Flue gas desulfurization

Flue gas desulfurization is technology that employs a sorbent, usually lime or limestone, to remove sulfur dioxide(SO₂) from the gases produced by burning fossil fuels. Flue gas desulfurization is current state-of-the art technology for major SO₂ emitters, such as power plants.

Source(s) of sulfur

Fossil fuels such as coal and oil contain significant amounts of sulfur. When burned, this sulfur is generally converted to SO₂. This happens under normal conditions of temperature and especially oxygen content of the flue gas. However, there are two circumstances under which this may not be the case.

The first is if there is insufficent oxygen. Instead of being oxidized to SO₂, the sulfur will instead be in a reduced oxidation state. A typical compound would be hydrogen sulfide (H₂S). This compound acts much like SO₂ and much of the following discussion is applicable.

The other case is where there is too much oxygen and the SO₂ is further oxidized to sulfur trioxide (SO₃). Actually, too much oxygen is only one of the ways that SO₃ is formed. Gas temperature is also an important factor. At about 800°C, formation of SO₃ is favoured. Another way that SO₃ can be formed is through catalysis by metals in the fuel. This is particularly true for heavey fuel oil, where significant amounts of vanadium are present. However SO₃ is formed, it does not behave like SO₂ in that it forms a liquid aerosol known as sulfuric acid mist (SAM) that is very difficult to remove. Generally, about 1% of the sulfur dixoide will be converted to SO₃. SAM is often the cause of the blue haze that often appears as the plume dissipates. Increasingly, this problem is being addressed by the use of wet electrostatic precipitators (WESP's).

Scrubbing of SO₂

From the above section, it is apparent that most of the sulfur in the fuel will leave the boiler or combustion unit in the form of SO₂. So most of the emphasis on will be on removing SO₂ after it leaves the boiler. It should be pointed out that there are other methods than FGD to remove SO₂. One of the most important is a fluidized bed boiler or reactor, where lime is added with the fuel. This will react with the SO₂ before it leaves the boiler and keep it from being emitted up the stack. In a conventional boiler however, the SO₂ must be treated after it leaves the boiler.

SO₂ Chemistry

SO₂ is a gas at room temperature and at the temperatures found in boilers. As such, its solubility in water is governed by Henry's Law. This law states that the concentration of SO₂ dissolved in water is proportional to the partial pressure of the SO₂ in the gas phase, according to Equation 1:

Henry's law constant is merely the proportionality constant between P_SO2 and [SO_2(aq)]. It is sometimes expressed as solubiilty and sometimes as volatility. Equation 2 shows the solubility form:

Unfortunately, the equilibrium concentration of SO_2(aq) is very low. This means using water as a scrubbing liquid would be impractical because the volumes involved would be astronomically high.

To increase the solublity, according to Equation 2, the partial pressure of SO_2(g) can be increased. This will directly increase [SO_2(aq)]. However, this too is impractical, as the purpose of the scrubber is to reduce the SO_2(g) partial pressure.

That leaves finding some way of reducing [SO_2(aq)]. By Equation 2, if SO_2(aq) were reduced, so would SO_2(g), and SO₂ would be removed from the gas stream .The question then becomes how to reduce SO_2(aq). The answer lies in the fact that SO₂, besides being a gas, is also a weak acid. In fact, it is because of its acidity that SO₂ is a pollutant that needs to be removed in the first place. If it is not scrubbed before going up the stack, it will be scrubbed in the atmosphere and fall as acid rain.

As an acid, it will react with a base thereby removing SO_2(aq) according to the following reactions. Note that Equation 3, being a weak acid reaction, does not proceed very far to the right, while Equation 5 proceeds far to the right provided XOH is a strong enough base.

Thus, [SO_2(aq)] will be reduced as the Reaction 5 proceeds, and with it, so will the partial pressure of SO_2(g). The reduction of SO_2(g) partial pressure is another way of saying flue gas desulfurization. Due to the equilibrium constants involved, the reaction itself proceeds quite far to the right. The equilibrium constants are not what limit the extent of Equation 5 however. FGD reactions are heterogeneous, with mass transfer required in order to transport the SO₂ from the gas phase to the aqeuous phase, and from the gas/liquid interface into the liquid bulk phase before the chemical reactions can take place. Mass transfer requires surface contact area and mixing. This is the purpose of the scrubbing equipment.

Scrubbing Equipment

A wet scrubber uses a high energy liquid stream to contact the gas stream and effect mass transfer. Mass transfer requires high surface area, and it is the purpose of the scrubber to provide it. There are three main kinds:

Venturi scrubber

A venturi scrubber is a converging/diverging section of duct. The converging section accelerates the gas stream to high velocity. When the liquid stream is injected at the throat, which is the point of maxiumum velocity, the turbulence caused by the high gas velocity atomizes the liquid into small droplets, which creates the surface area necessary for mass transfer to take place. The higher the pressure drop in the venturi, the smaller the droplets and the higher the surface area. The penalty is in power consumption.

Packed scrubber

A packed scrubber consists of a tower with small objects inside. These objects can be in the shape of saddles, rings or some highly specialized shapes designed to maximize contact area between the flue gas and liquid. Packed towers typically operate at much lower pressure drops than venturi scrubbers and are therefore cheaper to operate. They also typically offer higher SO₂ removal efficiency. The drawback is that they have a greater tendency to plug up.

Spray tower

A spray tower is the simplest type of scrubber. It consists of a tower with spray nozzles, which generate the droplets for surface contact. Spray towers are typically used when circulating a slurry (see below). The high speed of a venturi would cause erosion problems, while a packed tower would plug up if it tried to circulate a slurry.

Scrubbing Reagent

As explained above, pure water is not suitable for scrubbing SO₂. An alkaline solution must be used. Depending on the application, the two most important are lime and caustic soda. Lime is typically used on utility boilers, as it is the most cost effective. The problem is that it results in a slurry being circulated through the scrubber instead of a solution. This makes it harder on the equipment. A spray tower is typically used for this application. The use of lime results in a slurry of calcium sulfate (CaSO₄) that must be disposed of. Fortunately, this by-product calcium sulfate can be used in the building products industry, where it is known by its mineral name, gypsum.

Causic soda is is more expensive than lime, but has the advantage that it forms a solution rather than a slurry. This makes is easier to operate. It produces a solution of sodium sulfite/bisulfite (depending on pH), or sodium sulfate that must be disposed of. This is not a problem in a kraft pulp mill for example, where this can be a source of makeup chemicals to the recovery cycle.

External links

• Schematic of desulfurization plant