By Chen Yinbiao & Zhang Jianli for Cornerstone
Water is indispensable to human survival. In recent years, due to climate change, population growth, environmental pollution, and other factors, the lack of freshwater resources in many countries and regions has become a greater concern. Water scarcity is increasingly affecting global sustainable development; hence resolving water shortage problems has become a common focus of nations around the world.
With abundant seawater on Earth—accounting for more than 97% of the total water volume—desalination has been demonstrated as an option to turn the vast oceans to a major potential source of freshwater. The widespread application of desalination is feasible and could be an important component of solving the global water crisis.
Today’s Desalination Industry
Large-scale development of desalination technology began in the mid-20th century, when it was primarily used in the Middle East, a region with extreme water scarcity. As desalination technology matured, application gradually expanded geographically. According to statistics from Global Water Intelligence and the International Desalination Association, as of June 2013 about 17,277 desalination plants had been built globally, reaching a total freshwater production capacity of 80,900,000 m3/d. These projects include a seawater desalination capacity of about 47,730,000 m3/d—about 59% of the total capacity—addressing water supply problems for more than 100 million people worldwide.
China, in particular, is facing water scarcity, with per capita water resources at only one-fourth that of the global average. In recent years, desalination technology has developed rapidly in China, and major progress has been achieved in research and development, equipment manufacturing, and water production capacity. Desalination equipment made in China has been exported to countries such as Indonesia and India. According to statistics from the Desalination Branch of China Water Enterprises Confederation, as of October 2013, China had constructed a seawater desalination capacity of 858,600 m3/d, mainly supplying municipal users and the power industry (see Figure 1).1
Figure 1. Statistics on desalination capacity development in China1
For energy, China employs primarily coal-fired power generation. Most power plants are located in regions with extreme freshwater scarcity. Water consumption is a key factor that can affect the efficiency of power generation and could restrict the development of power plants. Water shortages are especially serious in the northern areas of China, and the siting of new power plants in many northern regions must be based on access to adequate water resources. In recent years, new power plants built in coastal areas have adopted industrial closed-loop water systems, direct seawater cooling condensers, and other water-saving measures. Even so, there remains a huge demand for freshwater. For example, Shenhua’s Hebei Guohua Cangdong Power Plant has four coal-fired units currently in operation, with a combined electrical capacity of 2520 MW. Direct seawater cooling is used for all the condensers. Approximately 3,200,000–4,400,000 m3 of freshwater is consumed every year as feedwater for the four boiler units, as well as desulfurization and other processes. This massive water consumption requirement is completely filled through desalination, achieving zero consumption of freshwater and effectively converting this power plant into a supplier of freshwater to help alleviate water shortages in the surrounding regions (see Figure 2 for a photo of the desalination facility). At the same time, cogeneration of power and water can effectively reduce the production costs for both water and electricity. When power plant water consumption demand is met through desalination, the building of power plants in coastal areas is no longer restricted by whether freshwater resources can be obtained on land. This approach is conducive to the development of coal-fired power generation that is harmonious with the environment.
Figure 2. Shenhua’s Hebei Guohua Cangdong Power Plant desalination facility
Current Status of Desalination Technologies
Chief Desalination Technologies
The water supply for desalination is not influenced by the seasons or by the climate, and thus can be considered a good-quality, stable water source. Today, desalination is mainly based on two technical routes: 1) distillation desalination based on multi-stage flash evaporation (MSF) and/or low-temperature multi-effect distillation (MED) and 2) membrane desalination based on seawater reverse osmosis (SWRO).
In MSF, heated seawater is evaporated in multiple flash chambers with sequentially reduced pressure; the condensed vapor is freshwater. In the MED process, multiple evaporators connected in series are used to evaporate seawater: The vapor from each previous evaporator is turned into heated vapor for the next evaporator and then condensed into freshwater. In SWRO, seawater is pressurized to force water to pass through osmotic membranes while the salt does not pass through. The main technical parameters of these three desalination technologies are shown in Table 1.
Table 1. Comparison of parameters in three main desalination technologies
Due to high energy consumption, the share of MSF technology in the desalination market has been declining year by year. MED and SWRO are being widely applied in newly built desalination projects. MED and SWRO both have advantages: Equipment investment and water production costs for SWRO are lower, but this technology has a higher requirement in terms of the quality of the seawater. The pre-treatment technology is rather complex and its adaptability to seawater temperature is poor. It has no obvious advantages in investment and operational energy consumption when applied in northern China with lower seawater quality. MED technology has a wider adaptive range for seawater temperature and water quality, and is characterized by high heat transfer efficiency, low pre-treatment requirement, simple operation, high reliability, and good water quality, although the capital costs are higher.
Technical Advantages of MED
The “low-temperature” in low-temperature MED refers to its highest evaporation temperature, which is generally lower than 70°C. Since the heat that is input can be used repeatedly, the process offers high thermal efficiency, low energy consumption, and low water production costs. Additionally, due to the low-temperature characteristic, the equipment experiences reduced scaling and corrosion and very high operation reliability. MED technology is especially suitable for power and water cogeneration projects; it can use exhaust from the low-pressure steam turbine to reduce water production costs, an approach that also offers important technical advantages. MED technology has received increased attention in recent years, with a constantly expanding installation scale and continuously increasing market share. From 1997 to 2002, MED only accounted for about 25% of desalination by distillation. Between 2003 and 2008, its share of the world market for desalination by distillation grew to 42%. In 2013, MED technology accounted for 33% of China’s total desalination capacity (see Figure 3).1
Figure 3. 2013 breakdown of the desalination technologies capacities in China
Cogeneration of Power and Water
Cogeneration of power and water is known as dual-purpose water production in the international desalination industry and is the preferred option for desalination for coal-fired power plants. Employing this form of cogeneration can significantly reduce desalinated water costs and investments and it has become the principal model for existing large desalination projects.
There are many benefits to integrating power plant operation and desalination. First, since coal-fired power plants can be water-intensive, the use of desalination to produce freshwater can meet the power plant’s freshwater needs. Second, when the desalination process uses the plant’s low-grade steam, it provides the heat needed for desalination, simultaneously providing cooling for the plant. This results in an increase in the plant’s efficiency and reduces desalination costs. Third, the water produced from desalination is high in purity, and further softening (i.e., removing calcium and other metal cations that could cause buildup) is simple compared to other sources of water. Thus the cost of using desalinated water as boiler feedwater would be lower than water that must be treated with a traditional water-softening process.
In addition, during cogeneration of electricity and water, the power plant’s cooling water discharge facilities can be shared with concentrated seawater from desalination, which reduces project capital investments. Finally, when the concentrated seawater is mixed with the power plant’s circulating cooling water for discharge, it can reduce the impact on oceans because the discharge temperature will be lower.
MED Technical Innovation and Application of Cogeneration
For newly built and expanded coal-fired power plants within the Shenhua Group that are distributed in coastal areas with water resource scarcity, the demands for desalination technologies are constantly increasing. Driven by the company’s strategic development needs, Shenhua Guohua has selected the MED desalination technology that is suitable for cogeneration of power and water. Through research, development, and application, Shenhua Guohua now has proprietary, large-scale MED equipment and has mastered the design of such equipment.
The large-scale MED equipment developed by Shenhua Guohua uses a horizontal-pipe falling film evaporator (see Figure 4 for the full process). The generator turbine exhaust steam enters the heat exchanger for the first effect (i.e., stage) evaporator as heating steam and condenses after releasing sensible and latent heat; some of the seawater is evaporated after absorbing heat. The steam from the first evaporator is guided into the second-stage evaporator and condenses in the next stage, a step which is repeated for each sequential evaporator, to yield several times more condensed water than can be obtained from single-stage evaporation. When the generator exhaust has a higher temperature and pressure, a thermal vapor compressor can be used to increase the pressure of some of the low-pressure steam generated through seawater evaporation and transfer it to the first stage as the heating source, which can greatly improve the efficiency and reduce the costs of water generation.
Figure 4. Shenhua Guohua’s MED process
The technical crux of low-temperature MED desalination includes efficient heat transfer, material selection and corrosion control, discharging non-condensed gas, avoiding scaling, good product water quality, etc. The technical research and development strategy of Shenhua Guohua consists of mastering all these key technological aspects so as to provide desalination equipment that can meet user demands, mainly including guarantees on performance indicators, operational costs, reliability, and service life. Through R&D the mechanisms of vacuum heat transfer and flow with small temperature differences and multiphase flow have been mastered. In addition, large-scale MED design and key equipment structure design methods have been obtained to develop large-scale MED series equipment that can produce 12,500–25,000 m3/d of freshwater.
Through research on MED heat transfer and flow characteristics, a heat transfer and flow resistance coupling calculation method has been established, and heat transfer and flow characteristics computational software for large low-temperature MED equipment has been developed. This software provides effective tools for the selection of MED working media parameters and the optimized design of the heat exchanger. Meanwhile, MED technical parameter selection and calculation software has been developed as well. This software can work out a large number of technology schemes as well as conduct a techno-economic analysis targeting the initial conditions and user demands of different projects, to obtain the technical configuration with optimal economics.
Through fundamental experimental research, a liquid spray system with high distribution efficiency has been designed, and a single-shell double tube evaporator has been developed; this design promotes the flow of steam in the tubes and improves overall heat transfer. Using a 100-m3/d MED pilot plant, the reliability of the design calculation software was verified through heat transfer tests. The optimized structural design direction of the MED evaporator was also explored by conducting various heat-transfer flow-comparative experiments on the pilot plant.
The MED evaporator is a large thin-walled vacuum container. Shenhua Guohua cooperated with manufacturers in China to overcome technical problems—such as welding and deformation control in large thin-walled containers, expansion of thin-tube plates, installation of large tube bundles, installation and deformation correction of the flow guide plates, avoidance of acid pickling, field assembly and welding of multiple-effect evaporators—and formed a comprehensive set of standards in enterprise manufacturing techniques. Through the overall development effort, large-scale MED installation, troubleshooting strategies, and new technologies involving start-up, operation, normal shutdown, and emergency shutdown have been mastered. In addition, domestic industrial standards have been established. At the same time the desalination anti-sludging agent and the chemical cleaning system and method independently developed by Shenhua Guohua have reduced the operational costs for MED equipment.
The MED technology that was independently researched and developed by Shenhua Guohua has been successfully applied to Shenhua’s Hebei Guohua Cangdong Power Plant for the cogeneration of power and water project; one 12,500-m3/d unit and one 25,000-m3/d unit are currently operating, both of which were domestically manufactured. The exhaust steam from the turbine of the coal-fired power generator is used as a heat source for desalinating water, which meets not only the needs of the power plant, but also provides water for the Port of Huanghua in Cangzhou, Hebei, as well as industrial users in nearby steel plants. As the quality of the water produced through desalination is higher than other sources of freshwater, the cost for follow-up water treatment is reduced, and the desalinated water offers a price advantage for industrial users. At present, Guohua Cangdong Power Plant’s annual external freshwater supply capacity is close to 10 million m3, which has effectively relieved the freshwater resource scarcity in the Port of Huanghua.
The Challenges of Desalination Technology
The cost of energy is the most important factor influencing the cost of desalinated water. For example, in MED technology, when the designed ratio of product water quantity to heating steam consumption reaches 10 (e.g., about 0.1 tonne of steam is consumed for every 1 m3 of freshwater produced) about 1.2 kWh is also used and the energy cost accounts for about 40–50% of the water production cost. Therefore, reducing energy consumption for desalination is an important focus. There are two central aspects to reduce energy consumption: First, develop new desalination technologies and equipment. Second, integrate and optimize desalination technologies and make the most rational allocation of resources to maximize efficiency in energy utilization, and even achieve zero discharge in the desalination process. For example, heat-membrane cogeneration technology—the use of a combination of distillation methods and membrane methods (namely, MED-SWRO or MSF-SWRO)—meets different demands in water quality, lowers desalination costs, and improves the energy utilization rate.
In addition to saving energy, it is also possible to utilize unconventional energy resources and renewable energy, such as solar, wind, geothermal, and tidal energy,2 to reduce the consumption of traditional fossil fuels. The Perth Seawater Desalination Plant,3 which began operation in 2007 and uses wind energy, has a freshwater production capacity of 144,000 m3/d, accounting for 17% of the total water supply of Perth. It is the largest desalination plant in the world powered by renewable energy.
The need to continue advancement of the desalination industry, while placing greater emphasis on minimizing environmental impacts, is receiving increased attention. Many large-scale coastal power plants applying desalination in northern China have begun discharging concentrated seawater to salt fields to make salt. In addition, some research and application related to extraction of raw materials such as potassium, bromine, and magnesium from concentrated seawater is ongoing. There have also been developments in promising technologies with reduced brine discharge volumes, such as forward osmosis technology. Forward osmosis uses the hyperosmosis drive liquid to extract freshwater from seawater through a selectively permeable membrane: The discharged brine concentration can reach over 20%, which is conducive to the follow-up recovery processing of the salt water resource. Forward osmosis technology has demonstrated promise in development projects.
Desalination is an important means of providing a sustainable supply of freshwater. However, it continues to face challenges in reducing costs and increasing operational efficiency. We believe the necessary future developments and improvements to desalination technology are as follows:
The industry must develop new processes, new equipment, and new materials to additionally reduce the cost of desalinated water. Larger-scale MED and SWRO desalination plants must be demonstrated.
Through the integrated application of new technologies and new processes, the scope of application of desalination can be expanded. Desalination will be incorporated into national or regional water supply systems in China, making it possible for desalinated water to be transported long distances to municipal water supply systems and even be integrated into the modern agricultural sector.
In the future, greater emphasis will be placed on the concept of zero discharge in the desalination industry. The industry should strive to reduce energy consumption and brine discharge, as well as create a recycling industry chain and reduce the impact on the marine environment, through the integration of technologies and comprehensive utilization of resources.
The cogeneration of power and water is the best option to solve the freshwater needs of thermal power plants in coastal areas. The integration of desalination with power plants creates the possibility of converting a thermal power plant from a water consumer into a freshwater supplier to nearby coastal cities.
With further improvements, desalination technologies can play a greater role as a convenient, energy-efficient, and environmentally friendly means of providing freshwater to meet the world’s growing demand for freshwater and water security.
The content included in Cornerstone is based on the opinion of the authors, and does not necessarily reflect the views of the World Coal Association or its members.
- Desalination Branch of China Water Enterprises Confederation. (2013). 2012–2013 China desalination annual report.
- Gude, V.G., Nirmalakhandan, N., & Deng, S. (2010). Renewable and sustainable approaches for desalination. Renewable and Sustainable Energy Reviews, 14, 2641–2654.
- El Saliby, I., Okour, Y., Shon, H.K., Kandasamy, J., & Kim, I.S. (2009). Desalination plants in Australia, review and facts. Desalination, 2009, 249, 1–14.