Analysis of thermal drying technology for sludge treatment

In order to meet the requirements for subsequent disposal of sludge, the water content of conventional mechanical dewatered sludge needs to be further reduced. Thermal drying of sludge refers to the process of removing moisture from sludge by evaporation through the heat transfer between sludge and a heat medium.

2 Application principles 


Select the type of drying and process technology according to the needs of disposal and actual conditions. The thermal drying process should be combined with waste heat utilization. It is not appropriate to set up a separate thermal drying process. It can make full use of the biogas heat energy generated during the anaerobic digestion of sludge, waste heat from waste incineration, waste heat from thermal power plants or other waste heat to dry sewage mud.


3 Sludge thermal drying process and equipment 
 

3.1 General process flow  

The sludge thermal drying system mainly includes storage and transportation systems, drying systems, exhaust gas purification and treatment, electrical automatic control instrumentation systems and auxiliary systems.


The storage and transportation system mainly includes silos, sludge pumps, and sludge conveyors. The drying system is based on various types of drying process equipment. The exhaust gas purification and treatment includes the condensation and treatment system of the exhaust gas after drying. The electrical self-control The instrument system includes electrical and control equipment that meets the system's measurement and control requirements. The auxiliary systems include compressed air systems, water supply and drainage systems, ventilation and heating, and fire protection systems.


3.2 Process and equipment 

1) Traditional process equipment types 


According to different heat transfer methods, sludge drying equipment is divided into two methods: direct heating and indirect heating. Considering the safety of the system and the prevention of secondary pollution, indirect heating is recommended.


2) Traditional drying process equipment 


Currently, there are six types of sludge drying process equipment including fluidized bed drying, belt drying, paddle drying, horizontal rotary disk drying, vertical disk drying, and spray drying. Process equipment. The drying process and equipment should be comprehensively considered in terms of technological maturity and investment and operation costs, and selected in accordance with the requirements of different sludge treatment and disposal projects.

① Fluidized bed drying 


The temperature of the sludge particles in the fluidized bed drying system is generally 40 ° C to 85 ° C, the system oxygen content is <3%, and the heat medium temperature is 180 ° C to 220 ° C. Indirect heating is recommended. Heat transfer oil is often used as the heat medium. Various heat sources such as natural gas, fuel oil, and steam can be used. The fluidized bed drying process can be used to fully dry the sludge, or it can be semi-dried. The sludge particles in the final product are more evenly distributed, with a diameter of 1 to 5 mm.


The fluidized bed drying process equipment has a single unit of evaporating water of 1,000 ~ 20,000 kg / h, and a single unit of sludge treatment capacity of 30 ~ 600 t / d (water content of 80%). Can be used in sewage treatment plants of various sizes, especially for large and extra large sewage treatment plants. The drying effect is good and the processing capacity is large; there are successful domestic engineering experiences that can be used for reference. However, the investment and maintenance costs are high; when the sand content of sludge is high, anti-wear measures should be taken.


② Belt drying 


The working temperature of the belt drying is from ambient temperature to 65 ° C, and the system oxygen content is <10%; direct feeding without the need for dry mud back mixing. The belt drying process equipment can be used for both full drying of sludge and semi-drying of sludge. The water content of the mud can be freely set and used flexibly. During partial drying, the moisture content of the sludge particles can generally be between 15% and 40%, and the dust content in the sludge particles is very small. When fully dried, the moisture content is less than 15%. In 3 ~ 5 mm. The belt drying process equipment can use direct or indirect heating methods, and can use various heat sources, such as natural gas, fuel oil, steam, hot water, heat transfer oil, cooling water from gas engines, and exhaust gases.


There are two ways of belt drying: low temperature and medium temperature. The single-unit evaporation water volume of the low-temperature drying unit is generally less than 1000 kg / h, and the single-unit sludge treatment capacity is generally less than 30 t / d (water content is 80%), which is only suitable for small sewage treatment plants; the single-unit evaporation water volume of the medium-temperature drying unit can be It can reach 5000kg / h. When fully dried, the single-machine sludge treatment capacity can reach up to about 150 t / d (water content is 80%), which can be used in large and medium-sized sewage treatment plants. Because the main equipment runs at low speed, there are few wearing parts, and the maintenance cost of the equipment is very low; high temperature and high concentration of dust are not generated during operation, and the safety is good; it is flexible to use and can use a variety of heat sources. However, the volume of the equipment is relatively large under unit evaporation; the large amount of circulating air, the larger the thermal energy consumption.


③ Paddle Drying 
 

The paddle type drying adopts a hollow paddle and a shell with a hollow interlayer, and has a high heat transfer area and a material volume ratio. The sludge particle temperature is <80 ℃, the system oxygen content is <10%, and the heat medium temperature is 150 ℃ ~ 220 ℃. Generally, indirect heating is used, and the heat medium is preferably steam, or heat transfer oil (heated by burning biogas, natural gas or coal). The dry sludge does not need to be mixed back. The moisture content of the outlet sludge can be adjusted by the rotation speed of the shaft, which can be fully dried or semi-dried. The particle size of the fully-dried sludge is less than 10 mm, and the semi-dried sludge has a loose mass.


The paddle-type drying process equipment can evaporate water up to 8000 kg / h and the single-machine sludge treatment capacity is about 240 t / d (80% water content), which is suitable for sewage treatment plants of various sizes. The structure is simple and compact; high temperature and high concentration of dust are not generated during operation, and the safety is high; successful domestic engineering experience can be used for reference. However, sludge is easy to stick to the paddle blades and affects heat transfer, resulting in a decrease in thermal efficiency. The blades need to be targeted for design.


④Horizontal turntable drying 


Horizontal turntable drying can be either full drying or semi-drying. The particle temperature of the whole drying process is 105 ° C, and the particle temperature of the semi-drying process is 100 ° C; the system oxygen content is <10%; and the heat medium temperature is 200 ° C to 300 ° C. With indirect heating, saturated steam is the preferred heat medium, followed by heat transfer oil (heated by burning biogas, natural gas or coal), and high pressure hot water can also be used. The sludge needs to be back-mixed, and the moisture content of the back-mixed sludge is generally lower than 30%. Fully-dried sludges are particles with uneven particle size distribution, and semi-dried sludges are loose clusters.


The horizontal rotary disk drying process equipment has a single unit of evaporating water of 1000 ~ 7500 kg / h and a single unit sludge treatment capacity of 30 ~ 225t / d (water content of 80%), which is suitable for sewage treatment plants of various sizes. The structure is compact, the heat transfer area is large, and the equipment covers a relatively small area. However, there may be the phenomenon of sludge adhesion. After drying, it will become loose clusters, which need to be granulated before being sold as fertilizer; there is no engineering application in China for the time being.


⑤Vertical disc drying 


Vertical disc drying is also known as pearl granulation process, which is only suitable for sludge drying. The particle temperature is 100 ℃ ~ 40 ℃, the system oxygen content is <5%, and the heat medium temperature is 250 ℃ ~ 300 ℃. With indirect heating, the heat medium is generally only heat-conducting oil (heated by burning biogas, natural gas or coal). The back-mixed dry sludge particles are mixed with mechanical dewatered sludge, and the dry particles are coated with a thin layer of wet sludge to reduce the water content to 30% to 40%. The particle size distribution of the dried sludge particles is uniform, and the average diameter is between 1 and 5 mm. No special particle size distribution equipment is required.


The stand-alone evaporating water volume of vertical disc drying process equipment is generally 3000 ~ 10000 kg / h, and the stand-alone sludge treatment capacity is from 90 ~ 300 t / d (water content is 80%), which is suitable for large and medium-sized sewage treatment plants . The structure is compact, the heat transfer area is large, and the equipment occupies a relatively small area. The sludge has uniform drying particles, and there are many adaptable digestion channels. It is only suitable for full drying and has high requirements for heat transfer oil; it has not been applied in China for the time being.


⑥ spray drying 


The spray-drying system uses an atomizer to disperse the raw material liquid into droplets, and the droplets are dried with hot gas (air, nitrogen, superheated steam or flue gas). The stock solution can be a solution, an emulsion, a suspension or a paste. The dried product can be made into powder, granule, hollow ball or granule according to requirements.


Spray drying adopts co-current direct heating, which can be used for sludge semi-drying or full drying without the need for sludge back-mixing. After the dewatered sludge is atomized by the atomizer, the particle size of the atomized droplets is between 30 and 150 μm. As the heat medium, sludge incineration of high temperature flue gas is preferred, followed by hot air (produced by burning biogas, natural gas or coal, etc.), and high pressure superheated steam can also be used. When sludge incineration of high temperature flue gas is used, the inlet temperature is 400 ° C to 500 ° C, the exhaust temperature is 70 ° C to 90 ° C, and the sludge particle temperature is less than 70 ° C. The particle size distribution of the dried sludge particles is uniform and the average particle diameter It is between 20 ~ 120μm.


The single machine evaporation capacity of spray drying process equipment is generally 5 ~ 12000 kg / h, and the single machine processing capacity can reach up to 360 t / d (water content is 80%), which is suitable for sewage treatment plants of various sizes. Short drying time (in s), high heat transfer efficiency, high drying strength When sludge incineration of high temperature flue gas is used, the drying strength can reach 12 ~ 15 kg / (m3⋅h), the temperature of the dried sludge particles is low, and the structure Simple, flexible operation, high safety, easy to realize mechanization and automation, small footprint. However, the dust content in the exhaust gas from the drying system is high and there is a foul odor, which requires two-stage dust removal and deodorization treatment.


3.3 Exhaust gas purification and treatment 

The tail gas of the sludge after drying includes water vapor and non-condensable gas (odor gas), which needs to be separated first. Water vapor is condensed by a condensing device for post-treatment, and non-condensable gas (odor) is discharged. The drying tail gas condensing device may use a spray tower or a condenser.


4 Design and process control  


4.1 Design and Operation Control Points 


1) The selection of the degree of sludge thermal drying should follow the following principles: use the technical characteristics of the drying process itself; the entire drying through the heat transfer between the sludge and the heat medium and the investment and operating cost of the subsequent disposal system should be the lowest; Consider the adaptability of sludge morphology (looseness and particle size) to sludge transportation, feed systems and subsequent disposal equipment.


2) According to the cost of drying heat source, the order from low to high is as follows: ① flue gas; ② coal combustion; ③ steam; ④ fuel oil; ⑤ biogas; ⑥ natural gas. In general, indirect heating can use all energy sources, and the difference between them is only in temperature, pressure and efficiency. The direct heating method is limited due to different types of energy. Among them, coal-fired furnaces and incinerators have a large amount of flue gas and corrosive pollutants, making them difficult to use.


3) The three main factors related to the explosion of drying equipment are the temperature of oxygen, dust and particles. Different processes will have some differences, but in general the safety factors that must be controlled are: the oxygen content of the fluidized bed and vertical discs is less than 5%, and the oxygen content of the belt, paddle, and horizontal rotary discs Less than 10%, dust concentration is less than 60 g / m3, and particle temperature is less than 110 ° C.

4) The methane concentration in the wet sludge silo is controlled below 1%, and the temperature of the dry sludge particles in the dry sludge silo is controlled below 50 ° C.


5) To avoid the impact of open transport of wet sludge on the environment, sludge pumps and pipelines should be used to seal and transport the wet sludge into the dryer. The outlet of the drier must be provided with an accident storage bin or an emergency discharge port for temporary storage or discharge when the sludge drier is out of operation or abnormal operation.


6) Sand and gravel mixed with sludge have a negative impact on the safety of the drying equipment. For sludge with a large sand content, the wear of the heat exchange surface can be reduced by measures such as increasing the wear margin and reducing the speed of rotating parts. Especially when using heat transfer oil as the heat medium, great care must be taken.


4.2 Secondary pollution control requirements 


The water vapor and non-condensable gas (odor) that evaporate after the sludge is dried need to be separated. Water vapor is condensed by a condensing device and treated. Wastewater from the incineration plant should be reused after treatment. When the wastewater needs to be discharged directly into the water body, its water quality shall meet the requirements of the Comprehensive Wastewater Discharge Standard GB 8978.


In order to prevent the odor from leaking out during the sludge drying process, the drying device must be completely closed, and the inside of the sludge dryer and the sludge drying room must maintain a slight negative pressure. After drying, the sludge should be stored under seal to prevent the odor from volatilizing due to the high temperature of the sludge. The control and prevention of odorous pollutants in drying plants shall comply with the provisions of the "Emission Standard for Odorous Pollutants" GB 14554.


The noise of the drying plant shall meet the requirements of the "Environmental Noise Standard for Urban Areas" GB 3096 and the "Noise Standard for Boundaries of Industrial Enterprises" GB12348. Regulations. Noise source control measures should be taken as priority in the drying plant noise control. Noise control at various locations in the plant area should be based on sound insulation, supplemented by noise reduction, vibration isolation, and sound absorption.


5 Evaluation and analysis of investment and operating costs  


Investment costs are determined by factors such as the complexity of the system and the localization rate of the equipment. In general, if there is available waste heat energy, when domestic equipment is used for thermal drying, the unit investment cost is 100,000 to 200,000 yuan / t sludge (80% moisture content), and some chemical equipment is imported equipment, and the unit investment cost is At 300,000 to 400,000 yuan / t sludge (80% water content).


The operating cost of sludge thermal drying is determined by many factors, such as the price of the drying heat source, the moisture content of the final dried sludge, whether a separate exhaust gas purification system needs to be constructed, etc., which are difficult to convert to specific amounts.

Attachment: Specific comparison of various drying technology equipment, as shown in the table.

Specific comparison of various drying technology equipment