**3.1. Liquid hot water (LHW) pretreatments**

The liquid hot water (LHW) treatments are also called hot compressed water treatments, hy‐ drothermolysis [22, 23], aqueous or steam/aqueous fractionation [24], uncatalyzed solvolysis [25, 26], and aquasolv [27]. LHW is based on the use of pressure to keep water in the liquid state at elevated temperatures (160–240 °C). This process changes the biomass native struc‐ ture by the removal of its hemicellulose content alongside transformations of the lignin structure, which make the cellulose more accessible to the further enzymatic hydrolysis step [1, 28]. Differently from steam-explosion treatment, LHW does not use rapid decompression and does not employ catalysts or chemicals. Nevertheless, as with the acid treatment, LHW depolymerizes hemicelluloses to the liquid fraction. In this case, sugars are removed mostly as oligosaccharides, and the formation of the inhibitors furfural and 5-hydroxymethyfurfu‐ ral (HMF) is at a slightly lower level [28], depending on the process conditions. To avoid the formation of inhibitors, the pH should be kept at between 4 and 7 during the pretreatment, because at this pH, hemicellulosic sugars are retained in oligomeric form, and monomer for‐ mation is minimized. The removal of hemicellulose also results in the formation of acetic acid in the liquid fraction.

LHW pretreatment, whose most important parameters are the biomass moisture content, the operation temperature, and the residence time [29], is usually done in a pressure tank reac‐ tor where two streams can be obtained after filtration of the biomass slurry: a solid, cellu‐ lose-enriched fraction and a liquid fraction rich in hemicellulose-derived sugars. The solid phase is therefore constituted by cellulose and lignin along with residual hemicellulose. There are three types of rector design for LHW pretreatment. For co-current reactors, the bi‐ omass liquid slurry passes through heat exchangers where it is heated to the appropriate temperature (140–180 °C) and kept for 10–15 minutes as the slurry passes through an insu‐ lated plug-flow snake-coil, followed by the slurry-cooling concomitant to heat recovery via the countercurrent heat exchange with the incoming slurry. Flow-through technologies pass hot water at 180–220 °C and approximately 350–400 psig. The resulting pretreated biomass has enhanced digestibility and a significant portion of the lignin is also removed. In counter‐ current pretreatment, the biomass slurry is passed in one direction while water is passed in another in a jacketed pretreatment reactor. Temperatures, back pressures, and residence times are similar. In the flow-through pretreatment reactor, water or acid is passed over a stationary bed, and removes some of the biomass components including lignin. Although LHW can result in the partial depolymerization and solubilization of lignin, the re-conden‐ sation of lignin-derived, soluble compounds is also observed. Flow-through systems have been reported to be more efficient in terms of hemicellulose and lignin removal in compari‐ son to batch systems for some types of biomass via the addition of external acid during the flow-through process [30].

There have been many studies on the use of LHW for the pretreatment of corn fiber [28, 30-33], wheat straw [34, 35], and sugarcane bagasse [36, 37]. Studies on woody biomass from *Eucalyptus* [38-40], and olive tree biomass [41] have also been reported.

Several works have reported about the optimal LHW pretreatment conditions in terms of temperature and residence time. For the pretreatment of corn stover the best conditions were reported to be 190 °C and 15 min, resulting in a 90% of cellulose conversion after en‐ zymatic hydrolysis [33], while for wheat straw the optimum treatment temperature was found to be 188 °C during 40 min, which resulted in 79.8% of cellulose conversion and re‐ leasing of 43.6% of hemicellulose derived sugars to the liquid fraction. Nonetheless, when response variables were analyzed separately, the best conditions for the recovery of hemi‐ cellulose-derived sugars from wheat straw, at up to 71.2%, were found at 184 °C during 24 min, whereas the optimal conditions for a cellulose conversion of 90.6% were found to be 214 °C during 2.7 min [35].

ilarly to what is observed for LHW, the hemicellulose fraction is extracted by direct contact with water-saturated steam and due to the high temperatures and high solids concentration (around 50%), lignin and biomass polysaccharides can be extracted and degraded, releasing derived products such as furfural, HMF, and derived acids at high concentrations [36].

Sugarcane and Woody Biomass Pretreatments for Ethanol Production

http://dx.doi.org/10.5772/53378

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Sugarcane bagasse pretreatment by steam using high solids content (≥ 50%) at 200 °C for 10 min allowed a poor xylan recovery of 12%; nevertheless, an SSF yield of 79% was ob‐ served and the dissolved xylan content was found to be 89%. When the steam treatment was carried out at 220 °C for 2 min, the xylan recovery was increased to 48%, and the SSF yield and the dissolved xylan content were 85% and 88%, respectively, indicating the effi‐ ciency of high temperatures coupled with very short pretreatment times for high solids

Aiming to improve the recovery of xylan sugars, most of the steam treatment studies report the use of SO2 as the catalyst. When this procedure was used for sugarcane bagasse, it al‐ lowed for the recovery of 57% of hemicellulose-derived sugars and minimal amounts of sug‐ ar-degradation compounds were formed. The overall highest sugar yield achieved from the bagasse cellulose enzymatic hydrolysis was 87% [43]. There are several reports on the use of steam pretreatment associated with rapid decompression as a pretreatment for several bio‐ mass types. In this chapter, this type of treatment will be addressed as a steam-explosion

LHW and steam pretreatments are attractive from a cost-savings perspective, as they do not require the addition of chemicals such as sulfuric acid, lime, ammonia, or other catalysts. Moreover, the reactors do not require high cost materials and maintenance due to their lowcorrosion potential. Additionally, these treatments do not alter the biomass glucan content, as a glucose recovery rate of 97% was observed for sugarcane bagasse that was pretreated by both methods [36]. The main differences between the features of the two treatments re‐ lates to hemicellulose extraction, which is higher for the LHW, and the biomass load, which is higher for the steam pretreatment, with the obvious corresponding advantages and disad‐ vantages. In contrast to steam pretreatment, LHW allows for a higher xylan recovery associ‐

Steam explosion is one of the most used methods for lignocellulosic biomass pretreatment. This process was initially developed in 1926 by the Masonite Corporation, Canada, for the production of fiberboard from wood [44]. From 1970–1980, the process was adapted to treat wood and agriculture residues aiming at improving the cellulose enzymatic hydrolysis and cattle feed production from lignocellulosic materials. A batch-type device was available from Iotech Corporation at pilot-plant scale in 1983 and a continuous device was available

**3.3. Advantages and disadvantages of LHW and steam pretreatments**

ated with the lower formation rate of inhibitors.

from Stake Technology, both from Canada, in the '80s.

**4. Steam-explosion pretreatment**

concentrations [36].

treatment.

For sugarcane bagasse, top-performing LHW runs are favored by high temperatures (≥ 220 °C) and a short residence time (≤ 2 min) associated with low solids concentrations (≤ 5%), reaching 87% of simultaneous saccharification and fermentation (SSF) conversion, and 81% of xylan recovery. However, it is reported that the use of LHW using a solid concentration of more than 1% can significantly decrease the ethanol fermentation rate due to inhibition [36].

The LHW pretreatment of *Eucalyptus* biomass was studied in two steps as follows: in the first step the pretreatment in which a temperature range from 180 to 200 °C was studied gave the highest total xylose recovery yield of 86.4% at 180 °C for 20 min. In the second step of the pretreatment, a temperature range from 180 to 240 °C was studied for intervals of time up to 60 min. The authors concluded that the efficiency of LHW for the cellulose con‐ version rate was more sensitive to temperature than residence time. The optimum reaction conditions for the second step of the pretreatment with minimal degradation of sugars were found to be 200 °C for 20 minutes, where the total sugar recovery from *Eucalyptus grandis* after 72 h of enzymatic saccharification reached 96.63%, which is superior to the yield from a single-step pretreatment with hot water or diluted acid [40].

LHW pretreatment of olive tree biomass resulted in a 72% glucose yield from cellulose hy‐ drolysis after 72 h of saccharification using 2% of solids concentration during pretreatment, while for higher solids content the glucose yields were strongly affected reaching 70%, 60%, 57%, and 43% when using 5%, 10%, 20%, and 30% of solids, respectively [41]. A two-stage process which combines the LHW for hemicellulose removal and a treatment for delignifica‐ tion (e.g. ammonia pretreatment) has also been suggested for further improvement of enzy‐ matic hydrolysis [42].

#### **3.2. Steam pretreatment**

The steam treatment is quite similar to LHW, with the major difference between the process‐ es being related to the contact of the liquid phase with the biomass. For LHW, the biomass is in direct contact with the liquid phase at the bottom of the reactor, which prevents the use of high solids content, while in the steam pretreatment, the biomass is at the top of the reactor and not in direct contact with the liquid phase, in a similar manner to that of an autoclaving process. Using steam pretreatment means that it is possible to use a higher solids content of 50% or more, whereas for the LHW, in most cases, the solids content is lower than 10%. Sim‐ ilarly to what is observed for LHW, the hemicellulose fraction is extracted by direct contact with water-saturated steam and due to the high temperatures and high solids concentration (around 50%), lignin and biomass polysaccharides can be extracted and degraded, releasing derived products such as furfural, HMF, and derived acids at high concentrations [36].

Sugarcane bagasse pretreatment by steam using high solids content (≥ 50%) at 200 °C for 10 min allowed a poor xylan recovery of 12%; nevertheless, an SSF yield of 79% was ob‐ served and the dissolved xylan content was found to be 89%. When the steam treatment was carried out at 220 °C for 2 min, the xylan recovery was increased to 48%, and the SSF yield and the dissolved xylan content were 85% and 88%, respectively, indicating the effi‐ ciency of high temperatures coupled with very short pretreatment times for high solids concentrations [36].

Aiming to improve the recovery of xylan sugars, most of the steam treatment studies report the use of SO2 as the catalyst. When this procedure was used for sugarcane bagasse, it al‐ lowed for the recovery of 57% of hemicellulose-derived sugars and minimal amounts of sug‐ ar-degradation compounds were formed. The overall highest sugar yield achieved from the bagasse cellulose enzymatic hydrolysis was 87% [43]. There are several reports on the use of steam pretreatment associated with rapid decompression as a pretreatment for several bio‐ mass types. In this chapter, this type of treatment will be addressed as a steam-explosion treatment.
