In recent years, China's power industry has developed by leaps and bounds, and chemical cleaning work has been carried out on power station boilers according to the requirements of the Ministry of Power Industry. Chemical cleaning not only enables the boiler to operate safely, saves fuel, improves the metal strength of the heat transfer surface, but also effectively increases the evaporation rate of the boiler and prolongs the service life of the boiler. It has great practical significance for saving energy and improving economic benefits. At present, some power station companies fail to select the correct chemical cleaning method in the process of boiler pickling and passivation, which causes boiler scale and corrosion, which causes the metal to swell and change its metallographic structure and reduce the strength, thus causing the boiler to overheat. And the tube burst, causing economic losses. Therefore, in order to ensure good water vapor quality in boiler operation and avoid scaling corrosion of the furnace tube, boiler cleaning and passivation are essential.
Guangdong Yudean Jinghai Power Generation Co., Ltd. has two 600MW coal-fired generating units, which were put into operation on February 18, 2007 and June 28, 2007 respectively. The boilers are supercritical parameters, variable pressure DC furnaces, and hedging combustion methods. , solid waste slag, single furnace, one reheat, balanced ventilation, open air layout, all steel frame, full suspension structure 烟 type bituminous coal boiler.
According to the DL/T794-2001 guidelines for chemical cleaning of boilers in thermal power plants, our company has used chemical cleaning of the unit with hydrogen peroxide and EDTA sodium salt before the installation is completed. From the effect of the EDTA cleaning process, EDTA chemical cleaning has obvious advantages compared with the commonly used pickling process such as hydrochloric acid.
The system is simple, the number of temporary pipelines is small, and the entire program time is short.
The waste liquid can be recycled, and the amine salt can be recrystallized and reused to avoid environmental pollution and save money.
A suitable corrosion inhibitor is added to the EDTA solution, which is extremely corrosive to metals. At the same time, pickling and passivation are in place. The detached iron oxide forms a solution with the cleaning liquid, which reduces the phenomenon of cleaning and plugging after the afterglow, and produces a good process effect.
The water consumption is about half that of hydrochloric acid pickling.
(3) Since the EDTA waste liquid can be recovered, the recovery rate can reach more than 90%, so that the cost is only about 60% of hydrochloric acid pickling.
At present, the main components of the deposits in the thermal power unit boiler using demineralized water as supplementary water are iron compounds such as Fe34, Fe23 and FeO, and may also contain small amounts of copper, calcium and magnesium compounds. In the aqueous solution, the iron oxide has the following hydrolysis equilibrium: the size of the constant, it can be determined whether a stable complex can be formed under such conditions, that is, whether the complex descaling can be achieved under the pH condition. Commonly used ions in the furnace, such as Ca'Mg'Fe'Fe'C, the stable pH value of the complex is 7.010.5. EDTA has a strong ability to clean the sediment under the condition of low pH. During the cleaning process, when the pH of the cleaning solution is greater than 8, the cleaning ability of the cleaning liquid is significantly reduced, and the cleaning is completed into the passivation pH range, so the EDTA cleaning can achieve the descaling passivation step by step.
1 Brief description of cleaning process According to the requirements of the chemical cleaning guidelines for boilers in thermal power plants (DL/T794-2001), combined with the actual situation of the unit, the cleaning process is divided into two parts: alkaline washing and pickling. This chemical cleaning alkaline washing adopts the most environmentally-friendly degreasing cleaning process: hydrogen peroxide cleaning and degreasing; acid washing adopts EDTA sodium salt cleaning process, and EDTA cleaning waste liquid treatment adopts the method of first recycling and comprehensive treatment.
2 cleaning range, cleaning process parameters 2.1 alkaline washing (H202 degreasing cleaning) cleaning range condenser steam side, chemical finishing bypass, shaft seal heater and bypass, 85 low water side and bypass, 1/2 Oxygen water tank, high and medium pressure water supply pipeline, condensate water pipe, 31 high water side and bypass, 56 low steam side, 31 high steam side, economizer, spiral water wall, vertical water wall, start separator, water storage tank Wait.
2.2 EDTA cleaning range deaerator tank, high and medium pressure water supply pipeline, 31 high water side and bypass, condensate pipeline, shaft seal heater and bypass, 85 low water side and bypass, economizer, spiral water wall, vertical Water wall, starter separator, water storage tank, etc.
First circuit: deaerator water tank - water supply front pump - temporary pipeline - shaft seal heater and bypass - 85 low plus bypass - deaerator water tank; second circuit: deaerator water tank - water supply front pump ―31 high-pressure heater and bypass-water supply console-economizer-spiral water wall-vertical water-cooling wall-starting separator-storage tank-temporary pipeline-deaerator tank.
2.3 Chemical cleaning process parameters The chemical cleaning process parameters are shown in Table 1.
Table 1 chemical cleaning process parameters cleaning steps process requirements test items and end indicators analysis interval water rinse water clear clarification export water quality transparent clarification continuous test alkali wash HO2:. 05% 0.1% temperature: normal temperature cycle cleaning: h immersion: about 1012hH2O2 concentration per hour test water rinse demineralized rinse outlet water quality transparent clarification detection H2O2 concentration near the end point continuation total iron concentration end EDTA residual concentration > 0.5% The acidity, pH value, temperature and total iron concentration are measured once every 60 times during cleaning. This cleaning is to use a certain concentration of sodium salt solution of ethylenediaminetetraacetic acid (EDTA) as the cleaning solution to dissolve the metal surface by the complexation of EDTA. Sediment. EDTA has different forms under different pH conditions: under appropriate conditions, the EDTA complexing element Y4- is complexed with metal ions to promote the dissolution of the deposit. The iron oxide releases a corresponding amount of OH- during the complexation and dissolution process (such as Fe3O4+4H2O-2Fe3++FeM+8OH), and the pH automatically increases as the sediment dissolves. According to the complex formed by metal ions and EDTA at a certain pH value, DL/T561 thermal power plant water vapor chemical supervision guide DL/T794-2001 thermal power plant boiler chemical cleaning guide DL/T889-2004 electric power infrastructure construction heat Equipment chemical supervision guidelines 3 cleaning process 3.1 alkali washing before washing the temporary system, condenser steam side and low steam side, the furnace low pressure system for water washing; for the condenser, low water side and high and low steam side After alkali washing and alkali washing, the water is rinsed. After the hydrogen peroxide cleaning liquid is drained, the condenser is replenished to a high water level, and the condenser and the low-steam side water washing step are flushed until the effluent hydrogen peroxide concentration is about 0, and the outlet water quality is transparent and clear; The water side of the high water side and the furnace body system are washed with alkali and water after alkali washing.
Preparation of cleaning EDTA cleaning mother liquid According to the volume of the cleaning system and the small test, calculate the amount of EDTA required, reducing agent, corrosion inhibitor and other dosage, and prepare 12% 18% EDTA cleaning mother liquid, the pH value is 5.56.0. (1) Add EDTA cleaning mother liquid to the cleaning system, replenish water to the water level of the deaerator water tank 1/3 water level, water tank 0 water level, heat up to 120130 °C according to the heating test loop heating, and maintain the temperature for 1012h. The oxygen tank water level is maintained at slightly above 2/5 and the storage tank is below 2/3 water level. When the residual concentration of EDTA and the concentration of iron ions tend to stabilize, remove the monitor tube and observe whether the rust has been thoroughly cleaned. After the monitoring tube is cleaned, adjust the pH of the cleaning solution to 8.59.5 to enter the passivation stage, recirculate for 68 hours, and EDTA cleaning ends. After the cleaning is completed, continue to circulate. After the system temperature is lower than 100 °C, discharge the EDTA cleaning solution.
During the heating process, the temperature of the economizer inlet and the infrared thermometer are used to monitor the temperature of the inlet and outlet of the heater and the temporary pipeline of the water storage tank to the deaerator tank, and the measurement is performed once in 30 minutes. When it is found that the temperature difference between the high and the high inlet and outlet is greater than 30 ° C or the heating rate is slower, reduce the 2 high steam and reduce the mixing and heating of the oxygen tank, thereby reducing the temperature difference between the high and the inlet and the inlet, and improving the overall system. temperature.
During the cleaning process of each circuit, the EDTA effective concentration, total iron concentration, pH value and temperature parameters are strictly monitored. The above items were analyzed once every 60 minutes; when the EDTA effective concentration, total iron ion concentration, pH value, temperature and time were all stabilized, it was determined that the cleaning was near the end point.
EDTA waste treatment After the EDTA cleaning is completed, continue to circulate, open the boiler all the holes and the flue wind deflector (if the conditions are available, start the fan), quickly reduce the system temperature to below 100 °C, stop the water supply front pump The EDTA cleaning solution in the cleaning system is quickly returned to the storage tank, and the time is controlled within 2 hours.
EDTA recovery: Concentrated sulfuric acid (pH control at 0.81.2) was added to the EDTA cleaning solution to cause precipitation of the EDTA by acid action, and the mixture was uniformly stirred with compressed air and allowed to stand for 12 days. The upper clarified liquid is checked to determine that the EDTA concentration meets the discharge standard and is discharged to the industrial wastewater system for treatment. The EDTA crystals in the tank were rinsed with demineralized water and dried in bags.
4 chemical cleaning results The cleaned metal surface is clean, basically no residual oxide and welding slag, no secondary floating rust, no pitting, no over-washing phenomenon of obvious metal coarse crystal precipitation, no copper plating, metal surface Forming a complete passivation film; the average corrosion rate of the corrosion indicator sheet is less than 8g/(m2h), the total amount of corrosion is less than that of the open deaerator water tank, and there is no residue in the internal cleaning; the condenser hot well is opened, and there is no residue in the internal cleaning; after cleaning Inspection of the water wall and other parts showed that the metal surface was clean, smooth and non-copper-plated, showing a gray passivation film. The corrosion rates of No. 20 steel and BHW-35 steel were 7.35g (m2h) and 7.5g (m2h), respectively. And passivation effect is good.
5 Conclusion This cleaning has better removed iron oxide, copper scale and calcium and magnesium scale deposits, and the corrosion to metal is also very small. After cleaning, the metal surface has a good anti-corrosion protective film, no need to passivate separately. The treatment and cleaning of the temporary device is relatively simple, the cleaned waste liquid is extracted after purifying the pure EDTA and can be directly discharged with water, but the price of such a cleaning agent is relatively high.
Direction: Power plant chemistry.
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