Article pubs.acs.org/EF
High-Temperature Corrosion Properties of Boiler Steels under a Simulated High-Chlorine Coal-Firing Atmosphere Yacheng Liu,† Weidong Fan,*,† Xiang Zhang,‡ and Xiaojiang Wu‡ †
School of Mechanical and Power Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China Shanghai Boiler Works, Limited, Shanghai 200245, People’s Republic of China
‡
ABSTRACT: High-temperature corrosion of the superheater and reheater tubes in utility boilers frequently occurs upon firing high-chlorine coal, which severely impacts the safety of the boiler operation. In this work, the corrosion mechanism of several commonly used boiler steels, including T91, 12Cr1MoVG, and TP347H, was investigated. First, the types and amounts of the main corrosive elements in the flue gas and the heating surface deposits from high- and low-chlorine coals were predicted with the aid of thermodynamic equilibrium modeling. The main corrosive elements that vaporized and subsequently condensed from the flue gas were determined to be Na, Cl, and S. Varying the wall temperature of the heating surface did not dramatically change the forms of these elements but slightly influenced their amounts. Second, lab-scale experiments were conducted in a tube furnace to estimate the degree of corrosion of various boiler steels under simulated conditions upon firing high- and low-chlorine coals. A sophisticated wall temperature control method was employed to account for the influence of different heating tube surface temperatures (560 and 610 °C) on corrosion at the same simulated flue gas temperature (950 °C) around the tube. The results show that more severe material degradation is caused by firing high-chlorine coal together with a higher tube wall temperature and the resistance to corrosion was highest for the austenitic steel TP347H, which had the highest chromium and nickel contents.
1. INTRODUCTION Recently, a substantial reserve of Sha Erhu (SEH) coal, which is characterized by high levels of chlorine and alkali metals, was located in the Xinjiang district of China. Chloride ions are strongly oxidizing and have a low activation energy, and most metal chlorides have low melting points and easily deliquesce.1 Thus, chlorine-induced high-temperature corrosion frequently occurs in the superheater and reheater, which can cause severe boiler tube rupture. In light of these restrictions on the use of high-chlorine coal, investigation of the characteristics of chlorine-induced corrosion is imperative for exploiting these abundant reserves. In addition to corrosive elements in the flue gas, ash and minor salt deposits on the boiler metal surface can also accelerate corrosion. The consequences of corrosion include not only a decrease in the effective thickness of the heating tube surface but also the loss of heat transfer as a result of a buildup of corrosion products and deposition on the surface of the tubes. As the main element implicated in hightemperature corrosion, coals that contain more than 0.3% chlorine on a dry coal basis are generally accepted to be potentially corrosive to pulverized coal boiler units. Substantial chlorine-related research2,3 has indicated that Cl can form gaseous hydrochloric acid (HCl) or volatile alkali chlorides that react with boiler metal surfaces, and the presence of inorganic trace elements in the flue gas was verified by molecular beam mass spectrometry. Chou et al.4 conducted high-temperature chlorine corrosion experiments in a pilotscale furnace for 1000 h, and their measurements revealed different corrosion rates for coals with the same chlorine content but varying ash content, and a higher ratio of volatile NaCl versus HCl following Cl partitioning was inferred to cause more severe corrosion during pulverized-coal combus© XXXX American Chemical Society
tion. In addition, Cl as the main contaminant contributes to forming molten or partially fused deposits on the convection heating surfaces, which can boost the rate of ash deposition and the potential for corrosion. However, research on Cl-related high-temperature corrosion5,6 has primarily focused on biomass combustion, which involves a relatively lower metal temperature in the convection zone compared to the combustion of coal. Furthermore, the proportion of ash and sulfur in most biofuels is lower than in coal, while the potassium content is typically higher. That is, potassium is the dominant alkali metal element in biofuels, whereas sodium is the dominant element in coal. In light of these distinctly different influences on corrosion, investigation of the high-temperature corrosion characteristics of the metal tubes of the convective heaters in the supercritical or ultra-supercritical utility boilers upon firing high-chlorine coal is meaningful. Many laboratory-scale tests for investigating Cl-related high-temperature corrosion have been conducted under isothermal conditions, and synthetic ash deposition has been applied to a steel sample.7,8 However, in actual boiler environments, the steel material and the real ash deposition are exposed to a steeper temperature gradient across the hot flue gas, the deposit layer, and the tube steel wall to the steam in the heating tube. Literature reports have shown that the thermal gradient from the flue gas to the metal surface greatly impacts corrosion rates.9,10 Lindberg et al.9 developed a laboratory test setup for studying the corrosion of superheater materials that accounts for the temperature gradient. However, the main focus of their research was on the composition and Received: October 24, 2016 Revised: March 2, 2017 Published: March 15, 2017 A
DOI: 10.1021/acs.energyfuels.6b02755 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels morphology of synthetic ash deposits. The alkali chlorides evaporated from hotter particles in the deposit and condensed on colder particles closer to the cooled metal surface or even on the metal surface itself. The formation of a partially or completely molten layer in the outer, hotter region closer to the flue gas was also observed in the experiments, and the presence of a molten phase at the alloy/deposit interface indicated that severe corrosion had occurred. To study the high-temperature corrosion performance of boiler steels, several new technologies have been adopted, such as electrochemical noise techniques11 and an online corrosion probe measurement device, which measures the resistance between the steel surface and the formed ionic layer and relates the measured signal to the corrosion rate to qualitatively describe the steel loss over time.12,13 Uusitalo et al.14 performed high-temperature corrosion tests on ferritic steel and austenitic stainless steel with various coatings or no coating, and the results indicated that the corrosive attack proceeded through the formation of alkali chromate and was severe as a result of active oxidation. Decreasing the steam temperature may be an effective countermeasure, but it reduces the electrical efficiency of the power plant. Hence, materials with increased corrosion resistance are urgently needed for combustion of high-chlorine coal. The aim of this study was to evaluate the high-temperature corrosion characteristics of boiler steels (T91, 12Cr1MoVG, and TP347H) in the convection zone of a utility boiler from the firing of high-chlorine coal (SEH coal). A sophisticated wall temperature control method was used in the lab-scale corrosion experiments, and a tube furnace was also employed to simulate the boiler conditions, which are characterized by a temperature gradient at the metal tube in the presence of corrosive gases and ashes. The corrosion tests were conducted on the three metal tube materials at a fixed flue gas temperature (950 °C), and the influence of two metal temperatures of 560 and 610 °C was investigated. The types and amounts of corrosive elements formed in the flue gas and in the heating surface deposits from firing high-chlorine coal were predicted with the aid of thermodynamic equilibrium modeling. The predicted gas composition at the flue gas temperature was used as the simulated flue gas atmosphere in the corrosion test. Meanwhile, the same experiments were also performed with Da Nanhu (DNH) coal, which is characterized by low chlorine and high sulfur contents, to compare the corrosive mechanism with SEH coal.
Table 1. Compositions of the Two Fuels Used in the Thermodynamic Equilibrium Modeling coal moisture (wt %, ad) ash content (wt %, ar) ultimate analysis (wt %, daf) C H O N S Cl in dry ash (mol/kg) Si Al Fe Ca Mg Na K Ti S Mn
SEH coal
DNH coal
23.93 8.05
20.84 17.2
73.37 3.12 21.51 1.06 0.33 0.61
80.99 3.11 13.76 0.95 1.14 0.05
0.2695 0.1691 0.0127 0.5669 0.2952 0.0898 0.0099 0.0141 0.0628
1.1082 0.6282 0.1062 0.5111 0.2868 0.2362 0.0391 0.0214 0.1629 0.0029
condensed-phase compositions, and the condensed phase was thought to have a higher melting point and be less corrosive. Thus, the condensed phase in the first-step calculation was removed from the input data for the next calculation. For the second step, the equilibrium gas phase at 950 °C was applied as the input to simulate the formation of ash on the wall from the gas phase. The equilibrium was calculated at 560 and 610 °C as an approximation of the hightemperature heating surface. 2.2. Experimental Section. 2.2.1. Material Data and Ash Deposition. In the present study, the corrosion resistance of three different boiler steel materials was investigated using a hightemperature corrosion test. The chemical compositions of the three steels are given in Table 2. Material A is a martensitic steel T91 alloy consisting of 9.5% chromium and 1.05% molybdenum, which improves the strength by inhibiting creep, pitting, and crevice corrosion. Pearlitic steel B, 12Cr1MoVG, contains small amounts of alloying elements, such as chromium (1.2%), and from macroscopic observation, an effective protective oxide layer is not built up at room temperature. A similar steel surface is observed for T91, but for material C, a better oxide layer is formed that can resist oxidation. The austenitic steel TP347H (C) has a high chromium content of 18.64% in addition to 11.12% Ni, which contributes to a high resistance to corrosion in a reducing atmosphere and stress corrosion cracking. Furthermore, Nb is added to increase the resistance to various corrosive attacks, such as intergranular attack.16 Ash samples were procured according to the standard ash preparation method, and the raw coal was ground to pulverized coal with a particle size of less than 70 μm. The predicted equilibrium results for firing SEH coal indicate that a high content of vaporized sodium chloride exists in the flue gas, which then condenses on the metal surface and causes high-temperature corrosion. Thus, to preserve a high level of sodium in the coal ash, the coal samples were ashed at 500 °C in a muffle furnace for 72 h. Meanwhile, the effects of other crystalline phases in the coal ash on the corrosion process could be considered in comparison to synthetic ash deposition. During the corrosion experiments, ash deposition was divided into two layers under the effect of the thermal gradient, in which the inner layer was the condensed phase. 2.2.2. High-Temperature Corrosion Testing. Lab-scale corrosion tests were performed in a horizontal tube furnace equipped with an alundum tube (shown in Figure 1), which is the same as the setup used by Li et al.17 The experimental system was primarily comprised of three parts: preparation of simulated flue gas, sophisticated metal wall
2. RESEARCH METHODS 2.1. Modeling Procedures. The thermodynamic software package HSC Chemistry, version 6.0, was used for the modeling. The thermodynamic equilibrium calculation was based on the principle of minimum Gibbs free energy. All compounds in the database were considered except for nitrogen, where only N2 was considered because thermodynamic equilibrium modeling does not satisfactorily predict nitrogen chemistry in combustion conditions.15 For the thermodynamic equilibrium modeling, a two-step calculation procedure was employed to predict the types and amounts of corrosive elements formed in the flue gas and in the heating surface deposits from high- and low-chlorine coal. In the first step, the required amount of air for SEH and DNH coal was calculated for an excessive air ratio of 1.2. The calculated air amount was then input into the software together with all compositions listed in Table 1 for SEH and DNH coal. The chemical equilibrium was calculated at 950 °C because this is a reasonable temperature for the flue gas in the horizontal flue of a utility boiler. The equilibrium results included gas-phase and B
DOI: 10.1021/acs.energyfuels.6b02755 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 2. Compositions of the Three Materials Investigated (in wt %) position
steel type
C
Si
Mn
Cr
Ni
V
Mo
A B C
T91 12Cr1MoVG TP347H
0.08−0.12 0.08−0.15 0.04−0.10
0.02−0.5 0.17−0.37
