Fiberglass Reinforced Plastics for Air Quality Improvement at Power Plants

Ben R. Bogner AOC, LLC

USAMarch 2011

AbstractThe recent growth in flue gas desulfurization (FGD) projects at power plants around the world has spurred the use of Fiberglass Reinforced Plastic (FRP) composites to contain and handle the corrosive liquids and gases. Air quality regulations in most countries require that sulfur dioxide from coal fired power plants be removed before the combustion products are vented to the atmosphere. FRP has been selected for use in numerous sections of the FGD portion of coal fired power plants; this paper will discuss the parts of the power plant that benefit most from the use of FRP. FRP composites are increasingly selected over high nickel alloys and stainless steels based on cost and performance. The thermal resistance, chemical resistance, cost and durability issues associated with FRP will be discussed and examples will be given.

IntroductionCoal is burned to produce steam that powers the electric generators at the power plant. Coal is a good source for energy but it also contains 2 – 6 % total sulfur. The sulfur content is related to the source and the type of coal being burned. The sulfur in the coal converts to sulfur dioxide (SO2) during combustion in the furnace. After leaving the plant, SO2 and much smaller amounts of sulfur trioxide (SO3) form sulfurous acid and sulfuric acid when mixed with water in the atmosphere. Sulfur acids, both sulfurous acid and sulfuric acid in the plant exhausts, are harmful to people’s health and the overall environment if not removed from power plant exhausts. The sulfur dioxide is harmful to peoples breathing, particularly those with respiratory problems while the S02 in the air creates acid rains that can acidify lakes and damage buildings and vegetation.

The major chemical reactions taking place in the furnace during coal combustion are listed below:

Coal plus air yields heat, carbon dioxide, sulfur oxides and nitrogen oxides.

Coal + air => Heat + CO2 + SO2 + SO3 + NOx

The majority of the sulfur oxide produced is in the form of sulfur dioxide. The most common way of tying up or reacting with the sulfur dioxide from the exhaust stream is to react the sulfur dioxide with calcium carbonate (CaCO3) in the form of limestone. This reaction is shown below:

Sulfur oxide gases are mixed with limestone; limestone forms calcium salts that are solids and can be separated from the gas stream.

SO2 + SO3 + 2 CaCO3 => CaSO3 + CaSO4 + 2 CO2

The limestone used is typically greater than 95% calcium carbonate and should be relatively free of abrasive contaminates.

These scrubber solutions are quite corrosive to concrete and carbon steels as they contain other corrosive ingredients besides the sulfur acids. Chloride ions (up to 20,000 ppm) are corrosive to many materials of construction; the chloride ion quickly attacks carbon steel, common stainless steels and Portland cement concrete.

DiscussionMaterials of construction for sulfur dioxide scrubbers & stack liners

Because of the corrosive nature of the gases in the flue gases, other materials of construction have been used. High nickel alloys such as Alloy C-276 often selected for use in these types of applications because of their excellent resistance to sulfuric and hydrochloric acids. Even though it meets the corrosion requirements solid Alloy C-276 is seldom selected because of its high cost. Instead, high nickel alloy cladding over steel is chosen over the solid Alloy C-276 parts to significantly reduce costs while maintaining the needed corrosion resistance. This is often called “alloy wallpapering over steel” and it more economical than solid alloy parts.

Acid resistant brick linings are used sometimes because of their inherent resistance to the acids. They may be lower cost on a materials basis but cost their overall cost is often higher due to labor costs; they also add quite a bit of weight to the scrubbers and to the stacks.

FRP composites have had over fifty years proven performance in the chemical process industries and are also used. FRP is commonly used in acid plant construction and in scrubbers removing sulfuric acid and hydrochloric acid; both of these applications are harsher and more corrosive environment that those of seen by FGD equipment.

One definition of a fiber reinforced polymer composite is: A matrix of polymer materials that is reinforced by fibers or other reinforcing material. For power plant application the polymer is usually a vinyl ester resin and the reinforcing fibers are typically glass fibers (fiberglass).

Since these FRP composite work well in scrubbers they are now the preferred material of construction based on a combination of performance and cost. Table 1 shows the relative difference between various materials of construction for a thirteen meter diameter tank. A vinyl ester fiberglass tank had a lower installed cost when compared to rubber lining or Alloy C-276 clad steel tanks.

Several other metals have been used in FGD units over the years including various grades of stainless steel; the stainless steels have led to a number of failures due to chloride corrosion, which is the result of crevice corrosion and pitting. So far, the only alloy that has consistently been resistant to the complex scrubbing solutions and gases has been Alloy C-276. See Table 1.

Table 1Costs of Comparisons of

13m Diameter x 30m Tall Tank

Material Installed Cost Cost Ratio

FRP made with Vinyl Ester Resin $750,000 1.0

Rubber-Lined Steel $865,000 1.15

C-276 Clad Steel $1,300,000 1.75

Alloy C-276 $1,500,000 2.0

Applications for FRP in Power Plant Scrubbers and Chimney (Stack) LinersOne of the oldest applications for sulfur dioxide scrubbing applications at power plants has been FRP piping; the piping is usually constructed of a vinyl ester resin with an abrasion resistant coating. The abrasion resistant coating is required because the limestone slurries are somewhat abrasive. FRP piping has been shown to provide longer service life that metal piping. The abrasion resistant linings use silica carbide, silica or alumina oxide as fillers in the corrosion resin layer to improve the abrasion resistance.

Figure 1FRP Pipe at Power Plant

Photo Courtesy of PSEG Power

Scrubber vessels made of FRP are growing around the world as are scrubber internals. FRP scrubber internals offer design flexibility at an economic cost relative to other materials of construction. Using infusion molding technology, fiberglass internal sections can be made to exacting physical tolerances and the higher fiberglass levels of this process means the parts have high stiffness and strength. The temperature of the scrubbing solutions is typically in the 60-80 oC range, which is far below the upper temperature limits of a conventional bisphenol-A epoxy vinyl ester resin. Ducting from scrubber to stack is often the preferred alternative.

There have been over 75 FRP chimney liner installed and over 50 FRP sulfur dioxide units constructed around the world. Figure 2 is a photo of a scrubber internal section for wet FGD units.

Figure 2

Sulfur Dioxide Scrubber Internal Section

Photo courtesy of RL Industries

Stack LinersThe concrete chimneys need to be protected from the moist, corrosive gases leaving the power plant. Some form of corrosion resistant lining needs to be used. FRP liners are often the lower cost alternative for this application. Figures 3, 4 & 5 show an individual filament wound liner section, the sections assembled at the plant prior to installation and the installation in the liners in the chimney. The same corrosion requirements exist for the chimney liners as exist for the scrubbers; these conditions are mild for epoxy vinyl ester resins. The liners operate at temperatures in the 60-80 oC range, they do in the scrubber but the liners can occasionally experience upset conditions short periods of time. At AOC, we have exposed our laminates made from our brominated epoxy vinyl ester to a variety of temperature excursions. For example, liners can handle exposure to hot gases for 30 minutes at temperatures of 260 oC with significant loss of properties.

Figure 3Photo of Individual Chimney Liner Section

Figure 4Photo of Liners Sections at Power Plant

Figure 5Installation of Chimney Liner Sections

While many liners are exposed to moderate temperatures of 60-80 oC, some liners are operated at ~150 oC and excursions for one hour up to 180 oC. For this type of exposure, a high temperature epoxy vinyl ester novolac is required. Figure 6 shows 72 meter high, 1.4 meter exhaust chimney installed in Norway.

ConclusionsCoal fired power plants are one of the largest generators of sulfur dioxide in the world. Globally, environmental regulations are requiring plants to be fitted with SO2 removal equipment. This trend of requiring sulfur dioxide removal is accelerating. This market offers a growing opportunity for fiberglass engineers and fabricators in India.

There are a variety of technologies for removing sulfur dioxide, including wet and dry scrubbing processes. Scrubbing SO2 from plant exhausts is an expensive process because the plants are large and the materials of construction are costly. The cost of the fitting scrubbers will ultimately be passed on to the electricity users. FRP composites offer excellent corrosion resistance to the process conditions experienced in wet scrubbers and they provide this resistance at an affordable cost. Scrubbers and chimney liners made from FRP have a proven history in the power industry.