
9B.
A general survey of fuels with an emphasis on biofuels (bioethanol, biodiesel,
biogas, hydrogen
and other alternative fuels - which may or may not renewable?
Doc Brown's
chemistry revision notes: GCSE chemistry, IGCSE chemistry, O level
& ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old
students of chemistry
Biofuels and alternative fuels e.g.
bioethanol, biogas, biodiesel, hydrogen and other
cleaner fuels like hydrogen should prove useful for the AQA GCSE chemistry,
Edexcel GCSE chemistry & OCR GCSE chemistry (Gateway & 21st Century) GCSE (9–1),
(9-5) & (5-1) science courses. Doc
Brown's GCSE/IGCSE/O Level KS4 science–CHEMISTRY Revision Notes Suitable for Advanced A Level/IB/US grades 10-12
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All my
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INDEX of Advanced A Level revision notes on the chemistry of ALCOHOLS
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Sub-index for this page
(1) Introduction
to biofuels
(2)
More on
making ethanol from biomass
(3)
Biodiesel - another biofuel
derived from plant material
(4)
A comparison of three
alternative fuels for motor vehicles - advantages and disadvantages
(5) Is bioethanol a carbon neutral fuel?
(6) A
wider ranging fuel survey which overlaps with (5)
See also
Biotechnology and
enzymes (GCSE chemistry -
enzyme notes)
Biotechnology and ethanol production
(GCSE chemistry - alcohol notes)
and
Biotechnology and genetic engineering
(GCSE biology notes)
AND
Chemistry and uses of alcohols - ethanol and in
the GCSE 9-1 Physics Notes .....
Energy resources & uses, general survey & trends, comparing sources of renewables, non-renewables
& biofuels
and
Carbon cycle,
nitrogen cycle, water cycle, decomposition - decay investigation, biogas
gcse biology
GCSE/IGCSE/O Level Oil Products & Organic Chemistry INDEX PAGE
See also
Alcohols, Ethanol, manufacture, physical properties and chemical reactions
9b. (1) What is a biofuel? - renewable energy sources
-
A
discussion on biofuels and using plant materials for fuels
- As well as specific crops like sugar cane or
vegetable oils, all sorts of other organic materials can be used to make
biofuels e.g. animal droppings, farm waste, landfill rubbish
-
What is a biofuel? What does 'carbon
neutral mean'?
- Biofuels are alternative fuels to fossil
fuels produced from plant material or animal dung i.e. from once
living materials.
- They are referred to as renewable because
more can be grown, recycling the carbon absorbed in photosynthesis.
- They can be burnt to produce steam to
drive turbine to power electricity generation
or used as fuel in road vehicles.
- Ideally they are 'carbon neutral',
that is the carbon dioxide the they give off on combustion is matched by
the plant absorbing the same amount of carbon dioxide.
- The absorbed
carbon dioxide is then utilised in photosynthesis to regenerate the
plant material ie remaking the same amount of biofuel again. So, its a
sort of mini–carbon cycle.
-
Renewable Biofuels that can be burned to heat water
to make steam to drive a turbine and generator.
- Biofuels are renewable
energy sources and come in a variety of forms eg woodchips (trees or
waste from timber products), alcohol (ethanol
from fermenting sugar cane), biodiesel (from vegetable oil) and
biogas (methane from anaerobic digestion of sewage waste) and are all
derived from plant materials eg crops or bacterial digestion/decay of
waste organic material.
- To be
considered a carbon neutral fuel, the fuel must be produced, and used
efficiently, with no net emission of the greenhouse gas carbon dioxide which
contributes to global warming.
- The theoretical 'carbon neutral' idea behind
using biofuels is that the carbon dioxide released on burning is
re–absorbed by plants and utilised in photosynthesis to create the next
fuel crop. But, even though this sounds fine in principle, there are
still environmental issues eg in Brazil and other countries, huge areas
of ecological valuable natural rain forest (habitats, species rich) are
being cut down to grow crops for biofuels.
TOP OF PAGE
and sub-index
(2) More on
making ethanol from biomass
Full details on ways of
manufacturing ethanol
- Waste biomass is obtained from the parts
of plants that have no particular use and would normally be thrown away
or just burned.
- Examples, which apply to any country,
include wood pulp/dust/shavings/chippings, straw, rice husks, corn
stalks etc.
- What these examples of biomass have in
common is they all contain a high percentage of cellulose, a much more
stable molecule that the carbohydrates easily broken down to
sugars and fermented with yeast to produce ethanol.
- So yeast fermentation cannot be used on
this kind of biomass material. Instead, genetically modified E. coli
bacteria are used to break down the cellulose and convert it into
ethanol. A good example, I hope, of a non-controversial use of GM
biochemistry.
- The optimum conditions for this process
are 35oC and pH 6, so near body temperature and very slightly
acidic.
- The optimum conditions for the E. coli
induced reactions are illustrated by the graphs below, i.e. the graph
peaks show where the fastest rate of reaction happens, therefore showing
the most economic reaction conditions for the process.
-
and
- There is no reason why making ethanol
from waste biomass isn't a viable and sustainable route for making
ethanol compared to the fermentation of sugar.
- Both processes have similarities e.g.
they both use renewable plant materials, both use a relatively simple
reaction chamber system.
- Some advantages for the biomass method
include (i) you don't need to grow more crops, conserving land use, (ii)
you are using material that might otherwise be thrown away, but, the
amount of biomass waste material is small compared to the mass of the
original crop material, so large scale production of ethanol by this
method is limited.
-
Ethanol is an example of a renewable
biofuel from plant material
('bioethanol')
- See also 9a.
Chemistry of alcohols - ethanol
-
What is 'gasohol'?
Ethanol and
petrol can be mixed to make a motor vehicle fuel
- Countries like Brazil, that have no oil
reserves of their own, and importing oil is costly, so they are very
interested in the manufacture of biofuels, i.e. fuels that can be
derived from plant crop materials.
- Some ethanol is used in blends of petrol
( a 'gasohol') eg in Brazil (called 'alcool') with
a cheap labour force (local population or migrant labour) and large land
areas growing sugar cane.
- A typical composition of 'gasohol' might
be 10% ethanol (ethyl alcohol) and 90% unleaded petrol (gasoline).
- It is less polluting than ordinary
petrol, it burns more cleanly-efficiently.
- It also makes the imported oil go a bit
further!
- The renewable sugar cane crop is crushed and the sugar
extracted with hot water and the solution fermented with yeast.
- The alcohol is fractionally distilled
from the filtered fermented solution and can then be used as a biofuel.
- Details of the fermentation process are on the 'Ethanol
Chemistry' page.
-
Making renewable fuels like
bioethanol from plant materials
- Benefits of using plant or animal materials
to produce
fuels
- Naturally, they are all renewable.
- Drawbacks and risks to using plant
materials to produce fuels
- Large areas of agricultural land or
woodland required - the land is often prepared for crop growing by
clearing large areas of forest by burning - polluting smoke,
removal of wildlife habitat, reducing biodiversity.
- Slow growth eg trees for wood fuels,
fast to burn! could demand be matched?,
- or slow industrial processes like
fermentation.
TOP OF PAGE
and sub-index
(3) Biodiesel
- another biofuel
derived from plant material.
- Biodiesel can be made from
vegetable oils
(and animal fat or waste cooking oil) which contain glycerol esters of
long–chain fatty acids.
- These vegetable oils/fats like rapeseed oil and
soybean oil can re–esterified
(transesterification) into methyl esters to make a fuel that can be used
directly as diesel fuel or mixed with regular diesel fuel.
- Vegetable oils are suitable for diesel
fuel and release lots of energy on combustion just like petrol or
conventional diesel.
- Biodiesel has similar physical and
chemical properties to ordinary diesel from crude oil and burns in
conventional diesel engines.
- The simple word equation for processing
vegetable oil into biodiesel fuel is ...
- oil/fat + methanol ==> biodiesel +
glycerol
- glycerol ester + methanol (an alcohol)
==> methyl ester + glycerol (another sort of alcohol)
- Pros – advantages of using
biodiesel
- Biodiesel, is in theory, another 'carbon
neutral' renewable fuel.
- Biodiesel is readily biodegradable, so
less harmful to the environment if spilled compared to hydrocarbon oils
which take much longer to break down.
- Existing diesel engines don't need
converting.
- Cons – disadvantages of using
biodiesel
- Its relatively expensive to make
(small scale production compared to the petrochemical industry based on
crude oil).
- There won't be enough to replace
diesel from crude oil.
- Farmers (especially in third world
countries) may switch from essential food production to producing
plant oils to make biodiesel, thereby increasing food prices and maybe
creating food shortages.
- 'Alternative fuels'
- Think in terms of use of renewable
resources, storage and use of the fuels, their products of combustion.
- As already mentioned,
ethanol is one of the more recent 'alternative
fuels' to traditional fossil fuels like coal, gas and oil.
- If an efficient source of
hydrogen
production could be found, this could be another fuel, this is
especially clean burning, because all it produces is water!
TOP OF PAGE
and sub-index
(4) A
comparison of three alternative fuels for motor vehicles - advantages and
disadvantages
Comparison of
three alternatives to vehicle fossil fuels – their 'pros' and
'cons' |
FUEL (all renewable in theory) |
Advantages of the biofuel, the 'pros' |
Disadvantages of the biofuel, the 'cons' |
ETHANOL
(bioethanol)
Note that ethanol made from ethene
is NOT a renewable method because ethene is made from cracking
hydrocarbons from crude oil. |
Advantages
of bioethanol
a)
Can be cheaply produced from sugar cane/beet on an industrial
scale by fermentation.
See also 9a.
manufacture of ethanol
b) Ethanol is easily stored and distributed as
a liquid fuel e.g. petrol may contain 5-20% of distilled ethanol
(labelled E5 to E20) - but any remaining water must be first
removed - dehydrated ethanol. E100 is the azeotropic mixture
containing the maximum of 95.6% ethanol and 4.4% water. c)
Bioethanol is theoretically carbon neutral. In other words the
carbon dioxide produced on burning ethanol is taken in, and
balanced, by plants via photosynthesis, so the overall level of
carbon dioxide in the atmosphere is not increased, minimising
global warming.
d) The only products on combustion are carbon dioxide
and water with no pollutants like sulfur dioxide
2CH3CH2OH +
6O2 ===> 4CO2 + 6H2O |
Disadvantages
of bioethanol
a)
Slow rate of mass production
b) Requires large areas of
agricultural land or cleared woodland AND farmers may switch from
growing food crops – increasing food prices and possible causing
food shortages in third world countries.
c) Does produce the greenhouse
gas carbon dioxide (and water), theoretically reabsorbed by
plants, but fast to burn and plants slow to grow.
d) Ethanol fuels are not widely
available. e) Car engines
need modifying to work with highly concentrated ethanol fuels
e.g. the azeotropic mixture of 95% ethanol.
f) Ethanol costs five times more to
produce than 'straight–run' gasoline (and hydrogen is even more
expensive). Quite a lot of energy is needed to distil the
fermented mixture to obtain the concentrated ethanol.
g) Is bioethanol a truly carbon neutral fuel?
See extra
discussion note below this table |
HYDROGEN |
Advantages
of hydrogen
a)
Endless supply of water
b)
Water is the only product of burning hydrogen, so its very clean
non–polluting combustion. Hydrogen is the most clean burning of
any fuel.
2H2 + O2 ===>
2H2O
See also
use of hydrogen in fuels cells |
Disadvantages
of hydrogen
a)
Efficient large scale technology not yet developed to produce
hydrogen on a large scale eg from electrolysis
using solar power electricity – photovoltaic power system, wind
turbines or hydroelectric power.
b) Although water is cheap and
plentiful, it requires expensive electrical energy to
electrolyse water to split it into hydrogen and oxygen.
c) Hydrogen, as a gas it is more difficult to
store, needs a larger storage space (volume) and distribute safely due to it being highly flammable,
easily ignited, so risk of explosion if leaked.
d) You need a special expensive?
hydrogen burning car engine that is not widely available.
e) As already mentioned, hydrogen is an explosive gas and
difficult to store and transport safely from the point of view
of distribution to consumers.
f) Most hydrogen used in industry is
actually made from fossil fuel hydrocarbons! (see making
hydrogen for the
Haber Synthesis of ammonia). |
BIOGAS
(usually methane) |
Microorganisms can be used to break down organic waste e.g.
sewage waste, under
anaerobic conditions to produce biogas, which is mainly the
hydrocarbon methane gas, CH4. You can use a variety
of materials to be broken down e.g. animal waste, dead plant
material.
Advantages
of biogas
a) The biogas can
be burned like any other fuel to produce heat. The heat can be
used to generate steam to drive a turbine and electrical
generator. This is quite handy for small scale electricity
production in remote areas far from a national grid supply. It
could also power road vehicles to.
b) Theoretically it is a
renewable resource and carbon neutral The decomposed
plants are replaced by new crops, and, with the animal waste
from eating plant material, the carbon is recycled by carbon
dioxide formation on burning. The growth of new crops removes
and balances
the same carbon dioxide by the process of photosynthesis in
plant leaves.
c) The raw materials for biogas
are relatively cheap and readily available, mainly from
agricultural sources.
d) Burning biogas is relatively
clean fuel, although it produces carbon dioxide and water on
combustion it does not produce much sulfur dioxide, oxides of
nitrogen or carbon/hydrocarbon particulates |
Disadvantages
of biogas
a) At the moment biogas cannot be
produced on a huge scale. |
TOP OF PAGE
and sub-index
(5) Extra note on
ethanol/bioethanol
-
Is bioethanol a carbon neutral fuel?
- To be a carbon neutral fuel, ethanol must be produced
and used with no net emission of the greenhouse gas carbon dioxide.
- So, we can test this idea out for ethanol by looking
at the theoretical (albeit over-simplified) processes involved.
- If you set out the chemical equations involved, you
can present a case that ethanol fuels can be produced in a 'carbon neutral'
way.
- (i) Synthesis of sugars by plants using
photosynthesis using sunlight energy.
- 6CO2 + 6H2O
===> C6H12O6 + 6O2
- This is removing 'carbon' from the atmosphere.
- (ii) Fermentation to convert the sugar into
ethanol using the enzymes in yeast.
- C6H12O6 ===>
2CH3CH2OH + 2CO2
- (iii) Complete combustion of ethanol in a fuel
mixture fuel to
carbon dioxide and water.
- 2CH3CH2OH + 6O2
===> 4CO2 + 6H2O
- (iv) Adding up (ii) + (iii) gives the opposite of
photosynthesis.
- C6H12O6 +
6O2 ===> 6CO2 + 6H2O
- This is returning 'carbon' to the atmosphere and
equates with the reverse of photosynthesis.
- (v) Therefore, theoretically, in terms of 'carbon
mass', you 'growing'
(absorbing CO2) as much carbon in plants as you are 'burning' in
fuel combustion,
- or to express the overall process in terms of
energy, and you are releasing the renewable sunlight energy as heat
energy from ethanol combustion
- BUT, this assumes all these processes (ii) and
(iii) are 100% efficient, which they are not AND, life is never that simple.
- Apart from the
theoretical equation argument set out above, there are lots of other factors
to consider involving both economic and environmental issues - think of the
sequence in manufacturing a biofuel like bioethanol.
- Growing sugar source crop requires fertiliser and
harvesting require machinery and energy.
- There are raw material and energy costs to
produce fertilisers based on ammonia.
- In fact, the hydrogen needed for ammonia
comes from coke (carbon in coal) or methane (natural gas) by
reacting them at high temperatures with steam - carbon oxides are
by-products.
- The processes simplified are:
C + H2O
===> CO + H2 or
CH4
+ H2O ===> CO + 3H2
- before
N2 + 3H2
===> 2NH3 !!!
- Note that more carbon oxides are released if
fossil fuels are used to provide the energy for any of these process
either in manufacturing or transport of the final fertiliser
product.
- You can only get a maximum of 95% ethanol (with
~5% water in an azeotropic mixture) from fractional distillation and
this requires energy.
- There is also an extra cost to dehydrate the
fractional distillate to 100% pure ethanol.
- There are further costs in blending fuels and
transportation to petrol stations.
- Green energy eg. electricity from solar panels
and wind turbines may off-set this, but this is not significant for mass
production of fertilisers or bioethanol at the moment.
- Quite simply, whatever you may read, using
bioethanol is NOT a carbon neutral fuel system, BUT it MAY be better
than using fossil fuels.
- There is one more point, often neglected in
discussing biofuels. The crops require lots of land to provide
sufficient biomass to be economic.
- The including lots of deforested land with loss
of biodiversity and degradation of soil.
- AND this land is therefore not used to grow
crops to feed people (often the poorest) of these 'forested'
countries.
- So, there are serious question marks about the
widespread bulk use of biofuels such as bioethanol.
|
TOP OF PAGE
and sub-index
(6) A
wider ranging fuel survey which overlaps with discussion (5) above
Fuel
Consideration Factor
|
Hydrogen
H2
|
Methane
(Natural Gas)
CH4
|
LPG Liquefied Petroleum
Gas, mostly propane C3H8 &
butane C4H10
|
Methanol
CH3OH
An oxygenate fuel
|
Ethanol
CH3CH2OH
An oxygenate fuel
|
Petrol and diesel fuels
(biodiesel theoretically has a much lower carbon footprint -
renewable) |
Coal
Mainly Carbon but
contains some Sulphur
|
Suitability for
road transport
|
Large storage volume. Not as convenient
as petrol - difficult to liquefy
|
Large
storage volume. Not as convenient as petrol
|
Not
as convenient as petrol but more dense than hydrogen or methane gases
|
Hygroscopic,
absorbs water, causes corrosion
|
Hygroscopic,
absorbs water, causes
corrosion
|
Very suitable |
NO,
too inconvenient, polluting and back to the steam engine!
|
Safety in use
All gas fuels are more hazardous -
flammable and explosive compared to liquids - though petrol is volatile
|
No
more flammable or explosive than other gas fuels but more dangerous than
liquids
|
No
more flammable or explosive than other gas fuels but more dangerous than
liquids
|
No
more flammable than other gas fuels but more dangerous than liquids
|
Volatile,
very flammable, toxic liquid, not as dangerous as gaseous fuels
|
Volatile,
very flammable, toxic liquid, not as dangerous as gaseous fuels
|
Petrol more volatile than diesel. more flammable. |
Very
safe, not flammable but a bit ‘mucky’!
|
Sources, availability and
cost of supply
|
Not
readily available, made from water by electrolysis –
needs energy source - Sun! but is renewable
|
Large
naturally occurring oil and gas reserves at the moment, not renewable
|
Large
naturally occurring reserves at the moment, not renewable
|
More
costly, has to be synthesised from CO + H2
|
More
costly, has to be synthesised from ethene + water, can be from fermentation of
plant material, so can be renewable
|
Big
reserves, easily transported (very limited
quantities of biodiesel available) |
Big
reserves, easily transported
|
Energy Density
J/ kg
|
Very
high
143000kJ/kg
|
High
55600
kJ/kg
|
High
50000
kJ/kg
|
Lower
22700
kJ/kg
|
Lower
29717
kJ/kg
|
High 45000
kJ/kg |
Moderate
32800 kJ/kg
|
Can existing car
engines be adapted
|
Yes
|
Yes
|
Yes
|
Yes
|
Yes
|
No need to. |
Not
applicable!
|
Ease of storage and
distribution-can existing network be modified
|
Can be piped safely and can be mixed
with natural gas
|
Can be safely piped direct to homes
|
Stored
under pressure, much more dense than natural gas
|
Readily
stored and transported just like petrol or diesel
|
Readily
stored and transported just like petrol or diesel
|
Readily stored and transported. |
Readily
stored and transported
|
Effect on Environment
CO
toxic
SO2,
NO2 irritant, toxic, acid rain
Unburned
CxHy carcinogens, hydrocarbon/carbon particulates
|
Product
on combustion water, cleanest possible fuel, no pollutants emitted
|
Produces less
carbon dioxide than other fossil fuels (per unit of energy), less polluting (less CO, NO2) than petrol or
diesel, but still a big contributor to ‘Greenhouse Effect’
|
Produces
carbon dioxide, a contributor to the ‘Greenhouse Effect’, less polluting than
petrol or diesel
|
Cleaner burning
than alkane hydrocarbon fuels, produces less CO, unburned CxHy, NO2
air pollution
|
Cleaner burning
than alkane hydrocarbon fuels, produces less CO, unburned CxHy, NO2
air pollution
|
Not good - large CO2 emissions and particulate pollution,
particularly from diesel and CO, unburned CxHy, NO2
air pollution (biodiesel is less polluting -
cleaner burning) |
Very
polluting, lung irritant, SO2 causes acid rain, carcinogens, large
quantities of greenhouse gases as well as air pollution
|
The idea of bioethanol
and biodiesel is to attempt at some kind of 'carbon neutrality' by being
produced from renewable resources via photosynthesis. This gives them a
much less carbon footprint
See also 9a.
Chemistry and uses of alcohols - ethanol and in
the GCSE 9-1 Physics Notes
Energy resources & uses, general survey & trends, comparing sources of renewables, non-renewables
& biofuels
and
Carbon cycle,
nitrogen cycle, water cycle, decomposition - decay investigation, biogas
gcse biology notes
Multiple Choice Quizzes and Worksheets
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