Excessive sulfur dioxide produced by industrial production has increased the concentration of SO2 in the air, leading to acid rain in many parts of the world, especially in Western Europe, North America, and India. SO2 pollution and acid rain endanger the health of residents, corrode building materials, damage the ecosystem, and cause huge economic losses.
Most of the sulfur dioxide comes from industrial waste gas from smelting sulfur-containing ore, burning fossil fuels, producing sulfuric acid, phosphate fertilizer, etc., and waste gas from motor vehicles.
Relevant research shows that 90% of the current sulfur dioxide emissions come from burning coal. Thermal power plants are big coal consumers. Therefore, the focus of reducing the total sulfur dioxide emissions is on the desulfurization of coal-fired power plants. The mainstream desulfurization method currently used is the limestone-gypsum wet flue gas desulfurization(FGD).
How does FGD work?
Currently, the limestone-gypsum wet flue gas desulfurization is the most mature and widely used desulfurization method in the world. The limestone slurry made of limestone powder and water is pumped into the absorption tower to fully contact and mix with the flue gas. The sulfur dioxide in the flue gas, the calcium carbonate in the slurry and the air blown from the lower part of the tower undergo neutralization reaction and oxidation reaction to form calcium sulfate.
Calcium sulfate reaches a certain degree of saturation and crystallizes to form dihydrate gypsum, which is discharged, concentrated, dehydrated to a water content of less than 10%, and sent to the gypsum silo for storage. Desulfurization flue gas is demisted by a demister, heated by the heater, then discharged into the atmosphere through the chimney.
The desulfurization rate of limestone-gypsum wet desulfurization is up to 99%. More than 95% of coal-fired power plants use this method to implement desulfurization, which is the most effective and main technical means to alleviate sulfur dioxide and acid rain pollution.
What factors of limestone will affect the performance of GFD?
As the adsorbent in the FGD system, limestone plays a vital role in the entire reaction. So what factors of limestone will affect the desulfurization efficiency of the FGD system?
(1)Purity of limestone powder:
Generally, the mass percentage of CaCO3 in the limestone is used to show the purity of the limestone. This figure in limestone powder for desulfurization should be higher than 90%. If it is too low, there will be problems in operation due to more impurities, such as increased absorbent consumption and transportation costs, decreased gypsum purity, and increased waste disposal costs.
(2) The particle size of the limestone powder:
The particle size of the limestone powder determines the dissolution surface area. The finer the limestone powder, the larger dissolution surface area. The particle size is determined by the sieve through which the limestone powder sample passes.
At first, the typical specification of limestone particle size is that the limestone powder can pass through a 200-mesh sieve, and the pass rate is 70%. However, FGD system designers and chemists are beginning to realize that this kind of limestone powder is too coarse, which will result in low limestone utilization and low desulfurization efficiency. Nowadays, it is the standard requirement that limestone powder passes through a 325-mesh sieve with a pass rate higher than 90%.
How to get high quality limestone powder for desulfurization?
We can use measuring instruments to select high-purity limestone mines. In terms of particle size, we can precisely control it through the grinding process. There are two common desulfurization limestone grinding processes: one is dry milling and the other is wet milling.
There are many choices of milling equipment using the dry milling process. Commonly used are the Raymond mill and the three-ring medium-speed mill.
Raymond mill (Vertical mill machine、Vertical pulverizer)
Limestone powder is poured into the machine from the feeder on the side of it. The grinding roller device suspended on the star rack of the main engine revolves around the vertical axis and rotates itself at the same time. Limestone fragments are shoveled up by a scraper blade and sent between the grinding rollers and the rings to be ground into powders.
After grinding, the limestone powders are blown up by the wind from the bottom fan and sorted by the analyzer in the grinding chamber. The unqualified limestone powders fall into the grinding chamber for regrinding, and the qualified ones go to the cyclone collector with the wind flow. Then, they are discharged through the powder outlet, collected by a bag vacuum cleaner into the limestone powder bin.
The limestone powders in the silo are quantitatively delivered to the limestone slurry tank by the feeding device, mixed with water to make a certain concentration of limestone slurry, and then transported to the absorption tower by the slurry pump.
- The entire Raymond mill is vertical, occupying a small area. The complete set is strong, and it can form an independent production system from the rough processing, conveying, milling to storage.
- It has an electric concentrated control operation system and unmanned operation can be realized in the computer operation room.
- The transmission device of the main engine adopts closed reducer that makes the mill operate smoothly and reliably.
- It has good sealing performance and can be operated under negative pressure, which can reduce dust emission and ensure a clean and environmentally friendly production.
- The finished limestone powders are uniform in particle size. Its pass-through rate is high, as high as 99%, which is difficult to achieve by other grinding equipment.
- The wearing parts are made of high manganese steel, which makes sure a long service life.
- The noise is loud during operation, silencers can be installed to reduce the noise.
- The hourly capacity is small. When the required output is large, multiple devices can be operated at the same time.
Derived from the pursuit of quality, Forte has always been innovative. It learned from advanced experience at home and abroad and launched the HLM German version of the ultra-fine mill. It works on the same principle as the Raymond Mill, but has been upgraded in many places.
- The use of DMC pulse single dust collector guarantees a more satisfying cleaning effect, reducing energy consumption.
- The innovative design of the grinding roller and grinding ring slows down the material’s sliding speed, prolongs the rolling time, and makes the limestone powder finer and more uniform.
- The refined impeller device can increase the output of finished products by more than 50% at the same energy consumption, which enhances the productivity advantage.
Tricycle Medium-speed Micro Mill:
When the Tricycle Medium-speed Micro Mill is running, the main engine drives the main shaft and each layer of the turntable to rotate through the reducer, so that dozens of grinding rollers roll and rotate in the ring raceway. The raw materials are scattered to the periphery under the action of centrifugal force, fall into the raceway, and are crushed, pressed and ground by the rollers, After the materials are crushed in the first layer, they fall into the second layer for finer grinding, and finally the third layer for the required particle size.
- Safe and reliable operation. There is no rolling bearing and no screws in the grinding cavity. So there is no damage caused by the bearing, fragile seals, and loosen screws.
- Energy saving and environmental protection. The crushing and grading of limestone are carried out in a closed system. The dust containing gas is treated by dust removal. All these make this milling machine an ideal equipment for environmental protection.
- High fineness index. The equipment adopts graded grinding to make the finished limestone powder finer. The fineness of the finished product can reach 3000-mesh at one time.
- Environmental friendly and pollution-free. The use of the high efficiency pulse type dust collector makes sure there is no dust pollution during the operation of the whole set of equipment.
- Mufflers and anechoic rooms are equipped to reduce noise.
- This type of mill is large in size and covers a large area.
- The clearance between the assembly parts is very small, which easily causes poor lubrication.
- Due to the looseness of machine parts, vibration and heat of the machine, leakage will appear at the pipe joint of the sealing surface of the machine.
The main milling machine used in wet grinding is wet ball mill. The limestone is mixed with water during grinding in a wet mill to form limestone slurry directly, which is pumped into the hydraulic rotator for separation. The limestone slurry with qualified limestone particle size enters the limestone slurry tank and is pumped into the absorption tower by a slurry pump. If it is not, it will be sent back to the grinder for regrinding.
- It can be operated continuously and has a large production capacity, which can meet the needs of modern industrial large-scale production.
- It has good sealing performance and can be operated under negative pressure to avoid flying dust and environmental pollution.
- The simple and sturdy structure makes it easy to operate and maintain. And the worn parts are easy to check and replace.
- The comminute proportion is large. And it is easy to adjust the product’s fineness
- The design of the discharge port is reasonable and flexible, and customers can choose iron remover according to their own needs.
- Fully hydraulic operation, easy to use, easy to realize automation.
- A lot of power will be consumed when starting. Therefore, it is suggested that the ball mill should not be stopped and started without special circumstances during operation to avoid more time and power waste.
- High power consumption.
- When the milling balls impact and grind the limestone, the balls themselves and the lining plate in the cylinder will also be worn. Regular inspection and spare lining plates and grinding media are needed.
Power plant desulfurization is an effective way to alleviate the problem of acid rain. Its efficiency is directly affected by the quality of limestone powder. Therefore, it is crucial to obtain high quality limestone powder. That calls for the appropriate grinding equipment. Through the above comparison, we can see that these two kinds of grinding equipment have their own advantages and disadvantages. When selecting a desulfurization limestone milling machine, we should take the fineness requirements, work efficiency, production environment and other aspects into consideration.
FAQs about limestone milling
What are the problems in the operation of limestone mill? How to solve them?
1. The particle size of limestone is much coarser than the specified particle size.
|The V-belt is loose.||Tighten the V-belt and adjust other parts appropriately to reach the satisfactory product size.|
2. The bearing in the mill locks up.
|Heating problem||Rest the milling machine for a while|
|Lack of oil in bearing.||Lubricate in time.|
|Dust problem.||Regularly inspect and clean the bearing.|
|Damaged bearing.||Replace new bearing in time so as not to damage other parts.|
3. No powder or low powder output.
|The powder locker is not adjusted properly and the seal is not tight, causing reverse suction.||Check and adjust the seal of the powder locker.|
|The shovel blades are worn out, and the materials cannot be shoveled.||Replace with new blades.|
|Air duct is blocked.||Clean up the materials in the air duct before refilling.|
|The pipeline is leaking and the sealing is not good.||RSeal the pipeline.|
4. The finished limestone powder is too coarse or too fine.
|The blades of the analyzer are seriously worn and cannot grade limestone powders.||Replace the blades;|
|The air volume of the fan is not appropriate.||too coarse, turn down the fan to reduce the inlet air volume. too fine, increase the inlet air volume.|
5. The host is noisy and vibrates greatly
|Little limestone feed leads to severe blades wear, making them unable to shovel limestone fragments.||Adjust the feed rate and replace with new blades.|
|The grinding rings and grinding rollers are seriously deformed.||Replace the grinding roller and ring.|
What should the staff pay attention to when the limestone mill is working?
- 1The staff should memorize the maintenance instructions presented by the manufacturer, and regularly maintain the equipment.
- 2Pay attention to counting the service time of the roller grinding device. It should be replaced after a certain period of time. Clean the bearings during replacement.
- 3Lubricate regularly and thoroughly so as to avoid damaging some parts. For example, if the bearing is not lubricated thoroughly, the pulverizer will lock up.
- 4Feed evenly, not too much or too little. Too much feeding will block the air duct, reduce the output, and easily burn the electric motor. Too little material is easy to wear the blade.
- 5Adjust the air volume according to the fineness of the limestone powder.
- 6If you want to stop the machine for maintenance, stop feeding first, and then the host continue to work for about one minute to grind the residual limestone powder. Then turn off the host motor and analyzer motor. Finally, turn off the fan motor to blow away the remaining powder.
What is the desulfurization gypsum used for?
At present, the desulfurization gypsum is mainly used to produce gypsum board together with natural gypsum.
Why choose limestone as absorbent in FGD?
Limestone powder is used in desulfurization of coal-fired power plants because coal contains sulfur, which appears in the form of sulfur dioxide or sulfur trioxide when burning at high temperature. The calcium oxide produced after high-temperature combustion of limestone is alkaline and can react with sulfur dioxide and sulfur trioxide by acid-base neutralization. Therefore sulfur dioxide and sulfur trioxide is removed from the flue gas.
Other alkaline substances can also be used, such as magnesium hydroxide. But limestone is cheap. In terms of economic benefits, limestone is a better choice.