Currently biomass pretreatment is still a necessary step to establish a cheap sugar platform for bioethanol and biochemical products. An ideal pretreatment technology should target the three basic requirements: simple process,cost effective, and high sugar recovery.
If we examine all the pretreatment technologies published so far, few of them meet such fundamental requirements. To design a pretreatment approach, a fundamental understanding of biomass cell structure,cellulose and lignin chemistry,and transport phenomenon is necessary. The following picture has been used and cited widely to visually demonstrate the effect of pretreatment for those non-scientific people. Unfortunately, I am afraid there is somewhat misleading. People who saw this pretreatment representation may think the lignin is fragmented and cellulose Cristal is damaged after treatment. In reality, it is not this situation for the majority of the pretreatment technologies published so far. Even through severe pulping process, some lignin is still linked to carbohydrate and cellulose structure is still unaffected to an striking extent.
The more appropriate visual demonstration may be shown as follow, i.e. after pretreatment, the hemicellulose, some soluble lignin, and some cellulose in amorphous regions are extracted or removed, leaving the lignocellulosic matrix with somewhat alteration.
The key for the pretreatment is to open channels/pores due to the removal of these chemical components and somewhat alteration or dislocation of the cell wall structure. As a result, it allow enzymes or chemicals more easily to transport into cell structure and function. No doubt, a relatively severe treatment causes more damage to the cell wall structure. The question is how much more degradation occurs for removed hemicellulose and cellulose (monomer sugars).
To meet the above three requirements for a pretreatment, the chemical selected should has the ability to participate and remove dissolved lignin. One of the process parameters: temperature needs to be set carefully to avoid lignin redistribution and re-deposition/re-adsorption; Because the monomer sugar formation and degradation occur simultaneously, an appropriate process configuration and operation need to be balanced to avoid more sugar degradation.
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