Optimization of enamel coatings on heat exchanger plates used in rotary air preheaters of thermal power plants

Today, due to decrease of fossil fuels and high costs of electricity generation, productivity in thermal power plants has become highly important. Rotary air preheaters are critical elements that reduce electricity generation costs by increasing the efficiency of thermal power plants. These heaters transmit heat of combustion gases to the cold combustion air in order to preheat it before the injection into the boiler. Thus, an additional energy is not consumed in order to heat combustion air required for the burning of fuel. Rotary air preheaters provide heat conduction via the heat exchangers plates. Heat conduction is the most important parameter affecting the efficiency of rotary air preheaters. Higher heat conduction increases the efficiency of rotary air preheater, and thus thermal power plant efficiency is also increased. In the light of this information, improvement of the design of the heat exchanger plates, selection of suitable materials and coatings can be summarized as the studies that can be done for improving the efficiency of heat exchanger plates. Sulfuric acid corrosion is the most significant problem, which is encountered on the cold stage of rotary air preheaters. Formation of sulphuric acid on the metal surfaces can cause a severe sulphuric acid corrosion of heat exchanger plates and thereby significantly shorten the lifetime of the components of the heat exchanger plates. In addition, the integrity of corroded parts can be broken, which can cause the disruption of heat conduction. This leads to a drop in efficiency in thermal power plants, and therefore causes an increase in the cost of electricity generation. From this point of view, heat exchanger plates must be coated with a sulfuric acid-resistant protective coating. A possible way to limit this phenomenon is to discharge the flue gases at a temperature above the temperature of the Sulfuric Acid condensation (known as said above as "dew point" temperatures and proportional to the content of sulfur trioxide present in the flue gases themselves), jeopardizing however the energy saving and the efficiency of the boiler operation. Unfortunately, the sulfuric acid corrosion is not only problem affecting the rotary air preheaters. Ash and unburned carbon particles deposited on the heat exchanger plates causes clogging of the channel between these plates. This phenomenon reduces the heat transmission quality. It is necessary to coat with a protective coating to prevent fouling of heat exchanger plates. Corroded heat exchanger plates should be changed as new ones. Avoid the frequent replacement of the heat exchanger plates operating at low temperatures, it means not only to save the direct cost due to the supply of new plates but also to avoid the others direct costs involved with the shutdown of the unit for the plates replacement. Increase of the operating life and the delay of plates replacement, has forced the technology to target a very well selected materials to be used in the manufacture of heat exchanger plates. For example, for the "hot" air preheaters layers (those still operating above the dew point), carbon steel is used for the exchange plates; this material is significantly corroded by sulfuric acid but the working temperature to which it is submitted, generally does not permit the acid condensation and the corrosion is quite limited. For the "intermediate" air preheaters layers (where they exist), either the carbon steel or the corten steel are used. Corten is a steel similar to the carbon steel where small amount of copper has been added. This element gives some resistance to the corrosion of the sulfuric acid. For the "cold" air preheaters layers (those that theoretically should work to the limit of the acid temperature condensation or below it), enameled steel is use for the heat exchange due sulfuric acid corrosion. Enamel coatings can be used as a successful protective coating for heat exchanger plates thanks to their high wear resistance, hardness, heat transmission coefficient and sulphuric acid resistance. Enamel is a material, which can be produced by mixture of frits and other constituents. Enamels main constituent is silica (SiO2). However, silica cannot be used as its original state due to its high melting point and thermal expansion coefficient. Also silica (SiO2) adhere to steel very poorly. Therefore, silica (SiO2) must be modified by adding various constituents in order to obtain an enamel. These various constituents can be categorized as refractories, fluxes, adhesion agents, opacifiers and coloring agents. Refractories are added to mixture of enamel to give amorphous structure and improve the mechanical strength. Alumina can be given as an example of refractories. Alumina (Al2O3) increases the hardness of the enamel. In addition, alumina (Al2O3) increases chemical and abrasion resistance of the enamel. Fluxes are added to mixture of enamel to reduce the melting and firing point of the enamel. Borax (Na2B4O7) and alkaline oxides such as oxides of sodium (Na2O), potassium (K2O), lithium (Li2O), calcium (CaO), magnesium (MgO) can be given as the examples of fluxes. Adhesion agents are added to mixture of enamel to ensure good adhesion steel/enamel interface. Adhesion agents are always added to ground-coat enamel. Nickel oxide (NiO), molybdenum oxide (MoO), cobalt oxide (CoO), copper oxide (CuO) can be given as the examples of adhesion agents. For an aesthetic appearance, opacifiers and colouring agents are added to mixture of enamel. Titanium dioxides (TiO2), antimony oxide (Sb2O5), zirconium oxide (ZrO2), tin oxide (SnO) are some of the examples of opacifiers and colouring agents. The heat exchanger plates used in thermal power plants, the problems encountered in these plates, enamel coating structure, enamelling types and enamel coating process were given in the literature section of the thesis. In the experimental studies section, surfaces of low carbon steel plates cleaned and thus they became ready for enameling. In addition, low carbon steel plates were characterized by determining chemical composition, and investigation of surface roughness, contact angle and the hardness values. After the characterization of substrate materials finished, the enamel coating process started. As a first step in the enamel coating process, the enamel mixtures were prepared for wet and dry enamel coating methods. For wet enamel coating method, frits and other additives are mixed with water to obtain enamel slurry. For dry enamel coating method, frits mixed only with silicon oil to prevent powder agglomeration. The prepared wet enamel was applied on steel substrate by using air-assisted spraying method. The prepared powder enamel was applied on steel substrate by using electrostatic powder spraying method. For each method, enamel coating thickness value was 150 ± 10 µm and 250 ± 10 µm. After the coating process, enamel coated samples were fired. Enamel coatings were characterized by using X-ray diffraction (XRD), optical microscope, scanning electron microscope (SEM). Bubble structure of enamel coating was examined by using optical microscope. Steel/enamel interface was examined by using scanning electron microscope (SEM). Hardness and surface roughness values of the enamel coatings were determined. In addition, testing apparatus were prepared for impact and sulfuric acid corrosion resistance tests. Adhesion strength of the enamel coatings examined by using impact test equipment and micro scratch tester. Adhesion strength, thermal shock resistance and sulfuric acid corrosion resistance of the enamel coatings were investigated depending on the coating method and thickness. In addition, the coating methods were compared for the same thickness. As a result of this study, it was observed that enamel obtained by wet enamel coating method (air-assisted spraying) contains higher number of bubble in their structure compared to the enamel obtained by dry enamel coating method (electrostatic powder spraying). It was also found that bubble formation increases with increase in coating thickness. In addition, it was found that the most successful enamel coated samples were the ones which obtained by wet enamel coating method (air-assisted spraying) and 150 ± 10 µm thickness. This enamel coating has great sulfuric acid corrosion resistance and high adhesion strength. Moreover, it was understood that the hardness and sulfuric acid corrosion resistance of the steel surface could be improved with enamel coating.