Pump 'gas' from biomass: the journey from dream to reality (7)

Pretreatment

The objective of pretreating lignocellulosics is to alter the cell wall structure of biomass to extract hemicellulose and meanwhile make the cellulose more accessible and amenable to hydrolytic enzymes that can generate fermentable sugars.

Effective pretreatment technologies need to address several important criteria, including: minimize inhibitory compounds formation (e.g. furfural, HMF, aldonic acids, etc.), lignin alterations, minimal energy, capital and operating costs.

Good pretreatments will have a major influence on downstream process such enzyme loading, enzymatic hydrolysis rates, fermentation toxicity, product concentration, mixing power, and waste treatment.

Certainly, limit chemical addition or consumption should be considered for pretreatment because itself is alslo a costly step. But overall cost from the whole process should be more critial than the pretreatment itself.

So far, many pretreatment technologies have been proposed and studied using either inorganic catalysts or organic solvents. Under acid pretreatment conditions, most of the process have been conducted at a temperature above 170 C, which usually causes a problem for lignin re-adsorption or precipitation on the surface of cellulose fiber after cooling down (See Fig). The temperature of 170 C is close or higher lignin glass transition points. As a result, some hydrophobic lignin will melt, re-distribute or migrate. If they are not removed before cooling down, they will deposite on the surface of cellulose fiber forming lignin coating, which will act as a barrier to the cellulase enzymes and also adsorb cellulase enzymes to limit the efficacy of hydrolysis and increase enzyme loading.

(Zeng and Ladisch, 2007)

Therefore, ideally, chemicals used for pretratments have the ability to participate in lignin fragmentation and/or prevent lignin from re-condensation/re-adsorption.

Alkaline pretreatments of biomass using lime, NaOH, and ammonium can avoid the above problems. But some new problems occur. For example, lime pretreatment will generate a lot of gypsium to be dealt with and cause 20-30% sugar loss. Liquid ammonium pretreatment need a costly system to recover the ammonium. Sodium hydroxide or carbonate pretreatment will introduce large amount of sodium ions in the process to be recovered.

Some organic solvents are effective pretreatment agents. The separation and recovery of the solvents are big issues from technical and economic perspectives such as acetone, ethanol, formic acid etc. In addition, the reaction of these solvents during pretreatment will lead to the consumption of the chemicals, increasing chemical costs.

Additionally, differences in cell-wall structure and chemistry impact how biomass responds to chemical pretreatments. Several authors have indicted that the recalcitrance of softwood resources is greater than hardwoods which is exhibited in reduced digestability by cellulase. The exact chemical constituents and ultrastructures that contribute to this effect needs to be well understood. Future fundamental research into these issues promises to have a far-reaching beneficial effect in accelerating the development of low-cost sugar-based biofuels or biochemicals.