At present, most coal-fired generator sets in my country use limestone-gypsum wet desulfurization process. This process can reduce the flue gas temperature to 45-55°C. These low-temperature saturated wet flue gas directly enters the atmosphere through the chimney, and will condense. Formed into tiny droplets, resulting in "white plume". Although the pure "white plume" has no effect on environmental quality, it affects the perception of the environment, and is sometimes mistaken for toxic and harmful exhaust gas. Although the tiny water droplets formed by the condensation of wet smoke are not harmful, they will cause some trouble to the lives of the surrounding residents. The Environmental Protection Agency often receives similar complaints. Therefore, many coal-fired power plants regard the elimination of "white plume" as one of the important contents of ultra-low emission transformation. Let's briefly talk about the application of the new carbon fiber modified graphite tube heat exchanger in the process of plume de-whitening.

1. Introduction to the new carbon fiber modified graphite tube heat exchanger

1. Impermeable graphite is a new type of structural material made of artificial graphite and synthetic resin through impregnation, pressing, casting and other methods. The difference between it and all other non-metallic corrosion-resistant materials is:

(1) It has excellent thermal conductivity. Graphite has a high thermal conductivity, which is 5 times larger than that of copper and aluminum, and 2 times that of carbon steel. It is incomparable to other non-metallic materials (not as good as fluoroplastics, glass lined, etc.). It is used to make heat exchangers. Ideal material.

(2) It has excellent chemical stability and corrosion resistance. Except for a few strong oxidizing acids and alkalis, graphite is stable to most acids and alkalis.

(3) The linear expansion coefficient is small, and the linear expansion coefficient is generally in the range of (5~27)×10-6×1/℃, and the sensitivity to temperature changes is small. The equipment made with it can maintain the original at high temperature Shape and mechanical strength

(4) Pressed graphite (graphite tube) is not easy to foul, because the nature of graphite itself and the smooth inside and outside of the graphite tube determine that the "affinity" between the graphite tube and most media is extremely small, so dirt is not easy to adhere to the surface .

(5) The mechanical processing performance is good, and various mechanical processing can be performed except for rolling and forging.

(6) The density of graphite is small, and the equipment made is lighter than the corresponding metal equipment.

2. New carbon fiber modified graphite tube heat exchanger

The new type carbon fiber modified graphite tube heat exchanger is a national patent product independently developed by our company. Some high-strength carbon fiber filaments are added to the graphite raw materials. The carbon fiber modified graphite tube made of carbon fiber modified graphite tube maintains the traditional graphite tube heat exchange. In addition to the advantages of being good and not hanging on the wall, the impact resistance and seismic resistance of the stone pipe are greatly improved, and the safety performance of the application in the impact and vibration section is significantly enhanced. Its main features are as follows:

a. Both ends of the tube sheets are fixed tube sheets, and the tube sheet and the heat exchange tube are connected by a highly elastic seal (patented by our company) to realize the expansion and contraction of a single tube, avoiding the traditional products due to tube sheets, heat exchange tubes and Stress failure caused by different linear expansion coefficients of adhesives.

b. The scope of application is expanded. The operating pressure has been increased from 0.3MPa to 1.0MPa, and the operating temperature has been increased from 130°C to 180°C. In chemical production, as long as the pressure is not higher than 1.0MPa, and the temperature is not higher than 180℃, it can basically be used.

c. Patented replaceable tube technology, simple structure, convenient maintenance, single tube can be replaced, avoiding the scrapping of the equipment due to pipe blockage of traditional products, and greatly extending the service life of the equipment.

2. The formation mechanism of "white plume"

The direct emission of wet flue gas after wet desulfurization produces "white plumes" because in the desulfurization process, the desulfurization slurry directly contacts the high-temperature flue gas, causing heat and mass transfer. On the one hand, the moisture evaporates and increases the moisture content of the flue gas; on the other hand, the flue gas temperature decreases and the flue gas's ability to carry water vapor decreases. After the flue gas reaches saturation, it will carry some small droplets. After these saturated wet flue gas with small droplets are removed by the demister to remove most of the droplets, if they are directly discharged into the atmosphere through the chimney, since the ambient temperature is lower than the flue gas temperature, the moisture in the saturated wet flue gas will condense into The small droplets form a "white plume".

IMG_256

Figure 1 shows the change trend curve of humidity in the air with a relative humidity of 100% with temperature. Point A is the initial state of the emission of wet flue gas, points B and C are different ambient air conditions, AB and AC are respectively saturated The humidity curve intersects at points B1 and C1. Whether there will be white smoke and the length of white smoke after the wet smoke is discharged from the chimney mouth is not only related to the moisture content of the smoke itself, the temperature of the smoke (point A), but also the temperature of the surrounding air , Humidity (that is, the position of point B, C is set on the change of flue gas state on the temperature and humidity diagram in Figure 1). After the wet flue gas at point A is discharged from the chimney mouth, it changes along the saturation humidity curve to point B1. In this process, the moisture in the wet flue gas condenses into small droplets, that is, a "white plume" is formed. The point changes along a straight line to the environmental state point B, and the "white plume" disappears in the process. The process of changing from point A to point C, where the ambient temperature is lower, is similar, except that the length of the "white plume" is longer. Therefore, as long as the line connecting the initial state point of the wet smoke gas and the environmental state point intersects the saturated humidity curve, a "white plume" will be produced. And the lower the ambient temperature and the higher the humidity, the longer the "white plume".

Three, governance methods

1. Install a "Lulong brand" vertical tubular graphite heat exchanger (flue gas cooler) at the entrance of the flue gas desulfurization tower. Circulating water is used as cooling water between the tubes, and the high temperature flue gas Heat exchange, the high-temperature flue gas releases heat and cools, and then enters the desulfurization tower to carry out the desulfurization process. The low-temperature circulating water is heated to high-temperature water for the subsequent flue gas reheater to clean the flue gas after heating the tower.

2. After the flue gas is desulfurized, it enters a "Lulong brand" vertical tube-type graphite heat exchanger (called flue gas condenser), exchanges heat with the low-temperature water outside the tube, and then enters the flue gas reheater. Perform flue gas reheating process.

3. The low-temperature net flue gas that has been condensed enters the last "Lulong brand" vertical tubular graphite heat exchanger (flue gas reheater). The high temperature circulating water from the flue gas cooler in and between the graphite tubes is carried out. Heat exchange, the high-temperature water releases heat to cool down and then returns to the circulating pool pump, and enters the flue gas cooler under the drive of the pump to start the next cycle heat exchange process; the low-temperature net flue gas absorbs heat and rises up (>65°C) and then the chimney Empty.

3. Reasons for adopting new carbon fiber modified graphite tube heat exchanger

1. The new carbon fiber modified graphite tube heat exchanger has high thermal conductivity, good heat exchange effect and corrosion resistance.

2. The inner and outer walls of the graphite heat exchange tube are smooth, not easy to scale, and do not hang on the wall, which can effectively avoid the effect of heat exchange caused by the scaling of circulating water and the crystallization of salt substances contained in the flue gas;

3. Compared with the fluoroplastic capillary heat exchanger, the heat exchange efficiency is high. The graphite heat exchanger has a smaller area and lower price under the same conditions; and the smooth graphite tube has a large diameter, which is compared with the fluoroplastic capillary heat exchanger. , There is not much requirement for cooling water (fluoroplastic cooling water needs desalinated water, otherwise the cooling water will easily block the heat exchange tube due to scaling), so the overall cost of graphite heat exchanger is very low.

Fourth, the conclusion

In summary, although "white plume" has no impact on environmental quality, it affects the perception of the environment and needs to be managed. At present, the direct heating method is mainly used to eliminate the "white plume", which will increase the energy consumption of the unit. Through the process of condensation and then heating, not only part of the waste heat can be recovered to make up the energy consumption of the unit, but also some condensed water can be recovered for comprehensive utilization, which is an energy-saving "white plume" management mode. At the same time, the graphite heat exchanger The application in this process will become wider and wider, and the advantages of graphite heat exchangers, such as corrosion resistance, no pipe blocking, long service life, and comprehensive cost, will become more and more obvious, and will be accepted by more owners.