Urban sludge characteristics incineration and drying technology and design points
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1. Urban sludge characteristics
Indicators for characterizing sludge characteristics include moisture content and solid content, volatile solids, nutrients, toxic and hazardous substances, dehydration properties, and chemical kinetic properties.
(1) Moisture content and solid content. Urban sludge has a high water content and a low solid content. In general, the smaller the solid particles, the more organic matter they contain and the higher the moisture content of the sludge.
(2) Volatile solids. The volatile matter of the sludge is an organic compound of oxygen, sulfur, nitrogen and other elements and a mixture of saturated and unsaturated aromatic hydrocarbons. Volatile solids are commonly used to characterize the amount of organic matter in the sludge. The more volatile solids, the worse the sludge stability. The amount of volatiles precipitated in different types and different regions of the sludge is very different. Under normal circumstances, the precipitation ratio of industrial sewage sludge volatiles is greater than the precipitation ratio of domestic sewage sludge.
(3) Nutrients. Li Yanxia's statistical analysis of urban sludge nutrients in 29 sewage treatment plants in a Chinese city found that the average organic matter content of municipal sludge not including industrial sludge reached 384.0g/kg, total nitrogen, total phosphorus and total potassium, nitrogen and phosphorus, The total potassium nutrient content reached an average of 48.3 g/kg. It can be seen that the sludge is rich in nutrients and can be applied to the farmland as a fertilizer, but it should be noted that it pollutes the soil.
2. Sludge incineration method
Sludge incineration can be divided into direct incineration and dry incineration according to the process flow.
(1) Direct incineration
Direct incineration is the direct injection of mechanically dewatered sludge into an incinerator for incineration. This process is suitable for sludges with low moisture content and high calorific value. At present, direct incineration is used more abroad.
(2) Drying and burning
Drying incineration is to dry the sludge after mechanical dewatering, reduce the water content, improve the incineration efficiency, and eliminate the need to add auxiliary fuel or add a small amount of auxiliary fuel during the operation of the incinerator.
Chongqing has studied the calorific value of sludge in the urban sewage treatment plant of the main city. According to the experimental data, the highest moisture content of the sewage sludge in the region with self-sustaining combustion characteristics is 68%, and the sludge is directly incinerated. Auxiliary fuels are guaranteed to be fully incinerated and meet the requirements, and the operating costs are quite high [4-5]. At present, sludge in most parts of China generally has the characteristics of low calorific value and high water content. Therefore, it is more appropriate to adopt the treatment method of “first dry re-incinerationâ€.
(3) Sludge drying and incineration
Direct incineration of dewatered sludge is not only difficult but also extremely uneconomical. The calorific value of the dried sludge is close to the calorific value of lignite. The incineration of the dried sludge not only completely treats the sludge, kills the pathogen, minimizes the sludge volume, and recycles its heat energy. Fly ash produced after sludge incineration can be used to improve soil, build roads, and manufacture bricks and ceramics. More than 60% of the sludge in Japan is treated by dry incineration, and more than 10% of sludge in Europe is treated this way.
Sludge incineration mainly consists of separate incineration and mixed incineration. Sludge mixed incineration is a reasonable disposal method for blending dry sludge with fuel such as garbage, cement kiln fuel cabinet and power plant coal.
3. Municipal sludge drying technology
In the late 1980s, due to the unfavorable factors of dewatered sludge in agriculture, landfill and sea, and the successful application of sludge thermal drying technology in Europe and the United States, sludge drying technology is very popular in Western countries. Promote it quickly. Sludge drying is a deep dewatering operation on sludge after mechanical dewatering. The dried sludge can be used as a fertilizer, a soil conditioner, and an alternative energy source. The purpose of drying is to further dewater the sludge, from 80% water dehydration after mechanical dewatering to 10% to 50% water content. On the one hand, effective volume reduction is carried out, on the other hand, sludge storage and transportation are facilitated. use. The dried sludge has a relatively low water content and is relatively stable, and microorganisms and pathogens are also greatly reduced. The sludge drying technology mainly includes heat drying, solar drying, microwave heating drying, ultrasonic drying and heat pump drying. At present, the most widely used and most mature is the thermal drying technology. According to the contact mode of the heat medium and the sludge, the heat drying technology can be divided into three types of direct drying method, indirect drying method and direct indirect combined drying method.
(1) Direct drying method
Direct drying is the use of a combustion device to supply hot air and flue gas to the drying equipment. The sludge is in direct contact with hot air and flue gas, and the water in the sludge is evaporated under high temperature. The high heat transfer efficiency and evaporation rate of this technology can increase the solid content of sludge from 25% to 85%~95%. However, due to direct contact with the sludge, the heat medium will be polluted, and the discharged waste water and water vapor must be disposed of after being harmlessly treated. At the same time, the heat medium and the dry sludge need to be separated, which brings certain difficulties to operation and management. . Flash dryers, tumble dryers, belt dryers, and fluidized bed dryers are all direct drying types. Among them, the direct heating drum dryer is the most commonly used direct drying equipment.
(2) Indirect drying method
Corresponding to the direct drying method is the indirect drying method. The steam or hot oil supplied by the heating device first heats the container, and then transfers the heat to the sludge through the surface of the container to evaporate the water in the sludge. Indirect drying technology mainly includes disc drying, membrane drying, hollow paddle drying, and coating drying technology. The technology effectively avoids the plastic stage of the sludge, and the sludge organic matter is not easily damaged, and also has the advantages of factory operation, small land occupation, high degree of automation, and easy operation.
(3) Direct-indirect combined drying technology
Direct-indirect combined drying technology is the integration of convection and conduction technology, designed turbine thin layer dryers, Schwing's secondary drying system, new fluidized bed dryers developed, and belt dryers from Enviryix. Types.
The turbine thin layer drying process uses both heat conduction and heat convection. The effective heat convection accounts for about 40% of the total heat transfer, and the heat transfer accounts for more than 60%. The advantage is that the moisture content of the sludge after drying treatment is less than 5%, and the moisture content of the sludge after dewatering is only 20%~25% of the volume of 70%~80%, and the reduction rate of >70% can ensure the complete elimination of microorganisms and pathogens. And protect the plant nutrients and bio-energy in the sludge from running economics. In the process of energy consumption, the sludge drying process in the lower limit of European sludge drying energy consumption, no exhaust gas emissions, condensed water can also be recycled. Will not cause secondary pollution.
4. Design points
(1) The nature of the sludge is special. When the water content is in the range of 40% to 60%, the fluidity characteristics of the sludge will change significantly and suddenly become sticky, which is called the sludge viscous zone. The viscous sludge brings great inconvenience to the transportation. If the sludge is dry, this situation will also affect the normal operation of the dryer. Therefore, sludge viscous zone should be considered during sludge drying.
(2) The calorific value and solid content of sludge are important factors affecting the thermal energy balance of the incineration system. The reduction in solids ratio directly leads to an increase in the amount of sludge. With 1 t of dry sludge, when the solid content decreased from 25% to 20%, the solid content decreased by only 5%, but the wet sludge volume increased by 25%. This will undoubtedly increase the load on the dryer and affect the heat balance of the entire system.
(3) The SiO 2 content in the sludge is particularly important for the wear of the drying system and the incineration system equipment. Sludge with a high sand content will greatly increase the degree of wear of the equipment in the fluidized state. Therefore, improving the efficiency of the grit chamber in the previous process of dewatering sludge, reducing the sand content of the dewatered sludge from the source, can fundamentally solve the wear problem of the equipment. The use of wear resistant materials or wear resistant coatings to increase the wear resistance of the equipment can also reduce the incidence of the above problems.
5 Conclusion
(1) Under the influence of the sustainable development strategy universally advocated, sludge is attracting more and more attention as a carrier of recyclable resources and energy.
(2) Urban dewatered sludge has the following characteristics: water content is above, not easy to dehydrate, high volatile content is rich in plant nutrients, and contains a large number of pathogens, heavy metals and toxic organic substances and other toxic and hazardous substances have a certain calorific value, can be It is used for heat recovery.
(3) Contact mode of heat medium and sludge The heat drying technology can be divided into three types of direct drying method, indirect drying method and direct indirect combined drying method. Direct heating drum dryer is the most commonly used direct drying equipment, which reduces the burden of tail gas treatment and improves system safety. Indirect multi-disc drying technology is a commonly used indirect drying equipment, the safety of the drying process. High, exhaust can meet very strict emission standards Turbine thin layer drying process is the main direct-indirect combined drying technology, using a combination of convection and heat transfer, making the process energy consumption is very low, and the operation is very economical.
(5) The terminal treatment methods after sludge drying include land use, sludge farming, landfill, energy utilization and comprehensive utilization. Among them, land use and energy utilization are considered to be the development trend of sludge treatment in the future. With the emergence of new technologies such as sludge low temperature heat treatment, sludge gasification, sludge coal water slurry, etc., the terminal treatment method of sludge is gradually developing towards diversification, safety and economy.