Flue Gas Desulphurisation

Flue-gas desulphurisation (FGD) is the process of removing sulphur dioxide and other associated emissions from the exhaust gases of fossil-fuel power stations. A similar approach is also being utilised for other applications such as scrubbing the exhaust gases of large ships.

Background

Fossil fuels (coal, oil) contain an amount of sulphur - the exact quantity depends upon the geological source and the degree of processing or refinement undertaken. During combustion, >95% of the sulphur is transformed to sulphur dioxide (SO₂). As SO₂ is a toxic gas associated with increased respiratory symptoms and disease, and is a precursor to the formation of acid rain and other airborne particulates, it must be removed from emissions.

The remaining sulphur can be oxidised to sulphur trioxide (SO₃) if there is excess oxygen present in the combustion chamber. The SO₃ will almost immediately combine with any moisture within the system to form sulphuric acid (H₂SO₄). As approximately 1% of the SO₂ produced will also form this aggressive acid, the internal environment is extremely challenging for material selection.

Emission control systems

Wet scrubbers

The wet scrubber process is based on a simple chemical reaction to remove the SO₂. The flue gas is passed through an aqueous slurry of lime (CaO) and limestone (CaCO₃), where it reacts to form calcium sulphate (CaSO₄·2H₂O) - better known as gypsum, which can be recovered and used in plasterboard applications.

The standard process configuration draws flue gases through a spray tower under an induced draft. A series of nozzles create a fine mist of the limestone slurry to ensure intimate mixing during counter-flow. The creation of acids can occur, and in a high volume process it can be difficult to prevent flow from one section of the process to another - resulting in corrosion issues in apparently benign sections.

Irrespective of the specific design, all systems will include sections for the preparation, handling and pumping of the limestone slurry, as well as further sections for recovery and treatment of the effluent. Therefore, candidate items for corrosion resistant alloys include pumps, valves and piping in addition to the main vessels.

Sea water scrubbers

Seawater has been increasingly used as a solution to treat flue gases, particularly if power stations are located adjacent to the coastline or estuaries. The flue gas is circulated through a scrubber system as previously described, but seawater is used instead of an aqueous lime slurry. The SO₂ is absorbed into the water, relying upon dilution and the buffer effect of seawater containing natural bicarbonate (HCO₃) to neutralise any acidity.

These type of systems lend themselves to application in large ships. They can be operated as open loop systems when in open water, but may need to be operated as a closed loop system when in port, in which case the seawater is dosed with an alkali to achieve the same neutralising effect.

Dry scrubbers

These systems deploy the slurry as an extremely fine mist that is instantly dried as soon as it comes into contact with the hot flue gases. The resulting fine particles are carried along in the neutralised gas stream and rely upon a filter system (filter bags, electrostatic) to capture and remove them from the emissions.

Material selection

The internal environment of FGD systems can be very corrosive, and large variations in slurry compositions may also exist between different installations. Therefore, material selection needs to consider each project on an individual basis reflecting the plant size and design, operating costs and composition of the likely flue gas.

The main material challenge has been combinations of uniform, pitting and crevice corrosion originating from acid attack when SOx condensates are formed at various points of the FGD system:

• When the flue gases entering the system are initially quenched, sulphuric and sulphurous acids are formed and can be very aggressive. The pH of the acidic media in the entry duct can be 1 or less.
• As the limestone slurry flows into the absorber vessel, conditions reach equilibrium and the pH is significantly increased towards 4.0-5.5.
• After scrubbing, the exiting gas can still contain some acid, mostly as a fine mist. In the outlet ducting, cooling gases will generate very acidic condensates on the duct walls.
• Similar condensation can also occur on the walls of the exit chimney/stack, which can be exacerbated as the flue gas mixes with the moist atmosphere as it leaves the chimney.

Given this variation in conditions, the choice of materials will vary through the system as well as from site to site.

Progression of materials

Initially, series 3xx austenitic stainless steels were widely used, with Alloy 316L being subsequently replaced by Alloy 317L. This alloy benefits from significant additions of Mo (3.0-4.0%) and increased Cr content to raise its Pitting Resistance Equivalent number (PREN) from 25 to 31. However, such alloys are only really sensible for less aggressive areas - where temperatures, acidity and content of halides are low.

Duplex stainless steels (such as Alloy 2205) were subsequently introduced, particularly in plate form for fabrication of the main vessels. With a PREN of 34, they represent the next logical step up in corrosion performance and have historically been the most widely specified material. Under cost pressure, this category of materials has developed into 'lean duplex' stainless steels offering comparable performance but with lower alloy contents (and cost).

Nickel alloys will offer better corrosion performance still - but at a cost point that will be several factors more than stainless steels. C276 has been widely reported as suitable for this application, often utilised as thin sheets ('wall-papering') of ductwork and chimneys in front of a lower-cost structural metal. Titanium has been shown to possess very good corrosion resistance in FGD environments and has been used for structural members and bolting, although its cost precludes large-scale use.

Sitting in the middle of this diverse choice of metals are super duplex stainless steels. They are generally the metal of choice for many pump and valve applications - both as the cast body, and as machined bars for the moving parts - due to their combination of corrosion performance and mechanical properties.

Their composition has been developed to achieve a PREN > 40, which is only matched by metals several times more expensive. The proof stress is typically twice that of austenitic stainless steels, which is valuable in load-bearing components and can be exploited by designing smaller sections requiring less material.

Within the family of super duplex grades, Ferralium^® 255 benefits from increased levels of Cu addition that has been shown to enhance pitting corrosion resistance beyond that indicated by the PREN.

Ferralium^® performance data

Both super duplex grades we stock, Alloy 32760 and Ferralium^® 255, clear a PREN above 40 and perform strongly in chloride and seawater service. In hot concentrated sulphuric acid, Ferralium^®'s higher copper content gives it a distinct advantage: in immersion testing in 70% wt sulphuric acid at 37°C for 48 hours, corrosion rates were Ferralium^® 0.05 mm/year, Alloy 32760 2.00 mm/year and Alloy 316L 3.00 mm/year. For chloride and seawater duty, Alloy 32760 is an excellent and widely specified choice; where reducing acids such as sulphuric dominate, Ferralium^® is the grade to select.

A more specific laboratory test was undertaken simulating the conditions of a typical FGD system. Metal samples were exposed to a corrosive environment, elevated temperature and a flow of gas. In this specific environment Ferralium^® performed better than competing metals.

Test conditions: 45,000 ppm Cl^- [0.003% FeCl₃, 0.11% KCl, 0.5% MgCl₂, 1.1% CaCl₂, 0.02% CaF₂, 5.56% NaCl, 200 g/l CaSO₄·2H₂O] at 66°C, pH c. 2.5, with SO₂/O₂ (1:1) bubbled through solution.