BOWL MILLS MAINTANACE -BLUE PRINT

A Four-Step Plan:

Blueprinting a pulverizer isn’t rocket science, but it does require close attention to the details. Here is our four-step plan to restore and improve performance of your pulverizer, regardless of its age.

STEP-I :

**Ensure that the grinding elements are in good condition.

 Make sure that the grinding surface profiles are optimum. That means using the original design grinding profiles for your mill. The majority of coal pulverizers sized around 120,000 pph use three grinding elements, referred to as journals, rolls, or tires. For best results, all three grinding elements should be replaced in matched sets. The concentricity, physical dimensions, and contours must be exactly the same. This is especially important when maximum preload pressure is required to produce maximum coal fineness and/or with lower-than-original design HGI. We have seen mills assembled with unmatched sets of three grinding journals using maximum spring pressure. The result of such setups: The main shafts break because of the unbalanced load. Matched sets of grinding elements and exactly the same size rolls with exactly the same contour are important for maximum reliability.

The grinding surfaces also must be in good condition and parallel (Figure 3). Don’t expect optimum performance if the grinding elements are well-worn or the tires are "flat" (Figure 4). Unusual wear patterns are often the result of uneven spring frame tolerances, alignment issues, pressure variations, geometry, and/or eccentricity issues. 

  **Walk the line. The profile of the roll should be parallel with the grinding ring profile.

***Perfectly round. In an MPS mill, the tire and table profiles must match, and the tires should not have flat spots.

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STEP-II:

**Set the correct grinding pressure.  

Check your mill to confirm that the grinding roll or spring frame preload pressure is set correctly. Our experience with both RP and MPS pulverizers has been that mills designed for a throughput of about 120,000 pounds of coal per hour, an HGI of about 45 to 50, and coal fineness exceeding 75% passing 200 mesh will require about the same force on the grinding elements. It is reasonable to expect that grinding coal will take about the same amount of grinding element pressure regardless of the type of medium-speed, vertical spindle pulverizers you use.

In our experience the spring frame of an MPS mill tuned for maximum true capacity will be set at about 20 tons minimum force on the grinding tires. A bowl mill spring or hydraulic preload for this size of mill will also be about 20 tons of pressure. Lower-HGI fuel and greater than passing of 200 mesh requires the maximum pressure of the grinding elements. Keep in mind that in operation there is no metal-to-metal contact, and all coal grinding results from the pressure applied coal particle – to – coal particle on a bed of coal squeezed between the grinding elements.

Internal clearances are also very important. For example, a bowl mill spring canister can be set to the needed preload, but if it is not adjusted for the "button" to roll with assembly minimum clearance, then the preloading does not come into play until the roll rides up on a deeper bed of coal . Ensuring sufficient grinding pressure is absolutely essential, and it begins with setting this critical tolerance.

** Keep your distance. The "button" clearance between the spring canister and the journal assembly is a critical tolerance.

For a spring frame mill, the hydraulic preload must be balanced across the mill and the grinding elements perfectly centered in the assembly

* *Balanced load. The MPS spring frame hydraulic preload must be carefully balanced and the spring frame centered for optimum mill performance. 

STEP-III:

** Set the correct pulverizer throat clearance. An oversized pulverizer throat will require more than optimum primary airflow to minimize coal rejects. The pulverizer "free annular jet" of vertically flowing airflow, in our experience, must be adjusted for a minimum of 7,000 fpm under normal operation. Throats that are oversized will result in either excessive coal rejects (not tramp metal or pyrites, but raw coal).

Compounding the problem, high primary airflow is the main cause, in our experience, for poor fuel fineness, poor fuel distribution, and reduced furnace performance. Right-sizing the flow area of the pulverizer throats and matching them for compatibility with the coal pipes and burner nozzle sizes is essential for the best furnace performance. Furthermore, remember that there will be minor variation in mill capacity, fuel quality, and mill inlet airflow rates that must be considered when sizing the pulverizer throat flow area.

The vertically flowing air must be of sufficient velocity to suspend the granular coal bed in the grinding zone. Some designs use mechanical means to keep the coal above the under-bowl pyrite section, while others use airflow. Reducing coal rejects by mechanical means entails increasing the height of the "bull ring" extension ring or the extension of flat surfaces above the rotating throat to trap or dam coal particles mechanically so that they remain above the throat.

We prefer the optimum throat area fluidic solution to suspend the coal bed and reduce the potential for fires beneath the bowl or grinding table. Keep in mind that if the fuel is above 17% moisture and the air/fuel ratio is about 1.8, then the under-bowl primary air temperature will be above 450F. Any coal that falls through the throat opening will combust unless it is removed in mere minutes. Combustion of coal particles beneath the grinding zone is not a serious problem, as long as the mill is in operation. But if a mill trips or a boiler has a main fuel trip, then fires in the pyrites zone (beneath the grinding zone) are the most common cause of pulverizer "puffs," in our experience. A fire beneath the grinding zone provides the ignition temperature to initiate a mill "puff" when restarting a mill after a trip or restarting it after a main fuel trip when coal remains in the bed.

For safety as well as for performance reasons, properly sizing the mill throats is extremely important . The optimum throat area is determined by calculating the free annular jet area when the desired air/fuel ratio (usually 1.8 lb air/lb fuel) is known. Also, the throat area must be properly designed to be compatible with the flow areas of the burner coal pipes and coal nozzles

**Optimum design. Ensure optimum arrangement of the mill throat and the coal flow path to improve mill performance.

**Close tolerances. Pulverized coal mills with throats that are too wide will have corresponding low throat velocity in the mill grinding zone that contributes to excessive coal rejects and fires. This is an example of an oversized mill throat.

STEP-IV:

**Properly maintain the classifier.

Once the grinding zone is blueprinted and put in first-class condition, the next component to examine is the classifier. The best furnace combustion performance is governed by uniform coal combustion by the burners and satisfactory coal fineness. Adequate fineness for both western and eastern fuels (Powder River Basin or bituminous) is a minimum of 75% to 80% passing 200 mesh and zero to 0.1% remaining on a 50 mesh screen (Figure 9). To achieve this fineness, the pressurized mill classifier must perform two functions:

·   **      It separates particles small enough to be supplied to the burners (mean particle size about 40 to 55 microns) from larger coal particles that need return to the grinding zone for regrinding.

·     **    It balances the distribution of coal to each coal pipe.

**Why use a classifier? A classifier recirculates coarse coal in the grinding zone and balances the flow of coal to each burner line to the furnace.

The flow of coal particles through a classifier is several times the amount of coal flowing to the burners because of the large amount of coal recirculated within a pulverizer. For example, if a pulverizer is operating at 100,000 lb/hr coal feed to the burners, as much as 300,000 lb/hr or more may be flowing through the classifier for regrinding. For this reason the surface smoothness and inverted cone clearances are extremely important for good pulverizer performance.

Our experience over the years has helped us develop a number of proven minor enhancements for achieving best classifier performance. The critical dimensions indicated in Figure 10 include:

·         Surface smoothness of the classifier cone.

·         Synchronized classifier blade angles and lengths.

·         Inverted cone to classifier clearances.

·         Classifier outlet cylinder length and flared opening.

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**Better than good. Areas of the classifier where performance can be improved.

Other improvements that should be considered when overhauling a classifier include these:

·         Smooth surfaces in the upper turret section for good fuel spinning and uniform distribution (no surface discontinuities, such as "pad eyes").

·         Ensure the free movement and closure of the discharge doors (trickle valves).

·         Confirm the sound and good condition of the classifier cone assembly (no holes should be worn through).

·         Ensure the good mechanical condition of the classifier blades.

** Mill design guide. An internal view of  a typical vertical-spindle pulverizer and specific areas where special attention to dimensional tolerances and assembly dimensions can improve mill performance.

COAL PULVERISING IN BOILERS

Coal Pulverizing in Boilers:

     Pulverizing coal for a boiler is very important factor in overall cycle efficiency. There are many types of pulverizers available, but proper selection will ensure consistent boiler and cycle efficiency. This helps in reduction of carbon-dioxide emission per million units of electricity generated.

     Boilers for steam generation in power plants and process industries use coal as fuel. The percentage of boilers operating with coal as fuel outnumbers the boilers using all other fuels combined. Coal is pulverized before firing for achieving a stable and efficient combustion. Many types of pulverizers are used in boilers by different designers.

History of pulverization:

         The history of pulverization dates back as early as 1824 and was envisioned by Carnot in a coal fired engine. In 1890 Diesel made use of pulverized coal in his diesel engine. Pulverized coal firing was first developed in the cement industry and then migrated to the power and process industries. Actually Thomas Alva Edison and the Niepce brothers of France were pioneers in pulverized coal firing. This technology gained momentum after World War I in the power generating industry. It was John Anderson, chief engineer of power plants at the Wisconsin Electric Power Company who introduced pulverized coal firing in power stations.

            Pulverized coal is the most efficient way of using coal in a steam generator. The coal is ground so that about 70 % will pass through 200 mesh (0.075 mm) and 99 % will pass through 50 mesh (0.300 mm). A pulverized coal boiler can be easily adapted for other fuels like gas if required later without much difficulty. However, during the design stage it is possible to make boilers firing multiple fuels. With pulverization technology, large size boilers could be designed, manufactured, erected, and run much more efficiently.

Types of pulverizers:

         Mainly there are three types of pulverizer used in industry: The slow speed mills like ball tube mills, the medium speed mills like bowl, ball and race, roller mills fall in this category, and the third type is the high speed impact mill. The slow speed and medium speed mills are selected for coals ranging from sub-bituminous to anthracite. The high speed mills are used mainly for lignite.

The purpose of a pulverizer in a coal fired boiler:

• To supply pulverized coal to the boiler as per requirement of steam generation
• Transport the pulverized coal from pulverizer to the burners in the boiler
• To remove moisture in coal to an acceptable level for firing in boiler
• To remove high density inorganics from coal during pulverization
• To classify coal particles to the required level of fineness, normally 70 % through 200 mesh and less than 2% on 50 mesh .
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Coal parameters affecting pulverizer output :

             While selecting a pulverizer, the coal characteristics play an important role. The Hardgrove index, total moisture, input coal size, output fineness, and mill wear have direct impact on the mill output.

• The Hardgrove index of coal tells us about the ease with which it can be pulverized. A higher Hardgrove index indicates the coal is easier to grind. 50 HGI normally is taken for calculating the base capacity of the mill. When coal with HGI higher than 50 is fed to the pulverizer, the output will be higher than base capacity, and below 50 HGI, the output will be lower.
• The total moisture in coal has a high effect on mill output. The higher the moisture, the lower the output.
• Higher pulverized coal fineness increases the recirculation in the mill and the output reduces.
• The inlet size of the coal also affects the mill output directly.
• Mill air flow variations result in changes in mill outlet temperature and fineness as well as capacity.

Ball tube mill:

           Ball tube mills are either pressurized or suction type. In the pressurized type, the hot primary air is used for drying the coal and to transport the milled coal to the furnace. In this type, leakage in the mill area is high.

         In the suction type, the exhauster is used for lifting the milled coal from the pulverizer to the furnace through a cyclone. The tube mills have a large circular drum, with adequate ball charge, which is rotated at about 70% of the speed at which the ball charge would be held against the inner surface by centrifugal force. In this mill the grinding balls can be replenished on the line.

           Normally the ball mill designers use three types of balls with three different diameters. These balls reduce in size as the mills operate and so the highest size ball is normally used for recharging. In earlier days, most of the ball mills had a single inlet and outlet, but now designers use both ends to feed coal and also for taking out pulverized coal. The control systems are well made to understand the requirement of ball charge and the output from the mill. Ball mills are always preferred to be operated at full capacity because the power consumption of this type of mill is very high at lower loads when compared with other types. Ball mills can be designed for a very high capacity like 75 tons per hour output for a specific coal.

Vertical spindle mill:

             There are many different varieties of vertical mills. Designers use large steel balls ranging from 2 to 6 or more between two grinding rings for pulverizing. There are also other types like conical rollers with shallow bowl; deep bowl, etc. where load is applied on the rollers and the bowl rotates while pulverizing. These types of mill are designed normally up to 60 tons per hour for a specific coal; however there are vertical mills with 90 tons per hour output. A vertical spindle mill is also designed for pressurized and suction type requirements. Boiler designers use this type of mill for poor quality coal as this type of mill rejects foreign materials like stones and other high density materials. The power consumed by the mill per ton of coal ground is only two-thirds of the ball mills. However if the primary air fan power is also taken into account, in the case of a pressurized mill the power consumption is lower only by about 15%.

High speed impact mill:

            This type of mill uses a central horizontal shaft which has a number of arms, and a beater of different design is attached to these arms to beat the coal to be pulverized. High speed impact mills are mainly used in pulverizing lignite. Today all boiler designers opt to use ball or vertical spindle mill for coal other than lignite.

             While selecting the type of mill boiler designers must clearly understand the coal characteristics, the overall system being used, and the maintenance requirement. It is always seen that if the advantage of the mill alone is considered, then the overall boiler economics can prove to be different.

 

COAL HANGUP IN BOWL MILLS

Coal Hang-up in Bowl Mill:

      Depending on the quality of the coal, sometimes the smooth flow to the coal mill can be disrupted. The mill will then trip on flow protection, upsetting boiler parameters and demanding immediate operator corrective action.

        Bowl mills are used for pulversing coal in pulverized coal fired boilers. The coal to the mill is fed by a coal feeder from the coal bunker. There are many type of feeders used for coal feeding like the belt feeder which can be in volumetric or gravimetric mode, the chain link feeder, the drag feeder, etc. Row coal from the yard is sized in crushers and stored in coal bunkers. It is sometimes experienced, depending upon coal quality, that the smooth flow to the coal mill can get disrupted. The mill will trip on no coal flow protection, and such a mill trip can upset boiler parameters and require corrective action.

        For taking corrective action during a coal hang-up in bowl mills, the operator will have to know the reason for such a hang up, how the plant will respond, what he has to respond to, and what the local operator will have to do. This will reduce the chance of mill trip and bring the boiler back to normal condition as early as possible.

Specific causes:

The reasons for coal hang-up to mills can be many, like large raw coal size, jamming of the feeder, jamming of the coal chute from the bunker due to high moisture in the coal, foreign material at raw coal inlet to feeder, etc.

Plant responses:

      The boiler fuel input coming down due to coal flow hang-up the mill slowly gets unloaded. This can be inferred from the response of the boiler.

• Mill differential pressure comes down

• Mill outlet temperature will rise

• Boiler steam pressure will start falling

• No coal flow alarm will appear

• Coal feeder and mill will trip after some time

• Mill outlet temperature high alarm will appear

Operator responses:

On seeing the changes in the operating parameters of the mill the operator will have to take corrective action.

• Reduce the load on the boiler and inform the steam consumer accordingly

• Maintain boiler pressure and temperature of steam

• Start reserve mill and stabilize, if reserve mill is on maintenance, then ask local operator for the reason for hang-up and his action plan

• Keep steam consumers informed of your action

• Once coal flow is restored through the reserve mill or the same mill, steam consumer can be asked to increase load

Local operator responses:

         In this case the local operator will have to play a major role in helping the control room boiler operator by performing the following checks and deciding further course of action.

• Check whether the hot air and cold air gates are closed, if not then close both

• Check hot air to raw coal and feeder is closed, if not close this

• Close the raw coal feeder inlet gate if not already closed

• After making sure all the above are closed, open the inspection door of the mill

o Check if the coal pipe from feeder to mill is clear and make sure they are made clear

o Check raw coal feeder and if chocked then get it cleaned

o Check the raw coal feeder shear pin and replace if needed

o Now operate on no load and check for smooth running

o Check the coal chute from bunker to feeder is clear, that is from the closed feeder inlet gate onwards

• Check the bunker for coal, if empty organize for filling immediately

• If mill has any remnant materials then clean the mill and restart the mill and check for smooth operation

• Now clear the mill for resuming operation