Sludge thermal drying technology
Sludge drying is the process of water evaporation from sludge, which mainly includes thermal drying, solar drying, microwave heating drying, ultrasonic drying and biological drying, among which the most widely used and mature is the thermal drying of sludge Technical sludge thermal drying can be divided into direct thermal drying, indirect thermal drying, and direct-indirect combined thermal drying technology according to the way the heat medium contacts the sludge.
Adaptability Analysis of Sludge Thermal Drying Technology
As one of the main methods of sludge treatment, thermal drying of sludge has a history of nearly 100 years abroad. There are many types of sludge thermal drying technology, and the problems of safety, stability and cost are outstanding. Therefore, it is urgent to solve the problem of sludge drying by choosing a reasonable process route to treat sludge safely, stably, at low cost and on a large scale problem. The following mainly analyzes the adaptability of sludge thermal drying technology from four aspects: energy consumption, safety, environmental friendliness and flexibility.
1. Energy consumption analysis
The consumption of drying energy is the most important technical indicator of the drying process, which generally accounts for more than 80% of the operating cost of the drying system, including heat and electricity. It is usually measured by the heat and electricity consumption per kg of water evaporated.
(1) Heat energy consumption
The heat energy consumption mainly includes the theoretical heat (about 2 595 kJ / L) required for sludge heating and water vaporization and the heat energy loss of the drying system, which accounts for 60% of the total cost of drying. Factors affecting heat energy loss: ① Heat source type, transmission, storage and utilization conditions. ② Sludge particle size, viscosity, initial moisture content, final moisture content and heating and condensation of heat medium. If the dry mud back-mixing process is used, the sludge is repeatedly heated and cooled, causing a large loss of heat energy. ③ Process type and drying efficiency. The indirect heating method has a higher drying efficiency when the sludge moisture content is high, but it is more difficult to remove the last 20% ~ 30% of the water; the direct heating method has a higher drying efficiency for the sludge with a water content of less than 50%.
(2) Power consumption
Electricity consumption refers to all the electrical energy spent by the drying system, including the drying system, wet mud and dry mud transfer and storage system, process water system, heat source heating and working fluid circulation system, safety control and lighting system, etc., accounting for the total cost of drying About 20%.
2. Security analysis
During the sludge drying process, safety accidents such as spontaneous combustion of the sludge and equipment explosion occur from time to time, and safety has become one of the most important issues for drying. As early as 1994, the European Union promulgated the “Safety Standards for Potential Explosion Hazards” [ATEX95 (94/9 / EC) and ATEX137 (1999/92 / EC)] [15, , and it has been enforced in the EU since 2003 . The safety problem of sludge drying exists in the entire drying process, and its safety factors mainly include dust concentration, oxygen content, ignition energy and moisture content.
(1) Dust concentration
In the process of sludge drying, sludge transportation, mixing, screening, crushing, storage and other operations will generate a lot of dust. The smaller the particle size of the dust, the larger the specific surface area, the easier it is to ignite, and the stronger the explosion. The dust concentration is usually based on dust with a particle size of less than 150 μm. It is generally considered that the lower limit of the dust explosion concentration of sludge is within 60 g / m 3 and is affected by factors such as oxygen content, moisture content and dust properties. The dust concentration varies greatly in different drying processes, and the dust concentration in different locations of the same process is also different. In addition, the dry mud back mixing process will greatly increase the amount of dust.
(2) Oxygen content
At a certain dust concentration, ignition energy and different types of protective gases (nitrogen, carbon dioxide, steam, etc.), the minimum oxygen content (LOC) that can cause combustion is 5%, 6%, and 10%, respectively . LOC is also related to the influence of dust concentration and temperature, moisture content, dust properties, etc. Generally, the oxygen content is required to be less than 12%. Since the minimum ignition energy of the dust is small, and most processes cannot further reduce the dust concentration, reducing the oxygen content in the drying system has become the main means to avoid risks. Generally, gas inertization is used to reduce the oxygen content in the drying system. The inertization gas is divided into four types: steam, carbon dioxide, nitrogen, and flue gas according to the inertization efficiency from high to low.
(3) Ignite energy
When the dust in the drying system reaches a sufficient concentration and satisfies the oxygen content condition, the dust needs a certain amount of ignition energy to ignite. The ignition energy of sludge dust is very low, ranging from a few millijoules to hundreds of millijoules. Any small friction sparks, static sparks, mechanical collisions and electric sparks may provide ignition energy. Ignition energy is also related to dust concentration and oxygen content. Usually the temperature of the drying system is greater than 85 ℃, and the ignition energy is as low as about 100 mJ. Therefore, as long as the dust concentration and oxygen content conditions are met, the ignition energy is always present and difficult to eliminate.
(4) moisture content
Refers to the moisture content in the drying system, expressed in weight percent. The higher the moisture content of the desiccant system, the higher the degree of gas inertization, the higher the ignition energy required for the sludge, and the lower limit of the dust explosion concentration and LOC will also increase accordingly. When the moisture content reaches a certain level, even if the dust concentration and oxygen content are high, the sludge cannot be ignited. Therefore, the moisture content has become an important means to reduce the dust concentration, increase the ignition energy, and reduce the oxygen content, thereby improving the safety of the drying system.
3. Environmental friendly analysis
Sludge contains a large amount of organic components and harmful substances, and its nature is extremely unstable and perishable. During its thermal drying process, it will release malodorous gas and cause secondary pollution. According to the principle of sludge treatment, it must be properly handled. The exhaust gas composition of sludge drying is shown in Table 1. Benzene, toluene, ethylbenzene, and xylene (BTEX) are an important type of pollutants released during the sludge drying process. Among them, benzene is highly carcinogenic and is harmful to human health.
Very harmful. According to the experimental study on the release characteristics of BTEX in the process of sludge drying, most of BTEX begins to release when the drying temperature is higher than 150 ℃, and the release amount gradually increases as the drying temperature increases. Therefore, a suitable drying process and a reasonable drying temperature can reduce the release of harmful substances from sludge. For example, the direct heating and drying technology adopts the design of gas circulation and reuse, and the indirect heating and drying technology.
4. Flexibility analysis
The flexibility of the sludge drying technology refers to the performance of the drying technology to adapt to the fluctuation of the sludge quality and initial moisture content of the sludge under the premise of efficiently and stably meeting the moisture content of the dried product.
(1) Sludge quality and initial water content
Sludge is divided into municipal sludge, industrial sludge, water supply sludge, river and lake sludge, etc. according to its source. The sources of sludge are different. The initial moisture content of the sludge, the particle size, viscosity of the sludge and the special properties of the sludge (corrosion, abrasiveness, toxicity, oil content, etc.) are quite different. And have a greater impact on safe operation. If part of the drying technology cannot directly dry the sludge dewatered by machinery, it is necessary to use dry mud back mixing to reduce the water content to about 50%, and then enter the drying machine for further drying.
(2) Moisture content of dried product
Sludge drying is only the first step in the utilization of sludge resources, and the moisture content of dried sludge should be determined according to the needs of the subsequent disposal and utilization of dried products. For example, sludge composting, incineration, land use, building material utilization, etc. have certain requirements on the moisture content of the sludge and are not the same. The sludge drying process should be reasonably selected according to the principle of “determining drying by disposal”.Adaptability Comparison of Commonly Used Sludge Thermal Drying Technology
According to the characteristics of energy consumption, safety, environmental friendliness and flexibility of sludge thermal drying technology, the adaptability of the five commonly used drying technologies is compared.
The direct thermal drying process has the characteristics of high mass and heat transfer rate, high drying efficiency, good treatment effect, etc., but its energy consumption is high, the amount of tail gas and wastewater treatment is large, the process is complicated, and the investment and operating costs are too high. economic developed area. Compared with direct drying, indirect drying has obvious advantages in energy consumption and environmental performance. Energy consumption and waste gas and wastewater treatment costs are lower, and it is more suitable for economically underdeveloped areas. However, considering the operating cost, no matter what kind of drying process, the waste heat and waste heat available nearby should be preferentially selected, such as the flue gas of thermal power plants and incineration stations.
Future development trends and suggestions
On November 26, 2010, the Ministry of Environmental Protection issued the “Notice on Strengthening the Prevention and Control of Sludge Pollution in Urban Sewage Treatment Plants”, which clearly requires the simultaneous treatment of “water” and “sludge” in sewage plants. Development brings great opportunities. According to the principles of reduction, stabilization, harmlessness, and recycling of sludge treatment and disposal, it is an ideal treatment method to recycle sludge after thermal drying, and it has good development prospects. According to the adaptability characteristics of the sludge thermal drying technology, the rational selection of the type of drying technology, the optimization of the drying process, and the development of a drying technology suitable for China’s national conditions and the Chinese market are the most important issues that need to be resolved in the future. Due to its strong adaptability, the rotary disk drying technology is recommended as the leading process of sludge thermal drying in China in the future, and from the aspects of expanding the processing scale, optimizing the heat source, reducing process steps, improving system safety, and avoiding secondary pollution, etc. Carry out improvements and optimizations to better meet the development needs of sludge treatment in China.
In recent years, many emerging drying technologies focusing on energy conservation have emerged, such as biological drying, hydrothermal drying, solar drying, microwave drying, etc., or are in the experimental stage, or the use conditions are limited, or large-scale Processing, the development is not yet mature, and it cannot replace the traditional thermal drying technology, and will still be used as a supplement to the mainstream drying technology. In the future, China’s sludge treatment should be based on the continuous optimization of traditional thermal drying technology, and be committed to the development of new technologies to promote the continuous progress of China’s sludge treatment business, and truly realize the resource utilization of sludge, turning harm into profit and waste. For treasure.
Heat pump dryer belt dryer
At present, under the background of global warming, the development of a low-carbon economy and the limitation of greenhouse gas emissions have gradually become a global consensus. The Chinese government made it clear that it will vigorously develop a low-carbon economy. On the eve of the Copenhagen Climate Conference, it clearly announced its carbon emission reduction targets, and proposed to achieve a target of reducing carbon dioxide emissions per unit of GDP by 40% to 45% by 2020 compared with 2005. Sludge drying technology is the mainstream technology of sludge resource treatment and disposal technology at this stage. The research and development of sludge treatment and disposal technology based on new energy and energy-saving technologies in China is not only a direct need to control environmental pollution and achieve environmental protection goals, but also an important content for the development of low-carbon economy at home and abroad To achieve the goal of reducing carbon emissions in China.
At present, the total annual wastewater discharge in China exceeds 40 billion tons, and the annual discharge of sludge is about 5.5 to 6 million tons, and it is increasing. It is estimated that by 2020, China’s sludge output will reach 8000t / a, and with the development of large urbanization and urban cluster areas, China’s sludge treatment will eventually move to tertiary treatment such as drying or incineration. Reducing the moisture content of sludge is the key to solving many problems encountered in the current sludge treatment process, and it is also a prerequisite for sludge resource treatment (agricultural use, incineration, etc.) treatment, disposal and utilization.
The use of heat pump dehumidification to recover the enthalpy energy of the low-grade heat source generated in the drying process, after heating up, is used as a heat source for material drying and dehydration. It is a new energy-saving drying technology developed in the 1980s. At present, the technology has been applied in various fields, such as wood, chemical products, food drying, vegetable dehydration and sludge drying, etc., and has achieved good results. In developed countries, the proportion of heat pump drying technology in the drying industry has accounted for 50%. In China, this technology accounts for less than 10%. Vigorously developing our country’s heat pump technology is an inevitable trend. In view of the problem of sludge and manure drying, heat pump drying technology has two major advantages of low operating cost and no pollution to the environment. At present, Europe, America and Japan are also carrying out research on heat pump drying sludge and have a heat pump sludge drying system in operation. Drying temperature is an important factor restricting the efficiency and application of heat pump drying equipment. Under the constraints of heat pump drying, the design of the sludge belt dryer is an important measure to optimize the design parameters of the sludge heat pump drying system and improve the operating efficiency of the system.
The belt dryer used in the heat pump sludge drying research is composed of several independent unit sections: each section includes a circulating fan system, a heat pump drying and heating system, an evaporative dehumidification system, and bypass circulating air and dehumidified dry air are sent Air system, dehumidification air supply system.
After years of research and development, our company has developed a low-temperature dehumidification heat pump sludge dryer. It can fully realize the reduction, stability, harmlessness and resources treatment of sludge; the final sludge particles can be used as mixed fuel, incineration, construction materials, biofuel, etc .; suitable for domestic sludge, printing and dyeing, papermaking, electroplating, chemical , Leather, various types of sludge drying system.
One is suitable for drying from 80% moisture content to 10% (10-50% moisture content can be adjusted according to customer needs) to make strip sludge; one is suitable for plate and frame filter press to form particles The sludge with a moisture content of 60% to 30% (the moisture content can be adjusted according to customer needs).
The two devices meet different customer needs. The device has the following characteristics:
(1) Large reduction: 80% moisture content to 10% (10% -50%, can be adjusted according to customer needs); or from 60% to 30% (10-50%, according to customer needs) Tune).
(2) Closed drying, no odor discharge;
(3) Power consumption 0.25-0.4kw.h / 1kg.H2O;
(4) Fully automatic operation, simple operation and maintenance;
(5) Low temperature drying below 75 ℃, safe;
(6) No mechanical static friction and no hidden danger of dust;
(7) The service life is more than 15 years;
(8) Discharge 10% -50% adjustable;
(9) The area is small and the cost of civil construction is low.
Product performance of Dryfree heat pump sludge dryer
1. The sludge can be fully “reduced, stabilized, harmless and recycled”, and the final sludge particles can be used as fertilizer, fuel, incineration, construction materials, biofuel, landfill cover, land Use etc.
2. The continuous mesh belt drying mode is suitable for various types of sludge drying systems (including large-sand sludge), and has a long service life.
3. The mud cake with a water content of 80% can be dried into sludge particles with a water content of 10%; the sludge reduction capacity is 1 / 4-1 / 5, and the calorific value of the sludge after urban sludge drying can reach 3000kcal / kg.
4. Low temperature (40-75 ℃) fully enclosed drying process, no tail gas emissions, no odor treatment system; low temperature drying can fully avoid the volatilization of different types of organic matter in the sludge to avoid the volatilization of malodorous gas (chain alkane The temperature of volatilization with aromatic hydrocarbons is 100-300 ℃, the temperature of naphthenic hydrocarbons volatilization is mainly 250-300 ℃, the temperature of volatilization of nitrogen-containing compounds, amines and oximes is mainly 200-300 ℃, alcohols, ethers The volatilization temperatures of ketones, fatty ketones, amide nitriles, etc. are all above 300 ° C. In addition, the volatilization temperatures of aldehydes and anilines are mainly at 150 ° C, and the volatilization temperatures of lipids are at 150-250 ° C).
5. The entire drying process can be carried out in a closed environment, and no gas will be discharged into the external environment, which will not cause secondary environmental pollution; there is no pollutant discharge in the drying process, and the hygienic conditions of the drying workshop are good; concentrated water Cooling mode, good cooling effect, excellent working temperature in workshop.
6. The system is safe to operate, there is no hidden danger of explosion, and there is no need to flush the nitrogen; the oxygen content of the sludge drying process is less than 12%, the dust concentration is less than 60g / m3, the particle temperature is less than 70 ° C, and there is no dust in the whole drying process 2 m / s).
7. The transmission speed of the mesh belt is controlled by frequency conversion, and the moisture content of the sludge discharge can be adjusted (10-30%) to meet the requirements of various types of technology.
8. Adopt leading heat pump dehumidification technology, energy saving is more than 40%, power consumption per 1kg.H20 is about 0.3 ~ 0.4kw.h; traditional sludge drying equipment 1kg.H20 needs to consume 1kw.h energy, in addition to electricity , Cooling water, pharmaceuticals, etc .; use the low-peak staggered power mode, which can save more than 60% energy.
9. The equipment covers a small area and is easy to install; the equipment covers an area of about 3.5m2 per 1000kg of water evaporated.
10. The daily processing capacity of a single drying line can reach 45 tons (80% moisture content mud cake), and the daily processing capacity of 55% moisture content mud cake can reach 140 tons. It can be suitable for sludge dispersion or centralized processing mode. Reduce the volume and reduce the amount of mud biscuit treatment, save the cost of sludge transportation (depending on the road, the transportation cost per ton of sludge is more than 100 yuan) and reduce the environmental pollution during transportation.
11. Fully automatic operation, easy operation and management; dry and wet bulb temperature and humidity control, frequency conversion control of feed volume, molding control, solid content control of discharge, material control, manual control, etc.
Post time: May-28-2020