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Sludge is the product of sewage treatment. The main characteristics of sludge are high water content (up to 99% or more), high organic content, easy to decompose and smell, which requires sludge drying treatment. The current sludge treatment process Among them, the drying method of sludge treatment still ranks first. Today, the editor summarized some technical solutions for sludge drying for your reference.
1. What does the product temperature mean in the drying process?
Sludge is a kind of ultra-fine powder with high organic content. The purpose of sludge drying is first to reduce and sanitize. Regardless of the final disposal method, sludge drying itself does not change the properties of the sludge, that is, temperature does not cause degradation of the sludge product or quality problems. In view of this, whether from the perspective of the quality of the sludge product or the efficiency of the dryer, the higher the temperature, the better. However, due to safety issues, most drying processes tend to reduce the temperature of the product as much as possible, that is, reduce the ignition energy of the so-called dust explosion.
However, according to research, the ignition energy of sludge dust is very low. When the concentration of oxygen and dust reaches a certain level, the ignition energy is as low as a few to a dozen millijoules at a temperature of about 100 degrees. When the ignition energy reaches 1 joule, 70-80 degrees is enough to form a combustion. When the dust concentration is higher, even the environment of 20-30 degrees may be at risk. The spontaneous combustion and explosion of many silos belong to this situation. In order to ensure a certain treatment efficiency in the drying process, the temperature must exist, and it cannot be very low, the typical value is between 105-125 degrees. The safety of the process can only start from reducing the dust concentration and suppressing the combustion atmosphere. It is incorrect to simply rely on lowering the product temperature to ensure safety.
2. Why do we need to analyze the sludge composition for drying?
Based on experience, a certain analysis of the sludge composition is necessary to determine the drying process, obtain the best design parameters, and confirm the working conditions.
The wet mud testing content related to the drying process includes: moisture content, viscosity, fat content, acid-base corrosivity, sand content, etc.
Dry sludge testing related to the final disposal of sludge includes: heavy metal content, organic matter content, calorific value, bacterial content, etc.
3. Why is it said that sludge drying is the first step in resource utilization?
Regardless of whether sludge comes from industry or municipality, a feasible goal of its treatment is to return all pollutants from industry as raw materials to the process. In fact, all pollutants are raw materials lost in the intermediate process. In most cases, the medium that causes the loss is water. Removing water will allow a large number of potential pollutants to be reused.
The pollutants contained in sludge generally have a high calorific value, but due to the existence of a large amount of water, this part of the calorific value cannot be used. If the sludge with high moisture content is incinerated, not only the calorific value cannot be obtained, but also a large amount of supplementary fuel is needed to complete the combustion.
If the moisture content of the sludge is reduced to a certain level, combustion is possible, and the heat obtained from the combustion can meet the needs of part or all of the drying.
For the same reason, whether in the manufacture of building materials or the use of legends, reducing moisture content is the key. Therefore, it can be said that sludge drying or semi-drying is actually the first step in the utilization of sludge resources.
4. What is the solid recovery rate of the cyclone separator?
In many thermal convection systems, all or part of the products must be collected by cyclone for sludge drying. Due to the different air volume and pressure of each process, the particle size and proportion of the particles recovered by this method are different, resulting in the design The difference is huge. Generally speaking, the solid recovery rate of the cyclone separator is between 95-98%. The higher the solid content, the smaller the particle size of the product, and the difficulty of capturing will increase.
5. What are the necessary process steps involved in drying?
The purpose of sludge drying is to remove part of the moisture in the wet sludge to adapt to different disposal requirements.
Drying means that a certain amount of heat energy is transferred to the moisture contained in the material in a unit of time. The moisture is vaporized after being heated and separated from the material. The material that has lost moisture and the vaporized moisture are collected separately. This is The process of drying.
Describe this process from the perspective of equipment, including feeding, drying, gas-solid separation, dust trapping, moisture condensation, solid transportation and storage, etc.
If the nature of the material (viscosity, moisture content, etc.) may cause the instability of the drying process (such as adhesion, agglomeration, etc.), it is necessary to adopt a process in which part of the dried product is mixed with the wet material (return, Back mixing of dry mud). At this time, before loading and after solid transportation, equipment for transportation, storage, separation, crushing, screening, lifting, mixing, and loading should be added accordingly. 6. Why is it necessary to distinguish between indirect and direct heating methods for drying?
The division of direct and indirect heating methods lies in the difference in the form of heat source utilization, specifically, whether it is directly used as a medium or indirectly heats the heat exchange medium.
Drying is accomplished by heat, which is generally produced by burning energy. The heat generated by combustion exists in the flue gas. There are two types of utilization of this part of the heat:
(1) Direct utilization: Directly introduce high-temperature flue gas into the dryer, and exchange heat through the contact and convection of the gas and wet materials. This approach is characterized by high heat utilization efficiency, but if the dried material has pollutant properties, it will also cause emission problems. Because the entry of high-temperature flue gas is continuous, it also causes the same flow rate and The waste gas that has been in direct contact with the material must be discharged after special treatment.
(2) Indirect utilization: the heat of high-temperature flue gas is transferred to a certain medium through a heat exchanger, which may be heat transfer oil, steam or air. The medium circulates in a closed loop and has no contact with the dried material. The flue gas after the heat is partially used is discharged normally. There is a certain heat loss in indirect utilization.
For the drying process, direct or indirect heating has different thermal efficiency losses and different environmental impacts. It is an important content of project environmental assessment and economic investigation.