FUTURE TRENDS IN BIOETHANOL
PRODUCTION
Sana Javed
UG-3 (Section A)
Submitted to: Sir Tahir Baig
Submission date: January 1st, 2013
FUTURE TRENDS IN BIOETHANOL PRODUCTION
The vast majority of all fuels and carbon-containing chemicals are produced from fossil
resources such as petroleum and natural gas. Studies predict that most of these limited fossil
resources will be able to meet our needs just for a few more decades before depletion.
Furthermore, the combustion of these fossil fuels produces greenhouse gases (GHG), which
could lead to global warming. As a consequence, the global society must gradually shift from an
economy based on fossil resources to the one based on sustainable/renewable resources. Biomass
could play a significant role in this. Depletion and global warming fears have lead to the
substantial increase in the trend to produce ethanol by fermentation of biomass. [1]
Bio-ethanol, therefore, is ethanol produced by fermentation of biomass such as crop plants, and
is structurally similar to chemically produced ethanol.
1. BIOETHANOL PRODUCTION
The biotechnological processes are responsible for the majority of ethanol that is currently being
produced. About 95% of ethanol produced today comes from agricultural products. Ethanol
production from sugar crops (sugarcane, sugar beet
etc.) comprises 40% of this, and nearly 60%
corresponds to starch-containing crops (corn, wheat,
rice etc.).
Bioethanol can be produced by fermentation of
simple sugars or polysaccharides like starch or
cellulose that can be converted to sugars. Thus,
carbohydrate sources (feedstocks) can be classified
into three main groups; simple sugars, starches and
lignocellulosic biomass.
[2]
Different countries use
different sources of simple sugars and starch,
depending upon the availability, cultivation and costs of crops in the respective geographical
regions. Bioethanol production involves three basic steps:
1. Obtaining simple (fermentable) sugar solution
2. Fermenting sugars into bioethanol
3. Purification/separation of bioethanol
The first step is the main difference between different ethanol production processes for different
source materials used:
1.1 SIMPLE SUGARS
Simple sugar crops such as molasses require only milling to make the stored sugars available for
fermentation, thus it is relatively the simplest method to transform sugars into ethanol. The
bioethanol production process from simple sugars, as a result, involves only milling of source
material, followed by its fermentation through yeast (mostly Sachharomyces species), and final
steps of purification. Currently, fermentation is carried out mainly by the fed-batch process, but
may also be carried out by continuous fermentation.[3]
1.2 STARCH
For crops containing starches, such as wheat and corn, an additional step of saccharification
(breaking complex sugars into monosaccharides) is performed before fermentation because yeast
cannot ferment starch
into ethanol directly.
During this process,
the starch is gelatinized
and is subjected to
enzymatic
hydrolysis
using
α-amylase,
converting
it
simple
glucose
monomers.
glucose
into
This
is
then
fermented to produce
bioethanol in the same
way as
the
simple
sugars are. Ethanol production from starchy crops, particularly corn, is well-established and
widely used all over the world. [4]
1.3 LIGNOCELLULOSE
Polysaccarides present in ligocellulosic materials such as cellulose and hemicellulose contain a
great potential to be used as feedstocks for bioethanol production. Lignocellulose is present in
plant parts such as wood, husks and agricultural plant materials which are considered as wastes.
The technologies involved in producing ethanol from these sources are more complicated than
the others, and the production costs are also higher when compared to those of sugarcane,
sugarbeet, corn and wheat. But, despite this, lignocellulosic ethanol production is expected to
become the main source of feedstock for bioethanol manufacture in the near future due to
concerns about wheat and corn shortages and their use as staple foods. The steps involved in
bioethanol production from lignocellulose include
1. Pretreatment: To render celluloses and hemicelluloses more accessible to enzymes and
yeast by reducing particle size mechanically.
2. Enzymatic hydrolysis: To break down complex polysaccharides into simple sugars using
fungal cellulolytic enzymes.
3. Fermentation: To transform simple sugars into bioethanol by yeast.
4. Purification: To separate and concentrate ethanol using distillation and purification
processes.
Fungi naturally produce and secrete enzymes to digest cellulose. These cellulolytic enzymes
directly convert cellulose polysaccharides into simple sugars which can be fermented by yeast.
Species such as Candida may be used in addition to Saccharomyces in this case to carry out
efficient fermentation. [4, 5]
2. BIOETHANOL: THE FUTURISTIC APPROACH
Bioethanol production has increased strongly in the recent times due to the growing awareness of
issues such as environmental pollution, limited-resource depletion and global warming.
Countries such as the United States, Brazil, China, Japan and India are particularly eager in
producing bioethanol from different resources such as sugarcane, molasses, sugarbeet, corn,
wheat etc. depending upon the regional availability and costs of these raw materials.[6]
Crop- derived bioethanol market throughout the world is ever-increasing, particularly in the
countries producing it. Its demand is expected to continue increasing as more awareness grows,
ultimately replacing it as the primary source of vehicle fuel.
2.1 CHALLENGES & TECHNOLOGICAL TRENDS
The popularity of sugar and starch-based bioethanol production, however, is currently marred by
various issues. The fact that its raw materials (feedstocks) essentially are sources of human and
animal food also makes bioethanol production from these feedstocks controversial. The
increasing global food shortage and the elevating food prices, has thus resulted in this technology
to attract excessive criticism. [1]
Other than the above mentioned drawback, the approaches of bioethanol production also face
other challenges which need to be addressed for the future use of bioethanol as an alternative to
using limited depleting resources. Producing ethanol from grains such as corn has several
environmental impacts, including the soil erosion, biodiversity loss, and pollution due to organic
compounds and oxides of Nitrogen. It also requires a significant area for crop plantation to be
used as feedstock. For these reasons, the nations worldwide are searching for new technologies
and processes for the bioethanol production, which do not have these adversities. [7]
2.2 PROSPECTIVE SOLUTIONS: LIGNOCELLULOSE
Consequently, bioethanol production from lignocellulosic biomass is a very important alternative
being considered for large scale production. However, while technologies for bioethanol
production from sugar and starch are very much elaborate and well-established, the technologies
to produce it from lignocellulosic biomass are still under the process of being developed. Despite
the substantial amount of progress in cellulose-derived ethanol research and establishment, many
challenges are still to be overcome.
These include the development of an economically feasible hydrolysis step of cellulose. The
fungal cellulolytic enzymes used to break down cellulose to simple sugars are too costly to be
utilized at a commercial level. This drawback can be overcome by developing new methods of
pretreatment, such as the use of genetically engineered cellulolytic enzymes which are not as
costly.
High amount of energy consumption for cellulosic biomass pretreatment also remains a
challenge, which needs to be overcome. Scale-up of the process is one of the major challenges
for production at a commercial level, which can be overcome by optimization of the whole
process from yeasts to enzymes to other factors such as economy. [8, 9]
Once these challenges are overcome, lignocellulosic ethanol has the potential to grow as a
separate industry in the future, replacing the use of non-renewable sources altogether. Other than
lignocelluloses based approach, processes for bioethanol production, such as those based on the
use of algae may be developed.
2.3 PROSPECTIVE SOLTIONS: ALGAE
Algae have a much different makeup than does most feedstocks used in bioethanol, such as corn
and sugarcane. Still, algae can be used as a feedstock to produce bioethanol due to the fact that
they are naturally high in
polysaccharide
amounts
and possess thin cellulose
walls. This provides the
benefits
of
high
polysaccharide content of
simple sugars and starch,
while
having
thin
cellulose walls requiring
reduced quantity, and thus
the costs, of the cellulolytic enzymes. This approach has not been given much attention, but due
to increasing concerns about depletion of limited resources, it has finally started to gain more
focus from scientists.
The technique involves growing starch and cellulose accumulating algae species such as
Chlorella vulgaris in the presence of sunlight, and harvesting this grown algae biomass. This is
followed by separation of polysaccharides from rest of the biomass. Then, the familiar steps of
breakdown of polysaccharides to simple sugars, fermentation by yeast, and separation and
purification follow. [10]
Several scientific and technical barriers, like other techniques, exist in this approach of
bioethanol production, which may be overcome through advances in photobioreactor design
among other developments. Once the challenges are overcome, algal ethanol may become
commercialized reality.
3. CONCLUSION
Depleting nonrenewable resources raise the need of developing alternates to these resources that
would conserve the natural resources, while meeting our needs. Bioethanol is such an alternative,
which can be produced by renewable sources such as crops, wood, algae etc. through the steps of
pretreatment, fermentation and purification. Although most of these techniques are still in
developmental stages, they hold a promising potential to one day be able to replace the
nonrenewable resources as vehicle fuel and chemical products.
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