Desulfurization wastewater recycling zero-discharge treatment method combined with flue spray drying

Abstract

The invention relates to the technical field of coal-fired power plant water treatment and environmental protection, and discloses a flue spray drying combined desulfurization wastewater recycling zero-discharge treatment method which comprises the following steps: performing pressurized dissolved gas and heating modulation on desulfurization wastewater to construct an overheated dissolved gas multiphase working medium; coaxial nozzles are used for cooperating with high-temperature sheath gas to jet towards a main flue, and low-density hollow salt particles are formed through flash evaporation phase change and a bubble expansion mechanism; the coal-fired fly ash penetrates through a jet beam by utilizing an inertia difference based on Stokes quasi numbers, and low-density hollow salt particles are guided to deflect along with airflow to enter a salt-rich branch; and after gas film protection of the virtual impactor is introduced, fine fly ash is further separated and waste salt is recycled by pneumatic cyclone classification. The density of the waste salt is changed through hollow granulation, online separation of the waste salt and the fly ash is achieved in combination with a flow field inertia screening mechanism, the problems of nozzle blockage and scaling are solved through sheath gas and a gas film structure, and zero-emission recycling treatment of the high-salt wastewater is achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of coal-fired power plant water treatment and environmental protection, in particular to a desulfurization wastewater resourceization zero-emission treatment method combined with flue spraying drying. BACKGROUND

[0002] The wastewater generated by the wet desulfurization process of coal-fired power plants has characteristics such as high salt content, complex composition and strong corrosion. Realizing zero-emission treatment of the wastewater is of great significance for environmental protection control of power plants. Using the waste heat of the boiler tail flue for spraying drying and evaporation of desulfurization wastewater is one of the main technical routes for current desulfurization wastewater zero-emission, because it has a short system process and low operating cost. However, this process has technical problems in product resource utilization and equipment operation stability.

[0003] In terms of product disposal and resource utilization, the direct spraying evaporation process sprays the wastewater into the main flue after atomization. The dried and separated salt particles are mixed with the original fly ash in the flue gas. Because the solid salt particles formed by conventional evaporation are similar to the fly ash in particle size distribution and density characteristics, they are captured together in the subsequent dust removal equipment and are difficult to separate by conventional physical means. This mixed state not only causes the chlorine ion content in the fly ash to exceed the standard, reducing the utilization value of the fly ash, but also makes the waste salt mixed in the fly ash become a difficult-to-handle mixed solid waste, which cannot be recovered as an independent resource product, restricting the economic and environmental benefits of desulfurization wastewater treatment.

[0004] In terms of atomization efficiency and drying mechanism, the existing technology mainly uses traditional double-fluid nozzles, which rely on the external shear force of compressed gas to break the liquid film to achieve atomization. This mechanical shearing method produces a wide range of liquid droplet size distribution, and part of the large particle size droplets are difficult to completely dry in the limited flue residence time, resulting in the accumulation of incompletely dried droplets at the bottom of the flue. In addition, the atomization effect dominated by external airflow shear is affected by the gas-liquid momentum ratio and the change of flue gas temperature field. When the unit load fluctuates or the exhaust gas temperature decreases, the evaporation rate of the droplets decreases significantly, which limits the adaptability of the process under variable working conditions.

[0005] In terms of equipment operation stability, in the high-temperature and high-dust environment inside the flue, high-concentration fly ash is easily absorbed by the backflow at the nozzle end face, causing nozzle orifice ash deposition and plugging, which destroys the atomization flow field. At the same time, due to the viscosity of part of the incompletely dried droplets, if they come into contact with the flue wall or the inner wall of the branch pipeline during the flow process, they will stick to the wall and form a salt accumulation layer over time. This not only increases the corrosion risk of the equipment, but also affects the flow area and flow field distribution of the flue, resulting in frequent shutdown and cleaning of the system, which reduces the operation rate and reliability of the desulfurization wastewater zero-emission system. SUMMARY

[0006] In view of the technical problems of incomplete drying of liquid droplets leading to wall sticking and scaling, difficulty in separating dry products from coal fly ash leading to waste salt being unable to be resourcefully utilized, and wide particle size distribution of the two-fluid nozzle spray, and evaporation efficiency being limited by the heat transfer temperature difference in the flue desulfurization wastewater evaporation process in the prior art, the present application provides a desulfurization wastewater resourceful zero-emission treatment method combined with flue spray drying, which solves the problems of difficulty in gas-solid separation and equipment operation stability in the high-salinity wastewater zero-emission process.

[0007] To achieve the above object, the present application is implemented by the following technical solutions:

[0008] A desulfurization wastewater resourceful zero-emission treatment method combined with flue spray drying, comprising the following steps:

[0009] Step S1, softening pretreatment, pressurized gas dissolution and heating conditioning are performed on the desulfurization wastewater to build a superheated gas-dissolved multiphase working medium in a thermodynamic superheated state;

[0010] Step S2, the prepared superheated gas-dissolved multiphase working medium is introduced into the center channel of the coaxial nozzle assembly, and high-temperature sheath gas sprayed from the outer channel of the coaxial nozzle assembly is matched to perform orthogonal or reverse injection into the main flue; by using flash phase change and sheath gas isolation effect, a composite flash jet beam containing low-density hollow salt particles is constructed on the cross section of the main flue;

[0011] Step S3, the coal fly ash in the main flue penetrates the composite flash jet beam by using the inertia difference of particles based on the Stokes number, while the low-density hollow salt particles in the composite flash jet beam are guided to deflect along the streamline and enter the salt-rich branch channel under the action of aerodynamic drag, forming a salt-containing deflected airflow separated from the coal fly ash;

[0012] Step S4, the salt-containing deflected airflow is guided to enter the inside of the salt-rich branch channel through the virtual impactor inlet structure, and after completing the final drying in the channel, it is introduced into the aerodynamic cyclone classification unit to further separate the residual fine fly ash by using the solid phase density difference, and finally the waste salt solid product is recovered.

[0013] Preferably, in step S1, the heating conditioning process controls the stagnation temperature of the superheated gas-dissolved multiphase working medium before entering the coaxial nozzle assembly through an online heating adjustment device, and the stagnation temperature value is determined based on a temperature control calculation model to give the working medium the required superheating potential energy before spraying.

[0014] Preferably, in step S1, the specific implementation of the pressurized gas and heated conditioning is: pressurizing the softened wastewater to a stagnation pressure, and setting the stagnation pressure to a multiple interval of the internal injection environment pressure of the main flue; regulating the gas-liquid mixing ratio in the pipeline environment maintaining the stagnation pressure, so that the fluid forms an unsaturated or saturated state of gas; using the stagnation pressure to drive gas molecules into the intermolecular gap of the liquid phase, providing a gas phase nucleation core for the flash evaporation behavior in the subsequent injection process, thereby constructing the superheated gas-liquid multiphase working medium.

[0015] Preferably, in step S2, the synergistic high-temperature sheath gas injection process includes: setting a high-temperature sheath gas to form an outer gas phase sheath flow, and controlling the axial velocity of the outer gas phase sheath flow to be higher than the flow velocity of the flue gas in the main flue; wrapping the superheated gas-liquid multiphase working medium with the outer gas phase sheath flow in the initial jet section to establish a momentum exchange interface between the gas-liquid two phases; based on the momentum exchange interface, using the momentum flux ratio to control the delivery trajectory of the composite flash evaporation jet beam, and the injection parameters are determined according to the momentum flux ratio calculation model.

[0016] Preferably, in step S2, the flash evaporation phase change specifically manifests as: when the superheated gas-liquid multiphase working medium leaves the coaxial nozzle assembly and enters the low-pressure environment of the main flue, the fluid pressure suddenly drops from the stagnation pressure to the ambient pressure; using the pressure drop to induce the gas molecules dissolved in the liquid phase to precipitate and form bubble nuclei, establishing a volume change difference between the internal bubble expansion rate and the external salt shell contraction rate; using the volume change difference to drive the incompletely solidified salt shell to expand outward, and finally solidifying to form low-density hollow salt particles.

[0017] Preferably, in step S3, the specific process of using the inertial difference based on the Stokes number involves: analyzing the velocity vector distribution of the flow field at the intersection area of the main flue and the salt-rich branch channel; determining the background fluid parameters based on the flow field distribution characteristics, and using the particle relaxation time calculation model to determine the response speed of the particles to the flow field changes; substituting the response speed into the Stokes number calculation model to set the dynamic separation boundary of the coal fly ash and the low-density hollow salt particles.

[0018] Preferably, in step S3, by controlling the average suction air flow velocity at the inlet of the salt-rich branch channel, the Stokes number corresponding to the coal fly ash is maintained in an interval greater than 1, and the inertial force dominates to maintain the original motion direction, and the main flue mainstream flue gas flows downstream; at the same time, the Stokes number corresponding to the low-density hollow salt particles is maintained in an interval less than or close to 1, and the viscous resistance dominates to deflect along the curved streamline and enter the salt-rich branch channel.

[0019] Preferably, in step S4, the virtual impactor inlet structure is configured with a gas distribution jacket on its outer side and a diverging flow guide structure to connect the main flue; hot clean gas is uniformly injected into the channel interior through the annular slits or porous permeable materials on the inner wall surface, forming a continuous gas film protection layer on the inner wall surface of the virtual impactor inlet structure and the salt-rich branch channel; the gas film protection layer separates the salt-containing deflected airflow from the channel inner wall surface and provides a dry heat source.

[0020] Preferably, in step S4, the operating parameters and structural dimensions of the pneumatic cyclone classification unit are determined based on a cyclone cut particle size calculation model to match the apparent density characteristics of the low-density hollow salt particles.

[0021] Preferably, in step S4, based on the density difference between the low-density hollow salt particles and the coal fly ash, the centrifugal separation factor applied to the low-density hollow particles is enhanced by reducing the inlet channel width of the pneumatic cyclone classification unit or increasing the system air volume, so that the low-density hollow salt particles are captured into the bottom waste salt collection bin, and the purified gas is discharged through the central exhaust pipe.

[0022] The present application provides a flue spray drying combined desulfurization wastewater resource utilization zero discharge treatment method, which has the following beneficial effects:

[0023] 1. The present application forms a hollow microcrystalline structure with low density in the drying process by constructing a superheated gas multiphase working medium and inducing flash phase change, establishing the apparent density difference between hollow waste salt and solid coal fly ash; based on the density difference and Stokes criterion, an inertial screening mechanism is constructed in the main flue flow field, and pneumatic screening is used to make high-density fly ash remain inert with the main flue gas, while low-density waste salt is deflected into the branch along the curved flow line, realizing online separation of waste salt and fly ash and separate recovery of waste salt.

[0024] 2. The present application uses a coaxial high-temperature sheath gas to wrap the jet core, establishes a momentum isolation layer at the nozzle outlet, prevents the backflow of coal fly ash from adhering to the nozzle end face; at the same time, a virtual impactor structure is used at the inlet of the salt-rich branch channel and hot clean gas is injected, forming a continuous gas film protection layer on the inner wall surface, physically isolating the wet salt mist from the channel wall surface and providing a dry heat source, solving the problems of nozzle blockage and wall scaling caused by incomplete drying of droplets and fly ash adhesion in the flue evaporation process.

[0025] 3、The application utilizes the pressure drop mechanism of superheated solution gas working substance in low pressure environment to promote the rapid precipitation and expansion of internal dissolved gas of liquid phase, and realizes the internal explosion atomization and rapid surface hardening of liquid drops by using the volume change difference between the internal bubble expansion rate and the external liquid drop surface shrinkage rate; the mechanism overcomes the limitation of wide atomization particle size distribution caused by pure gas flow shear of traditional two-fluid nozzles, strengthens the gasification and evaporation efficiency of liquid drops, and ensures that the desulfurization wastewater completes phase change drying in limited flue space and time. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 The overall structure and process flow diagram of the desulfurization wastewater resource zero-emission treatment system of an embodiment of the application;

[0027] Figure 2 The desulfurization wastewater resource zero-emission treatment method flow chart combined with flue spraying drying of an embodiment of the application.

[0028] Among them, 10, wastewater pretreatment unit; 20, superheated solution gas working substance preparation circuit; 21, high-pressure delivery pump; 22, gas-liquid mixer; 23, online heater; 30, sheath gas supply unit; 40, orthogonal hot jet inertial screening module; 41, main flue; 42, coaxial nozzle assembly; 50, salt-rich branch channel; 51, virtual impactor inlet structure; 60, pneumatic cyclone classification unit; 70, tail gas treatment unit. DETAILED DESCRIPTION

[0029] The technical solutions in the embodiments of the application will be described clearly and completely below with reference to the drawings in the specification of the application. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the application.

[0030] Referring to the drawings in the specification of the application, Figure 1 The application provides a desulfurization wastewater resource zero-emission treatment method combined with flue spraying drying, which is implemented by relying on an integrated fluid treatment and classification system.

[0031] The treatment system mainly includes a wastewater pretreatment unit 10, a superheated solution gas working substance preparation circuit 20, a sheath gas supply unit 30, an orthogonal hot jet inertial screening module 40, a salt-rich branch channel 50, a pneumatic cyclone classification unit 60, and a tail gas treatment unit 70.

[0032] The outlet of the wastewater pretreatment unit 10 is connected to the inlet of the superheated gas working medium preparation circuit 20 through a pipeline. The superheated gas working medium preparation circuit 20 is configured with a high-pressure delivery pump 21, a gas-liquid mixer 22, and an online heater 23. A compressed gas source outside the system is connected to the gas phase inlet of the gas-liquid mixer 22, and the high-pressure delivery pump 21 pressurizes and delivers the pretreated wastewater to the liquid phase inlet of the gas-liquid mixer 22. The online heater 23 is arranged downstream of the gas-liquid mixer 22 and is used to heat the gas-liquid two-phase fluid to a preset thermodynamic state.

[0033] The orthogonal hot jet inertial screening module 40 is installed on the side wall of the main flue 41 of the coal-fired unit. The inside of the main flue 41 flows high-temperature flue gas containing coal fly ash. The orthogonal hot jet inertial screening module 40 includes a coaxial nozzle assembly 42, which is respectively connected to the end outlet of the superheated gas working medium preparation circuit 20 and the outlet of the sheath gas supply unit 30 through a pipeline. The jet axis direction of the coaxial nozzle assembly 42 is arranged orthogonally or at a large angle reversely to the main flow direction of the flue gas in the main flue 41.

[0034] The salt-rich branch channel 50 is arranged on the side or the lower side of the main flue 41 opposite to the coaxial nozzle assembly 42. The inlet end of the salt-rich branch channel 50 adopts a virtual impactor inlet structure 51, and the opening cross-sectional area of the virtual impactor inlet structure 51 is greater than the cross-sectional area of the jet beam formed by the coaxial nozzle assembly 42. The outlet end of the salt-rich branch channel 50 is connected to the tangential inlet of the pneumatic cyclone classification unit 60. The bottom discharge port of the pneumatic cyclone classification unit 60 is connected to a waste salt collection bin, and the top exhaust port is connected to a tail gas treatment unit 70 or is connected back to the downstream area of the main flue 41.

[0035] Referring to the accompanying drawings, Figure 2 , the present application provides a flue spray drying combined desulfurization wastewater resource utilization zero-emission treatment method, which comprises the following steps:

[0036] S1, the desulfurization wastewater is pretreated, pressurized, and heated to form a superheated gas multiphase working medium in a thermodynamic superheated state;

[0037] S2, the superheated gas multiphase working medium prepared in step S1 is introduced into the central channel of the coaxial nozzle, and the high-temperature sheath gas in the outer channel is jetted orthogonally or reversely into the main flue, and the flash phase change and sheath gas isolation effect are used to form a composite flash jet beam containing low-density hollow salt particles on the cross section of the main flue;

[0038] S3, the coal fly ash in the main flue penetrates the composite flash jet beam by using the inertial difference based on the Stokes number, and the low-density hollow salt particles in the composite flash jet beam are deflected along the flow lines and enter the lateral branch under the action of aerodynamic drag, forming a salt-containing deflected airflow separated from the coal fly ash;

[0039] S4, guiding the salt-containing deflected gas flow to enter the salt-rich branch channel through the virtual impinging inlet, and after completing the final drying, sending it into the pneumatic cyclone classification unit to further separate the residual fine fly ash using the solid phase density difference, and finally recovering the waste salt solid product.

[0040] The following will be combined with the above process, the fluid working medium modulation, flow field construction parameters and classification equipment structure details involved in each step will be described in detail.

[0041] Referring to the drawings Figure 2 In the flue spray drying desulfurization wastewater resource utilization zero emission treatment method, the softening pretreatment, pressurized gas and heating of the desulfurization wastewater in step S1 build a superheated gas multiphase working medium in a thermodynamic superheated state, which specifically includes the following substeps and technical implementation details:

[0042] S11, anti-fouling softening and basic pressurization of wastewater. The original desulfurization wastewater is introduced into the wastewater pretreatment unit 10, and sodium carbonate or sodium hydroxide reagent is added to remove calcium and magnesium hardness ions in the water body to prevent pipeline fouling under high temperature working conditions. The softened wastewater is transported to the pipeline by the high-pressure delivery pump 21 in the superheated gas working medium preparation circuit 20, and is pressurized to a preset stagnation pressure . The stagnation pressure is set to 2.0 to 5.0 times the environmental pressure of the area where the orthogonal thermal jet inertial screening module 40 is located, so as to establish the pressure potential for driving the dissolution process and jet atomization.

[0043] S12, establish a gas-liquid non-equilibrium dissolution system. In the pipeline with a stagnation pressure , the compressed gas source is introduced into the gas-liquid mixer 22 in the superheated gas working medium preparation circuit 20. The high-pressure delivery pump 21 transports the wastewater to the liquid phase inlet of the gas-liquid mixer 22, and the compressed air or nitrogen gas is transported to the gas phase inlet of the gas-liquid mixer 22. Control the gas-liquid mixing ratio to form a gas-unsaturated or gas-saturated state in the gas-liquid mixer 22. Under the action of the stagnation pressure , the gas molecules dissolve in the liquid phase interstitial space to form a high-pressure gas solution fluid, which provides a gas phase nucleation core for the subsequent flash evaporation process.

[0044] S13, build a superheated thermodynamic state. The above high-pressure gas solution fluid is transported to the online heater 23 downstream of the gas-liquid mixer 22 for non-boiling heating. The heat source medium uses power plant steam or high-temperature flue gas. The stagnation temperature of the superheated gas multiphase working medium before entering the coaxial nozzle assembly 42 is controlled by the online heater 23 to meet the thermodynamic superheat criterion.

[0045] The thermodynamic superheat criterion is determined by a temperature control formula, which is:

[0046]

[0047] In the formula: represents the stagnation temperature of the superheated gas-liquid multiphase working medium at the outlet of the online heater 23, with the unit of Celsius (°C); represents the injection ambient pressure inside the main flue 41, with the unit of Pascal (Pa); represents the saturation boiling temperature of the fluid at the pressure , with the unit of Celsius (°C); represents the preset minimum superheat threshold, with the value range of 15°C to 40°C, used to ensure that the fluid enthalpy is sufficient to overcome the surface tension work; represents the stagnation pressure of the superheated gas-liquid working medium preparation circuit 20 pipeline, with the unit of Pascal (Pa); represents the saturation boiling temperature of the fluid at the pressure , with the unit of Celsius (°C).

[0048] Through the above formula control, it is ensured that the working medium remains in a single-phase liquid state or a dense gas-liquid mixed state in the delivery pipeline and the online heater 23, avoiding premature boiling.

[0049] S14, preset dissolved gas expansion internal energy and phase change fragmentation power. Through the modulation of the above steps S11 to S13, the superheated gas-liquid working medium constructed before entering the coaxial nozzle assembly 42 has high-energy metastable state characteristics. This feature is reflected in the fact that the fluid has both thermodynamic sensible heat that causes it to undergo phase change in a low-pressure environment and the potential energy of the dissolved gas expansion that is precipitated from the liquid phase due to pressure drop. These two forms of energy are released simultaneously in the subsequent injection process, causing volume expansion inside the liquid droplets and promoting the formation of a thin-walled hollow spherical structure of the precipitated salt before solidification on the particle surface, thereby reducing the apparent density of the waste salt particles and meeting the physical property requirements for inertial separation based on the Stokes number in the subsequent step S3.

[0050] In the flue spray drying desulfurized wastewater resource utilization zero-emission treatment method, the superheated gas-liquid multiphase working medium is introduced into the central channel of the coaxial nozzle in step S2, and the high-temperature sheath gas in the outer channel is injected orthogonally or reversely into the main flue, forming a composite flash jet beam containing low-density hollow salt particles on the cross section of the main flue by using flash phase change and sheath gas isolation effect, which specifically includes the following sub-steps and technical implementation details:

[0051] ​S21, constructing coaxial layered isolation flow field. The coaxial nozzle assembly 42 in the orthogonal thermal jet inertial screening module 40 adopts a double-layer concentric sleeve structure. The superheated gas solution multiphase working medium prepared in step S1 is transported to the central channel of the coaxial nozzle assembly 42 via the superheated gas solution preparation circuit 20 to form an inner layer liquid phase jet core. The sheath gas supply unit 30 introduces the heated clean air or nitrogen into the annular outer channel of the coaxial nozzle assembly 42 to form an outer layer gas phase sheath flow.

[0052] The sheath gas supply unit 30 controls the sheath gas outlet temperature to be not lower than the temperature of the flue gas in the main flue 41, preventing the inner layer working medium from condensing too early. At the nozzle outlet cross section, the outer layer gas phase sheath flow wraps the inner layer liquid phase jet core and simultaneously enters the main flue 41. The axial velocity of the outer layer gas phase sheath flow is set to be higher than the flow velocity of the flue gas in the main flue 41, so that a momentum isolation layer is formed at the initial segment of the jet, pushing away the flue gas flow field carrying the coal fly ash, and constructing a local low-dust area on the cross section of the main flue 41, preventing the coal fly ash from adhering to the liquid droplets at the initial stage of jet breakup.

[0053] S22, matching the jet penetration depth and trajectory. In order to ensure that the composite flash jet beam can penetrate the boundary layer of the main flue 41 and be delivered to the inlet area of the salt-rich branch channel 50, the jet trajectory is controlled by using the momentum flux ratio.

[0054] The injection parameters are determined by using the momentum flux ratio calculation formula, which is:

[0055] ;

[0056] In the formula: represents the jet momentum flux ratio, dimensionless; represents the homogeneous density of the gas-liquid mixed fluid at the outlet cross section of the coaxial nozzle assembly 42, with the unit of kilogram per cubic meter (kg / m 3 ); represents the average axial injection velocity of the composite fluid at the outlet of the coaxial nozzle assembly 42, with the unit of meters per second (m / s); represents the density of the transversely flowing flue gas in the main flue 41, with the unit of kilogram per cubic meter (kg / m 3 ); represents the main flow velocity of the transversely flowing flue gas in the main flue 41, with the unit of meters per second (m / s).

[0057] By adjusting the delivery pressure of the high-pressure delivery pump 21 in the superheated gas solution preparation circuit 20 and the gas supply flow of the sheath gas supply unit 30, the value of is controlled to be within the range of 10 to 80. When When the value is in this range, the composite flash jet is deflected under the action of the transverse airflow, and the end of its jet core trajectory is aligned with the virtual impactor inlet structure 51 of the salt-rich branch channel 50.

[0058] S23, triggering depressurization flash evaporation and inducing hollow crystallization. When the superheated dissolved gas multiphase working fluid leaves the coaxial nozzle assembly 42 and enters the lower-pressure main flue 41 environment, the fluid pressure drops sharply from the stagnation pressure to the ambient pressure. Flash evaporation phase change occurs inside the fluid, and gas molecules dissolved in the liquid phase precipitate and form bubble nuclei. As the water on the droplet surface evaporates rapidly, a solid salt shell forms on the droplet surface. Because the volume expansion rate of the dissolved bubbles inside is greater than the contraction rate of the salt shell on the outer surface, the internal gas phase pressure forces the incompletely solidified salt shell to expand outward, eventually forming hollow spherical salt crystals with cavities and dense walls after drying. This hollow structure reduces the apparent density of the waste salt particles, increasing the difference in kinetic characteristics between them and the solid coal fly ash particles in the main flue 41.

[0059] In the zero-discharge treatment method for desulfurization wastewater resource utilization combined with flue gas spray drying, step S3 utilizes the difference in particle relaxation time and the Stokes criterion to achieve online inertial screening of coal fly ash and waste salt particles in a positive alternating field. This specifically includes the following sub-steps and technical implementation details:

[0060] S31, establish separation criteria based on particle dynamic response characteristics. In the intersection region of the main flue 41 and the salt-rich branch channel 50, due to lateral air intake, streamline curvature and velocity gradients are formed in the flow field. To quantify the differences in the ability of different particles to follow streamlines in this type of flow field, particle relaxation time and Stokes number are used as core evaluation indicators. Since the waste salt particles generated in step S2 have a hollow, thin-walled structure, while coal fly ash is mainly composed of solid aluminosilicate spheres, their apparent densities differ significantly even with similar geometric particle sizes. This difference in physical properties determines the dynamic response speed of the particles in a variable-speed flow field.

[0061] The particle relaxation time calculation formula is used to determine the particle's response speed to changes in the flow field. The particle relaxation time calculation formula is as follows:

[0062] ;

[0063] In the formula: Represents particle relaxation time, measured in seconds (s), and characterizes the time scale required for particle velocity to adapt to fluid velocity adjustment. The apparent density of the particles is expressed in kilograms per cubic meter (kg / m³). When calculating waste salt particles, the apparent density of the hollow material is used, while when calculating fly ash particles, the density of the solid material is used. Represents the geometric equivalent diameter of the particle, in meters (m). represents the Cunningham slip correction factor, dimensionless, used to correct the drag characteristics of micron-sized particles in a non-continuous medium; represents the dynamic viscosity of the mixed gas flow in the sorting region, in units of pascal seconds (Pa·s), which depends on the mixed temperature of the main flue gas and the sheath gas.

[0064] Based on the above relaxation time, further set the separation limit based on the Stokes number formula, the Stokes number formula is:

[0065] ;

[0066] In the formula: represents the Stokes number, dimensionless, used to determine whether the particle is separated from the streamline; represents the average suction flow velocity at the inlet of the salt-rich branch channel 50, in units of meters per second (m / s); represents the characteristic opening width of the virtual impactor inlet structure 51, in units of meters (m).

[0067] S32, perform density-dominated streamline selection and screening. By adjusting the operating load of the aerodynamic cyclone classification unit 60 or the tail gas treatment system connected downstream of the salt-rich branch channel 50, control the suction flow velocity at the opening of the virtual impactor inlet structure 51 So that the flue gas streamline in this area is diverted with a preset curvature and flows into the salt-rich branch channel 50. At this time, the two kinds of particles in the mixed flow field show different motion trajectories based on the difference in Stokes number:

[0068] For coal-fired fly ash particles, due to their large apparent density The calculated relaxation time is large. Under the condition of the set characteristic opening width and suction velocity , the value of corresponding to the fly ash particles is greater than 1. At this time, the inertial force plays a leading role, and the fly ash particles cannot respond to the rapid bending of the streamline, maintain the original motion inertia direction, cross the diverted streamline and cross the capture area of the virtual impactor inlet structure 51, and continue to flow downstream with the main flue gas in the main flue 41.

[0069] For waste salt particles, due to the hollowing modulation of step S2, the apparent density is significantly reduced, and the calculated relaxation time is small, so that the corresponding The value is less than or close to 1. At this time, the viscous resistance plays a leading role, and the waste salt particles can adapt to the change of the flow field velocity vector, deflect along the curved streamline, and enter the virtual impactor inlet structure 51, and finally be transported to the aerodynamic cyclone classification unit 60 through the salt-rich branch channel 50. By controlling The value is located in the separation interval of fly ash and waste salt, realizing online inertial separation based on the apparent density difference.

[0070] In the flue spray drying desulfurization wastewater resource utilization zero emission treatment method, step S4 carries out non-contact transportation and gas-solid separation purification of the wet salt mist entering the salt-rich branch channel through the streamline selection, specifically including the following substeps and technical implementation details:

[0071] S41, a virtual capture channel isolated by a gas film is constructed. The virtual impactor inlet structure 51 at the inlet end of the salt-rich branch channel 50 is designed as an expanded flow guide structure, and a gas distribution jacket is provided outside the structure. An annular slot or a porous permeable material is provided on the inner wall surface of the virtual impactor inlet structure 51 along the flow direction, and the annular slot or the porous permeable material is connected to the gas distribution jacket.

[0072] The external hot gas source (which can be connected to the sheath gas supply unit 30 or an independent hot air system according to the specific process) injects hot clean gas with a certain pressure into the gas distribution jacket. The hot clean gas is uniformly injected tangentially through the annular slot or the porous permeable material, and a continuous gas film protection layer is formed on the inner wall surface of the virtual impactor inlet structure 51 and the salt-rich branch channel 50. The gas film protection layer physically isolates the main flow gas carrying the waste salt particles from the inner wall surface of the channel, preventing the direct impact of the slightly wet salt droplets that have not completely solidified on the wall surface to cause adhesion and scaling. At the same time, the hot clean gas provides additional heat enthalpy, so that the salt particles continuously exchange heat with the gas flow during the transportation in the salt-rich branch channel 50, until the moisture content of the salt particles is reduced to the preset drying standard, realizing the drying during the pneumatic conveying process.

[0073] S42, the cyclone classification for hollow low-density particles is implemented. The dried salt-containing gas flow is introduced tangentially into the aerodynamic cyclone classification unit 60. Since the waste salt particles generated in steps S2 and S3 are hollow thin-walled structures, their apparent density is significantly lower than that of conventional solid particles, resulting in smaller centrifugal sedimentation force in the cyclone field. In order to ensure the separation efficiency, the cutting particle size of the aerodynamic cyclone classification unit 60 needs to be calculated and parameter matched according to the physical characteristics of the hollow salt particles.

[0074] The cyclone cutting particle size calculation formula is used to determine the key operating parameters and structure size of the aerodynamic cyclone classification unit 60, and the cyclone cutting particle size calculation formula is as follows:

[0075] ;

[0076] In the formula: The cutting particle size of the pneumatic cyclone classification unit 60 is represented by meters (m), which is the particle diameter at which the separation efficiency is 50%. The design goal is to set this value to be smaller than the minimum average particle size of the waste salt particles. The dynamic viscosity of the airflow at the internal operating temperature of the aerodynamic swirl classification unit 60 is expressed in Pascal-seconds (Pa·s). The width of the inlet channel of the aerodynamic swirl classification unit 60 is represented by meters (m). This represents the effective number of rotations of the airflow within the cyclone separator. It is dimensionless and ranges from 3 to 8. This represents the tangential velocity of the airflow at the inlet of the aerodynamic swirl classification unit 60, expressed in meters per second (m / s). The apparent density of hollow waste salt particles is expressed in kilograms per cubic meter (kg / m³). 3 This value is determined by the hollow structure formed in step S2; This represents the density of the carrier gas within the aerodynamic swirl classification unit 60, expressed in kilograms per cubic meter (kg / m³). 3 ).

[0077] Based on the above formula, given that ( The value of ) is relatively small due to the hollow structure of the waste salt, in order to obtain the required fine cutting particle size. In the structural design and operation of the aerodynamic swirl classifier 60, the width of the inlet flow channel is reduced. Alternatively, increase the system's induced draft to increase the inlet tangential velocity. Technical measures are implemented to enhance the centrifugal separation factor applied to the low-density hollow particles. Under the action of the enhanced swirling field, the hollow waste salt particles are thrown against the wall of the container and slide down to the waste salt collection bin at the bottom. The purified gas is discharged through the central exhaust pipe and connected to the tail gas treatment unit 70 or returned to the downstream area of ​​the main flue 41 to complete the recovery of waste salt.

[0078] To verify the feasibility and effectiveness of the above technical solution, the following example of a 300MW coal-fired unit desulfurization wastewater zero-discharge retrofit project will be used to illustrate the typical operating parameters of the present invention.

[0079] S51, Pretreatment and Working Fluid Preparation Parameter Settings. The TDS (Total Dissolved Solids) concentration of the desulfurization wastewater to be treated ranges from 40,000 mg / L to 60,000 mg / L. After suspended solids are removed by the wastewater pretreatment unit 10, the wastewater enters the superheated dissolved gas working fluid preparation loop 20. The high-pressure delivery pump 21 increases the working fluid pressure and maintains it at 1.5 MPa to 1.8 MPa. The online heater 23 heats the gas-liquid mixed working fluid to 150°C to 160°C. This temperature setting is higher than the flue gas temperature of 120°C to 140°C in the main flue duct 41, utilizing the temperature difference to assist evaporation; on the other hand, it is lower than the saturation temperature at the corresponding pressure (the saturation temperature at 1.5 MPa is approximately 198°C), keeping the working fluid in the liquid phase in the pipeline and preventing two-phase flow blockage before the nozzle. At the same time, the gas-liquid mass ratio in the gas-liquid mixer 22 is controlled at 3% to 5% to ensure that sufficient expansion gas nuclei are formed inside the tiny droplets.

[0080] S52, jet trajectory and penetration depth control. The sheath gas supply unit 30 supplies hot, clean air at 160°C to 170°C to the outer ring of the coaxial nozzle assembly 42. This temperature is slightly higher than the flue gas temperature to prevent acid condensation on the nozzle end face. The jet velocity at the outlet of the coaxial nozzle assembly 42 is set. The velocity of the flue gas within the main flue duct 41 is 60 m / s to 80 m / s. The flow rate typically fluctuates between 10 m / s and 15 m / s. Based on the momentum-flux ratio calculation formula in step S2, the jet velocity and flow rate are adjusted to achieve the desired momentum-flux ratio. The value remains between 25 and 40. Within this parameter range, calculations show that the composite jet can effectively penetrate the flue gas boundary layer, delivering its core stream to the inlet area of ​​the salt-rich branch channel 50, which is 2 to 3 meters away from the nozzle outlet, while avoiding droplets directly impacting the opposite flue wall due to excessive momentum.

[0081] S53, a screening and recycling operation based on the Stokes criterion. Online sorting zone, coal fly ash particle density. The value is 2300 kg / m 3 The average particle size is 20 μm; the apparent density of the waste salt particles after cavitation treatment is... Approximately 700 kg / m 3 The average particle size is controlled between 18μm and 22μm (on the same order of magnitude as fly ash particle size to ensure that density difference dominates the separation behavior). The induced draft load of the aerodynamic cyclone classification unit 60 is adjusted to control the intake airflow velocity at the virtual impactor inlet structure 51. The value is 8 m / s. Calculated using the Stokes number formula from step S3:

[0082] Coal fly ash particles The value is approximately 2.4 ( ), the inertia force dominates, and the fly ash particles cannot respond to the streamline deflection and pass through with the mainstream flue gas;

[0083] The hollow waste salt particles, due to the significant reduction in density and the particle size remaining within a controllable range, can correspondingly have a value of about 0.7 ( ), and the fluid drag force dominates, so that the waste salt particles can follow the deflected streamline into the salt-rich branch channel 50.

[0084] S54, gas film protection and secondary purification. The annular slit of the virtual impactor inlet structure 51 injects 160°C auxiliary hot air tangentially to the inner wall surface, and the flow rate accounts for 12% of the total branch air volume, forming a stable heat insulation and anti-sticking gas film. In the aerodynamic cyclone classification unit 60, according to the cutting particle size formula in step S4, for the low-density hollow salt particles, the inlet tangential velocity is set to 20 m / s, and the narrow-slit type inlet flow channel width is matched. Actual operation monitoring shows that the waste salt capture rate under this condition is greater than 95%, and the fly ash mass fraction in the recovered product is less than 3%, meeting the resource utilization standards of industrial-grade waste salt.

Claims

1. A method for zero-discharge treatment of desulfurization wastewater combined with flue gas spray drying, characterized in that, Includes the following steps: S1. The desulfurization wastewater is pretreated by softening, pressurized dissolved gas, and heated to form a superheated dissolved gas multiphase working fluid in a thermodynamically superheated state. S2. The superheated dissolved gas multiphase working fluid prepared in step S1 is introduced into the central channel of the coaxial nozzle assembly, and the high temperature sheath gas in the outer channel of the coaxial nozzle assembly is injected into the main flue orthogonally or in reverse. By utilizing the flash phase change and sheath gas isolation effect, a composite flash jet containing low-density hollow salt particles is formed on the cross section of the main flue. S3. Utilizing the inertial difference based on the Stokes number, the fly ash from the main flue is made to penetrate the composite flash jet, while the low-density hollow salt particles in the composite flash jet are deflected with the streamline and enter the salt-rich branch channel under the action of aerodynamic drag, forming a salt-containing deflected airflow that is separated from the fly ash from the fly ash. S4. The salt-containing deflected airflow is guided into the salt-rich branch channel through the virtual impactor inlet structure, and after the final drying is completed, it is introduced into the aerodynamic cyclone classification unit to further separate the residual fine fly ash by utilizing the solid density difference, and finally recover the waste salt solid product.

2. The method for zero-discharge treatment of desulfurization wastewater combined with flue gas spray drying according to claim 1, characterized in that, In step S1, the heating modulation specifically includes controlling the stagnation temperature of the superheated dissolved gas multiphase working fluid before it enters the coaxial nozzle assembly by online heating, wherein the stagnation temperature is determined by a temperature control formula.

3. The method for zero-discharge treatment of desulfurization wastewater combined with flue gas spray drying according to claim 1, characterized in that, In step S1, the pressurized dissolved gas and heating modulation specifically include: The softened wastewater is pressurized to a stagnation pressure, and the stagnation pressure is set to 2.0 to 5.0 times the injection environment pressure inside the main flue. In a pipeline environment that maintains the stagnation pressure, the gas-liquid mixing ratio is controlled so that the fluid is in a state of either dissolved gas unsaturation or dissolved gas saturation. The stagnation pressure is used to drive gas molecules into the interphase spaces of the liquid phase, providing gas-phase nucleation nuclei for the subsequent flash evaporation process, thereby constructing the superheated dissolved gas multiphase working fluid.

4. The method for zero-discharge treatment of desulfurization wastewater combined with flue gas spray drying according to claim 1, characterized in that, In step S2, the coordinated high-temperature sheath gas injection specifically includes: The high-temperature sheath gas is set to form an outer gas phase sheath flow, and the axial velocity of the outer gas phase sheath flow is controlled to be higher than the flow velocity of the flue gas in the main flue. The outer gas sheath flow is used to envelop the superheated dissolved gas multiphase working fluid in the initial jet section, thereby establishing a momentum exchange interface between the gas and liquid phases. Based on the momentum exchange interface, the trajectory of the composite flash jet is controlled by the momentum flux ratio, wherein the injection parameters of the composite flash jet are determined by the momentum flux ratio calculation formula.

5. The method for zero-discharge treatment of desulfurization wastewater combined with flue gas spray drying according to claim 3, characterized in that, In step S2, the flash phase change specifically includes: When the superheated dissolved gas multiphase working fluid leaves the coaxial nozzle assembly and enters the main flue environment, the fluid pressure drops sharply from the stagnation pressure to the ambient pressure. By utilizing the sudden drop in pressure to induce the gas molecules dissolved in the liquid phase to precipitate and form bubble nuclei, a volume change difference is established where the expansion rate of the internal bubbles is greater than the contraction rate of the external salt shell. The volume change difference forces the incompletely solidified salt shell to expand outward, eventually solidifying to form the low-density hollow salt particles.

6. The method for zero-discharge treatment of desulfurization wastewater combined with flue gas spray drying according to claim 1, characterized in that, In step S3, the utilization of inertial differences based on Stokes criterion numbers specifically includes: In the intersection region of the main flue and the salt-rich branch channel, the velocity vector distribution of the flow field is analyzed; The background fluid parameters are determined based on the analyzed flow field velocity vector distribution, and the particle response speed to the flow field change is determined using the particle relaxation time calculation formula. Substituting the response rate into the Stokes number formula, the kinetic separation boundary between the coal fly ash and the low-density hollow salt particles is set.

7. The method for zero-discharge treatment of desulfurization wastewater combined with flue gas spray drying according to claim 6, characterized in that, In step S3, by controlling the average intake airflow velocity at the inlet of the salt-rich branch channel, the Stokes number corresponding to the coal fly ash is made greater than 1, and the original inertial direction of motion is maintained by inertial force and flows downstream with the mainstream flue gas in the main flue; the Stokes number corresponding to the low-density hollow salt particles is made less than or close to 1, and the viscous resistance causes them to deflect with the curved streamline and enter the salt-rich branch channel.

8. The method for zero-discharge treatment of desulfurization wastewater combined with flue gas spray drying according to claim 1, characterized in that, In step S4, the virtual impactor inlet structure specifically includes: A gas distribution jacket is provided on the outside of the virtual impactor inlet structure, and an expansion-type flow guide structure is used to connect it to the main flue. Hot clean gas is uniformly injected into the channel through the gas distribution jacket via an annular slit opened on the inner wall or by a porous permeable material. A continuous air film protective layer is formed on the inner wall surface of the virtual impactor inlet structure and the salt-rich branch channel; The air film protective layer physically isolates the salt-containing deflected airflow from the inner wall of the channel and provides an accompanying drying heat source.

9. The method for zero-discharge treatment of desulfurization wastewater combined with flue gas spray drying according to claim 1, characterized in that, In step S4, the key operating parameters and structural dimensions of the pneumatic cyclone classification unit are determined using the cyclone cut particle size calculation formula, thereby matching the apparent density characteristics of the low-density hollow salt particles.

10. The method for zero-discharge treatment of desulfurization wastewater combined with flue gas spray drying according to claim 9, characterized in that, In step S4, based on the density difference between the low-density hollow salt particles and the fly ash from coal combustion, the centrifugal separation factor applied to the low-density hollow particles is enhanced by reducing the inlet channel width of the pneumatic cyclone classification unit or increasing the system induced draft, so that the low-density hollow salt particles are captured in the waste salt collection bin at the bottom, and the purified gas is discharged through the central exhaust pipe.

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