Drying Technology
The goal of sludge treatment is to prevent the organic matter, various pathogens and other harmful substances contained in the sludge from forming secondary pollution sources, causing environmental pollution and endangering human health.
In order to ensure that the toxic and harmful substances in the sludge will not cause harm to humans and the environment, developed countries have adopted a series of sludge disposal methods, including landfilling, agricultural use and incineration, of which incineration is the most prominent. Sludge drying and incineration can not only make full use of the heat in the sludge to reduce the volume of the sludge to the greatest extent, but also kill all germs and pathogens during the incineration process, and at the same time the toxic and harmful organic residues can be oxidized and decomposed in this way. Therefore, it is an ideal method to treat sludge through incineration. Sludge drying and incineration way is more common in Japan, Europe and the United States.
Sludge Drying and Incineration
The development of urbanization makes the contradiction between population and land resources gradually emerge. In the past, the burying of sludge seemed to reduce the volume of sludge, but in fact the toxic substances in the sludge caused secondary pollution to the land, and the land used for burying the sludge could no longer be used for agricultural product planting and house construction and so on. Therefore, the sludge burial treatment method has exacerbated the contradiction between population and land resources to a certain extent.
The advantages of dry incineration of sludge compared with landfill treatment are obvious, which are mainly reflected in:
①reduce the area of sludge;
②incineration can decompose some non-degradable organic matter in sludge at high temperature;
③The incineration ash contains organic elements and can be used in the manufacture of other products, which is conducive to resource recovery.
Therefore, the dry incineration of sludge can be widely used in many countries. Although sludge drying and incineration has significant advantages, there are still some disadvantages, such as certain dangers in the construction process. In order to prevent the occurrence of dangerous accidents in the process, attention should be paid to each link of the construction process, and necessary protective measures should be designed and installed together during construction. For this reason, our country's scientific researchers should step up to further optimize the design of drying incineration equipment to reduce the risk of equipment failure.
The sludge drying incineration process is mainly: wet sludge storage system → wet sludge conveying system → sludge drying system → dried sludge conveying system → sludge incineration system → exhaust gas purification system → discharge.
Sludge Drying System
The heating method in sludge drying process can be divided into direct drying and indirect drying. Direct drying is to introduce hot flue gas or hot air directly into the dryer, and exchange heat through the contact of high-temperature steam and wet sludge as well as convection. This drying method is characterized by a high heat utilization rate, but because the dried material has the nature of pollutants, it leads to the problem of exhaust gas emission. The inflow of high-temperature gas is continuous, so the waste gas with the same flow rate that has been in direct contact with the sludge must be specially treated before it can be discharged. In the past few years, the sludge drying process used a rotary drum sludge dryer to directly dry the sludge. The process mainly originated in Japan and Germany.
Sludge incineration is a disposal method that uses the incinerator to oxidize the organic matter in the sludge at high temperature under aerobic conditions to completely mineralize the sludge into a small amount of ashes. This method is the most thorough treatment method for sludge treatment, and it is widely used in developed countries. use. In the 1960s, sludge incinerators were mostly multi-chamber incinerators. Due to the gradual increase in auxiliary fuel costs and stricter gas emission standards, multi-chamber incinerators have gradually lost their competitiveness. At the same time, fluidized bed incinerators are easy to disassemble and maintain, with high efficiency in sludge incineration and good air preheating. Such advantages are widely popular. Most of the sludge incinerators currently in operation in China are fluidized bed incinerators. The working principle is that the fuel and materials are in a fluidized state under the action of the fluidizing air in the furnace, and the fuel is burned in the fluidized state. The fluidized bed incinerator has a large heat capacity and good material mixing, and is highly adaptable to fuel. The strong turbulence and material circulation in the bed can increase the residence time of combustion, so the fuel is fully and thoroughly burned and the combustion efficiency is high. The sludge incineration fluidized bed is usually a cylindrical reactor. The lower part of the reactor is designed in a conical shape and is closed by a chassis with nozzles.
Flue gas purification is the last step before the gas and smoke in the incinerator enter the environment, and it is very important for environmental protection. The flue gas purification device can not only purify the gas and dust, and purify the toxic gas in the gas or other dust that is harmful to the environment, but also has the detection function. Before the gas is emptied, the gas can be detected. Only when the indicator meets the atmospheric emission standard can the emission be carried out.
Analysis of Key Points in Treatment Process Design
1. Transportation of sludge. The water content of sludge is generally about 80%, which is difficult to transport. For the treatment of sludge, the state of the sludge must first be considered. Some sludges have low water content and are viscous and agglomerated, which is convenient for transportation; some sludges have high water content. It needs to be comprehensively considered and combined with the actual situation for transportation.
2. How to maintain the temperature of the drying equipment during the sludge drying process. If the heating value is not enough, the sludge drying efficiency is not high. In addition, temperature stability must be considered. The sludge usually contains some combustible gas, which may cause an explosion in the sludge drying project under unstable temperature conditions. Therefore, it is very important to maintain temperature stability and thermal balance, and some inert substances can usually be added to the drying equipment.
3. Before the sludge is dried, the sand content of the sludge needs to be considered. Sludge with high sand content may cause a certain degree of wear on the drying equipment, thereby shortening the service life of the machine. Therefore, the sludge should be sanded before drying, and the sanding rate should be in accordance with the process requirements.
4. Some sludge has a special nature. When the moisture content reaches 40% to 60%, the flow characteristics of the sludge will change significantly, such as suddenly becoming viscous, which is the so-called sludge viscous zone. Viscous sludge brings great inconvenience to transportation. Therefore, it is necessary to consider avoiding sludge stickiness during the drying process of sludge.
In addition, the thermal energy balance is also crucial to the entire sludge incineration system. Subtle changes in the elemental composition of sewage sludge and the moisture content of the sludge will affect the thermal energy balance. To find this thermal energy balance point, a detailed analysis of the nature of the incineration sludge is necessary. When the low calorific value of the sludge is lower than 3350kJ/kg, it cannot be burned by itself, and auxiliary fuel must be added. Both the calorific value and solid content of sludge are important factors that affect the thermal energy balance of the incineration system. The decrease in solid content can directly lead to an increase in the amount of sludge. Based on the amount of 1t of absolutely dry sludge, when the solid content is reduced from 25% to 20%, the solid content is only reduced by 5%, but the amount of wet sludge increases by 25%. This will undoubtedly increase the load of the dryer and affect the heat balance of the entire system.
Recycling and energy recovery from waste and biomass are more and more important in waste management as well as global energy supply. Shincci low-temperature Belt Waste Dryer is ideal for the pre-treatment of RDF, SRF, Municipal Solid Waste, Biomass and Sewage sludge. The air recirculating system, as well as the possibility to generate the heat required from biomass considerably reduce operating cost and carbon footprint. Shincci sludge dryer makes sludge incineration process more economic and safe, besides, it makes conveying easier.
