Biofuels from Biomass - Novel Combustion Concepts for Sustainable Energy Development - page 39

6.3 Fermentation

The cost of ethanol is highly dependent on the ef ciency of conversion of biomass

to sugars and then fermentation to ethanol. This depends on the substrate, bacteria
used, concentration of enzyme and nutrients, pH, oxygen supply, and pressure.
A major hurdle to lowering the ethanol price is the lack of ability to ferment both
C5 and C6 sugars. C6 sugar technology is well developed, but research is being
undertaken to find a microorganism that will ferment both sugars and give a high
yield of ethanol.
A variety of process methods have been suggested in the production of cellulosic
ethanol. In separate hydrolysis and fermentation (SHF), the liquid and solid phases
are separated after pretreatment. In the case of dilute acid pretreatment, the solid
stream is washed and then subject to enzymatic hydrolysis. The resulting sugar
solution is then fermented to ethanol with conventional yeast microbes. A suitable
pentose fermenting strain can covert the liquid C5 sugar stream into ethanol after a
detoxi cation step. These separate processes allow optimization for each step so

you can get the highest yields possible. Alternatively, enzymes could be added to
the whole pretreatment slurry without separation of the solid and liquid fractions,
followed by fermentation of the C5 and C6 which is called separate hydrolysis and
co-fermentation (SHcF). This is economically very attractive, but the fermentation
step is much more challenging.
As cellulase enzymes are inhibited by their hydrolysis products, a favoured
process is simultaneous sacchari

cation and fermentation (SSF) which keeps sugar
concentrations low. Despite the need to lower the temperature from optimum
enzyme ef ciency to accommodate the fermentation microorganisms, SSF was

shown to achieve higher rates, yields, and concentrations than SHF by overcoming
the effects of end product cantly reduces

equipment demands and the presence of ethanol prevents contamination by
unwanted microbes (Brethauer and Wyman 2010 ).
A new line of development has been consolidated bioprocessing of lignocellu-
lose. This combines simultaneous sacchari

inhibition. In addition, SSF signi

cation with fermentation of the resulting
sugars in a single process mediated by a single microbial mixture which produces
the enzymes necessary for hydrolysis and has the pathways necessary to ferment the
sugars produced under the same conditions. CBP has the potential to reduce the cost
of conversion of biomass to ethanol signi cantly although a microorganism that

produces ethanol at high yield has yet to be found (Xu et al. 2009 ).
Another area of development for fermentation is the concept of continuous
fermentation. All commercial fermentation processes at the current time use a batch
process whereby the sugar solution is put into the fermentation vessel and left to
ferment until the ethanol concentration reaches a certain level. The vessel is drained
and a new fermentation broth is created. In a continuous fermentation system, the
substrate is constantly fed into the vessel at the same rate as the fermentation
product is removed to keep the volume constant. The advantage of this technique is
that

it shows higher productivities and reduced vessel downtime, filling, and