Coal: A Lesson in Economic Geology

From the lecture series: The Nature of Earth — An Introduction to Geology

By John J. Renton, PhD, West Virginia University

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.

Image of a coal mine from above
(Image: Mark Agnor/Shutterstock)

Where does coal come from? It comes from land plants—in particular, it comes from wood. If you hold a lump of coal in your hand, fundamentally 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. Due to low oxygen content and because the water in them moves at an incredibly slow pace—it isn’t being aerated— swamps are good at preserving the woody tissues, or any organic matter for that matter. There are also all kinds of carbon material around that sucks up oxygen and makes carbon dioxide.

This is a transcript from the video series The Nature of Earth: An Introduction to Geology. Watch it now, on The Great Courses Plus.

The microbial activity is directed primarily by the pH of the water. Microbes do their best job at about pH seven. To preserve enough of the material to make good coal, the pH of the swamp has to be at least less than three and preferably less than two, 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 too high, maybe a few hundred degrees centigrade. It goes through a type of baking process where it changes from peat into the various forms of coal. Many people, including myself, have tried to reproduce that process, and I’ve never been able to take woody 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.

Learn more about the rifting of continents and spreading of the sea floor

The Invention of the Steam Engine

Portrait of James Watt by John Partridge 1806
The steam engine was invented in the early 1700s by Thomas Newcomen. (Image: Antonia Reeve/Public domain)

Coal as a major energy source 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 get it operating. 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. 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.

Learn more about how minerals are classified 

How to Measure the Energy of Coal

To understand coal there are only two things you have to know: Rank and coal quality. Rank is all about carbon, while 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. Other components are 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%, we get what is called lignite. Around the world, lignite is called brown coal because it looks like coal, but is brown in color. If we continue to drive off some more volatiles and refine it up to 85%, then we’ve got bituminous coal. If if it refined further, we drive it up to greater than 95% carbon, then we have anthracite. Peat is the lowest rank of coal and 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. It may sound like a big number, but it’s pretty low. At the top of the list, anthracite has about 15,000 BTUs per pound. That’s an enormous difference as you go up the rank because BTU content increases. If you’re in the business of buying coal, to burn to make heat, to make steam, to drive a turbine, or to make electricity, you want to buy the highest rank of coal you can.

Coal ranking system used by the United States Geological Survey
The energy in different types of coal. (Image: Stanley P. Schweinfurth/Public domain)

Learn more about igneous, sedimentary, and metamorphic rocks

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 from the plant itself. But the ash is an interesting acquisition. 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 and plants in general just don’t want. Two in particular are aluminum and silicone. Unless they get rid of them somehow, these elements get deposited in the leaves. The leaves block up and as a result, the plant dies. It gets stashed 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 material 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 lump of high-quality coal would be any coal that has less than 10% ash, 1% sulfur. The dividing line between medium and low 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.

Learn more about the composition of magma

The EPA Steps In

Up until 1970, you could burn anything you wanted to make steam to produce electricity. The problem is that coal would be a great fuel if it wasn’t for sulfur. 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. Coal must have less than 1.2% sulfur. Knowing the sulfur content, not just the energy output, is extremely important in coal mining and the responsible maintenance of environmental effects.

Common Questions About Coal

Q: What creates coal?

Millions of years of pressure with heat force oxygen out of plant and animal matter to result in rich carbon deposits of coal.

Q: How many types of coal exist?

There are four main types of coal: anthracite, which is most useful as it burns at a higher heat, lignite, bituminous, and peat.

Q: Where in the world is the most coal found?

The largest deposits of coal are found in Russia, China, India, and the US, although it is found in excess across the world.

Q: Will coal ever run out?

This is a complex question based on our uses of gas, oil, and whether we invest more in nuclear. But if our use of coal continues and grows, based on energy use we could run out of coal in as little as 150 years.

This article was updated on October 13, 2020

Keep Reading
Soldier Survives 70-Foot Plunge into Hawaiian Volcano
Environmental Protection Bill Expands Death Valley National Park
Fossils: Life Cast in Ancient Stone