The mode of action of the catalysed steam pre-treatment method is similar to the auto-hydrolytic process described in the preceding section with the major difference being the impregnation of the lignocellulosic biomass with acidic gases or liquids (i. e. sulphur dioxide (SO2), sulphuric acid (H2SO4), nitric acid (HNO3) and hydrochloric acid (HCl)) which act as process catalysts prior to applying the steam pre-treatment.

The implementation of acid catalysis has been demonstrated to exhibit a higher pre-treatment efficiency than the use of the steam pre-treatment method alone. Com­pared with the use of the steam pre-treatment alone, the catalysed route has been observed to lead to a greater hemicellulose (near complete) removal with a reduced generation of inhibitory compounds [32]. This in turn results in an increased con­version potential of the post-treated fractions via an improvement of the biomass digestibility using enzymatic methods. An improved sugar recovery, especially as pentoses in the post treated aqueous phases has been shown by [13].

As highlighted in Sect. 3.2.1, due to the characteristic low reactivity and perme­ability of the fibres of softwoods to the penetrating steam used in the auto-hydrolytic process [13], acidic conditions could thus be employed with the use of this particular type of lignocellulosic biomass to enhance the conversion efficiencies obtainable with the use of the steam pre-treatment method. In addition, the use of the acid catalysts on such biomass materials have been seen to facilitate the use of lower steam temperatures and shorter exposure times [33]. As demonstrated in [31], more than 85 % of the original hemicellulose derived sugars was recovered in the aqueous phase after an acid catalysed steam explosion of Douglas fir wood chips at relatively lower steam severity conditions (175 °C, 7.5 min.) with SO2 (4.5 %, w/w) used as the process catalyst. In that study, it was also observed that all of the hexose components obtained from the biomass using low — and medium severities were readily converted to ethanol [31].