By Manzoor Qadir, United Nations University and Vladimir Smakhtin, United Nations University
Link to Qianyun
Water scarcity is among the top five global risks affecting people’s well-being. In water-scarce areas, the situation is grim. Conventional sources like snowfall, rainfall, river runoff and easily accessible groundwater are being affected by climate change, and supplies are shrinking as demand grows.
In these countries, water is a critical challenge to sustainable development and a potential cause of social unrest and conflict. Water scarcity also impacts traditional seasonal human migration routes and, together with other water insecurity factors, could reshape migration patterns.
Water-scarce countries need a fundamental change in planning and management. We are looking at how to do this, through the creative exploitation of unconventional water resources.
From Earth’s seabed to its upper atmosphere, we have a variety of water resources that can be tapped. But making the most of these requires a diverse range of technological interventions and innovations.
Catching Fog
Water embedded in fog is increasingly seen as a source of drinking water in dry areas where fog is intense and happens regularly. Fog can be collected using a vertical mesh that intercepts the droplet stream. This water then runs down into a water collection, storage and distribution system.
Different types of screen materials can be used in fog collectors, like aluminium, plastic, plexiglass and alloy. The success of a system like this depends on the geography and topography, which need to be conducive to optimal fog interception. But this could work in dry mountainous and coastal regions.
With active engagement of local communities and technical support from local institutions, fog water harvesting is a low-maintenance option and a green technology to supply drinking water. Fog water collection projects have been implemented in different parts of world, including Chile, Eritrea, Israel and Oman.
Cloud Seeding
Under the right conditions, rain enhancement through cloud seeding has the potential to increase the volume of water harvesting from air. This technology involves dispersing small particles into clouds or in their vicinity. These particles act as a starting point for raindrops or ice crystals, promoting their formation. In turn, this makes it more likely to rain or snow.
Application of cloud seeding technology in different countries has shown, precipitation can be increased by up to 20% of the annual norm depending on the available cloud resources and types, cloud water content and base temperature. As only up to 10% of the total cloud water content is released to the ground as precipitation, there is a huge potential for rain enhancement technologies to increase rainfall in dry areas.
Minimising Evaporation
As dry areas receive small amounts of rainfall, micro-catchment rainwater harvesting may help in capturing rainwater on the ground, where it would otherwise evaporate.
There are two major types of micro-catchment rainwater harvesting systems. One is water harvesting via rooftop systems where runoff is collected and stored in tanks or similar devices. This water is used domestically or for livestock watering.
The second is water harvesting for agriculture, which involves collecting the rainwater that runs off a catchment area in a small reservoir or in the root zone of a cultivated area. The catchment surface may be natural or treated with a material that stops the soil absorbing water, especially in areas with sandy soils. Because of the intermittent nature of runoff, it is necessary to store the maximum amount of rainwater during the rainy season so it can be used later.
Desalinating Seawater
The process of desalination removes salt from seawater or brackish groundwater to make them drinkable. This allows us to gather water beyond what is available from the water cycle, providing a climate independent and steady supply of high-quality water.
Seawater desalination has been growing faster because of advances in membrane technology and material science. These advances are projected to cause a significant decrease in production costs by .
For more Alloy Water Cycle Technologyinformation, please contact us. We will provide professional answers.
More places are expected to become reliant on desalinated water because of its falling costs and the rising costs of conventional water resources. While at present desalination provides approximately 10% of the municipal water supply of urban coastal centres worldwide, by the year this is expected to reach 25%.
Iceberg Harvesting
Towing an iceberg from one of the polar ice caps to a water scarce country may not seem like a practical solution to water shortages, but scientists, scholars and politicians are considering iceberg harvesting as a potential freshwater source.
Moving an iceberg across the ocean is technically possible, based on a theoretical four-part process. It would require locating a suitable source and supply, calculating the necessary towing power requirements, accurately predicting melting in transit, and estimating the economic feasibility of the entire endeavour. Countries like United Arab Emirates and South Africa are considering iceberg towing as an option to narrow gaps in their water demand and supply.
Water and climate change are interconnected, so climate change increases the likelihood of extreme droughts in dry areas. Harnessing the potential of unconventional water resources can help increase the resilience of water scarce communities against climate change, while diversifying water supply resources.
We need to identify and promote functional systems of unconventional water resources that are environmentally feasible, economically viable, and support the achievement of water related sustainable development, in the Sustainable Development Agenda and beyond.
Manzoor Qadir, Assistant Director of the Institute for Water, Environment and Health, United Nations University and Vladimir Smakhtin, Director of the Institute for Water, Environment and Health, United Nations University
This effect is reflected in the decreasing boiler water specific conductivity limits shown in Figure 1.
Mechanical carryover is exacerbated by other factors including rapid load swings that cause drum surges and boiler water contamination by impurities, e.g., organic compounds, that may generate foam. An example of foam-induced superheater fouling appeared in Part 2 of this series.6
Vaporous carryover is defined as the “fraction of substances entrained from boiler water into the steam by the substance’s volatility, [where] only above a drum pressure of about 2,300 psi does vaporous carryover start to become significant for most of the solids dissolved in boiler water. Below this pressure, it is nearly all mechanical carryover with the exception of a few substances like silica [and] the copper oxides/hydroxides…” 3
Silica does not cause corrosion but will precipitate on turbine blades and reduce aerodynamic efficiency. For older steam generators with copper alloy feedwater heater tubes, feedwater chemistry should be carefully controlled to maintain the copper concentration below 2 ppb.3 Copper is of much less concern nowadays with the phase-out of many coal-fired units in favor of combined cycle and renewable generation. The copper in older units came from corrosion of copper alloy feedwater heater tubes. Personnel at some plants have replaced copper-alloy tubes with ferrous materials including stainless steel.
In virtually all modern steam-generating power units, main- and reheat-steam attemperation water is taken from the discharge of the boiler feed pump. Not only must feedwater be very pure to protect the boiler, but its use for attemperation offers a direct path for contamination of the steam system and turbine. Ammonia, or in some cases an organic amine or ammonia/amine blend, are the only suitable chemicals for feedwater pH control, as solid alkalis such as sodium hydroxide can cause severe and prompt turbine blade and rotor corrosion. For units with steam surface condensers, a condenser tube leak directly contaminates boiler water and steam, apart from any mechanical carryover effects.
The deposition of impurities is influenced by the behavior of steam as it passes through the turbine. Steam, of course, expands and decreases in pressure as it flows through and transfers energy to the turbine. Silica and copper become less soluble and precipitate on turbine blades. Neither of these compounds is corrosive, but both will reduce the turbine’s aerodynamic efficiency. The loss of even a few megawatts capacity during a peak demand period can be quite costly.
Issues related to offline corrosion in the low-pressure (LP) section of power turbines often represent the most critical issue. Even with units on world-class chemistry programs, trace quantities of salts such as sodium chloride and sodium sulfate carry over into steam. As the steam releases its final superheat energy in the last stages of the LP turbine, moisture droplets begin to form. This region is commonly known as the Phase Transition Zone (PTZ). Salts will concentrate in the moisture and deposit on the blades and rotor. Within normal operation, these salts are not corrosive. However, during unit outages if condenser vacuum is broken and the LP turbine is exposed to ambient air, humidity and oxygen will moisten and activate the salts, which then may initiate pitting on blades, blade attachments and rotors. And this effect is exacerbated if water remains in the condenser hotwell.
With repeated unit cycling, the pits can evolve into microcracks, followed by corrosion fatigue (CF) and stress corrosion cracking (SCC). The potential outcome is blade failure with the turbine in operation, a catastrophic proposition. Nitrogen blanketing has been suggested to protect the LP turbine, but this method has difficulties, including issues related to confined space entry. Less expensive and safer is dry air humidification (DHA) to keep any salts from moisturizing and attacking turbine components.7
For units in which alkalizing amines are employed for feedwater pH control, these organic compounds can decompose in high-temperature superheaters and reheaters to form small-chain organic acids and carbon dioxide. For decades, debate has raged back and forth about the potential effects of the acids on LP turbine components. The corrosion influence appears to be slight, but the carryover of the acids into condensate can depress pH and potentially influence flow-accelerated corrosion (FAC) in the feedwater system.8
Also well-known is mechanical degradation of turbine blades by iron oxide particles that exfoliate from superheater surfaces. This condition has been exacerbated by much more frequent unit cycling due to the need for conventional and combined cycle power plants to follow renewable load production.
A large portion of makeup water preparation and steam generator chemical treatment is designed to minimize impurity carryover to steam. This is especially important for turbine protection. In the next installment, we will summarize the appropriate analytical instrumentation to monitor chemistry throughout the steam-generating circuit.
Brad Buecker is president of Buecker & Associates, LLC, consulting and technical writing/marketing. Most recently he served as Senior Technical Publicist with ChemTreat, Inc. He has over four decades of experience in or supporting the power industry, much of it in steam generation chemistry, water treatment, air quality control and results engineering positions.
Want more information on solar power system supplier? Feel free to contact us.