By Mark M Green
Carbon is a great element. It loves to bond with itself leading to all that fat around our midsections. Carbon also bonds with nitrogen and oxygen giving us, for just three examples of many, our skin, and hair, and muscles, and those incalculably useful enzymes that catalyze all the chemical processes in our body that keep us alive. A special love affair exists between carbon and oxygen, which shows itself with every exhale of our breath
Each carbon atom contained in what we eat and drink is sent to enzyme driven machines in our bodies. The carbon is pried lose from whatever other atoms it was combined with in the food, eventually replacing those atoms with two oxygen atoms to form carbon dioxide, CO2. Because carbon dioxide is an exceptionally stable molecule, a large amount of energy is released for every carbon dioxide molecule formed. We exist on that energy, warming our bodies and keeping us moving and thinking.
I well remember my honeymoon many years ago at Mrs. Burke’s cottage on Inishbofin off the west coast of Ireland. As usual, the Irish claimed this was the coldest wettest summer in their memory. But the advice to get some peat for the fireplace told us there was a great deal of experience with summers of this kind. I don’t recommend burning peat. The stuff never seemed to light up, at least in my hands. I recommend waiting a few hundred million years for some appropriate geological changes to convert the peat to coal. Luckily a great deal of peat formed a very long time ago and has been around and has changed to coal. My wife and I got our hands on some of it and boy did that coal light up in Mrs. Burke’s old fireplace. We were lucky not to burn the house down. That coal was hot stuff, hotter than any fire I’ve ever seen made from wood.
Coal burns so much hotter than peat because coal has a higher percentage of carbon. The ransformation from peat to coal is caused by geological events over millions of years, even hundreds of millions of years. First there were bogs, large areas of dense plant growth in warm, humid, very wet conditions. With new plants constantly growing, and old ones dying, dense layers of plant debris accumulated over thousands of years. And as things go in our world, parts of our planet rise up and parts subside and in those many years these bogs did exactly that, going from a submerged state to rise again to a level to see some sun and form a bog and then subside strong as all the rings work together to resist any deformation. The material is best again. New layers of plant debris became covered with silt, sand and mud. The layers of the plant debris, decomposed at first by microorganisms in a process known as diagenesis, became the seams of coal so familiar to miners. The layers of decomposed plant material became buried deeper and deeper in the earth, compressing the layers, stopping the diagenesis and exposing the material to greatly increased pressures and higher temperatures. These geologic changes are called metamorphic development, or coalification.
During coalification the coal becomes richer and richer in carbon so that in the highest rank of coal, the percentage of carbon in the coal rises to well over 90%. Coal technologists express these changes as the ratio of hydrogen to carbon, H/C, which becomes a smaller number as the rank of the coal increases. For comparison, this ratio for methane, natural gas, of so much interest from fracking, is 4 while for anthracite the highest rank of coal can be lower than 0.4. By comparison wood is typically near 1.5. The coalification process takes the derived plant material from peat to lignite to subbituminous coal to bituminous coal to anthracite.
As coalification proceeds under pres-sure and heat, the carbon atoms in the decayed plant matter are transformed to what chemists call aromatic carbon. An aromatic carbon is bound to three other atoms, usually two carbon atoms and a hydrogen atom. And as the tens of millions of years pass, the hydrogen is expelled from this arrangement and replaced with another aromatic carbon atom to make sheets of aromatic carbons in which every carbon atom is attached only to other carbon atoms. This is the reason that the critical ratio H/C becomes smaller. The ultimate state of pure aromatic carbon is graphite (H/C is zero) entirely composed of these sheets of aromatic carbon atoms, a state that is rarely reached in nature because of the very high temperatures necessary to form graphite. It is these sheets of aromatic carbon in graphite slipping over each other that give graphite its special properties as a lubricant.
But there is still another state for pure carbon. At extremely high pressures, as for example buried below about 160 kilometers in the earth, the carbon atoms seek an arrangement that takes up less space. A network of rings form, each with six carbons and this network arrangement is very described by adapting a Greek word, adamas, meaning unconquerable, diamond.
In petroleum there is an abundance of hydrogen bound to carbon. Individual strings or chains of aliphatic carbon atoms are covered in a blanket of hydrogen atoms allowing these chains to easily slither by each other as if they were snakes in a pit. The H/C ratio varies between 1.5 and 2.0, far higher than in coal. This kind of arrangement makes a fluid and so petroleum can flow through permeable rocks to the underground reservoirs where we discover it and from which we can recover the petroleum. But petroleum is the same as coal in one regard. Both contain carbon and it is the carbon we are after and we’ll get it the cheapest easiest way we can. Apparently that is now the kind of carbon with hydrogen atoms attached, petroleum and methane with the added benefit that a higher H/C ratio means less carbon dioxide is produced when the fuel is burned and therefore less contribution to climate change.