EGEE 101
Energy and the Environment

Catalytic Converter, Scrubbers, & Low NOx burners

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Scrubbers - To Remove SO2 from the Flue Gas

The formula for this process looks like this: SO2 + CaCO3 + 1/2O2 + 2H2O CaSO4 + 2H2O + CO2

 Babcok & Wilcox Absorber Module. Described the paragraphs below.
Babcock & Wilcox Absorber Module.
Credit: Babcock & Wilcox Company

The schematic above is of a device known as a scrubber. Their role is to take the flue gas with the products of combustion and remove the SO2 emissions prior to the flue gas being released out of the stack. This wet device also has the advantage of reducing some of the particulate matter. There is a dry version as well, but the wet scrubber is my favorite!

  1. SO2 and Flu Gas go into the Babcock & Wilcox Absorber Module.
  2. Multiple Interspatial Spray Levels. A fine spray of water is used to create a mist through which the flue gas must flow up (counter-current flow).
  3. Absorption Zone. The mist absorbs the SO2...
  4. which then falls to the bottom of the tank into a pool of slightly acidic water, after which lime is added.
  5. Oxidation Air Supply. Air is also added, which then oxidizes the product, allowing it to be converted into a solid product that can either be disposed of or sold as gypsum.

Most of you are, quite literally, surrounded by a version of this solid byproduct much of the time - GYPSUM is the primary material in the drywall, or wallboard, that makes up the interior walls of most new houses, apartments, and buildings today. If you're sitting in a place right now that has a smooth or textured painted surface or has wallpaper, then it's a pretty safe bet that gypsum lies underneath.

Catalytic Converters—to remove NOx from the flue gas

We have discussed approaches to reducing NOx from automobiles. One approach used was the use of a catalytic converter. The same technology can be used to remove NOx from power plants. The catalyst works in exactly the same way but on a much larger scale. The buildings can be 8 stories high and are the size of an apartment complex. A building that size will hold millions of dollars worth of catalyst that has to be replaced every few years. So, NOx reduction is an expensive process.

 Schematic of a Catalytic Converter. See caption for details.
In a similar approach to the catalytic converter on the gasoline car, reducing NOx via a catalyst can be used for NOx removal from flue gas. The catalyst is close to the boiler, as the gases need to be hot for the catalyst to work. There is a greater challenge to reducing NOx under these conditions because of particulates and SO2 in the flue gas.
Credit: LANL

Low NOx Burners - To Reduce the Formation of NOx in the Boilers

Don’t worry too much about the complexities of low NOx burners. It is enough that you know that they work by controlling the mixing of the air (oxygen) and fuel, and lowering the temperature of the flame (by controlling the mixing). You need to produce a radical-rich zone so that you can attack the formed NOx. This has to happen early in the flame or else the nitrogen in the char will most likely form NOx. Hopefully, the result will be more ubiquitous nitrogen and we will release less NOx into the atmosphere.

 NOx Reduction Boiler.  System View and a 45degree Rear-View Turbine view.  See caption for more details.
Here you see a couple of images portraying low-NOx burners. By careful mixing of the air and the fuel, you can lower the flame temperature and lower the NOx emissions. The right image illustrates the different air flows coming from the burner, and the swirling process that enhances the mixing of the fuel and the air (mostly nitrogen and oxygen).
Credit: left: IEA, right Fluent

Fluidized Bed Combustion

 Picture of a valley with grass and trees.
Here an old coal strip mine has been reclaimed
using the ash and lime particles (with a gypsum
layer) from the fluidized bed. This also helps
prevent acid mine drainage from this site and from
the culm pile.

There is a method of burning coal that uses existing technology to produce electricity with very low emissions of both NOx and SO2: Fluidized Bed Combustion. The fuel need not be coal - fuel flexibility, in fact, is one of the reasons that this is an attractive boiler configuration, but most PA fluidized beds tend to be in the anthracite region, where there is abundant and FREE fuel: Culm Piles (or so called "gob piles" in Western PA).

These "manmade mountains" of reject coal actually contain a significant quantity of coal (recall these culm piles help produce acid mine runoff - from the pyrite contained in the anthracite). By removing the culm pile, producing electricity, employment, and taxation and cleaning open-pit operations (more on that in a bit) these operations significantly improve the local area, environmentally and economically.

The reject coal is cleaned then combusted. The relatively large pieces of coal burn in the presence of limestone. Suspending the particles with air flowing from underneath enables the coal to be combusted slowly, the ash forms most of the bed (where the suspended material is is called the "bed") along with some coal and lime (from the limestone CaCO3 heat-----‡ CaO (Lime) + CO2). This gives the coal a long residence time in the bed, allowing lower temperature combustion (thus less NOx) and the S is not emitted into the atmosphere because it is captured in-situ (in place) by the limestone. CaO + SO2 + 1/2 O2-‡ CaSO4 (gypsum)

Like the output from S scrubbers explained above, the gypsum becomes a solid and can be used to fill in the old strip mining holes to restore the damage from old abandoned mines. But, the output from this process produces a less pure form of gypsum than from scrubbers, so this gypsum is not usable for wallboard materials (it is just a coating over the lime particle).