**7.1. Alkaline pretreatment**

This pretreatment is similar to the Kraft pulping process used in the pulp and paper indus‐ tries. Nevertheless, sodium, potassium, calcium, and ammonium hydroxides have been em‐ ployed for the pretreatment of lignocellulosic biomass, sodium hydroxide has been the most studied reagent [146-148]. However, calcium hydroxide is advantageous due to its low cost, higher safety besides its recovery as insoluble calcium carbonate through reaction with car‐ bon dioxide [149]. Lime pretreatment has been used in studies carried out with several lignocellulosic materials, such as sugarcane bagasse [150], switchgrass [151], rice straw [152] and poplar wood [153].

The main effect of alkaline pretreatments is the biomass lignin removal thereby reducing the steric hindrance of hydrolytic enzymes and improving the reactivity of polysaccharides. It is believed that the mechanism involves saponification of intermolecular ester bonds between xylans and lignin, increasing the material porosity. The addition of air/oxygen to the reaction mixture dramatically improves delignification, especially in the case of materials with high lig‐ nin content. The removal of acetyl groups from hemicellulose by the alkalis also exposes the cellulose and enhanced its enzymatic hydrolysis [2]. The alkali pretreatment also causes partial hemicellulose removal, cellulose swelling and cellulose partial decrystallization [149].

In the alkaline process the biomass is soaked in the alkaline solution and mixed at a mild controlled temperature in a reaction time frame from hours to days. It causes less sugar deg‐ radation than the acidic pretreatments. The necessary neutralizing step, prior to the enzy‐ matic hydrolysis, generates salts that can be partially incorporated to the biomass. Besides removing lignin the pretreated material washing also removes inhibitors, salts, furfural and phenolic acids.

#### **7.2. Ammonia fiber expansion (AFEX) pretreatment**

Another pretreatment that deserves attention is the ammonia fiber expansion (AFEX), which is a physicochemical process very similar to steam explosion, in which lignocellulosic bio‐ mass is exposed to liquid ammonia at high temperature and pressure for a period of time, with a subsequent quick reduction of the pressure [154]. In a typical AFEX process, the dos‐ age of liquid ammonia is 1-2 kg of ammonia/kg of dry biomass and the temperature and res‐ idence time are around 170 °C and 30 min, respectively [2].

The AFEX technology has been used for the pretreatment of several lignocellulosic materials including wood, switchgrass, sugarcane bagasse and corn stover [154-158]. Over 90% hy‐ drolysis of cellulose and hemicellulose was obtained after AFEX pretreatment of bermuda‐ grass (approximately 5% lignin) and bagasse (15% lignin) [157]. Although hardwood pretreatment, like poplar, requires harsher AFEX conditions to obtain equivalent sugar yields upon enzymatic hydrolysis, poplar (*Populus nigra x Populus maximowiczii* hybrid) AFEX-pretreated at 180 °C, 2:1 ammonia to biomass loading, 30 minutes residence time by using various combinations of enzymes (commercial cellulases and xylanases) achieved high glucan and xylan conversion (93 and 65%, respectively) [159].

This process presents some disadvantages, such as the use of ammonia solvent itself, that should be recycled and handled with caution to make the process environmentally feasible, and also from an economic point of view the ammonia consumption needs to be minimized [47]. However, there are some advantages in this pretreatment, like the feasibly solvent re‐ cover and the hydrolysate from AFEX is compatible with fermentation microorganisms without the need for conditioning [160].

#### **7.3. Biological pretreatment**

**7. Other pretreatment processes**

**7.1. Alkaline pretreatment**

and poplar wood [153].

phenolic acids.

**7.2. Ammonia fiber expansion (AFEX) pretreatment**

idence time are around 170 °C and 30 min, respectively [2].

According to the foregoing, this chapter has covered the most relevant pretreatment techni‐ ques for sugarcane and woody biomass as well as the new trends in this field. Below, we present other pretreatments, such as alkaline, ammonia fiber expansion and biological, which are also of relevance. Other important methods such as organosolv, ammonia percolation, and

This pretreatment is similar to the Kraft pulping process used in the pulp and paper indus‐ tries. Nevertheless, sodium, potassium, calcium, and ammonium hydroxides have been em‐ ployed for the pretreatment of lignocellulosic biomass, sodium hydroxide has been the most studied reagent [146-148]. However, calcium hydroxide is advantageous due to its low cost, higher safety besides its recovery as insoluble calcium carbonate through reaction with car‐ bon dioxide [149]. Lime pretreatment has been used in studies carried out with several lignocellulosic materials, such as sugarcane bagasse [150], switchgrass [151], rice straw [152]

The main effect of alkaline pretreatments is the biomass lignin removal thereby reducing the steric hindrance of hydrolytic enzymes and improving the reactivity of polysaccharides. It is believed that the mechanism involves saponification of intermolecular ester bonds between xylans and lignin, increasing the material porosity. The addition of air/oxygen to the reaction mixture dramatically improves delignification, especially in the case of materials with high lig‐ nin content. The removal of acetyl groups from hemicellulose by the alkalis also exposes the cellulose and enhanced its enzymatic hydrolysis [2]. The alkali pretreatment also causes partial

In the alkaline process the biomass is soaked in the alkaline solution and mixed at a mild controlled temperature in a reaction time frame from hours to days. It causes less sugar deg‐ radation than the acidic pretreatments. The necessary neutralizing step, prior to the enzy‐ matic hydrolysis, generates salts that can be partially incorporated to the biomass. Besides removing lignin the pretreated material washing also removes inhibitors, salts, furfural and

Another pretreatment that deserves attention is the ammonia fiber expansion (AFEX), which is a physicochemical process very similar to steam explosion, in which lignocellulosic bio‐ mass is exposed to liquid ammonia at high temperature and pressure for a period of time, with a subsequent quick reduction of the pressure [154]. In a typical AFEX process, the dos‐ age of liquid ammonia is 1-2 kg of ammonia/kg of dry biomass and the temperature and res‐

The AFEX technology has been used for the pretreatment of several lignocellulosic materials including wood, switchgrass, sugarcane bagasse and corn stover [154-158]. Over 90% hy‐

hemicellulose removal, cellulose swelling and cellulose partial decrystallization [149].

oxidative reactions using hydrogen peroxide or ozone will be dealt with elsewhere.

70 Sustainable Degradation of Lignocellulosic Biomass - Techniques, Applications and Commercialization

Biological pretreatment employs various types of rot fungi, being the white-rot fungi the most effective for biological pretreatment of lignocellulosic biomass. The aim of biological pretreatment processes are the lignin degradation by microorganisms, through the action of lignin degrading enzymes such as peroxidases and laccases [2]. The most investigated fun‐ gus for lignin degradation is *Phanerochete chrysosporium* [161].

The biological pretreatment of sugarcane straw was evaluated by screening eight microor‐ ganisms, including bacteria and fungi, for an incubation time of 30 days. The fungus *Asper‐ gillus terreus* was found as the most effective strain, resulting in 92% reduction in the lignin content [162]. The pretreatment of sugarcane straw was also evaluated using the fungus *Car‐ iporiopsis subvermispora* with the objective to reduce cooking times and chemicals load for the organosolv pulping. The pretreatment was effective regarding the decomposition of lignin, however high cellulose losses were pointed as negative side effects [163]. Another study evaluated the pretreatment of sugarcane bagasse with the white-rot fungus *Pleurotus sajorca‐ ju* PS 2001 using a 45 days incubation time, in order to modify its lignin content. However, in this case, the aim of the study was to provide a more digestible substrate for the produc‐ tion of cellulases by the fungus *Penicillium echinulatum* [164].

The pretreatment of the Japanese red pine *Pinus densiflora* was studied using three white-rot fungi. The fungus *Stereum hirsutum* was able to selectively degrade lignin resulting in a less recalcitrant biomass after eight weeks of pretreatment. As consequence, the sugar yields ob‐ tained after the hydrolysis of the pretreated red pine with commercial enzymes was 21% higher when compared to non pretreated control samples [165].

The main advantages of such processes are the low capital cost, low energy, no requirement for chemicals, fewer hydrolysis and fermentation inhibitors produced during pretreatment and mild environmental conditions [166]. However, the biological processes require a very long residence time, when compared to other pretreatment techniques and result in very low reaction rates. Additionally, most microorganisms consume part of the substrate as a nutrient for its growth during the pretreatment, which affects negatively the sugar yield at the end of the process [3]. In addition, the consumption of lignin also reduces the biomass energy utilization. At present, the use of biological pretreatments may represent a competi‐ tive option only if associated with other pretreatment techniques, in order to reduce the en‐ ergy requirement of the total pretreatment process [167]. In future, if less recalcitrant genetically modified plant materials are available, biological pretreatments may represent an important alternative.

Table 4 lists the pretreatment options presented in this chapter and its general effects in the bio‐ mass composition and structure. All pretreatments cause an increase in the surface area, which responds for the increased enzymatic digestibility of the treated materials. However, the sub‐ stantial decrease in cellulose crystallinity is only observed for the treatments using ball milling and IL. This effect is of paramount importance for the increased rates and yields of cellulose en‐ zymatic hydrolysis. The acid, LHW and steam explosion pretreatments are more effective on hemicelluloses and on the modification of the lignin structure, which also cause a higher for‐

> **Removal of hemicellulose**

Acid ++ - +++ ++ +++ +++ Alkali - - + +++ ++ ++ LHW ++ - +++ + ++ ++ Steam explosion ++ - +++ + +++ ++ Ball milling ++ +++ - - - - WDM +++ + - - - - Extrusion ++ + - - - - Ionic liquid +++ +++ + ++ + nd

**Removal of lignin**

**Modification of lignin**

Sugarcane and Woody Biomass Pretreatments for Ethanol Production

**Formation of toxic compounds**

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

73

mation of inhibitors in comparison to milling, extrusion and IL pretreatments.

**Reduction of CrI**

+++ expressive effect; ++ moderate effect; + low effect; - no effect; nd: not determined

perennial energy grasses, are used with different pretreatment types.

existing methods or for the development of new and more advanced options.

**Table 4.** General effects of different pretreatments on the composition and structure of the biomass.

Table 5 presents sixteen biomass ethanol plants (pilot, demonstration and commercial scale) which are operating or under construction. It is also presented, for each case, the feedstock and the biomass pretreatment that is used in these facilities. At the current scenario the ma‐ jority of the units have implemented processes that generate a hemicelluloses rich stream: three units use diluted acid, three units use LHW and three units use steam-explosion pre‐ treatment. Two units describe its process as a thermal-mechanical pretreatment which could also generate of a hemicelluloses rich stream. One unit applies a mild alkaline pretreatment that precludes lignin separation and the remaining four units have not disclosed the choice of pretreatment. A variety of feedstocks, such as pine wood chips, wood wastes, forest resi‐ dues, garden waste, wheat, barley and oat straw, corn cob, corn stover, corn straw as well as

As the pretreatment step accounts for a substantial part of the biomass ethanol production cost, it is expected that the research in this field will continue to seek for improvements of

**Pretreatment**

SSA: Specific surface area CrI: Crystallinity index LHW: Liquid hot water WDM: Wet-disk milling

**Increase of SSA**
