High-temperature-resistant fiber reinforced plastics (FRP) commonly are used in gas scrubbers to remove acidic byproducts and other pollutants from the flue gas. Additionally, epoxy vinyl ester resins (EVERs) are used as matrices in composite applications in which resistance to corrosive environments and excellent mechanical and thermal performance are major property requirements.
For economic reasons, plants have a strong interest in using FRP in parts that are exposed to a corrosive environment and high temperatures. Recent advances in "high-temperature" novolac-based EVERs have provided an almost 68F (20C) increase in heat resistance compared to the established novolac-based resins. This increased resistance allows the resins to be used under a 390F (200C) operating temperature.
Incinerator systems often require the hot gases to be cooled and fed into a quench chamber. There, the gas is sprayed with water to reduce the temperature from approximately 430F (220C) (with the possibility of occasional peaks as high as 535F/280C) to approximately 140F (60C). When the gas enters the scrubbing vessel, it undergoes a washing cycle. Because combustion gases commonly are acidic, the first washing cycle liquid normally is a base, followed by either an acid or a pure-water wash stage.
The gas often contains strong, highly corrosive mineral acids such as sulfuric, hydrochloric and hydrofluoric acids. While the gases are sprayed with water ," at near-to-ambient temperature ," the containment wall materials are intermittently splashed with water, exposing the vessel materials to a hot, wet corrosive atmosphere. The gases can experience thermal shocks caused by the cooling effect of the water and the heating effect of the gases.
Design engineers have evaluated the performance of a number of construction materials under these severe-service conditions.
Although rubber linings resist corrosion and thermal shock, the high temperatures lead to the rubber's degradation.
High-nickel-content alloys also have been used. In the short term, these alloys show a strong resistance to heat and thermal shock, but do not hold up to the hot, wet mineral acid attack in the long term, resulting in short life spans. High-nickel alloys often last only one year before severe corrosion of the vessel walls occurs.
Brick lining is the traditional material of choice because it resists the acids, the heat and the thermal shock. However, brick lining is very expensive. Repairs are costly and time consuming.
FRP-based systems using the standard novolac EVERs resist corrosive environments and thermal shocks; however, these systems are challenged by the temperature requirements of the application. The new novolac EVER discussed previously has a higher thermal resistance, opening a new application window for FRP in scrubber applications.
Table 1 summarizes the performance of these four material options.
Attaining a successful FRP design
Many design options are available. While such options depend, in part, on geometry and design conditions (including upset temperature for short interval), they can include:
Full (glass-reinforced) laminate.
Laminate with one or more thermal insulation barriers (e.g., air annulus).
Highly conductive carbon fiber reinforcement or weakly conductive glass fiber reinforcement.
Protective surface veil and/or chemical-resistant barrier.
Standard resin or resin with conductive filler (e.g., graphite aimed to give improved thermal conductivity).
Years of study and the testing of a significant number of applications with high temperatures have strongly suggested that the prerequisites for a successful economic design are the combined experience and cooperation of the resin manufacture, FRP fabricator, engineering contractor and end-user.*
To develop a successful FRP application, the plant must locate a suitable resin and a suitable laminate design, and must consider temperature distribution through the laminate.
EVERs, based on epoxy resins, are modified to allow the resins to be handled and cured by the same methods as traditional unsaturated polyester (UP) resins ," i.e., via a free-radical mechanism with styrene as the co-curing monomer. The use of the epoxy structure gives the final cured resin its high performance and the cure characteristics of UP resins.
EVERs, when cured, contain terminal cross-linking sites, allowing the whole length of the polymer to stretch and uncoil when stressed. This capability makes EVER-based composite moldings tougher and more resistant to thermal and mechanical shock. Unlike EVERs, UP resins cross-link along the length of the entire chain, making them less resistant to stresses and more brittle.
The new resin
The recently commercialized resin discussed previously offers an increase in the glass transition temperature (Tg) to 375F (190C) and to 390F (200C) after post-cure. This temperature capability compares favorably to widely used novolac EVERs with a Tg of 330F (165C). The new resin's molecular backbone was altered to optimize heat resistance, adapting the co-monomer ratio to the needs of the application.
The resin has an epoxy novolac structure and chemical resistance characteristics comparable to resins currently in use. The key difference, however, centers on the acid resistance increase, which makes the new resin highly suitable for use in corrosive environments, including those conditions typically found in the gas inlet sections of scrubbers.
The resin's higher Tg makes it suitable for service in contact with gas temperatures as high as 410F (210C). It has a 3 percent tensile elongation and can easily withstand the cyclic stress induced by the thermal shocks typically found in scrubber quench applications. Table 2 summarizes these resin properties.