A Comparative Study on Several Anti-Corrosion Materials for Power FGD System ()
1. Introduction
As a result of stringent environmental protection regulations regarding SO2 emissions, SO2 is now being removed from flue gases by methods of flue gas desulfurization (FGD) technology in power plants. There are many factors that need to be considered in the installation of a wet FGD system, especially in the selection of corrosion resistant materials, to insure long maintenancefree operation. These include initial costs, availability of alloy and lining materials, and operation expenses. Dew point, acidity, temperature, halide concentration, wet/dry condition and gas velocity all contribute to the aggressive corrosion conditions of FGD system. Because the inner temperatures of chimney for FGD system without gas to gas heater (GGH) are approximately 40˚C ~ 50˚C, corrosion of carbon steel at dew point is very severe. In addition, chimney vibration and temperature change during FGD operation are very harmful to internal anti-corrosion materials. When wet FGD systems begin to be used in power industries, some high-performance anti-corrosion materials must be required to prevent equipment failure due to corrosion [1-3].
In the past few years, cellular glass boards and protective coatings are applied extensively in inner surface of chimney of FGD system. However, heavy corrosion of many chimneys with use of the anti-corrosion method has taken place only after several month of production. Cellular glass board is a kind of inorganic thermal insulating material with a “honeycomb” cell structure. The limitations of glass materials and fabrication processing often lead to the low mechanical strength and high absorbing-water rate of cellular glass board. Some coatings such as epoxy modified silicone, silicate, polyurea have been used to prevent corrosion of FGD chimneys, but they are quickly damaged after using due to the low adhesion strength and cracking at the high temperature. The damage images of internal steel stack in FGD chimneys are shown in Figure 1 and Figure 2, respectively. Several alloys have been used for prevention of corrosion in wet limestone FGD systems with varying degrees of success. Nickel alloys containing high levels of molybdenum are found to be the best, but were relatively expensive. Titanium has good resistance to dilute sulfuric
Figure 1. Corrosion picture of steel stack of FGD chimney using cellular glass.
Figure 2. Corrosion picture of internal steel FGD chimney after cleaning away.
acid, but is weak resistance to concentrated sulfuric acid at high temperature. FGD equipment made from fibre reinforced plastic (FRP) are currently being installed in coal-fired power plants in North America. Don Kelley compares the cost and reliability of FRP with other materials in wet FGD appli-cations [4-6].
Inorganic-organic hybrid composites are rapidly emerging as alternatives to traditional anti-corrosion materials as they combine the chemical and mechanical properties of both inorganic and organic components [7]. The performance of corrosion resistance and stress resistance of lining materials of FGD chimneys can be improved by the incorporation of the new hybrid composite. The current demand for corrosion resistance is driving the use of thr new hybrid composites in wet FGD systems. In the paper, we comparatively discuss the properties of epoxy modified silicone coating, vinyl ester flake mastic and silicon-contained heterocyclic polymer with enhanced fibers (organic-inorganic hybrid composite) when they are used in corrosive condition of FGD system.
2. Experimental Method and Materials
The silicon-contained heterocyclic polymers (Good Co.) with 15% glass fibers were cured by a sulfone-containing amine curing agent (NERC) to form a reinforced hybrid composite. The epoxy modified silicone coating (Caiyi Chemical Co.) and vinyl ester flake mastic (Guangmin Chemical Co.) were prepared and comparatively analyzed with the reinforced hybrid composite with 15% glass fibers. The three mixtures were poured into a mold coated by a fluoropolymer, and the resulting samples were kept at 25˚C for 7 days. The samples were removed from the mold and cut to the specimen dimensions required for testing.
The samples were applied to the surface of steel plate with size of 150 mm × 75 mm, and the thickness of coatings is 1.00 ± 0.05 mm. The free films for immersion test were stripped from the steel plate with coated silicon oil. The samples were placed in a high-temperature and high-pressure reactor for corrosive test. Firstly the test was operated in 40˚C mixed acid with 8% H2SO4 and 5% HCl for 24 hours, and then temperature raised to 80˚C for 24 hours, and finally temperature increased to 160˚C for 2 hours. The process above was defined as a corrosive period, and these samples were subjected to 24 corrosion cycles. The above corrosion cycle is similar to the serious corrosion condition of FGD system.
Water or acid absorption of the materials was determined by immersion test of free film. The test was used to evaluate the corrosion resistance of anti-corrosion materials using the gravimetric method. Water or acid absorption was calculated based on the weight gain of the specimens immersed in corrosive solution. All of the mass gain or loss results were the average of the three specimens in parallel. Weight gain of sample was plotted as a function of immersion time.
DSC measurements were carried out using a Perkin– Elmer differential scanning calorimeter. Samples were placed in aluminum pans and cured using a dynamic temperature scan from 25˚C to 100˚C with a heating rate of 5˚C/min and 10˚C/min under air atmosphere. The conversion rate of hybrid composite were determined from the analysis. The SEM (scanning electron microscopy) micrographs of samples before and after corrosion test were investigated with an XL30 ESEM instrument, and the imaging analysis was operated at 20 kV. A gold film was sputtered atop the surface of specimen to make it electrically conductive. Measurement of adhesion strengths was performed according to ASTM C633-01 standard. Two f25 mm steel sticks were stuck together with polymers, and were continually drawn along the face by using an AG-5000A drawing machine until two steel sticks were divided. The divided force of two steel sticks is adhesion strength of composite or coating. The average values of adhesion strength of three same samples were recorded. Coefficient of thermal expansion (α) was measured according to GB/T2572-2005 standard. The column specimen with diameter of 6 mm was prepared and α was measured by Anter model 1101 instrument. Limiting Oxygen Index (LOI) of materials was measured using a Dereke instrument on sheets with the size of 120 mm × 60 mm × 3 mm according to the ASTM D2837/77 standard.
3. Experimental Results and Discussion
3.1. Analysis of Curing Conversion
Figure 3 shows the conversion rate of the studied samples as a function of the temperature at different heating rate. Temperature and time dependence of the curing conversion rate can be determined from the curves. The different conversion rates of hybrid polymer contribute to different levels of cross-linking in microstructure and different amounts of internal stress formed in curing process, which is related to corrosion resistance of composites. The results indicate that the conversion rate of curing reaction of hybrid composite at 5˚C/min is higher than that at 10˚C/min. When conversion rate of hybrid composites increases to 80%, temperatures for curves at 5˚C/min and at 10˚C/min are 73.5˚C and 78.5˚C, respectively. When temperature increases from room temperature to 90˚C at 5˚C/min, the conversion rate of hybrid polymer is very high (98.4%). The analysis reveals that the hybrid polymer can be cured with JK-GW12 curing agent safely and fully only after 16 minutes at heating rate of 5˚C/min. The high curing conversion rate of hybrid composite at lower temperature is beneficial for engineering application of the materials at ambient temperature [8,9].