Material Selection

Corrosion performance advantages of fibre reinforced plastic in wet FGDs

By Kevin Lambrych, Manager, and Michael Stevens, Principal Scientist, INEOS Composites LLC, USA

The amount of gas, vapor, and particulate pollution released into the atmosphere has grown steadily with the industrialization of our global society. Over the past 100 years, governments throughout the world have implemented legislation and regulations relating to the release of air pollutants into the atmosphere. The implementation of these air pollution control (APC) standards drive significant investment in the installation and maintenance of APC systems for coal-fired power plants worldwide. According to the International Energy Agency’s 2018 Outlook, coal will continue to fuel one third of electricity generation worldwide, with significant growth occurring in markets such as India over the next five years. Durable and cost-effective pollution control equipment is important for profitable operation of coal-fired power plants. Equipment designs based on fiber reinforced plastic (FRP) are not only less expensive than alloys, but also more durable for the corrosive chemical environments found in flue gas desulphurization (FGD) equipment.

Materials of construction

There are several different designs of dry and wet FGD systems available for coal-fired power plants. Wet FGD systems commonly achieve higher SO2 removal efficiency (over 99 per cent) compared to dry FGD systems (40-90 per cent). The wet FGD chemical process used to capture SO2 creates an acidic, high chloride, high sulfide environment inside wet FGD absorbers. The materials selected for this construction of wet FGD equipment must be able to withstand this extremely corrosive environment.

FRP has a long history of success in APC for power and a range of other industries. APC equipment made from FRP has cost and durability advantages compared to alloys, with case histories supporting its use dating back to 1973.

Typical wet FGD applications for FRP include stack liners, storage tanks, limestone slurry piping and low pH high chloride scrubber systems. Since the latter half of the 1970s, much of this FRP equipment has been fabricated with corrosion resistant glass fiber and epoxy vinyl ester thermoset resins. Epoxy vinyl ester resins (EVER) are specified due to their superior mechanical properties and chemical resistance compared to traditional unsaturated polyester resins (UPR). More recently, FGD absorbers, ductwork and stack lines typically use FRP based on flame retardant novolac epoxy vinyl ester resin (NEVER).

Figure 2: Large FGD absorber FRP vessels (left) are typically fabricated on site due to their size. The SO2 absorber vessel above has a diameter of 180 feet (55 meters) and a height of 60 feet (18.3 meters). Absorber discharge tanks (right) with 254,000 gallons (960,000 liters) volume, 36 feet (11 meters) diameter, 433 feet (132 meters) height, and up to 176°F (80°C) operating temperature

When considering the design of new projects, engineers need to balance the budget limits set during planning with equipment fabrication costs at the time of execution. The selection of construction materials for new equipment or to repair old equipment is no small task.

FRP is typically chosen for its performance and cost advantage over alloy. FRP equipment can also be easily relined or new equipment economically fabricated on site. New alloys with higher concentrations of nickel or molybdenum and chrome have significantly improved the corrosion performance of alloys, but these have also increased their price. History has shown specialty metals used in anti-corrosion applications can swing greatly in price over short periods of time. Within the past 15 years, we have seen this volatility complicate budgets and planning for large industrial projects.

Figure 3: Large post absorber FGD ductwork made with EVER-based FRP. Ductwork diameter is commonly between 18 feet and 28 feet (5.4 meters to 8.4 meters) with a flue gas operating temperature of 122ºF to 140ºF (50° C to 60°C).

FRP does not suffer the same type of price volatility that has plagued alloys over the past decade. Furthermore, data in Table 1 shows that equipment made with FRP costs considerably less than the same equipment made with alloy.

To understand better how the cost of fully fabricated FRP and alloy equipment compare, refer to Figure 1.  The cost ratios for a fully fabricated 6,000 gallon (22,700 liters) vessel made from various materials is presented. Compared to FRP, it is readily seen that 2205, C-22 and C-276-based alloy tanks are significantly more expensive.

Other corrosion resistant materials of construction such as coated steel and rubber-lined steel are cost competitive with FRP. However, FRP has a much better lifecycle cost than these materials because it requires less maintenance over the life of the equipment. Depending on the environment, FRP can have a 20-year service life or even longer.

Figure 4: Field fabrication of a tank wall section by filament winding. Vertical winding is pictured on the left and horizontal winding is pictured on the right.

Choosing the right FRP

When designing FRP-based pollution control equipment, the first critical step is selecting the correct resin for the chemical service. When choosing a resin, the design engineer should consult the resin selection guide of a knowledgeable resin manufacturer whose products are well established and backed with experience and case histories. If the chemical environment in question is not represented in the resin selection guide, the designer should contact the resin supplier directly for a resin recommendation.

In corrosive chemical environments, FRP structural layers must be combined with a corrosion barrier to protect structural layers from chemical attack. The corrosion barrier is a resin rich layer (90 per cent resin vs glass w/w) that interfaces directly with the chemical environment backed with layers of resin and chopped strand mat (75 per cent resin vs glass w/w). If needed, the corrosion barrier thickness can be increased to improve overall FRP corrosion performance. The use of improved CR reinforcements like ECR-glass over traditional E-glass in the corrosion barrier and structural layers is commonly recommended as a best practice by industry consultants to maximize FRP equipment life.

FRP applications and case histories

EVER-based FRP has been used for wet FGD processes in absorber vessels (Figure 2), slurry piping, ductwork (Figure 3) and stack liners. The most prominent applications are limestone slurry piping followed by stack liners. FRP pipes based on epoxy vinyl ester resin have been successful in more than 150 plants dating back to 1977. From 2004 to 2010, it was used in more than 70 stack liners, 75 limestone slurry piping systems and over 25 FGD scrubbers.

Figure 5: Schematic representation of the filament winding technique. Here horizontal filament winding is represented.

Field fabrication – Filament winding

In Figure 4, large FRP vessel sections are filament wound on a large mandrel mounted in either a vertical or a horizontal position. To illustrate this manufacturing technique, a schematic representation is presented in Figure 5.

In Figure 5, a corrosion barrier is first constructed on the mandrel surface. A continuous strand of glass filament is drawn through a resin application bath and impregnated with resin. The impregnated glass filament is next wound onto the mandrel to build up the tank wall section then cured.

In Figure 6, a completed chimney stack section and an aerial view of the remote manufacturing site can be seen. Figure 6 gives a good perspective of the capabilities of this remote fabrication technique for producing very large vessels.

Figure 6: Remote manufacturing site for filament winding fabrication of FRP FGD stack liners.

Field assembly – Ring oblation

Ring oblation of shop-made vessel walls is a technology that is well proven and has been in use for more than 30 years. Tank walls up to 57 feet (17.4 meters) in diameter can be oblated, nested together, and shipped to the project site more easily than a fully assembled vessel. Ring oblation is feasible due to the strong but flexible character of FRP composites. In this method, tank wall sections are filament wound in a controlled shop environment, cured, and compressed into an oblate shape. This oblate shape allows tank wall sections to better fit on a trailer for simplified transport via roadways to the project site. Compression of the tank wall causes no harm to the corrosion barrier or detriment to long-term performance of the completed vessel. Upon arrival, the FRP tank wall sections are allowed to relax into their original shape, the sections stacked, fitted together, and bonded.

Conclusion

The process conditions found in FGD absorbers are some of the most corrosive chemical environments found in industrial applications today. EVER and NEVER-based FRP is well suited for many of the operating temperatures and chemical environments, particularly when there are high concentrations of chlorides and sulfides. EVER-based FRP and lining systems have been used successfully to repair degraded alloy equipment.

Additional advantages of FRP equipment are that it can easily be relined and repaired in place, shipped economically, and fabricated and assembled remotely at the project site. FRP has enjoyed a long history in APC applications. Given its economic and performance advantages compared to alloy, the expectation is that it will continue to develop as an important material of construction in APC applications.

Figure 7: Field assembly of an “oblated tank” for aggressive caustic service.

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