Analysis of application, technical points and operation status of garbage pneumatic conveying system
The pneumatic conveying system of garbage adopts suction dilute phase conveying, that is, a fan is installed at the end of the system, and negative pressure air is used as the power source, and the domestic garbage is transported and collected quickly from many points to one point through underground pipelines. Compared with the traditional way of garbage collection and transportation, the pneumatic garbage transportation system has the characteristics of high efficiency, environmental protection, sealing, concealment and intelligence, which can realize the "point-to-point" transportation of garbage from the delivery port to the centralized treatment station, effectively solve the problems existing in the traditional way of garbage collection and transportation, such as drip filtration, odor emission, traffic congestion and labor consumption, and is an innovative measure to solve the current problem of garbage collection and transportation.
The garbage pneumatic conveying system includes three parts: garbage throwing system, pipeline conveying system and central collection station, which are respectively responsible for garbage throwing, storage, transportation, collection and transfer and central system control. The garbage throwing system is divided into indoor throwing system and outdoor throwing system according to the position of throwing port. The user puts the garbage into the throwing port, and the garbage is temporarily stored in the garbage storage section. Under the command of the control system, the valves cooperate with each other to open and close, and the garbage in the storage section falls into the pipeline and starts to be transported under the action of the wind force formed by the exhaust fan. Finally, the garbage and air enter the central collection station, and the garbage is compacted and collected after solid-gas separation, and the gas is discharged into the atmosphere after dust removal and deodorization. According to the garbage handling capacity, the garbage pneumatic conveying system can be divided into fixed system and mobile garbage pneumatic conveying system, and the fixed system can be divided into large fixed system (10~30t/d) and small fixed system (2.5~10.0t/d). China's residential areas produce a large amount of garbage, and usually adopt fixed systems. According to the actual planning and layout, the scheme of combining large fixed systems with small fixed systems is adopted.
Since 1950s, western countries began to study the vacuum transportation of municipal waste. Up to now, the pneumatic garbage transportation system has been widely used in residential areas, commercial areas, airports, hospitals, Expo sites, Asian Games villages and comprehensive areas all over the world. After more than 70 years of research and development, some experience and achievements have been accumulated in the theoretical research and operation management of garbage pneumatic conveying technology at home and abroad. The garbage pneumatic conveying technology started late in China. At present, it is only used in a few buildings in Beijing, Shanghai, Guangzhou, Tianjin, Zhuhai, Yantai and other cities to collect and transport domestic garbage or medical waste. Therefore, it is necessary to further explore and improve the operation parameters, operation management methods, fault problems and solutions of each link of the domestic garbage pneumatic conveying system.
1 Application of garbage pneumatic conveying system
In this study, a total of 66 cases of garbage pneumatic conveying projects were collected worldwide, which were distributed in four continents and 22 countries in Asia, Europe, America and Oceania. Among them, Asia (accounting for 50%) and Europe (accounting for 40%) built more garbage pneumatic conveying projects, followed by America (9%) and Oceania (1%). In Europe, the garbage pneumatic conveying project covers a wider range and is distributed in 10 countries including Sweden, Denmark, Netherlands, Finland and Norway. In Asia, it is mainly concentrated in China, Japan, South Korea, Singapore and Saudi Arabia.
Most garbage pneumatic conveying projects are connected to the garbage sorting system at the front end (accounting for 90%). Before entering the pneumatic conveying system, the garbage is usually divided into 2~4 categories and then transported separately. Hospitals generally adopt two types of garbage classification mode, that is, garbage and other wastes. Domestic garbage generated in residential areas and commercial areas is usually divided into two or four types, recyclable garbage and other garbage, or kitchen garbage and other garbage, or organic garbage, paper, plastic (bottles) and other garbage before pneumatic transportation.
As far as application scenarios are concerned, most of the completed garbage pneumatic conveying projects are used for conveying domestic garbage in residential areas (accounting for 45.59%), followed by commercial areas, comprehensive areas and hospitals. The garbage disposal capacity of pneumatic system is mostly within 7~100t/d, which is determined by many factors, such as local overall construction layout, actual garbage output of users, garbage characteristics and application scenarios.
2 The technical points of the existing garbage pneumatic conveying project
2.1 garbage outlet
In the building, at least one set of indoor outlets is installed on each floor; Outside the building, it is suggested to set up a group of outdoor throwing outlets at intervals on the road, and the radiation range of the throwing outlets should be ensured to facilitate the surrounding users to throw garbage. For example, the Shanghai World Expo has an outdoor outlet every 70~100m on the road, Tianjin Sino-Singapore Eco-City has an outdoor outlet every 200m in the indoor and outdoor area, and an outdoor outlet every 400m along the public pipe network.
The indoor throwing port is usually designed as a bucket, which can effectively throw long garbage on the wall when it is opened and blocked from the pipeline. The size of the outdoor garbage outlet is generally 400mm×400mm, which is smaller than the inner diameter of the conveying pipeline (mostly 500mm), thus limiting the entry of large-volume garbage and reducing pipeline blockage. Considering that the garbage in the indoor throwing port will have a great impact on the discharge valve at the bottom when it falls, it is suggested to set an inclined storage section, which has a certain impact and buffer effect on the falling garbage. When the garbage pneumatic conveying system is running, there will be a strong suction force in the pipeline. If the user throws garbage at this time, the strong airflow entering through the throwing port may cause personal injury. Therefore, it is necessary to install a safety device at the throwing port.
It is recommended to set a bottom valve at the indoor delivery port. The bottom valve is located at the top of the storage section, and it works with the discharge valve. When the system sucks the garbage in the storage section, the discharge valve is opened and the bottom valve is closed. After the garbage suction is completed, the discharge valve is closed and the bottom valve is opened, thus avoiding the influence of the airflow generated during garbage suction on users.
For the outdoor delivery port, it is recommended to install an electronic door lock. When the system is running, the delivery door will automatically lock and refuse to accept the delivery of garbage. When the garbage is pumped, the user can deliver the garbage. The garbage stored in the garbage outlet is easy to breed mosquitoes and flies, produce foul smell and affect the living environment. For the indoor feeding port, a fan and a deodorizing device are usually installed at the top of the vertical collecting pipe, so that a slight negative pressure is generated in the vertical collecting pipe to prevent the stench of garbage from being emitted. In addition, many devices also prevent odor emission by enhancing the sealing performance of the garbage box, such as installing a garbage bin top cover or baffle, adding an air cleaning system, and sterilizing equipment. Schematic diagram of garbage pneumatic conveying system.
2.2 Transmission pipeline
2.2.1 pipe size and material
The diameter and material of the main pipeline and the branch pipeline of the garbage pneumatic conveying system are generally the same. At present, most of the garbage pneumatic conveying pipelines used in the world adopt DN500 size. Mirimat Company reduced the pipe diameter, adopted DN300 size pipe, and installed a special deformer at the front end of the pipe, which can compress the garbage for transportation. The small-diameter conveying pipe can save installation cost and energy consumption, which is an innovative design, but its adaptability to domestic garbage materials in China needs engineering verification. The wall thickness of the conveying pipeline is 6 ~ 25mm, and the wall thickness of different pipe sections is determined according to the amount of garbage transported. Generally, the wall thickness of the pipeline is larger at the position with greater impact and wear. For example, the garbage will collide and decelerate and accelerate again at the bend, and the pipe wall needs to be thickened at the bend (usually 8~20mm). The closer the pipeline is to the central collection station, the thicker the pipe wall is. Generally, low carbon steel pipe is used. When the wall thickness exceeds 20mm, alloy steel pipe is considered. In the current garbage pneumatic conveying pipe network, the limit length of the main pipe is less than 2km. In order to save ground space, pneumatic pipes are generally laid underground, and the buried depth of the pipes is 3 ~ 3.5 m..
2.2.2 Pipe components
The supporting parts of the pipe network mainly include sectional valves, elbows, tees, manholes, manholes, etc. The segmented valve is located on each branch pipeline, and the other end is connected with the main pipeline, and the operation of a branch pipeline is controlled independently by controlling the opening and closing of the segmented valve. The manhole is located in the manhole, and an manhole should be set every 50m or 100m on the straight pipeline, and an manhole should be set about 2m downstream of the elbow and tee. Pressure and wind speed detection equipment should also be set at the manhole to determine the pipeline blockage and clean it up in time. Elbows and tees can make the long-distance transportation of garbage more flexible and convenient. The angles of elbows are 90, 60, 30 and 15, and the turning radius of elbows should not be less than 1.8m.
2.2.3 transport parameters
Transport ratio, air velocity and garbage transport velocity are three key parameters of pneumatic transport. The transport ratio refers to the mass flow ratio of garbage and air in the pipeline. The greater the transport ratio, the better it will improve the efficiency of garbage transport. However, if the transport ratio is too large, it may cause blockage at the same air velocity, and the requirements for transport pressure will also become higher. Therefore, the transport ratio is affected by the physical properties and transport conditions of garbage. In the current garbage pneumatic conveying system, the conveying ratio in the conveying pipeline is 0.5~5.0, which is usually 1.5 according to the project experience. When the garbage is pumped, the number and frequency of fans are adjusted to form an appropriate airflow speed in the pipeline, which is greater than the garbage transmission speed. Generally speaking, the airflow speed in the pipeline is stable at 30 ~ 40m/s, and the garbage conveying speed is 17 ~ 21m/s.. During transportation, the garbage will collide with the pipe wall, decelerate and re-accelerate at the bend, and cause pressure loss when passing through the separator and other parts. It is necessary to ensure that the power provided by the fan can withstand the pressure loss during transportation. The vacuum degree of the garbage pneumatic transportation system is usually 0.02~0.05MPa.
2.3 Central Collection Station
2.3.1 structure of central collection station
The structural forms of central collection station include aboveground, semi-underground and underground, and the choice of its structural forms mainly depends on the planning and layout of the project and the amount of garbage generated. According to the architectural experience of the central collection station in the middle area of Tianjin Sino-Singapore Eco-city, the above-ground central collection station is generally divided into two floors: the first floor is mainly equipped with packing area, central control room and fan room; The second floor is mainly equipped with dust removal room, storage room, separator and high and low voltage distribution room. The semi-underground first floor is below the ground, and is mainly equipped with packing area, dust removal room, fan room and high and low voltage power distribution room, while the second floor is mainly equipped with control room, storage room and separator.
The underground floor of underground central collection station is mainly equipped with fan room, control room, packing area, etc. Only the lifting platform part is considered for the above-ground buildings. The advantage of the above-ground central collection station is that the container does not need to be lifted, which is more convenient for logistics and transportation. At the same time, the two-story building is above ground, which reduces the use of lighting, reduces the operating cost, and can also avoid groundwater and rainwater from entering the station. However, its disadvantage is that it covers a larger area and requires higher noise control.
The semi-underground central collection station has the advantages of isolating foul smell, simple garbage pipeline in the station, and easy noise control. Its disadvantages are that the floor space is increased, garbage trucks need to go underground to operate, containers need to be lifted, and the underground floor needs lighting during the day. Compared with the above-ground collection station, the equipment, construction cost and operation time are increased.
Underground collection station is suitable for small-scale garbage collection, and the garbage separator and container of small fixed system are integrated, without considering the climbing height of garbage pipeline. In the middle area of Sino-Singapore Eco-city, two large fixed systems collect more garbage (23.63t/d and 17.09t/d), both of which adopt the above-ground collection station structure, while two small fixed systems collect less garbage (3.06t/d and 8.11t/d), adopting underground structure and semi-above-ground structure respectively. The collection stations of foreign garbage pneumatic conveying systems mostly adopt semi-underground or all-underground structures. Because the gases emitted by garbage are mainly NH3, H2S and other gases with high density, which mainly gather in the lower part, this semi-underground or all-underground collection station can concentrate the malodorous gases in the collection station and prevent them from being emitted to the surrounding environment.
2.3.2 Garbage separator
The garbage separator is a kind of equipment that can separate garbage from waste gas. The top of the garbage separator needs to be equipped with a rotating grille for secondary solid-gas separation to further separate light garbage and particles in the airflow, so as to separate the garbage and particles floating on the top of the garbage separator from the air and avoid such garbage from damaging the fan. The garbage separator is usually equipped with a standby machine, and a steering valve needs to be installed before the separator. On the one hand, when the garbage separator fails, the garbage can be transferred to the standby garbage separator for treatment, so as not to affect the operation of the pneumatic conveying system; On the other hand, to realize the classified treatment of transported garbage, different kinds of garbage can be transported to the corresponding collection station through the steering valve.
2.3.3 Negative pressure power system
The negative pressure power system mainly includes wind turbine, check valve, main valve, venturi tube and dynamic pressure gauge. The front end of the wind turbine is provided with a main valve, which is closed before the system is started, and opened after the fan is turned on for about 2s. Setting the main valve can effectively reduce the starting load of the fan, thus protecting the fan. The wind turbine is composed of several centrifugal variable frequency fans of the same model connected in series, and each fan is connected with a one-way valve to prevent the resistance of the whole system from increasing due to the stop of a fan. The wind speed generated in the pipeline when the fan is running is monitored in real time by venturi tube and dynamic pressure gauge. Generally speaking, the number of fans depends on the transportation distance and the amount of garbage. The longer the transportation distance, the more garbage, the greater the power required and the more fan equipment needed.
2.3.4 Gas treatment system
The gas components of domestic waste pneumatic conveying system mainly include dust, inorganic gases such as H2S and NH3, and organic waste gas. The gas treatment system includes two steps: dust removal and deodorization. Usually, there is one dust removal room and three deodorization rooms. Bag filter is used for dust removal, and the malodorous gas is transported to the dust removal room through the pipeline. The volume in the dust removal room is enlarged, and the flow rate is reduced to meet the flow rate requirement of dust removal. After bag dust removal, the gas enters the deodorization room, and the deodorization is treated by three-stage activated carbon. The odor is repeatedly treated by activated carbon for three times, so that the waste gas in the garbage can be fully treated and discharged up to standard. Both bag dust removal and activated carbon deodorization need to be replaced regularly. Usually, the service cycle of activated carbon is 1.5 months. In addition, it is suggested to monitor the pressure at the front and rear ends of the dust removal and deodorization system in real time to ensure that the pressure loss in this part is within a reasonable range.
3 Operation status of garbage pneumatic conveying system
3.1 Operating cost of garbage pneumatic conveying system
At present, there are not many studies on the economic cost and environmental impact analysis of garbage pneumatic conveying system. The existing research shows that compared with the traditional garbage collection and transportation methods, the construction cost and operation cost of the garbage pneumatic conveying system are higher.
Taking Helsinki, Finland as an example, Teerioja et al. compared the economy of traditional vehicle garbage collection and transportation with that of pneumatic system, and found that the pneumatic system is about six times more expensive than the current vehicle collection and transportation system, and the construction cost and operating cost of the traditional collection and transportation method are about 33 euros /t and 40 euros /t respectively, but the construction cost of pneumatic system is very high, accounting for 83% of its construction cost, while the operating cost only accounts for 17%, that is, the construction cost reaches 343 euros/t. During the Shanghai World Expo, the unit disposal cost of garbage pneumatic conveying system is 298 yuan /t, which is much higher than the traditional collection and transportation mode of 80~120 yuan/t.
Zhang Jing and others analyzed the operating cost of the garbage pneumatic conveying system in Tianjin Sino-Singapore Eco-city. The results show that after the system runs at full capacity in 2022, the unit operating cost of the garbage pneumatic conveying system in Eco-city is about 220 yuan /t, which is also higher than the traditional garbage collection and transportation. With the increase of labor costs, it is estimated that the unit costs of the two garbage collection and transportation modes can basically converge by 2035.
The cost of the pipeline, central collection station, delivery port, negative pressure power system and other components required by the garbage pneumatic conveying system is much higher than that of traditional collection and transportation facilities such as garbage sheds, garbage bins and trucks, and the operation and maintenance of the garbage pneumatic conveying system is also very important. Regular inspection can avoid greater losses in the later period to a certain extent. When a fault occurs, professional technicians and professional equipment are needed for maintenance, which leads to higher construction cost and operation cost of the garbage pneumatic conveying system. However, in the long run, the mature operation of pneumatic conveying system will reduce a lot of labor costs. At the same time, the advantages of pneumatic conveying system are more reflected in the environmental impact, such as increasing garbage recycling, saving land, increasing livable comfort (noise, secondary pollution and reduction of mosquitoes and flies), etc. Therefore, it is necessary to further transform the above-mentioned environmental and social benefits into recycling resource value, land value and human value through quantitative model, and incorporate them into the economic cost accounting system of the whole life cycle of pneumatic system.
3.2 Fault problems and optimization measures in the operation of garbage pneumatic conveying system
In the actual operation process, due to human factors, climate, operation management and other factors, the system will have some failures, such as pipeline blockage, pipe wall wear, valve out of control, sensor failure, delivery port failure, air leakage in cylinder and trachea, etc.
Pipeline blockage is one of the most common faults in pneumatic conveying system, which is mainly caused by three reasons: first, the nature of garbage, which contains substances that are difficult to transport, large in volume and easy to agglomerate and entangle; Second, pipeline factors, pipeline design problems or quality problems themselves make garbage accumulate at the deceleration places of bends, valves and other unqualified pipelines; Third, the pressure loss is too large. Due to the failure and air leakage of pipelines or other systems, or the climbing height is too high and the turning radius of elbows is too small, the pressure loss is too large and the forward movement of garbage is blocked. In view of the problem of pipeline blockage, the eco-city owners in China and Singapore mainly adopt three ways to deal with it, one is to clear it manually, the other is to flush it with high-pressure water injection, and the third is to dredge it with pipeline robots.
In addition, the pipeline buried in the ground is easy to cause pipeline corrosion, and the collision and friction between garbage and the pipeline wall during transportation will cause pipeline wall wear. Generally, the elbow is the easiest to wear, and the wear parts are mainly concentrated on the outer wall surface of the elbow at 30 ~ 60, and the wear degree is 10~100 times that of the straight pipe. This is mainly due to the sudden change of the moving direction of garbage at the elbow, which will keep the original moving direction under the action of inertia, thus colliding with the elbow, causing impact wear and seriously affecting the service life of the elbow. In addition, in the actual operation of the pneumatic system of Tianjin Eco-city, it is found that there is serious water accumulation in the pipeline, especially in rainy days, and the cause of water accumulation is still being investigated. In addition, some points of the system will be out of control. When the temperature is low, electromagnetic valves, sensors, discharge valves and other equipment are easy to freeze, and it is impossible to pump garbage when it is running automatically.
It can be seen that in practical application, the construction and stable operation of garbage pneumatic conveying system is not easy, and there are still some problems to be solved urgently, such as: high construction and operation and maintenance costs; Technology monopoly, most of the pneumatic conveying systems in the world are provided by Enwater Company, and a few are provided by other foreign enterprises, so the domestic technical experience is insufficient; Problems such as pipeline blockage and wear are easy to occur during operation. In the future, it is necessary to further improve and upgrade the garbage pneumatic conveying system in order to achieve wide application in China. Based on the application and operation status of the existing pneumatic waste conveying system, this study puts forward the following suggestions:
(1) Suggestions for improving the garbage throwing system: The shape and size of the throwing port can be designed according to the characteristics of throwing garbage, so as to limit the entry of large-volume garbage, or compressors and crushers can be added in the throwing port to crush and compress the thrown garbage; To standardize the user's garbage throwing behavior, we can set up a throwing sign at the throwing mouth, install intelligent equipment such as garbage identification system and voice prompt, or set up a specialist to remind and supervise.
(2) Suggestions for improvement of pipeline transportation system: Optimize pipeline design to minimize friction resistance and pressure loss; Thickening the thickness of elbow, changing the shape of elbow, installing elbow accessories, etc. to reduce the wear and impact of garbage on elbow and prolong the service life of elbow; Choose a transportation route with short transportation distance, few bends, large curvature radius of bends and small slope change; The pipeline should be made of more wear-resistant and corrosion-resistant composite materials, and the anti-corrosion treatment of the pipeline surface should be strengthened, while ensuring the quality of the pipeline, and preventing problems such as joint dislocation and pipeline air leakage.
(3) Other suggestions for improvement: add insulation and cooling facilities to reduce the influence of external environmental changes such as temperature and humidity on the system; To improve the economic analysis, the current cost analysis mainly considers labor cost, energy consumption cost, material cost, etc., which can not fully reflect the social and environmental benefits of the pneumatic conveying system. We can consider establishing more quantitative models, and incorporate more dimensions such as land saving value, garbage resource recycling value, environmental value (congestion, noise, smell, etc.) and residents' comfort value (convenience of delivery) into the cost calculation system, so as to reflect the cost and value of the system more comprehensively, thus being targeted.
4 Conclusion
In recent years, China has continued to promote the classification and resource recovery of domestic waste, and the garbage collection operation is an important part in the process of garbage treatment and resource recovery. The traditional collection and transportation methods will gradually fail to meet the needs of garbage collection and treatment, and more efficient and environmentally friendly collection and transportation technologies will be adopted in the future. The garbage pneumatic conveying system fundamentally solves the problems existing in traditional garbage collection and transportation, such as labor and material consumption, mixed loading and transportation of garbage, secondary pollution, etc. It is an efficient and sanitary way of domestic garbage collection and transportation, and gradually highlights its remarkable advantages in garbage collection and transportation.
At present, many cities in China have started to build and use pneumatic conveying system of garbage with reference to foreign technology. However, due to the high cost and differences at home and abroad, it is not applicable. There are still many practical and technical problems to be solved and overcome in order to realize pneumatic conveying of garbage in China. Domestic research, improvement and innovation of pneumatic conveying technology of garbage are the only way to realize pneumatic conveying of garbage in China in the future.
Source：https://mp.weixin.qq.com/s/bS8y0zuMrbib1RQ0yBZTWQ