Power Plant Turns Waste into Treasure
Chen is a veteran driver of a sewage treatment plant. At 9 o'clock every morning, he drove a closed transport truck to start his day's work.
The sewage treatment plant is the largest sewage 'reservation station' in Cangshan District, Fuzhou City, Fujian Province, and 200,000 tons of sewage have to be 'reported' here every day. The sludge formed after sewage treatment will be compressed into a piece of mud cake qualified indicators with a moisture content of less than 60%. Chen's job is to transport these mud cakes.
Following Chen's vehicle, the reporter came to the destination of these mud cakes-the power plant. Here, the mud cake and coal are blended together, and then through the high temperature 'forging' of the boiler, not only the harmless treatment can be realized, but also the electricity can be generated, which solves the old problem that the urban sludge has nowhere to go.
From the garbage that everyone avoids to the 'sweet and pastry' that can generate electricity, how do power plants turn waste into treasure?
Coal Slime Residue Blending
Let the sludge 'burned'
Sludge is a sedimentation material produced in the process of sewage treatment. It has high aqua content, large volume, perishable, foul smell, and contains toxic and harmful substances such as germs.
The relevant person in charge of the sewage treatment plant introduced that after the process of concentration, tempering, dehydration, and compression, the sewage plant produces 50 to 70 tons of wet mud cake every day, and then sends it to the power plant along the designated route through a dedicated airtight transport vehicle for harmless resource-processing.
In the circular coal tank of the power plant, several transport vehicles lined up to unload the sludge, and the workers were busy at operating the forklift to mix the sludge with coal.
'Sewage can be discharged after it reaches the standard. However, if the sludge produced is improperly disposed of, it is easy to make secondary pollution to water and soil, causing the city to fall into the 'sludge trap'.' Yao Wei, deputy plant director of the power plant said.
If a large amount of sludge is not treated in a harmless manner, it will face the dilemma of nowhere to go.
How can wet sludge burn? Yao Wei said that sludge itself has a certain calorific value, which can be called fuel to some extent. However, the calorific value of the sludge is relatively low, and it can only be burnt up through technical treatment.
The first stage is blending and stirring. The sludge and coal are blended in a ratio of 1:3, allowing the sludge to 'take a bath' in the coal, and it will be stained with a thick layer of coal and become a slime mixture.
Then there is smashing and burning. The coal mill pulverizes the slime mixture and sprays the mixture directly into the boiler through a conveying pipe for combustion and power generation.
To burn the slime mixture, a high-temperature boiler is required. Because the slime mixture can only be burned as a fuel at a high temperature above 800°C. Only thermal power plant has the boiler temperature as high as or over 800°C can process the slime mixture.
At present, the sludge processed by power plants mainly comes from sewage treatment plants in industrial areas and printing and dyeing plants. From 2017 to the end of July 2021, the power plant has accumulated 71,900 tons of mixed sludge, and the recent average daily treatment is nearly 80 tons, effectively alleviating the pressure of urban sludge disposal.
The maximum daily sludge treatment volume will reach 300 tons.
“The combustion of mixed sludge for power generation consumes part of the sludge, but due to technical limitations, it is difficult for power plants to absorb more sludge.” Yao Wei said.
Although the coal slurry has been dried, the water content is still high, so the blending ratio cannot be too high
During the transportation of the coal slurry mixture, it is easy to block the coal hopper of the coal mill, which affects the safety of the machine unit, so it cannot be mixed and burned on a large scale.
In addition, sludge loading and unloading, mixing and other processes are mostly open-air operations, and the smell is unpleasant, which brings secondary pollution to the environment.
Therefore, the power plant launched a new upgraded version of sludge treatment technology-sludge pre-drying carbonization treatment technology.
The project includes Shincci sludge drying system and an in-plant supporting system.
The urban domestic sludge can be directly sent to the boiler to be burned for power generation after thermal drying and crushing, and the odor, waste water and other pollutants generated in the process can be completely enclosed and get harmful treated.
The biggest feature of the new project is the use of pre-drying carbonization treatment technology. After the sludge is sent to the power plant, it is directly discharged into a fully enclosed negative pressure sludge silo without mixing with coal. Instead, the sludge is dried directly by extracting high-temperature flue gas from the tail of the boiler with the help of Shincci dryer. The dried sludge is crushed sent to the furnace for incineration, and the incineration products are purified by the flue gas purification equipment at the tail of the boiler and discharged after reaching the standard.
'The new process adopts advanced processes such as fully enclosed, integrated, negative pressure, etc., which had greatly improve the efficiency of sludge disposal, and realized harmless, volume-reduced and resource-based treatment for large-scale wet sludge, and at the same time bring obvious economic benefits.' Chen said.
The maximum daily sludge treatment volume increased to 300 tons. Calculated on the basis of 6400 hours of operation of the integrated machine, the annual sludge treatment capacity is about 80,000 tons; each ton of wet sludge can be dried to save 65.60 kg of coal, and if 80,000 tons of sludge are processed a year, it can save 5248 tons of fuel coal and increase power generation by 19 million kilowatt-hours.
The reporter learned from the Municipal Urban and Rural Construction Bureau that last year, the annual dewatered sludge in the urban area was about 100,000 tons, which was safely disposed of through heat and power blending, composting and other methods. The municipal sewage treatment plant fully implements the sludge transfer joint single management system, which records in detail the amount of sludge produced, transported and disposed of, and effectively guarantees the safe disposal of sludge.