In this paper, the relationship between pollution discharge and energy saving is illustrated by charts from three aspects: pollution discharge scale, pollution discharge rate calculation and waste heat utilization. The purpose is to regard sewage discharge as an important content of boiler water quality management and attract the attention of relevant operation managers.

Keywords: waste heat utilization and energy saving. 1. Overview: 1. Pollution discharge is divided into continuous discharge and regular discharge. Continuous blowdown, also known as surface blowdown, is the continuous discharge of boiler water with high concentration of salt (about equal to dissolved solids) near the water surface in the boiler drum, so that the alkalinity and dissolved solids of boiler water meet the requirements of boiler water quality standards; Periodic blowdown, also called intermittent blowdown, regularly discharges suspended solids, water slag and other sediments from the lowest point of boiler water circulation system (bottom of boiler drum and lower header). Proper blowdown is an effective measure to keep the boiler water quality good, reduce boiler scaling and prevent metal corrosion and steam pollution. But improper sewage discharge and unreasonable operation may damage valves and pipelines, increase sewage discharge and waste fuel. In severe cases, it will form scale corrosion, which will affect heat transfer, reduce the strength of pressure components or cause serious water shortage in the boiler, endangering the safe operation of the boiler. Therefore, boiler blowdown is of great significance, and designers, installers, operators and managers should pay special attention to it to reduce blowdown losses and save energy. Second, scale and energy saving People know that the thermal conductivity of scale is about 1.2w/m? 6? At 65438 0℃, the thermal conductivity of steel plate is about 48w/m? 6? At 1℃, the thermal resistance of scale with a thickness of 1mm is equivalent to that of a steel plate with a thickness of 40 mm Scale not only affects the heat transfer, but also reduces it by half. The calculated wall temperature of metal increases obviously with the increase of scale (when q= 175× 103w/m3, it is/. Therefore, the scale will make the lower part of boiler drum with high heat load overheat and bulge, and the water wall tube will deform and burst, and it will also form corrosion under the scale, which will reduce the strength of pressure components and seriously affect the safe operation of boiler. At the same time, due to the influence of scale on heat transfer, a lot of fuel will be wasted. For example, the scale thickness and fuel loss are shown in the following table: scale thickness (mm)0.5 123456 fuel loss (%)1.12.244.76.36.88.2 For 4t/h boilers, secondary bituminous coal is used. If the scale with a thickness of 1.5mm is accumulated, 740 kg/h×3%=22.2kg of fuel will be wasted per hour, and 6000×22.2= 133.2 tons of coal will be wasted after running for 6000 hours throughout the year. Third, the calculation of sewage discharge rate is directly related to the quality of water supply. The greater the alkalinity and salt content of feed water, the more sewage the boiler needs. The index of boiler blowdown is expressed by blowdown rate, that is, the percentage of blowdown water (Q pollution) to boiler evaporation (Q steam). It is expressed by the following formula: K= Q sewage /Q steam × 100% When the water quality of the boiler is stable, according to the mass balance relationship, the amount of a substance brought into the boiler with feed water is equal to the sum of the amount discharged by sewage and the amount taken away by saturated steam. Then (Q sewage +Q steam) ×S supply =Q steam ×S steam +Q sewage ×S sewage type, where S supply, S steam and S sewage respectively represent the content of a substance in water supply, saturated steam and sewage, and the S value in the formula can be calculated according to the salt content or the content of a component (such as alkalinity and chloride ion). Then K= Q pollution /Q steam =(S to-S steam) /(S pollution -S to) × 100% Because it is convenient to determine chloride ions and there is a fixed proportional relationship between chloride ions and salt content, chloride ions are usually used instead of salt content to calculate boiler discharge rate. Then: K= (S gives -S steam) /(S sewage -S gives) =(Cl- gives -Cl- steam) /(Cl- sewage -Cl- gives) where Cl- gives, Cl- sewage and Cl- steam respectively represent the chloride ion content in feed water, sewage and saturated steam, and sewage is boiler water. In the formula, S boiler water and Cl- boiler water respectively represent the contents of substances and chloride ions in boiler water. For large-capacity boilers, the steam-water separation device has good effect and low steam humidity. In this way, the salt content in saturated steam is much lower than that in feed water, so the salt content in steam can be ignored when calculating the discharge rate of this kind of boiler, that is, K=S feed water /(S pollution -S feed water) = Cl feed water /(Cl pollution -Cl feed water) × 100%. For most industrial boilers, especially the drum volume is small, the steam-water separation device is simple, and the steam humidity is usually 3%. In this case, the salt content in the steam cannot be ignored when calculating the boiler discharge rate. Because k =(Cl- give -Cl- steam) /(Cl- pollution -Cl- give) = Cl- give /(Cl- pollution -Cl- give) -Cl- steam /(Cl- pollution -Cl- give) < Cl- give /(Cl- pollution -Cl- give). It can be seen that if the salt content in steam is ignored, the calculated sewage discharge rate will be larger (the difference is greater than the steam humidity). For every increase of 1% in the discharge rate of industrial boilers, the fuel consumption will increase by 0.3%. The fuel consumption of a 4t/h boiler burning secondary bituminous coal is about 740kg/h and the steam humidity is 3%. If the salt content in the steam is ignored in the calculation of the sewage discharge rate, the calculated value of the sewage discharge rate will increase by at least 3% (humidity value), the fuel consumption will increase by 0.9% correspondingly, and the wasted fuel will be 740kg/h×0.9% = 6.66kg/ hour.