5.2 Biochemical Structural Aspects of Lignocellulosic Biomass
For Review
To begin this part of Lesson 5, review the Biomass Carbohydrate Tutorial from the previous lesson. It will be important to remember all of the terminology for carbohydrates.
So, at this point, we’ve talked a bit about what lignocellulosic biomass is composed of, what various carbohydrates are chemically, and how to pretreat various biomass sources. Now, we will discuss the use of enzymes in biomass conversion, particularly in cellulose conversion. I’ll first introduce you to cellulases, and then we'll look at a model of enzymatic hydrolysis of cellulose and enzymes for hemicellulose and lignin.
For cellulases, we’ll discuss what they are, provide a brief history, look at glycosyl hydrolases, and, finally, cellulases.
The processing of cellulose in lignocellulosic biomass requires several steps. We’ve discussed pretreatment, where cellulose, lignin, and hemicellulose are separated. Hemicellulose is broken down into xylose and other sugars, which can then be fermented to ethanol. Lignin is separated out and can be further processed or burned depending on the best economic outcome. The first step of processing is then on the cellulose.
Producing Ethanol from lignocellulosic biomass:
Cellulose is pretreated so that hemicellulose is broken down to xylose, and other sugars are fermented to ethanol. Lignin is separated and burned: energy exceeds processing requirements.
Cellulose goes through enzymatic hydrolysis to produce glucose, which then goes through fermentation to produce 5% ethanol, which is distilled to produce 100% ethanol.
Pretreatment helps to decrystallize cellulose. However, it must be further processed to break it down into glucose, as it is glucose (a sugar) that can be fermented to make ethanol, and the liquid product must be further processed to make concentrated ethanol. So, we are focusing this lesson on the enzymatic hydrolysis of starch and cellulose.