Method for removing ammonia nitrogen in coal gas ash water

Abstract

The invention discloses a method for removing ammonia nitrogen in coal gas grey water. The method comprises the following steps: pretreatment: precipitating to remove hardness and adding alkali to adjust the pH value to 9-11, so that NH4 < + > is converted into NH3H2O; treating in a stripping tower, heating the pretreated grey water, enabling the grey water to enter the stripping tower from the middle part of the stripping tower, enabling low-pressure steam to enter countercurrent contact from the bottom of the stripping tower, and decomposing NH3H2O into NH3 at 90-110 DEG C to escape; gas-liquid separation and ammonia water recovery are carried out, tower top mixed gas is condensed and cooled to form ammonia water, part of the ammonia water flows back, and part of the ammonia water serves as a by-product. According to the method, by limiting key parameters such as the pH value, the temperature and the steam consumption, the ammonia nitrogen removal rate reaches 97% or above, the method has the advantages of being efficient, low in cost, high in adaptability, capable of recycling resources and the like, the problems that an existing method is low in efficiency, high in cost and prone to being affected by water quality are solved, and the method can be widely applied to ash water treatment in the coal gas industry.

Description

Technical Field

[0001] This invention belongs to the field of coal gasification ash water treatment technology, specifically relating to a method for removing ammonia nitrogen from coal gasification ash water using a stripping process. Background Technology

[0002] my country boasts abundant coal reserves, and coal-to-gas, as a key pathway for the clean and efficient utilization of coal, plays a vital role in diversifying energy supply and ensuring national energy security. It not only effectively alleviates the tight supply of natural gas but is also widely used in urban gas supply, industrial fuel, and chemical raw materials, providing solid energy support for the stable development of the economy and society.

[0003] However, coal gasification inevitably generates large amounts of ash water, which contains high concentrations of ammonia nitrogen. Ammonia nitrogen, a potent pollutant, will cause devastating damage to aquatic ecosystems if discharged directly without effective treatment. It can lead to eutrophication, causing algae and other plankton to proliferate uncontrollably, consuming large amounts of dissolved oxygen, deteriorating water quality, and ultimately causing fish and other aquatic life to die from oxygen deprivation, severely disrupting the aquatic ecological balance. Furthermore, ammonia nitrogen can be converted into nitrite under certain conditions, posing a potential threat to human health. Long-term consumption of water contaminated with ammonia nitrogen may increase the risk of diseases such as cancer. Therefore, from the perspective of environmental protection and sustainable development, the efficient removal of ammonia nitrogen from coal gasification ash water is urgent and a crucial step in achieving green development in the coal gasification industry.

[0004] Currently, common methods for removing ammonia nitrogen from coal-to-gas ash water include ion exchange, biological methods, and chemical precipitation. However, these traditional methods have revealed many problems in practical applications and are difficult to meet the stringent requirements for ammonia nitrogen removal from coal-to-gas ash water.

[0005] Ion exchange primarily removes ammonia nitrogen from ash water by exchanging ions with ion exchange resins. While this method offers advantages such as relatively simple operation and high removal efficiency, its drawbacks are also significant. Firstly, ion exchange resins are expensive and have a limited lifespan, requiring frequent replacement, which undoubtedly increases treatment costs considerably. Secondly, the complex composition of coal gas ash water means that impurities such as suspended solids and organic matter can easily poison the ion exchange resins, causing them to lose their exchange capacity and thus affecting treatment effectiveness, increasing maintenance difficulty and costs.

[0006] Biological methods utilize the metabolism of microorganisms to convert ammonia nitrogen into harmless substances such as nitrogen gas or nitrates. Common biological methods include nitrification-denitrification, aerobic ammonia removal, and anaerobic ammonia oxidation. While biological methods offer advantages such as lower cost and environmental friendliness, they have significant shortcomings in treating coal gasification ash water. The quality of coal gasification ash water fluctuates greatly and its composition is complex. Harmful substances such as phenols and sulfides can inhibit or even toxicize microbial activity, preventing normal microbial metabolism and significantly reducing ammonia nitrogen removal efficiency. Furthermore, biological methods require stringent reaction conditions, demanding strict control of parameters such as temperature, pH, and dissolved oxygen. They are difficult to operate and have long treatment cycles, making them unsuitable for the continuous and efficient treatment needs of coal gasification production.

[0007] Chemical precipitation involves adding chemical agents, such as magnesium chloride and disodium hydrogen phosphate, to the ash water to react with ammonia nitrogen and form insoluble precipitates, thereby removing ammonia nitrogen. This method is fast and can reduce ammonia nitrogen concentration in a short time, but it also has several drawbacks. First, the large amount of chemical agents used leads to high treatment costs. Second, it generates a large amount of chemical sludge, which, if not properly treated, can cause secondary pollution to the environment. Third, this method has high requirements for water quality; other components in the coal gas ash water may interfere with the precipitation reaction and affect the ammonia nitrogen removal efficiency.

[0008] Therefore, existing ammonia nitrogen removal methods generally suffer from high costs, low efficiency, and susceptibility to water quality issues when treating coal gasification ash water, making it difficult to meet the growing environmental and production demands of the coal gasification industry. Thus, developing an efficient, stable, economical, and adaptable ammonia nitrogen removal method for coal gasification ash water has become a critical issue urgently needing to be addressed in the field of coal gasification ash water treatment. Summary of the Invention

[0009] The purpose of this invention is to provide an efficient, low-cost, stable and reliable method for removing ammonia nitrogen from coal gasification ash water, in order to solve the problems of low ammonia nitrogen removal efficiency, high cost and susceptibility to water quality in the existing technology, meet the increasingly stringent environmental protection requirements and production needs of the coal gasification industry, achieve compliant discharge of coal gasification ash water and recycling of water resources, and promote the green and sustainable development of the coal gasification industry.

[0010] Specifically, this can be achieved through the following technical solutions: A method for removing ammonia nitrogen from coal-to-gas ash water includes the following steps: 1) Pretreatment: The coal gas ash water is subjected to sedimentation and dehardening treatment to remove suspended solids and calcium and magnesium ions, and alkaline solution is added to adjust the pH value to 9-11, so that NH4+ in the ash water reacts with OH⁻ to generate NH3·H2O; Specifically, the ash water discharged from the coal gasification system is first settled in a descaling tank to reduce the concentration of suspended solids and calcium and magnesium ions, thus slowing down scaling on the stripping trays. The pH is adjusted by adding a 20% sodium hydroxide solution to reduce the NH4+ concentration in the ash water. + It reacts with OH⁻ to produce ammonia monohydrate (NH₃·H₂O). The reaction equation is: NH₄ + + OH⁻→NH₃・H₂O. The more ammonia monohydrate is generated, the better it is for the removal of ammonia nitrogen in the subsequent stripping tower. After passing through a plate heat exchanger, it enters the stripping tower.

[0011] In this step, pH value is one of the key factors affecting the ammonia nitrogen removal efficiency. In the stripping process of this invention, the pH value of the ash water is controlled between 9 and 11. When the pH value is below 9, the conversion of ionic ammonium to molecular ammonia is incomplete, leading to a decrease in ammonia nitrogen removal efficiency; when the pH value is above 11, although it is conducive to the escape of ammonia, it increases the amount of alkali used, increases the treatment cost, and may also cause corrosion to the equipment. Through experimental comparison, it was found that when the pH value is 9.5, the ammonia nitrogen removal efficiency is about 30% higher than that when the pH value is 9.

[0012] 2) Stripping tower treatment: The pretreated ash water is heated by a plate heat exchanger and then enters the stripping tower from the wastewater inlet distributor in the middle of the stripping tower. Low-pressure steam enters from the bottom of the stripping tower and comes into countercurrent contact with the ash water in the tower. Under the temperature conditions of 90-110℃, the monohydrate ammonia is decomposed into free ammonia and escapes with the steam. Specifically, ash water is mixed with flash steam and heated to 110°C before entering the stripping tower through the wastewater inlet distributor in the middle of the tower. Low-pressure steam enters from the bottom of the stripping tower and comes into countercurrent contact with the ash water inside the tower. In the stripping tower, the low-pressure steam heats the ash water, disrupting the stability of NH3·H2O and causing NH3 to escape. The reaction equation is: NH3·H2O → NH3↑ + H2O. The stripping section of the stripping tower uses anti-clogging internals to prevent impurities in the ash water from clogging the tower plates, ensuring the stable operation of the stripping process. The rectification section uses random packing to increase the gas-liquid mass transfer area and improve the separation efficiency of ammonia and nitrogen.

[0013] In this step, temperature has a significant impact on the stripping process. Generally, higher temperatures accelerate the volatilization rate of ammonia, which is beneficial for ammonia nitrogen removal. In this invention, the operating temperature inside the stripping tower is controlled between 90-110℃. When the temperature is below 90℃, the ammonia escape rate is slow, and the ammonia nitrogen removal effect is poor; when the temperature is above 110℃, although the ammonia nitrogen removal efficiency will improve, it will increase steam consumption, raise operating costs, and may lead to higher pressure requirements for the equipment.

[0014] Furthermore, the steam consumption directly affects the stripping effect and operating costs. Insufficient steam consumption cannot provide enough heat and mass transfer driving force, resulting in incomplete ammonia nitrogen removal; excessive steam consumption leads to energy waste and increased operating costs. In actual operation, the appropriate steam consumption is determined through experiments and experience based on factors such as the flow rate of ash water and the concentration of ammonia nitrogen, generally controlling the mass ratio of steam to ash water between 0.1 and 0.3.

[0015] The gas-liquid ratio refers to the volume ratio of steam to ash water, which affects the gas-liquid mass transfer efficiency. A suitable gas-liquid ratio ensures sufficient contact between steam and ash water, improving the ammonia nitrogen removal efficiency. In this invention, the gas-liquid ratio is controlled between 10 and 30. When the gas-liquid ratio is less than 10, the gas-liquid contact is insufficient, resulting in low ammonia nitrogen removal efficiency; when the gas-liquid ratio is greater than 30, although the gas-liquid contact is sufficient, it increases the load on the equipment and operating costs.

[0016] 3) Gas-liquid separation and ammonia recovery: The ammonia gas and steam mixture escaping from the top of the tower is condensed and cooled to form ammonia water. The ammonia water then enters the water cooler for further cooling and enters the reflux tank. Part of the ammonia water is sent out as a by-product, and the other part of the ammonia water is returned to the stripping tower as the top reflux liquid.

[0017] Specifically, the mixture of ammonia gas and vapor escaping from the top of the tower first enters the ammonia water condenser, where cooling causes the vapor in the mixture to condense into water, while the ammonia gas dissolves in the water to form ammonia water. The ammonia water then enters a water cooler for further cooling before entering the reflux tank. In the reflux tank, a portion of the ammonia water is sent out as a byproduct for use in fertilizer production and other fields; the other portion is returned to the stripping tower as top reflux liquid to increase the ammonia concentration gradient within the stripping tower and enhance the ammonia nitrogen removal effect.

[0018] Further, the alkaline solution mentioned in step 1) is a 20% sodium hydroxide solution with a pH value controlled between 9.5 and 10.

[0019] Furthermore, in step 2), the operating temperature of the stripping tower is controlled between 90-100℃, the mass ratio of steam to ash water is controlled between 0.1-0.3, and the gas-liquid ratio is controlled between 10-30.

[0020] Furthermore, in step 2), the ash water heating uses flash steam as a heat source and heat exchange is carried out through a plate heat exchanger.

[0021] Furthermore, in step 3), the reflux rate of the reflux liquid at the top of the column is controlled at 2-4 m3 / h.

[0022] Furthermore, the stripping tower is internally divided into a rectification section and a stripping section. The rectification section is filled with random packing to increase the gas-liquid mass transfer area and improve the ammonia-nitrogen separation efficiency. The stripping section adopts anti-clogging tower internals to prevent impurities in the ash water from clogging the tower plates.

[0023] Furthermore, the anti-clogging tower internals have an inclined tower plate and downcomer structure, and the random packing is stainless steel Pall ring packing.

[0024] Compared with the prior art, the present invention has the following technical effects: 1) Compared with existing ammonia nitrogen removal methods, the stripping process of this invention has a higher ammonia nitrogen removal efficiency. By optimizing the process flow and controlling key parameters, the ammonia nitrogen removal rate can reach over 97%, far exceeding traditional methods such as ion exchange, biological methods, and chemical precipitation. Furthermore, this invention has strong adaptability to water quality, effectively treating coal gasification ash water with complex composition and large fluctuations in water quality. The treated water quality is stable and can meet the requirements for subsequent biological treatment or compliant discharge.

[0025] 2) In terms of cost control, this invention has significant advantages. On the one hand, by making reasonable use of steam, such as using flash steam as a partial heat source, steam consumption is reduced, thereby lowering operating costs. On the other hand, the equipment structure of this invention is relatively simple, with low investment costs, and the equipment is easy to maintain and manage, further reducing overall costs. For example, compared with the traditional ion exchange method, the operating cost of this invention is reduced by more than 30%.

[0026] 3) This invention is environmentally friendly. Through a highly efficient ammonia nitrogen removal process, ammonia emissions are reduced, thus lowering pollution to the atmosphere and water bodies. Simultaneously, the recovered ammonia water can be exported as a byproduct for use in fertilizer production and other fields, achieving resource recycling, reducing waste generation, and meeting the requirements of sustainable development. Attached Figure Description

[0027] Figure 1 This is a flow chart of the stripping and ammonia removal process added to the coal gasification process of this invention; Figure 2 This is a schematic diagram of the stripping tower of the present invention; Figure 3 This is a graph showing the change in ammonia nitrogen removal rate with stripping time in an embodiment of the present invention; Figure 4 This is a graph showing the change in ammonia nitrogen removal rate as a function of the pH value of the ash water in an embodiment of the present invention. Figure 5 This is a graph showing the change in ammonia nitrogen removal rate with ash water temperature in an embodiment of the present invention. Figure 6 This is a graph showing the change in ammonia nitrogen removal rate with steam consumption in an embodiment of the present invention. Figure 7 This is a graph showing the change in ammonia nitrogen removal rate with gas-liquid ratio in an embodiment of the present invention. Detailed Implementation

[0028] The present invention will be further described below with reference to specific embodiments and accompanying drawings.

[0029] Example 1: Construction of the stripping tower experimental setup The stripping tower is constructed using high-quality 316L stainless steel to ensure excellent corrosion resistance and mechanical strength, adapting to the complex chemical environment of coal gasification ash water. The tower is cylindrical with an inner diameter of 500 mm and a height of 8000 mm. It is divided into a stripping section and a rectification section. The stripping section, 4000 mm high, utilizes patented anti-clogging internals with inclined trays and a highly efficient downcomer structure, effectively preventing suspended solids and impurities in the ash water from clogging the trays and ensuring stable stripping. The rectification section, 3000 mm high, is filled with randomly packed stainless steel Pall ring packing, which has a large specific surface area and high porosity, significantly increasing the gas-liquid mass transfer area and improving ammonia-nitrogen separation efficiency. The tower body is equipped with multiple interfaces, including a wastewater inlet distributor interface located in the middle of the tower for uniformly distributing ash water; a low-pressure steam inlet distributor interface located at the bottom of the tower to ensure that steam can fully countercurrently contact the ash water; a gas phase outlet interface at the top of the tower for discharging the mixed gas of ammonia and steam; and a liquid phase outlet interface at the bottom of the tower for discharging the treated ash water.

[0030] Example 2: Coal-to-gas ash water treatment experiment 1) Ash Water Preparation: Ash water samples are collected from the ash water discharge outlet of the coal gasification plant and transferred to an equalization tank. First, the initial pH of the ash water is measured to be 7.5 using a pH meter. Then, an appropriate amount of 20% sodium hydroxide solution is added to adjust the pH to 9.5 to promote the conversion of ionic ammonium to molecular ammonia. The reaction equation is: NH4+ + + OH - → NH3・H2O. The supernatant after precipitation is the pretreated ash water, which is used for subsequent stripping experiments.

[0031] 2) Stripping Operation: Low-pressure waste heat steam from the flash tank is introduced into the stripping tower at a flow rate of 300 kg / h through the low-pressure steam inlet distributor at the bottom of the tower, where it comes into countercurrent contact with the ash water. The ash water flow rate is 1000 kg / h, and the steam-to-ash water mass ratio is 0.25. Inside the stripping tower, the temperature is maintained at 95℃. The heat of the steam decomposes the NH3·H2O in the ash water, releasing NH3. The reaction equation is: NH3·H2O → NH3↑ + H2O. The mixture of ammonia gas and steam escaping from the top of the tower enters the ammonia water condenser for cooling. The cooled ammonia water then enters a water cooler for further cooling before entering the reflux tank. In the reflux tank, a portion of the ammonia water is sent out as a byproduct, while the other portion is returned to the stripping tower as reflux liquid at a reflux rate of 3 m³ / h.

[0032] 3) Data Acquisition: During the experiment, samples of the influent and effluent were collected every 30 minutes. Ammonia nitrogen concentration was measured using a Hach DR3900 spectrophotometer, and the data were recorded. Simultaneously, pH values ​​were measured using a Mettler Toledo FiveGo pH meter, temperature was measured using a PT100 temperature sensor, and the flow rates of steam and ash water were recorded using an electromagnetic flowmeter. Furthermore, the steam consumption of the steam generator was recorded every 2 hours to ensure the completeness and accuracy of the experimental data. Real-time monitoring and recording of these data allowed for a comprehensive understanding of the changes in various parameters during the stripping process, providing a basis for subsequent experimental result analysis.

[0033] The process flow diagram of the stripping and ammonia removal process added to the coal gasification process in this invention is as follows: Figure 1 As shown in the schematic diagram of the stripping tower structure... Figure 2 As shown, since the schematic diagrams of the process flow section and the stripping tower structure can be obtained by simple modification using existing technology, they are not described in detail.

[0034] Experimental Results and Analysis 1. Changes in ammonia nitrogen removal efficiency over time like Figure 3 As shown, within the first 2 hours after the experiment began, the ammonia nitrogen removal rate rapidly increased from approximately 30% to 80%. As time progressed, the rate of increase in the ammonia nitrogen removal rate gradually slowed, stabilizing after 2.5 hours and eventually reaching over 97%. Therefore, 2.5-3 hours was selected as the optimal stripping time.

[0035] 2. Effect of pH on ammonia nitrogen removal efficiency like Figure 4As shown, pH value significantly affects ammonia nitrogen removal efficiency. At low pH values, the conversion of ionic ammonium to molecular ammonia is incomplete, resulting in low ammonia nitrogen removal efficiency. As pH increases, the ammonia nitrogen removal rate significantly improves. However, when the pH value is too high, the improvement in ammonia nitrogen removal rate is not significant; instead, it increases alkali usage and treatment costs, and may also cause equipment corrosion. Maintaining the pH value between 9.5 and 10 ensures effective ammonia nitrogen removal while also considering cost and equipment safety.

[0036] 3. Effect of temperature on ammonia nitrogen removal efficiency like Figure 5 As shown, increased temperature accelerates the volatilization rate of ammonia and increases the mass transfer driving force, which is beneficial for ammonia nitrogen removal. Within a certain range, the ammonia nitrogen removal rate increases significantly with increasing temperature. However, excessively high temperatures increase steam consumption, raise operating costs, and may lead to higher pressure requirements for the equipment. Considering all factors, controlling the operating temperature within the stripping tower between 90 and 100℃ is more appropriate.

[0037] 4. Effect of steam dosage on ammonia nitrogen removal efficiency like Figure 6 As shown, insufficient steam usage cannot provide enough heat and mass transfer driving force, resulting in incomplete ammonia nitrogen removal; excessive steam usage not only wastes energy and increases operating costs, but also reduces ammonia nitrogen removal efficiency. Experiments have determined that a suitable steam usage of 200-250 kg / h can reduce operating costs while ensuring effective ammonia nitrogen removal.

[0038] 5. Effect of gas-liquid ratio on ammonia nitrogen removal efficiency like Figure 7 As shown, a suitable gas-liquid ratio ensures sufficient contact between steam and ash water, improving ammonia nitrogen removal efficiency. If the gas-liquid ratio is too small, insufficient gas-liquid contact results in low ammonia nitrogen removal efficiency; if the gas-liquid ratio is too large, although sufficient gas-liquid contact is achieved, it increases the equipment load and operating costs. Controlling the gas-liquid ratio between 20 and 30 achieves good ammonia nitrogen removal performance and economic benefits.

[0039] Through experiments in this embodiment, the ammonia nitrogen removal rate can reach over 97%, proving the high feasibility of the stripping process used in this invention for ammonia nitrogen removal from coal gasification ash water. Compared with existing technologies, under the same experimental conditions, the ammonia nitrogen removal rate of this invention is 20-30 percentage points higher than that of ion exchange, 30-40 percentage points higher than that of biological methods, and 40-50 percentage points higher than that of chemical precipitation. In terms of operating costs, the steam consumption of this invention is reduced by 20%-30% compared with traditional stripping processes, and the equipment investment cost is reduced by 10%-20%, fully demonstrating the technical and economic advantages of this invention.

Claims

1. A method for removing ammonia nitrogen from coal-to-gas ash water, characterized in that, The method includes the following steps: 1) Pretreatment: The coal gas ash water is subjected to sedimentation and dehardening treatment to remove suspended solids and calcium and magnesium ions, and alkaline solution is added to adjust the pH value to 9-11 to reduce the NH4+ in the ash water. + It reacts with OH⁻ to produce NH₃·H₂O; 2) Stripping tower treatment: The pretreated ash water is heated by a plate heat exchanger and then enters the stripping tower from the wastewater inlet distributor in the middle of the stripping tower. Low-pressure steam enters from the bottom of the stripping tower and comes into countercurrent contact with the ash water in the tower. Under the temperature conditions of 90-110℃, the monohydrate ammonia is decomposed into free ammonia and escapes with the steam. 3) Gas-liquid separation and ammonia recovery: The ammonia gas and steam mixture escaping from the top of the tower is condensed and cooled to form ammonia water. The ammonia water then enters the water cooler for further cooling and enters the reflux tank. Part of the ammonia water is sent out as a by-product, and the other part of the ammonia water is returned to the stripping tower as the top reflux liquid.

2. The method for removing ammonia nitrogen from coal-to-gas ash water as described in claim 1, characterized in that, The alkaline solution mentioned in step 1) is a 20% sodium hydroxide solution with a pH value controlled between 9.5 and 10.

3. The method for removing ammonia nitrogen from coal-to-gas ash water as described in claim 1, characterized in that, In step 2), the operating temperature of the stripping tower is controlled between 90-100℃, the mass ratio of steam to ash water is controlled between 0.1-0.3, and the gas-liquid ratio is controlled between 10-30.

4. The method for removing ammonia nitrogen from coal-to-gas ash water as described in claim 1, characterized in that, In step 2), flash steam is used as the heat source for heating the ash water, and heat exchange is carried out through a plate heat exchanger.

5. The method for removing ammonia nitrogen from coal-to-gas ash water as described in claim 1, characterized in that, In step 3), the reflux velocity of the liquid at the top of the column is controlled at 2-4 m / s. 3 / h.

6. The method for removing ammonia nitrogen from coal-to-gas ash water as described in claim 1, characterized in that, The stripping tower is divided into a rectification section and a stripping section. The rectification section is filled with random packing to increase the gas-liquid mass transfer area and improve the ammonia-nitrogen separation efficiency. The stripping section adopts anti-clogging internals to prevent impurities in the ash water from clogging the tower plates.

7. The method for removing ammonia nitrogen from coal-to-gas ash water as described in claim 6, characterized in that, The anti-clogging tower internals have an inclined tower plate and downcomer structure, and the random packing is stainless steel Pall ring packing.

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