Chlorine Conditioning

#Remove chlorine based deposits and stop corrosion

A new approach based on the sulfur – chlorine balance

Chlorine conditioning

During the combustion of renewables and waste, chlorine is released at various stages within the boiler. This chlorine reacts with alkali metals
present in the fuel, forming alkaline chlorides.
These chlorides cause significant corrosion and deposits within the plant.

Pentol’s chlorine conditioning process addresses this issue by using SO3 gas to establish a sulfur-chlorine equilibrium.
By balancing the chlorine with SO3, high-temperature corrosion is halted, and corrosive deposits on the superheaters are removed. This process extends the boiler’s runtime and enhances its efficiency.

With a corrosion rate of 5mm per year we had to change our super heaters every 18 months

During the combustion stage, fly ash is generated and dragged by the gas current and given its rather plastic and semi-molten state, clings to the walls of the furnace, 2nd and 3d pass as well as the super heaters.
The deposits grow quickly and reduce the heat exchange, increase the delta p and are very corrosive in nature. The operator is forced more often than intended to shut down the unit for cleaning, whereas the deposits found are very hard and difficult to remove.

Along with the growing deposits on the pipes and walls inside the boiler, Combustion temperature is rising and the process efficiency is reduced.

Contributing to these effects are
• Halogenated acids (HCl, HF)
• Sodium based compounds

Most affected are superheaters with a steam temperature of around 500°C, where we found corrosion rates of 5 mm per year or even more.

The circle of corrosion

The corrosion mechanism

Low melting temperatures allow molten salts to condensate on pipes with relatively low temperatures (eg. superheaters). The alkalichlorines diffuse through the deposits and attack the pipes. Iron is transported through the deposit to the surface, where it is released to the flue gas as ironoxide, while the chlorine diffuses back to the cooler pipe.
Inside the deposits, the chlorine concentration is increased and destruction of the pipes guaranteed. The sticky deposits are not only highly corrosive, they also prevent a good heat transfer from the flue gas to the steam.

Sulfur / Chlorine balance

Tip the balance by adding SO3

The sulfur / chlorine balance has been researched for years.
The sulfurization process is described in the following reactions:

2 KCl + SO₃ + H₂O –> K₂SO₄ + 2 HCl

2 KCl + SO₃ + ¹/₂ O₂ –> K₂SO₄ + Cl₂

Similar reactions take place with sodium or calcium. Sulfurization of NaCl and KCl through SO3 are most efficient if the alkali chlorides are gaseous. The best
temperature range for the sulfurization process starts at 600 °C.
For an ideal sulfurization of the alkali chlorides in the flue gas according to the reactions described, a molar ratio of Cl/S = 2 is sufficient. Because of the nonlinearities of the technical process, a higher ratio will be required in real
world applications.

Pentol’s approach to reduce chlorine corrosion is based on our unique SO3 generator units that have been widely installed on electrofilters for coal fired power plants.

SO3 is injected directly into the furnace by means of injector probes. Alkali sulfates are formed during the cooling process of the combustion while the chlorides remain in gaseous form and are washed out from the flue gas. With a full sulfurization of the alkali chlorides in the flue gas, alkali chlorides can no longer condense on the super heater and take part in the corrosion process.
By balancing the SO3 injection with the chlorines, the growing process of deposits is reduced to a minimum and with the “glue” removed, these deposits are easier to remove during regular outages.

Chlorine Conditioning

Process description

Dosing point

To achieve the perfect S/Cl molar ratio, SO3 is injected into the furnace by means of injector probes.

Liquid, elementary sulphur is stored in a steam heated tank. By means of a dosing pump, the sulfur is transported into the sulphur burner. Together with preheated air, the sulphur is converted to SO2. After the sulphur burner, the SO2/air gas mixture is passing the 2-stage converter unit, oxidizing the SO2 to SO3.

Now the SO3 is available for reaction. No SO3 is stored in any place and the correct quantity is produced automatically by the system. SO3 reacts in a much more active way than SO2 or ammoniumsulfate and the dosing rate is considerably lower.

With Pentol SO3 injection, the conversion rate of sulfur to SO3 is 97 %. Compared to traditional sulfur injection, the consumption rate is therefore 50 – 100 times lower and SO2 emission is not increased.