The alternative to fossil fuels might be non-fossil biofuels, that is, molecules assembled recently by living organisms. The argument goes that if the world used biofuels to the exclusion of fossil fuels, leaving the remaining fossil fuels sequestered underground as they had been for hundreds of millions of years, this would limit the amount of carbon in circulation, cap the atmospheric supply of CO2, and limit the effects of global warming. Biofuels therefore deserve to be explored.
It’s not our objective to delve too deeply into organic chemistry – the chemistry of carbon compounds – but some definitions are important. One word we’ve already used often is hydrocarbon. Hydrocarbons are molecules that consist of carbon and hydrogen. The carbon atoms join together in various patterns while the hydrogens dangle from the carbons. There are many related categories of molecules which have additional species of atom in them in some configuration or another. For example, alcohols resemble hydrocarbons but have a hydroxyl (-OH) group bound to a carbon atom. Meanwhile carbohydrates consist of carbon, hydrogen, and oxygen atoms with a hydrogen to oxygen ratio of 2 to 1 as in water. Sugars are an important subset of the carbohydrates. There are many other interesting sorts of molecules. Organic chemistry and biochemistry are very rich subjects.
From the perspective of provisioning energy for human use, what matters most with biofuels is the energy it costs to create these organic molecules, and the ability to use them by tearing them apart to liberate some of that energy again. This energy liberation is done by combustion or burning, a chemical reaction in which a fuel reacts with an oxidant. Most often, the oxidant is oxygen gas (O2) although combustion is sometimes taken to include reactions with other elements such as halogens. The result of combustion is heat energy and smaller waste molecules, such as CO2 and H2O. Although humans mastered fire tens of thousands of years ago, we are not yet very good at turning CO2 back into longer organic molecules, but we do try and the results are the re-formed fuels, also known as synfuels. Fortunately other organisms had the secret of CO2 recycling figured out long ago, and if it weren’t so we would not exist.
Nourishment and Growth of Living Organisms
Life forms are not magically exempt from the laws of physics. The phenomenally complicated processes that take place in living organisms (and which modern biology has yet to fully grasp) all obey the laws of physics and chemistry. All earthly life forms are based on carbon, they require oxygen, hydrogen, and a handful of other elements for key processes, and they need a source of energy. It is the organisms’ nourishment mechanism that distinguishes them – what they use as their sources of carbon, energy, and electric charge. Open courseware material from the Massechusetts Institute of Technology presents a beautiful summary of this subject. Although this section may seem wildly out of place in a discussion of biofuels, it presents a fundamental overview of what processes are possible for the creation of useful fuels by living organisms, and just as burning fuels in engines has to follow the laws of thermodynamics, so too does the creation of the fuels.
As a first cut, organisms are usually sorted according to their cell structure, namely whether they have a nucleus (eukaryotes) or lack a nucleus (prokaryotes). For the life thermodynamics of interest here, there is an additional distinction between self-nourishers (autotrophs) and those nourished by others (heterotrophs). Each come in three basic flavours.
Autotrophs are those organisms that get their carbon for growth and production of new cells from CO2. They need an electron donor to balance electric charges. The three autotrophic nourishment mechanisms are:
Oxygenic photosynthesis by plants and certain bacteria and algae. Their energy comes from sunlight and the electron donor is water. They can produce carbohydrates and oxygen. These organisms thrive in aerobic light conditions.
Anoxygenic photosynthesis by sulfur bacteria uses energy from sunlight, but the electron donors are not water. Instead these organisms use hydrogen, hydrogen sulfide, or metal ions as the electron donor. These organisms live in anaerobic light conditions.
Chemosynthesis by chemoautotrophic bacteria. Although they share with other autotrophs the characteristic of using CO2 as their carbon source, their energy and electron sources are reduced mostly inorganic compounds (CH4, H2, NH4, H2S, Fe2+). These organisms prefer anaerobic dark conditions.
Heterotrophs get their energy and carbon by burning reduced organic compounds. They require an electron acceptor. The three heterotrophic nourishment mechanisms are:
Aerobic respiration by aerobic eukaryotes and prokaryotes. These use carbohydrates as both their carbon and energy sources. Oxygen is the electron acceptor. CO2 is a waste product. These organisms need aerobic conditions to survive.
Fermentation by eukaryotes and prokaryotes. In this mechanism, carbohydrates are again the carbon and energy sources, but the electron acceptors are organic compounds – part of the energy source is oxidized and the rest reduced. This takes place in anaerobic conditions. The products are organic acids and alcohols.
Anaerobic respiration is only known among some prokaryotes. Again, carbohydrates are the carbon and energy sources, but the electron acceptor is not O2 but rather an oxidized compound such as (SO42-, Fe3+, NO3+, etc.). Otherwise this is similar to aerobic respiration.
Reference will be made to at least some of these processes as the production of biofuels is described.
Possible biofuel production mechanisms
Fossil hydrocarbons are the most familiar and desirable fuels today. Biochemical processes do not yield hydrocarbons, but any substance they can make that behaves similarly enough in our engines, turbines, and heating systems would be welcome as a substitute fuel. Thus, making “biofuels” means harnessing organisms (usually plants but sometimes algae, yeasts, bacteria, fungi, or other organisms) to prepare fuels that we like. Practical biofuels tend to fall into two general categories: alcohols and diesels.
Before beginning that exploration, it is important to learn a key performance criterion for fuels that is especially applicable to biofuels, namely “energy balance”. That is the ratio between the energy obtained from the fuel divided by the energy consumed in its production. In the posting on “The world’s energy sources and sinks”, it was noted that about 27% of the world’s energy is consumed in its production and distribution, and that reflects mostly the energy cost of recovering and distributing fossil fuels, but includes additional losses in generating electricity. So the “energy balance” of fossil fuels taken together is about (1-0.27)/0.27 = 2.7. Note this is not a uniform result. For example, extracting gas from America’s shales or oil from Canada’s tar sands, are far more energy-hungry processes than taking oil from a well in Kuwait. It should be obvious that any primary energy source whose energy balance is less than unity is worse than useless. Such an energy “source” should instead be considered a secondary energy storage medium. In the search for biofuels, the emphasis should be on ones that have a high energy balance.