Is coal ash flammable?

In this article, we will discuss the following question: “Is coal ash flammable?”, and other vital questions regarding coal wastes and their toxicity.

Is coal ash flammable?

No, coal ash is not a fire hazard and can’t ignite easily. Coal ashes are the residues from burning coal. If they were good combustibles, factories would use them in their furnaces, since most coal is burned to generate energy in power plants.

What is coal ash?

Coal ash is also known as coal combustion residuals (CRCs), they are a set of compounds resultant from the burning of coal in coal-fired power plants. Coal ash is waste.

A coal power plant is a facility that burns coal to generate electricity. They generate one-third of the world’s electricity, especially in China and the U.S.

The components found in coal can vary depending on the type of coal that’s burned.

Coal ash is composed of many things, particularly: fly ash, flue gas desulfuration material, bottom ash, and boiler slag.

Fly ash is the most abundant compound, representing 60% of the total waste, and is mostly made of silica. It can fly away easily due to the thin nature of its dust.

The Flue material, although, is made of calcium compounds. It’s a leftover from the process of reducing sulfur dioxide emissions from a coal-fired boiler.

The last two components are larger particles that can be used in the manufacturing of portland cement.

Coal waste is the second largest industrial waste stream in the U.S, losing only to mining wastes. Coal itself comes from mines as well.

There are also many heavy metals that can be impregnated in coal ash, especially:

  • Arsenic
  • Lead
  • Mercury
  • Cadmium 
  • Chromium 
  • And Selenium
  • Aluminum
  • Antimony
  • Barium
  • Beryllium
  • Boron
  • Chlorine
  • Cobalt
  • Manganese
  • Molybdenum
  • Nickel
  • Thallium
  • Vanadium
  • Zinc

Most of these can be very toxic, even in short-term exposure. Coal can come from different places, so the distribution of these metals in coal ash can vary. 

Most of the coal ash generated in the U.S is stored in sludgy basins (coal ash ponds), after being mixed with water. The problem with this kind of storing process is that it can leak, leading to catastrophic events.

Coal ash destination

Coal ash can rather be recycled or disposed as garbage.

In the U.S, about 38% of the coal ash is recycled for agriculture and industrial uses, as a source of fertilizer, and additive for the production of Cement.

About one-third of coal ash waste is disposed of in dry landfills. It is considered the less bad form of disposal, it’s easier to control and can’t leach easily.

It can also be mixed with water and put on ponds, which are similar to lakes. This end is given to about one-fifth of the residues. In this case, a big pond is constructed using earth and clay to create walls, and the waste is thrown on.

This kind of disposal can be deadly. In Brazil, one of these earth barriers broke in a city named “Brumadinho”, reaping the life of 270 people, in 2018. This was the worst incident involving mining in the world and the second bigger industrial disaster in the century.

Is coal ash hazardous?

Coal ash is not considered hazardous waste in the U.S, but simply a solid waste, which is the same designation given to domestic garbage. 

Hazardous wastes (the official ones) have a very different regulation. The industries that produce them have much more responsibility, and the waste is much more expensive since it can require complete destruction.

If people are exposed to enough concentration of toxic materials of coal ash, it becomes a hazardous agent. The more direct danger is considered to be aimed at workers from facilities that burn coal and the people who live close by.

The Environment Protection Agency (EPA), based in the U.S, declared that someone who lives close to a wet ash pond and drinks water from a well may have 1 chance in 50 to acquire cancer due to exposure to Arsenic, for example.


Arsenic is considered the worst carcinogenic agent in heavy metals. Since it’s bioaccumulative in many organisms, it’s stored in animals that are higher in the food chain, where Humans occupy the top. 

Arsenic can be a real problem in our food. It already is in many regions of Asia, where eating rice can be worrisome in the long term.

Each heavy metal component in coal ash has its own hazard associated. Most of them serve no natural purpose in our body, so any concentration of them is considered harmful.

The large spill in Tennesse

In 2008, a billion gallons of coal ash slurry spilled out of a power plant in Tennessee (U.S), swamping 15 homes. About 900 workers were dispatched to clean it up but it took 5 years to finish. Many of them got sick and some even died from the exposure.

Two hundred of those workers had sued their contractor, alleging they were misled about the dangers of being exposed to coal ash. A jury agreed with them and said that the contractor had certainly jeopardized the worker’s health. The case is still ongoing.

This is considered the largest industrial spill in American history.

Industrial wastes and the environment

Any country that has good environmental regulations will assure that industries and other kinds of entrepreneurship are responsible for the waste they generate, forever.

The thing about solid wastes is that they should be easier to handle. Factories must oversee their processes, 

Coal ash flammability

Coal ash is the result of burning coal. It can’t ignite again.

In chemistry, when something burns it is because the carbon compounds in the fuel reacted with the oxygen in the air, producing the byproducts: water, carbon dioxide (CO2), heat, and light.

But how about that grey ashes we see flying around any fire?

Well, that explanation about combustion we’ve just given above works for combustions that occurred as a whole, often in controlled environments, in which almost all the fuel was turned into those byproducts.

Normally, uncontrolled and little-controlled fires produce a lot more byproducts, including that dark mist many fires have. You can try seeing that by gently shaking a lighted candle. 

If you burn a piece of paper, all those remaining ashes you see are materials that couldn’t burn. Some of those compounds can still burn, but only if more heat is provided. 

If the heating and other conditions from the burning of the piece of paper weren’t enough the first time, if we burn it again using the same circumstances and temperature, the fire won’t catch.

This means that coal ash can’t burn because it already has. When we burn something it means that the oxygen reacted with the carbon until the form of CO2, or that something got in the way and made some of the fuel not react, leaving some hydrocarbon behind, graphite, and other things.

But the thing is: if coal ash could ignite again under industrial conditions, industries would use it again. Instead, they decide to recycle or dispose of it using other means. 

If it can’t be burned in an industry, it won’t ignite after the disposal either.


Coal ash is not flammable because it’s barely combustible. A pile of coal ash would not ignite easily even in big furnaces, so it won’t after the disposal either. But it really is toxic for people and the environment.

Frequently Asked Questions (FAQS): Is coal ash flammable?

Is coal ash good for the garden?

Coal ash is often used as agricultural fertilizer, spread in the ground. Even so, you shouldn’t use them in your garden.

When big farmers use it, they are aware of the constitution of the coal ash, and its risks to the environment. We do not possess the means to find out how polluted with heavy materials a pile of coal ash is. 

Doing so could contaminate the water sources from beneath, and could prejudice your land since the total amount of chemicals is unknown.

Is coal ash biodegradable?

No. Most constituents of coal ash are inorganic, which can’t be easily processed by microbes (which is, essentially, what biodegradability is). Also, coal ash is very toxic to the environment.

Is coal ash radioactive?

Practically, yes. Coal has traces of certain radioactive compounds, but this is expected from mining. Coal ash also carries some of these compounds, but we can’t be sure if they are radioactive enough to be a concern.

Radioactivity is everywhere in our life. It’s in the sunlight, in our water, food, walls, and even in our own bodies. But radioactivity is only a problem when it’s high enough.

The problem is: we can’t give a certain answer. It all depends on where the coal was mined, and the specific amount of radiation the resulting coal ash would have. Specific radioactive tests are required for every case.


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