Source: Thermal Power Plant Technology Alliance
Authors: Wang Fengtao, Tian Zhengxue, Liu Zhenguo
National Energy Liaocheng Power Generation Co., Ltd
Abstract: In recent years, the denitrification reducing agent urea method is commonly used in thermal power plants to eliminate liquid ammonia, a dangerous source. However, in the renovation design, there are some specific issues that are easily overlooked by the designer. Taking Company L as an example, the urea ammonia production system has been running for more than 2 years, which has exposed a series of problems such as large vibration of the discharge pipe, air bubbles coming out of the overflow pipe of the dissolving tank, large vibration of the trap tank, inability to isolate the hydrophobic pipe, corrosion and leakage of the gas phase valve, and unreasonable setting of the sewage discharge time of the hydrolyzer. Through the efforts of the technical staff of Company L, some of the problems have been optimized, and the inspection effect is good after operation.
Keywords: denitrification; Urea; parent control; discharge pipe; dissolving tanks;
The denitrification reducing agent urea method transformation project is a key safety and environmental protection project of thermal power plants after the transformation of desulfurization, denitrification (liquid ammonia) and dust removal. The National Energy Administration has repeatedly issued relevant documents requiring thermal power plants to carry out the transformation of denitrification reducing agent urea method to eliminate the major dangers caused by the large storage of liquid ammonia in thermal power plants. Company L's machine assembly machine has a large capacity, high NOx concentration at the flue gas inlet, and a large liquid ammonia storage capacity, which was the highest level of major hazard source of GD Group at that time. In 2016, Company L carried out a feasibility study on the transformation of denitrification reducing agent urea method, started construction in March 2017, and gradually put it into operation in August 2018. Because the project is a relatively early large-volume liquid ammonia to urea project in China, the design experience is less, and the design scheme is not mature enough, so there are some problems, which are introduced in detail below.
Company L's urea ammonia production system consists of a discharge system, a dissolving tank, a storage tank, a hydrolyzer, a trap, an ammonia supply pipeline, an ammonia/air mixer and other equipment. The hydrolyzer adopts the parallel mode of mother tube, that is, the gas pipelines of the export products of the three hydrolyzers are connected with each other, and the product gas is transported from the two ammonia supply mother pipes to the SCR (selective catalytic reduction technology) area of the four units to participate in denitrification. Among them, the 1# and 2# units (phase I) share 1 ammonia supply mother pipe, and the 3# and 4# units (phase II) share 1 ammonia supply mother pipe. The design output of a single hydrolyzer is 1300 kg/h, which is currently the largest hydrolyzer in China. Figure 1 is a simplified diagram of urea ammonia production system.
Fig.1 Schematic diagram of urea ammonia production system
2.1 The discharge pipe vibrates greatly
Company L's urea is transported by tanker, and the urea in the tanker is transported to the dissolving tank through the stainless steel discharge pipe with miscellaneous compressed air. The stainless steel discharge pipe is fixed on the wall and the ground fixing bracket, and the bracket is connected with the wall and the ground by expansion screws. Between the discharge port and the urea dissolving tank, the discharge pipe is turned 2 90° bends. When discharging, under the thrust of compressed air, the 90° elbow of the discharge pipe vibrates greatly, and the anchor bolts on the wall and the fixed bracket vibrate many times, which seriously affects the safety of unloading.
2.2 Air bubbles come out of the overflow pipe of the dissolving tank
In the process of urea entering the dissolving tank, urea dissolution increases the surface tension of the water, and a large amount of gas enters the urea solution, producing a large number of bubbles on the surface of the solution. These bubbles accumulate more and more in the upper part of the dissolving tank, accumulate to a certain extent, and are discharged from the discharge port. There are generally 2 places in the upper discharge of the dissolving tank: a) the overflow pipe of the dissolving tank. Air bubbles emerge from the overflow pipe of the dissolving tank, causing a large smell of ammonia in the plant, endangering the physical and mental health of the personnel on duty. b) The fan on the top of the dissolving tank (some designs have it, some don't). After the bubbles enter the fan, they accumulate at the outlet of the fan, and as the bubbles continue to burst, they will turn into a liquid urea solution at the outlet of the fan, blocking the outlet of the fan. The urea solution clumps together and cannot be discharged, and when it accumulates to a certain extent, it will splash out of the fan drain pipe. The fan drain pipe is facing the urea discharge area, and the splattered urea solution will pose a threat to the urea unloading personnel and vehicles.
2.3 The trap vibrates greatly
Traps are used to collect water from the heat trace pipes in the hydrolyzer, dissolving tank, and urea station area. Since the temperature is higher than 100°C, if it is not cooled with demineralized water, liquid water cannot be formed, and all of it will be discharged into the atmosphere from the exhaust pipe of the trap. Therefore, the design is to add demineralized water to the trap tank, cool the trap to 84°C, and return it to the unit deaerator. However, when the demineralized water at a low temperature comes into contact with the hydrophobic water at a high temperature, a violent heat exchange occurs, causing the trap to vibrate violently. The operator has done a special test to reduce the temperature of the trap to 70°C, but the vibration is still very large, which seriously affects the safe operation of the trap. If the tank temperature is lowered too much, a large amount of desalination is required, which can also lead to uneconomical operation.
2.4 Hydrophobic tubes cannot be isolated
The water from the hydrolyzer, dissolving tank, and the heat trace pipe in the urea station area is all collected into the hydrophobic tube and then into the trap. The consequences of this design are: a) when the hydrophobic tube leaks, it cannot be isolated and repaired, and the leakage of the mother tube can only be plugged under pressure; b) all the traps enter the trap tank from the mother pipe, so that the heat exchange between the high temperature trap and the low temperature demineralized water has a violent heat exchange, resulting in the violent vibration of the trap tank.
2.5 Corrosion leakage of gas phase valves
The valves on the gas side of the hydrolyzer and the ammonia supply mother pipe, including the valve body and valve core, are all made of 316L stainless steel, and the valve seal is soft sealed. The operation of the urea ammonia production system is less than 1a, and the gas phase valve has successively leaked. Later, the valve on the ammonia supply mother pipe was disassembled, and it was found that the surface of the valve core was corroded into pits, as shown in Figure 2. After the urea ammonia production system was put into operation, there was also a problem of blockage of the ammonia supply pipeline due to valve leakage, which caused the unit to be delayed for 40 hours.
2.6 The setting value of the hydrolyzer blowdown time is unreasonable
Fig.2. Corrosion leakage of vapor phase valves
2.7 All 3 hydrolyzers cannot be put into automatic operation
The original design of the three hydrolyzers adopts the dual-use and one-standby operation mode, but due to the change of flue gas NOx concentration boundary conditions, the instability of coal-fired coal quality, and the response to heavy pollution weather in the Beijing-Tianjin-Hebei air pollution transmission channel, three hydrolyzers will be put into operation when the three units are actually operated. However, the designed adjustment logic is for the dual-use and one-backup mode, and under the existing adjustment logic framework, it is impossible to realize the automatic operation of all three hydrolyzers.
In addition, there is no isolation valve in the branch pipe of the ammonia mother pipe, and when the heat tracing branch pipe leaks, all the heat tracing pipes can only be isolated for maintenance, and there is a risk of crystallization in the ammonia mother pipe.
3.1 Optimization of discharge pipe vibration
After many demonstrations by the company's technical personnel, it was found that the reason for the large vibration of the discharge pipe was on two 90° elbows. The compressed air exerts a positive thrust on the pipe at the elbow, causing the pipe to vibrate regularly. In response to this reason, the maintenance personnel spliced two 90° elbows into a small angle and large elbow with a short straight pipe section, and the forward thrust generated by the compressed air entered the dissolving tank along the small angle and large elbow. After this transformation, the thrust generated by the compressed air is used more to transport urea, which reduces the energy loss along the pipeline, and the urea for unloading a tanker for 30 t is shortened from the previous 2.5 h to 1.5 h, which not only eliminates the potential safety hazard of pipeline vibration, but also saves compressed air and reduces energy consumption. The actual scene of the transformation of the two discharge pipes is shown in Figure 3.
Fig.3 Real picture of the transformation of 2 discharge pipes
3.2 Optimization of bubbles from the overflow pipe of the dissolving tank3.3 Optimization of the problem of trap tank and steam pool
For the problem of steam emitting from the drain pipe of the trap tank, the company's technical personnel modified the drain nozzle, changed the nozzle to upward, and set up 2 bends to prolong the condensation residence time of the steam, and the condensate was recovered to the trap tank, and the effect after the transformation is shown in Figure 5. After the retrofit, the amount of steam discharged through the tank evacuation pipe was reduced by about 50%, but the full recovery of the steam was not possible.
For new construction projects, the issue of traps and sphobless tubes should be reconsidered in the design. The drain should be controlled separately, and the water from each channel should enter the trap tank separately, and the direction of entering the trap tank should be evenly dispersed, and the distribution pipe should be added in the trap tank, which greatly increases the contact area between the trap and the desalinated water, and reduces the intensity of heat exchange, so that the problem of large vibration of the trap tank is also solved. Considering the cost of construction, if it is not convenient to realize that each drain can enter the trap tank separately, at least the dissolving tank, hydrolyzer, and urea hydrolysis workshop are separated from the heat and drainage, and isolation doors are set on each branch road to facilitate maintenance. In addition, the temperature of the trap tank was redesigned, and the model was constructed to calculate the optimal ratio of hydrophobic and desalinated water to achieve the optimal recovery rate of heat and water in the trap.
Fig.4 Real view of wind turbine transformation
Fig.5. Real view of the renovation of the drain pipe of the trap tank
3.4 Optimization of corrosion leakage problem of gas phase valves
In the past two years, the problem of corrosion leakage of urea hydrolyzer gas phase valve has attracted the attention of more and more peers, including urea hydrolyzer suppliers. At present, it is generally believed that 316L stainless steel cannot withstand the corrosion of ammonia mixture after urea hydrolysis. A common measure is to upgrade the valve spool material, such as urea grade 316L stainless steel, 2205 duplex stainless steel, 2507 duplex stainless steel. Through material upgrading and seal improvement, the corrosion and leakage time of the vapor phase valve is effectively prolonged, and the service life of the valve is prolonged.
3.5 Hydrolyzer blowdown time fixed value optimization
The hydrolyzer blowdown time needs to be determined according to the operating status of the hydrolyzer. When the load of the unit is high and the output of the hydrolyzer is large, the sewage discharge time and sewage discharge times of the hydrolyzer should be appropriately increased. The load of the unit is low, the output of the hydrolyzer is small, and the sewage discharge time and sewage discharge times of the hydrolyzer are appropriately reduced. The number of sewage discharge times and sewage discharge time of the hydrolyzer of Company L are as follows: from July to September, when the unit load is high, each hydrolyzer discharges sewage twice a week for 8 min each time, and in other months, each hydrolyzer discharges sewage once a week for 10 min each time. The evaluation criterion for the sewage discharge effect is that the hydrolyzer level fluctuation is small and the hydrolyzer runs smoothly. Under the existing regulations, if there is an upward trend in the fluctuation of the hydrolyzer level, the time and frequency of sewage discharge can be appropriately increased.
3.6 Hydrolyzer adjustment logic optimization
Since the interruption of ammonia supply to the hydrolyzer on July 11, 2019, the technicians of Company L have been studying the way to optimize the adjustment logic of the hydrolyzer, and have tried many methods, but they have not achieved the expected results. In March 2020, after full discussion by various professional and technical personnel, it was decided to optimize the adjustment logic of the hydrolyzer as follows: on the basis of the existing adjustment logic, the adjustment logic of any existing hydrolyzer (taking the 2# hydrolyzer as an example) is the main one, and the adjustment of the ammonia steam outlet regulating valve of the other two hydrolyzers (1#, 3# hydrolyzer) is controlled by the adjustment logic of the selected hydrolyzer. Add the "input" and "exit" buttons and the "hydrolyzer ammonia injection valve command bias" dialog box on the ammonia steam outlet valve of the 1# and 3# hydrolyzer, when the 1# and 3# hydrolyzers need to be automatic, the opening of the ammonia steam outlet valve of the hydrolyzer is adjusted to be roughly consistent with the 2# hydrolyzer, and then click the "input" button, the hydrolyzer will automatically accept the control of the 2# hydrolyzer adjustment logic, and increase by entering positive and negative values in the corresponding hydrolyzer ammonia injection valve instruction bias dialog box. Reduce the opening of the ammonia regulating valve of the corresponding hydrolyzer. When the hydrolyzer needs to be adjusted separately, click the corresponding "exit" button, and the ammonia steam outlet valve of the hydrolyzer will accept the adjustment logic control of the hydrolyzer itself. The effect after transformation is shown in Figure 6. After the optimization of the hydrolyzer adjustment logic, the hydrolyzer can be fully put into automatic adjustment, even if the NOx concentration at the inlet of the unit increases rapidly, the operation is relatively stable, which reduces the workload and working pressure of the operation duty personnel, and achieves good results.
In addition, when the heat trace pipe is inconvenient to maintain and cannot be isolated, it can be solved by adding an isolation door at the appropriate location during the design.
Through the optimization of the above common problems, the hidden dangers that affect the safe and stable operation of the urea ammonia production system have been eliminated, the reliability and safety factor of the urea ammonia production system have been greatly improved, and some common problems of the urea ammonia production system of the parent control have been effectively solved, and all the hydrolyzers have been put into automatic operation. After the parent urea ammonia production system is put into operation, as long as the whole plant unit is not suspended, there is no chance of all the shutdown, therefore, when designing the urea ammonia production system, it is necessary to fully consider the operability of maintenance, and facilitate maintenance when the equipment leaks or fails.
Fig.6. Effect of hydrolyzer adjustment and transformation
About author: Wang Fengtao, born in 1988, male, from Jining, Shandong, graduated from Shaanxi Normal University in 2017 with a master's degree in business administration, and is an engineer.