Research on the comprehensive utilization of marine water resources based on marine energy

First, the research background

According to the different forms of existence, ocean energy can be divided into tidal energy, tidal (ocean) flow energy, wave energy, salt difference energy, temperature difference energy, etc., these narrow marine energy theoretical reserves of up to 2.1×106 TWh/a [1], greatly exceeding the sum of the current human use of various energy (power). In addition, there are generalized marine energy, such as offshore wind energy, marine biomass energy, marine geothermal energy and so on. Marine energy in the narrow or broad sense described above is renewable and clean energy. However, there is also a huge amount of "marine (water) chemical energy" in seawater. The effective use of this special energy and the "resource" of massive amounts of seawater are new research areas and new technological problems proposed in this paper. Specifically, this paper will focus on a new scheme for the comprehensive utilization of marine energy, seawater desalination and seawater resources with research value, that is, seawater desalination through marine energy power generation, while obtaining valuable freshwater resources, further use the electricity converted by the ocean to complete the electrolysis hydrogen production and alkali production process of concentrated brine, and directly obtain hydrogen, chlorine, soda ash, caustic soda and other chemical products from seawater, so as to complete the whole process of asking for energy from the sea, fresh water from the sea, and asking for resources from the sea. Open up new ways for the comprehensive utilization of marine energy and sea water resources.

Seawater desalination is a realistic choice to solve the shortage of freshwater resources in coastal (offshore) areas, which has important social significance and economic value, and the desalination technology industry itself has matured. The use of marine energy to achieve marine energy marine use, local energy extraction, nearby power generation, to achieve coastal water shortage areas or waterless islands, islands and reefs of seawater desalination supply, further realize the integration of offshore renewable energy technology and seawater desalination technology, expand the industrial chain, is expected to further reduce the cost of seawater desalination. The concentration of sodium chloride in concentrated brine after desalination is very close to the concentration of sodium chloride in the electrolytic salt process solution. In terms of marine power generation technology, domestic and foreign research institutions and enterprises, including Zhejiang University, Guangzhou Institute of Energy Research of the Chinese Academy of Sciences and other units, have done long-term and extensive work in the field of marine energy power generation, and power generation equipment and technologies such as tidal energy and wave energy are becoming more and more mature. However, the simultaneous use of electricity generated by marine energy and concentrated brine, a by-product of seawater desalination, to complete the electrochemical production process that usually consumes a lot of electricity in inland manufacturers is a new concept, and there are no direct, specific research and implementation cases. Therefore, the following will analyze the above-mentioned schemes of the use of marine energy power generation and seawater to desalinate the remaining concentrated brine (usually discarded from the sea) two kinds of offshore resources, through electrolysis to hydrogen production and other chemical products of the process, cost and application technology issues, to explore the feasibility and effectiveness of this scheme, assess its potential economic and social benefits.

Second, the feasibility analysis of seawater desalination and the resource utilization of its concentrated brine by-products

China is a country with a shortage of freshwater resources, with more than 400 of the country's more than 660 cities running out of water, of which 108 are severely water-scarce [2]. The lack of water resources has become an important factor restricting social and economic development. The use of seawater desalination technology to obtain fresh water can be unaffected by time and space and climate, and the water quality is good and reasonable, which can provide a stable municipal water supply and industrial water supply for coastal areas. However, with the implementation of large-scale reverse osmosis membrane desalination project, the discharge of its by-product concentrated seawater has attracted widespread attention. Concentrated seawater not only has a high salinity content, but also introduces some chemicals during the pretreatment of seawater, which can affect or even pollute the environment if not handled properly [3], especially on the ecological environment of (semi-enclosed) sea areas. Therefore, the Chinese government attaches great importance to the comprehensive utilization of seawater, and in the overall idea of the Opinions on Accelerating the Development of the Seawater Desalination Industry (Guo Ban Fa [2012] No. 13), it is clearly proposed that "seawater desalination treatment and comprehensive utilization of resources are combined". When the goal of 6×106 ~ 8×106 t/d of seawater desalination in 2020 is achieved, the output value of the concentrated seawater resources industry will reach more than 37 billion yuan, which will not only obtain good comprehensive economic benefits, but also make important contributions to solving the source of urgent minerals in China and protecting the marine environment, and its prospects are very broad. In the case of a 1×105 t/d seawater desalination plant, the concentrated seawater discharged per day is about 6×104 t, of which the salt content is about 6×103 t, and if extracted as a by-product, the cost of seawater desalination can be reduced by 20% [4]. The unit discharges about 2.19 ×107 t of concentrated seawater a year, of which the total amount of chemical resources will exceed 2×106 t. In summary, it can be seen that the direct return of concentrated seawater into the sea will cause waste of resources and pollution of the sea area, and the use of desalinated seawater is not only an extension of the comprehensive utilization chain of marine water resources, but also fully has the technical feasibility and the potential to create new value.

3. Schemes for the comprehensive utilization of marine water resources based on tidal or ocean current energy

Tidal energy or ocean current energy refers to the kinetic energy of seawater flowing in the horizontal direction, and the causes of the two are not the same. The former is most concentrated in the coast or near the island, corresponding to a large flow rate and a large energy density. The latter is prevalent in vast oceans, usually with small but large total flows. Tidal energy or current energy is highly regular, has little impact on the marine environment, and is suitable for large-scale development [5]. Therefore, it is of great practical significance and possibility to carry out the comprehensive utilization of marine water resources based on tidal power generation on the coast or on the island, which is convenient for local energy extraction and sea energy.

(1) The utilization of seawater resources based on tidal current energy

1. Desalination of tidal power generation

For the vast majority of islands far from the mainland, the lack of freshwater resources is a common problem. Conventional desalination methods are inseparable from the support of electrical energy, and long-distance transmission of electricity on islands often has many limitations and is sometimes difficult to achieve. These islands tend to have natural current energy or ocean current energy nearby. Therefore, converting its kinetic energy into electricity through tidal (ocean) flow energy generation equipment, and then driving the seawater desalination device to obtain fresh water, is an effective solution to the problem of fresh water away from the mainland islands.

2. Seawater concentrated brine treatment based on power flow energy

As mentioned earlier, if the concentrated seawater obtained from desalination is directly discharged into the sea, it will not only pollute the surrounding sea area, but also cause a great waste of resources. Seawater is rich in sodium chloride and is an important raw material for the preparation of caustic soda. Hydrogen and chlorine, as by-products of the preparation of caustic soda, have very important economic value, especially hydrogen as an energy source for hydrogen fuel cells and hydrogen energy new energy vehicles, and its important value is self-evident.

The core part of the chlor-alkali industry is the electrolysis process. It is estimated that the electrical energy consumed by the electrolytic process accounts for 53.2% of the overall energy consumption in the entire production process, and many alkali plants have their own small power plants, which reduce the production cost while meeting the consumption of electrical energy in the production process [6]. However, most of the captive power plants are thermal power plants, which have a large environmental impact in the power generation process, and electrical energy consumption has gradually become a limiting factor affecting the preparation of caustic soda.

This makes it possible to combine tidal (ocean) flow power generation with the chlor-alkali industry, and also facilitates the use of concentrated seawater for chlor-alkali production. Combining tidal power generation, seawater desalination and chlor-alkali production locally or nearby to form a new comprehensive utilization system for marine energy should be an important step in the development and utilization of marine energy.

(2) Analysis of the process of comprehensive utilization of marine water resources based on tidal (ocean) flow energy

Figure 1 is a flowchart of the comprehensive utilization of marine water resources. Raw seawater water intake through a series of treatment processes to obtain fresh water, alkali products, hydrogen, chlorine and concentrated hydrochloric acid and other products, the process of electric energy are provided by tidal (ocean) flow energy generation equipment.

Figure 1 Flowchart of integrated utilization of seawater resources

First of all, the pretreated seawater is desalinated by the reverse osmosis membrane method to obtain concentrated seawater with a concentration of 6% to 8% sodium chloride [7], and the reverse osmosis membrane method has the advantages of low investment and low energy consumption [8].

Then, the resulting concentrated seawater is obtained by electrodialysis to obtain exquisite concentrated seawater. In industry, caustic soda, chlorine, and hydrogen are prepared by electrolysis of saturated sodium chloride solutions, so concentrated seawater refining is required to increase the content of sodium chloride in the solution. Electrodialysis, as a well-established seawater concentration method [10], can increase the concentration of sodium chloride in concentrated seawater to meet the requirements of alkali production.

Finally, electrolysis of saturated sodium chloride solution prepares caustic soda, chlorine and hydrogen. In the electrolysis process, the refined concentrated seawater enters the ion membrane electrolyzer from the anode side, and the fresh water obtained by desalination of part of the seawater is added to the cathode side, and the light brine and chlorine gas are obtained on the anode side after electrolysis, and the electrolyte and hydrogen are obtained on the cathode side.

The light brine and chlorine gas generated by the anode are cooled by a titanium cooler and then separated by the gas liquid. The outflow of light brine is removed from hypochlorous acid through the dechlorination process and can be reused in the refining process of concentrated seawater. After the chlorine is cooled and dried, it can be liquefied and stored, and it can also be synthesized with the hydrogen generated by the cathode to synthesize hydrochloric acid and use it in the power solution process. After the hydrogen is dried, it can enter the high-pressure hydrogen storage tank for storage and storage, and can also enter the hydrochloric acid production process. In addition, the electrolyte obtained from the cathode is evaporated to obtain a solid caustic soda product.

(3) Economic analysis of process operation

Combined with the operating environment of tidal (ocean) flow energy generation equipment, this paper takes the seawater desalination equipment with a production capacity of 2.5×103 t/d as an example, and 10 a is the operating time period to evaluate the cost of the whole process flow. The seawater desalination process proposed in this paper is a mature reverse osmosis membrane method, so its operating cost is no different from that of conventional seawater desalination equipment. The operating cost of the equipment consists of two parts: fixed cost and operating cost. The fixed cost of seawater desalination equipment includes depreciation, financial expenses and other expenses; operating costs mainly include electricity, maintenance, membrane replacement costs, pharmaceutical costs, labor costs, etc. [11].

There are many similarities between offshore wind power generation systems and tidal power generation systems in investment, operation and maintenance, so this paper uses the feed-in tariff of offshore wind power to estimate the on-grid price of tidal energy for cost calculation with strong reference. According to the literature [12], according to the Notice of the National Development and Reform Commission on Adjusting the Benchmark Feed-in Tariff of Onshore Wind Power for Photovoltaic Power Generation (Fa Jie Price [2016] No. 2729), the benchmark feed-in tariff for offshore wind power projects is 0.85 yuan /kW · h。 Combined with the calculation of operating costs in the literature [11], it can be seen that under the conditions designed in this paper, the cost of desalination is 12.4 yuan /t.

Electrodialysis was used to concentrate concentrated seawater after desalination, converted into sodium chloride to calculate its electricity consumption of 170 kW · h/t [13], for 0.85 RMB /kW · h Calculated by the electricity price, the electricity cost of obtaining sodium chloride is 144.5 yuan /t. At present, the raw salt price of chlor-alkali industrial production is about 250 yuan / t, so the use of concentrated concentrated seawater as the original solution for chlor-alkali production can reduce the cost of raw materials to a certain extent.

A seawater desalination plant with a capacity of 2.5×103 t/d can produce 3×103 t of concentrated seawater per day, so it is necessary to make full use of concentrated seawater, and the caustic soda capacity needs to be 5×104 t/a. According to literature [14], the electricity consumption per ton of caustic soda by ion membrane electrolysis method is 2340 kW·h, the steam consumption is 0.9 t, and the consumption of sodium chloride is 1.6 t, combined with the conversion of the electricity price and sodium chloride price above, the energy consumption cost per ton of caustic soda is 2310 yuan. Taking 10 a as the time period, plus equipment depreciation and operating costs, the total cost per ton of caustic soda is about 2594 yuan.

In this paper, the comprehensive utilization scheme of marine chemical energy based on tidal (ocean) flow energy generation is proposed, and the cost of seawater desalination is 12.4 yuan /t, and the cost of caustic soda is 2594 yuan /t. According to the cost calculation of seawater desalination and caustic soda production in the literature [11] and literature [14], it can be seen that the cost of seawater desalination under normal conditions is 8.808 yuan / t, and the cost of caustic soda is 1281 yuan / t. Compared with the conventional seawater desalination and caustic soda production processes, the main reason for the current cost gap between the two is the price of electricity. With the advancement of technology and the expansion of scale, the cost of power generation will be reduced and the economic value of this scheme will be further revealed.

Fourth, the downstream application of seawater resources

Saturated brine can generate hydrogen, chlorine and caustic soda through electrolysis, all of which are important chemical raw materials. Caustic soda is the main product of the chlor-alkali industry, has important industrial value, and is known as the "mother of chemical industry". Chlorine and hydrogen are important co-products of the chlor-alkali industry and play a key role in the development of the chlor-alkali industry.

Caustic soda is widely used in detergents, soaps, papermaking, printing and dyeing, textiles, pharmaceuticals, dyes, metal products, basic chemicals and organic chemical industries. The domestic demand structure of caustic soda has remained basically stable, and the downstream consumer market is still dominated by traditional industries such as alumina, papermaking, printing and dyeing, and chemical fiber [15].

Chlorine is an important co-product of the chlor-alkali industry, widely used, is an important tap water disinfectant, but also the main raw material of bleach powder, bleach, in the electronics industry is widely used in the manufacture of ultra-large scale integrated circuits. Chlorine downstream products are also quite abundant. With chlorine as the source, chloroal alcohol can be used to produce propylene oxide, downstream production of polyether and tetrachloroethylene, tetrachloroethylene can be partially produced for refrigerant export; chloropropylene can be produced, epichlorohydrin; phosgene can be generated, and diphenylmethane diisocyanate (MDI) can be produced, and with the products of another production line, polyurethane can be further produced; trichloroethylene can be produced for direct sale [16].

Hydrogen is widely used in oil refining and fuel cells. It plays an important role in improving light oil yield and improving oil quality. In 2017, China's total refining volume reached 7.7×108 t, and the hydrogen consumption was about 6.16 ×106 ~ 1.078×107 t [16]. Fuel cells have great commercial value and have broad prospects in the fields of transportation, energy, military and aerospace. Hydrogen fuel cells are the key direction of fuel cell applications. Hydrogen fuel cell vehicles, distributed generation, emergency power supply and other industries with hydrogen fuel cells as the core have begun to emerge [17].

In addition, hydrogen also plays a huge role in the far-reaching naval and civilian bases. It is reported that China Aerospace Science and Technology Corporation has established China's first military-civilian fusion hydrogen energy engineering technology research and development center. If according to the marine energy desalination, ice making, gas and seawater kinetic energy and chemical energy resource utilization scheme proposed in this article, the power generation, desalination and hydrogen production equipment system is completed in the offshore and far-reaching seas of China, relying on islands and reefs rich in marine energy, making it a frontier supply base for far-reaching sea automation, mobility, unmanned charging, water filling and gasification, and a production and storage base for seawater chemical raw materials and fishery ice, which can not only promote the research and development and engineering application of high-end technology and equipment in frontier fields such as renewable marine energy and hydrogen energy utilization. At the same time, it can also play a major supporting role in safeguarding the security of China's maritime frontier and maritime rights and interests.

5. Conclusion

Combining tidal power generation with desalination and chlor-alkali production, according to local conditions, can make full use of marine resources, reduce dependence on land energy, and provide the possibility of establishing related industries on islands and seas.

It increases the scope of use of power flow energy, provides important economic value for the use of power flow energy, and adds a new driving force to the industrialization of power flow energy generation.

Fresh water from desalination can serve chlor-alkali production, provide domestic water for islands, or store freshwater as freshwater replenishment stations to provide freshwater to ships.

The in-situ utilization of concentrated seawater not only reduces the environmental pollution of seawater desalination by-products, reduces the cost of chlor-alkali production, but also makes full use of marine chemical energy and improves economic benefits.

Hydrogen energy is recognized as a clean energy source and is known as the most promising secondary energy source in the 21st century. Obtaining hydrogen from the ocean is one of the development directions of the hydrogen industry in the future, and through the marine chemical energy utilization scheme proposed in this paper, high-purity hydrogen can be prepared, providing a possibility for the construction of offshore hydrogen refueling stations.