An anti-fouling and corrosion resistant ceramic coating for heat exchanger tubes

Corrosion of boiler tubes remains an operational and economic constraint for example in Waste-to-Energy (WtE) facilities. This article presents an innovative anti-fouling and highly corrosion resistant ceramic coating that has been developed by the Tubacex Group for application to the external and internal surfaces of tubes frequently used in the heat process industries.

Corrosion and fouling of boilers or heat exchange systems can cause significant economic penalties and it is estimated that those problems cost industries billions of dollars per year. Therefore, minimizing operating costs and maintaining equipment reliability are primary goals in today's difficult economic climate. Heat exchangers are used to recover sensible heat from process streams (reactors, distillation columns etc.) to preheat the feedstock and minimize the external energy demand to provide the required heat for the process. Fouling in the exchanger train and the related reduction of heat transfer can cause significant energy loss and increase operating costs. Fouling can also become so severe that unit capacity and production limits are reached. Typically, units are then shutdown incurring high maintenance costs and production losses, as well as increased environmental and safety concerns.

Similarly, corrosion of boiler tubes remains an operational and economic constraint for example in Waste-to-Energy (WtE) facilities. The formation of low melting temperature salts of chloride and sulphate mixture react and dissolve the protective oxide films on the metal surface of the equipment (e.g. tubes). Current methods of protection remain costly: refractory lining of waterwall tubes, highly alloyed corrosion resistant materials or use of surfaces coating such as weld overlays of nickel-chromium based alloys have become popular.

Figure 1. Tubacoat ceramic coating applied on tube size ranging from 20 to 150 mm diameter

Among the various corrosion resistant and anti-fouling materials and coating systems used in the industries, ceramic coatings have the advantages of chemical inertness, high temperature stability, and superior mechanical properties compared to the other ones.

Additionally, they can demonstrate a strong bonding to the substrate they are deposited onto and be applied in thin layers will minimize the thermal insulating effect. Finally, glass-based ceramic coatings show very smooth surface states (roughness Ra <0.04 um), which play an important role against the formation of scale (e.g. adherence of ashes or coke).

The Tubacoat concept is based on the utilization of a silica-based ceramic coating that can be applied inside and outside of long and narrow metallic tubes of different substrate types (preferentially stainless steel). The process is comparable to a sintering process that has to be carried out under controlled temperature and atmosphere in a furnace.

The major Tubacoat properties are:

1. Microstructure, Morphology & Surface state
A ceramic slurry (suspension) is applied on the tube inner and/or outer surface before the sintering process. Suspension parameters and rheological properties are controlled to ensure a continuous and well spread on the steel surface. As a result, a very low coating roughness prevents from the settlement of particles onto the coating surface (fouling).

2. Thermal conductivity, Adhesion and Thermal Shock Properties
Thermal conductivity measurements and thermal shocks tests has been performed. No cracks in the coating, coating detachment or interlayer at the coatingsteel interface was observed, which suggests that the coating has a thermal expansion coefficient similar to that of the steel and had a strong chemical bond with the steel. Furthermore, and due to the thin ceramic layer applied the heat transfer is marginally affected through the ceramic coating.

3.Corrosion resistance
The glass-ceramic coatings possess superior coating properties as compared to conventional vitreous enamel.

Therefore, Tubacoat can be recommended as a highly corrosion resistant vitreous coating on steel tubes to transform it into a superior material of construction for power generation, chemical, oil, gas and allied industries.

Figure 2. Tubacoat typical thickness range: 100-150 μm and surface roughness Ra<0.04 μm

Industrial Applications

Energy from Waste
Energy conservation and environmental protection are currently very important issues worldwide.

Municipal solid waste (MSW) can be converted to an eco-friendly renewable resource that not only produces energy but also significantly reduces the greenhouse gas emissions from landfills. A wasteto - energy (WTE) incineration plant recovers energy from MSW and produces electricity and/or steam for heating. For a WTE plant, the energy efficiency can be improved and CO2 emissions be reduced by improving the heat efficiency of the boiler system because this is the main method for heat transfer. The variability in the heating value of the MSW and the high content of chlorine and light metals contribute to a highly corrosive atmosphere that shorten the life of the heat exchangers tubes used. For example, in the waterwall section, where water is evaporated and, especially in the steam superheater sections of the WTE, where the tube temperature is at its maximum.

As the hot combustion gas flow over the heat transfer surfaces, such as membrane water tubes (waterwall) superheater tube bundles, evaporator tubes and economizer tubes, heat is transferred from the gases to the water vapor within the tubes. The efficiency of conversion steam energy to electricity increases with higher steam temperatures. However, with increasing steam temperature, the heat transfer surfaces are subjected to severe high temperature corrosion, caused by both the metal chlorides in the ash particles deposited on the gas tubes and by the high concentration of HCl in the process. Fig. 4 shows the corrosion and fouling sensitive areas in a WTE facility. The typical working conditions are: TSteam: 300OC, P = 170 bar, TFumes: 850OC, inner media: Steam, Outer media: Alkaline ashes

Figure 4. Tubacoat typical application in Steam Reheater of a Waste-to -Energy Plant

Loss of heat transfer and subsequent charge outlet temperature decrease is a result of the low thermal conductivity of the fouling layer or layers which is generally lower than the thermal conductivity of the fluids or conduction wall. As a result of this lower thermal conductivity, the overall thermal resistance to heat transfer is increased and the effectiveness and thermal efficiency of heat exchangers are reduced.

The mechanism for corrosion is mainly determined by the most abundant deposits observed on the metal after corrosion, i.e. oxidation by metal oxides, sulfidation by metal sulphides, sulfidation/oxidation by mixtures of sulphides and oxides, carburization by metal carbides and chlorination of metals to metal chlorides. In general, there are two principal mechanisms of high temperature corrosion: active oxidation (above 450oC) and corrosion due to deposits by sulphation and by molten salts.

The Tubacoat ceramic coating was applied to the heating surfaces of boiler waterwall tubes of a Waste to Energy Plant in Spain. The effect of the ceramic coating on fouling resistance and energy efficiency was evaluated after 6 months, 1, 3 and 5 years of operation of the boiler. As seen in Fig. 5 a significant difference in the coated and uncoated areas of the boiler surfaces was observed.

The coated surfaces were relatively clean with only small amount of unwanted matters attached on a small part of boiler surface. The thermal efficiency of boilers was calculated before and after applying the composite ceramic coating and the amount of produced steam (production amount). It has been demonstrated that the developed ceramic coating has great potential to be applied in real boiler systems to improve their overall thermal efficiency.

In a crude preheat exchanger system, the hot preheat section is usually the area of greatest concern. The hottest exchangers or exchangers with the highest heatflux typically show the highest fouling rates. This is also the case for the hydrotreater feed/effluent exchangers and the slurry exchangers. fouling can lead to local hot spots resulting ultimately in mechanical failure of the heat transfer surface. Major detrimental effects of fouling include loss of heat transfer as indicated by charge outlet temperature decrease and pressure drop increase. Other detrimental effects of fouling may also include blocked process pipes, under-deposit corrosion and pollution. In addition, increased surface roughness due to fouling will increase frictional resistance to flow. Such effects inevitably lead to an increase in the pressure drop across the heat exchanger, which is required to maintain the flow rate through the exchanger, and may even lead to flow blocks.

Intermediate cleanings represent significant economic penalties, if not planned in the normal refinery operation. increases the fouling or coking tendency of the furnace. Both effects further increase fuel demand and energy costs. Mechanical cleaning, using high pressure jetting, is typical. Opening the equipment also increases the risk of damage and additional repair costs, as well as safety and environmental concerns.

Different fouling deposit structures can lead to under-deposit corrosion of the substrate material such as localised fouling, deposit tubercles and sludge piles. The factors that are most likely to influence the probability of under-deposit corrosion include deposit composition and its porosity and permeability. Even minor components of the deposits can sometimes cause severe corrosion of the underlying metal such as the hot corrosion caused by vanadium in the deposits of fired boilers.

The use of glass-ceramic coated equipment may be of primary importance to their producers, suppliers and consumers as this results in lowering of production costs, increasing productivity, improving reliability and waste reduction.

The coatings can be successfully used for various challenging applications across wide specytrum of industries such as: Power generation boilers (Coal, Biomass, Urban garbage): water walls, steam re-heaters, super heaters and heat recovery bundles, Tubular systems of Molten salt solar power plants, Ash corrosion and fouling in the Oil and Gas Industry(Visbreaker, Coke Unit, Coke Calciner, Heat Recovery Units…), Overhead Sulphur Condensers, Metal dusting avoidance(Syngas, Supercritical CO2, etc.) and Nitric Acid Condensers

As a glass-ceramic coating Tubacoat is not only new generation of coatings but also versatile engineering materials which increase the service life of different types of metallic substrates. They have a very potential and promising market. Among various coating systems for industrial and engineering applications, Tubacoat glass–ceramic coating has advantages that can be summarized as followed:

1. Superior adhesion designed to withstand extreme thermal, physical and chemical conditions within a boiler environment

2. Forming an impervious barrier, preventing corrosive products of combustion from attacking the metal tubes

3. Non-catalytic technology, molten ash will not sinter and adhere to the surface of the coated tube, preventing from the buildup of an insulation layer.

4. The glass-like film alloys for easy cleaning of the boiler tubes, improving heat transfer and maximizing boiler efficiency.