DESIGN CHALLENGES AND OPERATIONAL EXPERIENCE OF A MEGA MED SEAWATER DESALINATION PLANT IN TIANJIN

Transcription

1 DESIGN CHALLENGES AND OPERATIONAL EXPERIENCE OF A MEGA MED SEAWATER DESALINATION PLANT IN TIANJIN Authors: Presenter: T. Efrat, Yu Haimiao Tomer Efrat Deputy Manager, Thermal Process Dept. IDE Technologies Ltd. Israel tomere@ide-tech.com Abstract This article is a continuation of the article "Sliding Pressure Turbine Integrated with Seawater Desalination Facility (MED)" [1] presented at the IDA 2011 World Congress. The previous article described a unique MED-TVC plant design that is able to integrate with a sliding pressure turbine to supply motive steam to a thermal desalination process. This turbine is part of the Tianjin Beijing power station, located in the south Hangu District along the Bohai Sea. The Tianjin power plant is a highly efficient, large-scale engineering system, comprising power generation, seawater desalination and salt production from brine. It is managed as an integrative system and is a representative project of a circulating economy model in China, with the subentry of power, seawater desalination and in the future, salt production from brine, which are interdependent upon one another. To support this type of optimal power generation cycle along with a highly efficient desalination process, the Tianjin MED-TVC operates under varying working conditions. This is possible due to its robust design, and highly flexible operation to achieve optimum electricity and water production. The MED-TVC facility includes eight desalination units, each with a capacity of 25,000 m 3 /day, totaling a plant capacity of 200,000 m 3 /day. This article presents operational experience gained from this project, while focusing on: 1. The integration between the power plant and the MED desalination plant 2. Main design parameters of both phases of the desalination plant for enabling operational flexibility 3. Actual performance data of the desalination plant The International Desalination Association 2013 / Tianjin, China

2 I. INTRODUCTION The Tianjin Beijiang power station is located in the south, in the Hangu District along the Bohai Sea. The south of the Hangu District is on the northwest coast of the Bohai Sea, east of Tianjin and at the lower reaches of the Ji Canal. Figure 1: Location of the Tianjin Desalination Plant The Tianjin Beijing Power Plant is a large-scale engineering system with high efficiency, representative of the circulating economy model in China. The project is composed of a power plant, seawater desalination plant and salt production from brine, and is managed as an integrated system in which all three parts are interdependent on each other. The power plant is a 4 x 1000 MW ultra-critical coal fired power station and is coupled with a highly efficient thermal desalination plant designed by IDE Technologies. Steam extracted from the power plant turbine is used as motive steam for the desalination plant. The water produced by the desalination plant is used as boiler make-up water, industrial process water and potable drinking water supplied to the city. The brine from the seawater desalination plant is transferred to a salt field to produce salt

3 The Tianjin desalination plant makes an important contribution to local long-term water sustainability. Specifically, water from the desalination plant has enabled: End to economic activities coming to a standstill due to water shortages, increasing gross domestic production. Reduction of water extraction from the country's already overdrawn natural reservoirs of potable water. This prevents further degradation of the natural reservoirs by saline water intrusion and causes their levels to rise to hydrologically safe values. General increase in water supply dependability to all consumer sectors. Decrease in water hardness by mixing high-quality desalinated water with lower-quality water from the water supply system. This blend can then be routed to industry and households that stand to gain the most from enhancements in the quality of their water supply. This decreases scale formation in domestic and industrial water heaters, piping, appliances and other equipment, and therefore a reduction in detergents and scale-cleaning chemicals in the wastewater. One of the key features of the Tianjin power plant is the independent operation of the electrical power station. The 4 x 1000 MW ultra-critical coal fired power station utilizes a sliding pressure control system. In this system, the boiler steam is regulated using pressure control in proportion to the generator output. This operation mode enables the steam quality at the turbine inlet to be altered while maintaining constant volume flows. Figure 2: Tianjin Desalination Plant

4 The turbine governing valve can be kept open at all steady state conditions, but must be modulated slightly during load increase or decrease due to the lag in boiler response to a change in the load. Boiler control output allows higher reheated steam temperature due to higher temperatures of the high pressure turbine exhaust steam. At partial loads, a decline in primary boiler output brings about a decrease in boiler feed water pump output and energy demands. When compared to constant pressure control by way of a turbine governing valve, partial operating loads are more efficient in sliding pressure operation. II. POWER PLANT CONTROL PHILOSOPHY AND ITS EFFECT ON THE OPERATION OF THE DESALINATION PLANT The power plant at Tianjin is capable of varying its electrical power generation during times of peak water demand and vice versa. As a result, the desalination plants are designed to operate across a range of steam supply conditions and are able to adjust output between 40% and 110%. The steam supplied to the desalination plant is extracted from either the 5 th or 6 th low pressure turbine. In the event of a peak in power demand, low pressure steam is extracted from the 6 th bleed instead of the 5 th, at pressures of between 1.2 to 2.3 bara, which enables enhanced electricity production. On the other hand, in the event of a peak in water demand, mid-range pressure is extracted from the 5 th bleed with pressures ranging from 6 to 3.1 bara. These extraction pressures represent a load variance of 100% to 50% in power production over varying power plant efficiencies. In phase 1 of the MED plant, the steam is routed to a different line in accordance with the pressure of the steam supplied by the turbine. In the MED plants there are two thermo-compressor installed for handling wide spectrum of steam pressure. In phase 2, one varying nozzle thermo-compressor is the recipient of the entire pressure spectrum of the steam supplied by the turbine. The steam supplied to phase 2 by the turbines is in the range of bara. III. THE MED DESALINATION PLANT The Tianjin desalination plant has the largest capacity of any thermal desalination plant in China, with a nominal capacity of 200,000 m 3 /day. This capacity is obtained by eight MED units, each with a nominal capacity of 25,000 m 3 /day. The plants were erected in two phases of four units each. This capacity can supply the daily freshwater needs of approximately 600,000 people. However, it is important to look and evaluate the desalination plant not only by its size, but also for its contribution to the entire Tianjin Beijiang plant. When considering the Tianjin project as a single entity consisting of power, water and salt production, the design of the desalination plant provided an important challenge in supporting both efficient power and salt production. Supporting efficient power production is expressed in the ability to operate with a variety of motive steam pressures that are extracted from the turbine, while supporting efficient salt production is expressed in the ability to operate with a relatively high concentration factor, resulting in constant brine salinity of 6.6%. In addition, the MED type desalination plants in this project have the

5 ability to provide great flexibility in other operation parameters, such as production output, gain output ratio (GOR), as well as operation with varying seawater temperatures and levels of salinity. Figure 3: Tianjin MED Unit 3.1 Main Design Challenges of the Desalination Plant The main challenges faced during the design of the desalination plant stemmed from both the type of project, in which the desalination plant acts as the connecting link between the power generation and salt production plants, and from environmental aspects. The main challenges of the project related aspects arose from the need to be able to supply the full capacity of desalinated water while receiving variable steam pressure from the power plant turbine, and from the need to operate at a relatively high concentration factor to support efficient salt production. Among the environmental aspects the challenges arose from the need to operate at a wide range of seawater temperatures and salinity levels, and from the ambient conditions during the winter. This section provides a detailed description of the design challenges, and the measures taken to meet them. 3.2 Operation with Variable Motive Steam Pressure A TVC-MED desalination plant is typically designed to a specific motive steam pressure at which the plant produces its nominal capacity. A change in motive steam pressure also affects the operation of the plant; constant changes in the motive steam pressure affect the plant's ability to produce its nominal capacity while maintaining its temperature profile across the plant evaporator. However, when considering a desalination plant coupled with a sliding pressure turbine power plant, the most significant aspect of the operation flexibility comes from the ability to operate the plant with steam pressures varying from almost 0.3 to 6 bara from the power plant turbine. The plants' remarkable ability is due to its robust design including two types of thermo-compressors, a back-pressure line in phase 1 and a

6 varying nozzle thermo-compressor in phase 2. These configurations are the perfect match for sliding pressure turbines such as those installed at Tianjin as they allow the MED to be utilized over a much wider range of pressures, and increases the efficiency of the entire. Another degree of freedom in the operation of the combined cycle is achieved by extracting the steam from the power plant to the MED from cascading turbine bleeds. This flexibility allows regulating the amount of electricity produced relative to the amount of water that is to be desalinated. In practice, the motive steam pressure is provided to the desalination plant according to the power plant operating conditions and power generation requirements. A pressure indicator, installed at the battery limit of the desalination plant, continuously monitors the motive steam pressure, allowing the plant control system to react to the pressure changes by automatically operating the steam valves system and directing the steam to the proper thermo-compressor and/or adjusting the thermo-compressor position (in case of variable nozzle thermo-compressor). Figure 4 shows the thermo-compressor arrangement. Figure 4: Thermo-Compressor Arrangement According to the plant design, the change in motive steam pressure also affects the GOR of the plant in the range of 11 to 15. In terms of the plant water production, the plant design secures full water production of 25,000 ton/day at all motive steam pressures, which allows the operation of a thermocompressor. In case of the use of back pressure steam (pressure of below 1 bara), the plants achieve water production of 21,500 ton/day. The graph below shows the change in the plant design GOR and water production with variable steam pressure

7 Figure 5: Change in Plant Design GOR and Water Production 3.3 Operation with Variable Seawater Temperature and Salinity The ambient temperature in the Tianjin vicinity changes significantly during the course of the year and can vary between -25 C and 40 C. As a result, the temperature of seawater supplied to the desalination plant as feed water can vary between -2 C and 33 C. As the temperature of the feed water is essential for maintaining a proper temperature profile along the MED evaporator, the temperature of the seawater entering the MED falling film condenser (FFC) is continuously controlled by the plant feed water system illustrated in Figure

8 Figure 6: Plant Feed Water System The feed water system allows utilization of the heat rejected via the plant product and brine streams for heating the water during cold periods, using plate-and-frame heat exchangers. As the seawater temperature rises during the course of the year, the feed water gradually and automatically by-passes the heat exchanger and is introduced directly into the FCC. As the seawater temperature rises above about 29 C the plant starts using cooling water received from the cooling tower. Using the design presented, and by utilizing the plant s rejected heat for seawater pre-heating, it is possible to obtain a proper temperature profile across the MED evaporator during the entire year. The graph below indicates the change in the plant s incoming and rejected streams with variable seawater temperatures

9 Figure 7: Change in Plant Incoming and Rejected Streams with Variable Seawater Temperatures The ambient temperatures at the site also provide an additional challenge in the operation of the desalination plant during the cold winters, in which the temperature can drop down to -25 C. To address these extreme ambient temperatures, special measures were taken in the design of a winterization system for each of the plants including insulation, electric tracing, drainage system and a related control system. Special measures were taken for the operation of the plant, as well as for its shutdown procedure during sub-zero ambient temperature. The winterization system was designed to ensure complete drainage of the desalination plants during shutdown at these conditions to ensure that there is no freezing of stagnant water, which can result in pipe cracks due to ice expansion. In addition to its temperature, the salinity of the seawater can also vary between 2.7% and 3.3%. As brine salinity is essential for efficient salt production, the plant is designed to maintain a relatively high brine salinity of 6.6%. To reach this figure throughout the year, the brine salinity is controlled by sending a portion of the brine back to the feed water stream to maintain constant feed water salinity

10 IV. DESALINATION PLANT OPERATING DATA VS. DESIGN From the initial operation, the desalination plants in Tianjin were operated across the entire range of operating conditions described above. Table 1 shows the performance summary of the desalination plants, in the form of measured data versus design data. Table 1: Desalination Plant Performance Summary Description Unit Design Measured Maximum production % ton/day 27,500 28,250 Minimum production % ton/day 10,000 9,500 Distillate quality ppm 5 2 ER to 1.2 bar abs steam to 5 bar abs steam Power consumption kwh/ton V. CONCLUSIONS The Tianjin Beijing power station is an inspiring example of a hybrid power, water desalination and salt generation solution. This type of project, where the desalination plant acts as a connecting link between the power and salt generation, comprises a significant challenge for the desalination plant design in achieving high overall efficiency of all three parts of the project. This great challenge is met by the design of the MED-TVC desalination plant in Tianjin, which provides the required flexibility to an efficient power generation operation together with high efficient water production, as was presented in this article,. The operation experience gathered since the commissioning of the first desalination plant at the end of 2009, shows high performance parameters of the desalination plants, while successfully addressing the entire range of challenges presented

11 Figure 8: Tianjin MED Unit adjacent to Power Plant VI. REFERENCES 1. "Sliding Pressure Turbine Integrated with Seawater Desalination Facility (MED)" By Hagay Shemer, presented at the IDA 2011 World Congress