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246 8 Pre-combustion Air Emission Control

Corn is the well-developed feedstock in United States, while sugar cane is a tropical
and subtropical crop that is the primary feedstock in Brazil, India, and Colombia.
Cellulosic bioethanol is a more recent development that is not well commercialized
yet. Regardless of the feedstock, the sales price of bioethanol must be competitive
with that of petroleum gasoline. However, profit margins in bioethanol production
processes are still low. Nonetheless, our focus here is a briefly introduction rather
than an in-depth discussion.
Depending on the biomass feedstock, there are several major steps that may
apply to bioethanol production [26].

• Feedstock pre-treatment
• Hydrolysis
• Fermentation
• Separation
• Storage.

8.4.3.1 Feedstock Pretreatment

Feedstock pretreatment is necessary to convert most of the carbohydrates in the
feedstock into sugars. The first step is size reduction and breakdown of the cell wall
structures surrounding the target compounds, mainly cellulose. Some lignin is
dissolved in the solution. Sugar cane is crushed followed by juice extraction; corn
goes through dry grinding or wet milling according to the downstream conversion
technology. The feedstock is then pretreated using chemicals, such as dilute sulfuric
acid or ammonia.
After pretreatment, a large amount of water is separated from the hydrolysate
slurry. Meanwhile, other unwanted chemicals such as acetic acid and furfural are
also removed from the slurry. Then hydrolysate slurry is cooled by dilution water
and further chemically conditioned for next step.

8.4.3.2 Hydrolysis and Fermentation

In this step, the starch and cellulose are converted into sugars, primarily being glu-
cose, by cellulose enzymes. It is also called enzymatic hydrolysis. A cellulase enzyme
breaks down cellulose fibers and ultimately converts them into glucose monomers.
The resulting glucose and other sugars from feedstock pretreatment are to be
converted into bioethanol by fermentation. This process may take several days to
complete. The simplified chemical reactions for hydrolysis and fermentation are
Enzyme
C H 22 O 11 þ H 2 O ! 2C 6 H 12 O 6 ð8:18Þ
12