Research and Application of Closed-Loop Recovery of Steam Condensate in Coking Plants

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Research and Application of Closed-Loop Recovery of Steam Condensate in Coking Plants Yang Lijing (China Environmental Management Institute, Qinhuangdao 066004) Li Xiaohong, Ding Zhijun (Baotou Steel Coking Plant, Baotou 014010) The Baotou Steel Coking Plant is one of the units with high steam consumption within the Baotou Steel Group; its steam usage is around 140 t/h in winter and about 120 t/h in summer. As the coking plant is an old enterprise with a history of over 40 years, its steam and condensate recovery systems were not comprehensively planned or upgraded in conjunction with the modifications and expansions of its production facilities. Before this renovation, there were hardly any complete sets of condensate recovery systems throughout the plant, and a large amount of condensate was discharged directly into the sewer system. Due to reasons such as the failure of the steam traps, condensate is discharged along with steam, resulting in steam escaping everywhere in the plant throughout the year. This not only wastes energy but also causes environmental pollution. Therefore, the treatment of steam condensate in coking plants is of great significance. The effective utilization rate of steam for industrial production in our country is generally between 30% and 40%, which is lower than that in developed Western countries. In an era of rising energy prices and increasingly strained energy supplies, energy conservation has become particularly important. The calorific value of steam can be divided into latent heat and sensible heat; among the total calorific value of steam, latent heat accounts for about 80% of it. When using steam for indirect heating, the latent heat of steam is primarily utilized. During the heating process, as heat exchange takes place, latent heat is continuously released, and steam gradually turns into condensed water. To ensure the efficient operation of the heating equipment at its designed temperature, the condensed water generated must be removed from the equipment promptly. In order to guarantee that all of the steam undergoes phase change before it is removed, that is, only after it has completely turned into condensed water, a drain valve must be installed at the end of the system. The formation of steam condensate is similar to the process of producing distilled water; therefore, the quality of the condensate is very good, and it can even meet the water supply requirements of ordinary low-pressure boilers. At the same time, since most steam-using equipment discharges saturated condensate at the corresponding steam pressure, its temperature is very high, generally above 100°C; therefore, steam condensate has great value for recovery. 1 Determination of the recycling plan: The Baotou area is located in the upper reaches of the Yellow River. In recent years, it has become increasingly difficult to obtain water from the sources of the Yellow River, and water prices have risen. Coupled with the continuous increase in coal prices, the cost (price) of steam has increased significantly. The recovery of condensate water has become an aspect that enterprises must pay attention to in terms of energy conservation, and it is also an important challenge for Baogang. Therefore, in recent years Baogang has been constantly trying to adopt new processes and equipment for the recovery and utilization of condensate water and waste steam. There are mainly two forms of condensate recovery: open recovery and closed recovery. Open-loop recovery is a method in which condensate water is sent to a water tank connected to the atmosphere for recovery. Closed-loop recovery involves the efficient removal of the high-temperature condensate water generated after the latent heat of steam is released. This water is smoothly discharged by specialized continuous traps designed for closed-loop recovery, flows through sealed pipelines, and is then directed back to the closed-loop condensate recovery system through adjustments made by auxiliary devices such as boosters and multi-way distributors. Finally, it is automatically transported to the facilities that require water using high-temperature condensate pumps. Based on the actual conditions of the coking plant and after thorough research, it has been decided to implement closed-loop recovery for the steam condensate from the coking process. The temperature of the condensed water recovered in a closed system is significantly higher than that in an open system; as there is no leakage of secondary flash vapor, the amount of condensed water that can be recovered is 3% to 10% higher compared to open-system recovery. By using closed-loop recovery, re-contact between the condensed water and the atmosphere can be avoided, preventing gases such as O2 and CO2 from causing secondary pollution to the returned water or inducing oxidative corrosion in downstream equipment. The quality of the recovered condensed water is good, resulting in a longer service life for the water return system. 2 Advantages and disadvantages of the two recovery methods 2.1 Open-loop recovery The advantage of open-loop recovery is that it has little impact on the upstream heat exchange facilities, resulting in a more stable system. The disadvantages are as follows: (1) Due to the flashing of secondary steam, heat loss accounts for 50%–70% of the heat output by the trap, and the residual pressure energy is also completely lost; the amount of softened water lost constitutes 5%–20% of the amount of condensate water. (2) It causes severe thermal and noise pollution to the environment. Condensate water can be secondary polluted by oxygen and carbon dioxide in the atmosphere, causing corrosion to water tanks and downstream water-using equipment. 2.2 Technical challenges of closed systems: (1) The back pressure behind the trap increases, reducing its capacity and potentially affecting the heat exchange in the process equipment ； (2) When steam and water coexist, unsafe issues such as steam blockage and water hammer can occur, resulting in a **reduction in the actual conveying capacity ； (3) When condensate water from different processes enters the system, issues arise with poor return flow of low-pressure condensate water due to differences in back pressure ； (4) When the hydrophobic point and the recovery device cross obstacles or roads, there is a problem of insufficient ability of condensate water to climb upward ； (5) When individual traps leak severely, it will cause the back pressure of the entire system to increase, affecting the system’s balance ； (6) When transporting high-temperature condensate, the water pump impeller is prone to cavitation, leading to problems such as vacuum formation and equipment damage. The core components of a closed-loop condensate recovery system include the main unit, safety valve, butterfly valve, high-temperature condensate pump, magnetic flap level sensor, PLC control cabinet, and other elements. The main unit is composed of a pressure vessel, patented components such as an internal steam-water separator, a pressure regulator, a drain device, and a cavitation eliminator. Its technical core lies in applying the theory of micro-undercooling to ensure that the water entering the pump is free of vapor and does not vaporize, allowing the pump to operate continuously in a condition of transporting single-phase high-temperature water. This approach eliminates the factors that can cause cavitation in the pump, thereby ensuring that the impeller is not subjected to cavitation when transporting high-temperature water. This condensate collection device is a pressure vessel that captures high-temperature condensate in a sealed manner; there is no new steam leaking from secondary steam vents or drain valves, and no waste of water or thermal energy occurs. The steam that leaks from these vents and valves condenses into water inside the vessel. Furthermore, the use of a multi-channel combiner allows for the creation of local low-pressure areas within the combiner through entrainment, ensuring that the condensate from each channel can be discharged smoothly and combined into a single return water network for reuse ； The booster can adjust the flow pattern of the vapor-liquid mixture, and by utilizing the energy released from flash condensation, it enhances the ability of the condensed water to rise, thereby successfully resolving the issue of gas obstruction and allowing the condensed water to flow smoothly into the condensate recovery system. In actual processes, it is common to encounter situations where the pressure levels of condensate water vary. To enable the use of a single device for condensate water at different pressures, a multi-channel distributor should be installed at the point where the streams converge; this device uses the condensate water with higher pressure as a jet to force the condensate water with lower pressure into the closed-loop recovery system. When the condensate return pipe rises from underground to the surface, climbs over obstacles, or crosses roads, in order to improve its ability to rise, a self-priming booster needs to be installed at the junction between the horizontal and vertical sections. By adjusting the flow pattern of the vapor and liquid phases, and utilizing the pressure generated by flashing, this booster enables the condensate to rise to greater heights, thereby significantly enhancing its rising capacity and eliminating the phenomenon of \"impact\" in the vertical pipe. The amount of steam that can be recovered from the coking plant is shown in Table 1. Table 1: Recoverable steam condensate from coking plants (t/h), as well as steam consumption in various processes. Steam consumption in summer and winter: Pure benzene – 1012; Tar – 810. Recovery process 1 (ammonium sulfate, desulfurization, crude benzene) – 66; Recovery process 2 (ammonium sulfide, drum cooling, crude benzene) – 1315; Recovery process 3 (ammonium sulfate, desulfurization, crude benzene) – 99. Refrigeration stations: Recovery process 1 – 8; Recovery process 2 – 15; Recovery process 3 – 10. Deaerator for dry quenching of coke – 88. Total – 8760. As can be seen from Table 1, the amount of condensate that can be recovered from coking plants in summer is approximately 87 t/h, while in winter it is about 60 t/h. This represents considerable economic value and social significance. Although closed-loop recovery of condensate has many advantages, there is no precedent for its use at Baogang. Therefore, in the first phase of the renovation, pilot projects were only carried out in those sections of the coking plant where steam consumption is high and the steam usage points are concentrated, namely the double-recycling drum cooling, ammonia-sulfur, and refined benzene distillation units. Once these pilots were successful, the approach was then rolled out on a larger scale across the entire plant. 3 Selection of main equipment: Taking into account the production processes in the recovery workshop of the coking plant and the tar and benzene refining workshop, as well as the variations in the amount of condensate water recovered during winter and summer, and based on the technical data provided by the manufacturers, the estimated amounts of condensate water to be recovered in each system and the selection of main equipment are as follows: (1) The drum cooling section for secondary recovery. The condensate recovery rate is 3–5 t/h, using JF-CW5 type equipment. (2) The ammonia-sulfur recovery section. The condensate recovery rate is 6–10 t/h, using JF-CW 12 type equipment. (3) Benzene distillation section. The condensate recovery rate is 10 tons/h, using JF-C W 12 type equipment (explosion-proof). (4) Selection of air vents. A steam trap is a device that connects steam and condensate, and it plays a very important role in steam systems. If the strainer does not function properly, it will not only lead to significant waste of energy but also result in a decline in product quality; in severe cases, it can even bring the entire system to a halt. Given the actual conditions in our country, steam traps represent the area with the greatest potential for energy savings in steam systems. Generally, the investment in steam traps for the energy-saving renovation of steam systems accounts for 5% to 7% of the total investment, but they can contribute 40% to 50% of the overall energy-saving benefits. (5) The function of a steam trap is to continuously and promptly remove condensed water and other non-condensable gases such as air from the steam outside the system, while also preventing steam leakage. It is the boundary between steam and condensed water, and it is a crucial element in steam systems; it directly affects the amount of steam used during the heating process or the efficiency of the heating equipment. It is also related to the operation of the subsequent condensate recovery system, and even determines the effectiveness of the entire steam system and the condensate recovery system. (6) In steam systems, it is generally required to install steam traps in order to ensure the efficiency of the equipment and reduce energy waste. However, there is no universal trap; different traps must be selected depending on the specific application. In the renovation of condensate recovery systems in coking plants, the goal is to recover the waste steam from various heat exchangers. To ensure heating efficiency and cope with possible load fluctuations, mechanical steam traps are the most suitable choice. Inverted barrel-type traps offer the best performance, but they are expensive. Considering the actual conditions of this project, we chose the free-floating ball-type trap, which provides a better cost-performance ratio. A steam trap is a mechanical device; regardless of its brand or model, it can become ineffective due to improper installation or long-term use. Therefore, it is essential to regularly inspect, monitor, and adjust the trap, as well as carry out routine maintenance. 4 Reform plans for the condensate water process 4.1 Main parameters of the waste steam discharge unit (1) Main parameters of the waste steam discharge unit in the secondary recovery workshop. The second recovery workshop is the largest steam consumer in the coking plant. The main waste steam emission sources include the ammonia-sulfur unit, with a pressure of 0.15 MPa, a temperature of around 140°C, and a flow rate of about 10 t/h; as well as the insulation of the drum-cooled tar tanks and the heat tracing of process pipelines, for which the steam pressure is 0.12 MPa, the temperature is around 140°C, and the flow rate is about 3–5 t/h. (2) Main parameters of the waste steam emission unit in the benzene distillation section. The refined benzene distillation section is the main steam-consuming unit in the refined benzene plant; it features 5 waste steam branches and 9 condensate discharge points, with a total condensate volume of around 10 tons per hour. The actual situation on site is that none of the heat exchange equipment is equipped with traps; all condensate water is discharged directly into the sewer system. A large amount of thermal energy is not utilized effectively, resulting in significant energy waste and serious pollution of the factory’s environment as well. 4.2 Condensate recovery process: Based on the actual conditions on site, during this renovation, steam traps were installed on the waste steam pipes in various sections; these traps, along with self-priming boosters, are connected to a multi-channel collector. The collected condensate is then sent to a dedicated \"closed-loop condensate recovery system,\" from where it is pumped to the users for reuse. 4.3 Recovery of waste steam from the waste steam discharge unit (1) Recovery of waste steam from the ammonia-sulfur recovery section. Based on the actual conditions of the ammonia-sulfur unit, steam traps are installed at the waste steam discharge points, and self-priming boosters are used to raise the pressure and direct the flow to a multi-path collector; the condensate then enters JF-CW 10 for recovery. Since the temperature of the vapor condensate generated in the ammonia-sulfur unit itself is above 130°C, the recovered condensate can be directly fed into the waste heat boiler of this unit (the make-up water for the original waste heat boiler is softened water produced by the softening station, which is heated to 128°C using external steam before being fed into the waste heat boiler). After this renovation, all the condensed water recovered is used internally by the ammonia-sulfur unit, reducing the consumption of softened water and heated steam. (2) Recovery of waste steam from the secondary recovery drum cooling section. A drain valve is installed at the end of the instrument heating pipeline; the condensate rises to a higher level thanks to the self-priming booster JF-ZY, and then enters JF-CW5 for recovery. In the tar tank unit, steam traps are installed at the ends of the condensate water pipelines in each of the two tar tanks. The condensate water gathers in the main recovery pipeline, and thanks to the use of a self-priming booster JF-ZY, it is lifted to a higher level before entering the JF-CW5 system for recovery. A drain valve is installed at the end of the pipeline for heat tracing, where the condensate collects in a main collection pipe. It then rises to the multi-way distributor through a self-priming booster, and the condensate is sent to the JF-CW5 unit for recovery. The condensate water recovered from the above three units is used in winter to replenish the heating return water tank in the drum cooling area as heating water, and in summer it is fed into the condensate recovery pipeline of the ammonia-sulfur unit and sent to the waste heat boiler’s water replenishment tank. (3) Recovery of waste steam from the benzene distillation section. Strainers were installed on the condensate pipelines of each reaction tower in the pure benzene distillation section; condensates under identical operating conditions were combined into one stream and sent to the condensate recovery unit JF-CW 12. Since the pressure level of the waste steam emitted from the primary distillation tower is lower than that of the waste steam from other branches, it cannot be sent directly to the condensate recovery unit. Therefore, a separate start-up pump is installed to draw high-pressure steam from the main steam supply line, pressurize the waste steam from the primary distillation tower, and then send it to the recovery unit. The recovered high-temperature condensate is used in winter as make-up water for heating at the coking heat exchange station, and in summer it is fed into the workshop’s circulating water system to serve as make-up water for the hyperbolic cooling tower. 5 Energy-saving benefits: The steam consumption of the ammonia-sulfur unit in the recovery workshop is approximately 8 tons per hour; the temperature of the condensed water is 137°C (as measured), while the temperature in the closed-loop recovery system is around 135°C. All waste heat boilers are supplied with condensate water, and softened water as well as external steam used to heat the softened water are no longer utilized. The closed-loop recovery temperature for the condensate water from the instrument heating pipelines is around 130°C; the closed-loop recovery temperature for the condensate water from the pipeline heating pipelines is around 135°C. The closed-loop recovery temperature for the condensate water from the tar tank is also around 135°C. The total water volume from these three sources is approximately 5 tons per hour ； The closed-loop recovery temperature for the condensate water in the pure benzene workshop is around 140°C, with a flow rate of approximately 10 tons per hour. The value of the condensed water recovered each year is approximately 1.73 million yuan; the total investment in this project amounts to 1.883 million yuan, with an investment payback period of around 12 months. 6 Improvement effects: After the completion of the renovation project, the condensate water was recovered in a completely closed system, thereby creating a closed loop within the entire steam utilization system. There was no emission of flash vapor, which reduced internal pollution in the plant. Thermal energy was recovered and reused to the greatest extent possible, resulting in reduced consumption of soft water and steam; the energy-saving and environmental protection benefits were significant. The steam recovery system and heat exchange equipment in this device utilize indirect heat exchange; there is no direct contact with the materials. After the condensate water is recovered, its quality is maintained, preventing secondary contamination, and it can be reused in the boiler system without the need for further softening, thus saving on water treatment costs. It also eliminates the causes of oxygen corrosion in equipment and pipelines, thereby extending the service life of the system. The closed-loop condensate recovery unit features a compact structure, easy installation, fully automated operation through mechatronics, simple maintenance and management, and low operating costs. 7 Others (1) Construction of the project. Given the cold winter climate in the northern regions, and to prevent the facilities from freezing damage, we have installed the condensate recovery equipment as well as the related facilities indoors. During on-site construction, existing pipelines and pipe galleries were made use of as much as possible, reducing the project’s investment costs. (2) There are problems. During the summer, when the condensed benzene water is fed into the circulating water system, this causes the temperature of the circulating water to rise slightly, which is not favorable for the operational processes. Moreover, the thermal energy contained in the condensed water is lost needlessly. In the future, it could be considered to send the condensate back to the thermal power plant or nearby heat and water users via pipelines during the summer. It is also recommended to increase the number of waste heat boilers in the plant in order to improve the utilization rate of the recovered condensate water, thereby reducing the amount of steam supplied by the thermal power plant.