14.5.3.1 Liquid hot water batch pretreatment

In addition to uncatalyzed steam explosion pretreatments, other uncatalyzed pretreatment processes using pressurized liquid hot water without rapid decompression have been inves­tigated. Process conditions have been developed for cellulose hydrolysis at very high temper­atures of about 260°C (10, 43) and as a pretreatment approach for achieving hemicellulose hydrolysis at lower temperatures of about 200-230°C (44, 45). High yields of soluble sugars from the hemicellulose fraction of some biomass types (primarily herbaceous crops and agricultural residues) can be achieved, but liquid hot water processes generally liberate the sugars in an oligomeric form and thus require a secondary acidic or enzymatic hydrolysis step to produce fermentable monomeric sugars.

Another approach for liquid hot water pretreatment uses some chemicals as agents to control the pH in the range of 4-7 (10, 46). With some feedstocks, such as corn stover, there may be enough inherent buffering capacity from the feedstock that the target pH range is achieved without any requirement of pH-controlling chemicals. The reason for controlling the pH is to retain the released hemicellulose sugars in oligomeric form as a means of controlling sugar degradation losses and fermentation inhibitor formation (10). However, recent data suggests that this approach may not be effective in achieving high enzymatic digestibility of the cellulose (>80% glucose yield from the available cellulose) in pretreated corn stover (17). In general, liquid hot water pretreatments are attractive from a process cost-savings potential (no catalyst usage, low-cost reactor construction due to low-corrosion potential), but these cost savings can be offset by lower overall sugar yields and the need for enzymatic hydrolysis conversion steps (21).