Each potential energy generation method has its environmental considerations, examples include:

Sometimes not considered is the environmental impact possible from the entire fuel cycle; examples include water pollution from oil tanker spills, and pollution from mining. For example, the table below gives a comparison of CO2 emissions between fossil fuel plants (complete fuel cycle)

Fossil Fuel CO2 (g/kW·hr)
Coal 270
Oil 200
LNG 178

For purposes here emissions are divided into four types

  1. particulate: flyash and cinders
  2. gaseous: SO2, NOX, CO2 (greenhouse effect)
  3. solids: ash and sludge (from flue gas cleaning systems)
  4. thermal

Particulate emissions (e.g., flyash and cinders) from burning have historically received the greatest attention since they are easily seen. The particulate emissions are the easiest to control. The particulates are classified based on size since >10µm settle to ground but <1 µm do not settle but act as aerosols, for example, flyash is <100µm. Cinders are present where crushed coal rather than very fine pulverized coal is burned; typical of stoker and small cyclone furnaces. Mechanical and cyclone separators are used to control cinders. Flyash is typically controlled using electrostatic precipitators or fabric filters. Electrostatic precipitators have a collection efficiency of 99% but don't work well for flyash with a high electrical resistivity (as commonly results from combustion of low-sulfur coal). The large baghouse filters have a high maintenance cost since the cloth bags have a life of only 18 to 36 months. The baghouse filters are inherently large structures and result in a large pressure drop.

The main gaseous pollutants from combustion include sulfur oxides (SOX), nitrous oxides (NOX), and carbon monoxide (CO). Both SOX and NOX can create acid rain composed of H2SO4 and HNO3, respectively. The SOX, mostly SO2 and some SO3 can cause respiratory irritation. The NOX contributes to smog and ozone formation, and vegetation damage. The CO reduces the oxygen carrying capability of the blood, referred to as carbon monoxide poisoning. The Clean Air Act sets the federal standards for plant emissions; EPA provides minimal requirements, however, states can be stricter.

The greenhouse effect on global warming has received significant attention in recent years. The carbon dioxide (CO2) from the combustion process allows short-wavelength radiation from the sun to be absorbed by earth's surface. The re-radiated long-wavelength radiation from the earth is absorbed by the CO2; thus, the atmosphere becomes heated.

Thermal pollution is the introduction of waste heat into bodies of water supporting aquatic life. The addition of heat reduces the water's ability to hold dissolved gases, including oxygen which aquatic life requires; although aquatic life growth is usually enhanced by warm water. If the water temperature is greater than 95°F, then the dissolved O2 content is too low to support life.

Thermal pollution is a consequence of the second law of thermodynamics, roughly, it is impossible to perfectly convert heat into work. Concern with thermal pollution increased with the large nuclear plants constructed in the 1970's. This was due to the nuclear plant's large size and its lower thermal efficiency as compared fossil units, and the fact that all the heat rejection from a nuclear plant is via the cooling water whereas a fossil unit emits heat through the stack also. A thermal discharge index (TDI) can be calculated based on the plant thermal efficiency where a larger number is worse

Last updated: November 7, 1997

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