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6.2: Biochemical Structural Aspects of Lignocellulosic Biomass

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  • 6.2 Biochemical Structural Aspects of Lignocellulosic Biomass

    For Review

    To begin this part of Lesson 6, 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 (see Figure 6.3). We’ve discussed pretreatment, where cellulose, lignin, and hemicellulose are separated. Hemicellulose is broken down to 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.

    process of producing ethanol from lignocellulosic biomass, see text description in link below
    Figure 6.3: Preview to the process of producing ethanol from lignocellulosic biomass.

    Click for a text description of Figure 6.3.

    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.

    Credit: Liao, BEEMS Module B2

    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 a concentrated ethanol. So, we are focusing this lesson on enzymatic hydrolysis of starch and cellulose.

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