Coal and Fossil Fuel Plant Emissions Controls

Many would say that the coal industry is under attack.  The Environmental Protection Agency, politicians, the media, etc., are all trying to make coal clean.  The truth of the matter is that no fossil fuel is a “clean” fuel source.  Every type of fuel or energy has an issue that detracts from its use.  Wind and solar are unreliable, nuclear is termed “unsafe” and expensive to build, natural gas produces emissions just like coal does.  The difference between alternate fuel sources and fossil fuels is fossil fuel emissions can be removed or cleaned up from the flue gas.

The technology to do this is available and in use today.  Nitrous Oxides (NOx) make up a large portion of the emissions from natural gas and coal power plants.  Why are they bad?  They contribute to Acid Rain.  Nitrous Oxides in the atmosphere combine with water to form Nitric Acid.  This then falls back to the Earth as rain.  But there are ways to remove it such as specially designed burners and Selective Catalytic Reduction (SCR) units.  Low NOx Burners limit the amount of oxygen in the reaction so that NOx formation is low.  SCRs are a special treatment of the flue gas in which a building houses a special catalyst that converts NOx to Nitrogen and Oxygen.

SCR stands for:

• Selective – the SCR is selective for NOx. The only ions that react with the SCR are the oxides of Nitrogen.
• Catalytic – Catalytic because a catalyst is required for the reaction to take place. The reaction is also dependent on temperature.
• Reduction – basically this means that during the reaction the Nitrogen atoms gain a reaction and the oxygen loses an electron.

SCRs are normally placed on the outlet of the boiler so that the temperature inlet is warm and the complete flue gas is cycled through the SCR to maximize removal of the NOx.  Ammonia is the other part of the SCR and must be injected for the reaction to occur.  The Ammonia has a very low boiling point and when injected in the flue gas stream quickly evaporates to facilitate the reaction.  The basic reaction is as follows:

4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O

As can be seen, the NO (NOx) is combined with ammonia (NH₃) and oxygen to produce Nitrogen (N₂) and water (H₂O).  Nitrogen and oxygen are the main constituents of air and are inert.  So with this one static device (no moving parts), a major harmful constituent of the flue gas is removed.  There are other ways to remove harmful pollutants from the flue gas such as Wet and Dry Flue Gas Desulfurization units that were discussed in the Power Plant Environmental Upgrades blog.  As new technologies become available they are being implemented to make coal and natural gas flue gas cleaner.