Abstracted from A Scientists View of Almost Everything by Mark M. Green
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, replacing those atoms with two oxygen atoms to form carbon dioxide, CO2.
Coal, especially high rank coal like anthracite, is made of a great deal of carbon. When that coal is burned the carbon is combined with oxygen just as the carbon in the food we eat is combined with oxygen. A fire in the hearth burning coal parallels the “fire” in our gut. Both produce carbon dioxide and give off energy. In the hearth that energy takes the form of heat and light.
The transformation from plant matter 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 to subside again. Each time these happened 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 became a smaller number as the rank of the coal increases. For comparison, this ratio for methane, CH4, natural gas, is 4 while for anthracite the highest rank of coal the ratio can be less 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.
There is another change going on in coalification, a change in the type of carbon that is present. In the plant matter that begins the process, carbon is mostly in a form that chemists call aliphatic, carbon that is bound to four other atoms. For example a typical carbon in cellulose, the main constituent of wood, is bound to two other carbon atoms, and a hydrogen atom, and an oxygen atom. As coalification proceeds under pressure and heat, the carbon atoms in the decayed plant matter are transformed to what chemists call aromatic carbon. An aromatic carbon is bound to only 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 100 miles in the earth, the carbon atoms seek an arrangement that takes up less space. A network of rings form, each ring with six aliphatic carbon atoms, that is every carbon atom in the each ring is bonded only to four other carbon atoms, and this network arrangement is very strong as all the rings work together to resist any deformation. The material is best described by adapting a Greek word, adamas, meaning unconquerable-diamond.
The great surprise is that petroleum, which is as different from diamond as can be imagined, is also composed, like diamond, primarily of carbon atoms bound to four other atoms, but differs in that each carbon atom is bound to two other carbon atoms and two hydrogen atoms. Individual strings or chains of these 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 it can be recovered. It is the carbon in both coal and petroleum we continue to be after, but with the increasing realization of the climate change price we are paying and therefore the necessity to change the manner in which we gain energy for our activities.
However we will never replace carbon as the source of the energy to sustain our life in the process of converting the carbon in what we eat to the CO2 we breathe out.