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.

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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 35^oC 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.

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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

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

(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

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

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

(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

45000 kJ/kg

Moderate

32800 kJ/kg

Can existing car engines be adapted

Yes

Yes

Yes

Yes

Yes

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

Effect on Environment

CO toxic

SO₂, NO₂ irritant, toxic, acid rain

Unburned C_xH_y 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 C_xH_y, NO₂ air pollution

Cleaner burning than alkane hydrocarbon fuels, produces less CO, unburned C_xH_y, 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