LB is bulky and much of the volume is "empty" or saturated by air. Air in LB can be replaced by liquid water at high temperature and pressure. When water-saturated LB is suddenly exposed to low pres­sures, liquid water suddenly expands when vaporized forcing LB to disintegrate into fine particles. The Masonite process was invented in 1926 (Mason, 1926) employing this water to steam explosion process. Since then, the steam explosion pretreatment (SEP) has been a common technique (Mason, 1928). SEP is used to break the crystalline and lignocellulosic structure of biomass into its three major components, cellulose, hemicellulose and lignin. SEP enhances the resultant cellulose’s sus­ceptibility to enzymatic hydrolysis. High-pressure, satu­rated steam is applied to biomass for a brief period and then allowed to rapidly decompress to atmospheric pressure, hence the term explosion.

The explosion breaks up solid particles and is used as a standard practice in chemical pulping operations. The steam is vented and the biomass is discharged to a larger vessel for rapid flash cooling (Mosier et al., 2005). SEP is as much a mechanical process as it is a thermal process (Holtzapple et al., 1989). Regardless, the explosion per se, whether it causes particle disintegration or not, does not play a significant role in producing a product that is easily digested by enzymes (Brownell et al., 1986). A more likely mechanism at play is the treat­ment’s effect in removing hemicellulose (Mosier et al.,

2005) . Applying acid catalysts, usually SO2 (or sulfite), enhances this effect by reacting, in conjunction with wa­ter, within the interstitial spaces to form sulfuric acid and thus catalyze hemicellulose degradation (Gregg and Saddler, 1996b).

SEP effectiveness and the chemical changes that take place depend on residence time, temperature, chip size, and moisture content. Effectiveness is determined by the amount of hemicellulose solubilized and the rate of sub­sequent enzymatic hydrolysis. Optimal outcomes are obtained when pretreatment occurs at either high tem­perature or short residence time, such as 270 °C for 1 min, or at lower temperature and longer residence time, such as 190 °C for 10 min. Generally, initial treat­ment pressures range from 0.69 to 4.83 MPa and treat­ment temperature ranges from 160 °C to 260 °C.

At high temperatures water acts as an acid. Thus, dur­ing the treatment time, the hemicellulose hydrolyzes into soluble sugars. The hemicellulose is considered to auto — hydrolyze as a result of exposure to the acetyl groups in the organic acids formed at these high temperatures. Acetic acid is formed from the acetylated hemicelluloses. The pH during SEP is kept quite low, near pH 3—4. SEP degrades a significant portion of the hemicellulose (Sun et al., 2005). However, degradation of hemicellulose may not stop at this point. If the treatment conditions are severe, the solubilized hemicellulose may undergo a series of secondary reactions that yield furfural and HMF. These severe conditions may be high temperature or a long incubation time. Furfural and HMF are

Water with dissolved extracts

FIGURE 27.6 Schematic diagram illustrating three types of liquid hot water reactors: (a) cocurrent, (b) countercurrent, (c) flow through. (For color version of this figure, the reader is referred to the online version of this book.)

undesirable products that inhibit enzymatic hydrolysis and limit the effectiveness of fermentation.

Meanwhile, lignin is partially depolymerized, some lignin is redistributed within the material and some may be removed completely from the fibers, each of which contribute to an improved exposure of the cellu­lose domains (Chen and Qiu, 2010). The reduction in hemicellulose and partial removal of lignin exposes the cellulose surface and thus improves the ability of the enzyme to attack the cellulose microfibrils (Alvira et al., 2010). If the treatment conditions are severe, some degradation of cellulose to glucose can occur. One study reported an enzymatic hydrolysis efficiency of 90% over 24 h using poplar chips using SEP. This was significantly better than the control where the enzy­matic hydrolysis efficiency was 15% from untreated
poplar chips (Grous et al., 1986). That said, SEP is more effective on agricultural residues than in wood as a result of the lower acetic acid content in the hemicellu — lose portion of the biomass.

Adding a supplemental acid to the SEP reduces both residence time and temperature. Adding an acid such as H2SO4 (or SO2) or CO2, typically 0.3—3% (w/w), im­proves hydrolysis, decreases the production of inhibi­tory compounds and leads to a more complete removal of hemicellulose (Kumar et al., 2009a). For the effective treatment of softwoods, adding an acid catalyst is essential to make the substrate susceptible to enzy­matic hydrolysis. Adding a supplemental acid also im­proves the enzymatic hydrolysis of the residual solids and decreases the production of inhibitory compounds (Morjanoff and Gray, 1987).

These three parameters, the level of H2SO4 (or SO2) or CO2, the residence time and the temperature, are the most influential parameters on total sugar yield. For SEP treatment of sugarcane bagasse, the optimal condi­tions are 1% H2SO4, 220 °C; 30 s residence time, and a water-to-solids ratio of 2:1 (Holtzapple et al., 1989). After SEP treatment under these conditions sugar production was determined to be 65.1 g sugar/100 g.

A two-step SEP is a good pretreatment for softwood (Soderstram et al., 2003). In this case, the first step is to optimize the amount of hydrolyzed hemicellulose by employing low severity conditions where the biomass is treated at 180 °C for 10 min with 0.5% H2SO4. In the second step, the solid material from the first step is washed and impregnated again with H2SO4. SEP is applied again using more severe conditions. This time the biomass is treated at 180 °C—220 °C for a longer time, between 2 and 10 min, and with a higher concen­tration of acid catalyst, 1—2% H2SO4. These treatments appear to hydrolyze a portion of the cellulose and make it more accessible to enzymatic attack (Sassner et al., 2008). The most favorable conditions for Salix wood is to impregnate it with 0.5% H2SO4 at 200 °C for between 4 and 8 min. The yield is thus

55.6 g glucose and xylose per 100 g dry biomass (Sassner et al., 2008).

If one uses SO2 as the impregnating agent in spruce chips, the sugar yield is almost independent of impreg­nation time and slightly increases with decreasing chip size (Monavari et al., 2009). Shorter impregnation times result in slightly lower mannose yields in larger chips. The optimum pretreatment conditions when using SO2-catalyzed SEP for lodgepole and Douglas fir pine is 200 °C for 5 min with 4% SO2 (w/w) (Ewanick et al., 2007; Kumar et al., 2010).

Another option for an impregnating agent is to use a weak organic acid, in particular, lactic acid (Monavari et al., 2011). It was observed that it was not efficient and resulted in lower sugar yields in spruce, with or without the addition of SO2. However, using a weak organic acid is more environmentally friendly than us­ing an inorganic acid as it would biodegrade in a waste stream or be used for production of a biogas such as methane.

Particle size, by itself, is not a significant contributor to SEP effectiveness. Some studies report that larger par­ticle size may improve the outcomes from SEP (Cullis et al., 2004). In these studies, pretreated Douglas fir, a softwood, was milled to three particle sizes: <0.422 mm screenings, 1.5 x 1.5 cm and 5 x 5 cm. They were then steam exploded using SO2. It was observed that the largest particle size suffered less from pretreat­ment severity and had the highest cellulose recovery. It had larger quantities of solubilized carbohydrate and contained fewer furan degradation products. The smaller particle sizes produced outcomes containing more solubilized hemicellulose and lignin. If the resul­tant biomass is further refined to particles of a finer size by plate milling, with a 0.178 mm gap, the initially larger particle size showed a higher lignin removal with peroxide washing and a greater rate of enzymatic hydrolysis. This is likely due to the reduction in lignin redeposition as a result of treatment severity.

These findings were substantiated in studies of steam-exploded pine, another softwood (Ballesteros et al., 2000). The largest of the sizes exhibited a higher cellulose recovery and also a higher content of solubi­lized hemicellulose. Conversion of cellulose to glucose was only slightly higher from the larger particle sizes. However, the total recovered glucose, including the sol­ubilized glucose from the steam explosion, was much higher when starting with the largest particle size (Ballesteros et al., 2000).

Starting with a herbaceous feedstock, such as Brassica carinata residues, produced different results. Although the cellulose recovery was still higher for the 8 and 12 mm fractions, the smaller particle sizes performed better during enzyme hydrolysis. The 5—8 mm and 2—5 mm fractions yielded 100% while the 8—12 mm fraction produced 85% (Ballesteros et al., 2002). This suggests that lignin condensation is not as influential in herbaceous feedstock. It is a critical factor when pretreating softwood.

In softwood, the larger particle size produces a higher maximum glucose yield, over 80%, compared to smaller particle sizes with yields under 70% (Ballesteros et al.,

2002) . SEP-treated hardwood exhibited no difference in either enzyme digestibility or ethanol yield between disparate particle sizes, in particular between 2—5 mm and 12—15 mm (Negro et al., 2003). This indicates that the severity of the treatment plays a larger role than par­ticle size when using softwood. In these cases, smaller particles increase the lignin condensation and recalci­trance to enzyme hydrolysis.

Overall, SEP is an attractive pretreatment method considering the low amount of energy required to reduce the biomass size compared to mechanical comminution. Conventional mechanical size reduction methods require 70% more energy than SEP to achieve the same particle size reduction (Mittal et al., 2009). Additionally, SEP is attractive because there are no recy­cling or environmental concerns. This too lowers cost. SEP is thus recognized as one of the most cost-effective pretreatment methods for hardwoods and agricultural wastes. It has been extensively tested on a wide array of lignocellulosic feedstock. It has been observed that SEP is less effective for the pretreatment of softwoods.

The most significant limitations include the partial destruction of xylan, incomplete disruption of the lignin-carbohydrate matrix, low lignin removal, and lignin redistributes over the surface of cellulose (Chen et al., 2010). Additionally, there is a risk of producing un­desirable compounds such as furfural, HMF and other soluble phenolic compounds. These undesirables inhibit microbial growth and enzymatic hydrolysis. Thus, prior to fermentation, SEP-treated LB must be washed with water to remove these undesirable materials along with water-soluble hemicellulose. Unfortunately, this wash lowers the overall effectiveness as it washes away around 20—25% of the initial dry matter and a portion of the soluble sugars (Sun and Cheng, 2002).