Aqueous Phase Biomass Hydrolysis Route
Challenges in Aqueous — Phase Biomass Hydrolysis Route: Recalcitrance
4.1 Introduction — Two Ways to Produce Cellulosic Ethanol
There are two basic processes for conversion of lignocellulosic biomass to bioethanol: (1) cellulolysis process or aqueous-phase biomass saccharification and fermentation process, and (2) gasification or syngas to ethanol conversion process.
The first method of aqueous-phase biomass saccharification and fermentation involves the hydrolysis of cellulose and hemicellu — lose in biomass to sugars and then fermentation of the sugar solution with yeast to produce ethanol. The basic steps of this route are shown in Figure 4.1. Saccharification of the biomass or hydrolysis of polysaccharides to monosaccharides is the most challenging step in this process, and this can be accomplished by pretreatment of biomass followed by exposure to a cellulase enzyme cocktail, or by single-step direct acid hydrolysis using concentrated or dilute acid solution like aqueous sulfuric acid. This route will be discussed in Part 2 of the book in Chapters 4-10. This route is known as biochemical process as well.
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Pretreatment + Acid hydrolysis Figure 4.1 Basic steps of cellulolysis process or aqueous-phase biomass saccharification and fermentation process. |
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Figure 4.2 Basic steps of the gasification-syngas-ethanol process. |
The second method is the gasification or syngas to ethanol conversion process, and the key steps in this process are shown in Figure 4.2. This method is known as thermochemical process as well since pyrolysis of biomass to syngas (a mixture of carbon monoxide, carbon dioxide, and hydrogen) is the first step and an essential feature of the process. Then the gas mixture is converted to ethanol by using either a metal or biochemical catalyst. This second process will be discussed in Part 3 of the book in Chapters 11-13.