Sludge is the product after sewage treatment. The main characteristics of sludge are high moisture content (up to 99% or more), high organic matter content, easy to rot and odor, which requires the sludge drying treatment, the current sludge treatment process. In the middle, the proportion of the dry treatment of sludge treatment still ranked first. This article introduces and analyzes the progress of foreign sludge drying technology.
1. Introduction of sludge drying technology
As early as the 1940s, Japan and Europe and the United States had used direct heating drum dryers to dry sludge. After decades of development, the advantages of pollution drying technology are gradually emerging:
1 The sludge is significantly reduced in volume, and the volume can be reduced by 4 to 5 times;
2 Forming a stable product of granules or powders, greatly improving the properties of sludge;
3 The product is odorless and free from pathogens, which reduces the negative effects of sludge and makes the treated sludge more acceptable;
4 products have a variety of uses, such as fertilizers, soil conditioners, alternative energy sources.
Therefore, regardless of landfill, incineration, agricultural utilization, or heat energy utilization, sludge drying is an important first step, which makes sludge drying an increasingly important role in the entire sludge management system. Since the 1990s, the use of sludge drying technology to treat urban sludge has developed rapidly.
2. Sludge drying equipment
There are many different types of sludge drying equipment. Common types are:
(1) Direct heating. The principle is convection heating, on behalf of the equipment has a drum, fluidized bed, etc.;
(2) Indirect heating. The principle of conduction or contact heating, on behalf of the equipment spiral, disc, thin layer, disc, paddle, etc.;
(3) heat radiation heating type. Belt, spiral and so on.
3, the progress of sludge drying technology
The following combined with the actual investigation results in the United States, some of the technical points of the sludge drying, briefly introduced the progress of the market mainstream drying technology and equipment.
3.1 sludge bonding problem
The existing sludge drying equipment can be roughly divided into two types from the feed method and product form. One is the dry material backmixing system. The wet sludge is mixed with a certain proportion of dry sludge before feeding, and the water contains The rate is reduced to 30% to 40%, and then it enters the dryer. The product is a spherical particle, which is a process that combines drying and granulation; the other is the direct supply of wet sludge, and the product is mostly powdery.
Drying different sludges, such as industrial sludge and urban sludge, also has different equipment requirements. The equipment that can be successfully used to dry industrial sludge can be used directly in municipal sludge, but it is not necessarily successful. This is because the characteristics of urban sludge are very sticky, and there is a special phase of the adhesive phase in the drying process (the moisture content is about 60%). In this very narrow transition period, the sludge is very easy to agglomerate, the surface is hard and difficult to crush, and the inside is still muddy. This presents great difficulties for further drying and sterilization of the sludge. In order to overcome this difficulty and achieve a solids content of >90%, a dry material backmixing process results. Before the dryer feeds, a certain percentage of dry mud particles with a solid content of >90% are returned to the mixer (or coating machine) to be mixed with the wet sludge. In the process, dry particles act as pearl nucleuses, wet stains. The mud is only thinly wrapped outside the dried grains. The mixing ratio is controlled so that the moisture content of the mixture is reduced to 30% to 40%. This allows the sludge to directly pass through the adhesive phase, greatly reducing the adhesion of the sludge in the dryer. When drying, it only needs to evaporate the surface layer of the particles. Moisture makes drying easy and reduces energy consumption.
For their own needs, the direct heating system uses dry materials for back mixing. Early indirect heating systems used direct feed of wet sludge, and the abrasive wear of the equipment caused by the adhesion of wet sludge was quite serious, and caused some safety accidents, and some of these equipments were therefore discontinued. Later, some indirect heating systems such as Seghers's pearl process also used dry material back-mixing to successfully produce spherical particles, and the equipment worked well with low energy consumption. Its evaporation requires only 3100 kJ of heat energy per kilogram of water. There are also indirect heating systems, such as Fenton's patented indirect rotary chamber (IRC series), which still use wet sludge to feed directly, but it focuses on the problem of sludge bonding: it uses a double-helical propeller, between two sets of spirals Clean the surfaces with each other and use unequal pitch designs to minimize the sticking of sludge on the surface of the equipment. Practice has shown that good results have been achieved and the number of equipment in the entire sludge drying system has been greatly simplified.
3.2 Exhaust gas treatment and odor control
The management of sludge treatment abroad is very strict. It must be environmentally safe and cannot produce secondary pollution. Therefore, foreign sludge drying technology attaches great importance to tail gas treatment and odor control. In the early ESP direct heating system, the outside air was introduced into the dryer after heating, and the moisture in the sludge was evaporated and the sludge was transported. After leaving the dryer, the hot air is separated from the dried sludge particles, and then discharged after dust removal, thermal oxidation, and deodorization. Due to the large amount of hot air, the cost of exhaust gas treatment is very high, and this defect makes people once turned their interest to indirect heating systems. Later, Andritz's drum-type direct heating process adopted a gas recycling design that made this defect significantly improved. In the drying process, after the hot air is dusted, condensed, and washed, 85% of the drum is returned to the drum, and only 15% needs to be deodorized after thermal oxidation. This reduces the burden of tail gas treatment and, more importantly, greatly reduces the amount of external air introduced, maintains the oxygen content in the drum at a very low level, and thus greatly improves the safety performance of the system. For indirect heating systems, the amount of tail gas is much smaller, and the burden on the corresponding exhaust gas treatment is much lighter. After the condensing, washing, and discharging of the tail gas of the Siggas drying equipment, it is sent back to the combustion furnace, and the odor-causing compound is completely decomposed, so its exhaust gas can meet very strict emission standards. In addition, whether it is direct heating or indirect heating system, the drying equipment uses proper negative pressure inside to avoid the leakage of odor. The gas in the sludge storage room, drying plant, finished product warehouse and other structures of the factory are all pumped away and concentrated.
3.3 Equipment Security
In old-fashioned desiccators, fires or explosions occur quite frequently, and the safety performance of sludge drying equipment has been questioned. Most of the reasons for the fire or explosion have now been made clear. The three main factors associated with the explosion are the temperature of oxygen, dust and particles. Different process reports may be somewhat different, but in general the safety factors that must be controlled are: oxygen content <12%; dust concentration <60g/m3; particle temperature <110°C. The current sludge drying technology pays great attention to the safety of the equipment and has taken specific measures to improve the design and strengthen management. For the control of oxygen content, indirect heaters such as Segos drying equipment are additionally equipped with nitrogen protection to ensure that the oxygen content in the system is <2%; direct heaters, such as ANDRITZ's drum, pass through the gas as described above. Recycled to control the oxygen content <8%. The real-time monitoring of the oxygen content in the system is very important. In the ANDRITZ system, oxygen protection is set. Once the oxygen content exceeds 10%, the system will automatically stop. The key to controlling the pellet temperature is to control the residence time of the sludge in the dryer. The proper amount of moisture must be kept in the dried sludge to prevent the sludge from overheating and burning, so when the sludge reaches a certain dryness (eg 90%) Leave the dryer. This also makes it very important to solve the problem of sludge binding in the equipment. For dust control, the dry process using dry material backmixing is preferred, and for those indirect heating devices that produce powdered products, this issue should be addressed. In addition, sludge drying plants need to consider other safety factors: a factory designed with wet sludge silos must consider the generation of methane and minimize the storage time of wet sludge. In the design of Andritz, we will use methane from the wet mud silo. The concentration is controlled below 1%; the safety of the dry mudhouse is also taken seriously. To prevent spontaneous combustion, the temperature of the dried mud particles must be controlled below 40°C.
4 Conclusion
In the new period, sludge drying will continue to develop, improve and be welcomed. It is predicted that in the next 10 years in Europe, the amount of heat treated sludge will double. Sludge drying equipment is also being developed to large scale. For example, ANDRITZ has built the largest sludge drying plant in Europe - Bransands in the UK, with a processing capacity of 7 × 5000 kg/h for evaporated water. Segos was built in Barcelona. The world’s largest indirectly heated sludge drying plant has an evaporation capacity of 4×5000 kg/h. At the same time, the sludge drying equipment continues to improve in terms of safety performance including environmental friendliness. The efforts made by equipment developers in reducing energy consumption have resulted in significant improvements in the economic viability of sludge drying.
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