Analysis of the reasons for water presence in the pressurized gasification furnace for water-coal slurry and countermeasures

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Texaco’s pressurized gasification of water-coal slurry is a relatively successful technology in coal chemical applications. After our plant was the first to introduce it and master it, it has since been applied in numerous large-scale fertilizer production projects. For example: Shanghai Sanlian Supply, Weihe Large Fertilizer, Huaibei Large Fertilizer, etc. Through years of practical production experience, our factory has not only mastered this technology well but has also made further innovations and advancements in Texaco’s water-coal slurry pressurized gasification technology. In particular, significant progress has been achieved in understanding and addressing the issue of water presence during the operation of the gasification furnace, bringing our capabilities to world-class levels. 1 Brief description of the pressurized gasification process for water-coal slurry: Our plant primarily uses Beisu coal and Luoling coal, with local blended coal also being used. The coal slurry with a concentration of 66±1% is prepared by the coal grinding system. This slurry is pumped (at a full-load flow rate of 23.0 m3/h, corresponding to a dry-basis coal feed rate of 17.5 t/h), along with pure oxygen supplied via air separation (with a purity of around 99% and a pressure of 3.85 MPa; the oxygen demand is approximately 11,100 m3/h). The mixture is injected into the combustion chamber through process burners of domestic manufacture. There, partial oxidation-reduction reactions take place at a temperature of around 1350°C. The slag resulting from these reactions flows down through the downcomer into the quenching water area. Once cooled and solidified, the slag settles at the bottom of the quenching chamber and is periodically removed using a hopper and transported away by slag trucks. The high-temperature gases are cooled by water, becoming saturated water-gas. This saturated gas then enters the scrubber tower through the upcomer. After washing, the gas reaches a temperature of around 198°C and a pressure of 2.60 MPa, with a gas production rate of 45,000 m3/h (the components of the dry gas are shown in Table 1). This gas is then sent to another processing unit. Meanwhile, the black water coming out of the gasifier and scrubber tower undergoes three-stage flash evaporation and heat recovery before entering the sedimentation tank. The clearer gray water overflows into the gray water tank for reuse, while the sediment at the bottom is discharged to a sedimentation pond for further treatment. Its process flow is shown in Figure 1. http://www.zaoqiwang.com/upload/060518849019526.jpg 2 The phenomenon of water in the gasification furnace and its hazards 2.1 The essence of water in the gasification furnace There are two types of phenomena related to water in the gasification furnace: one is that the syngas, which contains a large amount of saturated water vapor from the gasification furnace, enters the scrubber; due to the drop in temperature, the water vapor condenses within the scrubber ； Second, when there is an abnormal operation in the gasifier, the syngas stream carries water droplets, which enter the scrubber tower along the syngas pipeline. It is normal for the former to contain water, in very small amounts ； The latter is abnormal; the amount is large. This article discusses the abnormal water carryover phenomenon. 2.2 Phenomenon of water in the gasification furnace (1) The liquid level in the gasification furnace drops sharply, preventing the formation of a quenching water zone and thus making it impossible to maintain normal operation of the gasification furnace. (2) The water output from the bottom of the gasification furnace decreased sharply, eventually dropping to zero; the water in the quench chamber was carried along with the synthesis gas stream into the scrubber tower. (3) The liquid level in the scrubber tower remains high, reaching 3.5 meters or even higher (the normal level is 2.9 meters); the water supply to the scrubber tower gradually drops to zero, resulting in a malfunction in the water system. (4) The temperature at the outlet of the washing tower is about 1–3°C higher, and the volume of syngas is about 1000 m3/h higher. This is due to a decrease in the liquid level in the gasifier; as a result, the high-temperature syngas spends less time in the chilled water zone, leading to less heat exchange and thus a higher syngas temperature. (5) The Venturi pressure difference rises to 0.10 MPa (normal: 0.07 MPa), with frequent fluctuations. The water carryover phenomenon in the gasifier occurred frequently during the period from 1996 to 1998 when the system was operating under overload conditions; it happened more than 10 times during each single operation of the gasifier. After 1998, thanks to a better understanding of this phenomenon, it occurred much less often, and when it did happen, it was easy to handle. 2.3 Hazards of water in the gasification furnace (1) When there is a significant amount of water in the gasification furnace, it causes the liquid level in the furnace to drop sharply, forcing the gasification system to stop operating, which can result in losses of nearly one million yuan. (2) An excessively high liquid level in the scrubber tower can cause water to enter the conversion system in subsequent processes, even leading to its shutdown; furthermore, if the catalyst becomes deactivated, replacing it will cost several million yuan. (3) The water content in the gasification furnace alters the sedimentation process of ash particles in the black water; as a result, the black water containing a large amount of these ash particles enters the flash evaporation system, increasing the solid content throughout the water system. This accelerates wear and tear on equipment and pipes, as well as promotes scaling and blockages in them. It not only affects the safety, stability, and long-term operation of the system but also increases maintenance costs, thereby raising the cost of the products. (4) Due to water carryover in the gasifier and the harsh operating conditions, the gasifier is unable to maintain high-load operation; as a result, the production capacity of each gasifier decreases, affecting the economic efficiency of the entire ammonia synthesis system. 3 Analysis of the mechanism behind water entrainment in the gasification furnace and preventive measures 3.1 Brief analysis of the mechanism for water entrainment in the gasification furnace The high-temperature syngas emerging from the combustion chamber enters the quenching water area at high speed; after absorbing a large amount of water vapor, it rises along the rising pipe ； Under the action of high-speed airflow, the liquid in the quenching chamber undergoes expanded boiling; the liquid level is low in the downcomer while it is high in the upcomer. The airflow passes through the downcomer and then rises rapidly along the gap between the upcomer and the downcomer, quickly surrounding and dispersing the liquid. This causes the liquid to break into numerous droplets of varying sizes, which then rise upward with the airflow at a certain speed. At this point, bulges form on the liquid surface ； When the gas flow velocity fluctuates sharply, circulatory currents occur as well; the airflow carries large amounts of water masses upward, but these carried droplets and water masses settle back down due to their own weight ； The water carryover phenomenon in the vaporization furnace occurs when the speed at which the gas stream carries the liquid droplets is greater than the settling speed of the droplets and water masses. As the liquid circulation intensifies, water carryover becomes more severe; once inertial water carryover occurs, large amounts of water masses and droplets are carried by the airflow into the scrubber tower, causing the liquid level there to remain high and preventing any replenishment of water. Of course, when the gasifier is operating normally, the water vapor carried by the airflow into the scrubber tower does not condense to an extent sufficient to cause water carryover. 3.2 Factors Affecting Water Carryover in the Gasification Furnace and Corresponding Measures (1) Gas velocity: Changes in gas velocity are an important factor that leads to water carryover in the gasification furnace. When the furnace operates at high load or when the furnace temperature is kept too high, the gas production rate increases rapidly, thereby increasing the gas velocity. This enhances the dispersion of liquid by the gas flow, and as a result, liquid circulation intensifies, leading to water carryover. (2) Load: The gasification furnace operates at a high load level (around 130% of the original design load), resulting in high pressure and temperature. The volume of gas flowing through it also increases, but the gaps through which the gas passes remain unchanged; this increase in gas velocity can cause water to be carried into the gasification furnace ； At low load, although no intense circulation occurs and the droplets dispersed by the airflow are smaller compared to high-load conditions, the speed at which the airflow carries these droplets is still greater than the speed at which they settle, which means that moisture will still be carried into the vaporization furnace ； Excessive frequency of load increases and decreases, as well as large adjustment amplitudes, can also cause water entrainment. Long-term practice has shown that it is advisable to maintain the load at 120% of the design value; adjustments to the load should be made as little as possible, and the rate of adjustment should not be too rapid. This not only prevents water from entering the gasification furnace but also contributes significantly to the long-term, stable operation of the system. (3) Liquid level: If the liquid level control in the gasification furnace is set high, the airflow spends more time passing through the chilled water area, which intensifies the dispersion of the liquid by the airflow. This enhances the circulation effect, resulting in water carryover in the gasification furnace. When dealing with water-containing substances, it is appropriate to lower the liquid level to prevent the formation of circulation, and then gradually increase the liquid level; generally, a liquid level of 2.5 to 3 meters is suitable. (4) Furnace temperature. In the operation of a gasification furnace, controlling the furnace temperature is quite critical; maintaining a high furnace temperature is beneficial for the gasification reaction, the amount of gas produced, and the carbon conversion rate ； However, if the furnace temperature is high, there is also more saturated water vapor in the quenching water; as a result, the gas volume increases and the gas velocity rises accordingly, which can also lead to the presence of water in the stream. Practice has shown that maintaining the furnace temperature at around 100°C above the ash melting point is most favorable for the gasification reaction, and it also helps to prevent water from entering the gasification furnace. (5) Pressure: According to the basic equation of the gasification reaction, it can be seen that increasing the system pressure is beneficial for the gasification reaction. However, too high a pressure will increase the velocity of the syngas, thereby increasing the tendency for water to be carried along with it; pressure fluctuations can also lead to the presence of water. When operating the gasification furnace, it is necessary to maintain stable oxygen pressure and system pressure; the oxygen pressure should be kept around 3.85 MPa, while the system pressure should be around 2.60 MPa. Maintaining stable oxygen and system pressures helps to prevent water from entering the gasification furnace and increases the gas production volume. (6) Ensuring an appropriate level of water flow at the bottom of the gasification furnace is an important indicator to determine whether there is water present in the furnace. When the water flow decreases to zero, the gas’s dispersing effect on the water in the quenching chamber increases, and the circulation phenomenon intensifies, which can lead to even more severe water contamination. Therefore, it is necessary to maintain a certain level of water flow at the bottom of the gasification furnace, so that the direction of the water flow is roughly opposite to that of the gas flow; this reduces the gas’s dispersing effect on the liquid and helps to prevent water contamination in the furnace to some extent. (7) Improvement of the gasifier internals: To address the issue of water entrainment in the gasifier, the number of large serrations at the bottom of the downcomer was increased. Thanks to these serrations, the airflow becomes more stable, and the dispersion of gas on the liquid is reduced, which also helps to prevent water entrainment. Near the syngas outlet of the gasifier, high-efficiency baffle plates are installed to enhance the sedimentation of droplets and water masses, and to prevent the presence of water in the stream. All these improvements have achieved good results. (8) Increase the circulation volume of the water system. Due to the increased load, the amount of cooling water used in operation was raised from 74 m3/h to 90 m3/h. Additionally, a filter was installed at the point where the cooling water enters the gasification furnace, and it is cleaned regularly to ensure a steady supply of cooling water. 4 Conclusions (1) Water entrainment in the gasifier is caused by the enhanced dispersion of liquid by the airflow when its velocity is too high, and the intensified liquid circulation leads to water entrainment. (2) The main factors affecting water entrainment include the gas flow rate, the liquid level in the vaporization furnace, the load, the furnace temperature, the oxygen pressure and system pressure, as well as enhanced liquid dispersion and intensified circulation effects. (3) Thanks to a rational understanding of the reasons for water presence in the gasifier and the implementation of proper preventive measures, the occurrence of water in the gasifier has been largely eliminated. This has reduced shutdowns caused by water presence as well as the need for maintenance on equipment and pipelines, thereby enabling the gasifier to operate safely, stably, and over extended periods of time, which in turn boosts the economic benefits of the enterprise. Author/Source: Zhu Dongmei, Nie Chengyuan, Sun Qingtao, Dong Jingguo (Shandong Yankuang Lunan Fertilizer Factory, Tengzhou 277527). This post was last edited by Ma Hai on 2009-3-15 at 18:15