by Dr Juan Carlos Nava
The widespread implementation of waste-to-energy (WTE) systems has been limited in principle by the lower efficiency of current system designs.1 Higher efficiencies can be achieved by operating at higher temperatures, which in turn translates into lower emissions and a more effective combustion of municipal solid waste (MSW). However, the major limitation to higher operating temperatures is the corrosion rates experienced by the heat transfer surfaces in the furnace cavity and in recovery convective paths.1
High-temperature chloridation and hot corrosion are the primary modes of component degradation in WTE systems.1–4 Chlorine-containing compounds in chlorinated plastics and in paper, cardboard and wood products generate flue gases with a relatively high HCl(g) content. The poor combustion characteristics of the MSW fuel result in incomplete combustion, leading to the condensation on heat transfer surfaces of aggressive deposits rich in alkali metals (Na, K) and heavy metals such as lead, tin and zinc. The alkali and heavy metals are condensed primarily in the form of sulphates and chlorides which melt at relatively low temperatures causing hot corrosion attack.
Low alloy steels are the preferred choice for heat transfer components in WTE boilers because of their affordability, excellent heat transfer capabilities, and adequate mechanical properties within the operating temperature range of the steam generating unit. However, these iron-based alloys are highly susceptible to high-temperature chloridation by flue gases containing HCl(g), resulting in the formation of low vapour pressure chlorides of iron (i.e. FeCl2 and FeCl3), with vapour pressures in the order of 1 x 10-4 torr at temperatures as low as 300ºC (578ºF), resulting in turn in the volatilization of the reaction product. The iron chloride vapour is then oxidized to iron oxide; free chlorine is pushed back to the metal surface due to thermophoresis, a transport mechanism by which gases are delivered to the metal surface under heat flux conditions, i.e. a hot gas and a cooler heat transfer surface. This oxidation mechanism is known as ‘active oxidation’,3 a self-supported oxidation process where the iron-based low alloy steel is rapidly wasted away due to the continuous reaction of chlorine with the metal surface.
Combating high-temperature chloridation
Surface modification technologies including thermal spray coatings, weld overlays and diffusion layers represent an option to plant operators to manage the accelerated wastage of low alloy steels due to high-temperature chloridation attack. In particular, thermal spray coatings represent a reliable and cost-effective approach to upgrade the metal component surface by adding effective alloying elements such as chromium at concentrations not practical in wrought or cast alloys.
Of these surface technologies, twin wire arc spray (TWAS) is the fastest application method and, under well-defined quality controls, can generate protective surface layers with reasonable life spans. Through its patent pending consumable manufacturing process, ArcMeltTM Company is capable of producing any possible alloy composition by the use of powder core wire technology. ArcMeltTM core wire consumables can be sprayed significantly faster when using a slightly modified spray gun and wire delivery system. This results in well-adhered coated layers with porosity levels below 5% and finely distributed oxides that impart lower build-up stresses. Improved surface coverage translates into shorter application times, meeting the most stringent outage schedules.
ArcMeltTM produces a composition marketed as AMC 3201 with 42% Cr- 8% Fe- Ni-balance. This chemical composition is similar to alloy type 45CT. To understand the merits of this composition in waste-to-energy applications we need to understand the fundamental process of alloy protection in high-temperature chloridating environments. Figure 1 shows the relative stability of metal chlorides in the temperature range 200–550ºC (400–1000ºF). This Ellingham diagram reveals that the most stable chlorides are those of chromium.
But in high-temperature chloridation, what matters is the volatility of the metal chlorides, and in this scenario, the important parameter is the vapour pressure of the chloride phase as a function of temperature. High vapour pressure compounds are usually those with low melting points. Iron chlorides are metal chlorides with the lowest melting points, i.e. 282ºC (539ºF) and 350ºC (662ºF) for FeCl2 and FeCl3, respectively. The stable chlorides of chromium and nickel, CrCl2 and NiCl2, have higher melting points, 540ºC (1000ºF) and 728ºC (1342ºF), respectively, with vapour pressures in excess of 1 x 10-4 torr at temperatures above 450ºC (842ºF), conferring a tremendous advantage compared with iron-based alloys.
Resistance to oxidation
The environments generated during the incineration of waste are not only potentially chloridating but also oxidizing. Table 1 lists the typical flue gas composition in WTE units as reported in the literature.
The flue gases generated during the combustion of MWS are also oxidizing with SO2(g) concentrations varying from 300 to 600 ppm in the presence of excess air (in the order of 7 v%). Thermodynamic calculations using the chlorine and oxygen indicators, log PCl2 and log PO2, respectively, for the gas compositions listed in Table 1 indicate that the formation of protective chromium-rich oxides.
The thermodynamic tendency to form this stable chromium oxide is what provides the resistance of this high-temperature resistant AMC 3201 alloy against most of the high-temperature oxidation phenomena, including high-temperature chloridation attack.
Upon exposure to the flue gases typical of WTE environments, the relatively low porosity of the coating structure will most likely be sealed by the formation of chromium-rich oxides as indicated by thermodynamic analyses and demonstrated in the scanning electron micrograph shown in Figure 2.
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| Figure 2. Oxidation resistance of ACM 3201 in still air at 816ºC (1500ºF) |
Sulphur dioxide poses a further problem
Additionally, the presence of SO2(g) in the flue gas can also react with sodium/potassium chlorides leading to the formation of the respective sulphate. The formation of alkali sulphates exacerbates wastage rates due of their tendency to react with residual alkali and heavy metals chlorides to form a low melting eutectic phase. This eutectic phase is highly ionic and it dissolves most of the protective oxides including chromia. However, the dissolution of chromia in sulphatic melts neutralizes the ionic character of the flux through the formation of alkali chromates arresting the wastage process.
Conclusion
Any material designed for long-term resistance to all possible scenarios of high-temperature oxidation in waste-to-energy applications needs to be such that long-term protection relies on the formation of protective chromium-rich oxides. Alloy formulation AMC 3201 has been designed with this purpose based on an understanding of all plausible environments generated during the incineration of municipal waste.
This article was prepared by Dr Juan Carlos Nava, DBA ME Technical Services, San Diego CA, on behalf of ArcMeltTM Company L.C. a subsidiary of CIC Group, Inc.
email: yj48cm@yahoo.com
web: www.arcmelt.com
References
1. B.A. Baker, G.D. Smith, L.E. Shoemaker, Performance of commercial alloys in simulated waste incineration environments.
2. Yuuzou Kawahara, Application of high temperature corrosion-resistant materials and coatings under severe corrosive environment in waste-to-energy boilers, Journal of Thermal Spray Technology, Volume 6, Number 2, 2007, pp. 202–213.?
3. Shang-Hsiu Lee, Nickolas J. Themelis, Marco J. Castaldi, High-temperature corrosion in waste-to-energy boilers, Journal of Thermal Spray Technology, Volume 16, Number 1, 2007, pp. 104–110.
4. D.O. Albina, K. Millrath, N.J. Themelis, ‘Effects of feed composition on boiler corrosion in waste-to-energy plants’, 12th North American Waste to Energy Conference (NAWTEC 12).
