**2.2. Trends in biomass conversion to fine chemicals and fuels**

One type of biomass which covers mostly of plant molecules is lignocellulosic biomass. Structurally, it composes of cellulose, hemicellulose and lignin. Cellulose and hemicellulose are mainly monomeric sugar linked to form polymer [4]. However, lignin consists of phenylpropane units, which cross-linked with tight and varied chemical bonds [4]. Thus, lignin complexity makes it difficult to be degraded as compared to almost unified type of bonding of cellulose and hemicellulose. Nonetheless, a few researchers have found ways to enhance

Lignin is separated from cellulose and hemicelluloses during kraft process, and only partial is utilised in combustion application as energy source [6]. In addition, through development of bioethanol production, it is expected that more lignin is produced as by-product which is also added to the mass number of lignin. Through advances of pretreatment technology and valorisation, lignin can be used as an alternative source for fine chemicals and raw material for fuel. In the progress of lignin utilisation for fuel production, hydrogenation of lignin becomes one of viable methods. Lignin contains functional phenolic compounds, but the difficulties of extracting the compounds remain a bottleneck to unlock this potential material for fuel production. For example, the degree of interaction between monomers (phenolic compound) varied due to the heterologous nature of each individual monomers [7]. Another reason would be the feasibility of pretreatment technologies that are needed to be strategised and to fully utilise the monomers present [7]. In addition, the monomers have to be separated from the strong linkages within lignin structure, so that conversion to fuel can be executed efficiently. In this chapter, lignin will be discussed of its structure and its different functional phenolic compounds. In addition, lignin depolymerisation or valorisation process to obtain individual monomers will also be presented. Further discussion will also include hydrogenation of lignin and the mechanism involved in the process. At the final part of this chapter, the future perspective of lignin hydrogenation that may lead to more innovative applications is discussed.

The present development of biomass conversion to energy and chemicals together with individual types of abundant biomass in nature has been discussed in terms of its source, avail-

In the abundance of biomass in the world's nature, traditional utilisation of biomass in order to survive has long been established. Primitively, when man knows how to create fire, biomass utilisation evolved from domestic usage to even larger application such as building houses, clothes production, paper making, etc. Yet the biomass utilisation seems endless and

Henry Ford was the first to design a model car that runs using ethanol, and it was also reported that Rudolph Diesel intended on using vegetable oil to power his car engine [8]. However, at

degradability of lignin through recent pretreatment technologies [4, 5].

290 New Advances in Hydrogenation Processes - Fundamentals and Applications

**2. Biomass conversion to fine chemicals and fuels**

ability, types of biomass and chemical characteristics.

continues to produce more relevant products.

**2.1. Historical background**

In developing technologies to fine tune biomass conversion to chemicals or fuels, many combinations of physical, chemical and biological approaches have been utilised. The strategy is to enhance accessibility to the main target component such as cellulose and hemicellulose or to separate main recalcitrant of the biomass which is lignin from the other component and treated individually as precursor for fuel.

For starter, all biomass should undergo pretreatment in order to achieve maximum conversion. During the past decades, the target is to produce fermentable sugar from complex lignocellulosic biomass which includes separating lignin from the complex biomass [16]. **Figure 1** shows a schematic flow of basic biomass conversion into value-added products.

During the course of chemical or biochemical conversion, an effective pretreatment is needed for maximum utilisation of the biomass. To summarise, a few criteria have been highlighted to achieve efficient pretreatment as indicated below:


**Figure 1.** Schematic flow of general conversion of biomass to value-added products. Adapted from Ref. [17] with permission from Elsevier.

The above criteria become a basis for choosing the right method of pretreatment of biomass in order to maximise its efficiency. Further details on pretreatment will not be discussed here and can be found intensively discussed in other publications [18].

Meanwhile, as the complexity of the biomass dissolved through pretreatment processes, the next step is to choose the right method to directly convert the simpler form of the material to the desired product based on physical, chemical and biological processing methods.

#### *2.2.1. Physical-based conversion*

In this method, for some biomass, pretreatment sometimes will not be necessary. For example, woody biomass through combustion process will produce heat and electricity. The initial combustion will produce steam at high pressure and eventually the steam is used to activate turbine plants that in turn will generate electricity. Such biomass-fired steam turbine plants are located at the industrial sites that commonly where the biomass is produced. Another example is gasification process, where the biomass is directly heated and broken down into flammable gas. The gas or called 'biogas' will be drawn into filtration system to clean and refine before subjected to usage for electricity production.

The fact that major biomass components such as cellulose, hemicellulose and lignin can be fractionated based on different temperatures is exploited with the merging technology of pyrolysis [19–21]. The process involves three stages or heat-based degradation. The first stage includes water elimination, structural deformation and alkyl group formation which is also called pre-pyrolysis. The second stage involves decomposition of components and formation of pyrolysis products. Finally, the last stage produces carbon residuals and bio-oil from charred biomass. Mainly, pyrolysis dealt with cellulose and hemicellulose component conversion, but very little is known about the contribution of this process to lignin fraction. The lignin is merely converted to low concentration of phenolics and char. As it becomes more evident that large amounts of hydrolytic lignin will be produced in future bioethanol plants, lignin has gained interest as a chemical feedstock or aromatic compounds such as catechols, guaiacol, syringol, phenol, furfural, and acetic acid [22, 23].
