Bioethanol
Ethanol fuel is ethanol (ethyl alcohol), the same type of alcohol
found in alcoholic beverages. It can be used as a fuel, mainly
as a biofuel alternative to gasoline, and is widely used in cars in
Brazil. Because it is cheap, easy to manufacture and process,
and can be made from very common materials, such as corn, it
is steadily becoming a promising alternative to gasoline
throughout much of the world.
Anhydrous ethanol, that is, ethanol with at most 1% water can
be blended with gasoline in varying quantities up to pure
ethanol (E100), and most spark-ignited gasoline style engines
will operate well with mixtures of 10% ethanol (E10).
Ethanol can be mass-produced by fermentation of sugar or by
hydration of ethylene from petroleum and other sources.
Current interest in ethanol mainly lies in bio-ethanol, produced
from the starch or sugar in a wide variety of crops, but there has
been considerable debate about how useful bio-ethanol will be
in replacing fossil fuels in vehicles. Concerns relate to the large
amount of arable land required for crops, as well as the energy
and pollution balance of the whole cycle of ethanol production.
Recent developments with cellulosic ethanol production and
commercialization may allay some of these concerns.
According to the International Energy Agency, cellulosic ethanol
could allow ethanol fuels to play a much bigger role in the future
than previously thought. Cellulosic ethanol can be made from
plant matter composed primarily of inedible cellulose fibers that
form the stems and branches of most plants. Dedicated energy
crops, such as switchgrass, are also promising cellulose
sources.
Chemistry
During ethanol fermentation, glucose is decomposed into
ethanol and carbon dioxide.
C6H12O6 → 2C2H6O + 2CO2
During combustion ethanol reacts with oxygen to produce
carbon dioxide, water, and heat: (other air pollutants are also
produced when ethanol is burned in the atmosphere rather than
in pure oxygen)
2C2H6O + 6O2 → 4CO2 + 6H2O + heat
Together, these two equations add up to the following:
C6H12O6 + 6O2 → 2CO2 + 4CO2 + 6H2O + heat
This is the reverse of the photosynthesis reaction, which shows
that the three reactions completely cancel each other out, only
converting light into heat without leaving any byproducts:
6 CO2 + 6 H2O + light → C6H12O6 + 6 O2
Sources
About 5% (in 2003) of the ethanol produced in the world is
actually a petroleum product. It is made by the catalytic
hydration of ethylene with sulfuric acid as the catalyst. It can
also be obtained via ethylene or acetylene, from calcium
carbide, coal, oil gas, and other sources. Two million tons of
petroleum-derived ethanol are produced annually. The principal
suppliers are plants in the United States, Europe, and South
Africa. Petroleum derived ethanol (synthetic ethanol) is
chemically identical to bio-ethanol and can be differentiated
only by radiocarbon dating.
Bio-ethanol is obtained from the conversion of carbon based
feedstock. Agricultural feedstocks are considered renewable
because they get energy from the sun using photosynthesis,
provided that all minerals, required for growth (such as nitrogen
and phosphorus), are returned to the land. Ethanol can be
produced from a variety of feedstocks such as sugar cane,
bagasse, miscanthus, sugar beet, sorghum, grain sorghum,
switchgrass, barley, hemp, kenaf, potatoes, sweet potatoes,
cassava, sunflower, fruit, molasses, corn, stover, grain, wheat,
straw, cotton, other biomass, as well as many types of cellulose
waste and harvestings, whichever has the best well-to-wheel
assessment.
Production Process
The basic steps for large scale production of ethanol are:
microbial (yeast) fermentation of sugars, distillation, dehydration
(required unless distillation is done repeatedly), and denaturing
(optional). Prior to fermentation, some crops require
saccharification
or
hydrolysis
into
carbohydrates.
Saccharification of cellulose is called cellulolysis (see cellulosic
ethanol). Other pre-production steps can be necessary for
certain crops like corn which requires refinement into starch and
liquification.
Fermentation
Ethanol is produced by microbial fermentation of the sugar.
Subsequent processing is the same as for ethanol from corn.
Production of ethanol from sugarcane (sugarcane requires a
tropical climate to grow productively) returns about 8 units of
energy for each unit expended compared to corn which only
returns about 1.34 units of fuel energy for each unit of energy
expended.
Carbon dioxide, a greenhouse gas, is emitted during
fermentation and combustion. However, this is canceled out by
the greater uptake of carbon dioxide by the plants as they grow
to produce the biomass. When compared to gasoline,
depending on the production method, ethanol releases less or
even no greenhouse gases.
Distillation
For the ethanol to be usable as a fuel, water must be removed.
Most of the water is removed by distillation, but the purity is
limited to 95-96% due to the formation of a low-boiling waterethanol azeotrope. The 96% m/m (93% v/v) ethanol, 4% m/m
(7% v/v) water mixture may be used as a fuel.
Dehydration
Currently, the most widely used purification method is a
physical absorption process using a molecular sieve, for
example, ZEOCHEM Z3-03 (a special 3A molecular sieve for
EtOH dehydration). Another method, azeotropic distillation, is
achieved by adding the hydrocarbon benzene which also
denatures the ethanol (to render it undrinkable for duty
purposes). A third method involves use of calcium oxide as a
desiccant.
Technology
Ethanol-based engines
Ethanol is most commonly used to power automobiles, though it
may be used to power other vehicles, such as farm tractors and
airplanes. Ethanol (E100) consumption in an engine is
approximately 34% higher than that of gasoline (the energy per
volume unit is 34% lower). However, higher compression ratios
in an ethanol-only engine allow for increased power output and
better fuel economy than would be obtained with the lower
compression ratio. In general, ethanol-only engines are tuned to
give slightly better power and torque output to gasolinepowered engines. In flexible fuel vehicles, the lower
compression ratio requires tunings that give the same output
when using either gasoline or hydrated ethanol. For maximum
use of ethanol's benefits, a much higher compression ratio
should be used, which would render that engine unsuitable for
gasoline usage. When ethanol fuel availability allows highcompression ethanol-only vehicles to be practical, the fuel
efficiency of such engines should be equal or greater than
current gasoline engines. However, since the energy content
(by volume) of ethanol fuel is less than gasoline, a larger
volume of ethanol fuel would still be required to produce the
same amount of energy.
A 2004 study, and paper published by the Society of
Automotive Engineers, present the possibility of a definite
advance over hybrid electric cars' cost-efficiency by using a
high-output turbocharger in combination with continuous dualfuel direct injection of pure alcohol and pure gasoline in any
ratio up to 100% of either. Each fuel is stored separately,
probably with a much smaller tank for alcohol, the peak costefficiency being calculated to occur at approximately 30%
alcohol mix, at maximum engine power. The estimated cost
advantage is calculated at 4.6:1 return on the cost of alcohol
used, in gasoline costs saved, when the alcohol is used
primarily as an octane modifier and is otherwise conserved.
With the cost of new equipment factored in the data gives a 3:1
improvement in payback over hybrid, and 4:1 over turbo-diesel
(comparing consumer investment yield only). In addition, the
danger of water absorption into pre-mixed gasoline and supply
issues of multiple mix ratios can be addressed by this system
Ethanol fuel mixtures
To avoid engine stall, the fuel must exist as a single phase. The
fraction of water that an ethanol-gasoline fuel can contain
without phase separation increases with the percent of ethanol.
This is shown for 25 C (77 F) in a gasoline-ethanol-water phase
diagram, Fig 13 of . This shows, for example, that E30 can
have up to about 2% water. If there is more than about 71%
ethanol, the remainder can be any proportion of water or
gasoline and phase separation will not occur. However, the fuel
mileage declines with increased water content. The increased
solubility of water with higher ethanol content permits E30 and
hydrated ethanol to be put in the same tank since any
combination of them always results in a single phase.
Somewhat less water is tolerated at lower temperatures. For
E10 it is about 0.5% v/v at 70 F and decreases to about 0.23%
v/v at -30 F.
Beginning with the model year 1999, an increasing number of
vehicles in the world are manufactured with engines which can
run on any fuel from 0% ethanol up to 100% ethanol without
modification. Many cars and light trucks (a class containing
minivans, SUVs and pickup trucks) are designed to be flexiblefuel vehicles (also called dual-fuel vehicles). Their engine
systems contain alcohol sensors in the fuel and/or oxygen
sensors in the exhaust that provide input to the engine control
computer to adjust the fuel injection to achieve stochiometric
(no residual fuel or free oxygen in the exhaust) air-to-fuel ratio
for any fuel mix. The engine control computer can also adjust
(advance) the ignition timing to achieve a higher output without
pre-ignition when higher alcohol percentages are present in the
fuel being burned.
Fuel economy
All vehicles have a fuel economy (measured as miles per US
gallon -MPG- , or liters per 100 km) that is directly proportional
to energy content. Ethanol contains approx. 34% less energy
per unit volume than gasoline, and therefore will result in a 34%
reduction in miles per US gallon. For E10 (10% ethanol and
90% gasoline), the effect is small (~3%) when compared to
conventional gasoline, and even smaller (1-2%) when
compared to oxygenated and reformulated blends. However, for
E85 (85% ethanol), the effect becomes significant. E85 will
produce lower mileage than gasoline, and will require more
frequent refueling. Actual performance may vary depending on
the vehicle.
Environment
Energy balance
All biomass needs to go through some of these steps: it needs
to be grown, collected, dried, fermented, and burned. All of
these steps require resources and an infrastructure.
Ethanol is not the only product created during production, and
the energy content of the by-products must also be considered.
Corn is typically 66% starch and the remaining 33% is not
fermented. This unfermented component is called distillers
grain, which is high in fats and proteins, and makes good
animal feed.
In Brazil where sugar cane is used, the yield is higher, and
conversion to ethanol is somewhat more energy efficient than
corn. Recent developments with cellulosic ethanol production
may improve yields even further.
Air pollution
Compared with conventional unleaded gasoline, ethanol is a
particulate-free burning fuel source that combusts cleanly with
oxygen to form carbon dioxide and water.
Use of ethanol,
produced from current methods, emits a similar net amount of
carbon dioxide but less carbon monoxide than gasoline. Current
production methods includes air pollution from the manufacturer
of macronutrient fertilizers. The production of ammonia to
produce nitrogen fertilizer consumed about 5% of the world
natural gas consumption while China uses coal for 60% of their
nitrogen fertilizer production.
If ethanol-production energy came from non-fossil sources the
use of ethanol as a fuel would add less greenhouse gas.
Manufacture
In 2002 , monitoring of ethanol plants revealed that they
released VOCs (volatile organic compounds) at a higher rate
than had previously been disclosed. Devices known as thermal
oxidizers or catalytic oxidizers can be attached to the plants to
burn off the hazardous gases. Smog causing pollutants are also
increased by using ethanol fuel in comparison to gasoline.
Greenhouse gas abatement
Corn ethanol has received much support on environmental
grounds primarily because of its role in reducing greenhouse
gas emissions. However, the evidence for this claim is mixed.
Another study has suggested that replacement of 100%
petroleum fuel with E85 (a fuel mixture comprised of 85%
ethanol and 15% petroleum) would significantly increase ozone
levels, thereby increasing photochemical smog and aggravating
medical problems such as asthma.
This value reflects increases in corn area and the use of 30% of
the corn crop for ethanol. It also apparently takes into account
anticipated improvements in corn yields and ethanol production.
The PNAS value is a 12% reduction in greenhouse gas
emission relative to the "net emissions of production and
combustion of an energetically equivalent amount of gasoline."
Land use
Large-scale 'energy farming', necessary to produce agricultural
alcohol, requires substantial amounts of cultivated land. Some
have claimed that land is acquired through deforestation, while
others have observed that areas currently supporting forests
are usually not suitable for growing any sort of crops. Related
concerns have been raised regarding a decline in soil fertility
due to reduction of organic matter a decrease in water
availability and quality, an increase in the use of pesticides and
fertilizers, and potential dislocation of local communities.
As demand for ethanol fuel increases, food crops are replaced
by fuel crops, driving food supply down and food prices up.
Alternatively, cellulosic ethanol can be produced from any plant
material, potentially doubling yields, in an effort to minimize
conflict between food needs versus fuel needs. Instead of
utilizing only the starch bi-products from grinding wheat and
other crops, cellulosic ethanol production maximizes the use of
all plant materials, including gluten. This approach would have
a smaller carbon footprint because the amount of energyintensive fertilisers and fungicides remain the same for higher
output of usable material. While the enzyme technology for
producing cellulosic ethanol is currently in developmental
stages, it is not expected to be available for large-scale
production in the near future. Moreover, the production of
ethanol for fuel raises a number of land scarcity issues,
regardless of what production method is employed. Many
analysts suggest that biofuel strategies must be accompanied
by fuel conservation restrictions.
Renewable resource
Ethanol is considered "renewable" because it is primarily the
result of conversion of the sun's energy into usable energy.
Creation of ethanol starts with photosynthesis causing the
feedstocks such as switchgrass, sugar cane, or corn to grow.
These feedstocks are processed into ethanol (see production).
The environmental and economic benefits of non-cellulosic
ethanol - including corn ethanol - have been heavily critiqued by
many. The main criticism dwells on the increasing costs of corn
for food as the demand for ethanol production increases. It
remains to be seen if ethanol production can overcome these
problems.
Current, first generation processes for the production of ethanol
from corn use only a small part of the corn plant: the corn
kernels are taken from the corn plant and only the starch, which
represents about 50% of the dry kernel mass, is transformed
into ethanol. Two types of second generation processes are
under development. The first type uses enzymes to convert the
plant cellulose into ethanol while the second type uses pyrolysis
to convert the whole plant to either a liquid bio-oil or a syngas.
Second generation processes can also be used with plants
such as grasses, wood or agricultural waste material such as
straw.
Economics
The science of Economics is generally defined as the study of
scarcity management. Absent scarcity and alternative uses of
available resources, there is no economic problem. As such,
the subject of economics involves the study of choices as they
are affected by incentives and resources. Since land and
agriculture have historically served the world as utilities for food
production, many believe the alternative use of agricultural
resources for ethanol fuel production imposes an artificial
scarcity of food on a global scale.
Currently, research on improving ethanol yields from each unit
of corn is underway using biotechnology. By utilizing hybrids
designed specifically with higher extractable starch levels, the
energy balance is dramatically improved. Also, as long as oil
prices remain high, the economical use of other feedstocks,
such as cellulose, become viable. By-products such as straw or
wood chips can be converted to ethanol. Fast growing species
like switchgrass can be grown on land not suitable for other
cash crops and yield high levels of ethanol per unit area.
(Source: Wikipedia)