Curbing Air Pollutants


Catalytic Converters

As previously mentioned, any combustion engine will tend to directly produce three pollutants: CO, NOx, and VOCs. One very successful way to immediately lower those pollutant concentrations in exhaust is to use a catalytic converter. The catalytic converter is put directly inline about a third of the way as the exhaust leaves the engine and travels for exit through the tailpipe. This is shown in the figure below.

The various metal catalysts inside the catalytic converter each have their role in doing three specific tasks:

  1. complete the oxidation of CO to CO2
  2. oxidize remnant VOCs to CO2 and H2O
  3. reduce any NOx back to N2

This is an example of heterogeneous catalysis - meaning the catalyst is in a different phase of matter than the reactants. In this case, the catalyst is solid phase - generally those metals absorbed onto ceramic or zeolite supports. The reactants are all gas phase. All the chemistry is done on the surface of the catalyst and it is for this reason that the surface area of the catalyst is maximized - lots of porosity to expose more surface.

All of this requires "just right" conditions of temperature and oxygen. This is typically handled with onboard monitoring of engine conditions along with an oxygen sensor. A car running optimally will have exhaust that is 99% or better CO2 and H2O. By-the-way, "optimally" also means the driver must help out as well. Any acceleration in the car will shift optimal emissions to less than optimal, NOx's will spike and make it out of the tailpipe. This is why most modern cars have an eco-mode for driving where the car helps you drive a bit more eco-friendly, thus reducing CO and especially NOx's in the atmosphere.


Scrubbers

Scrubbers is a general term used in pollution control systems. It refers to the inline unit that removes pollutants from expelled exhaust or waste. The process itself is called scrubbing. And, even thought the general term can mean a wide variety of systems each designed to remove specific pollutants, we will just look at scrubbers for removing SOx from exhaust gases. The most generally used type of scrubbing for sulfur oxides has a specific name: flue-gas desulfurization or FGD.

The process is to completely oxidize the gaseous sulfur oxides and get them into aqueous phase as sulfites and sulfates. Then those sulfites and sulfates are precipitated out as solids where they can be drained/filtered off and dried. Below is a diagram (modified wikipedia pic) showing the scrubber for FGD.

The key here is that the flue-gas has to rise through a rain of saturated aqueous CaCO3. Any ash is caught by the spray of water and pulled to the bottom pool. The SOx gases are pulled into the water droplets and react with the water and CaCO3 to form CaSO3 and CaSO4> - all of which are very insoluble in water. The solids sink to the bottom of the pool where they can be drained off and filtered. The calcium sulfate can be purified and then used as gypsum in the making of cement and wallboard.



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