Causes, hazards, and countermeasures for syngas deviation in Texaco quench-type gasifiers

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Introduction: Texaco water-coal slurry pressurized gasification belongs to the advanced second-generation coal gasification technology. Furnace types are mainly divided into quenching type and waste heat boiler type. The Texaco gasification units introduced in China, such as those at Lunan, Weihe, Shanghai Coking, and Huainan, all use quenching type gasifiers. Based on the actual operation conditions in manufacturers, there is a problem of syngas deviation, which is briefly described here for reference only by relevant technical personnel and operators. 1 Brief description of the process: The basic process of Texaco’s pressurized coal slurry gasification involves using a high-pressure coal slurry pump to feed the coal slurry into the burner. At the same time, high-pressure oxygen from the air separation unit is also fed into the burner; the oxygen passes through the outer annulus and the central tube of the burner, while the coal slurry passes through the inner annulus. Together, they are injected into the gasifier where they mix and get atomized. The gasification reaction takes place at a temperature of 1350–1400 °C. The resulting high-temperature syngas and molten slag flow together through the slag outlet, past the quenching ring and downcomer, and into the quenching water in the quenching chamber. The high-temperature syngas and molten slag are directly quenched by contact with chilled water. The purpose of this quenching is to cool the high-temperature gases down to the saturation steam temperature at that pressure, and to cool the molten slag so that it can settle, thereby achieving separation of the gas from the slag. The separated slag is passed through a slag breaker and regularly discharged into the slag pool via lock hoppers, where it is then removed by a slag skimmer and loaded for transport away. Quench water is drawn from the scrubber tower by a quench water pump, sent to the quench ring, and flows downward in a uniform manner along the inner wall of the downcomer. The water film formed by the quenching water on the inner wall of the downcomer not only prevents high-temperature gas streams and slag from coming into direct contact with the inner wall of the downcomer, thus protecting it, but also gradually reduces the temperature of the gas. The syngas, after being quenched in quench water, rises evenly in bubbles through the annular spaces of the downcomer and upcomer. After exiting the quench chamber, it is further cooled and dedusted via a Venturi scrubber and a scrubbing tower before being sent to CO conversion. 2 Reasons for syngas deviation 2.1 The installation of the downcomer does not meet the design requirements. The installation of the downcomer must comply with the design specifications. If the actual centerline of the downcomer deviates from the designed centerline, resulting in a large angle, the downcomer will tilt. As a consequence, the gas flow of the high-temperature syngas entering the water bath in the quench chamber along the downcomer cannot be distributed evenly as it rises, leading to flow deviation. If, during the production process, the downcomer tilts or deforms due to vibration and thermal stress, the result is the same as mentioned above: flow deviation will occur. 2.2 Misdirected jet from the burner: Since the burner has the dual function of atomization and creating an appropriate flow field, rather than merely performing the atomization function, when the burner emits a misdirected jet, it causes changes in the flow field within the reaction chamber. As a result, the high-temperature syngas flows to one side along the slag outlet, the quenching ring, and the downcomer, entering the water bath in the quenching chamber and thus causing a deviation in the flow pattern. 2.3 Partial blockage of the slag outlet: When there is partial blockage of the slag outlet, the exit passage for the high-temperature syngas becomes narrower. At this point, the center of the slag outlet deviates from the center of the original slag outlet, the center of the quenching ring, and the center of the downcomer. As a result, the particle velocities in the cross-section of the high-temperature syngas as it enters the quenching chamber through the downcomer differ, which prevents the gas flow from rising uniformly along the bottom edge of the downcomer in a bubbling manner; this leads to localized areas where the gas flow is too strong or to surging phenomena. 2.4 Local ash agglomeration or scale blockage may occur in the annular gap between the downcomer and the upcomer. If ash agglomeration or scale blockage forms at some point on the outer wall of the downcomer and the inner wall of the upcomer, the airflow encounters greater resistance at that location; as a result, the airflow will be directed toward areas with lower resistance, which inevitably leads to excessively high flow velocities in other parts of the annular gap. Worse still, some of the syngas even does not rise through the annular gap, but escapes in a certain direction from the bell jar at the bottom of the riser, entering the water-mixing space outside the riser. 3 Hazards of syngas drift: One day in 2005, at a fertilizer plant, the pressure difference between the reaction chamber of the gasifier and the syngas outlet of the quench chamber increased. The concentration of useful gas components in the syngas rose, as well as the temperature of the syngas exiting the gasifier. The liquid level in the quench chamber of the gasifier fluctuated violently and became uncontrollable. On-site operators heard knocking sounds and vibrations in the syngas outlet pipeline of the quench chamber, forcing the plant to reduce its operational load in order to continue production. A few hours later, the temperature reading from the thermocouple on the support plate showed abnormalities again; the amount of slag discharged by the slag removal machine decreased, and ultimately, production could not be continued, forcing a shutdown. After stopping the machine, the process burner was removed, and the slag outlet at the bottom of the gasification furnace’s reaction chamber was inspected visually at the burner flange at the top of the gasification furnace; it was found that partial blockage had occurred. The maintenance worker opened the manhole in the quench chamber; ash and slag had accumulated above the manhole, and the rising pipe had fallen off. Based on the above production examples, deviation of the syngas flow generally poses the following hazards: (1) When the syngas deviates in the water bath of the gasifier’s quench chamber, gas and liquid cannot come into maximum direct contact, resulting in inadequate mass and heat transfer. This leads to poor cooling of the gas, causing the temperature of the syngas exiting the gasifier to be too high, which affects the thermal balance of the system. (2) The syngas has severe ash content. Since the syngas does not come into sufficient contact with water, the dust carried in the gas stream is carried out of the quench chamber along with the gas flow and into the subsequent system, posing difficulties for dust removal and washing in that system. When the ash accumulation is very severe, dust quickly forms ash lumps at the temperature-sensing locations of the thermocouples on the refractory brick support plates in the quenching chamber, resulting in temperature measurement delays or false readings. (3) The liquid level in the quench chamber of the gasifier fluctuates greatly. When syngas is diverted, the liquid level in the gasifier quench chamber experiences severe fluctuations due to the disturbance caused by the diverted gas, and it even becomes difficult to control. (4) Water in the syngas or vibration in the quench chamber. When there is severe syngas bypass, a slightly higher liquid level in the gasifier quench chamber can cause water to be carried into the downstream system through surging, and vibrations will also occur in the gasifier quench chamber. Such vibration can easily cause the fixing bolts of the downcomer and upcomer to loosen or break, resulting in the upcomer falling off or the downcomer deforming; it can also damage other internal components. The detached rising tube will block the inlet to the slag crusher; a large amount of ash and slag will be blown by the airflow through the gaps along the bottom edge of the detached rising tube, accumulating outside it. This makes it difficult for the ash and slag to be discharged from the quenching chamber into the lockhopper, and they may enter the black water pipeline of the gasification furnace in large quantities, causing blockages in that pipeline and in the flash evaporation system, thereby hindering the operation of the system. 4 Accident identification and handling during production operations 4.1 Accident identification If the temperature of the gas exiting the syngas outlet of the gasifier is too high, if the temperature indicated by the refractory brick support plates in the gasifier is high, if there are severe fluctuations in the liquid level in the gasifier’s quench chamber, or if there are sounds of water hammering and vibrations in the syngas outlet pipeline of the quench chamber, it can be concluded that there is severe misdirection of the syngas flow. 4.2 Countermeasures: Once an accident is identified based on the aforementioned phenomena, if water hammer occurs, the liquid level in the quenching chamber of the gasifier can be reduced to its lowest level. If the pressure difference at the syngas outlet between the reaction chamber and the quenching chamber increases, the load can be reduced, and the oxygen-to-coal ratio can be increased appropriately to raise the furnace temperature (it is advisable to raise it by 50°C above the normal value). The central oxygen supply can be adjusted to lengthen the flame and reduce the pressure in the downstream system; the purpose of this is to melt any slag blocks that may be blocking the slag outlet and restore normal operation of the same. Depending on the specific circumstances, maintain reduced load operations for a period of time, and then increase the load once the system stabilizes. At this point, if the system is still unstable and the situation becomes increasingly severe and out of control, the operator should pay close attention to the various relevant safety interlock values; if the interlocks fail, the machine should be stopped immediately. After the system has finished processing, remove the process burner, open the manhole in the quenching chamber of the gasifier, and check for any blockages at the slag outlet, as well as any damage to the downcomer, upcomer, and other internal components. In the case of partial blockage at the slag outlet, personnel should be organized to carry out manual clearance ； In the event of damage to the riser, downcomer, and other internal components, it is necessary to contact the relevant departments and personnel to handle it. 5 Conclusion Texaco water-coal slurry pressurized gasification is a complex systems engineering project; the issues mentioned above are just some of those encountered in actual production. It is hoped that this article can provide some guidance for production operations. This post was last edited by logo911 on 2008-3-16 10:35.]