Comparison between the scrap pot process and the quenching process
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Waste boiler process: The gasification slurry is mixed with high-pressure oxygen in a burner and atomized before being fed into a gasification furnace at around 1400°C. The carbon and water contained in the coal slurry react rapidly at such high temperatures; the resulting hot syngas enters a radiant boiler and a convection boiler where it is cooled and initially dedusted. Subsequently, the gas passes through a venturi dust remover and a gas washing tower for further dust removal, before being sent to the gas purification workshop. The slag that flows with the hot air currents undergoes thermal radiation exchange with the boiler water inside the radiant tubes within the radiant boiler; thereafter, it reaches the bottom of the radiant boiler where it is cooled by ashwater to form granular glass material, which is then periodically discharged through a hopper into a slag pond for further disposal. The black water in the gasification furnace is discharged in a controlled amount; the fine carbon ash contained in this black water is returned to the mill after sedimentation and filtration (the waste boiler process is mainly used in power generation). By using radiant waste heat exchangers and convective waste heat exchangers, the heat from the high-temperature gas at around 1400°C and the slag generated during the gasification process is recovered, producing saturated steam that can be used in steam turbines for power generation within IGCC, thereby achieving heat recovery and utilization. The waste boiler process is the best option for IGCC projects. Quenching process: High-pressure oxygen and slurry are mixed and atomized through a nozzle before being injected into the gasification furnace. Under high temperature and specific operating pressure, carbon in the slurry undergoes partial oxidation with oxygen, resulting in crude gas composed mainly of CO, H2, and CO2. The high-temperature gas, carrying unreacted carbon particles and a small amount of slag particles, flows downward. It enters the quenching tank through the outlet at the bottom of the gasification furnace, where it comes into full contact with cold water. This causes the gas to cool down and become saturated, while the slag solidifies. The solidified slag and unreacted carbon particles enter the slag tank at the bottom of the quencher, where they are periodically removed under reduced pressure. The cooled gas and quench water flow from the quencher into the gas-water separator. The ash-containing water at the bottom of the separator is sent to a sedimentation tank for ash treatment after pressure reduction, while the gas coming out of the upper part enters a Venturi scrubber, where the ash and carbon particles in the gas are removed before it is sent to the downstream conversion unit. The high-temperature gas and slag generated during the gasification process enter a quenching chamber, where they come into contact with quenching water; as a result, the gas is cooled and the slag solidifies. In this process, heat mainly enters the water circulation system, resulting in significant heat loss. The quenching process is mainly used in coal chemical projects. Comparison between the waste heat boiler process and the quenching process: The waste heat boiler process involves using radial and convective waste heat boilers to recover as much sensible heat from the syngas as possible, in order to produce high-pressure steam or preheat other process fluids (such as the syngas going to the gas turbine and the nitrogen used for reinjection). This method can recover 15–18% of the energy corresponding to the lower heating value of the raw coal, enabling the efficiency of the hot gas to reach 90–95%. For combined cycle power generation, this improves the net power generation efficiency. It is more technically and economically reasonable to use a waste heat recovery process to recover the sensible heat from gas, with the gas after heat recovery being used to produce oxides (such as carbonyl compounds) or as fuel gas for combined cycle power generation. The quenching process involves cooling the raw gas with water after it passes through the gasifier, thereby bringing the gas to a saturated state. However, compared to the waste heat recovery process, this method results in a 5–8% decrease in the efficiency of the hot gas (in absolute terms), and it also reduces the net efficiency of combined cycle power generation by 4–5% (in absolute terms). Generally, the quenching process is used to produce chemical products such as hydrogen, ammonia, and methanol. Overall, for IGCC combined-cycle power generation units, the waste boiler process results in a roughly 15% higher utilization rate of the feed coal compared to the quenching process, offering significant technical and economic advantages.
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The raw gas at the outlet of the gasifier generally has a high temperature. The higher the temperature of raw coal gas, the greater its sensible heat energy. Harmful impurities in raw coal gas must be removed, and under current technical conditions, low-temperature wet purification remains the primary method for cleaning coal gas. Therefore, the gas must be cooled before purification; at the same time, a large amount of sensible heat in the gas is carried away by the cooling medium. If this sensible heat is not recovered, energy losses will be significant. Clearly, whether the sensible heat of gas can be fully utilized has a significant impact on the net efficiency of combined-cycle power generation. Heat recovery systems are highly complex; they can be arranged in various ways between the gasification furnace outlet and the users of the purified syngas. This requires the use of large and expensive heat exchangers. The most important heat recovery devices are the radiant waste heat boilers located after the gasification unit, as well as the convective waste heat boilers that follow them, which **increases the capital investment required for the equipment. 1. Waste heat boiler system – Full utilization of gas sensible heat system. This system involves using radial and convective waste heat boilers to recover as much of the sensible heat in syngas as possible, in order to produce high-pressure steam or to preheat other process fluids (such as the syngas going to the gas turbine and the nitrogen used for reinjection). This method can recover approximately 14% to 18% of the energy corresponding to the lower heating value of the raw coal, enabling the efficiency of the hot gas to reach 90% to 95%; for combined cycle power generation, this improves the net power generation efficiency. The waste heat boiler process is advantageous and reasonable for producing oxygen-containing compounds (such as carbonyl compounds) or for generating fuel gas for combined cycle power generation. This is because: (1) to produce the CO feed gas required for manufacturing carbonyl compounds, syngas does not need to undergo CO shift, and therefore steam for the CO shift reaction is also not required. (2) To produce fuel gas for cyclical power generation, the full recovery of the sensible heat of the gasified coal gas enables the production of high-pressure steam suitable for steam turbines, thereby significantly improving the net efficiency of the combined cycle power generation system. Due to the use of radial and convective waste heat boilers, the equipment is more complex, resulting in higher capital investment. 2. Efficient quenching process – Utilization of the sensible heat in the gas This system involves eliminating the radiant waste heat boiler and the convective waste heat boiler, and instead quenching the raw gas with water after it exits the gasification furnace; this process generates a certain amount of medium-pressure saturated steam through flashing, thereby making use of part of the sensible heat contained in the gas. Compared to the waste heat boiler process, this method results in a 5%–8% decrease in the thermal gas efficiency (in absolute terms), which will also reduce the net efficiency of combined cycle power generation by 4%–5% (in absolute terms). The quenching process is used to produce hydrogen, by converting the CO present in the gas into hydrogen through a transformation reaction; this hydrogen is then used in the synthesis of ammonia, methanol, or for hydroprocessing in oil refineries. The steam generated after quenching can be used as steam for other processes, enabling more efficient utilization of heat. This process features a simpler design due to the elimination of the radiation waste heat boiler and the convection waste boiler, resulting in reduced capital investment. Last edited by zjyang168168 on 2007-9-26 11:39.]
1. First, assuming that the gas generation rate in the quenching process and that in the waste heat boiler process are the same, with identical outlet temperatures, the heat difference between the two lies in the difference in water vapor content and the amount of steam generated as a by-product. The quenching process involves a higher water vapor content, around 50%, while the waste heat boiler process has a lower content, around 20%. However, a certain amount of high- and medium-pressure steam is generated as a by-product. It is unclear whether the thermal efficiency mentioned in the article takes into account the heat contained in the water vapor from the quenching process. 2. Due to a lack of familiarity with gas turbine technology, it is not clear whether the thermal energy of the water vapor in syngas can be utilized in gas turbines. 3. The waste heat boiler process results in a 5%–8% decrease in thermal efficiency (in absolute terms), which in turn leads to a 4%–5% reduction in the net efficiency of combined cycle power generation (in absolute terms). "I would appreciate it if you could explain how it was estimated.
I partially agree with some of the points mentioned above. I believe the main difference between the two processes lies in the method and pressure used for steam recovery; as for the efficiency of heat recovery, I don’t think there’s a significant difference. The key difference is the amount of heat removed when the temperature of the excess quench water rises by ~10 degrees. As is well known, waste boilers can generate large amounts of superheated and high-pressure steam, but the gas coming out of these boilers also needs to be washed to remove ash. The temperature of the gas after washing is around 170 degrees; subsequently, wet desulfurization is required, which necessitates reducing the temperature to normal levels. During this cooling process, it is basically not possible to produce steam as a by-product, and the only use for the heat is to preheat the boiler water or similar. After the quenching and wet washing steps in the rapid cooling process, the temperature remains relatively high, at around 230–240 degrees. During the cooling process, a large amount of medium-pressure and low-pressure steam can be recovered; the heat recovered in this way should be equivalent to the heat recovered from the waste boiler (slightly less). It is merely because medium and low-pressure steam cannot be used directly in combined-cycle power generation that the net power generation efficiency decreases significantly. The gas composition and quantity before the gas turbine should be essentially the same for both processes, with no differences. However, with an emergency cooling process, the power generation capacity of the steam turbine is reduced, while a larger amount of medium and low-pressure steam is produced as a by-product.
I basically agree with the opinion on floor 6. One more thing to add: in actual operation, the convection exchangers in the waste heat recovery system often get clogged, and this problem has not yet been effectively resolved. Based on this, foreign companies that have proposed waste boiler processes have implicitly admitted their failure with convective waste boilers.
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In short, for the chemical industry, it’s better to use a water cooling process; otherwise, Shell wouldn’t need to develop new cooling technologies. Shell’s technology was originally designed for IGCC applications, and trying to apply it in other industries is something only China would do – acting without proper investigation