Topic 4 · page 14 of 17

Alternative fuels and carbon neutrality

Alkane fuels are finite and release pollutants and carbon dioxide. An alternative fuel is judged by whether it is renewable and by its overall carbon dioxide emissions, counting how it is made as well as how it burns.

13 min read4.154.16

Read first: Pollutants from burning fuels

Why develop alternative fuels? 4.15

There are three reasons, and a good answer uses all three.

Sustainability. Crude oil is finite. It formed over millions of years and is being used far faster than it forms. A sustainable fuel can be replaced as fast as it is used, which means it is renewable.

Reducing emissions. Burning alkane fuels releases carbon monoxide, oxides of nitrogen, sulfur dioxide, carbon particulates and unburned hydrocarbons (Pollutants from burning fuels). A better fuel gives fewer of these.

Carbon dioxide and climate change. Burning any carbon-containing fuel completely gives carbon dioxide.

CX8HX18+252 OX2→8 COX2+9 HX2O \ce{C8H18 + 25/2 O2 -> 8CO2 + 9H2O}

CO₂ is a greenhouse gas. The Earth's surface warms and gives out infrared radiation. CO₂ in the atmosphere absorbs some of this infrared radiation and re-emits it, so less escapes into space. More CO₂ means a stronger greenhouse effect, which leads to global warming and climate change.

Carbon neutrality 4.16

Bioethanol

Bioethanol is ethanol made by fermenting sugar from crops such as sugar cane. The crop takes in CO₂ by photosynthesis. When the sugar is fermented, and when the ethanol is burned, CO₂ is released.

Step Equation CO₂
Photosynthesis (the crop grows) 6 COX2+6 HX2O→CX6HX12OX6+6 OX2\ce{6CO2 + 6H2O -> C6H12O6 + 6O2} 6 taken in
Fermentation CX6HX12OX6→2 CX2HX5OH+2 COX2\ce{C6H12O6 -> 2C2H5OH + 2CO2} 2 released
Combustion 2 CX2HX5OH+6 OX2→4 COX2+6 HX2O\ce{2C2H5OH + 6O2 -> 4CO2 + 6H2O} 4 released
The carbon dioxide balance for bioethanol. Photosynthesis takes 6 CO2 from the atmosphere. Fermentation releases 2 CO2. Burning the ethanol releases 4 CO2. Taken in 6, released 2 plus 4 equals 6, a net of zero.
On paper, 6 CO2 taken in and 2 + 4 = 6 CO2 released: carbon neutral.

Taken in: 6. Released: 2 + 4 = 6. On paper bioethanol is carbon neutral.

Comparing three fuels

Petrol Bioethanol Hydrogen
Source crude oil (a fossil fuel) fermenting sugar from crops electrolysis of water, or from methane
When burned releases CO₂ (and CO, NOₓ and others) releases CO₂ 2 HX2+OX2→2 HX2O\ce{2H2 + O2 -> 2H2O}: no CO₂
Where the carbon came from locked away for millions of years taken from the air recently by plants no carbon in the fuel
Carbon neutral? no close, but not fully only if it is made using renewable energy

Petrol is not carbon neutral because burning it releases carbon that was locked away underground, which adds to the CO₂ in the atmosphere. Hydrogen releases no CO₂ when it burns, but making it from methane, or by electrolysis using electricity from fossil fuels, does release CO₂.

Check yourself

  1. Give two reasons for developing alternative fuels, other than CO₂ emissions.
  2. Explain the link between CO₂ in the atmosphere and climate change.
  3. Define carbon neutral.
  4. Show by counting CO₂ that bioethanol could be carbon neutral.
  5. Explain why bioethanol is not completely carbon neutral in practice.
  6. A newspaper says "hydrogen is a clean fuel with zero emissions". Discuss this statement.

Answers cover these until you have tried the questions

  1. Any two of: crude oil is finite and a more sustainable (renewable) fuel is needed; burning alkane fuels gives pollutants (CO, NOₓ, SO₂, particulates and unburned hydrocarbons) so there is a need to reduce emissions.
  2. Burning fuels releases CO₂. CO₂ is a greenhouse gas: it absorbs infrared radiation emitted by the Earth's surface and re-emits some of it back, so the atmosphere warms. More CO₂ gives a stronger greenhouse effect, so global warming and climate change.
  3. A fuel is carbon neutral if there is no net release of CO₂ to the atmosphere: the CO₂ released when it is made and burned equals the CO₂ taken in when it was produced.
  4. Photosynthesis takes in 6 CO₂. Fermentation releases 2 CO₂ and burning the 2 ethanol releases 4 CO₂. Released: 2 + 4 = 6. Taken in = released, so the net release is zero.
  5. The CO₂ balance above only counts the chemistry. In practice fossil fuels are burned to run farm machinery, make fertiliser, transport the crop and distil the ethanol, so extra CO₂ is released.
  6. It is true that burning hydrogen gives only water (2 HX2+OX2→2 HX2O\ce{2H2 + O2 -> 2H2O}), with no CO₂. But the statement ignores how the hydrogen is made. Making it from methane releases CO₂, and electrolysis of water uses electricity that may come from burning fossil fuels. Hydrogen is only close to carbon neutral if it is made using renewable energy.