Process systems make-up in power industry requires demineralized water which does not cause corrosion and formation of deposits during operation of power plants. Water quality to a great extent determines economic efficiency and technical lifetime of energy generating systems, therefore, industry-specific requirements for the content of dissolved solids in water are the most stringent among all branches of industry.
To obtain water of the required quality multistage water treatment processes are used, the main of which are water demineralization technologies that prevent scale formation, corrosion and alkaline damage of boilers, as well as of the entire steam-water path.
Scale forms a solid insoluble residue on the heating surfaces, reducing thermal conductivity. The reason for scale formation is hardness salts and silicon transfer into the feed water. By heating the salts of these substances become insoluble and precipitate in the form of the scale. Thermal conductivity decreases sharply, there is a danger of overheating and efficiency loss of the unit. In addition, the cleaning and repair costs of the boiler , as well as of all heat and power equipment steeply rises.
Corrosion processes are caused by oxygen and carbon dioxide. Neglecting the corrosion hazard can cause gradual metal damage and even formation of fistulas and damage of the boiler. Also, due to an incorrectly calculated water treatment system and improper management of the water-chemical treatment such dangerous phenomena as alkaline softening and embrittlement can take place due to the increased alkalinity of the boiler water. Alkaline embrittlement, for example, occurs quickly and unnoticeably, causing severe accidents; all boiler elements are exposed to it.
Thus, water treatment system design, as well as development of the entire water chemistry specifications for the boiler equipment should comply with all general regulatory requirements (CO2 content in steam, relative alkalinity) and with those for each boiler type (pH, hardness, alkalinity of treated water, conductivity and alkalinity of boiler water, purge rate, etc.).
Ion exchange technologies, which appeared in the 60-s of the last century, are widely used in power industry. They are based on water passing through several stages of cationic and anion-exchange filters connected in series, which results in removal of dissolved mineral impurities from water. Depending on the requirements for the treated water quality one-, two- and even three-stage schemes are used. At the final cleaning stage, mixed-bed filters filled with a cation and anion mix are used. For regeneration of the ion-exchange filters ionite regeneration is regularly carried out, for which large amounts of reagents — acids and caustics — are used. Their consumption largely depends on the source water composition and the technology applied. Significant amounts of highly mineralized acid and alkaline effluents with a salinity of 10-60 g / l are formed as a result of the filter regeneration, which are sent to discharge after inactivation.
Taking into account current standards for wastewater salinity (not more than 1 g / l), regeneration effluents discharge requires multiply dilution with source water, which leads either to an increase of their quantity or water consumption or to penalties up to 10% of the demineralized water cost. As the ionites are very sensitive to organic substances, which are present in large quantities in natural waters, lime coagulation in clarifiers with subsequent filtration in mechanical filters is used as pretreatment before ion-exchange filters. As a result, a huge amount of solid waste (lime sludge) is formed, which cannot be disposed and is sent to waste dumps. In terms of the operating life of the most facilities, the design capacity of many sludge dumps is almost exhausted today.
Other disadvantages of the old water treatment technology are: high consumption of reagents (acids, alkalis, sodium hypochlorite, coagulant, etc.), leading to high operating costs and complicated logistics for the delivery of reagents to the chemical shop as these reagents are precursors and delivered by rail. And, therefore, additional costs for the delivery of reagents, the need for constant training of people to work with these (toxic) reagents is needed.
It should be noted, that at present boiler equipment being installed in Soviet times is gradually becoming “morally and physically” obsolete. An effective owner is faced with the task of reconstructing both boiler units and water treatment facilities. The issue of losses of excess gas volumes is also very important, the price of which currently makes us think about the need to use energy-saving technologies.
Under these conditions, the design of water treatment equipment stipulates not only meeting all existing requirements (hardness, pH, oxygen, carbon dioxide, etc.), but also needs focusing on the requirements of leading manufacturers of power equipment for feed and boiler water.
For example, the requirements for feedwater of domestic and Russian boilers do not include restrictions on alkalinity and salinity, however, these indicators are very important by designing water treatment equipment, since a decrease of alkalinity and salinity can reduce the purge rate by 5-6 times, and, gas consumption accordingly. European manufacturers take this dependence into account, therefore, they set alkalinity limits for both feed and boiler water.
In general, traditional ion-exchange methods for water demineralization have the following disadvantages:
• Large production areas are necessary for multi-stage systems which require a large quantity of the main process equipment (clarifiers, mechanical and ion-exchange filters) and auxiliary equipment (collecting tanks, pump stations);
• Large quantities of reagents (sulfuric acid, caustic, lime, iron sulfate). This not only leads to large operating costs for demineralized water production, but also requires an appropriate infrastructure – transport routes (usually railways), huge reagent facilities (reagent storage tanks, lime storage);
• Formation of large amounts of highly mineralized effluents and sludge, the disposal of which is often difficult;
• Complexity of process automation, the need for continuous monitoring and adjustment of working parameters. The treated water quality is dpendent on the operating parameters.
The vast majority of water demineralization plants, being currently in operation at energy facilities of our country and CIS countries, were put in operation in the 60-80s of the last century and are based on the use of ion-exchange technologies. Most of these facilities are obsolete and require serious reconstruction, both in terms of the process equipment and of automatic control systems.
Thus, the traditional scheme, including removal of suspended solids and two-stage Na-cationization, is a thing of the past, since it does not make it possible to reduce gas consumption due to alkalinity. This technology is being replaced everywhere with the combined method – Na-cationization – reverse osmosis (RO).
Reverse osmosis (RO) can simultaneously reduce hardness and alkalinity, the total salt content (conductivity), and it is also a barrier to silicon and copper and prevents from turbidity. In combination with Na-cationization as a pre-treatment or post-treatment, reverse osmosis (RO) gives an excellent result, proven by time and by a great number of installations. For urban waters in such cities as Kiev, Donetsk, Dnepr, Mykolayiv, Odessa, Zaporizhia, i.e., where the river serves as a source of water supply, Na-cationization – reverse osmosis (RO) is used more common; if the enterprise operates on artesian water, the following scheme is more efficient: reverse osmosis (RO) – Na-cationization.
It should be also noted, that there is a growing demand for the equipment that makes it possible to get feed water from technical river water (Dnieper river, Seversky Donets, Southern Bug, etc.), which is characterized by high oxidability values, suspended solids, bacteriological pollution and the presence of phyto- and zooplankton.
Further development is a baro-membrane technology at all stages
During reconstruction of existing demineralization facilities and practically always during designing new ones the most state-of-the-art and the most ecologically safe baro-mebrane methods are used.
The backbone of the baro-membrane methods is letting pressurized water pass through semi-permeable membranes that trap different impurities. One of the most innovative schemes for water demineralizing is currently a technology that includes the stages of preliminary filtration, ultrafiltration, reverse osmosis demineralization and electric deionization.
Preliminary filtration in the energy sector is traditionally carried out in sand-and-gravel filters. These filters occupy large areas, require high pressure when being washed and show large losses of washwater. For cleaning it is necessary to take the filter out of operation for the washing period, and therefore a redundancy is needed.
Innovative mechanical filtration filters use a vortex vacuum scanner technology with brush nozzles for highly efficient cleaning of the filter element (four-layer mesh). These filters take minimum space, spend no more than 1% of the instantaneous flow rate for flushing and, what is most important, they do not stop (and do not reduce) the feed of clean water during the process of self-cleaning. Such filters are fully automatic and can be easily integrated into a process control system. A high filtration rating (25 – 50 microns) allows you to significantly reduce washing water losses at the next stage (ultrafiltration) and to use UF membranes with a finer filtration rating.
The ultrafiltration stage is used to remove suspended solids, colloidal impurities, a part of organic contaminants from the treated water, as well as bacteria, algae and other microorganisms, the size of which exceeds hundredths of a micron. At its core, ultrafiltration is an analogue of coagulation in clarifiers and cleaning in mechanical filters, but it does not have disadvantages of the traditional technology.
So, the main advantages of ultrafiltration units are:
•No need in lime facilities – by operation of the ultrafiltration units only periodic acid and alkaline washing of the modules is required, however, the quantity of reagents is ten times less than in ion exchange technology;
•No need for exact compliance with process parameters (temperature, pH, flow rate), as required by the operation of clarifiers. At the same time, the quality of water treatment remains constantly high and does not depend on operating conditions or on the human factor;
• Significant (2-4 times) reduction of production areas for placement of the main and auxiliary equipment;
• Easy operation, a possibility for the process automation.
At the stage of reverse osmosis demineralization, dissolved impurities are removed from the water.
Depending on the required cleaning quality, either a one- or a two-stage scheme is used. As a rule, the residual salinity after the first stage is 5-20 mg / l. That corresponds to the water quality after the first stage of H / OH-ionization. If a deeper demineralization is necessary, a second stage of reverse osmosis is used.
The main advantages of the reverse osmosis method of water demineralization:
• High reliability of the method, providing a consistently stable demineralized water quality, which does not depend on seasonal fluctuations of the source water, process parameters and the “human factor”;
• High economic efficiency – replacing the first stage of ion-exchange demineralization with a reverse osmosis makes it possible to reduce the need for acid and caustic by 90-95%, which covers many times the cost increase associated with the energy consumption;
• The same as for ultrafiltration plants reduction of production areas, easy operation and process automation.
• After retrofitting of the water treatment systems in the heat power generation sector all new baro-membrane equipment can be placed in existing premises, even without dismantling the outdated system.
Industrial electric deionisation system
Recently, The method of electric deionization has been increasingly used for the final water demineralization in the power sector and microelectronics. Fundamentally and according to the principle of purification, electric deionization is an analogue of traditional mixed-bed filters (MBF). A distinctive feature of the electric deionization is that the filter regeneration is carried out due to the passage of electric current through the bed depth directly during the filtering process.
In the event of electric deionization, periodic filter stops for technically complicated regeneration with separate regeneration of ionites are not necessary; there is no need to use reagents.
Thus, the use of modern water treatment methods, such as ultrafiltration, reverse osmosis demineralization and electric deionization makes it possible to:
• Significantly reduce operating costs;
• Significantly reduce the consumption of reagents, go back from large reagent facilities;
• Improve the environmental safety of the plant;
• Provide stable top-quality water treatment;
• Introduce automation into the process.
All this makes the new technology more environmentally friendly than the existing one.
The cost of treated water and the projected payback period of the equipment.
A similar water treatment system is implemented on the river Seversky Donets and has a capacity of 50.0 m3/h with the use of the following scheme: preliminary filtration – microcoagulation – ultrafiltration (UF) – reverse osmosis (RO) – Na-cationization, which provides make-up for a 40 atm waste-heat boiler of the cogeneration plant.
One of our main goals was to reduce operating costs of demineralized water treatment by reducing the cost for replacing ion-exchange resins, mass consumption of reagents, such as lime, coagulants, acids, alkalis, etc. The estimated cost of purified water according to our technology was only 2. 9 UAH / 1 m3, while according to the existing traditional technology at the enterprise it was 17.5 UAH / 1 m3. The estimated payback period for innovative equipment is no more than three years.
Thus, modern water treatment technologies in the energy sector make it possible to reconstruct (upgrade) obsolete systems in existing premises, even without equipment dismantling. The result of introducing innovative technologies is energy saving, resource saving, environmental improvement, staff reduction, and the highest system reliability.