Page 293 - Synthetic Fuels Handbook

FUELS FROM CROPS 279

the sugars (hydrolysis) and then fermenting as much of the six- and five-carbon sugars as
possible to produce ethanol.
There are several different methods of hydrolysis: (a) concentrated sulfuric acid, (b) dilute
sulphuric acid, (c) nitric acid, and (d) acid pretreatment followed by enzymatic hydrolysis.

9.3.1 Ethanol Production
Ethanol is produced from the fermentation of sugar by enzymes produced from specific
varieties of yeast. The five major sugars are the five-carbon xylose and arabinose and the
six-carbon glucose, galactose, and mannose. Traditional fermentation processes rely on
yeasts that convert six-carbon sugars to ethanol. Glucose, the preferred form of sugar for
fermentation, is contained in both carbohydrates and cellulose. Because carbohydrates are
easier than cellulose to convert to glucose, the majority of ethanol currently produced in the
United States is made from corn, which produces large quantities of carbohydrates. Also,
the organisms and enzymes for carbohydrate conversion and glucose fermentation on a
commercial scale are readily available.
The conversion of cellulosic biomass to ethanol parallels the corn conversion process.
The cellulose must first be converted to sugars by hydrolysis and then fermented to produce
ethanol. Cellulosic feedstocks (composed of cellulose and hemicellulose) are more difficult
to convert to sugar than are carbohydrates. Two common methods for converting cellulose
to sugar are dilute acid hydrolysis and concentrated acid hydrolysis, both of which use
sulfuric acid.
Dilute acid hydrolysis 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 hydrolysis uses a
dilute acid pretreatment to separate the hemicellulose and cellulose. The biomass is then
dried before the addition of the concentrated sulfuric acid. Water is added to dilute the acid
and then heated to release the sugars, producing a gel that can be separated from residual
solids. Column chromatography is used to separate the acid from the sugars.
Both the dilute and concentrated acid processes have several drawbacks. Dilute acid
hydrolysis of cellulose tends to yield a large amount of by-products. Concentrated acid
hydrolysis forms fewer by-products, but for economic reasons the acid must be recycled.
The separation and concentration of the sulfuric acid adds more complexity to the process.
The concentrated and dilute sulfuric acid processes are performed at high temperatures
(100 and 220°C) which can degrade the sugars, reducing the carbon source and ultimately
lowering the ethanol yield. Thus, the concentrated acid process has a smaller potential for
cost reductions from process improvements such as acid recovery and sugar yield for the
concentrated acid process could provide higher efficiency for both technologies.
Another approach involves countercurrent hydrolysis, a two-stage process. In the first
stage, cellulose feedstock is introduced to a horizontal cocurrent reactor with a conveyor.
Steam is added to raise the temperature to 180°C (no acid is added at this point). After a
residence time of about 8 minutes, during which some 60 percent of the hemicellulose is
hydrolyzed, the feed exits the reactor. It then enters the second stage through a vertical
reactor operated at 225°C. Very dilute sulfuric acid is added to the feed at this stage, where
virtually all of the remaining hemicellulose and, depending on the residence time, any-
where from 60 percent to all of the cellulose is hydrolyzed. The countercurrent hydrolysis
process offers higher efficiency (and, therefore, cost reductions) than the dilute sulfuric acid
process. This process may allow an increase in glucose yields to 84 percent, an increase
in fermentation temperature to 55°C, and an increase in fermentation yield of ethanol to
95 percent.