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 the scrubbing the exhaust gases of large ships.
Fossil fuels i.e. 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 and so must be removed from emissions.
The remaining sulphur can be oxidised to sulphur trioxide (S03) if there is excess oxygen present in the combustion chamber. More likely is the formation of sulphur monoxide (SO), often associated with elevated temperatures or the presence of metal impurities in the fuel that can act as catalysts. The SO will almost immediately combine with any moisture within the system to form sulphuric acid (H2SO4). As c. 1% of the SO2 produced will also form this aggressive acid, the internal environment is extremely challenging for material selection. A
Other constituents of flue gas include soot, which are carbon deposits from incomplete combustion, and particulate matter from ash and other mineral impurities in the fuel. In addition, there are also nitrous oxides, nitrogen monoxide (NO) and nitrogen dioxide (NO2) that will form nitric acid on exposure to water and has been associated with the ‘acid rain’ phenomenon of power stations before abatement systems were installed.
2) How emissions can be controlled
a) Wet scrubbers
Typically, the wet scrubber process is based on a simple chemical reaction process 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(H20)2 ], better known as gypsum which can be recovered and used in plasterboard applications.
The standard process configuration is for the flue gases to be drawn through a spray tower under an induced draft. Here, a series of nozzles created 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 apparent benign sections. Alternative designs use venture-rods, where the flue gas helps to atomise the ‘scrubber liquid’, and packed bed scrubbers where the flue tower is packed with material in an attempt to maximise the contact area and time between flue gas and liquid.
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 the use of corrosion resistant alloys includes pumps, valves and piping in addition to the main vessels.
Achieving effective mixing between the slurry and the flue gases is critical in attaining high SO2 removal levels with the most common technique using a spray tower situated downstream of the electrostatic precipitator. Flue gases are then drawn into the tower by an induced draft fan (often the main flue fan) where they flow counter current to the limestone slurry spray, providing sufficient residence time for the chemical reactions.
b) 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, and relies 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 for instance. In which case, the seawater is dosed with an alkali to achieve the same neutralising affect.
c) Dry scrubbers
These systems are a variation on the theme of wet scrubbers, in that the slurry is deployed 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.
3) 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 sulphorous 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. This can be exacerbated as the flue gas can also mix with the moist atmosphere as it leaves the chimney to form droplets of acid, which can fall back into the chimney and cause corrosion on the chimney walls.
Given this variation in conditions, the choice of materials will vary through the system as well as from site to site. Based upon (negative) experience of corrosion, there has been a trend of using increasingly higher performance materials to avoid failure, maintenance costs or outage.
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. With the potential for conditions to vary considerably within the system, with multiple opportunities to form crevices, this additional corrosion resistance helps extend the service life but ultimately 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). Molybdenum alloy additions have been shown to provide good resistance to pitting, and Alloy 254 has also been specified. With a PREN of 43 it offers very strong corrosion resistance, but with high levels of Cr, Ni and Mo it can be prohibitively expensive for large-scale application.
Venturing further, nickel alloys will definitely offer better corrosion performance still – but at a cost point that will be several factors more than stainless steels. It has been more accessible as a solution when utilised as thin sheets – so-called ‘wall-papering’ of ductwork and chimneys in front of a lower-cost structural metal. C276 has been widely reported as suitable for this application in both trade and scientific papers. Similarly, titanium has been shown to possess very good corrosion resistance in FGD gaseous and liquid environments and has been used for structural members and bolting. However, its cost precludes its large scale use.
Sitting somewhere 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 sits Ferralium 255, which benefits from increased levels of Cu addition that has been shown to enhance pitting corrosion resistance beyond that indicated by the PREN.
As shown in Figure 1, Ferralium provides significantly better resistance to sulphuric acid than Alloy 316L and another popular super duplex grade S32760. This superior performance is supported by the iso-corrosion curve shown in Chart 1, where it sits above other stainless steels and is only bettered by the nickel-based Alloy 825.
From left-to-right, Ferralium – 0.05mm/year, S32760 – 2.00mm/year, Alloy 316L – 3.00mm/year
Chart 1: Iso-corrosion curve for metals exposed to Sulphuric Acid at a rate of 0.1mm/year.
With the encouragement of this general performance, a more specific laboratory test was undertaken simulating the conditions of a typical FGD system. Metal samples were exposed to both a corrosive environment, elevated temperature and a flow of gas. In this specific environment Ferralium performed better than competing metals.
Chart 2: Comparative corrosion performance of alloys in a simulated FGD process environment.
45,000ppm Cl– [0.003% FeCl3, 0.11% KCl, 0.5% MgCl2, 1.1% CaCl2, 0.02% CaF2, 5.56% NaCl, 200g/l CaSO4.2H20] at 66°C, pH c. 2.5, with SO2/O2 (1:1) bubbled through solution.