*2.2.3. Biological process-based conversion*

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

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

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

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

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,

the desired product based on physical, chemical and biological processing methods.

and can be found intensively discussed in other publications [18].

292 New Advances in Hydrogenation Processes - Fundamentals and Applications

refine before subjected to usage for electricity production.

guaiacol, syringol, phenol, furfural, and acetic acid [22, 23].

*2.2.1. Physical-based conversion*

permission from Elsevier.

In general, biological process-based conversion involves cellulosic or hemicellulosic sugar conversion to fuel. Most popular ever known is biofuel such as bioethanol production using microorganism(s) as the catalyst in the bioconversion process. There are two-step processes which include fermentation and pretreatment to obtain the primary sugars [32]. In many publications, cellulose and hemicellulose conversion to biofuel production have been discussed thoroughly and can be read elsewhere [32, 33], and it is necessary to briefly discussed on lignin bioconversion.

Though it has inhibitory effect on any cellulose/hemicellulose conversion, if present, lignin can be converted to value-added products such as stand-alone feedstock. Recently, a *Rhodococusc* sp. can convert lignin into triacylglycerols under nitrogen-limiting conditions [34]. Furthermore, it was demonstrated that lignin valorisation can be channelled by different catabolic pathways of few aromatic catabolising bacteria to produce precursors for fuel production [3]. However, the thermal properties of each aromatic bonding in lignin need an integrated thermochemical process in order to penetrate the bonds [35].
