Also referred to as auto-hydrolysis, the use of steam for the pre-treatment of lignocel — lulosic biomass is one of the most widely demonstrated and implemented methods in research and commercial facilities [7, 8]. This method employs the use of high — pressured saturated steam for the treatment of the lignocellulosic biomass. Steam pre-treatment is usually conducted at reaction temperatures of 160-260 °C using high pressures of 0.69-4.83 MPa with the period in which the biomass is exposed to this conditions ranging from several seconds to a few minutes [9]. A further clas­sification of the steam pre-treatment can be made depending on if the use of high pressured steam in the process is followed by a sudden reduction of the process pressures. The process is thus regarded as ‘Steam explosion’ where such abrupt de — pressurisation and cooling of the lignocellulosic biomass after a specified reaction period is implemented. This is since the abrupt pressure reduction results in the ex­plosive decompression of the biomass materials, thus facilitating a disruption of the lignocellulosic biomass cell walls and enhancing the accessibility of the biomass macromolecular contents. The conditions facilitated by the high-pressured steam (for both explosive and non-explosive methods) facilitates a disruption of the lignin sheath and enhances a solubilisation of the biomass hemicellulosic component (via hydrolysis), thus aiding the accessibility of cellulose to further conversion methods, that is, enzymatic hydrolysis [10]. For example, a 90 % enzymatic hydrolysis of Poplar biomass chips was achieved in 24 h after it was subjected to a steam explo­sion pre-treatment compared to only a 15 % hydrolysis obtained for the untreated chips [11].

During the steam pre-treatment, some of the biomass lignin is solubilised (which however re-polymerises on cooling and forms a part of the acid-soluble lignin frac­tion), with the biomass hemicellulose solubilised after the application of the steam pre-treatment and is subsequently recovered in the aqueous fraction (or is further degraded to other compounds, i. e. furfural) [ 12]. Most of the cellulose content of the biomass is preserved in the solid fraction, however, its hydrolysis to glucose could be obtained under high steam pre-treatment temperature conditions (i. e. >200 °C) [12]. The hydrolysis of the hemicelluloses is proposed to be brought about mainly by the action of acetic acid formed from the acetyl groups released during the steam pre­treatment [9]. In addition to this, other acids, that is, formic, levulinic and pyromucic acids, produced during the steam pre-treatment process as described [13], may also play an important role in the acid catalysed breakdown of the hemicellulosic glyco — sidic bonds [13]. Furthermore, water at high temperatures has been demonstrated to possess some acid properties, which could also enhance the hemicelluloses hydrol­ysis [14]. The acidic conditions provided by the steam pre-treatment could thus also lead to the degradation of available sugars in the biomass materials [15].

The potential benefits accruable with the use of steam pre-treatment over other pre-treatment systems have been widely demonstrated in the literature. Compared to the use of commonly applied chemical pre-treatment techniques, that is, sodium hy­droxide, calcium hydroxide (lime) and dilute sulphuric acid hydrolysis, higher treated products recovery as well as improved substrate availability for further processing has been obtained with the application of auto-hydrolysis [16-19]. The implementa­tion of mechanical biomass pre-treatment routes such as milling and novel methods like microwave radiation have also been demonstrated to less effective than the use of high-pressure steam for the pre-treatment of lignocellulosic biomass [18, 19]. Furthermore, the application of non catalytic steam pre-treatment methods was dis­cussed in to have a lower process energy requirements compared to mechanical comminution methods (70 % less, to achieve similar size reductions), with no or a low recycling or environmental costs attached [9].

The non-catalysed steam pre-treatment method (including steam explosion tech­niques) has been widely demonstrated both in the literature and in practice to be utilisable for a variety of lignocellulosic biomass, that is, from forest products and residues (including the use of shortrotation woody biomass) [17,18,20-22], purpose grown energy crops [9], as well as from agricultural residues [16, 19]. Regarding the use of woody biomass, it was seen that the use of younger biomass materials were easier to fractionate during the steam pre-treatment process and thus better substrates production for subsequent enzymatic hydrolysis, when compared to the use of older materials [13, 23]. This is, therefore, promising for the integration of steam pre-treatment with ongoing and proposed large-scale short rotation forest and fast growing purpose grown lignocellulosic biomass projects. The use of this method is however less effective for the pre-treatment of softwood (i. e. pine) where the acid catalysed route as described in Sect. 3.2.2.1 is better suited.