| With the increase of the world population,the development of industrialization and the environment deterioration,the shortage of freshwater resource has become a sharp problem that people need to face and solve.As a kind of method that different from saving water to obtain freshwater resource,desalination has developed rapidly.As a thermal seawater desalination method,low-temperature multi-effect distillation is most widely used in China,but it is an energy intensive process,which limits its development.On the other hand,as the main form of power generation in China,coal-fired power units have a lot of waste heat that is multi-form and multi-grade.In this paper,for the desalination system coupling with coal-fired power units,the cogeneration unit that using extraction steam from turbine for seawater desalination is analyzed.Based on this,three new types of low-temperature multi-effect distillation systems that using the heat of extraction steam,flue gas,and exhaust steam is proposed,and the performance of these systems are investigated.The coupling characteristics of the 600MW coal-fired unit and the low-temperature multi-effect distillation were analyzed.By changing the steam extraction amount and pressure,it can be seen that the increase of extraction amount will reduce the power generation and increase the coal consumption and the water production.The decrease of steam extraction pressure will reduce the water production and the coal consumption.For the low-temperature multi-effect distillation system,the sudden increase in the steam extraction amount not only causes an increase in the amount of freshwater,but also causes a sudden increase in the salinity of brine in each effect,and the sudden drop in the boiling temperature of each effect also happens.The dynamic response caused by the decrease in extraction pressure is just the opposite.In addition,the low-temperature multi-effect distillation system with a steam consumption of 50t/h has little impact on the power generation.With an increase in the extraction amount,the power generation of the unit is reduced although the peak shaving capacity is basically unchanged initially.The peak shaving capacity can reach up to 477 MW when the amount of steam extraction is less than 238 t/h,When the extraction amount is sufficiently large to make the exhaust steam from the low-pressure turbine reach the minimum allowable value,the peak shaving capacity of the unit sharply decreases.The maximum extraction amount of the unit under the BMCR can afford approximately 19 LT-MED systems.The maximum extraction amount under a 30%THA can afford approximately 3.75 LT-MED systems.Three new models of low-temperature multi-effect distillation systems were established,and the effects of basic operating parameters on power generation and water production were analyzed.Compared with the system with a water production of 12000m3/d,the maximum water production of the low-temperature multi-effect distillation system with flue gas as the preheating source is about 1.28 times of it,and the cost of water production is reduced from 3.14$/m3 to 2.45$/m3.When the water production is 12000m3/d,the steam consumption decreases from 50t/h to 36.1t/h,The extraction is 13.9 t/h less than that of the original system,with which the consequent increase of turbine power output is about 2.98 MW.It can be calculated as a reduction of the coal consumption about 1.53 g/kWh.When the flue gas is used as the heat source of the first effect,the maximum water production is about 5328m3/d,with the water production cost about 2.19$/m3.An extra power output of 10.75MW by the 600MW power generating unit can be acquired,and the coal consumption of power generation is reduced about 6.05 g/kWh.Results showed that using low temperature flue gas to heat condensed water can theoretically reduce coal consumption 1.26 g/kWh.The recovery of flue gas waste heat reduces the temperature of the flue gas entering the desulfurization tower,which can save about 42.35t/h of desulfurization water consumption.For the low-temperature multi-effect distillation system using flue gas to preheat seawater,the effects of temperature difference,TVC position,and feed seawater amount were studied by thermodynamic and economic analysis with GOR and water production cost as the objective functions.For the conditions that the highest evaporation temperature is 61.7? and the lowest is 46?,it is obtained that the total system investment is the lowest when the temperature difference between the effects is 2.5?.Different TVC installation locations also mean different water production performance.Under the conditions that the feed brine of each effect was the same,the system GOR gradually rises and the water production cost decreases with the backward movement of the TVC position.Considering the safe operation of the steam injector,the compression ratio is better to greater than 1.81,and the recommended installation position of the TVC is the fifth to seventh effects.When the TVC is placed at the 5th effect and the feed seawater flow rate of evaporator after TVC is 0,it is shown that the new proposed system is superior to the traditional system in both the water production and the water production cost.GOR increased from 11.02 to 14.45,and the water production cost fell from 2.187 $/m3 to 2.039 $/m3.The system exergy analysis shows that the exergy destruction in TVC is the largest and the enthalpy efficiency of the condenser is the lowest.In order to further strengthen the heat exchange between flue gas and seawater,based on the corrugated fins of the plate-fin heat exchanger,the effects of perforated radius,staggered ratio and breaking distance on the fin performance were studied.For perforated wavy fin,the enlarged hole can strengthen the flow turbulence,causing rapid mixing between the core and the fluid near the fin surface.The bigger the holes,the better the thermal-hydraulic performance.Within the scope of the study,the fins with a radius of 1.5 mm have better performance;For staggered wavy fin,the greater staggered ratio,which promotes heat transfer between the fluid and the fins.Then,the larger the staggered ratio,the better the thermal-hydraulic performance.When the staggered ratio is 0.4,although the performance evaluation criteria of the fin decreases with the Reynolds number,it is still the most advantageous within the research range;the discontinuous wavy fin has limited ability to improve heat exchange,but the increase of breaking distance promoted the turbulence near the fin,and the flow resistance is increased.Within the research range,the thermal performance of the discontinuous fin when the breaking distance is 1 mm is better.The aspect ratio also affects the thermal performance of the fins.Under different aspect ratios,the perforated wavy fins have the most outstanding ability to improve heat transfer,and the flow resistance is also the largest;the discontinuous wavy fins have limited heat transfer capacity,but the flow resistance has no obvious change at the same time;The heat transfer performance of staggered wavy fins is between them.Using the serration technique in the smaller aspect ratio can obviously improve the thermal-hydraulic performance of the wavy fin.When the aspect ratio is 0.45,the perforated wavy fins are the most advantageous,with a PEC value of 1.24.The results show that perforation is beneficial to the enhancement of the heat transfer and the improvement of the Nusselt number.Serration is beneficial to reduce the friction factor,and the breaking technique can reduce heat transfer area while enhancing heat transfer performance. |
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