**Sugarcane and Woody Biomass Pretreatments for Ethanol Production**

Ayla Sant'Ana da Silva, Ricardo Sposina Sobral Teixeira, Rondinele de Oliveira Moutta, Viridiana Santana Ferreira-Leitão, Rodrigo da Rocha Olivieri de Barros, Maria Antonieta Ferrara and Elba Pinto da Silva Bon

Additional information is available at the end of the chapter

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

**1. Introduction**

Lignocellulosic biomass, which is chiefly composed of cellulose, hemicellulose, and lignin, has now been recognized and established as a source of energy, fuels, and chemicals. Amongst the several options for the use of lignocellulosic materials for energy generation, the production of ethanol has attracted particular attention worldwide, becoming the target of intense research and development (R&D) over the past 40 years. However, the technology required for industrial conversion of these materials into ethanol, in an economic manner, has not yet been fully developed. As an example, the necessary biomass pretreatment step has been under thorough investigation, as the production of sugar syrups with high concen‐ trations and yields via enzymatic hydrolysis of biomass requires the pretreatment to be effi‐ cient to render the material accessible to the relevant enzyme pool.

A good choice for biomass pretreatment should be made by considering the following pa‐ rameters: high yields for multiple crops, harvesting times, highly digestible pretreated solid, high biomass concentration, no significant sugar degradation, formation of a minimum level of toxic compounds, high yields of sugars after subsequent hydrolysis, fermentation com‐ patibility, operation in reasonably sized and moderately priced reactors, lignin recovery, and minimum heat and power requirements [1].

© 2013 Silva et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Silva et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Different pretreatment methods produce different effects on the biomass in terms of its struc‐ ture and composition [2] (Figure 1). For example, the hydrothermal and acidic pretreatments conceptually remove mainly the biomass hemicellulose fraction and alkaline pretreatments remove lignin, whereas the product of a milling-based pretreatment retains its initial biomass composition. As such, the choice of pretreatment as well as its operational conditions deter‐ mines the composition of the resulting biomass hexose and pentose syrups. Furthermore, cel‐ lulose crystallinity is not significantly reduced by pretreatments based on steam, or hydrothermal, or acidic procedures, whereas ionic liquid-based techniques can shift crystal‐ line cellulose into amorphous cellulose, substantially increasing the enzymatic hydrolysis rates and yields. The activity profile of the enzyme blend and the enzyme load for an effective saccharification may also vary according to the pretreatment. Indeed, a low hemicellulase load can be used for a xylan-free biomass and a lower cellulase load will be needed for the hy‐ drolysis of a low crystalline and highly amorphous pretreated biomass material.

Sugarcane is one of the major agricultural crops when considering ethanol production, espe‐ cially in tropical countries. In Brazil, sugarcane occupies 8.4 million hectares, which corre‐ sponds to 2.4% of farmable lands in Brazil. The gross revenue of this sector is about US\$ 20 billion (54% as ethanol, 44% as sugar, and 2% as bioelectricity) [4]. In addition, up to 50% of all vehicles in Brazil are flex fuel cars, which corresponds to approximately 15 million cars [5]. Given the above, Brazil is an important player in this scenario, and, consequently, sugar‐ cane bagasse and straw are promising feed stocks for biomass ethanol. Brazil produced, in 2008, 415 million tons of sugar cane residues, 195 million tons of sugarcane bagasse, and 220 million tons of sugarcane straw, whereas the forecasted for the 2012 sugarcane production is 590 million tons, which would correspond to 178 million tons of bagasse, and 200 million tons of straw [6]. Although current R&D has been focused mainly on agricultural residues such as sugarcane residual biomass, woody biomass (hardwoods and softwoods) remains a very important feedstock for cellulosic ethanol production. It is estimated that 370 million dry tons of woody biomass can be sustainably produced annually in the United States. Woody biomass is also sustainably available in large quantities in various other regions of the world such as Scandinavia, New Zealand, Canada, Japan, and South America. Further‐ more, short-rotation intensive culture or tree farming offers an almost unlimited opportuni‐

Sugarcane and Woody Biomass Pretreatments for Ethanol Production

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

49

This chapter will address an overview of the pretreatments that have been studied for sug‐ arcane and woody biomass aiming at ethanol production using diluted acid, hydrothermal processes, steam explosion, milling, extrusion, and ionic liquids. Advantages and disadvan‐ tages of each method will be presented and discussed. The chapter will also discuss the in‐ ternational scenario regarding the existing research and technological choices for the

The use of mineral acids for biomass processing has a historical record dating back to 1819, when concentrated acid was used for wood saccharification aiming at ethanol production [8]. Nevertheless, different technologies using mineral acids have been developed over the last two centuries for converting plant biomass into monosaccharides [9, 10]. The use of acid for biomass pretreatment is conducted with diluted sulfuric or hydrochloric acid (1 to 5%) at 150 °C and pressures up to 10 atm [11]. The efficiency of hemicellulose removal in acid pre‐ treatments is approximately 90%, with sugar losses by degradation at around 1% [12].

The diluted acid pretreatment allows for the deconstruction of the lignocellulosic material structure and the release of sugar monomers, mostly derived from the hemicellulose. If acid pretreatment is carried out under mild conditions of acid concentration and temperature, the hemicellulose fraction can be extracted without significantly affecting the cellulose and lignin biomass content. Unlike cellulose, the hemicellulose is amorphous and branched, be‐ ing more accessible to hydrolysis agents. This structure allows for the diffusion of acids, which accelerate the hydrolytic process. Therefore, in diluted acid pretreatment, the hemi‐

ty for woody biomass production [7].

production of biomass ethanol.

**2. Diluted acid pretreatment**

cellulose is preferably removed and hydrolyzed.

**Figure 1.** Flow diagram for biomass ethanol production showing different pretreatments options and the composi‐ tion of the solid pretreated material. SSF: simultaneous saccharification and fermentation

As the pretreatment choice will also be affected by the type of biomass, the envisaged biore‐ finery model will need to consider the main types of biomass that will be used for the biore‐ finery operation so as to select an appropriate, and versatile pretreatment method [3]. To date, sugarcane and woody biomass, depending on the geographic location, are strong can‐ didates as the main renewable resources to be fed into a biorefinery. However, due to major differences regarding their physical properties and chemical composition, the relevant pre‐ treatments to be used in each case are expected to be selective and customized. Moreover, a necessary conditioning step for wood size reduction, prior to the pretreatment, may not be necessary for sugarcane bagasse, affecting the pretreatment energy consumption and costs.

Sugarcane is one of the major agricultural crops when considering ethanol production, espe‐ cially in tropical countries. In Brazil, sugarcane occupies 8.4 million hectares, which corre‐ sponds to 2.4% of farmable lands in Brazil. The gross revenue of this sector is about US\$ 20 billion (54% as ethanol, 44% as sugar, and 2% as bioelectricity) [4]. In addition, up to 50% of all vehicles in Brazil are flex fuel cars, which corresponds to approximately 15 million cars [5]. Given the above, Brazil is an important player in this scenario, and, consequently, sugar‐ cane bagasse and straw are promising feed stocks for biomass ethanol. Brazil produced, in 2008, 415 million tons of sugar cane residues, 195 million tons of sugarcane bagasse, and 220 million tons of sugarcane straw, whereas the forecasted for the 2012 sugarcane production is 590 million tons, which would correspond to 178 million tons of bagasse, and 200 million tons of straw [6]. Although current R&D has been focused mainly on agricultural residues such as sugarcane residual biomass, woody biomass (hardwoods and softwoods) remains a very important feedstock for cellulosic ethanol production. It is estimated that 370 million dry tons of woody biomass can be sustainably produced annually in the United States. Woody biomass is also sustainably available in large quantities in various other regions of the world such as Scandinavia, New Zealand, Canada, Japan, and South America. Further‐ more, short-rotation intensive culture or tree farming offers an almost unlimited opportuni‐ ty for woody biomass production [7].

This chapter will address an overview of the pretreatments that have been studied for sug‐ arcane and woody biomass aiming at ethanol production using diluted acid, hydrothermal processes, steam explosion, milling, extrusion, and ionic liquids. Advantages and disadvan‐ tages of each method will be presented and discussed. The chapter will also discuss the in‐ ternational scenario regarding the existing research and technological choices for the production of biomass ethanol.
