High-efficiency low-consumption water extraction process for desulfurized and purified flue gas

Abstract

The invention belongs to the technical field of flue gas treatment of coal-fired power plants, and particularly relates to a high-efficiency low-consumption water extraction process for desulfurized purified flue gas. Comprising the steps that saturated clean flue gas at an outlet of a desulfurizing tower enters a first-stage heat exchanger to be pre-cooled and indirectly exchanges heat with circulating cooling water, the temperature of the flue gas is reduced to 40-45 DEG C from 50-60 DEG C, and sensible heat is recovered; the pre-cooled flue gas enters a second-stage heat exchanger to be subjected to medium-temperature condensation, a fluoroplastic tube bundle heat exchanger is adopted to exchange heat with a cooling medium with the temperature of 30-35 DEG C, the temperature of the flue gas is reduced to 32-38 DEG C, and part of water vapor is condensed and recycled; and the flue gas subjected to medium-temperature condensation enters a third-stage heat exchanger for deep condensation, a porous ceramic membrane condenser with the pore diameter being 10-50 nm is adopted to exchange heat with cooling water with the temperature being 15-25 DEG C, the temperature of the flue gas is reduced to 26-30 DEG C, and water vapor is recycled through deep condensation. The water recovery rate, the heat recovery efficiency, the energy consumption control, the economical efficiency, the environmental protection benefit and the like are obviously superior to those of the prior art, and the method has important practical value and popularization prospect.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of flue gas treatment of coal-fired power plants, and particularly relates to a high-efficiency and low-consumption water extraction process for desulfurized clean flue gas. BACKGROUND

[0002] Coal-fired power plants are the main source of electricity, and coal power generation accounts for about 65% of the total power generation. Wet desulfurization is the most widely used flue gas desulfurization technology, and the desulfurization efficiency can reach more than 95%. However, the temperature of the clean flue gas after wet desulfurization is about 50-60℃, and it is in a wet saturated state, with a water vapor volume fraction of 15-17%. Direct emission causes huge waste of water resources and heat energy. According to statistics, the annual flue gas water vapor loss of thermal power plants is about 1 billion tons, which is equivalent to a loss of about 254 billion MJ of heat energy. In water resource shortage areas, flue gas water extraction technology is of great significance to achieve "zero water taking" operation of power plants.

[0003] The existing flue gas water extraction technologies mainly include indirect heat exchange condensation method, ceramic membrane condensation method, and direct contact condensation method. The main problems existing in the prior art are:

[0004] (1) Limited single-stage condensation efficiency: a single cooling temperature is used, which cannot simultaneously consider sensible heat recovery and deep condensation, and the water recovery rate and heat recovery efficiency are both not ideal;

[0005] (2) High energy consumption: the energy consumption of circulating water pumps and fans accounts for a large proportion, and the unit condensation water energy consumption is as high as 95-295 kJ / kg;

[0006] (3) Low grade of waste heat: the condensation water and cooling water have low temperatures (30-40℃), which are difficult to effectively utilize, resulting in secondary energy waste;

[0007] (4) Corrosion problem: metal materials are corroded by low-temperature acid, non-metallic materials have poor heat transfer performance, and the contradiction between material selection and cost is prominent;

[0008] (5) Lack of intelligent control: when the flue gas parameters fluctuate (temperature, flow, humidity), the system cannot be self-adaptively adjusted, resulting in a decrease in operating efficiency.

[0009] Therefore, there is a need for a desulfurized clean flue gas water extraction process that can efficiently recover flue gas moisture and waste heat, reduce system energy consumption, improve the utilization value of condensation water and waste heat, and have intelligent control capability. SUMMARY

[0010] The purpose of the present application is to solve the problems existing in the background art, and to provide a high-efficiency and low-consumption water extraction process for desulfurized clean flue gas. The process combines multi-stage cascade condensation, heat pump waste heat upgrading, and intelligent control to achieve a water recovery rate of ≥80%, a heat recovery efficiency of ≥50%, and a system self-consumption rate of ≤0.5%, while solving the corrosion problem and the problem of low-grade utilization of waste heat.

[0011] The technical scheme adopted by the present application is as follows: a high-efficiency and low-consumption water extraction process for desulfurized clean flue gas, comprising the following steps:

[0012] Step 1: saturated clean flue gas at the outlet of a desulfurization tower is introduced into a first-stage heat exchanger for precooling, and is indirectly exchanged with circulating cooling water, so that the temperature of the flue gas is reduced from 50-60℃ to 40-45℃, and the sensible heat is recovered;

[0013] Step 2: the pre-cooled flue gas is introduced into a second-stage heat exchanger for medium-temperature condensation, and is exchanged with a cooling medium at a temperature of 30-35℃ by using a fluoroplastic tube bundle heat exchanger, so that the temperature of the flue gas is reduced to 32-38℃, and part of the water vapor is condensed and recovered;

[0014] Step 3: the flue gas subjected to medium-temperature condensation is introduced into a third-stage heat exchanger for deep condensation, and is exchanged with cooling water at a temperature of 15-25℃ by using a porous ceramic membrane condenser with a pore size of 10-50nm, so that the temperature of the flue gas is reduced to 26-30℃, and the water vapor is deeply condensed and recovered;

[0015] Step 4: the cooling water of the second-stage heat exchanger and the third-stage heat exchanger is subjected to waste heat recovery after being raised in temperature by a heat pump system, and is used as a heat source for the first-stage heat exchanger or is used for preheating boiler feed water;

[0016] Step 5: the condensate water of each stage is collected and then subjected to condensate water collection, and is used as industrial water or boiler feed water after being subjected to water quality treatment.

[0017] Preferably, the first-stage heat exchanger is a stainless steel tube heat exchanger or a plate heat exchanger, and the material is 316L stainless steel or duplex stainless steel, the tube wall temperature is dynamically controlled to be 10-20℃ higher than the corresponding acid dew point temperature according to the SO2 concentration of the flue gas, the acid dew point is about 85-90℃ under the design condition, and therefore the tube wall temperature is ≥105℃.

[0018] Preferably, the fluoroplastic tube bundle of the second-stage heat exchanger is made of PTFE or PVDF material, and the tube bundle is arranged in a staggered manner, the flue gas side flow rate is controlled to be 8-12m / s, and the cooling water side flow rate is controlled to be 0.8-1.5m / s.

[0019] Preferably, the porous ceramic membrane condenser of the third-stage heat exchanger comprises a plurality of ceramic membrane tubes connected in parallel, the ceramic membrane material is alumina-based or titania-based ceramic, the porosity is 30-35%, and the flue gas is introduced into the outside of the membrane tube, and the cooling water is introduced into the inside of the membrane tube.

[0020] More preferably, the heat pump system is a compression heat pump, the refrigerant is R134a or R410A, and based on the typical performance of R134a refrigerant under the working condition, the heat pump COP value is greater than or equal to 2.5, and the condensation heat of the second-stage heat exchanger is increased to 55-65°C and the condensation heat of the third-stage heat exchanger is increased to 15-25°C.

[0021] More preferably, the intelligent control system is further included, which adjusts the cooling water flow of each stage and the operation parameters of the heat pump according to the flue gas inlet temperature, flow, moisture content and outlet temperature of each stage, so that the energy consumption of the system is minimized and the water recovery rate is maximized.

[0022] More preferably, the control strategy of the intelligent control system is as follows:

[0023] When the flue gas inlet temperature is higher than 55°C, the first-stage cooling water flow is increased;

[0024] When the flue gas inlet flow is increased by more than 10% of the design value, the cooling water flow of each stage is simultaneously increased;

[0025] When the flue gas moisture content is higher than 15%, the third-stage cooling water flow is increased;

[0026] When the ambient temperature is lower than 15°C, the power of the heat pump compressor is reduced or the heat pump is stopped, and the natural cold source is utilized.

[0027] More preferably, a demister is arranged between the outlet of the first-stage heat exchanger and the inlet of the second-stage heat exchanger to remove liquid droplets with a particle size greater than 30μm and prevent the pollution of the ceramic membrane.

[0028] More preferably, the condensation water of each stage in step 5 is collected and treated according to the water quality difference, specifically, the first-stage condensation water with good water quality is directly introduced into the boiler feed water system, and the second-stage and third-stage condensation water needs to be treated by ion exchange or reverse osmosis before use.

[0029] More preferably, the pressure drop of the flue gas side of each-stage heat exchanger is controlled to be 300-800Pa, the total system pressure drop is less than 1500Pa, and the power consumption of the fan is increased by less than 15% of the sum of the waste heat recovery amount and the condensation water recovery value.

[0030] Compared with the prior art, the present application has the following remarkable beneficial effects:

[0031] The present application adopts three-stage cascade condensation, which is optimized for sensible heat recovery, medium-temperature condensation and deep condensation, respectively, and the comprehensive water recovery rate reaches 80-85%, which is increased by 15-50 percentage points compared with single-stage condensation (32-70%). For a 660MW unit, the annual water recovery amount can reach 1.8-2 million tons, which is equivalent to the amount of fresh water saved, and is of great significance to power plants in water-deficient areas, and the water recovery rate is significantly improved.

[0032] The sensible heat temperature recovered by the first-stage heat exchanger of the application is up to 45-50 DEG C, which can be directly utilized; the low-grade waste heat of the second-stage and third-stage is raised to 55-65 DEG C by the heat pump, and has higher utilization value. The total heat recovery efficiency of the system is greater than or equal to 50%, which is significantly higher than the prior art (20-40%). About 10-15 thousand tons of standard coal (660 MW unit) can be saved per year, and the coal consumption for power supply is reduced by 3-4 g / kWh, the heat recovery efficiency is high, and the waste heat utilization value is large.

[0033] Although the application increases the heat pump system, the COP of the heat pump is greater than or equal to 2.5, and the system self-consumption rate is only 0.3-0.5% through intelligent control optimization operation, and the unit condensate water energy consumption is reduced to 50-80 kJ / kg, which is reduced by 50-70% compared with the prior art (95-295 kJ / kg). Considering the water recovery benefit, heat recovery benefit and operation cost, the investment recovery period is 4.5-5.9 years, and considering the rising of resource price and technical optimization, the dynamic recovery period can be shortened to 3.5-4.5 years, which is low in energy consumption and good in economy.

[0034] The application adopts a staged material strategy: the first stage is operated above the acid dew point, and 316L stainless steel with lower cost is adopted; the second stage adopts corrosion-resistant fluoroplastic; and the third stage adopts ceramic membrane. This material combination not only ensures the corrosion resistance, but also controls the cost, and the service life of the equipment is greater than or equal to 10 years, effectively solving the corrosion problem.

[0035] The first-stage condensate water is good in quality and can be directly used; the second-stage and third-stage condensate water are treated respectively, avoiding the overall water quality decline after mixing. The staged treatment reduces the water treatment cost by about 30-40%, and the condensate water is classified, and the treatment cost is low.

[0036] The intelligent control system of the application can be self-adaptively adjusted according to the flue gas parameter fluctuation (such as load change, coal type change, seasonal change), and keep the system high-efficiency operation. In winter, the natural cold source can be utilized, and in summer, the heat pump operation is optimized, and the average energy efficiency ratio is increased by 20-30% than the prior art all the year round, which is high in intelligent degree and strong in adaptability.

[0037] The three-stage heat exchanger is arranged in series, the heat pump system is compactly integrated, and the total occupied area is reduced by 20-25% than the single-stage system with the same processing capacity, which is high in system integration degree and small in occupied area.

[0038] The application greatly reduces the water content of flue gas emission, reduces the "white smoke plume" and "gypsum rain" phenomenon; cooperatively removes PM2.5 (research shows that 72% of PM2.5 is indirectly generated by water vapor); saves fresh water resources, supports the "zero water taking" operation of the power plant; saves energy and reduces emissions, reduces CO2 emission by about 20-30 thousand tons (660 MW unit) per year, and has remarkable environmental benefits.

[0039] In summary, the present application is superior to the prior art in water recovery rate, heat recovery efficiency, energy consumption control, economy and environmental benefits, etc. by innovative multi-stage cascade condensation, heat pump waste heat lifting and intelligent control technology, and has important practical value and popularization prospect. BRIEF DESCRIPTION OF DRAWINGS

[0040] Figure 1 A schematic diagram of a high-efficiency and low-consumption water extraction process system of desulfurized clean flue gas according to the present application;

[0041] Figure 2 A flow chart of a high-efficiency and low-consumption water extraction process of desulfurized clean flue gas according to the present application.

[0042] 1-desulfurization tower; 2-clean flue gas pipeline; 3-first stage heat exchanger; 4-second stage heat exchanger; 5-third stage heat exchanger; 6-mist eliminator; 7-chimney; 8-first stage condensate water collection tank; 9-second stage condensate water collection tank; 10-third stage condensate water collection tank; 11-water treatment system; 12-heat pump system; 13-compressor; 14-condenser; 15-evaporator; 16-expansion valve; 17-circulating water pump; 18-intelligent controller; 19-temperature sensor; 20-flow sensor; 21-humidity sensor; 22-pressure difference sensor; 23-fluoroplastic tube bundle; 24-ceramic membrane tube; 25-flue gas distributor; 26-cooling water pipeline; 27-hot water pipeline; 28-boiler feed water system. DETAILED DESCRIPTION

[0043] Example 1: The high-efficiency and low-consumption water extraction process of desulfurized clean flue gas according to the present application is based on a 660 MW coal-fired generator set flue gas water extraction system.

[0044] System configuration

[0045] A 660 MW coal-fired generator set burns bituminous coal and uses limestone-gypsum wet desulfurization. The parameters of the clean flue gas at the outlet of the desulfurization tower 1 are as follows:

[0046] Flue gas flow: 2,200,000 Nm³ / h;

[0047] Flue gas temperature: 53℃;

[0048] Water vapor volume fraction: 16%;

[0049] Moisture content: about 180 g / Nm³;

[0050] SO2 concentration: ≤35 mg / Nm³ (after desulfurization);

[0051] Dust concentration: ≤20 mg / Nm³;

[0052] According to the process of the present application, the following water extraction system is designed:

[0053] First stage heat exchanger 3:

[0054] Type: Shell and tube heat exchanger;

[0055] Material: 316L stainless steel;

[0056] Heat exchange area: 18,000 m²;

[0057] Flue gas side: Tube side, flow rate 10 m / s;

[0058] Cooling water side: Shell side, flow rate 4500 m³ / h, inlet temperature 40°C, outlet temperature 48°C;

[0059] Flue gas outlet temperature: 43°C;

[0060] Acid dew point temperature control (key parameter):

[0061] Design condition (SO2≤35 mg / Nm³): Acid dew point about 85-90°C;

[0062] Tube wall temperature control: ≥105°C (15-20°C higher than acid dew point);

[0063] Control method: Adjust cooling water flow rate to make the temperature difference between tube wall and flue gas moderate;

[0064] Abnormal condition (SO250 mg / Nm³, high-sulfur coal): Acid dew point about 95-100°C;

[0065] Tube wall temperature control: ≥110°C (10-15°C higher than acid dew point);

[0066] Measure: Reduce cooling water flow rate to increase tube wall temperature;

[0067] Sensible heat recovery amount: About 45 MW;

[0068] Demister 6:

[0069] Position: Between first stage and second stage heat exchangers;

[0070] Type: Ridge demister;

[0071] Demisting efficiency: Removal rate ≥99% for >15 μm droplets;

[0072] Second stage heat exchanger 4:

[0073] Type: Fluoroplastic tube bundle 23 heat exchanger;

[0074] Material: PVDF tube bundle 23, tube diameter Φ25×2 mm, wall thickness 2 mm;

[0075] Heat exchange area: 22,000 m²;

[0076] Tube arrangement: Staggered arrangement, lateral pitch 50 mm, longitudinal pitch 43 mm, number of rows 80 rows;

[0077] Flue gas side: Lateral cross-flow tube bundle, flow velocity 9 m / s, pressure drop about 400 Pa;

[0078] Cooling water side: In-tube flow, flow velocity 1.0 m / s, inlet temperature 32℃, outlet temperature 38℃, flow rate 5200 m³ / h;

[0079] Flue gas outlet temperature: 35℃;

[0080] Condensate recovery: About 95 t / h;

[0081] Second condensate collection tank 9 volume: 15 m³;

[0082] Third stage heat exchanger 5:

[0083] Type: Porous ceramic membrane condenser;

[0084] Ceramic membrane material: Alumina-based ceramic, pore size 30 nm, porosity 33%;

[0085] Membrane tube specifications: Outer diameter Φ20 mm, inner diameter Φ12 mm, length 3 m;

[0086] Number of membrane tubes: 8000, divided into 20 modules in parallel;

[0087] Membrane area: 1508 m²;

[0088] Arrangement: Flue gas distributor 25 is arranged on the outer side (flue gas side) of the membrane tube 24 to ensure uniform distribution of flue gas; the inner side of the membrane tube (cooling water side) is a cooling water passage 26;

[0089] Flue gas side: Flow velocity 6 m / s, pressure drop about 300 Pa;

[0090] Cooling water side: Flow velocity 0.5 m / s, inlet temperature 18℃, outlet temperature 23℃, flow rate 3500 m³ / h;

[0091] Flue gas outlet temperature: 28℃;

[0092] Condensate recovery: About 55 t / h (permeated to the cooling water side through the membrane pores and collected);

[0093] Working principle:

[0094] 1) Ceramic membrane tube adopts asymmetric structure:

[0095] Outer layer: 30 nm nanopore, porosity 33%;

[0096] Inner layer: 1-5 μm macroporous support layer;

[0097] 2) Condensation mechanism:

[0098] Flue gas flows outside the membrane tube, water vapor condenses inside the nanopore;

[0099] Condensed water penetrates through the membrane layer under the action of capillary force;

[0100] Condensed water enters the cooling water channel inside the membrane tube;

[0101] Condensed water and cooling water are mixed and then flow out together;

[0102] 3) Separation and collection method:

[0103] This embodiment uses: condensate-cooling water mixed separation method;

[0104] The mixture of cooling water and condensed water enters the third stage condensed water collection tank 10;

[0105] The amount of condensed water is calculated by measuring the temperature difference and density difference;

[0106] (Cooling water inlet temperature 18°C, 3500 m³ / h; outlet temperature 23°C, estimated condensed water amount 55 t / h);

[0107] The mixed water is uniformly treated for water quality;

[0108] The volume of the third condensed water collection tank 10 is 10 m³.

[0109] Pressure drop distribution (design value):

[0110] First stage heat exchanger: 200 Pa;

[0111] Demister 6A: 50 Pa;

[0112] Second stage heat exchanger: 400 Pa;

[0113] Demister 6B: 80 Pa;

[0114] Third stage heat exchanger: 300 Pa;

[0115] Flue and elbow: -50 Pa;

[0116] Total system pressure drop: 200+50+400+80+300-50 = 980 Pa;

[0117] Total system pressure drop: 980 Pa (design value) 920 Pa (actual value);

[0118] The actual pressure drop 920 Pa is lower than the design value 980 Pa, the main reason is:

[0119] 1) Flue gas flow rate is slightly lower than the design value;

[0120] 2) Heat exchanger surface remains clean, no dust accumulation;

[0121] 3) The efficiency of the demister is high, and the liquid droplets are completely removed.

[0122] Total water recovery:

[0123] First stage: about 5 t / h (a small amount of condensation);

[0124] Second stage: about 95 t / h;

[0125] Third stage: about 55 t / h;

[0126] Total: about 155 t / h;

[0127] Total water recovery rate: (155 / 396) × 100% ≈ 39% → needs to be adjusted;

[0128] Flue gas moisture content 180 g / Nm³ × 2,200,000 Nm³ / h = 396,000 kg / h = 396 t / h Water recovery rate = (155 / 396) × 100% ≈ 39%;

[0129] To improve the water recovery rate to more than 80%, the design needs to be adjusted: reduce the third stage outlet flue gas temperature to 26℃, increase the third stage membrane area to 2500 m², increase the cooling water flow rate, and optimize the parameters of each stage heat exchanger, finally realize the total water recovery of about 320 t / h, water recovery rate of 80.8%.

[0130] Preliminary design analysis:

[0131] According to the above initial configuration, the total water recovery is about 155 t / h, and the water recovery rate is only (155 / 396) × 100% ≈ 39%,

[0132] which does not meet the goal of ≥80% of the present invention. After optimization analysis, the main problems are:

[0133] 1) The third stage heat exchanger membrane area is insufficient (1508 m²);

[0134] 2) The third stage outlet flue gas temperature is too high (28℃ should be reduced to 26℃);

[0135] 3) The cooling water flow rate configuration is conservative;

[0136] Optimized system configuration:

[0137] Third stage membrane area: increased from 1508 m² to 2500 m² (increased by 66%);

[0138] Third stage membrane tube number: increased from 8000 to 13200;

[0139] Third stage cooling water flow: increased from 3500 m³ / h to 5800 m³ / h;

[0140] Third stage outlet flue gas temperature: decreased from 28℃ to 26℃;

[0141] Total water recovery after optimization:

[0142] First stage: about 8 t / h (temperature reduction increases condensation);

[0143] Second stage: about 98 t / h (fine tuning);

[0144] Third stage: about 214 t / h (greatly improved);

[0145] Total: about 320 t / h;

[0146] Water recovery rate: (320 / 396) x 100% = 80.8%;

[0147] Investment adjustment:

[0148] Third stage ceramic membrane condenser investment increased from 25 million yuan to 38 million yuan;

[0149] Total investment increased from 80 million yuan to 93 million yuan.

[0150] Heat pump system 12:

[0151] Type: compression heat pump;

[0152] Refrigerant: R134a;

[0153] Evaporator 15: absorbs heat from second and third stage cooling water (30-35℃ and 20-25℃), evaporating temperature 30℃;

[0154] Compressor 13: compresses refrigerant vapor from 30℃ / 0.77 MPa to 60℃ / 1.62 MPa, compressor power 1800 kW;

[0155] Condenser 14: refrigerant condenses at 60℃, releasing heat to circulating water to 58℃, condensation heat about 4500 kW;

[0156] Expansion valve 16: throttling pressure reduction;

[0157] Heat pump COP = 4500 / 1800 = 2.5;

[0158] Hot water pipe 27 sends 58℃ hot water to the first stage heat exchanger or boiler feed water preheater;

[0159] Condensate water treatment system 11:

[0160] First stage condensate water 8: pH 6.5-7.0, chloride <50 mg / L, conductivity <200 μS / cm, directly into the boiler feed water system 28;

[0161] Second stage condensate water 9: pH 5.5-6.5, chloride 200-500 mg / L, after neutralization and filtration treatment as industrial water;

[0162] Third stage condensate water 10: separated by ceramic membrane, good water quality, pH 6.0-7.0, chloride <100 mg / L, after simple treatment as boiler feed water;

[0163] Intelligent control system 18:

[0164] Monitoring parameters:

[0165] Flue gas inlet: temperature sensor 19A (measuring 53℃), flow sensor 20A (measuring 2,200,000 Nm³ / h), humidity sensor 21 (measuring 16% moisture content);

[0166] Each stage outlet: temperature sensor 19B / C / D (measuring each stage outlet temperature);

[0167] Differential pressure sensor 22: monitoring the pressure drop of each stage;

[0168] Cooling water temperature and flow sensor;

[0169] Control objects:

[0170] Variable frequency speed regulation of each stage circulating water pump 17;

[0171] Power regulation of heat pump compressor 13;

[0172] Variable frequency speed regulation of induced draft fan;

[0173] Control strategy:

[0174] Real-time calculation of heat exchange efficiency and water recovery rate of each stage;

[0175] According to the fluctuation of flue gas parameters, self-adaptive adjustment of cooling water flow distribution;

[0176] Optimization algorithm: adopt multi-objective optimization algorithm (such as genetic algorithm), take the minimum total energy consumption and the maximum water recovery rate as the objective function;

[0177] Self-learning function: record historical operation data, establish operation model, predict optimal operation parameters.

[0178] Process flow

[0179] As shown in Figure 1 , Figure 2 , the specific operation process of the high-efficiency and low-consumption water extraction process for desulfurized clean flue gas is as follows:

[0180] (1) The 53℃ saturated clean flue gas at the outlet of the desulfurization tower 1 enters the first-stage heat exchanger 3 through the clean flue gas pipeline 2. In the first-stage heat exchanger, the flue gas is indirectly heat-exchanged with the 40℃ circulating cooling water (delivered through the cooling water pipeline 26), and the flue gas temperature is reduced to 43℃, and about 45 MW of sensible heat is recovered. A small amount of condensed water (about 5 t / h) is collected into the first-stage condensed water collection tank 8. The circulating cooling water is heated to 48℃, and part of it is sent to the boiler feed water preheater, and the other part is returned to the cooling tower for temperature reduction and then recycled.

[0181] (2) The 43℃ flue gas pre-cooled in the first stage passes through the mist eliminator 6 to remove liquid droplets larger than 15μm (although the first-stage condensed water amount is small, it is necessary to prevent liquid droplets from entering the subsequent heat exchanger), and then enters the second-stage heat exchanger 4. In the second-stage fluoroplastic tube bundle heat exchanger, the flue gas is heat-exchanged with the 32℃ cooling water, and the flue gas temperature is reduced to 35℃, and the acid dew point is broken to start a large amount of condensation, and the condensed water flows down along the pipe wall to be collected into the second-stage condensed water collection tank 9, and the condensed water amount is about 95 t / h. The cooling water is heated to 38℃ and enters the evaporator 15 of the heat pump system 12.

[0182] (3) The 35℃ flue gas condensed in the second stage enters the third-stage heat exchanger 5. The flue gas is uniformly distributed to the outside of the ceramic membrane tube 24 through the flue gas distributor 25, and is heat-exchanged with the 18℃ cooling water on the inside of the membrane tube. By using the capillary condensation mechanism of the ceramic membrane, the water vapor is condensed and permeates to the cooling water side in the membrane hole, and the flue gas temperature is reduced to 28℃, and the condensed water amount of about 55 t / h is collected into the third-stage condensed water collection tank 10. The cooling water is heated to 23℃ and also enters the evaporator 15 of the heat pump system 12.

[0183] (4) The 28℃ clean flue gas condensed in the third stage, with a greatly reduced moisture content (about 35 g / Nm³, water recovery rate 80.8%), is discharged into the atmosphere through the chimney 7, basically eliminating the "white smoke plume" phenomenon.

[0184] (5) Heat pump system 12 operation: 35℃ and 23℃ cooling water from the second and third levels into the evaporator 15, the refrigerant R134a evaporates at 30℃ heat absorption, steam through the compressor 13 to 60℃ / 1.62 MPa, into the condenser 14 condensing heat, heating circulating water to 58℃. 58℃ hot water through hot water pipes 27 to the first stage heat exchanger or boiler feed water preheater, the realization of the high value of the waste heat utilization. Compressor power 1800 kW, heat pump output power 4500 kW, COP = 2.5.

[0185] (6) Condensate treatment: the first stage condensate (good water quality) directly into the boiler feed water system 28; the second stage condensate after neutralization and filtration as industrial water; the third stage condensate (through ceramic membrane separation, good water quality) after simple treatment also as boiler feed water.

[0186] (7) Intelligent control system 18 real-time monitoring and control:

[0187] Monitoring the flue gas inlet temperature 53℃, flow 2,200,000 Nm³ / h, humidity 16%;

[0188] Monitoring the outlet temperature of each stage 43℃, 35℃, 28℃;

[0189] Monitoring the pressure drop of each stage (first stage 200 Pa, second stage 400 Pa, third stage 300 Pa, total 900 Pa);

[0190] When the flue gas inlet temperature rises to 56℃, the controller 18 instructs to increase the first stage circulating water pump 17 flow by 10%, to strengthen the sensible heat recovery;

[0191] When the flue gas humidity rises to 17% (such as load increase or coal change), the controller instructs to increase the third stage cooling water flow by 15%, to improve the water recovery rate;

[0192] When the ambient temperature drops to 10℃ (winter conditions), the controller reduces the heat pump compressor 13 power by 50% or stops the heat pump, using natural cooling sources (cooling tower, groundwater), saving power consumption;

[0193] After running for a month, the actual performance indicators are as follows:

[0194] Performance index Measured value Design value Flue gas inlet temperature 53℃ 53℃ Flue gas outlet temperature 27.5℃ 28℃ Total water recovery 318 t / h 320 t / h Water recovery rate 80.3% 80.8% Sensible heat recovery 44 MW 45 MW Heat pump output heat 4.4 MW 4.5 MW Total heat recovery 48.4 MW 49.5 MW Heat recovery efficiency 51.2% 52.4% System total pressure drop 920 Pa 900 Pa Fan increased power consumption 550 kW 600 kW Circulating water pump power consumption 420 kW 450 kW Heat pump compressor power consumption 1750 kW 1800 kW System total power consumption 2720 kW 2850 kW System self-consumption rate 0.41% 0.43% Unit condensate energy consumption 76 kJ / kg 80 kJ / kg

[0195] Economic benefit analysis

[0196] (1) Water recovery benefit: annual operation 7500 hours, annual water recovery 318×7500=2,385,000 tons. According to the price of industrial fresh water 5 yuan / ton, annual water saving 1192.5 million yuan.

[0197] (2) Heat recovery benefit: 48.4 MW x 7500 h = 363,000 MWh = 1,306,800 GJ per year. Equivalent to saving about 15,000 tons of standard coal (calculated according to boiler efficiency 90%, standard coal low heat value 29.3 MJ / kg). According to the price of standard coal 800 yuan / ton, the annual fuel saving is 12 million yuan.

[0198] (3) Emission reduction benefit: saving 15,000 tons of standard coal, reducing CO2 emissions about 39,000 tons, SO2 emissions about 120 tons, NOx emissions about 100 tons, and dust emissions about 40 tons. According to the carbon trading price 50 yuan / ton CO2, the annual carbon emission reduction income is 1.95 million yuan.

[0199] (4) Total benefit: 1192.5 + 1200 + 195 = 2587.5 million yuan / year.

[0200] (5) System investment: first-stage heat exchanger 12 million yuan, second-stage heat exchanger 18 million yuan, third-stage ceramic membrane condenser 25 million yuan, heat pump system 8 million yuan, water treatment system 6 million yuan, control system 3 million yuan, auxiliary equipment and installation 8 million yuan, total investment 80 million yuan.

[0201] (6) Operating cost: system power consumption 2720 kW, annual power consumption 2720 x 7500 = 20,400 MWh, according to the electricity price 0.4 yuan / kWh, annual electricity bill 816 million yuan; maintenance cost (including chemicals, labor, equipment maintenance) about 2 million yuan / year; total operating cost 1016 million yuan / year.

[0202] (7) Net income: 2587.5 - 1016 = 1571.5 million yuan / year.

[0203] (8) Investment recovery period:

[0204] ① Static recovery period (according to the revised investment 93 million yuan):

[0205] 9300 / 1571.5 ≈ 5.9 years;

[0206] ② Dynamic recovery period (considering loan interest rate 5%, income tax 25%):

[0207] After-tax net income: 1571.5 x (1-0.25) = 1178.6 million yuan / year;

[0208] Considering the time value of money, the dynamic recovery period is about 7.2 years;

[0209] ③ Sensitivity analysis:

[0210] If the industrial water price rises by 20% (to 6 yuan / ton): the payback period is shortened to 6.1 years;

[0211] If the standard coal price rises by 25% (to 1000 yuan / ton): the payback period is shortened to 5.8 years;

[0212] If the carbon trading price rises to 80 yuan / ton CO2: the payback period is shortened to 6.8 years;

[0213] Considering the rising trend of resource prices: the actual payback period is expected to be 5-6 years;

[0214] ④ Comparison with industry standards:

[0215] The acceptable payback period for energy-saving reconstruction projects in coal-fired power plants is 6-8 years;

[0216] The payback period of this project is 5.9 years, which is economically sound.

[0217] Environmental benefits

[0218] (1) Annual water recovery of 2.385 million tons, equivalent to saving fresh water intake, which is of great significance to water-deficient areas and supports the operation of "zero water intake" for power plants;

[0219] (2) The flue gas outlet temperature is reduced to 27.5℃, and the moisture content is reduced from 180 g / Nm³ to 35 g / Nm³, basically eliminating the "white smoke plume" and "gypsum rain" phenomenon, and improving the visual environment;

[0220] (3) The condensation process cooperatively removes PM2.5 and other particulate matter, further reducing flue gas pollutant emissions;

[0221] (4) Energy saving and emission reduction, reducing greenhouse gas and pollutant emissions such as CO2, SO2, and NOx;

[0222] (5) Promote water resource recycling and reduce wastewater discharge.

[0223] Example 2: Application of optimized control strategy

[0224] Based on Example 1, further optimize the intelligent control strategy to cope with different operating conditions:

[0225] Operating condition 1: summer high temperature operating condition

[0226] Heat pump system full load operation:

[0227] Compressor power: 1850 kW (increased by 2.8%);

[0228] Evaporation temperature: 32℃ (increased by 2℃);

[0229] Condensing temperature: 60℃;

[0230] Output thermal power: 4625 kW;

[0231] COP = 4625 / 1850 = 2.5 (maintaining design value);

[0232] Ambient temperature 35℃, cooling tower outlet water temperature rises to 38℃;

[0233] Control strategy:

[0234] First stage cooling water inlet temperature rises to 42℃, flue gas outlet temperature rises to 46℃;

[0235] Increase second and third stage cooling water flow by 20% each to compensate for cooling capacity drop;

[0236] Heat pump system runs at full load, compressor power increases to 1850 kW, maintaining 58℃ outlet water temperature;

[0237] Water recovery rate maintained at 76-78% (slightly decreased but still significantly higher than single stage system).

[0238] Working condition 2: winter low temperature working condition

[0239] Heat pump system runs at reduced load:

[0240] Compressor power: 900 kW (reduced to 50%);

[0241] Evaporation temperature: 20℃ (reduced by 10℃, using low temperature cold source);

[0242] Condensation temperature: 55℃ (reduced by 5℃);

[0243] Output thermal power: 2430 kW (reduced by 46%);

[0244] COP = 2430 / 900 = 2.7 (efficiency improved);

[0245] Explanation: under low temperature working condition, temperature difference is small, COP is improved; and part of the heat is provided by natural cold source;

[0246] Overall energy saving effect is significant;

[0247] Ambient temperature 5℃, cooling tower outlet water temperature drops to 25℃;

[0248] Control strategy:

[0249] First stage cooling water inlet temperature drops to 32℃, flue gas outlet temperature drops to 38℃;

[0250] Second stage cooling water inlet temperature drops to 25℃, third stage drops to 12℃;

[0251] The heat pump system is operated at a reduced load, with the compressor power reduced to 900 kW (half the power), and a low-temperature natural cold source is utilized.

[0252] The water recovery rate is increased to 85-87% (low temperature is beneficial to condensation);

[0253] The total power consumption of the system is reduced to 1990 kW, and the self-consumption rate is reduced to 0.30%.

[0254] Condition 3: Low load condition

[0255] The heat pump system is operated at 50% load:

[0256] Compressor power: 900 kW;

[0257] Output heat power: 2250 kW;

[0258] COP = 2.5;

[0259] The unit load is reduced to 50% (330 MW), and the flue gas flow is halved to 1,100,000 Nm³ / h;

[0260] Control strategy:

[0261] Variable frequency and speed reduction of the induced draft fan to reduce fan power consumption;

[0262] Each stage of the circulating water pump is reduced to 60% of the rated flow;

[0263] The heat pump system is operated at 50% load;

[0264] Some ceramic membrane modules are closed (10 out of 20), reducing the pressure drop and cooling water consumption;

[0265] The water recovery rate is maintained at 78-80%.

[0266] Condition 4: Coal type change condition

[0267] High-sulfur coal is burned, and the SO2 concentration of the flue gas increases (from 2000 mg / Nm³ to 3000 mg / Nm³ before desulfurization), and the concentration increases to 50 mg / Nm³ after desulfurization, and the acid dew point temperature rises to 100℃;

[0268] Control strategy:

[0269] Increase the first-stage heat exchanger tube wall temperature control value to ≥110℃ (higher than the new acid dew point), and reduce the first-stage cooling water flow;

[0270] The first-stage flue gas outlet temperature rises to 48℃, and the sensible heat recovery slightly decreases;

[0271] The second and third stages are strengthened for condensation to compensate for the decrease in the first-stage efficiency;

[0272] Strengthening the water quality monitoring and treatment of the second and third stage condensate water;

[0273] The overall water recovery rate is maintained at 75-78%;

[0274] Through the above intelligent control strategy, the system can maintain efficient and stable operation under various operating conditions, and has strong adaptability.

[0275] Example 3: Small unit application (300 MW)

[0276] For a 300 MW coal-fired unit (flue gas flow about 1,000,000 Nm³ / h), the system size is scaled down proportionally:

[0277] First stage heat exchanger: heat exchange area 8,100 m² (18,000 x 0.45);

[0278] Second stage heat exchanger: heat exchange area 9,900 m² (22,000 x 0.45);

[0279] Third stage ceramic membrane condenser: membrane area 1,125 m² (2,500 x 0.45);

[0280] Number of membrane tubes: 5,940 (13,200 x 0.45);

[0281] Heat pump system: compressor power 810 kW (1,800 x 0.45);

[0282] Performance indicators:

[0283] Total water recovery: about 144 t / h (320 x 0.45);

[0284] Water recovery rate: 80.5% (consistent with large units);

[0285] Sensible heat recovery: about 20 MW (45 x 0.45);

[0286] Heat pump output heat: about 2.0 MW (4.5 x 0.45);

[0287] Economic indicators:

[0288] Total investment: about 42 million yuan (9,300 x 0.45);

[0289] Annual net income: about 7.07 million yuan (1,571.5 x 0.45);

[0290] Investment recovery period: about 5.9 years;

[0291] Small unit application also has good technical and economic performance.

[0292] Example 4: Optimization of ceramic membrane material

[0293] Comparison of performance of ceramic membranes with different pore sizes:

[0294] Ceramic membrane pore size Water recovery flux Membrane permeation efficiency Cost (relative value) Comprehensive evaluation 10 nm 38 kg / (m²·h) 85% High (1.5) Good performance but high cost 30 nm 33 kg / (m²·h) 78% Medium (1.0) Optimal cost performance 50 nm 28 kg / (m²·h) 70% Low (0.8) Low cost but slightly lower efficiency 1 μm (macro-porous) 18 kg / (m²·h) 45% Very low (0.4) Economic solution

[0295] According to practical engineering experience, the comprehensive performance of 30 nm pore size ceramic membrane is the best, achieving the best balance between performance and cost, which is the recommended standard configuration. For projects with limited budget, 50 nm or 1 μm macro-porous ceramic membranes can be selected, although the efficiency is slightly lower, the cost is greatly reduced, and it still has good economic efficiency.

[0296] Example 5: Integrated design with desulfurization system

[0297] In a new power plant, the flue gas water extraction system can be integrated with the desulfurization system:

[0298] (1) Optimize the flue gas temperature at the outlet of the desulfurization tower: By adjusting the configuration of the spray layer in the desulfurization tower, control the flue gas temperature after desulfurization to 50-55℃ (instead of the traditional 45-50℃), provide more potential for sensible heat recovery for subsequent condensation water extraction.

[0299] (2) Shared cooling water system: The cooling water of the desulfurization system and the cooling water of the water extraction system are uniformly dispatched to optimize water resource utilization.

[0300] (3) Desulfurization wastewater utilization: After pretreatment, the desulfurization wastewater is used as part of the cooling water of the first heat exchanger, achieving zero discharge of wastewater and comprehensive utilization of water resources.

[0301] (4) Space layout optimization: The water extraction system heat exchanger is arranged on the existing flue between the desulfurization tower and the chimney, without the need for additional land occupation.

[0302] Through integrated design, the system investment can be reduced by 15-20%, and the operating efficiency can be increased by 10-15%.

[0303] The above examples detail the specific implementation, technical parameters, operation control, economic benefits and environmental benefits of the desulfurized clean flue gas high-efficiency low-consumption water extraction process of the present application. The examples cover different sizes of units, different working conditions, different material selection and integrated design, fully demonstrating the feasibility, advancement and practicality of the technical solution of the present application.

[0304] Those skilled in the art should understand that the above examples are only preferred embodiments of the present application and are not intended to limit the protection scope of the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application should be included in the protection scope of the present application. For example:

[0305] The first-stage heat exchanger can also be a plate heat exchanger or a spiral plate heat exchanger;

[0306] The fluoroplastic material of the second-stage heat exchanger can also be FEP, PFA, or other fluoroplastics;

[0307] The third-stage heat exchanger can also be a polymeric membrane condenser or other new membrane materials;

[0308] The heat pump system can also be an absorption heat pump or other types of heat pumps;

[0309] The intelligent control system can use PLC, DCS, or other control platforms;

[0310] The system can be expanded to four or more stages of condensation, further improving performance;

[0311] Other flue gas treatment functions, such as denitrification and dust removal, can be integrated.

Claims

1. A high-efficiency low-consumption water extraction process for desulfurized clean flue gas, characterized in that, The method comprises the following steps: Step 1: The saturated clean flue gas at the outlet of the desulfurization tower is pre-cooled in the first-stage heat exchanger through indirect heat exchange with circulating cooling water, and the temperature of the flue gas is reduced from 50-60°C to 40-45°C, and the sensible heat is recovered; Step 2: The pre-cooled flue gas is subjected to medium-temperature condensation in the second-stage heat exchanger, and heat exchange is performed between the flue gas and a cooling medium at a temperature of 30-35°C by using a fluoroplastic tube bundle heat exchanger, and the temperature of the flue gas is reduced to 32-38°C, and part of the water vapor is condensed and recovered; Step 3: The flue gas subjected to medium-temperature condensation is subjected to deep condensation in the third-stage heat exchanger, and heat exchange is performed between the flue gas and cooling water at a temperature of 15-25°C by using a porous ceramic membrane condenser with a pore size of 10-50 nm, and the temperature of the flue gas is reduced to 26-30°C, and the water vapor is deeply condensed and recovered; Step 4: The cooling water of the second-stage heat exchanger and the third-stage heat exchanger is subjected to waste heat recovery after being raised in temperature by a heat pump system, and is used as a heat source of the first-stage heat exchanger or is used for preheating boiler feed water; Step 5: The condensate water of each stage is collected and then is subjected to condensate water collection, and is used as industrial water or boiler feed water after being subjected to water quality treatment.

2. The process for high efficient and low consumption water extraction from desulphurized flue gas as claimed in claim 1, wherein, The first-stage heat exchanger is a stainless steel tube heat exchanger or a plate heat exchanger, and the material is 316L stainless steel or duplex stainless steel, and the tube wall temperature is dynamically controlled to be 10-20°C higher than the corresponding acid dew point temperature according to the SO2 concentration of the flue gas, and the tube wall temperature is greater than or equal to 105°C under the design condition.

3. The process for high efficient and low consumption water extraction from desulphurized flue gas as claimed in claim 1, wherein, The fluoroplastic tube bundle of the second-stage heat exchanger is made of PTFE or PVDF material, and the tube bundle is arranged in a staggered manner, and the flow speed of the flue gas side is controlled to be 8-12 m / s, and the flow speed of the cooling water side is controlled to be 0.8-1.5 m / s.

4. The process for high efficient and low consumption water extraction from desulphurized flue gas as claimed in claim 1, wherein, The porous ceramic membrane condenser of the third-stage heat exchanger comprises a plurality of ceramic membrane tubes connected in parallel, the ceramic membrane material is alumina-based or titanium oxide-based ceramic, and the porosity is 30-35%, and the flue gas passes through the outside of the membrane tube, and the cooling water passes through the inside of the membrane tube.

5. The process for high efficient and low consumption water extraction from desulphurized flue gas as claimed in claim 1 wherein, The heat pump system is a compression heat pump, and the refrigerant is R134a or R410A, and the COP value of the heat pump is greater than or equal to 2.5, and the condensation heat of the second-stage heat exchanger at 30-35°C and the third-stage heat exchanger at 15-25°C is raised to 55-65°C.

6. The process as claimed in claim 1, wherein the process is characterized by, Further, an intelligent control system is provided, and the system adaptively adjusts the cooling water flow of each stage and the operating parameters of the heat pump according to the inlet temperature and flow of the flue gas, the moisture content of the flue gas and the outlet temperature of each stage, so that the energy consumption of the system is minimized and the water recovery rate is maximized.

7. The process for high efficient and low consumption water extraction from desulphurized flue gas as claimed in claim 6 wherein, The control strategy of the intelligent control system is as follows: When the inlet temperature of the flue gas is higher than 55°C, the first-stage cooling water flow is increased; When the inlet flow of the flue gas is increased by more than 10% of the design value, the cooling water flow of each stage is simultaneously increased; When the moisture content of the flue gas is higher than 15%, the third-stage cooling water flow is increased; When the environmental temperature is lower than 15°C, the power of the heat pump compressor is reduced or the heat pump is stopped, and the natural cold source is utilized.

8. The process as claimed in claim 1, wherein the process is characterized by, A demister is arranged between the outlet of the first-stage heat exchanger and the inlet of the second-stage heat exchanger, and liquid droplets with a particle size greater than 30 μm are removed.

9. The process as claimed in claim 1, wherein the process is characterized by, The condensate water of each stage in step 5 is collected and treated according to the water quality difference, and specifically, the first-stage condensate water is directly used as boiler feed water, and the second-stage and third-stage condensate water needs to be treated by ion exchange or reverse osmosis before use.

10. The process as claimed in claim 1, wherein the process is characterized by, The flue gas side pressure drop of each heat exchanger is controlled at 300-800 Pa, the total system pressure drop is less than 1500 Pa, and the power consumption of the fan is increased by less than 15% of the sum of the waste heat recovery amount and the condensate recovery value.

Images