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This post was last edited by I am kind on 2017-2-9 at 13:17. Summary: It discusses the issue of high energy consumption in the heating furnaces of the delayed coking unit at the Luoyang branch of Sinopec Corporation when the unit operates at reduced load, and analyzes the reasons behind this phenomenon. To reduce energy consumption, the approach of using a furnace idling operation was proposed. By adjusting the openings of the air dampers, flue gas dampers, and the permanent light ventilation openings in a coordinated manner, the device’s energy consumption was reduced from 1.8 t/h to 1.57 t/h, while still ensuring its normal operation. Keywords: delayed coking; heating furnace; furnace operation; energy saving. Delayed coking units have low construction and operating costs, high conversion rates and return on investment. Moreover, they possess technical and economic advantages in processing heavy oils and ultra-heavy oils with high sulfur content, high metal content, high specific gravity, and difficult processing characteristics, which makes delayed coking one of the important methods for the secondary processing of crude oil. The design capacity of the delayed coking unit at Sinopec’s Luoyang branch is 1.4 Mt/a, and it adopts the large-scale \"one furnace, two towers\" process developed by LPEC, along with a technology that allows for flexible adjustment of the recycle ratio. The level of energy consumption is closely related to the efficiency of the plant. The heating furnaces in coking plants primarily use fuel gas as a fuel source, and fuel gas accounts for over 60% of the total energy consumption in coking processes. Therefore, controlling the energy consumption associated with fuel gas plays an important role in improving the energy efficiency of the plant. Especially since production in a single furnace chamber has become required by the company, energy consumption has increased significantly, affecting the plant’s energy use and efficiency. Under the condition where production in a single furnace chamber is the norm, optimizing operations to reduce energy consumption has become a key issue in production. 1. Current production status: In accordance with the subsidiary’s production plan for 2015, the coking unit adopts a \"vaporization-coking\" processing route for Tahe oil. Based on the annual oil product balance, the coking unit needs to operate at ultra-low load levels from time to time. Under such operating conditions, the feed rate to the coking unit is kept at 85 t/h, with a recycle ratio of 0.15–0.2. This results in high energy consumption for the unit, around 23–27 kgeo/t per month, which represents an increase of about 20% compared to the energy consumption during the period when the unit operated with two chambers. During single-chamber production in the furnace of the unit, it is required that one furnace be in use for production while the other chamber remains in a hot standby state. The outlet temperature of the furnace in the production chamber is maintained at 490±5°C. To ensure that the outlet temperature of the standby furnace does not affect the reaction temperature in the coke tower, the outlet temperature of the standby chamber is kept at 495±5°C. The fuel gas energy consumption increased from 13.59 kgeo/t to 16.24 kgeo/t, an increase of 3.65 kgeo/t, representing a 26.86% rise in consumption; other media also saw increases to varying degrees. To ensure the reaction depth, the fuel gas in the production chamber has reached its limit of use; therefore, considering reducing the fuel gas consumption in the backup chamber is of great significance for controlling the total energy consumption. The air supply and fuel gas systems of the heating furnace are shown in Figure 2. Cold air enters the heating furnace after exchanging heat with an air preheater via a blower, while flue gas is drawn into the preheater by an exhaust fan to reduce its temperature before being discharged into the atmosphere through a chimney. Production is carried out at an outlet temperature of 490°C; the temperature inside the furnace chamber generally ranges from 640°C to 710°C. The temperature of the air after heat exchange before entering the furnace chamber is around 280°C. Entering the furnace chamber results in a decrease in its temperature, which requires more fuel gas for heating. Therefore, reducing the amount of air supplied to the furnace chamber can help minimize fuel gas consumption. 2. Impact of air intake volume on the device: To reduce gas consumption in the hot standby room, it is necessary to increase the temperature inside the furnace, provided that the temperature at the furnace outlet is maintained. The furnace temperature is supplied by the on-site main burner, the pilot light, and the hot air from the reheating furnace’s waste heat recovery system; at the same time, some heat is lost as it is drawn away through the flue dampers. Reducing the number of main burners and pilot lights in operation can significantly lower the gas consumption per unit of output; meanwhile, to maintain an appropriate furnace temperature, it is necessary to increase the blower temperature and reduce heat losses from the preheating chamber. Hot air is introduced into the furnace chamber by the furnace blower, which heats the air through an air preheater. The air preheater has a maximum allowable temperature; the blower temperature cannot be increased excessively, as this does not result in significant energy savings ； When the air supply volume is increased, in order to maintain proper negative pressure in the furnace, the flue dampers are opened wider, which results in more heat being removed from the furnace and higher heat losses; thus, the energy-saving effect is not significant ； However, by starting with minimizing heat loss from the hot standby room and allowing it to \"simmer,\" significant energy savings can be achieved. 2.1 Influence of air intake volume on the negative pressure of the device: The negative pressure in a heating furnace is primarily intended to ensure that the flue gases generated after combustion can be discharged promptly, while also preventing burns to personnel due to positive pressure when monitoring the furnace. During self-ventilation, the negative pressure is primarily controlled by the chimney height and the number and size of the quick-opening dampers ； During forced ventilation, the negative pressure in the heating furnace is primarily controlled by blowers and exhaust fans. During normal production, forced ventilation is primarily used; the blower’s operating level is maintained at around 50%, while the exhaust fan’s operating level is generally kept between 45% and 55%. At the same time, the flue dampers and air duct dampers are adjusted appropriately to control the negative pressure. To minimize the impact of cold air on the furnace chamber, attempts were made to gradually close the air dampers. Through repeated trials, it was found that when the air dampers are fully closed and the flue damper is set at around 15%, a negative pressure of approximately -15 to -20 KPa can be maintained, which meets the requirements for normal production. 2.2 Effect of air intake volume on oxygen content in flue gas The oxygen content in flue gas refers to the volume fraction of oxygen in it; generally, during production, this oxygen content is kept between 2-4% (V/V). Controlling the oxygen content is necessary to ensure that the fuel gas can burn properly and completely inside the furnace. There are four sources of oxygen available for the heating furnace: (1) air baffle. It is the source of oxygen during the period of forced ventilation for production, and it is the primary source; as driven by the blower, 90% of the oxygen comes from here. The downside is that the temperature is low, which reduces the furnace temperature and increases the energy consumption of the fuel gas. (2) Open the air damper quickly. Under natural ventilation, the main source of oxygen is available; in cases of abnormal ventilation, oxygen levels are supplemented relying on the quick-opening air valves at the bottom of the heating furnace, which constitutes abnormal ventilation. (3) Intake manual valve. Each firebox is equipped with a valve that allows manual control of the air flow entering it. It serves as an important means of regulating the amount of air that reaches the burner for combustion, especially under forced ventilation conditions. By adjusting this valve, it is possible to control both the oxygen level and the burning condition of the flame. (4) Permanent lamp air inlet. The air intake here does not serve as the main source of oxygen; rather, it functions as a supplementary source to replenish the oxygen level in the furnace. 3. Rectification measures: During the furnace holding process, through continuous experimentation, it was ensured that the oxygen level in the hot standby chamber remained at 2–4%, and the temperature at the furnace outlet was at 490±5°C. Meanwhile, the air dampers were gradually closed, and attempts were made to increase the opening degree of the permanent ventilation openings in batches in order to replenish the oxygen level inside the furnace. The ventilation holes for the perpetual lamps can adjust the air flow rate freely by rotating the sleeve. In practice, the diagram below shows the results of adjustments made to keep the hot standby room within an optimal operating range. Each coking heater has 16 burners; each burner is equipped with one ventilation hole for the perpetual lamp. There are two main nozzles per burner, one inner flame and one outer flame. With 4 such sets per room, there are a total of 128 ventilation holes. The calculations here are based on one furnace chamber as a reference. Table 1 Test adjustment data for the hot standby room. Table 1: Oxygen content in the test data from the furnace chamber. Air damper opening degree, flue gas damper opening degree, and permanent flame ventilation opening degree: 10–15%, 26–31%, 1/3; 5–10%, 15–20%, 1/2; 0%, 10–15%, 2/3. As can be seen from Table 1, when the air damper is completely closed, keeping the flue gas damper at 10–15% ensures normal combustion of the flame, enabling furnace idling and thus achieving energy savings. The specific measures for \"stopping the furnace operation\" are shown in Table 2: Table 2 Measures Taken for the Furnace Chamber. Sequence Number, Measure Content, Precautions: 1. Completely close the air duct damper AV1101 in the hot standby chamber to prevent large amounts of hot air at around 280°C from entering the chamber; oxygen supply to the furnace chamber shall come from the pilot light air holes. When closing the air duct damper, pay attention to adjusting the frequency of the blower. 2. Reduce the opening of the flue duct dampers KV1101 and KV1102 to about 10%, which will ensure proper negative pressure in the hot standby chamber. The adjustment range of the flue duct dampers should not be too large or too rapid. 3. The temperature in the furnace chamber of the hot standby chamber can be controlled by slightly adjusting the opening of the flue duct dampers; care should be taken to avoid a too rapid drop in furnace temperature. 4. Adjust the negative pressure and oxygen level in the production chamber promptly. The adjustments made in the hot standby chamber should ensure that there are no significant fluctuations in the production chamber. 4. Results of Implementation and Economic Benefits: Through this \"stopping the furnace operation\" approach, it is possible to completely shut down the main burners while maintaining an appropriate furnace temperature in the hot standby chamber. Additionally, only half as much fuel gas is required for the pilot light compared to normal operation in a single-chamber furnace, resulting in significant energy savings. This optimization approach has the following advantages: 1) Key innovation: Under conditions of ultra-low load operation of the plant, it requires no cost investment; there is no need to modify any pipelines or valves in the plant. Through clever operational optimizations, it is possible to significantly reduce the plant’s fuel gas consumption. 2) Practicality: The “stove retention operation” is simple and convenient; no on-site operation is required, which reduces labor intensity, and it is also easy to adjust and monitor. 3) The economic benefits are shown in Table 3: Table 3 Effect of idling the coking furnace. Table 3 shows the effects related to the type of chamber, the number of main burners, the number of pilot lights, and the total fuel gas consumption in tons per hour. Under normal operation, these values are 2161.80; under idling mode, they are 071.57. With a unit price of 2840 yuan per ton, idling one furnace chamber can result in a reduction of 165.6 tons of fuel gas consumption per month, thereby saving 470,300 yuan per month. 5. Conclusion During low-load single-chamber production, continuous optimization of the heating furnace’s operation helped reduce the energy consumption of the facility. With zero investment costs, savings of over several hundred thousand yuan per month can be achieved, and the return rate is as high as 100% – achieving innovative results of low investment and high returns. References