Steam explosion is a physio-chemical pretreatment, performed in a pressurized vessel at 190-270°C for 1-10 minutes at 200-450 psig pressure with sudden release of pressure to cause an "explosion" within the physical structure of the material. A potential drawback includes the processing of lignocellulosic biomass at elevated temperatures resulting in formation of inhibitory compounds that may further inhibit the fermentation process downstream.

Acid Pretreatment

Diluted- and concentrated-acids are used for acid pretreatment. Sulfuric acid, hydrochloric acid and phosphoric acid are the most common types of acids used. Sulfuric acid pretreatment for lignocellulosic biomass has been a popular method for many years. The use of diluted or concentrated acids is highly dependent on the properties of the lignocellulosic biomass (such as lignin content, crystallinity and available surface area). In addition, the pretreatment processing time is more dependent on the nature of the feedstock. For instance, hardwood and softwood have longer pretreatment times compared to grasses, primarily due to high lignin contents and high crystallinity indexes. The ability of concentrated-acid pretreatment to break lignin at lower temperature makes it a more suitable pretreatment than diluted acid pretreatment. The concentrated sulfuric acid pretreatment (70-77% concentration) is generally performed at 50°C. At higher temperatures, the formation of inhibitory compounds such as furfural and hydroxyl-methyl furfural takes place, inhibiting microorganisms such as

E. coli and S. cerevisiae during ethanol fermentation (Galbe and Zacchi 2002; Drapcho et al. 2008).