Pump 'gas' from biomass: the journey from dream to reality (2)

Chemical Properties of Biomass: Heterogeneous

Unlike corn or sugarcane, the chemical components of biomass also demonstrate heterogeneous propeties. The major chemical components of biomass are composed of cellulose (35-50%), hemicellulose (20-35%), lignin (10-30%), and extractives (<10%).>

Fig. 1. Chemical composition of some hardwood species

Fig. 2. Chemical composistion of some softwood species

Cellulose

Cellulose is one of the major but simplest chemical components of lignocellulosics, which is a linear polymer of D-glucose units linked by 1,4-beta-D glycosidic bonds with a degree of polymerization (DP) greater than 10,000 (Fig. 3).

Fig. 3. The structure of cellulose (Sjostrom, E. 1993)

However, cellulose chain has a strong tendency to form intra- and inter-molecular hydrogen bonds (Fig. 4) by the hydroxyl groups on these linear cellulose chains, which stiffens the chains and promotes aggregation into a crystalline structure (Fig. 5). These properties give cellulose a multitude of crystalline fiber structures and morphologies. Cellulose is also packed in hemicellulose and lignin, which make it difficult for chemical or enzymatical hydrolysis. Most native samples of cellulose also have varying degrees of amorphous cellulose, which is more reactive to chemical and enzymatic attack.

Fig. 4. Hydrogen-hydrogen bonds (Sjostrom, E. 1993)

Fig. 5. Crystal and amorphous structure (Sjostrom, E. 1993)

Therefore, to convert lignocellulosic biomass to monomer sugar-based biofuel or biochemicals, the critical step is to pretreat the biomass to reduce or remove the physical transport barriers in the cell wall structure, to loosen the cellulose crystal structure, and accordingly to improve chemical or enzyme accessibility and the rate of reaction.