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Why is coal the worst fossil fuel – emissions, climate, health, smog…?

Posted on May 15th, 2017 by in Chemical R&D

Coal fossil fuels

Beijing, China (Source, www.thebeijinger.com)

The photo above shows a street during daytime in Beijing, China, where over 70% of people wear masks on smoggy days.  Smoggy days occur frequently resulting in periodic highway and airport shutdowns.  The main cause of the smog is smoke and soot particles spewed from stacks of coal burning factories.  Among others, auto and truck emissions contribute to the air pollution of Beijing.  China produces 69% of its electricity by burning coal under lax environmental rules.  Similar descriptions can be made of numerous cities.  China, by no means, is the only country suffering from pollution caused by burning coal. Poland and India, where coal is the main fossil fuel, are among the most polluted countries in Europe and Asia (Figure 1).

India smog

Figure 1 Skyline of New Delhi India

In countries with far more stringent rules for clean air,  expensive scrubbers are required to remove particulate and chemical emissions.  The latter has been one of the subjects of gripe by the coal mining and burning industries.  The claims against particulate removal are thin and purely based on profiteering.  The new Administration is on its way to give a virtually free hand to the coal mining/burning industries.

The environmental damages caused by surface and subsurface coal mining are substantial because of the pollution of water resources, altering topology of the land and changing ecosystems and wildlife.  Residual ash from burnt coal must be exposed extremely carefully because of the toxicity (and radioactivity) of the ash and its ability to become airborne causing the contamination of air/water/land.  According to Greenpeace in the last 70 years of twentieth century 2.4 million hectares (5.9 million acres) of natural landscape, most originally forest, were left barren and contaminated by surface mining.  Underground mining also damages the environment by contaminating and lowering the water tables and changing the flow of groundwater and streams.

Over 99% of world coal reserves consist of Lignite, sub-bituminous and bituminous types, in the order of increasing carbon content.  More than half of coal reserves consist of sub-bituminous and bituminous types.  These three types of coal are those burnt as fuel at different proportions around the world depending on availability. The remaining 1% is the purest and highest carbon content coal called Anthracite.  Typically, sulfur content increases with decreasing carbon content:

Lignite Coal: 0.4 % by weight

Bituminous Coal: 0.7-4 % by weight

Anthracite Coal: 0.6- 0.77 % by weight

Pollution from Burning Coal

The Union of Concerned Scientists has published a report describing the pollution resulting from coal combustion. (Source: www.ucsusa.org/clean-energy/coal-and-other-fossil-fuels/coal-air-pollution).  The amount of pollutants emitting from the stacks of the coal burning plants are sobering.

“Coal plants are the nation’s top source of carbon dioxide (CO2) emissions, the primary cause of global warming. In 2011, utility coal plants in the United States emitted a total of 1.7 billion tons of CO2.  A typical coal plant generates 3.5 million tons of CO2 per year.

Burning coal is also a leading cause of smog, acid rain, and toxic air pollution. Some emissions can be significantly reduced with readily available pollution controls, but most U.S. coal plants have not installed these technologies.

  • Sulfur dioxide (SO2): Coal plants are the United States’ leading source of SO2 pollution, which takes a major toll on public health, including by contributing to the formation of small acidic particulates that can penetrate into human lungs and be absorbed by the bloodstream. SO2 also causes acid rain, which damages crops, forests, and soils, and acidifies lakes and streams. A typical uncontrolled coal plant emits 14,100 tons of SO2 per year. A typical coal plant with emissions controls, including flue gas desulfurization (smokestack scrubbers), emits 7,000 tons of SO2 per year.
  • Nitrogen oxides (NOx): NOx pollution causes ground level ozone, or smog, which can burn lung tissue, exacerbate asthma, and make people more susceptible to chronic respiratory diseases. A typical uncontrolled coal plant emits 10,300 tons of NOx per year. A typical coal plant with emissions controls, including selective catalytic reduction technology, emits 3,300 tons of NOx per year.
  • Particulate matter: Particulate matter (also referred to as soot or fly ash) can cause chronic bronchitis, aggravated asthma, and premature death, as well as haze obstructing visibility. A typical uncontrolled plan emits 500 tons of small airborne particles each year. Bag houses installed inside coal plant smokestacks can capture as much as 99% of the particulates.
  • Mercury: Coal plants are responsible for more than half of the U.S. human-caused emissions of mercury, a toxic heavy metal that causes brain damage and heart problems. Just 1/70th of a teaspoon of mercury deposited on a 25-acre lake can make the fish unsafe to eat. A typical uncontrolled coal plant emits approximately 77.3 kg of mercury each year. Activated carbon injection (ACI) technology can reduce mercury emissions by up to 90% when combined with bag houses. ACI technology is currently found on just 8% of the U.S. coal fleet.

Other harmful pollutants emitted annually from a typical, uncontrolled coal plant include approximately:

  • 51.8 kg of lead, 1.8 kg of cadmium, other toxic heavy metals, and trace amounts of uranium. Bag houses can reduce heavy metal emissions by up to 90%.
  • 720 tons of carbon monoxide, which causes headaches and places additional stress on people with heart disease.
  • 220 tons of hydrocarbons, volatile organic compounds (VOC), which form ozone.
  • 102 kg of arsenic, which will cause cancer in one out of 100 people who drink water containing 50 parts per billion.”

Sour crude oil grades contain quantities of sulfur comparable to coal.  It is, however, much easier and less expensive to remove the sulfur from sour crude oil than from coal where sulfur is locked inside its solid phase.

“Clean Coal” refers to a number of technologies used to reduce the harsh environmental impact of coal combustion gases, particulates, and heavy metals and capture part of the emissions.  It is costly and requires the disposal of significant quantities of captured hazardous materials.  The massive quantities of carbon dioxide produced during coal combustion are sometimes stored by injection into underground formations without an understanding of the long terms effects of the storage.

Let’s assume sulfur, nitrogen, heavy metals and particulates (fly ash) are removed from coal to levels comparable with petroleum-based fuels and natural gas.  Is a super clean coal environmentally comparable to those liquid fuels and natural gas?  That would be true, if carbon dioxide emissions were comparable.  The US Energy Information Administration (Source: www.eia.gov) has provided the data in Table 1 for the carbon dioxide emissions of all three fuels.

Table 1 Kilograms of CO2 emitted per million kilo Joules of energy for various fuels

Fuel Type

Combustion of coal emits nearly double the carbon dioxide that natural gas produces and therein lies the shortcoming of coal no matter how “clean” it might be.


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