1.2 Pretreatment

Lignocellulose containing biomass has to be pretreated prior to hydrolysis to improve the accessibility of the biomass. For this pretreatment, several processes are available: mechanical treatment for size reduction (e. g. chopping, milling, grinding), hydrothermal treatment (e. g. uncatalysed steam treatment with or without steam explosion, acid catalysed steam treatment, liquid hot water treatment) and chemical treatment (e. g. dilute acid, concentrated acid, lime, NH3, H2O2). Diverse advantages and drawbacks are associated with each pretreatment method (Mosier et al., 2005; Hendriks & Zeeman, 2009; Chen & Qui, 2010; Talebnia et al., 2010).

Steam explosion is a widely-employed process for this pretreatment. This process combines chemical effects due to hydrolysis (autohydrolysis) in high temperature water and acetic acid formed from acetyl groups, and mechanical forces of the sudden pressure discharge
(explosion). The steam explosion process offers several attractive features when compared to other technologies. These include less hazardous process chemicals and significantly lower environmental impact (Alvira et al., 2010). Typical operation conditions for steam explosion treatment of straw — temperature and duration of treatment — are summarised in Table 1.

According to Overend and Chornet (1987), the severity of the pretreatment can be quantified by the severity factor R0. The severity factor combines the temperature of the pretreatment (T in degree Celsius) and the duration of the pretreatment (t in minutes) thus:

The severity factor is based on the observation that it is possible to trade duration of treatment and the temperature of treatment so that equivalent final effects are obtained. However, it is not intended to give mechanistic insight into the process.