Assessment of Conversion Technologies for Bioalcohol Fuel ...

Feedstock Pretreatment
There are a number of possible pretreatment processes that can be applied to
cellulosic biomass (such as rice straw) to prepare the fiber for enzymatic
saccharification prior to fermentation and ethanol recovery:
Mechanical (grinding, milling, shearing, extruding)
Acid treatment (dilute or concentrated H 2 SO 4 )
Alkali treatment (sodium hydroxide, ammonia, alkaline peroxide)
Autohydrolysis (steam pressure, steam explosion, liquid hot water)
Acid Pre-Treatment–The acid hydrolysis of cellulose for the production of ethanol
was first incorporated in a commercial plant in South Carolina in 1910. The ethanol
yield was approximately 20 gallons/ton (Fieser and Fieser, 1950). Since that time the
acid hydrolysis process has been greatly improved.
Steam Explosion–This process uses high pressure steam (typically 200-450 psig)
for 1-10 minutes to break down biomass fibers. The resulting product is then
explosively discharged at atmospheric pressure to another vessel. Although this
process is nearly 75 years old, it has had a number of limitations until recently. A
relatively new development involves a continuous steam explosion process that
supports a higher processing temperature and reduces the residence time. This
process greatly reduces the need for chemicals (e.g. acids) typically associated with
this process.
Separation of Lignin and Cellulose from Sugars
Filtering–Different types of filtering media have been used to separate lignin and
cellulose from the free sugars. The free sugars are added to the fermentation tank
(Figure 3–Process 6).
Cellulose Hydrolysis
Cellulose must first be converted to sugars by acid hydrolysis or enzymatic hydrolysis
before these sugars can be converted to ethanol by fermentation processes.
Acid Hydrolysis–Two common methods under development for converting cellulose
to sugar are dilute acid hydrolysis and concentrated acid hydrolysis, both of which
typically use sulfuric acid (although other acids have also been tried). Dilute acid
hydrolysis usually occurs in two stages to take advantage of the differences between
hemicellulose and cellulose. The first stage is performed at low temperature to
maximize the yield from the hemicellulose, and the second, higher temperature stage
is optimized for hydrolysis of the cellulose portion of the feedstock. Concentrated acid
31