Transformation of High Ammonia Nitrogen Wastewater Treatment Process in Coal Gasification Plant

A large amount of high ammonia nitrogen wastewater is produced in the process of the coal chemical industry. Although there is a wastewater treatment device, the treatment effect is not ideal due to the complexity of sewage water quality and unreasonable process settings. High concentration of ammonia nitrogen has always been a coal gas. The key points and difficulties of chemical wastewater treatment. As my country’s requirements for environmental protection become higher and higher in recent years, coal gasification enterprises are facing severe challenges.

The gasification section of the coal gasification unit of a coal chemical enterprise adopts the Texaco coal-water slurry gasification technology, and the black and grey water circulation adopts a four-stage flash configuration of high pressure flash, medium pressure flash, low pressure flash and vacuum flash; Adopt low-temperature sulfur-tolerant shift technology.

The high content of N-NH3 and acid gas in the wastewater discharged from the gasification and transformation sections is likely to cause problems such as blockage and corrosion of equipment and pipelines in the device, affecting the long-term operation of the device and subsequent biochemical treatment. Therefore, in the original process foundation On, make full use of the medium pressure steam generated in the device.

The integrated optimization and transformation of the high-ammonia-nitrogen wastewater treatment process in the gasification and transformation sections were carried out. The double-tower pressurized stripping process was used to reduce the content of ammonia and acid gas in the wastewater, and the pH value of the greywater was controlled within a suitable range. The stable and long-term operation of the device is ensured, the device does not require an external heat source, and the ammonia in the system is recycled as a resource.

Existing coal gasification wastewater treatment technology

In view of the characteristics of high pollutant content and complex composition of coal gasification wastewater, coal gasification wastewater treatment at home and abroad generally adopts a combination of physicochemical and biochemical methods, which is divided into three stages: pretreatment, biochemical treatment and advanced treatment. See Table 1 for the wastewater treatment technologies.

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Principle of stripping process

The stripping process belongs to the pretreatment stage, which is one of the methods to remove ammonia and acid gas from ammonia-nitrogen wastewater by physical process. Ammonia nitrogen wastewater in coal gasification can be regarded as a multi-component aqueous solution composed of NH3, H2S, and CO2, etc., which form weakly acidic and weakly alkaline ammonium salts such as NH4HS, (NH4)2CO3, NH4HCO3 in water, and respectively produce NH3, H2S and CO2 after dissolving in water. molecular.

Therefore, the multi-component aqueous solution is a complex system with the coexistence of chemical equilibrium, ionization equilibrium, and phase equilibrium. Therefore, controlling the appropriate conditions of chemical, ionization, and phase equilibrium is the key to the treatment of ammonia nitrogen wastewater and the selection of suitable operating conditions. The main factors that affect the above three equilibria are the concentration and molecular ratio [3].

The content of NH3, H2S, and CO2 in the gas phase can be increased by heating in the stripping process, but their concentrations, solubility, and volatility in the liquid phase are different, and the equilibrium constants of the ionic reactions are different, which causes these three molecules to be removed from the liquid. The difficulty and order of phase inversion into the gas phase are different.

NH3, H2S, and CO2 exist in the form of corresponding ions (NH4+, CO32-, HCO3-, S2-) in the liquid phase. The form is removed by the gas phase, so that ammonia, hydrogen sulfide, and carbon dioxide are continuously transferred from the liquid phase to the gas phase, and finally achieve the purpose of purifying the water.

Brief description of the process flow

3.1 The original process

The original high-ammonia nitrogen wastewater treatment process is shown in Figure 1. The original process adopts a four-stage flash distillation process. The condensate and high-pressure flash condensate are converted into the medium-pressure flash tank, and the gas phase of the medium-pressure flash tank passes through the greywater heat exchanger in turn. , The medium pressure flash condenser enters the medium pressure flash separation tank, and the liquid phase of the medium pressure flash tank enters the low-pressure flash tank.

The gas-phase of the medium-pressure flash separation tank enters the shift stripper, and the liquid phase enters the deaerator; the gas phase of the low-pressure flash tank enters the deaerator, and the liquid phase enters the vacuum flash tank; the gas phase of the vacuum flash tank enters the vacuum flash condenser In the vacuum flash separation tank, the liquid phase enters the clarifier; the gas phase in the vacuum flash separation tank enters the vacuum pump, and the liquid phase is pumped to the deaerator.

The clarification tank is separated into the gray water tank, and part of the gray water is used as the lock bucket flushing water, the other part is used for wastewater treatment, and the rest is the deaerator, and the liquid phase of the deaerator returns to the gasification section. The part of the shift condensate is returned to the coal mill after being stripped by the stripper, and there is no outlet for ammonia nitrogen in the whole process, resulting in the accumulation of ammonia nitrogen in the device.

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It can be seen that the ammonia content of the high-pressure flash evaporation part exceeds 13000μg/g, and the ammonia content of the converted condensate liquid is as high as 6700 μg/g. The ammonia nitrogen content in the field operation is much higher than the design value, and the specific raw material composition is shown in Table 2. The ammonia content in the discharged wastewater was 1900 μg/g, far exceeding the designed 340 μg/g.

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  3.2 原工艺流程存在的主要问题

During the actual operation, the black and gray water circulation section of the gasification unit has high pH value and high ammonia nitrogen content in the discharged wastewater. The excessive ammonia nitrogen content causes great pressure on the subsequent biochemical treatment, which greatly increases the biochemical treatment cost of wastewater.

The analysis found that there was no outlet for ammonia nitrogen in the original process flow, and the ammonia nitrogen in coal gasification wastewater and synthesis gas passed through the black-grey water circulation system and the ammonia recovered from the transformation and purification sections entered the coal-water slurry preparation section in the form of an aqueous solution, resulting in ammonia nitrogen in the system. accumulation in. To this end, it is necessary to optimize the process and set up an outlet for the accumulated ammonia nitrogen to realize the recycling of ammonia.

3.3 Transformation ideas

The overall transformation idea is: on the premise of not using external heat, reasonably allocate the heat and power of medium and low-pressure flash evaporation, and add a deacidification tower, deamination tower, water recovery tower, ammonia absorption tower, and corresponding cooling towers. Change equipment reasonably configures the process and realizes the resource recovery of ammonia in the high ammonia nitrogen wastewater in the gasification and transformation sections.

The above transformation ideas provide an outlet for ammonia nitrogen in high ammonia nitrogen wastewater and realize high-quality recycling of ammonia nitrogen accumulated in the device.

The extracted ammonia gas is absorbed by desalinated water to make 10%~15% (w) ammonia water, which is directly used as a reducing agent for flue gas desulfurization; the ammonia nitrogen content of the treated wastewater can well meet the requirements of subsequent biochemical treatment.

The abundant energy in the flash evaporation process is used in the water recovery tower to purify the wastewater to the maximum extent, and the process operation is flexible and stable.

3.4 Process after transformation

The process flow after the transformation is shown in Figure 2, in which the red part is the newly added equipment. The black water of the gasification device first enters the high-pressure flash tank, and the gas phase of the high-pressure flash tank is condensed by the subsequent device to form the high-pressure flash steam condensate, which enters the flash condensate tank, and the high-pressure flash condensate enters the medium-pressure flash tank.

The vapor phase of the medium pressure flash tank is used as the heat source of the reboiler of the newly added deacidification tower and deamination tower, and can also be used as the heat source of the reboiler of the newly added water recovery tower according to the process adjustment. The condensate enters the flash condensate tank, and the medium pressure The liquid phase of the flash tank enters the low-pressure flash tank, and then enters the original process; the vapor phase of the flash condensate tank enters the middle of the newly added deacidification tower, and the liquid phase at the bottom of the tank enters the middle and upper part of the deacidification tower, and the temperature of the transformed condensate is lower. Enter the top of the deacidification tower as a cold reflux, and control the temperature of the top of the deacidification tower.

The acid gas from the top of the deacidification tower goes to the flare system for combustion, and the liquid phase at the bottom of the deacidification tower enters the deamination tower.

The vapor phase at the top of the deamination tower enters the primary condenser, the first sub-tank, the secondary condenser and the second sub-tank, in turn, to condense the water and enter the reflux tank as the reflux of the deamination tower, and the bottom of the deamination tower enters the original deaerator.

The vapor phase of the two-part tank is low-sulfur ammonia gas, which enters the bottom of the ammonia absorption tower, and the desalinated water enters the top of the ammonia absorption tower. The bottom of the ammonia absorption tower is 10%~15% (w) ammonia water product; Space, newly added water recovery tower.

Part of the greywater in the raw ash water tank enters the top of the water recovery tower, the components at the top of the water recovery tower enter the original deaerator, and the components at the bottom of the tower enter the biochemical treatment device after cooling. The vapor phase of the pressure flash tank is used as the heat source of the reboiler or the original low-pressure flash steam is used for direct steam stripping.

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Transformation effect

According to the stripping principle, the original high ammonia nitrogen wastewater treatment process was optimized and transformed, and the effect after the transformation is shown in Table 3.

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It can be seen from Table 3 that after the transformation, the ammonia content in the purified water at the bottom of the deamination tower is 100 μg/g, which is lower than the ammonia nitrogen content of the original design of 650 μg/g and the operating value before the transformation of 4700 μg/g, effectively reducing the ammonia in the device. content and acid gas content accumulation, the ammonia content of purified water at the bottom of the water recovery tower is 100μg/g, far lower than the original design value of 340μg/g and the operating value before the transformation of 1900μg/g, which meets the requirements of subsequent biochemical treatment, and the device can be auxiliary Produce 9373~14056kg/h low sulfur ammonia water (10%~15%). The renovation achieved the expected goal.

After the transformation, the temperature of the newly added deacidification tower is 157℃, and the steam flow rate of the reboiler is about 9000kg/h.

The temperature of the deamination tower is 160°C, and the reboiler consumes about 25,000kg/h of steam; the temperature of the ammonia distillation tower is 135°C, and the reboiler consumes about 43,000kg/h of steam; the medium-pressure flash tank can produce about 77,130kg /h, flash steam with a temperature of 175 °C, the flash steam generated by the medium pressure flash tank can fully meet the steam consumption of the device, and no external heat source is required.

5 Conclusion

After the transformation, the ammonia content in the waste water of the gasification and transformation sections was reduced to 100 μg/g. The transformation process provided an ammonia outlet for the gasification and transformation sections, which improved the flexibility of the adjustment of the original device and reduced the accumulation of ammonia in the original device. The operation period of the device is effectively improved, and the subsequent biochemical treatment requirements are met.

After the transformation, the process uses the flash steam of the medium-pressure flash tank as a heat source, and no external heat source is required, which achieves a good energy-saving effect; the device can also by-produce 9373~14056kg/h low-sulfur ammonia water (10%~15%), and obtain certain economic benefits.

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