The thermal energy we can harness by burning biofuels is yet another form of converted solar energy. How comes? Bioufuels are made of plants, with various degrees of processing. The oldest kind of biofuels known to humanity is firewood – the processing is pretty simple, on has just to cut it, dry it, and then cut to smaller pieces. More complicated processes are needed for making more sophisticated types of biofuels – for instance, for making biodiesel one needs to extract oil from oily seeds (may be the same cooking oil is extracted from), and next the oil, before being ready for pouring it into a fuel tank, has to be subjected to a process called “esterification.” Making ethyl alcohol (ethanol), a substance now added to gasoline, requires even more steps – first, one has to grow corn (maize), then extract starch from the seeds, then convert it to sugar by mixing it with special enzymes, then subject the sugar to alcoholic fermentation using yeast, and finally, to distill out the ethanol. But the first stage is always the same – before any processing may start, a plant has to grow first from a seed sown in soil.
The main problem with fuels is that by burning them we add CO2 to the atmosphere. No matter whether it is a fossil fuel, or biofuel – burning it always produces CO2. Yet, there is a difference. Before the biofuel can be obtained from a plant, the plant has to grow from a seed – it has to build its trunk, its stalk and its seeds – and where does it get the “bulding material” from? Well, the answer is simple: the two essential components the plant needs are CO2 and water, H2O. Plus sunlight. The plants use a biochemical mechanism, catalyzed by chlorophyll – a substance that makes all plants green – to combine, with the help of sunlight, CO2 and H2O molecules into larger molecules of “carbohydrates”. The simplest carbohydrates is glucose, a sugar. By combining several hundred glucose molecules into long chains, the plant make starch – and another material, cellulose, in which many glucose chains are bundled together, forming fiber-like molecules. Cellulose is a “scaffolding” material from which plants make cell walls, as well rigid structures needed for forming stalks. Another important carbohydrate is lignin. Cellulose and lignin together produce especially rigid structures – one that we all know is wood.
As follows from the above, plants when they grow remove much CO2 from the atmosphere – and when we burn biofuels, we don’t add new CO2 to it, we only return to it CO2 which has been taken away from it some time before. Therefore, if one day we managed to eliminate all fossil fuels and to use biofuels exclusively, CO2 would be only “cycling” – that released by burning would be used by next plant generations to build their stalks, leaves and seeds, and the concentration of CO2 in t4he air would stop growing.