The major factors influencing the un-catalysed steam explosion processes are res­idence time, temperature, moisture content and particle size [20]. The process conditions which result in the best substrates for hydrolysis and the least amount of soluble sugars lost to side reactions (i. e. sugar dehydration) usually considered to be the optimum conditions [13]. The use of steam temperatures ranging from 140 to 240 °C have been investigated in the literature and its influence on the overall process efficiencies is usually associated with the period in which the biomass is exposed to the steam pre-treatment. An optimum solubilisation (and hydrolysis) of the hemi- cellulosic component of lignocellulosic biomass was reported to be realised either by using a combination of high temperatures and short residence times (i. e. 270 °C, 1 min) or lower temperatures with longer residence times (i. e. 190 °C, 10 min) [24]. This is due to the fact that at lower steam pre-treatment temperatures (i. e. 190 °C) the recovery of the obtained hemisellulosic sugars in the lignocellulosic hydrolysate is maximised, with the acid-labile biomass polysaccharides partially converted to water soluble sugars [25]. On the other hand, the drastic conditions provided by the use of increased steam temperatures would most likely facilitate an enhanced accessibility of the macromolecules of the biomass material, but with inevitable sugar losses [13]. The employment of high steam temperatures has also been demonstrated to lead to an increase in the relative amount of acid-insoluble lignin in the post-treated materials [23]. With the use of steam temperatures (i. e. 220-240 °C) and residence times, con­densation reactions involving the by-products are derived from the biomass lignin and hemicelluloses. The acid-soluble lignin was observed to increasingly occur, re­sulting in the formation and accumulation of acid-insoluble polymeric materials [26]. This knowledge of the extensive condensation driven modification of the biomass lignin during steam pre-treatment has two important implications: That the formed polymeric materials could cause an apparent increase in the overall lignin yields (even potentially higher than the theoretical lignin yield, based on the content of the starting biomass), and the fact that part of these by-products are likely to remain in the steam pre-treated material even after employing additional washing steps (i. e. alkaline washing) [13]. A careful compromise must therefore be considered with the use of these two important conditions with opposite trends. The selection of the best temperature and time conditions could therefore be dependent on the other pa­rameters such as the subsequent processing and conversion steps to be applied after the biomass pre-treatment and the targeted fuel or chemical which is aimed to be produced. In general, an increase in the steam temperatures would correspond to a decrease in the carbohydrate yields, while longer reaction times have been seen to favour an increased lignin condensation and a degradation of pentosan, with acid hy­drolysis observed to predominate over degradation reactions with the use of shorter exposure times [13]. Furthermore, the use of an energy-material assessment would prove useful to ascertain the benefits (if any) of compromising high-energy inputs with the potential substrate conversion efficiencies.

The extent of biomass drying or water content is an important economic and technical parameter with the processing of lignocellulosic biomass. This is since the biomass costs could be substantially increased depending on the methods used in achieving a reduction in the biomass moisture content. To minimise processing costs, single fuel and chemical production systems as well as biorefineries would therefore ideally prefer the utilisation of cheap unprocessed biomass (containing high mois­ture content) or naturally dry biomass (containing ~5-15 % moisture). Investigations have been carried out on the influence of the use of lignocellulosic biomass with vary­ing moisture contents on the steam pre-treatment process efficiencies. Using ‘green’ freshly harvested and air-dried Aspen wood chips (3.2 mm, in the direction of the fibres), with moisture contents (oven dried basis) of 108.2 and 7.16 %, respectively, investigations carried out in [21] showed that statistically comparable reducing sugar yields (after enzymatic hydrolysis) were obtainable with the use of the different lev­els of water in the biomass materials. However, with an increase in the chip size, the samples with lower moisture contents (i. e. the air died chips) were observed to attain the steam temperatures within a shorter time, thus facilitating improved and quicker pre-treatment conversions. The steam requirements for the auto-hydrolysis process as examined by that study were therefore observed to increase with an increase in the lignocellulosic biomass size and moisture content.

Regarding the influence of particle sizes on the biomass steam pre-treatment, the application of mechanical size reduction schemes, that is, chipping and milling is usually carried out before the pre-treatment stage mainly to improve handling and to ease the biomass transportation from the acquisition site. The biomass size has also been described to be a critical parameter to be considered for the pre-treatment and conversion reactor designs [21]. The use of small biomass sizes were discussed to be preferable for most batch and continuous process operations due to the enhanced heat transfer facilitated with the use of such sizes for the biomass treatment [13]. However, much finer materials with smaller particle sizes (i. e. sawdust) have been seen to be difficult to utilise in batch units, with the use of plug flow reactors em­ployed to increase the pre-treatment conversion efficiencies [13]. An investigation on the influence of a range of lignocellulosic biomass (Brassica sp.) with particle sizes of 2-5, 5-8 and 8-12 mm respectively using process steam temperatures of 190 and 270 °C and a residence time of 4 and 8 min as carried out in [16] showed that an extensive size reduction was not desirable for optimum pre-treatment of the lignocellulosic biomass using steam for all the different temperature and residence time conditions examined. This was demonstrated by the higher cellulose concentra­tions and subsequent enzymatic digestibilities exhibited by the largest particle sizes (i. e. 8-12 mm) studied.

Another important aspect with the application of the non-catalytic steam pre­treatment route is the consideration of the need to employ the explosive decompres­sion step or not. Studies carried out in [22] using Aspen chips concluded that the explosive decompression step of the steam explosion process was not important and contributed little or nothing to hydrolysis of the lignocellulosic biomass and eventual accessibility of the biomass cellulose contents. Similarly, the use of steam explo­sion for the pre-treatment of green Eucalyptus chips was observed not to yield any considerable hydrolytic improvements; however, the use of high-pressured steam without an abrupt decompression step for the pre-treatment of air-dried Eucalyptus chips was seen to result in the production of poor substrates for further hydrolysis, thus suggesting that the use of the explosive decompression scheme is most suited for hardwood chips with a low moisture content [27]. The results of that study were however in somewhat in contrast to that presented in the patent proposed in [28], which discussed that the explosion step was essential for the production of hydrolytic substrates with improved macromolecular accessibility.