Carbon – an amazing atom
(This article first appeared in the NEC Newsletter, published by the New England Coalition on Nuclear Pollution.)
Carbon molecules are among the most interesting things in nature. A carbon atom can link to up to four other atoms directly. Since one or more of those other atoms could be other carbon atoms, linking to yet other atoms, the potential for molecules is endless. Many of the compounds formed are what we call organic, and scientists have identified over a million different types of these.
One of the really interesting things about organic compounds is that they can be changed into each other, and the ways to do this seem endless as well. An organic chemist once told me, “You can get from just about anywhere to just about anywhere else; all you need is to know how.”
Suppose we start with molecules of carbon dioxide and water. These can be combined by photosynthesis to form sugar; the chemical equation for this reaction is 6CO2 + 6 H2O → C6H12O6 + 6O2. This reaction requires a lot of energy, but that comes for free from the sun. The energy is trapped in the sugar molecule.
Sugar molecules can be combined to form cellulose, liberating a water molecule for each sugar molecule in the reaction; the formula, simplified because cellulose is a polymer, is C6H12O6 → C6H10O5 + H2O. Nearly all of the chemical energy from the sugar is trapped in the cellulose.
We can burn the cellulose; the simplified formula is C6H10O5 + 6O2 → 6CO2 + 5 H2O. This reaction releases the chemical energy in the cellulose, but it comes in the form of heat.
Interestingly, we are back to having carbon dioxide and water, exactly where we started. In the course of this process, we gained two big advantages, which are that we were able to store the energy of the sunlight in cellulose, and we were able to convert that energy into heat, which can be used to heat our homes, provide electricity, or fuel vehicles. When we let wood grow in a forest, harvest it, and use it for fuel, this is what we are doing.
There are a lot of other things we can do with cellulose, besides simply burning it as wood in a stove or fireplace. For example, we can gasify it, producing wood gas. One way to do this is to use a small amount of wood to heat a large amount of wood in a process called pyrolysis. The combustible products in the gas this produces include carbon monoxide, hydrogen, and methane. Ash and tars are also produced, but when the process is properly done, these are nearly all trapped, as they have commercial value. Wood gas can be burned in combustion turbines or internal combustion machines. It is a very clean fuel because it burns completely, and has almost no impurities.
As an alternative to burning wood gas, it can be catalyzed for other purposes. The Fischer-Tropsch process, which was invented in the early 1920’s, converts carbon monoxide and hydrogen to methane. The catalysts used for this are nickel, aluminum, or other metals or alloys, and the process is done at moderate pressure and temperature. Methane can be used as fuel or as a chemical feedstock.
We can go further using what is called the Mobil Process. This is a series of catalytic steps that start with carbon monoxide, hydrogen, and methane, and produce any of a series of chemicals called alkanes. Most people do not recognize the word alkane, but they do know the names of the individual alkanes themselves. Among them are propane, butane, octane, and many others.
We can start with biomass, in the form of wood, grass, algae, or a number of other things, and end with propane, a good substitute for natural gas, gasoline, diesel fuel, or such other products as feedstock for plastics. Five kilograms of wood can produce approximately one liter of the biodiesel produced in this way. This comes to about 120 gallons of diesel fuel per cord of wood. (See the article on Güssing.)
The process of producing wood gas is one of several technologies based on forest or agricultural products that are very clean. When it is gasified or completely burned, wood is much cleaner than petroleum because it does not have associated sulfur and other impurities. Combined with careful, sustainable forestry, use of wood in this way can actually improve our forests.