Summary of the Operation Status of the Yueyang Shell Powder Gasification Unit

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Author/Source: Hu Yimin (Sinopec Shell Gasification Company, Yueyang, Hunan 414003) 1. Introduction to the plant: The air separation unit utilizes Linde’s air separation technology and imported equipment; it produces 48,000 m3/h of oxygen, as well as 28,360 m3/h of ultra-high pressure nitrogen, 27,530 m3/h of medium pressure nitrogen, and 32,500 m3/h of low pressure nitrogen. The device mainly consists of an air compression and pre-cooling system, molecular sieves, a nitrogen circulation compressor, a refrigeration system, a distillation system, an internal compression system for liquid oxygen, a nitrogen product compressor system, and a liquid nitrogen backup system. The main equipment includes the main air compressor and nitrogen circulation compressor driven by a steam turbine, as well as a molecular sieve unit, air-cooled tower, cryogenic tank, expander, liquid oxygen pump, nitrogen compressor, and others. Coal grinding unit: Designed by China Huaneng Group, it uses medium-speed mills for coal grinding; the coal powder is transported and dried in a nitrogen atmosphere, with an oxygen content of less than 8%. 80% of the coal powder particles have a size between 5 μm and 90 μm, while 10% each have a size less than 5 μm and greater than 90 μm. The unit features 3 independent coal grinding and drying systems, each with a grinding capacity of 43 t/h; they operate in a 2-on-1 standby configuration, with the capacity of 2 of these systems being sufficient to meet the 100% load requirement of the gasifier. The device mainly consists of a raw coal bin, a weighing and feeding coal mill, a circulation fan, a sealing fan, an inert gas generator, a combustion fan, a screw conveyor, a coal powder tank, a limestone unloading, storage, and feeding system, filters, belts, etc. Powder coal gasification unit: It utilizes Shell’s pressure swing gasification technology for powder coal, featuring a high gasification temperature (1400–1600 °C), high conversion efficiency, low residual carbon content, and a wide range of applicable coals (provided that the composition of the coal remains stable over a certain period of time). The plant is designed to process 2000 t/d of coal with an ash content of 17.6%; its maximum processing capacity is 2500 t/d for coal with an ash content of 25%. The output of crude syngas is 142,000 m3/h [on a 100% (CO+H2) basis], and the effective gas composition in the crude syngas is 89% (CO+H2). The plant consists of 7 units, namely the coal pulverization pressurization and feeding system, gasification system, slag system, fly ash system, wet scrubbing system, wastewater stripping and clarification system, and the utility systems (nitrogen, liquefied gas, cooling water, process water, steam/condensate, instrument air/plant air, acid and alkali systems). The main equipment includes a gasification furnace, syngas cooler, ceramic filter, powder discharge tank and feed tank, slag collection tank, scrubber tower, slag skimmer, stripping tower, clarifier tank, various types of containers, pumps, pipelines, valves, instruments, electrical equipment, etc. Construction of the entire factory began in October 2003, trial operations started in January 2006, and it went into trial operation on December 19 of that same year. 2 Basic Information on Unit Commissioning 2.1 Air Separation Unit 1) Commissioning Process From March 9 to March 12, 2006, after adjusting the acceleration curve and resolving the oil leakage issue in the gearbox, the turbine began to warm up the pipes at 10:50 on March 12; a total of 3 tripping tests were conducted that day. The first shutdown occurred at a speed of 800 r/min to test the reliability of the logic; the second shutdown took place at 2000 r/min to assess its repeatability. The third time, the speed was increased to the designed shutdown speed of 5065 r/min, resulting in an interlocked shutdown, thereby completing the turbine testing. Starting at 9:00 on March 20, the main air compressor underwent 4 anti-surge tests, and the compressor testing was completed. The installation of air-cooling towers and molecular sieve packing began on March 22, and the liquid oxygen pumps (P2466A/B, P2467A/B) were operating normally on April 3. At 21:00 on April 30, 2006, the nitrogen circulation compressor completed 2 anti-surge tests ； At 15:00 on May 3, open valve HV2615 and begin purging the cryogenic tank ； At 10:00 on May 4, the expansion turbine was successfully tested, along with the anti-surge testing of the booster. At 10:00 on May 5, the expander operated in bare cooling mode, and this mode ended at 08:00 on May 7. From May 9 to May 13, 2006, airtightness inspection and leak repair in the cold box. From May 14 to May 21, the scaffolding inside the cold box was removed. From May 19 to June 1, pearlescent sand was loaded into cold boxes. On June 2, pearlescent sand was filled into the expander casing. At 11:00 on June 7, 2006, the air separation system was started up for the first time. At 11:38 on June 10, the expander was started; it shut down due to high vibration at 18:43, and was started several times thereafter. It shut down again at 1:03 on June 11 due to high bearing temperature. Upon inspection, it was found that the impeller was damaged. Driven again at 17:00 on June 15; the spare rotor failed to start at 9:32 on June 17 ； The axial spacing was adjusted on June 18, and the expander began to operate at 19:38 on June 20. At 8:22 on June 22, the liquid levels in the upper and lower towers were normal, and qualified oxygen and nitrogen were produced. The liquid oxygen pump P3568A is operating normally, while P3568B requires further tuning. The unit tripped at 21:15 on June 26 due to high expander bearing temperature. Upon inspection, both the impeller and shaft seal are damaged. During the expansion turbine maintenance, tests were conducted on the instrument air compressor and the nitrogen compressor. On July 1, 2006, the low-pressure liquid nitrogen pump was operating normally. On July 3, the high-pressure liquid nitrogen pump was tested successfully, marking the completion of the testing phase for the entire air separation unit. 2) Main problems encountered during testing: ① Oil leakage from the main air compressor and the nitrogen circulation compressor. The first oil leakage from the main air compressor was caused by low negative pressure in the oil tank ； The second oil leak was caused by the sealant used being oil-resistant ； The oil leakage in the compressor during the third nitrogen cycle was caused by an increase in the pore size of the sealing gas diffuser, which prevented the inlet pressure from being released and led to overpressure. All these problems have been resolved. ②A large temperature difference between the upper and lower cylinders of the turbine caused the shutdown. The reason is that the original acceleration curve does not include a holding phase, resulting in uneven heating of the cylinder block and large temperature differences. Later, the acceleration curve was modified, and a dwell time was set at 2200 r/min; this time could vary depending on whether the startup was cold or hot, and the problem was resolved. ③The vibration in the first-stage outlet pipeline of the nitrogen circulation compressor is high. The reason is that slight surge occurred during the speed-up process, coupled with a bracket design that did not meet the requirements. The solution involved enlarging the passage of the anti-surge valve, reinforcing the brackets, adjusting the operating conditions, and changing the opening degree of the inlet guide vanes from 35% to 50%. ④Trip due to temperature point fault or high vibration levels. Due to multiple single-point temperature interlocks in the design, 5 shutdowns occurred as a result of temperature point failures, and high vibration levels led to shutdowns caused by issues with the bearings in the gearbox. On August 15, the parking for maintenance was resolved by shifting the temperature points, using two-out-of-two and three-out-of-two logic; since then, there have been no more instances of tripping caused by open circuits in the instruments. ⑤The selection of the spring for the control valve actuator did not meet the requirements, which affected the proper progress of the testing; after replacement, everything worked normally. ⑥The expander impeller was damaged due to resonance. The impeller has been changed, the number of nozzles has been increased from 14 to 17, and 2 non-stationary areas have been established. ⑦Damage to the seal ring at the second-stage outlet of the circulating nitrogen compressor caused low outlet pressure and high temperature. The sealing ring has been replaced, and it performed well during the performance tests. According to the report by Shell’s experts, the size of the final impeller of the circulating nitrogen compressor did not meet the design requirements, and it was replaced in September 2007. ⑧The main oil pump of the instrument air compressor is unable to establish initial oil pressure. The oil circuit has been modified and an exhaust valve has been added, resolving the issue. 2.2 Coal Grinding Unit 1) Commissioning Process The process of the coal grinding unit is relatively simple. Commissioning involves heating up the inert gas generator, ensuring airtightness of the system, testing the moving equipment (screws, belts, fans), testing the instruments and control systems, attempting to grind limestone, and commissioning the coal grinder. By adjusting various parameters of the coal grinder, it is possible to obtain coal powder with the particle size required for gasification purposes. From the start of testing at the beginning of July to the completion of coal grinding tests on all three lines by the end of August, it took nearly 2 months. The commissioning was affected mainly by issues such as excessive vibration of the circulation fans requiring return to the factory for repair, leaks in the covers of the bag filters/coal dust filters as well as in the main units themselves, numerous leaks in the heat tracing pipelines, poor calibration of the particle size analyzer, faults with the ignition guns of the inert gas generators, and malfunctions of the dew point meters. The most significant issues are vibration problems of the circulation fans and filter leakage problems. 2) Main problems during commissioning: ① High vibration of the circulation fan. Less than 1 day after the circulation fan was started on July 5, it tripped due to high bearing temperature and high vibration levels. The bearing’s upper and lower components were inspected after disassembly, and the same issue persisted once the circulation fan was turned on. The rotor was disassembled and sent to the manufacturer for a new shaft, dynamic balancing was performed again, and bearings were replaced; the problem was resolved by the end of July. ②The coal mill experienced high vibration during its first trial run. On August 8, the B series coal grinder was tested; the vibration of the grinder was extremely high, so it was stopped immediately. The analysis shows that the high vibration was caused by too little coal being fed into the coal grinder for the first time (the designed amount is 500 kg). On the following day, another trial run was conducted: the coal feed amount of 800 kg was used to start the coal grinder, while the loading pressure was reduced to 0.1 MPa; the trial run was successful. ③Leakage from the bag filter, coal dust filter body, and cover. Due to construction errors during filter fabrication, the sealing plate between the circulating nitrogen side and the bag filter side was not fully welded, resulting in coal dust in the nitrogen escaping through the vent pipe. At the same time, an unreasonable design of the roof seal led to nitrogen leakage. The problems of coal dust overflowing from the vent pipe and leakage at the filter lid were resolved through corrective actions. ④Faults in the dew point meter, flame detector, and oxygen content analyzer. Dew point meters and flame detectors experience frequent malfunctions; the problems with dew point meters are mainly caused by coal dust contamination, while flame detectors may be damaged due to liquid present in the liquefied gas. Consider adding a separation tank to address this issue. 2.3 Gasification Unit 1) Test run passes through ① the gasifier. In January 2006, the boiler cleaning of the gasification furnace was completed, mainly by controlling the concentration of chemical agents and the temperature of the boiler water until the concentration of the analysis agents stopped decreasing to an acceptable level; this process was carried out twice in total. Starting from March 10, 2006, the drying of the gasifier was completed by the end of March. ②After the furnace drying was completed, an inspection of the water channels was carried out; it was found that the orifice plates in some of the water wall tubes were blocked. These tubes were cut open, cleaned, and rewelded. Debris left over from the construction of the gasifier’s internal components during overseas construction and domestic assembly was also discovered, such as welding rods, slag, and small metal pieces. After the first treatment, water was reintroduced to start the circulation process, and a second measurement of the water channels was conducted; new blockages in the pipes were detected. After another treatment, a third measurement was carried out, and new blockages in the pipes were still found. Therefore, backwashing was decided upon. After measurement, only a few pipes were found to be blocked; these were eventually cut open for cleaning and then rewelded. After the fourth measurement, the system met the required standards. The entire process of dealing with the water channels took nearly 3 months, and it was not completed until August 18th. ③Starting in mid-August, following the successful commissioning of the coal grinding system, tests on coal circulation were conducted. The aim was to verify the accuracy and reliability of the flow meters and density meters. By using these tests, correction factors for the fine coal flow meters were determined. After addressing issues such as the tendency of the flow meters to become damaged or unstable, as well as erosion and perforations in the fine coal pipelines, the testing of the four fine coal circulation lines was completed by December 6. Since the coal became too fine after two cycles of circulation and could no longer be reused, it had to be discharged; a temporary pipeline was used to connect it to the low-pressure section of the fly ash system, allowing the sequential control logic to be tested and thus completing the practical commissioning of this unit. ④Apart from the replacement and improvement of the cracked ceramic tube lining in the slag system, the commissioning proceeded smoothly; in September, tests for water circulation and sequential control of the system were completed, along with inspections of the relevant measuring instruments and logical controls. ⑤The wastewater stripping and clarification system allows for the testing of pumps, measuring instruments, control devices, and logical circuits through water-based commissioning tests; the comprehensive commissioning of the facility is to be completed by June 20. ⑥Test run of the quench gas compressor. Oil circulation was completed on September 25, 2006; upon attempting to start the compressor with dry gas sealing on October 3, damage was detected. It was not possible to rotate the shaft at a pressure of 0.8 MPa. By increasing the number and strength of the bolts used to secure the dry gas seal, and by adopting a method of rotating the shaft and starting the compressor under zero pressure, the turbine was successfully tested on November 10. Subsequent defects were rectified, and it was officially put into operation on November 28. ⑦During the first trial run of the burner cooling water system, the filter pressure drop was high, and the filtering area designed by the manufacturer did not meet the requirements specified in the basic design package. It was later resolved by expanding the area through improvements. ⑧The ignition test using a pilot burner began on November 29, and after multiple trials and adjustments to the liquefied gas orifice plate, it was completed on December 5. Meanwhile, starting from November 29, nitrogen filling simulation tests were conducted on the burner diesel pipelines and oxygen pipelines, to check whether the valve opening/closing times and the sequence control system were functioning properly, and to determine the preset opening degrees of the diesel flow control valves and pressure regulation valves, as well as the time settings for opening all valves. Ignition tests of the startup burners were carried out from December 6 to 8; a total of 7 trials were conducted, and ignition was successful on December 8 through adjustments to relevant parameters and timing settings. ⑨Starting from November 27, nitrogen was introduced into the oxygen lines of the 4 coal ash transfer lines at pressures of 0.7, 1.5, 2.5, and 4.0 MPa respectively, to test the opening and closing times of the oxygen valves. Simulation feeding tests began on November 29, during which the pressure in the coal powder conveying line was increased to 0.1, 1.5, 2.5, and 4.0 MPa respectively to test the opening times of the relevant valves as well as the sequential control logic; these tests were completed on December 2. ⑩A successful feeding was achieved on December 19, 2006. 2) Main issues during the commissioning process ① Static equipment. The ventilation cone is prone to damage: Due to inadequate process design, during the pressurization of pulverized coal, the actual pressure difference across the ventilation cone is significantly higher than the maximum designed operating pressure difference, which leads to its damage. By increasing the pressure difference display and adjusting the pressure increase rate, the pressure difference was strictly kept below 1.0 MPa, thereby resolving the issue of equipment damage caused by excessive pressure differences. Leakage in the burner flame shield: Due to design limitations that do not meet the requirements of the production process, it gets damaged after a few days of operation. This issue has been largely resolved by using different materials and increasing the water flow rate. But it was not tested under full load conditions. S1201/S1202/S1501 filters: Poor quality control during equipment manufacturing led to defects, which were later resolved. Heat exchanger leak: The E3051 nitrogen heater and the compressor oil cooler were leaking due to poor manufacturing quality; the issue has been resolved. High resistance of the burner cooling water filter: The design area was too small; after improvements by the manufacturer, it meets the requirements. ②Movable equipment. Quench gas compressor: Many problems were encountered during the commissioning process, such as rotor imbalance, insufficient number of fasteners for the dry gas seal, inadequate strength, the inability of the turntable to rotate under the design pressure, damaged ring gaskets, and high vibration levels. Boiler water circulation pump/Process water pump 2/Slag water pump: High bearing temperatures due to design issues; the problem was resolved by sending the process water pump back to the factory for repair. The boiler water circulation pump resolved the issue by addressing pipeline stress and alignment on-site. On-site repair of the slag pump was carried out to resolve the issue. ③Problems with supports, pipes, and valves: The number of pipe supports on site was insufficient, their strength was inadequate, or they did not meet the relevant requirements; in total, more than 300 such supports were repaired on site. Oxygen pipeline: Poor quality control during construction; issues such as excessively high weld bulges, defective welds, inadequate cleaning of grease, and incorrect use of pipes. The electric heating element has poor quality; the cold terminal has been replaced multiple times, and part of the electric heating element has also been replaced. Check valve leakage: The check valves in the oxygen pipeline and those for purging with nitrogen were of poor quality; they had to be sent back to the factory for repair before they became satisfactory. There are many flange leaks: the airtightness cannot be achieved properly, with many flanges leaking. There are issues with the quality of the gaskets, damage to the sealing surfaces, and problems related to installation; almost all of the gaskets had to be replaced. ④Instrument issue. Speedometers and densitometers: poor reliability and prone to damage. Biomass level gauge: It provided inaccurate measurements, with large fluctuations and was prone to interference; this issue was resolved by using a radioactive source for measurement. Fly ash level gauge, raw coal bin level gauge: prone to interference, subject to large fluctuations, and have poor stability. Biomass coal tee valve: Prone to getting stuck, often fails to open, and sometimes requires multiple attempts to open. A considerable number of valves did not meet the required switching times; this issue was resolved by increasing the pressure of the air supply and improving the cylinders. After being in use for some time, the pulverized coal balance valve suffered from severe internal leakage; even after carburizing treatment, the problem persisted. It is planned to change the valve’s structural design to resolve this issue. ⑤Process issue. The coal dust particle size is too fine: According to the design specifications of the coal grinder, under normal conditions the loading force is 13 MPa, the speed of the cyclone separator ranges from 80 to 150 revolutions per minute, and the volume of circulating air is 80,000 m3/h. In fact, in order to achieve the specified coal particle size, the coal grinder operates under boundary conditions, with a loading force of only 5 MPa, a separator speed of 50 r/min, and an air flow rate of 80,000 m3/h. Fly ash circulation: The work involved in fly ash circulation is extensive and time-consuming, and it is much more complex than expected. The entire coal ash circulation process for the 4 lines took 3 months to complete. Coal powder bridging: Bridging easily occurs during the transportation of coal powder, and this issue can be resolved by changing the pressure application method and timing. No diesel circulation pipeline: During testing, it was found that there was no circulation line in the diesel pipeline, which prevented the diesel from remaining clean; therefore, a circulation pipeline was added temporarily. Blockage of water channels: During the inspection of the water channels, it was found that some of the water wall tubes had high resistance; upon cutting open those tubes, debris was discovered to be causing the blockage. It took multiple attempts to clean them completely, and this process lasted 3 months. Blockage of the coal control valve: Caused by impurities in the coal; a filter fiber screen has been added. 3 Operation Status of the Gasification Unit 3.1 General Operation Status 1) Operation Conditions Since the successful commissioning of the gasification unit at Yueyang Sinopec Shell Gasification Co., Ltd. on December 19, 2006, the unit has been started up and shut down a total of 14 times to date. Among them, the air separation unit operated continuously for a total of 159 days, with the longest continuous operation period being 61 days. The gasification unit has operated continuously for a total of 96 days, with a maximum continuous operation period of 49 days. After the major problem caused by leaks in the burner flame shields, which led to frequent shutdowns of the plant, was largely resolved, the plant’s operation improved significantly starting in May 2007. In May and June, the operational rates were 28.3% and 56.3% respectively, while in July the highest operational rate was reached at 62% (based on the effective load of syngas; this figure rises to 78% when calculated based on coal). The operation rate is shown in Table 1. 2) Usage of raw coal: The gasification units use a variety of coal types; in total, coal from 7 mines has been used, including the Changcun Wangzhuang Mine in Shanxi Lu’an (HM081), Caishandong Coal Mine in Guang’an, Sichuan (HM181), Zhenxiong Mine in Yunnan (HM031), Jinsha Mine in Guizhou (HM138), Baisha Hongwei Nanyang Mine in Hunan (HM201), the refined coal from Baisha Hongwei Nanyang Mine in Hunan (HM201s), and Yongxing Mine in Yibin, Sichuan (HM198). Due to the small inventory levels of raw coal for different varieties, the raw coal is changed every 4 days on average. Sometimes switching the raw coal twice within 1 day poses difficulties for adjusting the gasification process. Through testing various types of coal, it was found that some coal grades such as HM081 tend to cause scaling, resulting in an extremely high outlet temperature of the syngas cooler, reaching up to 370 ℃. Increasing the frequency of the impactor had little effect; ultimately, the temperature was brought back within a controllable range by switching to a different coal grade, by which point the impactor frequency had already been increased to its maximum level. Secondly, when using refined coal, since the particles of raw coal are very small and easy to grind, even after adjusting the parameters of the coal grinder, the particle size of the pulverized coal remains too fine. This leads to instability in the pulverized coal delivery system and fluctuations in the temperature of the gasification furnace; in severe cases, it can cause damage to the refractory materials and other components inside the furnace. After comparing the 7 types of coal that have been used, the main problem with HM081 is scaling; the main issue with HM201/HM201s is extreme instability in the coal flow. Only 4 types of coal meet the requirements for use, namely HM181, HM031, HM138, and HM198. However, HM181 is no longer available, and the resources for HM198 have been allocated to another factory. Currently, only HM031 and HM138 are coal types that can be used safely ； I’ll consider trying HM196 the next time I drive. The main consumption of different coal types is shown in Table 2. ①Raw coal consumption: There is a significant difference in consumption among different types of coal. The highest HM201 consumption value reached 0.693 tons of coal per thousand standard cubic meters of effective gas, while the lowest HM081 value was only 0.628 tons; the former is 110.35% of the latter. Among the coal types that can still be used, the consumption is almost the same; HM138 has a slightly lower value, but it remains at 0.682 t, which is 116.4% of the designed coal consumption of 0.586 t. Even for the best-performing coal type, HM081, its coal consumption is 107.2% of the designed value. If the design consumption of 2,500 t of low-quality coal (0.734 t) is used, it meets the requirements. ②Medium-pressure steam consumption: The medium-pressure steam consumption reached 130% of the design value, mainly due to the air separation unit operating at a higher load while the gasification load was low. ③Limestone consumption: The limestone consumption was significantly lower than the design values; the highest addition rate in the used coal was 6%, which falls within an acceptable range. If it reaches 10% of the design maximum, operation becomes difficult, limestone transportation becomes a bottleneck, and the energy consumption for gasification also increases significantly. ④Oxygen consumption: The highest oxygen consumption was 0.626 t, while the lowest was 0.573 t for refined coal, representing 123.96% and 113.5% of the design values respectively. Since the oxygen level is significantly higher than the designed value, air separation will become a bottleneck when gasification operates at 100% load. The main reason is that the quality of the coal does not meet the standards specified for the coal at the time of design. It is usually characterized by an extremely high ash content. As can be seen from the above consumption figures, the quality of the coal not meeting the design requirements has resulted in significantly higher consumption rates of coal and oxygen compared to the designed values, thereby affecting the full-load operation of the gasification unit in subsequent stages. 3) Effect of ash content on syngas production: During the 13th operation, three types of coal were used from 17:00 on June 11, 2007, to 08:00 on June 21, namely Chongqing HM031, Sichuan Guang’an HM181, and Shanxi Lu’an HM081. During this period, the production load of the gasification unit varied significantly, mainly due to the change in coal used for gasification. The ash data for various coals are shown in Figure 1. The HM181 coal from Guang’an, Sichuan, is significantly higher than the HM081 coal from Lu’an, Shanxi, and the HM031 coal from Chongqing. The effects of ash content on the yield of usable gas are shown in Tables 3 and 4 respectively. As can be seen from Table 1, on June 14, 2007, when Shanxi Lu’an HM081 coal was used, the coal feeding rate to the gasifier was 1,050 t/d; on June 18, when Sichuan Guang’an HM181 coal was used, this rate increased to 1,261 t/d, representing a 20.10% increase compared to the previous value. However, the effective components in the coal increased by only 9.30%, resulting in a 9.93% increase in the output of useful gas. Compared with HM081 coal, HM181 coal lost 10.17% of its effective gas yield. The reason is that the ash content of HM181 is 7.4% higher than that of HM081. As can be seen from Table 2, on June 12, 2007, the coal feeding rate to the gasifier when using Chongqing HM031 coal was 987 t/d; on June 14, when using Shanxi Lu’an HM081 coal, the coal feeding rate was 1050 t/d, representing a 6.00% increase compared to the previous period. Meanwhile, the effective components in the coal increased by 7.21%, and as a result, the output of useful gas increased by 7.28%. The above two sets of data show that the ash content in coal has a significant impact on the yield of useful gas. The higher the ash content, the lower the effective gas production. Among the above three types of coal in equal quantities, HM081 yields the highest amount of useful gas. Therefore, it is very important to find a type of coal with a low ash content (13%–17%), reasonable cost-performance, and the ability to meet operational requirements. 3.2 Problems and Countermeasures in the Process 1) Low success rate of initial ignition of the start-up burner and tendency for damage: After more than 10 startups, it was found that the start-up burner often failed to ignite on the first attempt; sometimes 2 or even 3 attempts were required to achieve ignition, and it was damaged 3 times. The analysis shows that the reason is an insufficient flame length of the pilot burner, inappropriate settings of various system parameters resulting in a too low oxygen-to-oil ratio, as well as poor installation. After the burner was replaced, differences in the burner gaps caused large variations in the nitrogen back pressure during purging, making it difficult for the originally set process parameters to function properly. To this end, the liquefied gas pressure at the pilot burner was increased, the diameter of the orifice plate was enlarged, and the flow rate of combustion air was also increased accordingly; the flame observed through the burner opening was noticeably thicker and longer than before. By appropriately adjusting the relevant parameters to keep the oxygen-to-fuel ratio within an appropriate range during ignition, successful ignition on the first attempt is now generally achievable. The main causes of burner damage are poor assembly quality, such as O-ring leaks and improper gaps in the burner head, as well as dirty diesel; these issues have now been resolved. 2) Measures to address slag blockage: Local slag blockage is inevitable during the operation of the facility. Factors such as frequent changes in coal type, inappropriate amounts of limestone added, poor temperature control in the gasifier, rapid temperature changes, excessively low temperatures, and unstable coal properties can all lead to the formation of large chunks and fine slag that cause blockages. Larger slag pieces generally get stuck at the top of the downcomer at the bottom of the slag tank (V1402), while finer slag pieces get stuck at the outlet valve (14XV0015) of the slag collection tank (V1403). For large slag lumps, pressure difference is usually applied using a top-pressure and bottom-compression method to break them; for blockages caused by fine coal ash, temporary discharge pipes are used to increase the pressure to 1–1.5 MPa for discharge. During normal operation, it is easy to monitor and determine whether slag discharge is proceeding normally by observing the relevant trend charts and the changes in the weight of the slag extractor belt. Under normal circumstances, slag blockages can be resolved using the methods mentioned above. 3) Causes of ash blockage and solutions: There were 2 instances of ash blockage in total. The first occurrence happened shortly after the first batch of material was fed, and the reason was that low pipe temperatures made it easy for fly ash to cause blockages. The pipeline was cleared by temporarily adding 4 nitrogen purge points to the pipeline from the fly ash inflation tank (V1508) to the fly ash collection tank (V1507). The second ash blockage was more severe; all the fly ash got stuck in V1508, and the pipeline from V1508 to V1507 became blocked. As a solution, the valve (15XV0017) was removed, a temporary pipe was connected via its flange, pressure was applied up to 0.5 MPa to discharge the material, and at the same time, the pipeline from V1508 to V1507 was cleared using a purging method. Through analysis, it is found that when the moisture content in fly ash is high, a low temperature of the nitrogen gas used for transportation can lead to caking, thereby causing blockages in the ash flow. Therefore, the nitrogen outlet of the nitrogen heater (E1501) was raised to 100 ℃ (originally designed at 80 ℃), and no ash clogging issues have occurred since then. This major overhaul also involved insulating the pipelines from V1508 to V1507. 4) Overheating of the gasification furnace leads to damage to the refractory materials and pins. Since the gasification furnace lacks an automatic temperature control system and interlock protection system, when there are significant fluctuations in the coal powder delivery line, this causes sharp spikes in the furnace’s temperature. In such cases, the temperature readings indicated by 13TI9001/9002/9003 are inaccurate, and the fluctuations are so large that they cannot help operators make accurate judgments. This leads to damage to the gasification furnace, resulting in huge losses. An high-flow interlock for the steam volume of the gasifier (13FI0147) has been added, along with an automatic control function for 13FI0147. 5) The wastewater treatment unit does not operate optimally. Problems often arise during wastewater treatment, such as scaling of the packing in the stripping tower, high resistance that prevents an adequate increase in flow rate, poor control over the addition of flocculants, which results in wastewater of substandard quality that contaminates the recycled water tank and damages the circulating high-pressure pump, as well as issues with the quality of the filter cloth. Through efforts, the issue of drug concentration control has been resolved, and qualified filter cloth has been replaced. Regarding the scaling issue, considering changing the process to add equipment for the separate treatment of wastewater from the slag water system. 6) The powder discharge capacity of the coal powder discharge tank is insufficient, mainly due to an excessively long pressurization time of about 18 minutes. It is considered to increase the number of openings on each tube of the ventilation plate in the coal powder discharge tank in order to reduce the pressurization time, address the issue of poor powder discharge, and improve the powder discharge capacity. 7) Imbalance in liquid levels between the large and small vapor drums under high load conditions: When the effective gas load exceeds 70%, the steam generation in the syngas cooler is greater than that in the gasifier, resulting in a higher liquid level in the small vapor drum compared to that in the large vapor drum. At the same time, the balance valve (13XV0049) is opened, which affects the accuracy of measurement by 13FI0147. An additional flow-limiting orifice plate has been added to 13FI0147. 3.3 Problems with the equipment and countermeasures 1) Excessive vibration in high-pressure process pumps. High-pressure process pumps suffer from frequent failures, characterized by excessive vibration and high bearing temperatures. Analysis revealed that the seal water from the low-pressure process water pump returned to the process water tank, causing the inlet filter to become clogged frequently. This led to wear of the ring and impeller, ultimately resulting in excessive vibration. The return water is now directed to the clarifier, and the pump is operating well, resolving the issue of frequent pump failures. 2) Jamming of the pulverized coal hopper valve: During operation, the first pulverized coal hopper valve (12XV0231) got jammed and could not open or close automatically. As a solution, it was forced to stay in the fully open position, with the second hopper valve taking over control. Additionally, nitrogen was used instead of instrument air to raise the pressure to 0.9 MPa, allowing the valve to function properly; this arrangement enabled successful operation for over 40 days. Inspection revealed that the entry of a large amount of pulverized coal into the valve chamber caused increased resistance during valve rotation, leading to jamming. Currently, nitrogen is blown in to prevent coal powder from accumulating in the valve chamber. 4 Issues Requiring Further Resolution 4.1 Failure to Achieve Full Capacity Operation (2000 t/d) Although the unit has been in operation for about 500 days, the maximum coal load has only reached 78%, and the effective yield of syngas is only 67% of the designed level. To achieve full-capacity production, the following bottlenecks need to be addressed. Shell SGSI believes that the current flame shield of the burner protrudes only 60 mm into the gasifier. If the slag layer is too thick, it may cover the surface of the flame shield, leading to overheating and damage. In actual operation, the load reached up to 80% at its maximum; the plant operated with a load of nearly 1600 tons of coal for 2 days, with normal performance, before reducing the load due to requirements from downstream users. Next step: Maintain the load at 75% for about 1 week, then gradually increase it to 80% and keep it at that level for at least 1 week; if everything is normal, raise it to 90% for operation. If the flame arrester is damaged, consider ordering a flame arrester that protrudes 94 mm. Among the coal types currently in use, the best ones have an actual oxygen consumption of 113% of the designed value, while the worst ones have 123%. Given that the air separation unit operates at a maximum load of 105%, the gasification unit can only reach a load of up to 90%. Another improvement measure is to optimize operation in order to minimize the operating temperature of the gasifier. Due to the ash content in the coal reaching the design limit, the slag volume is the highest under the same load conditions. Based on actual operation experience, when the ash content plus the amount of limestone is 24%, the slag skimmer can meet at most 90% of the load requirement. In fact, the number of scrapers on the slag skimmer has been increased, and the chain guides have been improved. 4.2 The superheating temperature of medium-pressure steam does not reach the design value. The yield of by-product superheated steam specified in the coal gasification technology transfer contract should be 0.758 t/1000 m3 (CO+H2), whereas when using HM181 coal from Guang’an, Sichuan, the yield of by-product superheated steam is 1.15 t/1000 m3 (CO+H2), which is 52% higher. The by-product superheated steam yield per unit volume when using Shanxi Lu’an HM081 coal is 1.04 t/1000 m3 (CO+H2), which is 37% higher. The reason may be related to an excessively high flow rate of the quenching gas. The original design called for a cooling gas ratio of 1.1, while the current value is close to 2.0. An excessive amount of cooling gas reduces the quality of the hot syngas; the temperature of 13TI9002 is only 700–780 °C, which is lower than the designed value of 900 °C. At the same time, the flow rate of gas entering the syngas cooler is higher than specified, resulting in a large amount of low-quality heat that generates excess steam, and this in turn reduces the superheat level – sometimes it is only 320 °C, and at best it is 345 °C, which is a significant difference from the normal value of 395 °C. Improvement measures: Reduce the quench gas flow rate and increase the temperature of 13TI0019 to 700 ℃. 4.3 The dry-basis effective components in the syngas are below the specified value. The contract for the transfer of coal gasification technology requires that the content of dry-basis volume effective components in the syngas be greater than 89%, whereas the actual value is only 81%–82%. Improvement measure: Reduce the amount of nitrogen backblowing used. Select an appropriate coal type, adjust the amount of limestone added, lower the gasification temperature, control the CO2 concentration in the syngas as much as possible, and increase the production load. 4.4 Scaling in the inlet pipeline of the recycle gas compressor: There is currently no effective solution to the problem of pipeline scaling. Analysis shows that the main component of the scale is iron sulfide, indicating that corrosion is the cause. Whenever the vehicle is driven, the low temperature of the pipes facilitates the formation of sulfur corrosion. During this cycle, the combined operation lasted 49 days; the pressure difference remained normal, at around 20 kPa. Upon removing the filter screen, only a small amount of scale was found. However, if starting and stopping occur frequently, resulting in accelerated corrosion and significant scaling, considering adding a vertical separator to address this issue. 4.5 The online pH analyzers are unreliable. The pH analyzer for the circulating liquid at the bottom of the wash tower (16QIC0015) and the pH meter for the liquid exiting the bottom of the stripping tower (17QICO002) do not function properly; they get damaged shortly after being put into use, and even after replacing the electrodes they continue to fail to work, remaining unusable to this day. During operation, testing can only be done once per hour using pH test strips, which results in large errors; monitoring is not possible at any time, and automatic operation is also not feasible. Consider adding secondary dehydration and filtration to remove impurities as a solution. Additionally, internal leakage was caused by wear of the valve core in the pulverized coal discharge tank’s balance valve; carburizing treatment was applied to the valve core, which improved the situation, but the problem was not completely resolved. Next, considering changing the valve’s structural design along with spraying a wear-resistant material. The pulverized coal tee valve often fails to open when coal is fed into it. Although repairs have been carried out, it can only be used for a few times after each repair, and the problem has not been completely resolved yet. 5 Conclusion The Shell powder coal gasification technology features a mature process, high degree of automation, stable gasification, temperatures that can reach 1600 ℃, a high coal conversion rate, and the ability to handle coals with high ash content (up to 25%). The gasification furnace is easy to maintain with few repair requirements, but it demands high stability from the coal powder conveying line; the quality of the coal used must remain stable over a certain period of time, and high reliability of the instruments is also required. The operation and control of gasification furnaces are quite difficult, requiring extensive experience. Through commissioning and trial operation, and by continuously accumulating experience, it is believed that safe, stable, efficient, and economical operation of the pulverized coal gasification unit can certainly be achieved.