1 Project Overview
A power plant installs 2-600 MW domestic subcritical coal-fired generating units in the first phase and 2-660 MW domestic supercritical coal-fired generating units in the second phase, and adopts seawater DC water supply system for circulating water system. The power plant uses four units to pump steam and adopts seawater desalination process to produce fresh water and implement hydropower cogeneration. The steam required for the desalination plant with a daily output of 25,000 m3 of fresh water is provided by the final steam extraction from the turbine medium pressure cylinder of the first and second phase of the power plant, and the raw seawater is taken from the circulating water supply pipe. The desalination process adopts the cross-tube falling film low-temperature multi-effect distillation (LT-TVC-MED) equipped with vapor heat compressor (TVC), and the plant can be operated at 40%~100% working condition. The main equipment consists of 10-effect evaporators arranged horizontally in series, with vapor extraction at the end of the 7th effect. The evaporator adopts multi-support horizontal in-line arrangement on a steel frame. The main parameters of the device are shown in Table 1.
2 low-temperature multi-effect distillation technology cost analysis
The cost of low-temperature multi-effect distillation seawater desalination is a complex issue, affected by a variety of factors, such as the project's geographic location, climatic conditions, seawater quality, seawater with the seasonal distribution of temperatures and the availability of energy, and many other factors have an impact on the cost of seawater desalination of the water production. This paper provides a cost analysis for a specific program for a specific project.
The cost per unit of water for desalination project can be decomposed into fixed cost and variable cost. Fixed cost refers to the total cost does not change with the production of the various costs, mainly including wages or salaries, depreciation of fixed assets, long-term borrowing interest and other costs. Variable cost refers to the total cost of the same direction with the change of product output changes in the various costs, mainly including steam costs, power consumption costs, chemical consumption costs, labor costs and maintenance costs. This paper takes the low-temperature multi-effect distillation seawater desalination program with daily output of 25000 m3 of fresh water as the basis for cost calculation and analysis, and quantitatively reveals the change rule of seawater desalination cost and the influencing factors.
Basic data for cost calculation: steam parameters 0.55 MPa (a), 320 ℃; unit running at rated operating conditions, daily production of 25,000 m3 of fresh water, according to the annual amount of water production for the calculation of the unit cost of water; static investment of the device is about 220 million yuan, the amount of the loan according to the implementation of the proposed budget of static investment of 80% of the loan interest rate according to the same period of the bank lending rate; equipment Useful life of the equipment 30 a, depreciable life of 20 a, salvage value rate of 5%; equipment utilization rate of 98% per year; standard coal price of 640 yuan / t calculated steam costs 2.65 yuan / m3; power consumption 1.2 kW - h / m3, electricity price of 0.28 yuan / (kW - h); pharmaceutical costs of 0.28 yuan / m3; water ratio of 13.5 for costing.
After analyzing and calculating the basic scheme, the cost of fresh water per unit of water is about RMB 5.39/m3. The results of the calculation of various costs of desalination per unit of water are shown in Table 2.
Steam cost is the most important cost of seawater desalination unit, accounting for 49% of the total cost; followed by depreciation cost of fixed assets and financial cost, which are 22% and 12%, respectively; Repair costs, pharmaceutical costs, electricity, wages and benefits costs*** accounted for 17%. Among them, the steam cost is related to the annual utilization rate and the water-making ratio of the device; repair costs, depreciation of fixed assets and financial costs are based on the static investment amount for the fee calculation, and the proportion of the pharmacy fee, electricity and labor and welfare costs is relatively small and the cost is relatively constant, which does not have much impact on the change of the total cost.
Based on the above analysis, to determine the impact on the cost of the main factors are: static investment, steam costs, water ratio and annual utilization rate, the secondary factors for electricity, chemicals and labor welfare costs. The following basic program as the basis for analyzing when a single variable changes while other variables remain unchanged when the factors on the unit cost of water independently.
2.1 Engineering static investment
Dynamic investment in desalination project consists of static investment (including equipment purchase cost, installation cost, construction cost and other costs) and interest on the loan during the construction period.1/2 12NextEndPageFigure 1 shows that, when the static investment is increased from -20% of the base amount, the cost of the unit of water volume is increased from 4.98 yuan/m3 to 5.79 yuan/m3, and the unit cost of water volume is increased from 4.98 yuan/m3 to 5.79 yuan/m3. rises to 5.79 yuan/m3, an increase of 16%. Therefore, it is particularly important to control the static investment. Static investment in equipment purchases (including the main equipment and auxiliary systems) costs about 47% of the static investment in the project, so we must control the cost of the main equipment and auxiliary equipment to reduce the static investment in the project, thereby reducing the cost of water production.
2.2 Steam cost
Since the steam cost accounts for 49% of the total cost, it is the largest single cost that accounts for the proportion of water production cost. Low-temperature multi-effect distillation desalination steam cost is mainly reflected in the coal consumption, through the unit of water tons of standard coal consumption changes, to analyze the water production steam cost changes. The basic scheme uses the steam turbine to extract steam for water production, and the apportionment of costs is more complicated, and the steam cost varies greatly between different calculation methods. Because the heat method does not take into account the quality of steam, the use of this method for cost calculation is not scientific, the work capacity method and enthalpy drop method take into account the quality of steam, the calculation method is more reasonable, and the work capacity method and enthalpy drop method are similar to the results of the two calculations. Therefore, this paper takes the work capacity method as the calculation method of desalination steam cost, and the results are shown in Fig. 2.
From Fig. 2, when other factors remain unchanged, the steam cost changes from -20% to 20% of the baseline value, and the cost of the unit of water volume rises from 4.86 yuan/m3 to 5.92 yuan/m3 accordingly, and the rate of increase reaches 21.8%. Therefore, in order to reduce the cost of desalination, it is fundamentally necessary to take cost reduction measures from the steam source. If the water steam is spent steam or waste heat, the steam cost is negligible and the cost of water production will be very low. For the electric water cogeneration system, make full use of the waste heat of the power plant and unit pumping steam, can effectively reduce the cost of water production. Calculations show that the same desalination project when the use of four-stage pumping for water production, unit water steam cost of about 2.49 yuan / m3; and the use of spent steam water production unit water steam cost of about 1.18 yuan / m3, the cost savings effect is obvious.
2.3 Annual utilization hours
Figure 3 shows that the annual utilization rate from 60% to 100%, that is, the annual utilization hours from 5256 h to 8760 h, the cost of water from 6.73 yuan / m3 down to 5.35 yuan / m3, a decline of 26%. Therefore, in the application of engineering, strengthen equipment management, improve equipment health level, is to improve equipment utilization of the basis, but also to improve the economic efficiency of the device, reduce the cost of water production is one of the important ways. Low-temperature multi-effect distillation device due to the technical characteristics of low-temperature distillation, than other desalination technologies have more advantages, so that the equipment fouling and corrosion is reduced to a minimum, for the device in stable operating conditions can be a long time to provide the basis for the protection of the desalination device to improve the utilization rate. Only when the annual availability of the whole set of devices is greater than 95%, can effectively reduce the cost of water production.2.4 Water production ratio
The water production ratio of the device is defined as the ratio of the distillation unit product water and the external input of the total mass flow of steam (kg/kg). The water production ratio reflects the operating cost of the unit, usually the higher the water production ratio, the lower the energy cost per unit of fresh water production, i.e., the lower the amount of steam consumed. As seen in Figure 4, when the water production ratio is increased from 12.15 to 14.85, the cost of desalination unit water volume is reduced from 5.68 yuan/m3 to 5.15 yuan/m3, which is a 10% cost reduction.
2.5 Other Costs
Considering the coupling program of desalination unit and power plant project, the electricity cost adopts the cost tariff, and the calculation result of electricity cost accounts for 6% of the total cost of the unit of water volume. Distillation method desalination system operation process of electricity consumption fluctuation is small, can be optimized design to reduce the cost of electricity.
Pharmaceutical costs account for 5% of the total cost per unit of water, desalination system normal operation to add scale inhibitors, defoamers and reducing agents, dosing according to the amount of seawater fed into the proportionality of the addition, so the same water quality and water production, by changing the feeding method, increase the concentration rate of the reduction of seawater volume, dosing costs will decrease accordingly.
3 Cost reduction measures
3.1 Seek low-cost heat source, rational use of energy
Low-temperature multi-effect distillation seawater desalination costs in the highest proportion of the cost of steam, low-temperature operation of the characteristics of the low-temperature multi-effect distillation seawater desalination device can use low-grade heat source, seek low-cost heat source, the energy cost of the distillation process to a minimum to avoid the loss of quality and quantity of energy. Energy loss in quality and quantity is the main measure to reduce costs.
For the power-water cogeneration system, the use of high-parameter steam is detrimental to the efficiency of the whole system, and the optimal pumping parameters should be determined through the overall optimization of the power-water cogeneration system. The new unit can use the lower enthalpy turbine six pumping steam as the water heating steam to reduce the steam cost, thus reducing the cost of desalination. At the same time, the use of steam turbine pumping steam for water production, select the economic operating conditions for water production costs.
3.2 The use of waste heat utilization of new processes, to achieve the gradient utilization of energy
The gradient utilization of energy, including the use of energy according to the quality of energy and the use of two aspects of the gradient, according to the energy level of the equipment needs constitute a gradient utilization of energy relations, so that the total energy utilization rate to reach the highest level. Low temperature multi-effect distillation device heating steam pressure should be 0.025 ~ 0.032 MPa (a), the temperature is lower than 70 ℃, with the use of waste heat favorable conditions. The use of seawater desalination and waste heat recovery and utilization of coupled programs, need to be based on the type of waste heat, parameters, quantity and utilization of the possibility of comprehensive thermal efficiency and economic feasibility analysis to determine the use of the program.
(1) In thermal power plants, the exhaust smoke loss accounts for the largest proportion of the heat loss in the boiler, and lowering the exhaust smoke temperature and reducing the exhaust smoke loss play a decisive role in improving the thermal efficiency of the boiler. It can be seen that reducing the exhaust temperature of the boiler can save coal consumption. If the boiler exhaust smoke and desalination phase coupled with flue gas waste heat recovery and utilization, can also reduce the cost of water production.
(2) Under the mode of power-water cogeneration, the heat discharged from the condenser circulating cooling water of the power station is utilized to raise the temperature of the seawater of the seawater desalination plant in winter, which reduces the cost of water production and at the same time reduces the waste heat and the amount of wastewater discharged from the power plant.
(3) other waste heat utilization: power plant large-scale steam auxiliary engine exhaust steam waste heat and low-temperature multi-effect distillation seawater desalination device combined; steel plant or chemical plant process waste heat and seawater desalination technology combined.
3.3 Optimization of process parameters, improve the device water ratio
(1) the use of higher pressure steam turbine four pumping steam as a heating steam source, in order to make use of the effective energy of the pumping steam, reduce the evaporation of the final effect of the steam loss of the condensation heat loss, can be used with the steam vapor compression (TVC) of the low-temperature multi-effect distillation seawater desalination device (LT-TVC-MED), improve the thermal efficiency of the system while increasing the device's water production. System thermal efficiency while improving the device water ratio, reduce water production costs.
(2) In the desalination evaporation device total heat transfer temperature difference is certain, reduce the heat transfer temperature difference between the effect, increase the number of effects can improve the water ratio; on the other hand, because the water ratio with the increase of steam heat compressor suction temperature increases, reasonable determination of the TVC ejection parameter, optimize the ejection of the TVC in the device position, can improve the water ratio.
3.4 Reasonable equipment selection, reduce static investment in equipment
Equipment acquisition costs account for about 47% of the static investment, the cost of which determines the repair costs, depreciation of fixed assets and financial costs, etc., so you can reasonably set up standby equipment, the use of new low-cost materials and the use of optimized technology to reduce the static investment to reduce the cost of the unit of water.
3.5 Focus on operation and maintenance, improve equipment utilization
By controlling the operating parameters within a reasonable range, reduce the risk of scaling of the main equipment in order to reduce the downtime of the pickling; the main auxiliary equipment for condition monitoring, reduce the downtime of the failure; strengthen the daily maintenance of all the equipment to improve the utilization of the equipment.
3.6Reasonable design and equipment selection, reduce the cost of electricity
Select reasonable parameters to make the desalination plant running at high load conditions; optimize the system process and auxiliary equipment selection and configuration, select the appropriate equipment capacity safety margin and the use of frequency conversion equipment and other measures to reduce the equipment to reduce the cost of electricity, thereby reducing the unit of desalination of water and electricity costs.
4Conclusion
Low-temperature multi-effect distillation desalination cost is affected by a variety of factors, is a complex issue, according to the analysis of the composition of the cost of steam is the composition of low-temperature multi-effect distillation desalination system of the variable cost of the most important factors, accounting for 49% of the total cost of the cost of the cost of the cost of the cost of the cost of the cost of the cost of the cost of the cost of the cost of the cost of the most important factors, followed by the depreciation of fixed assets and finance costs, respectively, 22% and 12%; electricity costs and pharmaceutical costs accounted for a relatively small proportion of the cost of electricity and pharmaceutical costs. The cost of electricity and pharmaceutical costs account for a relatively small amount, accounting for 6% and 5% of the total cost, respectively.
Low-temperature multi-effect distillation desalination project investment cost analysis shows that: the static investment in the project equipment purchasing costs accounted for about 47%, so it is necessary to control the cost of the main equipment and auxiliary equipment through the device large-scale research, research on localization of equipment, as well as research and development of new materials, and other measures to reduce the project's static investment in order to reduce the cost of water production.
The cost of the factors affecting the availability of equipment and some design parameters related to the cost of water with the increase in annual utilization rate and reduce the cost of water, only when the annual utilization rate of the whole set of devices is greater than 95%, in order to effectively control the cost of water, and engineering design parameters on the cost of the unit of water is mainly manifested in the cost of desalination unit of water with the increase in the cost of making water ratio and reduce.
Reducing the cost of low-temperature multi-effect distillation desalination project water must be controlled from the design, manufacturing, operation and maintenance aspects to achieve the best equipment performance and lower cost of water, so as to promote the application of low-temperature multi-effect distillation desalination technology.
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