Biofuels

The promise of a biofuel is elegant: the carbon dioxide it releases when burnt is the same carbon dioxide the plant absorbed while growing, so the net addition to the atmosphere is zero. The chemistry behind that claim is worth knowing properly — including the reasons it is never quite true.

Renewable, non-renewable, sustainable

Three words that get used loosely in conversation and precisely in exams.

• Renewable — can be replaced over a relatively short period, so will not run out in the foreseeable future. Sometimes called infinite.
• Non-renewable or finite — supplies are limited and cannot be replaced within a short time. Fossil fuels are the obvious example.
• Sustainable — can be produced at the same rate as, or faster than, it is used. This is the key idea: renewability is about replacement, sustainability is about the rate.

Biofuels are developed to reduce pollution from fossil fuel combustion, to limit contributions to global warming, and because fossil reserves are running down. Whether they truly deliver on the first two points is a question of engineering as much as chemistry.

Bioethanol and the carbon-neutral claim

The route runs in two biological steps. First, green plants absorb carbon dioxide and convert it into glucose:

Then yeast ferments the glucose, anaerobically, into ethanol:

The word bioethanol means nothing more than ethanol made from a biological source. Chemically it is identical to ethanol from crude oil. What differs is the origin of its carbon — and that is what makes the carbon-neutral argument possible.

Bioethanol as a practical fuel

• It burns with an almost invisible flame, which is a genuine safety problem — you cannot see whether a spill is alight.
• Its specific energy is lower than petrol, so vehicles need a larger tank or more frequent refuelling.
• Blending it with petrol or diesel raises the energy content and makes the flame visible, which is why fuels are sold as blends rather than neat ethanol.
• It can be made from sugar cane, maize or waste plant material, so it is genuinely renewable — but the land used raises food-versus-fuel questions.

Biodiesel: transesterification

Vegetable oils are triglycerides — three long fatty-acid chains all attached to one glycerol backbone. You could in principle burn the oil directly, but the molecules are large and the oil is far too viscous to atomise properly in an injector.

The solution is to cut each chain off the glycerol and attach it to a small methanol molecule instead. Three big chains on one backbone become three separate, much less viscous methyl esters. This swap of one ester’s alcohol for another is called transesterification.

Two details about the catalysis are worth remembering:

• Acid catalysis works by protonating the carbonyl group, making the carbon more open to attack.
• Alkali catalysis works by deprotonating the methanol, generating a much more reactive methoxide. Alkalis (NaOH, KOH) are the more common industrial choice.
• Because the reaction is reversible, excess methanol is used to shift the position of equilibrium towards the products — a direct application of Le Chatelier’s principle.

Biogas

Biogas needs no crop at all. Microorganisms break down organic waste — food waste, animal slurry, sewage — in the absence of oxygen. These anaerobic conditions are essential; with oxygen present the waste would simply oxidise to CO₂ and water, and no methane would form.

The product is mostly methane and carbon dioxide, with traces of gases such as hydrogen sulfide. The proportions depend on what was fed in, and you can calculate them from a balanced equation.

Carbohydrates give a 50 : 50 mixture. Fats and fatty acids, which contain far more hydrogen per carbon, give a much richer gas — and a richer gas burns better.