Physical-chemical methods of pretreatment are remarkably more effective than physical ones. The most employed physical-chemical methods are presented in Table 4.3. Steam explosion (autohydrolysis) is the most commonly used method along with dilute-acid process for pretreatment of lignocellulosic materials. The use of saturated steam at high pressure causes autohydrolysis reactions in which part of the hemicellulose and lignin are converted into soluble oligomers with the help of some acids released from the biomass itself during the process. The

Physical Methods for Pretreatment of Lignocellulosic Biomass for Ethanol Production

Physical-Chemical Methods for Pretreatment of Lignocellulosic Biomass for Ethanol Production

Physical-Chemical Methods for Pretreatment of Lignocellulosic Biomass for Ethanol Production

factors affecting steam explosion pretreatment are residence time, temperature, chip size, and moisture content (Sanchez and Cardona 2008). In this process, the combined action of temperature and contact time of steam with the biomass is achieved. To quantify this effect, the severity index has been defined (Shahbazi et al., 2005; Soderstrom et al., 2003), and the pretreatment severity is described as a function of time t (in min) and temperature T (in degrees Celsius) related to a reference temperature of 100°C:

(4.1)

If the steam explosion process is carried out under acidic conditions (which increases the efficiency of cellulose hydrolysis), an additional term should be intro­duced in Equation (4.1) to consider the effect of pH on the combined severity (CS):

CS = log R0 — pH

The pH can be calculated from the amount of sulfuric acid added to the material and its water content (Soderstrom et al., 2003).

An important factor to be considered when pretreatment methods are used (including steam explosion) is the particle size of the lignocellulosic materials. The conventional mechanical methods require 70% more energy than steam explosion in order to achieve the same reduction in the particle size. During steam explosion, some inhibitors of the subsequent biological processes (enzymatic hydrolysis, fermentation) are formed. Inhibitors formation during the pretreat­ment requires the washing of pretreated biomass. This decreases the global yield of the saccharification due to the removal of sugars generated during the hemi — cellulose hydrolysis. Typically, 20 to 25% of the initial dry matter is removed by the washing water (Sun and Cheng, 2002). For this reason, the use of very small particles in some cases (e. g., herbaceous wastes) is not desirable if taking into account the economy of the process (Ballesteros et al., 2002).

Steam explosion is recognized as one of the most efficient methods for hard­wood (poplar, oak, birch, maple) and agroindustrial residues, but it is less efficient for softwood (pine, cedar; Sanchez and Cardona, 2008). For instance, an increase of 90% in the efficiency of the subsequent enzymatic hydrolysis of poplar chips pretreated by steam explosion was reported compared to 15% efficiency when pre­treatment of chips is not carried out (Sun and Cheng, 2002). In the case of cane bagasse pretreatment, Kaar et al. (1998) determined the conditions that maximize sugars concentration by varying the temperature within the range of 188 to 243°C and residence time within the range of 0.5 to 44 min. These authors concluded that these conditions strongly depend on the composition of the lignocellulosic mate­rial and it demonstrated the formation of furfural. On the other hand, it has been reported that the susceptibility of the pretreated substrate to the action of cellulases

Process Synthesis for Fuel Ethanol Production

is highly influenced by the steam pressure and vaporization time during the pre­treatment, as was demonstrated for rice straw. In particular, a steam pressure of 3.53 MPa during a short vaporization time (2 min) significantly increases the enzy­matic hydrolysis without any observable inhibitory effect (Moniruzzaman, 1996).

For the case of softwood that has an increased lignin content and is more difficult to degrade, a two-stage steam pretreatment has been proposed. In the first stage, the operating conditions are defined in such a way that the maximum amount of sugars derived from hemicellulose is obtained. In the second stage, more severe conditions are employed to degrade the solid fraction resulting from the first stage achieving a partial hydrolysis of cellulose. In both stages, the softwood sawdust is impregnated with dilute sulfuric acid. Shahbazi et al. (2005) propose a fractionation procedure for softwood based on steam explo­sion and alkaline delignification in order to produce ethanol and related co­products. An analogous fractionation procedure was utilized by Belkacemi et al. (2002) where the captured hemicellulose-rich liquor was enzymatically treated to produce xylose-rich solutions. Regarding the enzymatic hydrolysis of hemicellulose, Saha (2003) points out that there are no suitable commercial hemicellulase preparations that can efficiently hydrolyze feedstocks like corn fiber to monomeric sugars. This author also briefly reviews the microorganisms and enzymes that could be useful for degrading hemicellulose. Other analogous schemes involve two hydrolysis stages (Nguyen et al., 1999) or an initial treat­ment by steam explosion followed by an acid hydrolysis to completely degrade the xylans with a further acid recovery (Saska and Ozer, 1995).

One of the methods with better indexes is the pretreatment with liquid hot water (LHW) or thermohydrolysis. Laser et al. (2002) mention that under optimal conditions, this method is comparable to the dilute acid pretreatment, but with­out the addition of acids or production of neutralization wastes. In addition, this method presents elevated recovery rates of pentoses and does not generate inhibi­tors (Ogier et al., 1999). Nevertheless, solid load for this method is much less than for the steam explosion method, which is usually greater than 50%.

Another physical-chemical method is the ammonia fiber explosion (AFEX) process whose function is similar to steam explosion. The pretreatment with ammonium does not generate inhibitors for subsequent biological processes, so the washing with water is not necessary. In addition, a small particle size is not required. For this method, Dale et al. (1996) report experimental data correlat­ing conditions of the AFEX process, enzyme doses during cellulose hydrolysis, and the corresponding yields for several agricultural and lignocellulosic residues. Similarly to the AFEX method and steam explosion, CO2 explosion uses the same principle, but the yields are relatively low compared to the other methods (Sanchez and Cardona, 2008; Sun and Cheng, 2002).