Economic geology is the use of materials for the benefit of society. Two such materials, coal and petroleum, comprise 90% of all the energy budgeted in the United States.
Where does coal come from? It comes from land plants—in particular, it comes from wood. As a matter of fact, if you hold a lump of coal in your hand, basically what you’re holding in your hand is preserved wood. When the average tree falls in the woods it decomposes.
If you’re going to preserve it to make it into anything, you have to keep away from oxygen and microbial activity. The environment that does that is a swamp. The reason why swamps are so good at preserving the woody tissues—or any organic matter for that matter—is that the oxygen content is very low. Because the water moves very, very, very slowly, it’s not being aerated. There’s also all kinds of carbon material around that sucks up oxygen and makes carbon dioxide.
The microbial activity is directed primarily by the pH of the water. Microbes do their best job at about pH 7. To preserve enough of the material to make good coal, the pH of the swamp has to be at least less than 3 and preferably less than 2, which is often the case. Within the swamp, the woody tissues turn into peat.
I’ve never been able to take woody tissue or peat tissues of any kind and turn them into anything that even faintly resembled coal. The actual process is still sort of a mystery.
Next, the peat is buried—not very deep, you don’t want to heat it up too far, maybe a few hundred degrees centigrade. It goes through a sort of a baking process where it changes from peat into the various forms of coal. A lot of people, including myself, have tried to reproduce that process, and I, personally, have never been able to do it. I’ve never been able to take woody tissue or peat tissues of any kind and turn them into anything that even faintly resembled coal. The actual process is still sort of a mystery.
The Invention of the Steam Engine
Coal as a major energy source really had to wait until the invention of the steam engine. The steam engine was invented in the early 1700s by Thomas Newcomen. But Newcomen’s engine didn’t run very well. It took another inventor, James Watt, to really get it working. The first fuel that was used was wood. Very soon, they realized that wood didn’t have enough heat potential to provide the steam they needed to drive the engines. And thank God for that because, if it had, they would have cut down every tree in Europe. They looked around for another resource and settled on coal. Coal is what drove the industrial revolution. It drove the locomotive across the country. It drove the steamships. Coal remained the number one energy source until the early 1900s, when petroleum took over largely because of the introduction of the gasoline-powered automobile.
How to Measure the Energy of Coal
To understand coal there really are only two things you really have to know: you have to know what rank is and you have to know about coal quality. Rank is all about carbon. Quality is about ash and sulfur. Let’s talk about rank first. From a chemistry perspective, any organic material is made of two fundamentally different components: carbon and everything else. The everything else is lumped together as volatiles because they can be driven off by heating.
What’s so important about carbon? Wood, the source of all the coal, is around 45% carbon and 55% volatiles. Peat, the next step in the process, has roughly 55% carbon. As the peat slowly cooks, the volatiles are driven off, increasing the carbon content. If you increase the carbon content to around 65%, you’ve got what we call lignite. Well, around the world lignite is called brown coal because it looks like coal, but it’s brown in color. If we keep driving off some more volatiles and we get up to 85%, then we’ve got bituminous coal. And if we can drive up to greater than 95% carbon, then we have anthracite. Peat is the lowest rank of coal and, of course, anthracite is the highest rank—the only difference in the coal is simply the carbon content.
If you’re in the business of buying coal, to burn to make heat, to make steam, to drive a turbine, to make electricity, what you’re going to want to do is buy the highest rank of coal you can.
Well, so what? The so what is the energy. Energy is the measure of heat content. Heat content in coal is measured in BTUs per pound, or British Thermal Units per pound dry weight. Wood, for example, has about 4,500 BTUs per pound dry weight. That sounds like a big number but actually it’s pretty low. At the top of the list you get up to the anthracite, which has about 15,000 BTUs per pound. There’s a big difference as you go up the rank because BTU content increases. What does it mean? It means if you’re in the business of buying coal, to burn to make heat, to make steam, to drive a turbine, to make electricity, what you’re going to want to do is buy the highest rank of coal you can.
Sulfur, Ash, and Quality
For all living things—life as we know it—there are six required elements: carbon, oxygen, hydrogen, nitrogen, phosphorous, and sulfur.
There is something else to consider: quality. Coal quality is determined by ash and sulfur. These go hand in hand—when one goes up the other goes up, when one goes down the other goes down. For all living things—life as we know it—there are six required elements: carbon, oxygen, hydrogen, nitrogen, phosphorous, and sulfur. The sulfur comes right from the plant itself. How about the ash? Well, this is an interesting one. Picture a tree growing. It takes water out of the ground, passes the water up through the tree, takes out whatever nutrients it wants, and eventually the water gets to the leaves and is transpired out into the atmosphere.
But there are certain elements that trees just don’t want. As a matter of fact, there are certain elements that plants in general don’t want, and two in particular are aluminum and silicone. Unless they get rid of them somehow, they get deposited in the leaves. The leaves block up and as a result the plant dies. What do they do with it? They stash it away in the old dead wood cells. When you burn a log in your fireplace, that’s what you see in the way of ash. It’s simply the stuff that the tree stored in the wood that it didn’t want. If you take that wood and turn it into coal the ash just goes right along with that.
We usually break down quality into high, medium, and low quality. A high-quality coal would be any coal that has less than 10% ash, 1% sulfur. The dividing line between medium and low really depends upon what you’re using the coal for and how you’re using it, but the dividing line is around 30% ash, 3% sulfur. If you get more than 30% ash and 3% sulfur, you’ve got pretty poor coal.
The EPA Steps In
Up until 1970 you could burn any coal you wanted. You could burn anything you wanted to make steam to produce electricity. The problem is that coal would be a great, great fuel if it wasn’t for sulfur. Because here’s the deal. When you burn the coal in the firebox, the sulfur burns too. It burns, oxidizes, and turns into SO2, SO3—together they’re called SOxes. If the SOxes are allowed to get into the atmosphere and react with water in the atmosphere, they produce sulfurous and sulfuric acids, which then return to earth as acid rain. The acid rain sterilizes lakes and soils, killing plants and animals alike. In 1970 the Environmental Protection Agency Clean Air laws kicked into effect and stopped the burning of high-sulfur coal. Basically a coal must have less than 1.2% sulfur. So knowing the sulfur content, not just the energy output, is extremely important in coal mining and use.
From the lecture series The Nature of Earth: An Introduction to Geology
Taught by Professor John J. Renton, West Virginia University