to hydrolyze cellulose 200-230°C (Wyman, 1999).

In 1991, the Swedish Ethanol Development Foundation developed the CASH process. This is a two-stage dilute acid process that provides the impregnation of biomass with sulphur dioxide followed by a second step in which diluted hydrochloric acid is used. In 1995, this foundation has focused researches on the conversion of softwoods using sulphuric acid (Galbe & Zacchi, 2002).