Solutions to Magnesia Brick Degradation in Glass Kiln Regenerators Due to Petroleum Coke Use
In the petrochemical industry, the final residue from crude oil distillation and cracking is petroleum coke, with a carbon content of more than 95% by weight. Its chemical composition (by weight) is as follows: moisture 1.44%, ash 0.16%, carbon 88.87%, hydrogen 3.69%, nitrogen 2.27%, sulfur 0.87%, and oxygen 2.7%. Compared with residual oil, petroleum coke has higher carbon, nitrogen, and oxygen contents, but lower hydrogen content and calorific value.
Petroleum coke has a wide range of applications: approximately 40% is used as an alternative fuel in cement production, 22% as a raw material for carbon materials, 14% as fuel for thermal power generation, 7% as a carburizer in steelmaking, 1% as heating fuel, and 16% for other purposes.
Fuel costs account for about half of the total production cost of glass. Replacing heavy oil with relatively low-cost petroleum coke can significantly reduce production expenses. However, most petroleum coke used in the glass industry is imported, and much of it is low-cost material that cannot be readily utilized in its country of origin and contains high levels of sulfur, vanadium, and other impurities.
After switching from heavy oil to petroleum coke, changes in the thermal system-especially variations in the composition and acidity/alkalinity of furnace slag-have significantly affected the service life of refractory materials, particularly those used in regenerators. The service life of checker bricks has decreased from about 10 years to 2–5 years, or even less than one year.
To extend the service life of checker walls in glass furnace regenerators, studies were conducted on the chemical composition and alkali–sulfur ratio of furnace slag after the use of petroleum coke, the damage mechanisms of magnesia refractories used in regenerator checker walls, and the slag corrosion resistance of direct-bonded magnesia–chrome bricks, fused rebonded magnesia–chrome bricks, and fused rebonded high-purity magnesia–alumina spinel bricks. On this basis, a series of targeted countermeasures were adopted, which significantly prolonged the service life of checker bricks.
1.Chemical Composition of Glass Furnace Slag After Using Petroleum Coke
After a large glass enterprise replaced heavy oil with petroleum coke, chemical analyses were conducted on 14 slag samples collected from nine production lines and five small furnaces. The results are shown in Table 1. Here, the alkali–sulfur ratio refers to the molar ratio of R₂O to SO₃.
When the alkali–sulfur ratio is 1, R₂O reacts with SO₃ to form sulfates. When the ratio is greater than 1, the excess free alkali strongly corrodes aluminosilicate refractories. When the ratio is less than 1, excess SO₃ first reacts with CaO to form CaSO₄, and any additional SO₃ will strongly corrode basic refractories.
It can be seen from Table 1 that the slag has a high V₂O₅ content and that the alkali–sulfur ratio fluctuates widely.
2.Corrosion Study of Slag on Basic Refractories
2.1 Test Procedure
First, the microstructure of used magnesia bricks dismantled from the checker chamber of glass furnace regenerators fired with petroleum coke was analyzed using a scanning electron microscope (SEM) and compared with that of two unused bricks of the same grade to investigate the corrosion mechanism of magnesia bricks.
The average mass fractions of MgO, SiO₂, CaO, and Fe₂O₃ in the two unused magnesia bricks are 95.43%, 1.43%, 1.14%, and 0.76%, respectively. Their average bulk density, apparent porosity, and cold crushing strength are 3.02 g·cm⁻³, 14.2%, and 85 MPa, respectively.
Then, direct-bonded magnesia–chrome bricks, fused rebonded magnesia–chrome bricks, and fused rebonded high-purity magnesia–alumina spinel bricks were selected for slag resistance testing, using a neutral synthetic slag with a chemical composition close to that of Slag No. 6 in Table 1. The physical and chemical properties of the three types of test bricks are listed in Table 2.
The chemical composition (mass fraction) of the test slag is as follows: Al₂O₃ 10%, SiO₂ 30%, CaSO₄ 5%, Na₂SO₄ 30%, K₂SO₄ 2%, CaCO₃ 12%, Fe₂O₃ 7%, NiO 1%, and V₂O₅ 3%.
The three types of test bricks were machined into crucible specimens. Slag was placed inside each crucible, which was then placed in a high-temperature test furnace. The temperature was raised to 1450 ℃ at a rate of 1.5–2.5 ℃·min⁻¹, held at 1450 ℃ for 6 hours, and then allowed to cool naturally after the furnace was shut down.
The crucibles were cut along the axis of the slag cavity to observe the residual slag, as well as the penetration and corrosion of the slag into the crucible.
2.2 Results and Analysis
2.2.1 Microstructural Analysis of Used and Unused Magnesia Bricks
Figure 1 shows backscattered electron (BSE) images of the cross-section of an unused magnesia brick specimen of the same grade as those used in the checker chamber of glass furnace regenerators fired with petroleum coke. Since the brightness of a backscattered electron image is related to the atomic number, the phases in the image can be rapidly identified in combination with electron probe microanalysis (EPMA).
It can be observed that the unused magnesia brick exhibits a structure in which a porous matrix bonds the aggregate particles. The aggregate grains are relatively small (< 1 mm) and contain many pores, suggesting that sintered magnesia was used as the raw material. The main impurity phases in the brick are forsterite (M₂S) and monticellite (CMS), which almost completely enclose the periclase grains. Nevertheless, a direct bond is still barely maintained between the periclase grains.
Figure 1
Figure 2 shows backscattered electron images of a fractured magnesia brick taken from a regenerator under hot service conditions.
In the low-magnification image (Figure 2(a)), the dark gray phase is forsterite (M₂S), the light gray phase is monticellite (CMS), and the darker gray particles are periclase (M).
In the high-magnification image (Figure 2(b)), CVP represents calcium vanadophosphate, NAS represents nepheline, and "Glass" denotes the glassy phase.
It can be observed from Figure 2 that SiO₂ and CaO from carryover materials and ash slag have extensively penetrated into the brick. Under severe corrosion, M₂S (dark gray) and CMS (light gray) surround M (periclase) to form a three-dimensional continuous network, resulting in the complete breakdown of bonding between periclase grains.
Furthermore, the low-melting phases also contain vanadates, which further degrade the high-temperature performance of the magnesia bricks. The softening of these bonding phases at high temperatures consequently leads to the structural collapse of the checker bricks.
Figure 2
2.2.2 Corrosion Resistance of Direct-Bonded Magnesia-Chrome Bricks
After the corrosion test, the macrostructure of the direct-bonded magnesia–chrome brick crucible specimen remained generally intact; however, almost all the molten slag had penetrated into the specimen, leaving very little on the surface. Examination of the cut crucible cross-section revealed a large amount of yellowish-green material. This is attributed to sulfates penetrating the specimen interior and decomposing into SO₃ and R₂O, with R₂O reacting with Cr₂O₃ to form hexavalent chromium compounds.
The backscattered electron image near the corroded layer of the specimen cross-section is shown in Figure 3. Electron probe microanalysis of selected areas indicates the following:
Zone 1: sodium-calcium aluminosilicate glass phase and magnesia–chrome spinel
Zone 2: sodium aluminosilicate glass phase and magnesia–chrome spinel
Zone 3: periclase solid solution containing chromium and iron
Zone 4: magnesia–alumina–chrome spinel with a small amount of iron
Zone 5: periclase and a small amount of forsterite
Zone 6: periclase
These results indicate that the microstructure near the inner surface of the crucible underwent significant changes, mainly the complete reaction of chromite to form magnesia–chrome spinel solid solution and the penetration of the glass phase. In the deeper regions, the damage was primarily caused by the penetration of SiO₂.
Figure 3
2.2.3 Corrosion Resistance of Fused Rebonded Magnesia-Chrome Bricks
After the corrosion test, the macrostructure of the fused rebonded magnesia–chrome brick crucible specimen remained intact. A large amount of molten slag remained inside the crucible, while only a small portion penetrated into the specimen. No yellowish-green material was observed on the cut cross-section of the crucible, indicating that fused rebonded magnesia–chrome bricks exhibit excellent slag corrosion resistance.
Figure 4 shows the backscattered electron image near the slag–refractory interface on the specimen cross-section. Electron probe microanalysis of selected areas reveals the following:
Zone 1: sodium-aluminosilicate glass phase with a small amount of calcium derived from the slag
Zone 2: sodium-aluminosilicate glass phase with small amounts of calcium and iron
Zone 3: chromite
Zone 4: sodium-aluminosilicate glass phase with a small amount of calcium derived from the slag
Zone 5: magnesia-alumina spinel with small amounts of iron and chromium
Zone 6: silicate glass phase at the edge of chromite
These results indicate that fused rebonded magnesia–chrome bricks have excellent slag corrosion resistance. Microstructural changes were confined to a region within approximately 500 μm of the interface after corrosion. This is attributed to the spinel barrier layer formed during corrosion, which maintained the stability of the internal microstructure.
Figure 4
2.2.4 Corrosion Resistance of High-Purity Magnesia-Alumina Spinel Bricks
After the corrosion test, most of the slag remained inside the crucible specimen made of high-purity magnesia–alumina spinel brick, and the crucible showed only slight penetration and corrosion. However, a relatively large crack appeared in the specimen. Examination of the cut cross-section revealed a large amount of white salt, indicating that sulfates had penetrated into the brick.
Figure 5 shows the backscattered electron image near the slag–refractory interface on the specimen cross-section. Figure 5(b) illustrates slag penetration into the brick matrix:
Zone 3: sodium-aluminosilicate glass
Zone 4: magnesia–alumina–iron spinel and sodium-aluminosilicate glass
Zone 5: sodium-aluminosilicate glass with a small amount of calcium
Zone 6: magnesia–alumina spinel with a small amount of iron
It can be observed from Figure 5 that the microstructure of the magnesia–alumina spinel brick remained largely intact. Area composition analysis was conducted twice on an approximately 1 mm × 1 mm region of the uncorroded part of the brick. The results showed that the atomic percentage of Mg ranged from 14.35% to 14.58%, and that of Al ranged from 33.52% to 33.65%, indicating that the material is an alumina-rich magnesia–alumina spinel. The reaction between sodium sulfate in the slag and alumina released from the spinel may have contributed to the formation of cracks in the material.
Figure 5
Magnesia–alumina spinel bricks exhibit excellent resistance to the test slag. However, the dissolved Al₂O₃ in alumina-rich spinel may react with alkalis, and the dissolved MgO in magnesia-rich spinel may react with acidic components. Therefore, stoichiometric spinel should be used. High-purity magnesia–alumina spinel bricks are expensive and have poor thermal shock resistance, so they cannot yet be widely used as regenerator checker bricks in the glass industry. Nevertheless, as a chromium-free solution for glass furnace refractories, further in-depth research on magnesia–alumina spinel refractories is still needed.
Based on changes in slag composition, as well as the corrosion behavior and mechanisms of magnesia bricks, direct-bonded magnesia–chrome bricks, fused rebonded magnesia–chrome bricks, and fused rebonded high-purity magnesia–alumina spinel bricks, corresponding countermeasures were adopted in three stages:
First stage: The glass production process and petroleum coke quality were controlled. Ninety-five-grade magnesia checker bricks were discontinued, and 97-grade magnesia bricks and direct-bonded magnesia–chrome bricks were adopted instead.
Second stage: Direct-bonded magnesia–chrome bricks were replaced with fused rebonded magnesia–chrome bricks, and the proportion of fused magnesia in 97-grade magnesia bricks was increased.
Third stage: Customized solutions were developed according to the actual conditions of end users, providing optimal matching schemes with balanced cost-performance.
Through the gradual implementation of these strategies and measures, the service life of regenerator checker bricks has been steadily improved.
Since the glass plant initially used Al₂O₃ and substances containing B₂O₃ and P₂O₅ as fluxing agents, which caused corrosion and blockage of basic checker bricks, the use of these materials was stopped or significantly reduced. As creep of corroded 95-grade magnesia bricks was the main cause of checker wall damage, 95-grade magnesia bricks in the upper layer of the checker wall were eliminated. The use of 97-grade magnesia bricks was expanded in the top layer, and direct-bonded magnesia–chrome bricks were used in the middle layer. These adjustments alleviated the problem of excessively short service life of checker bricks in glass furnace regenerators.
Due to the insufficient corrosion resistance of 97-grade magnesia bricks and direct-bonded magnesia–chrome bricks, a higher proportion of fused magnesia was used, and a high-temperature firing process was adopted in the production of 97-grade magnesia bricks. Furthermore, direct-bonded magnesia–chrome bricks with inferior corrosion resistance were replaced with fused rebonded magnesia–chrome bricks. Owing to the excellent microstructural stability of fused rebonded materials, their corrosion resistance and service life are significantly better than those of similar products made with sintered magnesia. By replacing direct-bonded magnesia–chrome bricks with fused rebonded magnesia–chrome bricks, the service life of checker bricks can be further increased by 150%.
3 Conclusion
(1)The corrosion mechanism of refractories is determined by the characteristics of glass furnace ash and slag. When the ash and slag are alkaline (i.e., contain free Na₂O), basic refractories should be used. When they are acidic (i.e., contain free SO₃), neutral refractories, such as alumina–chrome bricks, can be adopted.
(2)When using petroleum coke, indicators including calorific value, volatile matter, ash content, moisture, sulfur content, vanadium content, and the sulfur and alkali contents in regenerator ash and slag should be tested. Based on these results, suitable refractories can be selected, and the glass production process can be controlled accordingly.
(3)If conditions permit, magnesia checker bricks should be produced using fused magnesia and the "three-high" process (high-purity raw materials, high-pressure forming, and high-temperature firing). The application of magnesia–chrome bricks should be expanded, and fused rebonded magnesia–chrome bricks should be used to replace direct-bonded magnesia–chrome bricks as regenerator checker bricks. Depending on actual service conditions, the top layer and part of the upper layer can be replaced with high-temperature fused magnesia–zirconia bricks, chrome–corundum bricks, chrome–zirconia–corundum bricks, or other suitable materials.