Zhengjian Liu

Stress corrosion cracking of 2205 duplex stainless steel in H2S-CO2 environment

Stress corrosion cracking (SCC) behavior of 2205 duplex stainless steel (DSS) in H2S–CO2 environment was investigated by electrochemical measurements, slow strain rate test (SSRT), and scanning electron microscopy (SEM) characterization. Results demonstrated that the passive current density of steel increases with the decrease of solution pH and the presence of CO2. When solutions pH was 2.7, the steel SCC in the absence and presence of CO2 is expected to be a hydrogen-based process, i.e., hydrogen-induced cracking (HIC) dominates the SCC of the steel. The presence of CO2 in solution does not affect the fracture mechanism. However, the SCC susceptibility is enhanced when the solution is saturated simultaneously with H2S and CO2. With elevation of solution pH to 4.5, the hydrogen evolution is inhibited, and dissolution is involved in cracking process. Even in the presence of CO2, the additional cathodic reduction of H2CO3 would enhance the anodic reaction rate. Therefore, in addition to the hydrogen effect, anodic dissolution plays an important role in SCC of duplex stainless steel at solution pH of 4.5.

Formation mechanism of the protective layer in a blast furnace hearth

A variety of techniques, such as chemical analysis, scanning electron microscopy−energy dispersive spectroscopy, and X-ray diffraction, were applied to characterize the adhesion protective layer formed below the blast furnace taphole level when a certain amount of titanium- bearing burden was used. Samples of the protective layer were extracted to identify the chemical composition, phase assemblage, and distribution. Furthermore, the formation mechanism of the protective layer was determined after clarifying the source of each component. Finally, a technical strategy was proposed for achieving a stable protective layer in the hearth. The results show that the protective layer mainly exists in a bilayer form in the sidewall, namely, a titanium-bearing layer and a graphite layer. Both the layers contain the slag phase whose major crystalline phase is magnesium melilite (Ca2MgSi2O7) and the main source of the slag phase is coke ash. It is clearly determined that solid particles such as graphite, Ti(C,N) and MgAl2O4 play an important role in the formation of the protective layer, and the key factor for promoting the formation of a stable protective layer is reasonable control of the evolution behavior of coke.

Zinc Accumulation and Behavior in Tuyere Coke

A case study of zinc oxide, which represents the first report on the occurrence, crystalline features, formation mechanism, and influence of this mineral in tuyere coke, was conducted in this study. A number of zinc oxides, some of which were in hexagonal wurtzite habit, were observed to distribute mainly in coke pores, cracks, surfaces, and around coke minerals. The accumulation of zinc in tuyere coke may enhance the degradation of coke and increase the generation and accumulation of coke fine in a blast furnace, which would cause bad effect on blast furnace operation. Investigations into zinc behavior in tuyere coke can be important for further interpretations of coke degradation in the high temperature zone of a blast furnace.

Reduction mechanisms of pyrite cinder-carbon composite pellets

The non-isothermal reduction mechanisms of pyrite cinder-carbon composite pellets were studied at laboratory scale under argon (Ar) atmosphere. The composite pellets as well as the specimens of separate layers containing pyrite cinder and coal were tested. The degree of reduction was measured by mass loss. The microstructures of the reduced composite pellets were characterized by scanning electron microscopy (SEM). It is found that the reduction processes of the composite pellets may be divided into four stages: reduction via CO and H2 from volatiles in coal at 673–973 K, reduction via H2 and C produced by cracking of hydrocarbon at 973–1123 K, direct reduction by carbon via gaseous intermediates at 1123–1323 K, and direct reduction by carbon at above 1323 K. Corresponding to the four stages, the apparent activation energies (E) for the reduction of the composite pellets are 86.26, 78.54, 72.01, and 203.65 kJ·mol−1, respectively.

Interfaces Between Coke, Slag, and Metal in the Tuyere Level of a Blast Furnace

An in-depth understanding about the reactions in the high-temperature zone of a blast furnace is significant to optimize both the current and future blast furnace process. The interfaces between coke, slag, and metal were observed using scanning electronic microscope with samples obtained from the tuyere level of a blast furnace. Two types of slag phases were identified, one originating from coke ash and the other from the bosh slag. Slag formed by coke ash was seen to cover the coke surface, which may hinder the reaction of coke with both gas and liquid iron. The reduction of FeO from the bosh slag (originated from the primary slag) occurs in the coke/slag interface with the reduced iron forming a metal layer surrounding the coke surface. The reduction of SiO2 occurs both in and outside the coke, and the reduced silicon reacts with iron to form iron silicide if the two species come into contact. Further study is proposed based on the results of this study.

Devolatilization Characteristics and Kinetic Analysis of Lump Coal from China COREX3000 Under High Temperature

A devolatilization study of two lump coals used in China COREX3000 was carried out in a self-developed thermo-gravimetry at four temperature conditions [1173 K, 1273 K, 1373 K, and 1473 K (900 °C, 1000 °C, 1100 °C, and 1200 °C)] under N2. This study reveals that the working temperature has a strong impact on the devolatilization rate of the lump coal: the reaction rate increases with the increasing temperature. However, the temperature has little influence on the maximum mass loss. The conversion rate curve shows that the reaction rate of HY lump coal is higher than KG lump coal. The lump coals were analyzed by XRD, FTIR, and optical microscopy to explore the correlation between devolatilization rate and properties of lump coal. The results show that the higher reaction rate of HY lump coal attributes to its more active maceral components, less aromaticity and orientation degree of the crystallite, and more oxygenated functional groups. The random nucleation and nuclei growth model (RNGM), volume model (VM), and unreacted shrinking core model (URCM) were employed to describe the reaction behavior of lump coal. It was concluded from kinetics analysis that RNGM model was the best model for describing the devolatilization of lump coals. The apparent activation energies of isothermal devolatilization of HY lump coal and KG lump coal are 42.35 and 45.83 kJ/mol, respectively. This study has implications for the characteristics and mechanism modeling of devolatilization of lump coal in COREX gasifier.

Formation mechanism of the graphite-rich protective layer in blast furnace hearths

A long campaign life of blast furnaces is heavily linked to the existence of a protective layer in their hearths. In this work, we conducted dissection studies and investigated damage in blast furnace hearths to estimate the formation mechanism of the protective layer. The results illustrate that a significant amount of graphite phase was trapped within the hearth protective layer. Furthermore, on the basis of the thermodynamic and kinetic calculations of the graphite precipitation process, a precipitation potential index related to the formation of the graphite-rich protective layer was proposed to characterize the formation ability of this layer. We determined that, under normal operating conditions, the precipitation of graphite phase from hot metal was thermodynamically possible. Among elements that exist in hot metal, C, Si, and P favor graphite precipitation, whereas Mn and Cr inhibit this process. Moreover, at the same hot-face temperature, an increase of carbon concentration in hot metal can shorten the precipitation time. Finally, the results suggest that measures such as reducing the hot-face temperature and increasing the degree of carbon saturation in hot metal are critically important to improve the precipitation potential index.

Corrosion Behavior of E690 High-Strength Steel in Alternating Wet-Dry Marine Environment with Different pH Values

The corrosion behavior and mechanism of E690 high-strength steel in marine environment with different pH values were studied through electrochemical technology and long-term alternating wet-dry cycle experiments combined with SEM and XRD. Results showed that the corrosion current density of E690 high-strength steel gradually increased with decreased pH. After long-term tests in alternating wet-dry marine environment with various pH values, uniform corrosion mainly occurred on E690 steel, accompanied by vast corrosion pitting. Weight loss analysis demonstrated that corrosion rate decreased with increased pH. Moreover, corrosion mechanism varied with pH, and hydrogen-evolution reaction greatly increased the E690 steel corrosion rate at low pH. Meanwhile, the compositions of corrosion products slightly differed with pH; these products consisted of Fe3O4, Fe2O3, α-FeOOH, β-FeOOH, γ-FeOOH, and amorphous substances. However, the rust-layer density varied. Cr in the rust layer promoted the densification of rust layer and improved the decay resistance of E690 steel.

Direct reduction of iron ore by biomass char

By using thermogravimetric analysis the process and mechanism of iron ore reduced by biomass char were investigated and compared with those reduced by coal and coke. It is found that biomass char has a higher reactivity. The increase of carbon-to-oxygen mole ratio (C/O) can lead to the enhancement of reaction rate and reduction fraction, but cannot change the temperature and trend of each reaction. The reaction temperature of hematite reduced by biomass char is at least 100 K lower than that reduced by coal and coke, the maximum reaction rate is 1.57 times as high as that of coal, and the final reaction fraction is much higher. Model calculation indicates that the use of burden composed of biomass char and iron ore for blast furnaces can probably decrease the temperature of the thermal reserve zone and reduce the CO equilibrium concentration.

Graphitization of Coke and Its Interaction with Slag in the Hearth of a Blast Furnace

Coke reaction behavior in the blast furnace hearth has yet to be fully understood due to limited access to the high temperature zone. The graphitization of coke and its interaction with slag in the hearth of blast furnace were investigated with samples obtained from the center of the deadman of a blast furnace during its overhaul period. All hearth coke samples from fines to lumps were confirmed to be highly graphitized, and the graphitization of coke in the high temperature zone was convinced to start from the coke surface and lead to the formation of coke fines. It will be essential to perform further comprehensive investigations on graphite formation and its evolution in a coke as well as its multi-effect on blast furnace performance. The porous hearth cokes were found to be filled up with final slag. Further research is required about the capability of coke to fill final slag and the attack of final slag on the hearth bottom refractories since this might be a new degradation mechanism of refractories located in the hearth bottom.