Different treatment methods for boiler water
Different treatment methods for boiler water
1、 Supply water treatment
1. Preprocessing
If the source water is surface water, the purpose of pretreatment is to remove suspended solids, colloidal substances, and organic matter from the water. Usually, by adding coagulants (such as aluminum sulfate) to the raw water, the impurities mentioned above are flocculated into large particles, which settle by their own weight and are filtered to produce clean water.
When using groundwater or urban water as supplementary water, the pre-treatment of raw water can be omitted and only filtration can be performed. Common clarifiers include pulse, hydraulic acceleration, and mechanical stirring clarifiers, while filtration devices include siphon filters, valve free filters, and single or double flow mechanical filters. Activated carbon filters can also be added to further remove organic matter from water.
2. Softening
Use natural or artificial ion exchangers to convert calcium and magnesium hard salts into non hard scale forming salts, preventing the formation of calcium and magnesium hard scale on the inner wall of boiler tubes. Strong alkaline water containing calcium bicarbonate or magnesium bicarbonate can also be solved by sodium hydrogen ion exchange or pretreatment (such as adding lime). In some industrial boilers, the salinity of the feedwater may not be significantly reduced, but this treatment is usually sufficient.
3. Desalination of seawater
With the continuous improvement of boiler parameters and the emergence of once through boilers, it is even necessary to remove all the salt in the boiler feedwater. At this point, the method of dilution should be adopted. There are many types of ion exchangers used in chemical desalination, but the most commonly used are cation exchange resin and anion exchange resin, also known as “cation resin” and “anion exchange resin”. In an ion exchanger, when saltwater passes through a resin, the cations and anions of the salt are removed by exchanging with the cations (H+) and anions (OH -) of the resin, respectively.
If the alkalinity of water is high, it is usually necessary to add a series of decarbonizers after the cation exchanger to remove carbon dioxide, in order to reduce the burden on the anion exchanger and improve the economic efficiency of the system operation. For particularly salty water, reverse osmosis or electrodialysis can also be used to desalinate the water first, and then put it into an ion exchanger for deep desalination. For high-pressure or higher pressure steam drum boilers and once through boilers, trace amounts of silicon in the feedwater need to be removed. For medium and low-pressure boilers, treatment needs to be carried out according to the content.
2、 Condensate water treatment
During the circulation of condensate water, leakage of cooling water from the turbine condenser and system corrosion products may contaminate and require treatment. The amount of condensate processed is related to boiler parameters, furnace type (without steam drum or separator), and condensate pollution. As the boiler parameters increase, the condensate water treatment capacity generally gradually increases. All supercritical boilers should be treated; For ultra-high pressure and subcritical boilers, the capacity is 25-100%; Steam drum boilers below high pressure generally do not require treatment. The commonly used condensate water treatment equipment includes cellulose coated filters and electromagnetic filters. After removing corrosion products such as copper and iron oxides from condensed water, it enters a mixed bed or a filter covered with powder resin for deep desalination.
3、 Boiler feedwater treatment
The dissolved oxygen in boiler feedwater can corrode the metals in the thermal system, and the corrosion products form copper and iron scales under high heat load in the boiler. It conducts heat in the high-pressure cylinder of the steam turbine, causing pipeline rupture and deposits, thereby reducing the efficiency of the steam turbine. Therefore, the softened or demineralized supplementary water and condensate water are generally deoxygenated before entering the boiler. The commonly used deoxidation methods include thermal deoxidation and vacuum deoxidation, sometimes supplemented by chemical deoxidation. The so-called hot degassing refers to the process where the solubility of gas in water decreases when the raw water is heated to boiling in the deaerator, and the gas escapes from the water and is released into the atmosphere. According to the working pressure, the most widely used thermal deaerators are 0.12 MPa and 0.6 MPa. When performing thermal deoxygenation, the water should be heated to saturation temperature, and the surface area of the deoxygenated water should be large (for example, using a spray or atomization diffusion device). This allows for the rapid release of escaping gases Vacuum degassing is usually carried out in the condenser of a steam turbine. Chemical deoxygenation is the addition of hydrazine or sodium sulfite to water to further reduce the oxygen content in the water.