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ball mill for coal grinding in power plant

coal pulverizer

coal pulverizer

Power generation industry studies have shown that coal pulverizers are an area where improved equipment reliability is badly needed. The Electric Research Institute (EPRI) has determined that 1% of plant availability is lost on average due to pulverizerrelated problems.1 EPRI also identified oil contamination and excessive leakage as two areas where pulverizer drive train failures account for 53% of pulverizer problems.

Pulverization is currently the favored method of preparing coal for burning. Mechanically pulverizing coal into a fine powder enables it to be burned like a gas, thus allowing more efficient combustion. Transported by an air or an air/gas mixture, pulverized coal can be introduced directly into the boiler for combustion.

This type of mill consists of a rotating tube filled with cast alloy balls. Coal is introduced through two hollow trunnions on each side of the tube. As the tube rotates, the balls tumble onto the coal, crushing and pulverizing it.

Grinding Action is carried out by a series of hinged or fixed hammers revolving in an enclosed chamber with wear resistant plates. The hammers impact on the coal, crushing it against the plates. Further pulverization is achieved as the smaller coal particles are ground through attrition against each other and the grinding face.

This mill uses hydraulically loaded vertical rollers resembling large tires to pulverize raw coal fed down onto a rotating table. As the table rotates, the raw coal is pulverized as it passes underneath the rollers. Hot air forced through the bottom of the pulverizing chamber removes unwanted moisture and transports the pulverized coal dust up through the top of the pulverizer and out the exhaust pipes directly to the burner. The more recent coal pulverizer designs are Vertical Roller Mills. Figure 2 shows a cutaway view of a Babcock and Wilcox MPS Pulverizer.

Mills A ball or roller between two races or rings provides the grinding surfaces on which pulverization occurs. One or both of the races may rotate against a ball or roll (in a Ring-Roll Mill the rolls may rotate while the ring is stationary). Ring-Roll (Bowl-Mill) and Ball-Race Mills comprise the majority of coal pulverizers currently in service at power generating facilities.

In this design the grinding rolls are stationary, while the ring (or bowl, as it is sometimes called) is rotated by a worm gear drive. Powerful springs force the grinding rolls against the ring, providing the pressure required to pulverize the coal.

Raw coal enters the top of the pulverizer through the raw coal feed pipe. The raw coal is then pulverized between the roll and rotating ring. Hot air is forced in through the bottom of the pulverizing chamber to remove unwanted moisture and transport the coal dust up through the top of the pulverizer and out the exhaust pipe directly to the burner. Coal that has not been pulverized into fine enough particles cannot be blown out of the top of the unit; it falls back to the ring and roll to be further pulverized.

The gears and bearings in the gearbox are oil lubricated. Fine coal particles and wear metals from grinding surfaces enter the lube oil through worn bearing and shaft seals, as well as being inhaled through reservoir vents. Historically, the design of pulverizers has been based on the expectation of few drive system problems under prescribed operation and maintenance. In practice, this has often been found not to be true.

Many coal pulverizer designs do not incorporate any filtration in their lube circuits. The pulverizers that do not incorporate filtration use coarse filtration such as 40-micron cleanable mesh or 200-micron cleanable, stacked disk filters. Such OEM-supplied filtration is often unable to keep up with the inherently high ingression rate. This results in contamination levels often exceeding ISO code 30/30, particularly on older designs. This high level of contamination can severely diminish bearing, gear, pump, and seal life, leading to premature need for replacement or rework. Coal pulverizer downtime can be a major factor in reducing overall plant availability and reliability.

Upgrade to Achieve Total Cleanliness Control (See diagrams on back page) The majority of pulverized coal particles are in the 4-30m range, with 70% of these particles smaller than 10m. Ingression rates vary with manufacturer, model, and age ofunit, with older units usually admitting contaminants faster than newer ones. Particulate contamination in the lube system can result in rapid damage to critical components.

In order to protect the coal pulverizer lube system components, Pall recommends maintaining a fluid cleanliness level of ISO 16/13 or better. This can be accomplished through the use of Ultipleat SRT AS grade (12(c) 1000) or finer filters. Ultipleat SRT filters, with their high particle removal efficiency and dirt-holding capacity, are ideally suited to cost-effectively control contamination in this high-ingression application.

When upgrading in-line filtration, a Pall Duplex Assembly is recommended so that elements can be changed out while the pulverizer is operating. Although putting filtration in-line is preferred, difficulty in getting system specifications from the OEM, combined with the typically low pump pressure associated with this application, may make kidney loop filtration a more viable alternative. Reservoir volumes typically vary from 15-300 gallons.

A 20% reservoir volume per minute flow through a kidney loop is generally sufficient to overcome the ingression rate of most applications. The high-viscosityof gear lube oil (2,200 SUS at operating temperature) along with the inherently rapid ingression rate usually associated with these units makes it necessary in most cases to utilize at least one UR619 housing with a UE619 element (12(c) 1000 or finer) for every 50 gpm of flow to provide superior filtration with long element life. Since the pulverizers come on- and off-line, it is important to size the system for the oil viscosity at the coldest possible ambient plant temperatures. Line diameters in the kidney loop should be large enough to facilitate flow of highly viscous lube oil.

Other applications where Pall high-performance filtration is useful include coal-carrying cars and conveyor belts. Many of these applications have both hydraulic and lube systems that are vulnerable to coal dust contamination. This equipment is required to transport the coal stored on-site to the coal pulverizers. Because this equipment is essential to theoperation of the power plant, it is critical that this equipment be free from contamination-related failure.

In March 2003, a major Canadian utility derated its Unit 4 due to a failure of the B mill. The mill was expected to be out of service for about a month. With lost production of approximately 864 MWh per day, the total estimated revenue loss was around $2,000,000. Repair costs for this outage added up to more than $400,000 due to the severity of the damage to drive train components. An analysis concluded that there were multiple causes of this problem, including poor predictive/preventive maintenance practices and poor oil cleanliness.

Pall provided a filter housing for a sixmonth trial to show that oil cleanliness could be improved to industry standards and maintained without the incurrence of substantial element costs. Oil cleanliness went from 20/19/17 to 18/16/13 in approximately 2 hours and has been maintained at this level since.

density clustering analysis of fuzzy neural network initialization for grinding capability prediction of power plant ball mill | springerlink

density clustering analysis of fuzzy neural network initialization for grinding capability prediction of power plant ball mill | springerlink

Ball mill of thermal power plant has high energy consumption and the grinding capability is usually used for representing the efficiency of ball mill. This paper proposes a density clustering analysis method of fuzzy neural network initialization for grinding capability prediction of power plant ball mill. The proposed method integrates the density clustering algorithm and the fuzzy neural network to predict grinding capability, where the density clustering algorithm is used to initialize the rules base of the fuzzy neural network. Furthermore, two parameters of the density clustering analysis can be determined by calculation formula, and the structure of the proposed model could be optimized by the training capability of neural network. The experiments are performed on two datasets obtained from the thermal power plant under the stable conditions. The experiments results verify that the proposed model has higher effectiveness. In addition, the proposed model has been put into practice and the field operation curve proves that the grinding capability could be predicted correctly.

Jiesheng W, Yong Z (2008) PID-ANN decoupling controller of ball mill pulverizing system based on particle swarm optimization method. 2008 Chinese Control and Decision Conference, Yantai, China; 14241428

Joaqun D, Salvador G, Daniel M, Francisco H (2011) A practical tutorial on the use of nonparametric statistical tests as a methodology for comparing evolutionary and swarm intelligence algorithms. Swarm Evol Comput 1(1):318

Quansheng D, Jizhen L, Zhifang W (2008) Design and experimental study on the pulverized coal concentration sensor based on -ray absorption method. Proceedings of 2008 I.E. Pacific-Asia Workshop on Computational Intelligence and Industrial Application 901906

Jiang, R., Wang, Y. & Yan, X. Density clustering analysis of fuzzy neural network initialization for grinding capability prediction of power plant ball mill. Multimed Tools Appl 76, 1813718151 (2017). https://doi.org/10.1007/s11042-016-4089-4

coal water slurry ball mill | ball mill for coal water slurry processing

coal water slurry ball mill | ball mill for coal water slurry processing

Coal water slurry ball mill (CWS ball mill) is the key equipment for coal water slurry fuel production, which is commonly used in coal water slurry power plant. Coal water slurry ball mill is grinding equipment specially used for coal water slurry production. Its specifications, structure and working conditions must meet the requirements of coal water slurry production scale, production process characteristics, rheological characteristics, product particle size, and coal type characteristics.

As the main equipment of the raw material grinding link in the coal water slurry production line, the fineness of the coal powder ground by the coal water slurry ball mill directly affects the quality of the subsequent products of the coal water slurry. A high-quality coal water slurry ball mill can guarantee the efficiency and fineness ofpulverized coal grinding. Therefore, it is easier to fully mix the pulverized coal and the solvent, and the utilization rate of the pulverized coal can be improved, and the occurrence of environmental pollution can be reduced.

Grinding is a key link in the process of coal water slurry preparation. Different from grinding in other industries, it not only requires the product to achieve a certain fineness, but more importantly, the product should have a good particle size distribution. The structure, operation condition and grinding process of coal water slurry ball mill meet the requirements of coal water slurry for production process. Therefore, coal water slurry ball mill is the most suitable grinding equipment for coal water slurry production line.

Slurry filtration. The coal water slurry finely ground by the coal water slurry ball mill flows through the vibrating screen for slurry filtering operation. The filtered coal water slurry enters into the slurry storage tank to obtain the finished coal water slurry.

Raw coal should be screened for coal water slurry production. Due to the different types of coal water slurry, the needs of users will be different, so the requirements of coal use will also be different. The purpose of coal preparation is to meet the users requirements for ash, sulfur, and calorific value of coal water slurry.

The coal water slurry should be made of cleaned coal or low-sulfur and low-ash coal. After coal is washed, 30%-40% of sulfur can be removed (60%-80% of pyrite sulfur is removed), ash (gangue, etc.) removed by 50%-80%, and the amount of original smoke and ash can be reduced. Therefore, the sulfur content and ash content in the slurry is low.

As a ball mills supplier with 22 years of experience in the grinding industry, we can provide customers with types of ball mill, vertical mill, rod mill and AG/SAG mill for grinding in a variety of industries and materials.

exploring ball size distribution in coal grinding mills - sciencedirect

exploring ball size distribution in coal grinding mills - sciencedirect

BSD affects milling efficiency.Ball top up policy in turn affects BSD.Wear rate models can predict BSD for a given top up policy.Fine BSD generally enhances milling rate for uncontaminated coal feed.

Tube mills use steel balls as grinding media. Due to wear in the abrasive environment it is necessary to charge new balls periodically to maintain a steady balanced ball charge in the mill. The amount and ball size distribution in this charge, as well as the frequency with which new balls are added to the mill, have significant effects on the mill capacity and the milling efficiency. Small balls are effective in grinding fine particles in the load, whereas large balls are required to deal with large particles of coal or stone contaminant. The steady state ball size distribution in the mill depends on the top-up policy.

The effect of the ball size distribution on the milling rate of coal has been measured as a function of ball size distribution. The change in ball size distribution as affected by wear and ball top-up policy has been modelled. From this a best ball top-up policy can be recommended that will ensure a close approximation to the desired steady-state ball size distribution that gives the required PF size distribution for the selected mill demand.

coal grinding - cement plant optimization

coal grinding - cement plant optimization

To achieve good combustion and satisfactory flame formation, coal needs to be dried and ground to a proper degree of dryness and fineness. Drying of moisture in coal is achieved normally by ducting part of the kiln exhaust gas through the mill with inlet temperatures of up to 300C. Inert kiln exhaust gases with oxygen content of about 3-5% are most suitable for the intended purpose due to high risk of fire/explosion in fine coal. However, the provision for inertization of coal mill circuit and fine coal bins (with CO2, N2 or CO2+N2 to replace O2 which promotes spontaneous ignition of fine coal) is strongly recommended to be available. Gas analyzers and explosion vents are essentially provided in mill circuits to monitor the potential of fire/explosion and mitigate fire/explosion incident respectively. Drying and grinding are generally being done in either air swept ball mill or a vertical mill. The selection of mill system will depend mostly on the factors like initial capital cost, drying and grinding capacity required, cost of energy (power) etc.

Coal Fineness: It is understood generally that the finer we grind, the easy it is for burning. However, the fineness required will always be directed by where to fire and what type of coal it is and lastly the fineness will be dictated by the risk factor involved in finer grinding high volatile coals. The recommended fineness for coal verses volatile matter percentage is depicted in graph beside.

As understood from the above graph, the relationship between 90-micron and 200-micron residue is quite important as well. So, it is generally recommended to have 200-micron residue as low as possible, because coarse particles delays ignition, gives long flames in kiln (coating & ring issues), CO formation at kiln inlet, higher preheater exit temperatures (EGT). As a rule of thumb, the residue on 90-micron sieve should not be less than half of the volatile content for safety purpose. ie. R90>=1/2. Volatile Content %.

Coal Moisture: The degree of drying, and therefore the required mill outlet temperature (from 65-80 0C) will depend upon the type of coal ground. Some residual moisture in fine coal is recommended (Graph below) to minimize the potential of spontaneous ignition of fine coal, which will again vary for different coal types as below:

While considering the safe mill outlet temperature, care should be taken to avoid the temperatures below dew point of mill outlet gases, so that the condensation inside the bag filter and consequent material jamming problems can be avoided.

Important Note: If you chose to use different types of coal (having different rank) simultaneously or use coal and Petcoke, remember to grind them separately as per above guidelines and feed them from different fine coal bins in required proportion to kiln and pre-calciner as required.

Coal Grinding Operation Objectives and KPIs: Highly energy intensive unit operation of coal grinding is intended to provide a fine coal as a fuel for calcination and clinkerization. Coal grinding operation is monitored for following parameters to ensure objectivity and economy of operation.

Note: Proximate and ultimate analysis are generally provided by coal supplier. However, Proximate analysis, Determination of calorific value, Ash analysis, Hardgrove analysis and Abrasion analysis are done as and when required in plant laboratory or by a third party agency.

Mill Load (Kw or Amps). Mill sound/filling % (in ball mills). Mill Inlet Temperature (0C). Mill Outlet Temperature (0C). Gas flow through mill (m3/h) or mill fan power (kw) Mill DP, or inlet/outlet draft (mmH2O). Separator DP (mmH2O, mbar) and temperatures (0C). Bag filter DP (mmH2O, mbar), Temperature (0C).

Position of Explosion vents. Operational readiness of quick shutoff dampers. Inertization section readiness (N2, CO2 pressure in bars) Mill Inlet Temperature (0C). Mill Outlet, bag filter outlet Temperature (0C). O2 + CO Percent at bag filter outlet and in fine coal bins. Bag filter hopper and fine coal bin temperature (0C).

Mill Feeding: Consists of following activities Coal Crusher: Generally, require when ball mill is used for grinding and raw coal size is on higher side(>25mm). Conveying to Hoppers: Covered belt conveyors, horizontal or inclined are most suitable and commonly used for conveying. Metal Detector and Magnetic Separator Arrangement of metal detector and magnetic separator is integral part of feeding system in vertical roller mills and roller presses. Both are installed on mill feeding belt conveyor. Magnetic separator, separates out small metallic impurities from mill feed. While as metal detector signals the presence of metallic debris, which can damage the grinding path and give rise vibrations issues. Mill Feeding Hoppers Hoppers for coal, petcoke serve the purpose of providing a buffer storage for mill feed and a convenient arrangement for feeding to weigh feeders. Hoppers are generally designed to hold the requirements of one shift or more. Coal hoppers are generally steel hoppers with conical steep bottom (inclination >700), wide opening for discharge as possible to ensure mass flow of coal, mounted on load cells and/or equipped with level sensors to guide filling in auto mode. De-dusting bag filters needs to be installed at the top to vent air when material is fed to a hopper. Mill Feeders: Feeders for coal mill are generally installed directly under hoppers with rod gate in between. The feeders are generally 2 to 3 m long and discharge on to conveyor or feeding chute to mill. For coal mill feeding, table feeders, belt feeders, chain feeders and weigh feeders have been used. However, weigh feeders are the most commonly preferred to feed and report production counters. Metering on Feeders: Metering can be either direct (gravimetric) or indirect (Volumetric). In direct method of metering the material passes over a load cell installed in weigh feeder/apron feeder and the travel speed is monitored with installed tachometer. Weight and speed together determines the mass flow rate of material in metric tons per hour (t/hr). Alternatively, in some of the arrangements direct system consists of weigh feeder and its pre-feeder. Feed rate is generally controlled with the speed of weigh feeder belt, which is driven by a variable speed drive. Feed rate is monitored and controlled by a control panel generally supplied with weigh feeder. Set points are passed to control panel from CCR by Operator. In indirect metering system feed rate is determined by measuring cross section of material and rate at which it flows and taking into account bulk density of material.

Calibration and Drop Test Facilities: Provisions for drop test for calibration of weigh feeders are commonly available in cement plants to validate production figures. Although the weigh feeder calibration is not required frequently unless there is a disturbance in mechanical system due to various reason including maintenance. However, it is a common practice to validate feed rate through drop test periodically. Weigh feeders generally come with self-calibration devices. A simple way of 'self-calibration' is to have the hopper mounted on load cells, so that a weight loss for predefined time will be used for calibration purpose, and in this case calibration of hopper load cells needs to done at least once a year with standard weights.

Cold Air Locking at Mill Inlet: This is very important for coal mill, as ambient air throttling the drying capacity of mill as well as increases the oxygen content of kiln gases making it riskier. Oxygen percentage of more than 12-14% is considered catalyst for fire/explosion risk. Rotary feeders (gravel gate), double flap valve are used to stop/minimize cold air leakage into mill system. Cold air leakage percentage can be determined by measuring oxygen percentage at inlet and outlet of the circuit element.

Ball Mill: Single chamber ball mills (with classifying liners and dam ring) with drying chamber and static or dynamic separator are commonly existing for coal grinding in cement plant for one or more kilns as per capacity. Ball mill is a cylinder rotating at about 70-80% of critical speed on two trunnions in white metal bearings or slide shoe bearings for large capacity mills. Grinding media consists balls of 3-4 sizes (60mm-30mm) in designed proportions with large sizes in feed end and small sizes in discharge end. About 27 to 35 % volume of mill is filled with grinding media. Equilibrium charge is that charge where compensation for wear can be done by balls of one size only usually the largest size in the compartment. Grinding media could be made of forged steel, cast steel or even cast iron. To economize grinding media consumption, presently grinding media used are high chrome steel balls. Mill shell is lined with lining plates to protect it from wear, high chrome steel liners are now commonly preferred to give longer life. Ground material is swept out of the mill by hot air /gas of significant velocity (5-6 m/s), through separator and coarse fraction is returned to the system for regrinding and fine material passed to bag filter for collection.

Vertical Roller Mills: In Vertical Roller mill 2 - 4 rollers (lined with replaceable liners) turning on their axles press on a rotating grinding table (lined with replaceable liners) mounted on the yoke of a gear box. Pressure is exerted hydraulically. This mill also has a built in high efficiency separator above the rollers to reduce circulation loads and consequently reducing differential pressure across the mill. Feed material is directed onto the center of the table and is thrown outward by rotation under the rollers by centrifugal action. Material gets partially ground and as it falls over the edge of the table, where it is picked up by hot gases, and is separated into coarse fraction falling back on grinding table and fine fraction is carried with hot gases to product collector. The mill is started either with the rollers in lifted-up position, or with the hydro-pneumatic system at low pressure. In grinding mode, actual metal to metal contact should be prevented by limit switches or a mechanical stop and by consistent feed. In VRMs the material cycle time is usually less than a minute against several minutes for a ball mill or tube mill. Thus, control response should be accordingly faster. In case mill feed fails action should be taken within no more than 45 seconds or excessive vibration will cause mill shut-down. Moreover, the vertical mills are subject to vibrations if material is too dry to form a stable bed. Therefore, provision is made for controlled spray water inside the mill During mill operation magnetic separator and metal detector should be always functional to ensure to exclude tramp metal which can damage the grinding surfaces.

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