With the continuous expansion of domestic thermal power units, the parameters and capacity of the units are constantly improving, and the chemical water treatment in power plants has undergone profound changes. Chemical water treatment in power plants has undergone profound changes in technology selection, equipment layout, process flow, control and monitoring, operation and maintenance, and production management.

1. Boiler feedwater treatment

The traditional pre-treatment of boiler feed water usually adopts coagulation and filtration treatment. The clarification treatment equipment for large domestic thermal power plants is mostly mechanical accelerated stirring clarification tanks, which have the advantages of fast reaction speed, convenient operation and control, and large output. In recent years, frequency conversion technology has been continuously applied to coagulation treatment, further improving the quality of pre treated effluent and reducing manual operations. In terms of the development of filters, the filtration technology using granular materials as filter media has gone through stages such as slow filtration, fast filtration, and multi-layer filter media, and has played a certain role in improving the pre-treatment water quality. However, due to the limitations of granular materials, the effluent quality, interception capacity, and filtration speed of filtration equipment are greatly limited. At present, new types of filtration equipment that replace granular materials with fiber materials are constantly emerging. Fiber filtration materials, due to their small size, large surface area, and soft material characteristics, have strong interface adsorption, pollution interception, and water flow regulation capabilities. Representative products include fiber ball filters, capsule extrusion fiber filters, pressure plate fiber filters, etc.

In terms of pre desalination treatment technology for boiler feedwater, the development of reverse osmosis technology (RO) has become a highlight. The characteristic of reverse osmosis is that it is not affected by changes in raw water quality. Reverse osmosis has a strong ability to remove silicon, and the COD removal rate can reach 83%, meeting the strict requirements of large units for silicon content. Reverse osmosis reduces the desalination burden of the ion exchange system in the next process by removing most of the ions in the water (usually around 90%), thereby reducing the discharge of acid and alkali waste liquids, lowering the salt content of discharged wastewater, and improving the economic and environmental benefits of power plants.

In the desalination treatment of boiler feed water, mixed beds still play an irreplaceable role, and the development of mixed beds itself is mainly reflected in two aspects: environmental protection and energy conservation. Filling bed electrodialysis (CDI) (EDI) is a precision desalination process that combines electrodialysis and ion exchange desalination technology. The regeneration of resin is completed by H+and OH - ionized by H2O, that is, the H+and OH - ionized in a DC electric field directly act as regenerants for the resin, without the need to consume acid and alkali agents. At the same time, the device has a strong ability to remove weak ions such as SO2 and CO2.

2. Boiler feedwater treatment

At present, the volatile treatment of boiler feedwater using ammonia and hydrazine is relatively mature, but it is more suitable for newly built units. After the water quality stabilizes, it can be converted to neutral treatment and combined treatment. Oxygenation treatment has changed the traditional deaerator and deaerator treatment, creating an oxidation-reduction atmosphere that can generate a protective film and cause corrosion at low temperatures. This method can also reduce the corrosion production of the water supply system, reduce drug usage, extend chemical cleaning intervals, and lower operating costs. The application of oxidative water chemistry operation mode is relatively popular in Europe, and it is basically in the research and trial stage in China. It must be emphasized that the operation mode of oxidizing water chemistry is only applicable to high-purity feedwater, and attention should be paid to the compatibility of system materials with it.

3. Boiler water treatment

The technology of in furnace phosphate treatment has a history of more than 70 years, and now 65% of drum boilers worldwide have used boiler water phosphate treatment. Due to the low parameters of previous boilers and outdated water treatment processes, a large amount of calcium and magnesium ions often appeared in the boiler water. To prevent boiler scaling, a large amount of phosphate had to be added to the boiler to remove the hardness of the water. As a result, the pH value of the boiler water was very high, and the problem of alkaline corrosion became particularly prominent. In this situation, coordinated phosphate treatment emerged and achieved certain anti-corrosion effects. But with the continuous improvement of boiler parameters, the "hidden" phenomenon of phosphates is becoming more and more serious, and the resulting acidic corrosion is also increasing. On the other hand, the boiler feedwater system of high parameter units has all adopted secondary desalination, and the condensate system is equipped with a precision treatment device. In this way, there is basically no hardness component in the furnace water, and the main function of phosphate treatment has shifted from removing hardness to adjusting pH value for corrosion prevention. Therefore, in the past decade, people have proposed low phosphate treatment and balanced phosphate treatment. The lower limit of low phosphate treatment is controlled within 0.3-0.5mg/L, and the upper limit generally does not exceed 2-3mg/L. The basic principle of balanced phosphate treatment is to reduce the content of phosphate in the furnace water to a low concentration that is only sufficient to react with the hardness components, while allowing for less than 1mg/L of free NaOH in the furnace water to ensure that the pH value of the furnace water is within the range of 9.0-9.6.

4. Condensed water treatment

At present, the vast majority of high parameter units with a capacity of 300MW and above are equipped with condensate polishing devices, mainly imported. The mainstream products of their regeneration systems are high tower separation devices and cone bottom separation devices. However, there are not many precision treatment devices that can truly achieve long-term ammonification operation, only a few such as Xiamen Songyu Power Plant. The operation cycle of the mixed bed in Songyu Power Plant is over 100 days, with a cycle water production capacity of over 500000 tons. From the perspective of environmental protection and economy, achieving ammonia operation will be the development direction of precision treatment systems in the future. In addition, in terms of equipment investment, equipment layout, and process optimization, consideration should be given to making the most of the existing public systems in the power plant, such as reducing the use of fans for resin regeneration and recirculation pumps for mixed beds. The system's programmable control and regeneration devices should be installed on the boiler feedwater side as much as possible to achieve centralized management.

On the other hand, powder resin (POWDEX) precision treatment systems with dual functions of filtration and desalination have gradually been applied, such as power plants in Fuzhou Huaneng Phase II and Nantong Huaneng Phase II. However, due to the high price of powder resin, which mainly relies on imports, the promotion and application of powder resin precision treatment equipment are limited to a certain extent.

5. Burial of circulating water

For thermal power plants that adopt closed loop cooling, the recycling of cooling water and the development of water quality stabilization technologies are the focus of water treatment work. The concentration rate of circulating water in developed countries has reached 6-8 times, while the concentration rate of circulating water in most domestic power plants is around 2-3 times. Domestic thermal power plants should make efforts to improve the efficiency of circulating water reuse. In order to avoid secondary pollution of environmental water by phosphorus based water treatment agents, low phosphorus and non phosphorus based formulations of scale inhibitors and dispersants, as well as multi-component copolymer water treatment agents, are gradually being applied. Thermal power plants that use open discharge cooling, especially coastal power plants that use seawater as cooling water, generally use chlorination treatment for cooling water. The common device used is a product from CapitalControl in the United States. However, some power plants also use electrolysis of seawater to produce sodium hypochlorite as a biocide. Such as Zhangzhou Houshi Power Plant, Beilun Port Power Plant, etc.

6. Wastewater treatment

At present, the layout of industrial wastewater treatment in large power plants in China basically follows the wastewater treatment mode of Baosteel Power Plant, which adopts a centralized collection and step-by-step treatment of wastewater. Generally, processes such as blast aeration oxidation, pH adjustment, coagulation and clarification, and sludge concentration treatment are mainly adopted. But the disadvantage of this treatment method is that it is difficult to treat incoming water with complex water quality and a large range of changes, and it affects the comprehensive recovery and utilization of wastewater. In recent years, the two-phase flow solid-liquid separation technology has gradually been applied. This technology uses a one-time dosing coagulation process to complete flocculation, sedimentation, clarification, scum removal, and sludge concentration in a combined facility. It separates sediment, suspended solids, algal suspended solids, and oil from the water in the same facility. This treatment technology improves the effluent quality, reduces treatment costs, and expands the scope of reuse.