The development of the SPORL process is based on the fundamental understandings of sulfite pulping [109]. Usually the SPORL pretreats the woodchips in an aque­ous sulfite solution at 160-180 °C and pH 2-4 for about 30 min. The woodchips are then fiberized (size-reduced) using a disk mill to generate fibrous substrate for subsequent saccharification and fermentation. With low pretreatment cost, excel­lent substrate digestibility, along with sulfite pulping and chemical recovery, and disk refining technologies that have long been practiced in the pulp and paper in­dustry, and existing industry infrastructure and commercial markets for high-value co-products from pretreatment-dissolved hemicellulose sugars and lignin, SPORL has low environmental and technological barriers and risks [112].

8.3.4.1 Mode of Action

Since the SPORL process is based on the sulfite pulping, this pretreatment chemistry is also similar to sulfite pulping. The major chemistry related to hemicellulose, cellulose, and lignin can be summarized as follows:

• A considerable amount of hemicellulose degradation and removal takes place during the pretreatment, as evidenced by the predominant Xyl content in pretreated effluent [113].

• Thedegrees of polymerization of xylan [114-117] and cellulose [118] are reduced.

• Sulfonation of lignin increases the hydrophilicity of lignin, which may promote the aqueous enzyme process.

• The degrees of dissolution of hemicellulose, degradation of cellulose, and sul­fonation and condensation of lignin are increased as reaction time and temperature increases, and pH decreases [111, 119].

It should be noted that the production of fermentation inhibitors HMF and furfural in the SPORL is significantly lower than those in dilute acid, which is favorable to the fermentation of pretreatment-dissolved sugars from cellulose and hemicellulose. Excellent performance of the SPORL with different wood species indicates that this process may be tree species independent [109].