2. Biofuels Flashcards
The two main biofuels currently used are what?
Bioethanol and biodiesel.
Sources of biofuel are called what?
Biomass.
For bioethanol, this biomass is sugar from plants, while for biodiesel, this is plant oils and animal fat. The biomass required for this can be replaced very quickly and this is what makes them a renewable resource.
How does biofuel production work?
When burnt, these biofuels produce carbon dioxide which is linked with climate change. However, plant biomass recaptures this carbon dioxide when more is grown as plants absorb carbon dioxide as they grow. This means that the carbon dioxide is recycled which again makes this a renewable process.
In order to make biofuels, we also need what?
Ethanol.
Ethanol is used as a biofuel by itself but it is also the most commonly used alcohol in the production of biodiesel. This key ingredient can be produced in which two ways?
Fermentation or the hydration of ethene.
What is the only way that bioethanol can be produced?
Fermentation.
Fermentation is the most commonly used method for producing alcoholic beverages but it isn’t the most used when it comes to biofuel production. Cracking produces ethene from crude oil which is then combined with steam (also known as ethene hydration) to make ethanol. This is currently the main method to make ethanol because it’s cheap and easy.
Ethanol produced from cracking isn’t a biofuel as it comes from fossil fuels, not an organic source. Bioethanol is only produced through fermentation.
How is bioethanol produced through fermentation?
Bioethanol is produced through the fermentation of glucose by enzymes in yeast.
What is most commonly used to create bioethanol?
Sugarcane and corn are the most commonly used raw ingredients, or feedstock, but their sugars are in the form of polysaccharides or starch. The grain must be milled to release the starch, which enzymes then convert to glucose.
What is glucose mixed with for fermentation?
The glucose is then extracted and mixed with yeast in an environment that is absent of oxygen. This is important because if oxygen was present, the ethanol would be oxidised to form ethanoic acid. Zymase, the enzyme in yeast responsible for fermentation, converts the glucose to ethanol and carbon dioxide.
Where does biodiesel come from?
Biodiesel comes from vegetable oils or animal fats and is a cleaner-burning replacement for petroleum-based diesel.
Not only is it better in terms of emissions, it’s also biodegradable and non-toxic which is a huge plus for the environment. Most biodiesel is produced from soybean and canola oil, but almost any rendered animal fats or vegetable oils can be used including waste oil from restaurants (go recycling!).
What does biodiesel consist of?
Biodiesel consists of long-chain fatty esters produced by combining alcohol with fatty acids and a catalyst. While biodiesel can be used alone, it is often blended with regular diesel to make it burn more cleanly. The most common blend is B20, containing 20% biodiesel and 80% petrol diesel.
What process converts fats and oils into biodiesel?
Transesterification is the chemical process that converts oils and fats into fatty acid esters, otherwise known as biodiesel.
Fats and oils are triglycerides with a molecular structure consisting of three hydrocarbon chains attached by ester functional groups to a glycerol. These react with short chain alcohols, usually methanol or ethanol, to produce an ester (the biodiesel) and glycerol. This is called transesterification.
What conditions does transesterification occur under?
This reaction takes place at low temperature and pressure with a strong basic catalyst such as NaOH or KOH to increase the rate of transesterification.
Why is bioethanol mixed with fuel?
With relatively strong intermolecular forces, ethanol is difficult to vapourise and this is a problem for starting a car on a cold day. Without aid, engines using only ethanol cannot be started below 20°C and this is the biggest issue for its use as a transport fuel.
However, its high oxygen content and octane value leads to higher efficiency and cleaner combustion which is why it is so frequently used as a fuel additive to petrol.
Ethanol has a low energy content and an even lower energy density. This means that the energy released when it is combusted, also known as the energy yield, will be less than traditional fuels and you would travel a shorter distance using only ethanol than you would with petrol.
What is the difference between biodiesels from plants and those from animals?
Biodiesel produced from plant oils form unsaturated esters with double bonds between carbon atoms, whereas biodiesel derived from animal fat forms saturated ester molecules.
Due to their shape, the unsaturated esters don’t pack as tightly together as the saturated ones do, which results in weaker intermolecular forces between neighbouring molecules. This means that biodiesel produced from plant oils tends to have lower melting points than biodiesel formed from animal fats.
