The three layers of insulation materials, namely, fire-facing bricks, backing bricks and insulation bricks, in the combustion chamber of the gasifier can effectively isolate the threat of high-temperature gas to the reactor shell. The reaction in the combustion chamber of the gasifier is intense, and the refractory bricks are washed by high-temperature gas, which causes continuous wear and thinning. The corrosion rate during normal operation is 0.02mm/d. However, when the coal type is abnormal, the erosion rate of refractory firebricks will be greatly increased, especially after the petroleum coke is mixed, the erosion of refractories bricks in the gasifier will be aggravated, which seriously restricts the safe and stable operation of the gasifier.
The thinning of refractory brick slag makes the furnace wall easy to overheat
Under normal circumstances, a solid slag film will be formed on the surface of the refractory brick to isolate the erosion of the fire bricks by the molten slag and high-temperature gas. First, after the coal slurry enters the gasifier, it burns and gasifies with oxygen to generate water gas with CO and H2 as the main components. After the reaction, most of the remaining ash and a small amount of residual carbon collide with the surface of the refractories bricks and are captured by the refractory brick wall. MgO, Fe2O3, and Al2O3 in the coal ash will combine with Cr2O3 to form dense spinel, which is the solid slag film. As the temperature of the ash slag away from the refractory fire bricks further increases, the ash slag close to the outer layer of the slag film gradually flows downward in a molten state and is finally discharged from the combustion chamber of the gasifier. Due to the existence of the slag film, the penetration of high-temperature coal gas and high-temperature molten slag is isolated. In addition, due to the role of backing bricks and insulation bricks, the furnace wall temperature of the gasifier is maintained at ~230℃. In the later stage, as the refractory bricks are thinned, the furnace wall temperature will gradually increase. Generally, the furnace wall temperature <300℃ can maintain operation.
During the operation of the full coal condition, the furnace wall temperature of the gasifier did not become abnormal, but after the petroleum coke was mixed, the furnace wall temperature of the gasifier rose slightly. When the blending ratio of petroleum coke is >30%, the wall temperature exceeds 300℃ several times. According to analysis, the reasons for the increase in wall temperature are as follows:
① The reactivity of petroleum coke is poor. In order to maintain the temperature of the gasifier and improve the reactivity of petroleum coke, a higher oxygen-coal ratio must be maintained to increase the operating temperature of the gasifier, which is an objective condition for the increase in wall temperature;
② Due to the high blending ratio of petroleum coke, the ash content in the furnace is low, resulting in thinning of the slag on the furnace wall. By checking the refractory firebricks in the gasifier, it was found that some of the bricks in the gasifier had no slag at all, and some slag areas did not form a slag film, while some refractory bricks had porous slag and did not form a slag film of a certain thickness. The main reason is the proportion of petroleum coke blending. When the ash content in petroleum coke is relatively low, although it can reduce the erosion of fire bricks, it is found in the actual operation process that after the petroleum coke is blended, the slag film of sufficient thickness is not enough to form on the refractory firebricks of the gasifier, and some fire bricks are exposed to the high-temperature gasifier reaction system. The ash joints of firebricks are the weakest link. The refractory mud in the ash joints will be washed away during the airflow entrainment process. The brick joints are first exposed to the environment, and the high-temperature water gas will enter along the brick joints of the refractory bricks, causing the furnace wall to overheat.
When dealing with the overheating of the furnace wall, measures to significantly reduce the reaction temperature of the gasifier are repeatedly adopted to make the ash slag re-hang the slag, which indirectly proves that the main reason for the overheating of the furnace wall is the excessive proportion of petroleum coke blending, the exposure of brick joints, and the backflow of airflow. In addition, in addition to a large amount of SiO2, CaO and Fe2O3, petroleum coke ash slag also contains a considerable amount of corrosive media, namely vanadium oxide (mainly V2O5), and the test shows that its content reaches 4.5% (w). The melting point of V2O5 is only 670℃, and when it coexists with Cr2O3, the lowest eutectic temperature is 665℃. Under gasification conditions, the refractories bricks exposed to the gasification environment system are easily melted without the protection of the slag film.
Combined with the actual situation, it is found that when the blending ratio of petroleum coke exceeds 40%, the furnace wall is prone to overheating and the operation is unstable. When the blending ratio is 30%, although the furnace wall temperature is slightly higher than that of the full coal working condition, preliminary calculations show that the gas production of the 30% blending ratio is slightly higher than that of the full coal working condition. Comprehensive considerations should be made that when blending petroleum coke, the blending ratio should be strictly controlled to <30% to avoid the occurrence of gas leakage in the brick joints.
The addition of petroleum coke leads to aggravated erosion of refractory bricks
After the addition of petroleum coke, the carbon conversion rate of the gasifier gradually decreases. Under the full coal working condition, the carbon conversion rate of the gasifier is only 98%. After the addition of petroleum coke (fine ash is not burned back), the carbon conversion rate of the gasifier drops from 98% under the full coal working condition to 94%, and as the proportion of the addition is >30%, the carbon conversion rate drops below 90%. When the carbon conversion rate is <88%, the wall capture efficiency of the gasifier decreases significantly. Although the capture efficiency of the furnace wall decreases, the residual carbon particles captured by the gasifier wall are slightly higher than those under normal working conditions. The captured residual carbon particles will consume oxygen and reduce the oxygen partial pressure on the surface of the refractory bricks.
Through observations into the furnace, it was found that this kind of erosion often occurs in the primary reaction zone, that is, the upper part of the burner chamber spreads to the dome, which is located in the primary reaction zone of the gasification reaction. The primary reaction zone of the gasification reaction belongs to the combustion reaction zone. The temperature in this area is relatively high, and the flame temperature reaches 2200℃. The ash and slag have good fluidity here, and the reaction is violent. It is not easy for the slag to form a stable slag film. It was also found that the situation of gasifier A is more serious than that of gasifier B.
Under normal circumstances, the Fe2O3 in the coal slag is reduced to FeO by residual carbon, and penetrates into the refractories bricks together with MgO and Al2O3 in the slag. The Cr2O3 and Al2O3 in the refractory firebricks react to form a dense layer of Mg-Al-Cr-Fe composite spinel, thereby achieving "slag against slag". However, in this device, due to the excessively high proportion of petroleum coke blending, the carbon conversion rate is low, and the slag contains a large amount of unreacted carbon elements. Excessive carbon elements lead to the occurrence of porous erosion of refractory firebricks. According to the observed erosion of refractories bricks and the analysis of process parameters during the operation of the device, the main reasons for the porous erosion of refractories bricks are as follows:
① In the gasification environment system of this device, due to the extremely low oxygen partial pressure, Fe2O3 in the slag of the gasifier is reduced to elemental Fe, and Mg-Al-Cr-Fe composite spinel cannot be formed, and the stable slag film is lost, which causes the molten slag after the reaction to directly corrode the surface of the refractory bricks;
② Under normal circumstances, the oxygen partial pressure in the gasifier is 10-8~ 10-10MPa, but there is a large amount of unreacted residual carbon in this device, which will further reduce the oxygen partial pressure in the gasifier system environment, making the formation of Cr2+ possible, and the Cr2O3 in the slag is reduced to elemental Cr and precipitated from the slag, so that the Cr2O3 in the high-chromium material is dissolved-reduced-precipitated in the slag, and the cycle continues, and the high-chromium material is severely corroded by the slag;
③ In this atmosphere, after the unreacted residual carbon contacts the fire bricks, it is easy to react to form chromium carbides, causing bubbling on the surface of the refractories bricks. Analysis of the operating data also found that the main reason why the situation of gasifier A is more serious than that of gasifier B is that the operation time of gasifier A mixed with petroleum coke is more than 2 months, while the operation time of gasifier B mixed with petroleum coke is less than 1 month.
The main reason for the porous erosion of refractory bricks in this device is that there is excessive unreacted residual carbon on the firebricks, which causes the oxygen partial pressure of the system to be extremely low, thereby inducing porous erosion of refractories bricks. To solve the problem of porous erosion of fire bricks from the root, we should also start from improving the carbon conversion rate, increase the reaction temperature of the gasifier, ensure that the carbon conversion rate is >95%, and at the same time appropriately increase the operating pressure of the gasifier, extend the residence time of the material, and maximize the carbon conversion rate.
