Summary of Technical Renovation for Texaco Gasification Unit

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1 Overview Our factory was the first in China to adopt Texaco gasification technology. In the early stages of operating the plant, we had insufficient knowledge of Texaco’s gasification technology, which led to a series of technical challenges and production failures in actual operation. To this end, we carried out a series of technical upgrades. At present, this device has achieved high-load, long-cycle, safe and stable operation. The technical upgrades carried out by our factory over the past few years are introduced as follows. 2. Clogging problem: Before 1997, the system frequently experienced clogs, and clearing them was difficult, dangerous, labor-intensive, and time-consuming, which hindered the safe and stable operation of our plant’s Texaco gasification unit at high loads over extended periods. To this end, we modified the pipeline from the gasifier to the high-pressure flash tank, the pipelines of the flashing system, the feed system for the sedimentation tank, and the quenching water pipeline. 2.1 Modification of the pipeline from the vaporization furnace to the high-pressure flash tank: The original design of this pipeline was for it to emerge from the bottom of the quench chamber, descend vertically by about 15 meters, and then turn upward to enter the high-pressure flash tank. Due to the long pipelines and numerous bends, there is high resistance to the flow of the black water in the pipes within the quenching chamber, resulting in low flow speeds. Especially when the atomization effect of the burners is poor or the gasifier contains a large amount of water, dust in the black water can accumulate inside the pipes, causing severe blockages. In response, in 1996 our factory changed the original pipeline design, which led from the gasifier outlet to the high-pressure flash tank in a vertical downward direction before turning upward, to a horizontal pipeline. Practice has shown that the renovation yields significant results, ensuring the flow rate of the black water in this pipeline and preventing blockages. 2.2 Modification of Flash System Pipelines 2.2.1 Flash Process and Equipment The originally designed flash system consisted of three stages of flashing: high-pressure flashing, medium-pressure flashing, and vacuum flashing. The first stage of flashing is high-pressure flashing; the black water coming from the vaporizer and scrubber tower first enters a high-pressure flashing tank where it is separated through flashing. The concentrated black water at the bottom of this tank proceeds to the second stage of flashing, which is medium-pressure flashing. The black water that has become concentrated at the cone bottom of this flashing tank then enters a vacuum flashing tank for the third stage of flashing. The black water with a high solid content at the cone bottom of the vacuum flashing tank is sent to a sedimentation tank feed pump, which pumps it under pressure to the sedimentation tank for reuse. The three flash tanks have a basically identical structure, namely an oval head at the top, a straight cylinder in the middle, and a cone at the bottom. 2.2.2 Modification details: After being separated through flashing, the black water that enters each flash tank accumulates at the bottom of the cone; as a result, its solid content increases. When the atomization effect in the burners is poor, the solid content of the black water entering the flashing system becomes even higher. Once this solid content reaches a certain level, it can cause blockages in the area at the bottom of the cone, in the pipelines connecting the various tanks, and in the inlet pipelines of the feed pumps in the sedimentation tanks. When the blockage is severe, it is necessary to manually open the bottom drain valves of each tank to clear it, and this process is extremely dangerous. To this end, at the end of 1996 we moved the black water outlet pipelines at the very bottom of the cones of each flash tank upward and connected them to the straight cylindrical section located slightly above the cones; the original outlet at the bottom of the cone was used for waste discharge, while the black water outlet of the vacuum flash tank remained at the bottom of the cone ； The feed pump in the sedimentation tank was removed, and the location of the original vacuum flash tank was moved from a lower position to one higher than that of the sedimentation tank. In this way, the black water in the vacuum flash tank enters the sedimentation tank directly due to the difference in levels, eliminating the need for a feed pump in the sedimentation tank. 2.2.3 Effects of the renovation: After the renovation of the flash evaporation system, blockages were largely avoided, labor intensity was reduced, and maintenance costs were saved. By eliminating two feed pumps for the sedimentation tanks, electricity consumption is reduced. 2.3 Renovation of the sedimentation tank feeding system The original process flow was ； The black water with a high solid content in the sedimentation tank enters the filter press feed pump from the bottom; after being pressurized, it is sent to the filter press. The filter cake is discharged via a conveyor belt, while the filtrate goes into the filtrate tank and is then pumped to the coal grinding system by a filtrate pump. The above process is adopted to minimize the discharge of water from the system and reduce heat loss. Practice has shown that this process has the following defects. (1) The solids in the black water cannot settle completely. Although various stabilizers and flocculants were used to treat the black water, the effects were not significant. The pipelines at the bottom of the sedimentation tank are still severely blocked. (2) The quality of the raw coal is unstable, and variations in coal quality cause fluctuations in the suspended solids content in the system’s black water. When the suspended solids content is too high, long-term operation of the system will cause blockages in the pipelines at the bottom of the sedimentation tank. Once this pipeline becomes blocked, solid particles will be carried throughout the water system, causing scaling and blockages in other equipment and pipes, which can lead to forced shutdowns in severe cases. To this end, we made the following modifications: we eliminated the filter press, filter liquid pump, and filter liquid tank. The wastewater discharged from the bottom of the sedimentation tank is sent to the first slag removal tank through a waste discharge pipe, while the clear liquid from the first slag removal tank then enters the second slag removal tank. A tracked slag skimmer is installed above the slag collection tank to regularly remove the fine ash and slag that have settled at the bottom. After the modification, blockages at the bottom of the sedimentation tank and shutdowns were significantly reduced; black water with a high solid content could be discharged in a timely manner, ensuring a healthy circulation in the water system. It simplifies operations, saves labor, and reduces power consumption. 2.4 Modification of the quench water pipeline: In the gasification process, the black water (quench water) from the scrubber tower is pressurized by a centrifugal pump and sent to the quench ring in the gasification furnace. The water passing through the quench ring spreads evenly over the inner surface of the downcomer, forming a thin water film that flows downward alongside the syngas. This prevents the high-temperature syngas from coming into direct contact with the inner surface of the downcomer, thus avoiding overheating and deformation of the downcomer, while also cooling and washing the high-temperature syngas. The amount of quench water directly affects the liquid level in the gasifier. Either too much or too little quench water volume can result in an excessively high or low liquid level in the gasification furnace, and the liquid level of the gasification furnace is an interlock parameter for its safety system. When the liquid level in the gasifier drops to the interlock value, the gasification system shuts down. Therefore, the quench water volume is an important control parameter to ensure the stable operation of the Texaco gasification unit. There was only one filter on the original quench water pipeline. Due to the large pore size of the filter screen, large particles in the black water pass through it and reach the quenching ring, causing blockages in the holes of the quenching ring and a reduction in the amount of water used for quenching, which in turn damages the quenching ring and the downcomer. In 1997, the quench water pipeline in the A# gasifier became clogged, resulting in a significant reduction in the amount of quench water. After parking, the downcomer of A# vaporizer was inspected, and it was found to be severely twisted and deformed. To this end, we installed a filter with a smaller pore size on the quench water pipeline, which allows for a second filtration of the finer particles in the black water. After the modification, the quench water can be properly filtered, and the gray particles in the black water are no longer carried into the quenching ring, thereby protecting the quench tubes and stabilizing the liquid level in the gasification furnace.