**3. Steam explosion (SP)**

#### **3.1. Description**

In steam explosion, biomass is exposed to saturated steam at high pressure (0.5–4.8 MPa) for a maximum period of 60 min followed by sudden reduction of pressure to atmospheric or lower, resulting in explosive decompression of biomass into component fiber and fiber bundles. The explosion is triggered by evaporation within biomass cells and sudden drop of pressure around the biomass. Exploded materials experience increase in water retention and pore size and specific surface area. Consequently, the bulk density is decreased. To improve penetration efficiency and swelling, biomass is pre-soaked before pretreatment. While the buffering effects of free moisture reduce heat transfer and increase energy demand, bound moisture softens fibers and increase pretreatment efficiency [22]. Thus, by carefully regulating water content of feedstock, substantial gains in sugar yield can be obtained during enzymatic hydrolysis, with collateral benefits in reduced energy demand [23].

The pretreated solids comprise unhydrolyzed cellulose, chemically-transformed lignin, and residual hemicelluloses. The liquid hydrolysate, on the other hand, contains solubilized hemicelluloses in oligomeric forms, with concentrations of monomers usually exceeding similar situations under LHW. Hemicellulose is hydrolyzed via the breakdown of both glycosidic and hemicellulose-lignin bonds. Hydrolysis of parts (acetyl groups and uronic acid substitutions) of hemicelluloses—via the catalytic actions of protons generated from the autoionization of water—occurs to form acetic and other acids which enhance further fractionation of hemicellulose [24], and trigger the release of carbonium ions from benzyl alcohol structures in lignin which cause the breakdown of some of the β-O-4 structures in lignin leading to reduced molecular weight [25]. Simultaneously, condensation reactions may take place in the presence of electron-rich carbon atoms, resulting in lignin repolymerization [25, 26], with the composition affected by pretreatment severity [27].

The process is affected by temperature, reaction time, material size, moisture content and efficient mixing of biomass. The explosion mechanism and time which are independent of the severity factor are also known to affect yields [28]. Increasing reaction time and temperature decreases the degree of polymerization of cellulose [29]. Though severe conditions contribute to reduction in crystallinity and increase in moisture retention, they do not necessarily lead to increased hydrolysis rates due to possibility of thermal degradation of cellulose. Similarly, xylose recovery is reduced for longer pretreatment times due to formation of degradation products. Further, severe conditions increase the intensity of repolymerization and condensation reactions from byproducts of lignin, hemicellulose, and extractives leading to increased molecular weights of lignin [30]. This development reduces substrate amenability to enzymatic hydrolysis caused by the covering of cellulose surface with the repolymerised lignin-like materials (pseudo-lignin). The problem of lignin repolymerization was overcome by Li et al. [31] who used a carbonium ion scavenger (2-napththol) to achieve solubilize lignin, resulting in improved recovery (91%) as against 51% for steam pretreated aspen wood without the additive.

#### **3.2. Applications**

the recovery of C5 sugars after the first pretreatment in a commercial-scale plant, prompting Inbicon to settle for a simpler, one-stage treatment processs [15]. Currently, the Inbicon demonstration plant, which is based in Kalundborg (Denmark), processes about 4 tonnes straw/h

LHW offers improved digestibility of cellulose by enzymes due to the solubilization of hemicelluloses and avoidance of inhibitors. Compared to steam explosion, LHW gives lower concentrations of solubilized hemicellulose and lignin products due to higher water input as well as higher pentosan recovery. Generally, catalysts/chemicals are avoided resulting in no/ low neutralization demands and byproduct/precipitate generation, with additional benefits such as reduced risk of reactor corrosion and explosion. Reactor cost is lower compared to methods such as AFEX [18]. The effect of particle size reduction on hydrolysis is low, thus,

There are however drawbacks in LHW related to hemicellulose fractionation into large fractions of oligomers, and xylose yields are generally low, which affect sugar and ethanol yields. There is a risk of sugar degradation into byproducts such as carboxylic acids and furans at severe conditions [19, 20]. A major cost involved in LHW pertains to high energy used to generate saturated liquid water. Consequently, solid loadings are restricted to about 20% [21].

In steam explosion, biomass is exposed to saturated steam at high pressure (0.5–4.8 MPa) for a maximum period of 60 min followed by sudden reduction of pressure to atmospheric or lower, resulting in explosive decompression of biomass into component fiber and fiber bundles. The explosion is triggered by evaporation within biomass cells and sudden drop of pressure around the biomass. Exploded materials experience increase in water retention and pore size and specific surface area. Consequently, the bulk density is decreased. To improve penetration efficiency and swelling, biomass is pre-soaked before pretreatment. While the buffering effects of free moisture reduce heat transfer and increase energy demand, bound moisture softens fibers and increase pretreatment efficiency [22]. Thus, by carefully regulating water content of feedstock, substantial gains in sugar yield can be obtained during enzymatic

The pretreated solids comprise unhydrolyzed cellulose, chemically-transformed lignin, and residual hemicelluloses. The liquid hydrolysate, on the other hand, contains solubilized hemicelluloses in oligomeric forms, with concentrations of monomers usually exceeding similar situations under LHW. Hemicellulose is hydrolyzed via the breakdown of both glycosidic and hemicellulose-lignin bonds. Hydrolysis of parts (acetyl groups and uronic acid substitutions) of hemicelluloses—via the catalytic actions of protons generated from the autoionization of

hydrolysis, with collateral benefits in reduced energy demand [23].

and at yields greater than 198 L ethanol/tonne of wheat straw.

**2.3. Positive attributes and drawbacks**

44 Fuel Ethanol Production from Sugarcane

**3. Steam explosion (SP)**

**3.1. Description**

large biomass flowrates can be handled effectively.

SE has been applied in combination with additives and pretreatment methods to improve yields and overall process economics. The major variations include the use of acids and bases as catalysts.

#### **3.3. Acid-catalyzed steam explosion (ACSE)**

In this process, SE is undertaken after the biomass is soaked with dilute acid or impregnated with SO2 or CO2 at low or atmospheric pressures for 0.5–25 h depending on the temperature (5–100°C). It favors solubilization of hemicelluloses into monomer units, making substrates more reactive while improving enzymatic hydrolysis of cellulose. Compared to dilute acid, SO2 impregnates biomass substrates better and more uniformly but requires harsher conditions to remove hemicellulose [32]. Both SO2 - and CO2 -based SE create the formation of pores of different sizes and shapes in the outer region of the cell wall of pretreated substrates, with the effect more noticeable in SO2 -based applications due to its higher combined severities under similar conditions [33]. Though CO2 has a lower solubility compared to SO2 , CO2 is highly available, less toxic and corrosive, and thus safer to apply.

A major positive attribute about ACSE is that most glucan and lignin are untouched and remain in solid form after pretreatment [34] though lignin presence hinders enzymatic hydrolysis [35]. Nonetheless, high sugar yields are generally obtained. Yields obtained by some investigators are given in **Table 1**.


**Table 1.** Results of acid-catalyzed SE of selected biomass.

The main disadvantages include the toxicity of SO2 in SO2 -catalyzed applications and the unavoidable release of degradation products. The acidic nature of pretreatment requires expensive reactors that can withstand corrosion. SO2 may be costly and as such on-site production could be an alternative for improving the financial viability [18]. The efficient use of co-products such as lignin and hemicellulose in process integration improves the economic health of the process considerably.

**3.7. Positive attributes and drawbacks**

SE Alkaline Sugarcane

**Table 2.** Examples of combined pretreatment including SE.

be heated to high temperatures in short times.

chemicals.

**First stage**

SE O2

SE H2

Dilute acid

SE is among the most cost-effective methods for and agricultural residues and hardwoods since it does not require external catalysts. It offers the possibility of pretreatment at high solids loading due to the high-energy content of steam and low water requirements which reduce capital expenditure. Moreover, excessive dilution of sugars in pretreated liquor is reduced while the downstream processing of waste solution is minimized or eliminated. Another advantage relates to the possibility of using large biomass sizes which can lead to lower energy intensity. Though particles smaller than 2 cm are usually used, a recent study using larger biomass size (2.5 cm) was found to improve saccharification yield and overall process economics more than smaller sizes (0.5–1 cm); however, smaller particles recorded higher pretreated sugar recovery [49]. Corrosion is reduced due to the non-usage/low-use of

**Second stage Biomass Results Reference**

substrates

ethanol yields

xylose yield degradability

industrial (SE) reactor

Enzymatic conversion of 85% in an

O2 + stabilizers Douglas-fir Effective lignin removal [42]

yield

in alkaline solution Douglas-fir 84% removal of lignin left in exploded

SE Laccase Wheat straw Effective removal of lignin phenols; high

SE WO Pine 96% cellulose yield; ~100% hemicellulose

straw

SE Fungi Wheat straw 75% of lignin degraded [45]

SE Rice straw Reduced inhibitor formation; enhanced

digestibility >88%

98% recovery of cellulose; glucan

Emerging Physico-Chemical Methods for Biomass Pretreatment

http://dx.doi.org/10.5772/intechopen.79649

[32]

47

[41]

[43, 44]

[46]

[47]

[48]

SE Organosolv Poplar Improved lignin removal; over

Despite the advantages, there are inherent drawbacks associated with SE. The formation of inhibitory products, especially furan derivatives, weak acids and phenolic compounds, negatively affect enzymatic hydrolysis and fermentation [50]. Severe conditions cause increased degradation of cellulose and hemicellulose. There is also a risk of condensation and precipitation of soluble lignin components which leads to reduced digestibility of the biomass substrates [41, 51], while disrupting the lignin structure. SE is less effective on softwood and unexploded materials are common. Further, pretreatment at high temperatures and pressures creates additional challenges in material handling, reactor operation, energy management and heat recovery [52]. Thus, scaling-up is a challenge since large volumes of biomass must

#### **3.4. Alkaline-catalyzed steam explosion**

Alkaline-catalyzed SE has received less attention compared to acid-based SE. The alkaline solution improves delignification of biomass, giving higher enzymatic degradability. Park et al. [40] pretreated *Eucalyptus* under alkaline environment and observed enzymatic digestibility (relative to uncatalyzed SE), leading to a maximum glucose recovery of 66.55%.

#### **3.5. Double-stage pretreatment involving SE**

The major target of the two-step process is to achieve higher delignification and increase biomass digestibility. In many cases, significant increase in glucose yields relative to SE application only, have been observed as outlined in **Table 2**.

#### **3.6. Industrial application**

SE is among leading pretreatment methods in terms of cost effectiveness and has been implemented at demonstration (e.g., BioGasol plant in Denmark; Green Plains's plants in USA) and industrial scale (e.g., Crescentino, Italy; Raízen and Iogen's plant in São Paulo, Brazil).


**Table 2.** Examples of combined pretreatment including SE.

#### **3.7. Positive attributes and drawbacks**

The main disadvantages include the toxicity of SO2

**Table 1.** Results of acid-catalyzed SE of selected biomass.

expensive reactors that can withstand corrosion. SO2

health of the process considerably.

**3.6. Industrial application**

**3.4. Alkaline-catalyzed steam explosion**

CO2 205, 15 Sugar cane bagasse

46 Fuel Ethanol Production from Sugarcane

SO2 205–225, 5–10 Spruce, pine, birch

H2

H2

and leaves

and aspen

**3.5. Double-stage pretreatment involving SE**

tion only, have been observed as outlined in **Table 2**.

in SO2

High glucose yield of 86.6% [36]

High fractionation efficiency of alkaline extractable lignin for hard woods, but low for

theoretical. Ethanol yield of 67% based on glucose content of raw material in SSF.

found higher than treatment without additive, with maximum yield of 264 g/kg DS obtained

unavoidable release of degradation products. The acidic nature of pretreatment requires

for ethanol/SE.

**Agent/catalyst T (°C), t (min) Biomass Observation Reference**

Sugarcane leaves High glucose yield of 91.9%

plant

softwoods.

220, 5 High glucose yield of 97.2%

SO<sup>4</sup> 185, 2 Rice straw Overall saccharification yield of 73% in a pilot

SO<sup>4</sup> 190, 10 Wheat straw Glucose and xylose yields of 102 and 96% of

Acetic/ethanol 180–225, 3–60 Wheat straw Sugar yield after enzymatic conversion was

SO2 190, 5 Sugarcane bagasse Moderately high glucose yield of 79.7%

duction could be an alternative for improving the financial viability [18]. The efficient use of co-products such as lignin and hemicellulose in process integration improves the economic

Alkaline-catalyzed SE has received less attention compared to acid-based SE. The alkaline solution improves delignification of biomass, giving higher enzymatic degradability. Park et al. [40] pretreated *Eucalyptus* under alkaline environment and observed enzymatic digestibility (relative to uncatalyzed SE), leading to a maximum glucose recovery of 66.55%.

The major target of the two-step process is to achieve higher delignification and increase biomass digestibility. In many cases, significant increase in glucose yields relative to SE applica-

SE is among leading pretreatment methods in terms of cost effectiveness and has been implemented at demonstration (e.g., BioGasol plant in Denmark; Green Plains's plants in USA) and

industrial scale (e.g., Crescentino, Italy; Raízen and Iogen's plant in São Paulo, Brazil).


[31]

[37]

[38]

[39]

may be costly and as such on-site pro-

SE is among the most cost-effective methods for and agricultural residues and hardwoods since it does not require external catalysts. It offers the possibility of pretreatment at high solids loading due to the high-energy content of steam and low water requirements which reduce capital expenditure. Moreover, excessive dilution of sugars in pretreated liquor is reduced while the downstream processing of waste solution is minimized or eliminated. Another advantage relates to the possibility of using large biomass sizes which can lead to lower energy intensity. Though particles smaller than 2 cm are usually used, a recent study using larger biomass size (2.5 cm) was found to improve saccharification yield and overall process economics more than smaller sizes (0.5–1 cm); however, smaller particles recorded higher pretreated sugar recovery [49]. Corrosion is reduced due to the non-usage/low-use of chemicals.

Despite the advantages, there are inherent drawbacks associated with SE. The formation of inhibitory products, especially furan derivatives, weak acids and phenolic compounds, negatively affect enzymatic hydrolysis and fermentation [50]. Severe conditions cause increased degradation of cellulose and hemicellulose. There is also a risk of condensation and precipitation of soluble lignin components which leads to reduced digestibility of the biomass substrates [41, 51], while disrupting the lignin structure. SE is less effective on softwood and unexploded materials are common. Further, pretreatment at high temperatures and pressures creates additional challenges in material handling, reactor operation, energy management and heat recovery [52]. Thus, scaling-up is a challenge since large volumes of biomass must be heated to high temperatures in short times.
