School chemistry revision 14-16 GCSE level chemistry notes: Biofuels: biogas, bioethanol, biodiesel & hydrogen

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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 chemistry students too

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(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 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

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.

renewable biomass energy store fuel diagram electricity power generation turbine generator transformer power lines

  • 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.

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(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.
  • (c) doc b 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.

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(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!

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(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'


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).


(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.

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(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.

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(6) A wider ranging fuel survey which overlaps with discussion (5) above


Consideration Factor




(Natural Gas)


LPG Liquefied Petroleum Gas, mostly propane C3H8 & butane C4H10



An oxygenate fuel



An oxygenate fuel

Petrol and diesel fuels

(biodiesel theoretically has a much lower carbon footprint - renewable)


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



55600 kJ/kg


50000 kJ/kg


22700 kJ/kg


29717 kJ/kg


45000 kJ/kg


32800 kJ/kg

Can existing car engines be adapted






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

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