Reasons for high wear of refractory bricks in Texaco furnaces and countermeasures

Thread Content

The damage to the refractory bricks in gasifiers includes bulk spalling, ablation damage, erosion damage, and chemical corrosion. 1. Block spalling mode: Block spalling is the mode that causes the greatest wear on the refractory bricks in gasification furnaces and has the most significant impact on their service life. Reducing or eliminating bulk spalling can significantly increase the lifespan of refractory bricks. When block-shaped pits on the surface of the refractory bricks have a depth of more than 1.5 mm and cover a certain area, it is considered that the damage to the refractory bricks is primarily characterized by block-like peeling ； When it is less than 1.5 mm, it is considered to be deep erosion caused primarily by ablation. The reasons for bulk spalling include the following: (1) Relative displacement between bricks. As the temperature of each layer of bricks changes during the heating or cooling process in the gasification furnace, differences in the rate of temperature change, along with varying constraints and resistances encountered during thermal displacement, lead to relative displacement between the bricks. This relative displacement generates frictional shear forces on the contact surfaces between bricks, resulting in a local tearing effect that causes surface cracks in the refractory bricks. These surface cracks expand with each subsequent displacement, and the erosion and diffusion of molten slag and reducing agents within these cracks cause the surface of the bricks to flake off; moreover, the process of brick displacement itself accelerates the erosion of slag within the cracks. 2. Brick joint erosion and slag erosion: The gaps between refractory bricks not only provide a pathway for the infiltration and erosion by high-temperature molten slag during operation, but this slag erosion itself also causes these gaps to widen over time. Both of these effects increase the surface area where the slag comes into contact with the sides of the refractory bricks, causing the sides of these bricks to be subjected to excessive stress during each cycle of thermal contraction and expansion. The slag erodes the furnace bricks not only radially but also circumferentially along the perimeter of the refractory bricks, within the gaps between them. Especially when circumferential cracks exist on the side of the refractory brick, the circumferential erosion rate is faster, causing massive flaking of the refractory brick surface. Therefore, circumferential cracks in refractory bricks have a greater impact and effect on their service life than radial cracks. 3. Ablation loss mode: The temperature field inside the vaporization furnace is a non-uniform, unstable, and even discontinuous temperature field. There are also many reasons for the occurrence of local high temperatures; as a result, it is easy for the surface of Cr2O3-Al2O3-ZrO refractory bricks to suffer ablation damage due to these high temperatures, and even local overheating and melting may occur. Under normal conditions, this ablation damage process proceeds slowly; it is only under extremely abnormal high temperatures and reaction conditions inside the furnace that the ablation process accelerates significantly. Based on observational analysis, the ablation of refractory bricks can be divided into high-temperature melting ablation and high-temperature redox ablation. The high-temperature melting ablation of the refractory bricks in the gasification furnace mainly occurs in the oxygen-rich zone, the flame licking zone, and under hyperoxia conditions in the gasification furnace. These three areas/conditions all belong to the local high-temperature zones within the gasifier. The main components of refractory bricks are Cr2O3, ZrO2, and Al2O3, and they are produced through high-temperature firing. Under normal temperature conditions inside the furnace, they exhibit good mechanical stability. Moreover, during operation, the surface of the refractory bricks is usually covered with molten slag; thus, the high-temperature gases inside the furnace do not come into direct contact with the surface of the refractory bricks. However, in the locally oxygen-rich areas and those where high-temperature air streams directly burn the surface of the refractory bricks, the surface structure of these bricks softens and their strength decreases; their resistance to wear and erosion as well as their structural integrity are reduced, and some of the bonding phases are directly burned away. The rate of ablation of refractory bricks is influenced by various factors, such as the operating conditions of the gasification furnace – including the oxygen-to-coal ratio, burner performance, slag outlet pressure difference, ash content of the feed coal, characteristics of the slag composition, the shape of the roof bricks, and the load on the gasification furnace. Meanwhile, low-melting-point impurities present in the refractory bricks also accelerate their melting and ablation rate. 4. Erosion damage mode: In a gasification furnace, in addition to being eroded by high-temperature slag, refractory bricks are also subjected to the scouring and wear caused by high-speed air currents and slag flowing along the wall surfaces, which leads to damage to the refractory bricks. Such damage is referred to as erosion damage. In the Deschamps gasifier, there are several types of erosion damage to the refractory bricks, primarily caused by high-speed gas flow, wear from flowing slag, and erosion by a mixture of gas and slag. 5. Chemical erosion: Since refractory bricks are formed by pressing together various refractory material particles, pores are inevitably present, and these pores accelerate the chemical erosion of the refractory bricks. The gases produced by the gasification furnace are primarily composed of H2 and CO. The fire-facing side of the refractory bricks is surrounded by these highly reducing gases, which penetrate inward through the pores and cracks in the bricks. There, they react with oxides such as SiO2 and Fe2O3 present in the bricks, resulting in the expansion of those pores or cracks and thereby damaging the structure of the bricks. Their reaction equations are as follows: SiO2 + H2 → H2O↑ + SiO↑ (1) Fe2O3 + 3CO → 2Fe↑ + 3CO2↑ (2) CO → C + CO2↑ (3) In reaction (1), the reaction between SiO2 and H2 produces H2O and SiO gas, which causes the pores in the refractory bricks to enlarge, the structure to become looser, and the strength to decrease, thereby reducing its resistance to erosion. Reaction (2) results in the formation of Fe and CO2 gases from the reaction between Fe2O3 and CO, which increases the pores in the refractory material and reduces its strength and erosion resistance. In reaction (3), CO causes carbon to be deposited in the pores of the refractory material under the catalysis of Fe2O3, leading to the spalling of the refractory material due to the resulting expansion. 6. Measures to prevent damage to refractory bricks: (1) Follow the furnace drying curve strictly to avoid displacement of the refractory bricks caused by rapid changes in furnace temperature. (2) Use fire-resistant mortar free of clay for building the furnace, so that the brick joints are no longer weak points prone to erosion. (3) Control the appropriate oxygen-coal ratio, select burners with good atomization performance, and maintain proper furnace temperature to prevent slag formation at the slag outlet. Choose coal varieties with low ash content and low levels of CaO and FeO in the coal. The dome is built with bricks in a hemispherical shape, and the load on the gasification furnace should be kept as low as possible. (4) Control the furnace temperature properly so that a thin layer of slag can form on the surface of the refractory bricks, providing protection for them. Furthermore, controlling the appropriate oxygen-coal ratio also helps to regulate the redox atmosphere inside the furnace. (5) Improvement of refractory bricks. Increasing the Cr2O3 content in refractory bricks improves their erosion resistance. The higher the Cr2O3 content, the stronger the resistance to erosion. The main components of the slag from the gasification furnace are metal oxides such as Fe2O3, Al2O3, SiO2, MgO, and CaO. Cr2O3 reacts with Fe2O3, Al2O3, FeO, and MgO in the slag to form a composite spinel (Mg,Fe)O(Al,Cr,Fe)2O3, which creates a dense layer on the surface of the refractory material and prevents further erosion by the slag. (6) Select coal with a lower CaO content. The erosion of refractory bricks by CaO in slag is relatively more pronounced compared to other oxides. Furthermore, the higher the Cr2O3 content, the lower the erosion rate, as shown in Figure 1. Therefore, increasing the Cr2O3 content in refractory bricks and using high-quality coal with a low CaO content can improve the erosion resistance of chromium-aluminum-zirconium bricks. However, a low CaO content in coal can result in a higher ash melting point, so the CaO content should not be too low either. Figure 1: Effect of CaO content in slag on the erosion of chromium-aluminum-zirconium bricks. (7) Increasing the bulk density of the refractory bricks reduces pore size, thereby minimizing the pathways for chemical erosion and enhancing resistance to slag erosion. By adjusting the particle size distribution of the refractory particles and adding an appropriate amount of ultra-fine powder to fill the gaps between them, the pores can be reduced in number and size. This helps to lower the porosity and minimize the penetration of slag and gases, thereby enhancing the refractory material’s resistance to penetration and erosion. (8) Under conditions that do not affect normal operation, minimize the operating temperature and reduce the frequency of starting and stopping. (9) Controlling the furnace temperature is quite critical for a gasification furnace. The maximum range for adjusting the methane content in syngas can be appropriately changed depending on the differences in the ash fusion points of various coal types (based on the components of the syngas and the slagging condition of the gasifier). Under normal circumstances, the maximum range for adjusting the methane content is (800–1200)×10⁻⁶, and the allowable deviation in methane content control is ±100×10⁻⁶. At the beginning of feeding material into the gasifier, the methane content should be kept at 900×10‑6. If the CO content in the syngas remains below 46% for 24 hours in a row, and it is confirmed that the furnace temperature is too high based on the slag condition, then the methane content can be increased by 100×10‑6 ； If the CO level remains below 46% for 24 consecutive hours, the value is increased by 100×10‑6, and this process is repeated until some of the CO readings reach above 46%. If the CO content in the syngas exceeds 48% for 4 consecutive hours, the methane content is reduced by 100×10‑6. If, after 4 hours of observation, the CO level remains above 48% in all cases, the operation area and the production office shall be notified to discuss countermeasures. The operator should pay attention to changes in the T3 temperature; if it is too high, they need to monitor changes in the CO content in the syngas, as well as the condition of the slag and the slag discharge process. Any abnormalities detected should prompt immediate adjustments to the methane content.