The science of what makes coal so dirty

Paths traced by water on the surface of a coal mine.
Paths traced by water on the surface of a coal mine.
Image: Harsha Vadlamani for Quartz
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The world can’t wait to rid itself of the need for coal. Without cutting coal use drastically, there’s little chance of hitting greenhouse-gas emission targets and avoiding the most catastrophic effects of climate change.

And eliminating the stuff isn’t just about greenhouse gases: Mining coal and then burning it produces other toxins, polluting our air, water, and soil.

What makes coal so dirty? To understand, we’ll have to don our lab coats and tackle just a little bit of chemistry.

Greenhouse arithmetic

The greenhouse-gas problem is relatively easy to grasp. All fossil fuels are made up of mostly carbon and hydrogen. When burned, the carbon is converted to carbon dioxide and the hydrogen to water. Each of those reactions produces a slightly different amount of heat.

C + O2 → CO2 generates 393 kJ of heat

H2 + 0.5 O2 → H2O generates 242 kJ of heat

The product we’re most concerned about is carbon dioxide, a greenhouse gas that traps sun’s heat in our atmosphere. Which means the better fossil fuel—the one that will produce the fewest CO2 emissions to generate the same amount of heat—is one that contains lots of hydrogen atoms for every carbon atom.

Natural gas wins that battle hands down. It contains mostly methane, a simple chemical with the formula CH4. That means there are four hydrogen atoms for each carbon atom, the maximum a single carbon atom can accommodate.

Coal’s chemical formula is far more complex (as we’ll see later). That’s because, unlike natural gas, coal is a mixture of many thousands of types of chemicals. But to understand its heat production, we can simplify coal’s formula to CH (that’s one hydrogen atom for each carbon atom).

The upshot is that coal produces twice as much carbon dioxide per unit of heat energy compared to natural gas.

Boggy start

Beyond the contribution of greenhouse gases to climate change, coal has other problems. Those have to do with how the stuff was formed.

“Coal is the most complex solid we’ve ever found and analyzed,” says Jonathan Mathews, a coal scientist at the University of Pennsylvania.

Many millions of years ago, some sort of natural event—maybe a flood, maybe a typhoon—buried vast forests underwater. As new layers of soil were deposited on top of the trees, depriving them of air, the interred wood slowly turned into peat bogs. More and more layers of sediment accumulated, increasing the pressure and temperature beneath, until finally, the bog turned into coal.

I’m being purposely vague about “many millions” because coal deposits in different regions can have different ages. Coal in the United States was created during the Carboniferous period, which lasted from 360 million to 300 million years ago. Australian coal, on the other hand, was formed during the Permian period, between 300 million and 250 million years ago.

Because it originally formed from plants, coal contains mostly carbon, hydrogen, oxygen, and nitrogen. Coal helped create the carbon-based branch of chemistry we call “organic chemistry.” When coal is heated in the absence of air, its complex mixture breaks down into simpler forms. These chemicals—such as benzene, toluene, naphthalene, anthracene, and phenol—form the basis for perfumes, explosives, and medicines.

Plants also have a whole host of other elements from the periodic table in much smaller amounts. Crucially, during its formation, coal can absorb still other elements found in its surrounding mud deposits or contaminated water. Depending on the geology of the region, the types and the amounts of these elements varies; more than half the periodic table of elements has been detected in different types of coal.

“The process through which coal forms compounds its complexity,” says Mathews. “That’s why almost every piece of coal found is chemically unique.”

When coal is burned, many of these elements are dumped into the atmosphere alongside other gases. These can travel for miles before they land on plants or in soil, where they could get absorbed into trees or crops and eventually be eaten by humans. Some of these elements can also end up in people’s lungs, where poisonous ones like tin, cadmium, and mercury can do real harm to nervous, digestive, and immune systems.

Despite regulations on the coal industry, these metals frequently end up in the environment. More than 40% of all mercury emissions in the US still come from coal power plants. In 2014, in the US alone, coal-related activities also released 40 metric tons of lead, 30 tons of arsenic, and 4 tons of cadmium.

Smoggy end

All those chemical pollutants only account for part of the problem, though. The more visible manifestation of coal’s environmental impact is smog: the result of the chemical reaction of coal burning. Because coal is a complex mixture of chemicals, it doesn’t burn as cleanly as natural gas—not all of the carbon and hydrogen gets converted neatly to carbon dioxide and water. Instead, coal’s smoke contains unburned or half-burned particles of carbon, sulfur oxide, nitrogen oxides, and lots of complex organic molecules formed in the process of combustion.

Each of these has its own way of causing harm. Let’s take them one by one.

Soot: Unburned or half-burned particles of coal can be categorized as soot. Its appearance (and to a large extent its chemical makeup) is similar to the soot found in fireplace chimneys: a fine black powder. The soot, which can carry any number of the contaminants listed above, is harmful to the lungs. But it’s even more damaging because it’s small enough to enter the bloodstream once inhaled. It can even end up in the brain. Coal and other solid fuels used in homes is a leading cause of air-pollution deaths in India. In India, soot is known to coat glaciers, which darkens them to trap more heat from the sun and melt faster.

Sulfur oxide: At high temperatures inside a furnace, sulfur in coal and oxygen in the air combine to form sulfur oxide, which is an irritant if breathed. When it combines with water, it forms sulfuric acid—creating acid rain. In the 1960s and 1970s, sulfur rain was a common phenomenon in the US and other countries. Since then, most power plants have been required to install equipment that pulls sulfur emissions from the smokestack, but some sulfur still slips into the atmosphere.

Nitrogen oxides: Like sulfur, nitrogen in coal combines with oxygen in the air to form a mixture of nitrogen oxides. These are irritants and can cause respiratory diseases like pneumonia. Nitrogen oxides are also chemically active, which means they mix with other pollutants in the atmosphere to create new ones, such as ozone.

Volatile organic compounds (VOCs): Coal mining and coal burning release harmful carbon-based compounds that persist in the atmosphere as gases. These are what nitrogen oxides can react with to form ozone and other pollutants. These chemicals are harmful to humans, other animals, and plants.

Carbon monoxide: Sometimes, instead of carbon combining with oxygen to form carbon dioxide, it will react to create carbon monoxide—a poisonous gas.

Over the years, regulations at coal power plants have helped cut some of these pollutants. Indian coal power plants, however, are seriously lagging. In 2015, the government set a deadline for 2017 for power plants to install equipment that will cut sulfur and nitrogen emissions. Few power plants had complied by the deadline, so now the government has extended it to 2022. That delay is likely to cause at least 26,000 premature deaths and the loss of many millions of work days each year.

More advanced countries already have these scrubbers installed—and some of them are even going a step further. Two coal power plants, one in Canada and one in the US, now capture most of the carbon dioxide they produce. In both cases, the captured carbon dioxide is pumped into the ground to extract oil. Once the technology gets cheap enough, emitters could simply bury their carbon dioxide underground without having to subsidize the process by selling oil.

That said, no regulation will be able to completely eliminate the noxious emissions of burning fossil fuels. Their harms can only be left behind if we stop extracting them from the ground.