Sulfites are found to be efficient agents for pretreating LB, in both hardwoods and softwoods (Zhu et al., 2009). In sulfite pretreatment to overcome recalcitrance of lig — nocelluloses (SPORL), the sulfite refers to any sulfite, bisulfate or combination. A combination may contain any two of the following three reagents: sulfite (SO§_), bisulfite (HSOL), and sulfur dioxide (SO2, or H2SO3). The specific combination to use depends on the pH of
the pretreatment liquor and the temperature (Zhu et al., 2009).

The first step in the process is to treat wood chips or another LB feedstock with a sulfite salt solution where the salt may be sodium, magnesium or calcium. This first step usually operates at a temperature between 160 °C and 190 °C and at a pH between two and four for 10—30 min. The second step is to fiberize the resul­tant biomass using a disk mill. This yields a fine fibrous substrate suitable for robust saccharification and fermentation (Shuai et al., 2010).

The typical acid charge on oven-dried wood is 0.5—1% for hardwood and 1—2% for softwood. The typical bisulfite charge is 1—3% for hardwood and

4— 8% for softwood (Zhu et al., 2010b). More than 90% of the cellulose was converted from SPORL-treated spruce chips. In this case the oven-dried wood chips were treated with an 8—10% bisulfate and 1.8—3.7% sul­furic acid combination at 180 °C for 30 min. The resul­tant material was treated with enzyme hydrolysis for 48 h using 14.6 cellulase and 22.5 b-glucosidase per gram of substrate. Shuai et al., and Zhu et al., have per­formed comparative SPORL studies using dilute-acid pretreatments for both softwoods and hardwoods (Shuai et al., 2010; Zhu et al., 2010b). In these studies, it was observed that SPORL is better at saccharification of hexoses and pentoses than was a dilute acid (DA) treatment. In one case, where oven-dried spruce was treated at 180 °C for 30 min with 1% H2SO4 at a 5:1 liquor-to-wood ratio, 87.9% of the hexoses and pentoses were recovered using SPORL versus a similar DA treat­ment where 56.7% of the saccharides were recovered.

About 92.5% of the cellulose was recovered in an SPORL process utilizing a 9% sodium sulfite (w/w of wood) and 77.7% for the DA (Shuai et al., 2010). In another study of aspen, or Populus tremuloides, a comparison was made between an SPORL pretreatment using a combination of sulfuric acid and sodium bisulfite and a dilute sulfuric acid (DA) pretreatment. It was observed that nearly 60% more ethanol was produced from the SPORL-treated wood than from the DA-treated wood. In both cases enzymatic hydrolysis was conducted using 10 FPU cellulase per gram glucan for 120 h (Zhu et al., 2010b).

Table 27.4 highlights a handful of comparisons between treatment methods and their effectiveness where the pretreatment conditions were L/W = 3, disk-milling solids loading = 30% (the solid contents of pretreated wood chips), and disk plate gap = 0.76 mm. Sodium bisulfite charge was 8% on oven-dried wood for the two SPORL runs; sulfuric acid charge was 2.21 (w/w) on oven-dried wood for the DA and low pH SPORL runs, and 0 for the hot water and high pH SPORL runs.

SPORL is an attractive pretreatment method due to several features. SPORL generates much less furfural and hydroxymethylfurfural (HMF) than does a simple DA pretreatment. SPORL significantly enhances fermentation yields by weakening the hydrophobic rela­tionship between lignin and enzymes and enhancing saccharification of cellulose. One of the products of SPORL is a sulfonified lignin, which has potential eco­nomic value as a directly marketable coproduct within existing markets and for opening new markets. The energy consumption in the size reduction process is reduced by an order of magnitude. Lastly, SPORL has demonstrated commercial scalability with low techno­logical and environmental risks (Zhu et al., 2010b).