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Optimum Operation and Maintenance of El Pulverizers
Operation within these parameters is critical to the following items:
Figure 1 - B & W El Pulverizer
The EL pulverizer is a medium speed ball-and-race mill working on the ball bearing principle. EL pulverizers are manufactured in 18 sizes ranging from EL-17 to EL-76. The numeric specification on EL pulverizers indicates the pitch circle of the balls in inches. EL pulverizers operate at capacities between 1½ (EL-17) to 20 (EL-76) tons per hour based on 50 H.G.I. coal. Most electric utility installations utilize the EL-50, EL-64, EL-70 or EL-76. The Babcock and Wilcox EL pulverizers can provide outstanding pulverizer performance when optimum operational and maintenance practices are adhered to. Optimum operation of the EL pulverizer requires the following:
•
Coal fineness of 75% ing 200 mesh and <0.1% remaining on 50 mesh. EL pulverizers that utilize ball bearing type grinding achieve very high 50 mesh fineness much easier than other types of pulverizers.
•
A maximum imbalance of ±5% deviation from the mean dirty airflow between each pulverizer’s separate fuel lines.
•
A maximum imbalance ±10% deviation from the mean fuel flow between each pulverizer’s separate fuel lines.
•
Pipe to pipe clean air balance within ±2% of the mean pipe velocity.
•
Air to fuel ratio of 1.75 to 1.8 pounds of air per pound of fuel.
•
Pulverizer to pulverizer mass air and fuel balance within ±5%.
• •
Pulverizer outlet temperature of 160°F or higher. Minimum fuel line velocity of 3,300 Fpm.
• •
Ensuring stable burner flames
• •
Ensuring full design pulverizer capacity is achieved
•
Uniform release and absorption of heat across of the furnace
•
Reduction in propensities
•
Maintaining furnace and boiler exit gas temperatures within design tolerances
•
Prevention of water-wall wastage and tube metal overheating
•
Maintaining design or better unit heat rate
Prevention of fuel line stoppages, pulsation or slugging Obtaining acceptable levels on unburned carbon in Flyash
furnace
slagging
and
fouling
Numerous variables influence distribution of air and fuel between a pulverizer’s separate fuel lines. These variables are as follows:
•
Size of pulverizer coal particles leaving the pulverizer (Fineness)
• •
System resistance of each individual fuel conduit
•
Velocity of air/coal mixture ing through each burner line
•
Maintenance of critical components such as the classifier, classifier cone, grinding rings, balls, cabbage cutter, orifices and burner components
Total airflow through the pulverizer which is controlled on a ratio of weight of air to fuel
Coal Fineness and Fuel Balance Utilization of Pre-NSPS burners required 70% ing 200 mesh and <1% remaining on 50 mesh. Many operators today are forced to utilize Low NOx burners. Low NOx burners require a higher degree of precision in delivery of fuel and air to the furnace. To compensate for fuel and air imbalances and lower exposure time of fuel carbon to free oxygen at temperatures above ignition points, coal fineness standards required reevaluation. Low NOx burner firing configurations require >75% ing 200 mesh with >99.9% ing 50 mesh. Coal fineness is not always required for acceptable NOx emissions. However, optimum combustion and flyash with <6% unburned Carbon (L.O.I.), requires improved fineness. Fineness levels of 75% ing 200 mesh and 99.9% ing 50 mesh improves fuel distribution as well as flyash unburned carbon. Higher coal fineness level will also promote flame stability at lower loads, increasing boiler turn down without the of oil flames.
Innovative Combustion Technologies, Inc. y 2367 Lakeside Dr. Ste A-1, Birmingham, AL 35244 www.innovativecombustion.com
Optimum Operation and Maintenance of El Pulverizers Coal Fineness and Fuel Balance As fineness increases, fuel balance improves. The finer the coal, the coal and air mixture characteristics are closer to a fluid than solids in suspension. The more fluid coal/air mixture makes achieving complete homogenization of the mixture leaving the pulverizer much easier. A more homogenous mixture of air and coal results is more even distribution between burner lines. EL Pulverizers utilize a classifier employing centrifugal separation to return coarse coal particles to the grinding zone for regrinding. The “Swirl” imparted by centrifugal classification separates coarse and fine coal particles. Homogenization of the stratified mixture leaving the pulverizer can be achieved with intense swirl. More massive coal particles (lower fineness) have more momentum when entrained in air at a certain velocity and are more easily stratified than finer coal particles that have less mass thus lower momentum. After separation of coarse and fine coal particles, fuel and air balance is further aggravated by airflow imbalance. Typically, burner lines that receive the largest quantity of coarse coal particles have the lowest air velocities. The graph below illustrates data collected on a unit with (5) separate pulverizers with differing maintenance intervals and classifier setting. This graph indicates improving fuel balance with higher fineness level despite other parametric differences. Relationship Between Fuel Line Distribution and Coal Fineness
Max. Fuel Imbalance %Dev. from the Mean
40.00% 35.00%
1. 2.
3.
Increase the raw coal feed to the pulverizer to the maximum pulverizer capacity. Make the transition from hot to cold air operation. Avoid making sudden changes in air flow through the pulverizer. Monitor pulverizer outlet temperature. After outlet temperature has returned to normal, continue operation until temperature stabilizes. Coal flow then may be reduced according to pulverizer demand. Ensure reductions in fuel flow are accompanied by proportional decreases in air flow to maintain a fuel rich mixture.
Pulverizer Skidding Pulverizer skidding is the sliding or “skidding” of pulverizer balls rather than rotation with upper and lower grinding rings. Skidding happens immediately without any type of warning. Skidding is usually the result of insufficient spring compression, also causing poor fineness, fuel balance and capacity. Skidding pulverizers are indicated by extreme noise in the immediate area of the pulverizer, sudden and large increase in pulverizer motor current (amps) and increased mill differential indicating a “plugged mill”. Suspension of pulverizer skidding warrants immediate pulverizer tripping. Heat generated by skidding can crack or break balls or the top grinding ring. Besides insufficient spring compression, flat spotted or banded balls and/or large coal chunks or tramp metal (from raw coal feed) jammed under the balls can cause skidding.
30.00% 25.00% 20.00% 15.00% 10.00% 5.00% 0.00% 62.00% 64.00% 66.00% 68.00% 70.00% 72.00% 74.00% 76.00% 78.00% 80.00% 82.00% Weighted Average Pulverizer Coal Fineness - %ing 200 Mesh Maximum Fuel Imbalance - %Dev. from the Mean
Linear (Maximum Fuel Imbalance - %Dev. from the Mean)
Coal Size Maintaining optimum sizing of raw coal to the pulverizer is mandatory in achieving capacity and fineness. Optimum raw coal feed size for EL pulverizers is ¾” to 1”. Raw coal feed should never exceed 1¼”, raw coal this size can plug throats and cause vibration. Raw coal that is too fine can plug the pulverizer or delivery components upstream of the pulverizer. Regular yard crusher inspection and maintenance is required to maintain optimum raw coal sizing. Mill Fires Sudden and large increase in pulverizer outlet temperature usually indicate a mill fire. External inspection during mill fires will reveal a hot mill housing that may peel or char off housing paint. Mill fires require immediate action to prevent further combustion. To extinguish a mill fire, air available for combustion must be reduced. During a mill fire, pulverizer air flow should not be increased. Increased air flow could result in explosion. The procedure applied to extinguish a fire on an in-service pulverizer is as follows:
2
Figure 2 - Grinding Ball Banding and Flat Spotting
Figure 3 - Normal Ball rotation and during skidding
Innovative Combustion Technologies, Inc. y 2367 Lakeside Dr. Ste A-1, Birmingham, AL 35244 www.innovativecombustion.com
Optimum Operation and Maintenance of El Pulverizers Spring Compression Springs provide compressive force required to grind coal and prevent rotation of the top ring. Insufficient spring tension decreases capacity, decreases fineness and could cause a grinding ring failure. Too much spring tension can cause unnecessary increase in mill power consumption. Spring compression (vertical length under load) decreases as grinding elements wear, this decreases force exerted on the top ring and requires periodical adjustment of spring bolts. Adjustment is facilitated by tightening spring adjusting nuts that move spring bolts up or down. During inspection, spring settings are determined by direct measurement from the pulverizer interior. Before and after each spring adjustment, spring setting and external spring bolt dimension shall be recorded. External measurement of the spring bolt is an indication of ring and ball total wear. This also allows determination and spring adjustment with the pulverizer on-line. Figure 4 documents required spring compressions.
Pulverizer Size
Total Spring Force (lbs)
Spring Setting (inches)
Wire Diameter (inches)
Spring Free Length (inches)
Number of Springs
EL-50
20,400
4 23/32
1½
8¼
4
EL-53
22,100
4 3/8
1½
8¼
4
EL-56
22,100
4 3/8
1½
8¼
4
EL-64
25,500
5 5/16
1½
8¼
6
EL-70
28,900
4 29/32
1½
8¼
6
EL-76
30,500
4 29/32
1½
8¼
6
Spring Compression Accurate maintenance and inspection records may facilitate prediction of spring compression change based on hours of operation. When adjusting spring setting by spring bolt external measurement, counter-clockwise turning of the spring bolt adjustment nut increases spring compression. One counter-clockwise turn of the adjusting nut will compress the spring bolts approximately 1/8”. Figure 6 illustrates a theoretical situation where records indicate an increase in total wear measurement every 1000 hours of operation.
Figure 6 - Spring Adjustment by External Measurement
Figure 4 - Desired Spring Compression for El Mills Compressed Length of Spring in Inches Pulverizer Size EL-50 EL-53 EL-56 EL-64 EL-70 EL-76
~Pounds of Force per Ball 1100 1200 1300 5 31/32 5 3/4 5 9/16 5 3/4 5 17/32 5 5/16 5 3/4 5 17/32 5 5/16 6 5/16 6 5/32 6 6 3/32 5 7/8 5 11/16 5 31/32 5 3/4 5 9/16
1400 5 11/32 5 1/16 5 1/16 5 13/16 5 1/2 5 11/32
1500 5 1/8 4 27/32 4 27/32 5 5/8 5 5/16 5 1/8
1600 4 15/16 4 5/8 4 5/8 5 15/32 5 1/8 4 15/16
1700 4 23/32 4 3/8 4 3/8 5 5/16 4 29/32 4 23/32
1800 4 1/2 – – 5 1/8 4 23/32 4 1/2
Not Recommended←
Number of Balls 12 13 13 15 17 18
→ Recommended
Figure 5 - Spring Dimensions on El Pulverizers
3
Ball Diameter 12 1/4 12 1/4 12 1/4 12 1/4 12 1/4 12 1/4
As a result of wear, balls become smaller and wear deeper pockets into the grinding rings. As balls wear and become smaller, available grinding surface decreases and space between the balls increases. Average gap between grinding balls should be approximately 5/8”. Fill-in balls are added when additional space between balls becomes large enough to accommodate extra balls to increase total grinding surface. EL-50 through EL-76 pulverizer grinding balls are 12¼”Ø when new. Worn balls, or sets of balls, of various size are usually classified and kept on hand. When balls are added, balls inserted must be within ¼” of the average diameter of other balls in the pulverizer. When average ball size is smaller than 9¼” diameter, the pulverizer should be overhauled and all balls replaced with 12¼” diameter balls.
Innovative Combustion Technologies, Inc. y 2367 Lakeside Dr. Ste A-1, Birmingham, AL 35244 www.innovativecombustion.com
Optimum Operation and Maintenance of El Pulverizers Common Pulverizer Modifications Less than desired pulverizer performance after mechanical and maintenance variables have been addressed may be due to marginal pulverizer capacity or coal quality different from the pulverizer design. Pulverizer modification is often required to achieve desired fineness and fuel balance.
•
or more will complicate pulverizer disassembly during routine maintenance. 16” FEED PIPE
OUTLET TO EXHAUSTER
Extended classifier blades. Increased blade length increases swirl rejecting more coarse particles and improving fuel distribution. Extended classifier blades can improve fineness between 5% and 15% ing 200 mesh. Figure 7 illustrates typical classifier blade extensions for an EL pulverizer and improvement in fuel distribution.
1” EXTENSION OF DEFLECTOR RING
COAL-AIR MIXTURE
INCREASED DOWNWARD MOMENTUM RETURNS HIGHER PERCENTAGE OF COARSE PARTICLES TO GRINDING ZONES OF PULVERIZER
NEW VANES 12.75" IN LENGTH 4" LONGER THAN ORIGINAL
ORIGINAL VANES 8.75" IN LENGTH
Figure 9 - Inverted Top Hat Extension
• PRE-MODIFICATION TESTING DIRTY AIR VELOCITIES
POST-MODIFICATION TESTING
RECOVERED SAMPLE (g)
+8.27% +13.2% -6.07% -18.3%
DIRTY AIR VELOCITIES MAXIMUM DEVIATION
MINIMUM DEVIATION
RECOVERED SAMPLE (g)
+1.55% +5.45% -2.33% -6.05%
DEVIATIONS ARE %DEVIATION FROM MEAN VELOCITY OR RECOVERY WEIGHT
Figure 7 - Improvement in Fuel Balance with Extended Classifier Blades
•
Installation of angled classifier blades will improve coal fineness by changing the diameter of the swirl. Blades are angled further towards the outside of the mill and the back of other classifier blades. Larger coal particles, which are accelerated more than fines due to higher momentum collide with classifier blades and are rejected.
Installation of raw coal deflectors. If raw coal deflectors are absent on EL pulverizers, raw coal is dumped on the outside of the grinding ring. This prevents coal from ing from the inside to the outside diameter of the grinding ring as required for efficient pulverization. The lower pulverizer ring rotates at approximately 90 Rpm on an EL-64,70 or 76 and 100 Rpm on an EL-50 at the shaft. Coal dumped on the outside of the grinding ring will not be able to overcome the centrifugal acceleration to through the balls. When this occurs, coal must be transported to the classifier by air through the throat and then circulated to the grinding zone through the tailing discharge. Improper circulation through the mill results in reduced capacity, high mill differential and reduced fineness. Figure 10 illustrates a typical raw coal deflector installed in an EL Pulverizer.
CLASSIFIER OUTLET RAW COAL FEED
Figure 8 - Angled Classifier Blades on an EL Pulverizer
PULVERIZER HOUSING
CLASSIFIER BLADES
SEAL WELD ACROSS TOP BAFFLE TWO 3/8” THICK GUSSETS
•
Extension of classifier outlet skirts or “inverted top hat”. This is performed to change the direction of the coal particles in a downward direction towards the classifier reject area. The increased downward momentum and 180° turn of the particles with higher mass allows less of these large particles to be carried to the fuel lines. A 1” extension of the inverted top hat is usually sufficient. Extension of 2”
4
TOTAL OF THREE 3/4” ROUND ROD BRACES AT ENDS AND CENTER OF DEFLECTOR
Figure 10 - Raw Coal Deflector for EL Pulverizer
Innovative Combustion Technologies, Inc. y 2367 Lakeside Dr. Ste A-1, Birmingham, AL 35244 www.innovativecombustion.com
Optimum Operation and Maintenance of El Pulverizers Pulverizer Vibration Non-optimum pulverizer air flow can sometimes cause EL pulverizers to vibrate. The mill level also plays a factor in vibration. Pulverizers that are operating fuel lean will vibrate much more than a pulverizer operating at correct air/fuel ratio. Excessive vibration after verification of mill coal level and air flow warrants an internal inspection. Mechanical factors that can cause excessive vibration are as follows:
• • • • •
resistance’s are not caused by dissimilar positions. Proper diff position is also required to maximize diff effectiveness and to promote efficient mixing of fuel and air at the burner. Optimum diff position is achieved when the diff is centered in the coal nozzle with ±1/8” tolerance and the diffs trailing edge is 1” beyond the coal nozzle tip. (Figure 11)
Ball flat spotting or banding Foreign objects in the pulverizer Broken grinding rings Excessive ring run-out Large raw coal sizing
Clean Air Balancing Balancing system resistance of fuel lines on clean air is the first step of an empirically derived approach to balancing pulverizer fuel and air. Clean air balance is defined as balance of airflow between a pulverizer’s fuel lines in the absence of fuel. This is achieved by forcing air through the pulverizer at normal operating mill outlet temperatures with primary air fans while the feeder remains off-line. Clean air balance is determined by measuring the velocity of air flowing through each individual fuel line with a standard Pitot tube. Primary airflow optimization will be discussed later in this report. In most cases, primary airflow is higher than desired and will be reduced to optimize performance. Prior to optimization of primary airflow it is prudent to perform clean air balancing. This will insure minimum fuel line velocities are maintained after optimization of primary airflow. Air velocity in all fuel lines must exceed 3,300 feet/minute (Fpm). Fuel line velocities above 3,300 Fpm will insure coal entrainment in primary air. Velocities below 3,000 Fpm allow coal to fall out, accumulate or “dune” in horizontal runs of fuel lines. The 3,300 Fpm minimum velocity includes a 10% safety margin above the absolute minimum line velocity of 3,000 Fpm allowing for some imbalance in air flow between pipes. Coal line accumulations may cause burner pulsation’s, flame instabilities and possible stoppages. Fuel lines are balanced by an iterative process utilizing 10 Gauge carbon steel trial orifices. Clean air balance with a maximum of ±2% deviation from the mean between all fuel lines on a pulverizer must be achieved. After optimum orifice configuration is determined, permanent 400 Series stainless orifices are installed. Typical coal line orifices are illustrated by Figure 12. Computer modeling is sometimes performed to determine orifice sizing. Following installation of orifices, clean air balance should always been verified by Pitot traverse of fuel lines. Clean air tests by Pitot traverse is also required to ascertain if any fuel line resistance’s not shown by drawings are present. Boilers with EL pulverizers utilizing pre-NSPS burners typically utilize a concentric ring coal diff. If so equipped, diffs on all burners must be at uniform position to ensure differences in fuel line system
5
Figure 11 - Proper Position of B&W Con Ring Diff
FLANGE
ORIFICE PLATE Trial Orifice - 10 Ga. CS Perm Orifice - 3/8" 400 Series S.S.
xx
STAMPED W/ORIFICE SIZE
FLANGE
ORIFICE
NOTE: ORIFICES MUST BE AS FAR UPSTREAM OR DOWNSTREAM OF TEST TAPS AS POSSIBLE. >10 PIPE DIAMETERS PREFERRED
Figure 12 - Typical Coal Lines Orifices
Innovative Combustion Technologies, Inc. y 2367 Lakeside Dr. Ste A-1, Birmingham, AL 35244 www.innovativecombustion.com
Optimum Operation and Maintenance of El Pulverizers Pulverizer Airflow High primary airflow can cause poor coal fineness as well as poor fuel balance. Primary airflow should be ramped against fuel flow. Optimum pulverizer airflow for a EL pulverizer is 1.75 to 1.8 pounds of air per pound of coal. The graph below illustrates a typical air to fuel relationship for EL pulverizers. Characterization of P.A. flow requires accurate control and measurement of primary air and fuel.
Primary Airflow - lbs./hour
Optimum Primary Air Flow Ramp 75,000 70,000 65,000 60,000 55,000 50,000 45,000 40,000 35,000 30,000 25,000 20,000 15,000 10,000 5,000 0
Minimum Primary Airflow to Maintain Minimum Fuel Line Velocity at 3,250 Fpm
0
5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
Coal Flow – Pounds per Hour Recommended -- 1.8 Air to Fuel Ratio
Figure 13 - Typical Primary Airflow Ramp
Pulverizer throat clearance must be verified prior to primary air flow optimization. Accumulation of pulverizer operating hours will result in throat gap enlargement as throat plates and lower grinding ring wear. As the gap becomes wider, velocity decreases, allowing smaller coal particles to fall through the throat. If throat gap is not closely monitored, serious coal dribble can result. If throat gaps are too small, mill differential will be high, possibly causing reduced capacity and trapping of pyrites such as rocks or tramp metal in the throat. Excessive coal dribble can be temporarily avoided with fineness and performance penalties by increasing primary air flow. Operation with increased primary air flow should only be used until proper maintenance can be performed. Figure 14 documents desired throat opening on EL pulverizers, sizes EL-50 through EL-76.
Figure 15 - EL Throat and Seal ring clearance
Flow nozzles or venturis usually provide the most accurate and reliable means to measure and control primary airflow. In many cases, the averaging Pitot tubes provide adequate accuracy for measurement of primary air flow. The much endorsed procedure of calibration by cold clean air flow should be abandoned. Calibration should be performed by measuring air flow at the P.A. fan discharge upstream of the averaging Pitot tube. At this location, Pitot calibration will be achieved at operating temperature of ~325°F rather than clean air at ~150°F.
Nominal Throat Size Pulverizer Size
Throat Opening 32nd of Inch
Open Area Ft2
EL-50
20
1.03
EL-53
20
1.14
EL-56
20
1.25
EL-64
22
1.48
EL-70
24
1.70
EL-76
30
2.00
Figure 14 - EL Pulverizer Throat Openings
Seal ring clearance is critical too prevent air from bying the throat to the pyrite section. Design seal ring clearances are as follows: EL-21 to EL-56: EL-64 to EL-76:
6
3/32” Maximum 1/8” Maximum
1/16” Minimum 1/16” Minimum
Figure 16 - Typical EL Pulverizer Averaging Pitot Tubes
Innovative Combustion Technologies, Inc. y 2367 Lakeside Dr. Ste A-1, Birmingham, AL 35244 www.innovativecombustion.com
Optimum Operation and Maintenance of El Pulverizers Pulverizer Airflow High primary airflow contributes to poor coal fineness by increasing classifier air velocities and reducing overall coal retention time in the grinding zone. Increased primary air flow increases velocities at the classifier outlet. Higher classifier outlet velocities will have sufficient energy to entrap larger coal particles. Lower velocities allow larger particles to be returned to the grinding zones. At higher velocity, coal particles have higher kinetic energy or momentum, increasing stratification of coal and air when centrifugal forces are imparted on coal particles by the classifier. The combined effect of poor fineness and separation of coarse and fine coal particles causes poor fuel balance. High primary air will also result in the following:
•
•
•
Poor flame stability at low load and flame ignition points unattached to burner nozzles. Primary airflow of 1.8 Lbs. air/Lb. fuel will maintain coal nozzle tip velocities within the best range of flame propagation speed for flame stability and improved combustion. Increased air heater exit gas temperatures. Higher dry gas losses are the result of increased tempering air usage. Higher air to fuel ratio requires lower pulverizer inlet temperature to maintain outlet temperature, decreasing hot air from the air preheaters. Additional dry gas losses are incurred by heat loss to heat additional ambient (tempering) air after injection into the boiler “boundary." Increased furnace exit gas temperatures. High primary air increases the differential in velocity between the primary air/fuel mixture and the secondary (combustion) air. This stages or delays combustion allowing increased heat release above the burner belt zone. Lower heat release in the burner belt zone results in lower waterwall heat absorption and a subsequent increase in furnace exit gas temperature.
•
All mating surfaces between the classifier and classifier cone sections must seal with no gaps. Gaps in mating surfaces will allow coal and air to by the classifier louvers thereby reducing classifier efficiency.
•
seal ring clearance. Air escaping through excessive clearance open will reduce air flow across the throat and could result in coal spillage or dribble.
•
Insure all segments of the seal ring are securely fastened to the housing unit. The loss of a segment will allow air to by the throat and cause nonuniform distribution of air around the periphery of grinding zone. The resulting friction from a loose segment in with a grinding ring will generate extreme heat possibly causing the ring to be damaged or a mill fire.
• •
Ensure the reject gate functions properly.
•
Inspect the grinding rings and balls for wear. Document the number and size of balls.
•
Document the vertical height between the grinding rings.
•
Insure all spring bolts are properly adjusted and uniform. Dissimilar compression’s of different springs will cause uneven loading of the grinding elements and poor grinding performance. Poor grinding performance could result in fineness and capacity reduction.
•
no retaining bolts are missing from the spring bolt shoes. These bolts hold springs and spring bolts in their sockets. Missing bolts will allow extra motion between the spring bolts, springs and shoes, contributing to excessive wear of these components.
•
Inspect spring bolt shoes. Excessively worn shoes may allow springs to push through the shoe eliminating proper spring setting.
•
Insure spring bolts are vertical. Out of line spring bolts may result in bent or broken spring bolts.
tailings discharge (cabbage cutter) seals with tailing section. Clearance between cabbage cutter and tailings discharge should be less then 1/8”.
• • • •
Higher mill fire or explosion propensity
Increased auxiliary power, due to increased P.A. Fan horsepower demand to convey additional primary airflow
•
•
Higher Nitrous Oxide emissions. Increased Nitrous Oxide emissions are facilitated by injecting more of the total air, for a given excess air, as primary air in the high temperature root of the burner flame rather than secondary (combustion) air.
Inspect the condition of the housing liners. Excessive erosion of the consumable housing liners may result in erosion of the mill housing.
•
Closely inspect the top and bottom rings for cracks. Cracks may worsen during operation that can ultimately lead to a broken grinding ring that can lead to breaking of the pulverizer’s main shaft.
•
Inspect the classifier blades. Make sure all the adjustable portion (thin gauge) of each blade or louver is at uniform insertion into the classifier. Ensure all nuts that secure the adjustable portion of the classifier blade to the stationary blade are tight. Nuts that loosen with vibration may result in dislodging of blades. Dislodged blades can damage the pulverizer, cause excessive vibration and decrease coal fineness.
Higher slagging or fouling propensity Increased coal line and burner nozzle wear
EL Pulverizer - Critical Inspection Items Pulverizers should be inspected on a monthly basis. During pulverizer inspection, the following items:
•
Pulverizer throat gap. Throat gap variation around the circumference should be less than 1/16” to ensure equal air distribution around the grinding zone perimeter. Throat gap should be measured with a taper gage for accurate measurement.
7
Innovative Combustion Technologies, Inc. y 2367 Lakeside Dr. Ste A-1, Birmingham, AL 35244 www.innovativecombustion.com
Optimum Operation and Maintenance of El Pulverizers • •
•
•
Inspect the inverted top hat and ensure it is not excessively worn. Ensure all swing valves open fully and seal securely in the closed position. Swing valves that do not open fully may bias dirty air flow and fuel flow between pipes. Air flow through a single pipe with a partially opened swing valve may result in low pipe velocities insufficient to maintain suspension of coal, causing a coal line stoppage. Inspect the primary air fan blades and inlet bells. the inlet bell to fan wheel clearances. Inlet bell to fan wheel clearance should be 1/8” at bottom and 1/4” at top. Inspect the primary air averaging Pitots tubes. Make sure the impact and static holes are perpendicular to the flow direction. Ensure the Pitots are not distorted and no holes are plugged.
and low outlet temperature is still experienced, available hot air temperature may be insufficient for the moisture content of a particular fuel. Required temperature can be calculated using the method shown in Figure 21. If the required temperature is not available, corrective action would require coal flow to be reduced or primary air flow increased. Higher air flow may cause decreased coal fineness, however, these cases are the exception and not the norm and should be implemented to prevent low outlet temperature.
Input Parameters Coal Flow : Air to Fuel Ratio : Primary Air Flow : Mill Oultet Temperature :
35,000 Lbs./hr. 1.80 # air/ # Coal 63,000Lbs./hr. 160 °F
h (water) @ ambient = h (vapor) @ mill outlet = Enthalpy Rise =
Ambient Temp. of Coal : Coal Moisture : of air : of Coal :
100°F 7.5% 0.24 0.30
68 Btu/Lb. 1130 Btu/Lb. 1062 Btu/Lb.
Heat Required to Dry Coal (Btu's)
Pulverizer Outlet Temperature Maintaining optimum mill outlet temperature is required to insure evaporation of fuel moisture and for proper ignition of coal at the burners. Pulverizer outlet temperature should be >160°F on EL pulverizers. Temperatures less than 160°F will not promote complete evaporation of fuel moisture. Low outlet temperature can lead to coal accumulation on fuel line pipe walls, poor combustion and poor fineness. Pulverizer ability to grind coal to fines is debilitated by fuel moisture resulting in undesirable fineness. Low outlet temperature results in a higher percentage of total primary airflow as tempering airflow rather than hot air. Tempering air is at ambient temperature and introduced through atmospheric damper or ducting from the forced draft fans. Tempering air that has not been heated by the air heaters, is injected into the boiler boundary resulting in higher dry gas losses. This also facilitates less air ing across the air heater which in turn results in a slight increase in air heater exit gas temperature. In some cases, low outlet temperature can result in lower than minimum fuel line velocities due to differential in velocity at varying temperature for a given weighted flow. When low outlet temperature is observed the following action should be taken: 1. 2. 3.
Enthalpy Rise of Coal = C oal Flow × [ 0.30 (Tin - Tout) + {Coal Moisture × (Enthalpy Rise)}] = 3,417,750Btu/hr. Heat Content of Primary Air (Btu's) Heat Req'd. to Dry Coal = (air) (air)××Mass MassPA PAFlow Flow××(Tin (Tin- -Tout) Tout) = 386.04°F Steam Properties Table - Data Taken From 1967 ASME Steam Tables Enthalpy of Vapor @ 1 Atmosphere Enthalpy (Btu/Lb.) Temp. (°F) 1124 145 1126 150 1128 155 1130 160 1132 165 1134 170 1136 175 1138 180 1140 185 1142 190
Enthalpy of Water (Liquid) @ 1 Atmosphere (14.7 PSIA) Temp. (°F) Enthalpy (Btu/Lb.) 40 8 50 18 60 28 70 38 80 48 90 58 100 68 110 78 120 88 130 98
Figure 21 - Pulverizer Heat Balance
calibration and functional checks of the temperature sensing devices. stroke and functional checks of the hot and tempering air dampers. Collect a raw coal sample and perform a proximate fuel analysis. Calculate the required temperature at the pulverizer inlet to maintain desired outlet temperature. Compare the required outlet temperature to actual pulverizer inlet temperature. If there are gross differences pulverizer air flow.
In some cases low pulverizer outlet temperature may be caused by insufficient air flow or extremely wet coal. This situation would be indicated by functional dampers that result in a fully open hot air damper and fully closed tempering air damper. If air flow is verified to be optimum
8
Innovative Combustion Technologies, Inc. y 2367 Lakeside Dr. Ste A-1, Birmingham, AL 35244 www.innovativecombustion.com
Optimum Operation and Maintenance of El Pulverizers
P.A. Temperature Required with Varying Coal Moisture 700°F 600°F 500°F 400°F
Typical correction for EL pulverizer capacity for coal grindability is illustrated by Figure 22.
EL Pulverizer Capacity Correction for Coal H.G.I. at a Specified Fineness 1.60 Capacity Factor (× base capacity at 50 H.G.I.)
Pulv. Inlet Temperature for 160° Outlet
The graph below illustrates increases in required pulverizer inlet temperature to maintain 160°F outlet temperature as coal moisture increases. This graph also illustrates increased pulverizer inlet temperature with lower pulverizer air to fuel ratio. In most cases 100% hot air will result in a maximum temperature of ~550°F. Examination of the graph will indicate that low air flow (1.5 air to fuel ratio, as an example) will result in low outlet temperature due to insufficient hot air temperature. At optimum air flow (1.8 air to fuel ratio), outlet temperature will likely not be achieved at coal moisture contents greater than 13%.
1.40 1.20 1.00 0.80 0.60 0.40 20 30 40 50 60 70 80 90
300°F 6%
8%
10%
12%
14%
Coal Hardgrove Grindability Index
% Moisture Content of Coal 1.8 Air-Fuel Ratio
1.5 Air-Fuel Ratio
Figure 22 - Effect of Coal H.G.I. on Pulverizer Capacity
Effect of Coal Grindability (H.G.I.) Design capacity of EL pulverizers is based on coal grindability index of 50 H.G.I. Pulverizer capacity for a specified coal fineness will vary directly with coal grindability below 50 H.G.I. As an example, an EL-76 rated at 40,000 Lbs./Hr. coal flow capacity with 50 H.G.I. coal will have a 32,000 Lbs./Hr. coal capacity with 40 H.G.I. coal. The converse is also true, that is, increased capacity can be achieved with higher coal grindability. Capacity correction for grindability above 50 H.G.I. is slightly less than proportional. The graph provided as Figure 22 illustrates the relationship of pulverizer capacity and coal grindability. As an example a capacity of 46,800 Lbs./Hr. coal flow could be achieved with 60 H.G.I. coal on an EL-76 with than design coal H.G.I. will require reduction in pulverizer capacity unless the pulverizer is modified to compensate for lower coal grindability. In some cases, design capacity can be achieved with lower grindability coal by increasing primary air to fuel ratio. This will achieve pulverizer capacity and unit electrical generation but with reduced fineness and boiler efficiency. Increasing air flow on coals that have both low H.G.I. and low volatile coal (20 to 29% V.M.) will cause significant flame instability requiring alternative actions.
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Optimum Operation and Maintenance of El Pulverizers Pulverizer Airflow Calibration of averaging Pitot tubes is performed to establish a Pitot tube “K” factor. “K” factors are obtained by measuring air flow while documenting differential pressure across the averaging Pitot tubes. Determination of “K” factors allow calculation of primary air flow for a given differential pressure across the averaging Pitot tubes. This should be performed at several pulverizer air flows to generate a curve. Typical test points include:
•
100% full pulverizer load P.A. flow (based on a 1.8 air to fuel ratio 75% of full pulverizer load P.A. flow Minimum pulverizer air flow calculated to yield fuel line velocities of 3,300 Fpm After collection of data, Calculate “K” factors utilizing the following formula:
• • •
K=
The figures below illustrate typical measures to improve the accuracy of primary air flow measurement elements. Flow nozzles, as shown by Figure 17, are the preferred and most accurate means to measure primary flow. Typical modifications to primary air ducting intended to increase velocity and smooth flow profiles by use of dams or angle iron are shown by Figures 19 and 20. If flow nozzles are not installed, installation of venturi section, as shown by Figure 18, is the preferred method to increase accuracy of existing averaging Pitot tubes.
W ΔP × δ
δ
Air Density at Pitot (Lbs./Ft³)
W ΔP
Mass Flow (Lbs. per Hour) Pitot Differential (“w.c.)
Figure 17 - Flow Nozzle installed at PA fan inlet
Calculated “K” factor at each of the three different air flows are averaged. No single “K” factor should deviate more than ±3% from the average. If “K” factors shift more than ±3% deviation from the mean, P.A. ducting must be modified to increase accuracy or Pitots must be replaced with an alternate flow measurement element. After determination of “K” factor, primary air flow can be calculated for given Pitot differential by rearranging the previous equation.
W = K × ΔP × δ
Figure 18 - Venturi to Improve Avg Pitot Accuracy
A primary air flow ramp can then be generated by rearranging the equation once again to calculate required Pitot differential for desired primary air flow.
⎛ W ⎞ ⎟⎟ ΔP = ⎜⎜ ⎝ (K ) • δ ⎠
2
Desired primary air flow is calculated by multiplying coal flow by 1.8 above minimum fuel line velocity. Figure 19 - Typical Air Dam in EL P.A. Ducting
P.A. Differential – "w.c.
Coal Flow Vs. Primary Air Differential – Utilizing P.A. “K” Factor of 2,746 3.50 3.25 3.00 2.75 2.50 2.25 2.00 1.75 Min. Air Flow to Yield 1.50 3300 Fpm in Pipes 1.25 1.00 0.75 0.50 12,000 17,000 22,000
Basis: P.A. Fan Discharge Conditions: 400°F 10" w .c. Static Prs. Density = 0.04736 Lbs./ft³
27,000
32,000
37,000
Coal Flow - Pounds per Hour Recommended – 1.8 Pounds of Air per Pound of Coal
Figure 20 - Angle Iron to Increase Velocity and Smooth Flow Profile Across Avg. Pitot
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Operation within these parameters is critical to the following items:
Figure 1 - B & W El Pulverizer
The EL pulverizer is a medium speed ball-and-race mill working on the ball bearing principle. EL pulverizers are manufactured in 18 sizes ranging from EL-17 to EL-76. The numeric specification on EL pulverizers indicates the pitch circle of the balls in inches. EL pulverizers operate at capacities between 1½ (EL-17) to 20 (EL-76) tons per hour based on 50 H.G.I. coal. Most electric utility installations utilize the EL-50, EL-64, EL-70 or EL-76. The Babcock and Wilcox EL pulverizers can provide outstanding pulverizer performance when optimum operational and maintenance practices are adhered to. Optimum operation of the EL pulverizer requires the following:
•
Coal fineness of 75% ing 200 mesh and <0.1% remaining on 50 mesh. EL pulverizers that utilize ball bearing type grinding achieve very high 50 mesh fineness much easier than other types of pulverizers.
•
A maximum imbalance of ±5% deviation from the mean dirty airflow between each pulverizer’s separate fuel lines.
•
A maximum imbalance ±10% deviation from the mean fuel flow between each pulverizer’s separate fuel lines.
•
Pipe to pipe clean air balance within ±2% of the mean pipe velocity.
•
Air to fuel ratio of 1.75 to 1.8 pounds of air per pound of fuel.
•
Pulverizer to pulverizer mass air and fuel balance within ±5%.
• •
Pulverizer outlet temperature of 160°F or higher. Minimum fuel line velocity of 3,300 Fpm.
• •
Ensuring stable burner flames
• •
Ensuring full design pulverizer capacity is achieved
•
Uniform release and absorption of heat across of the furnace
•
Reduction in propensities
•
Maintaining furnace and boiler exit gas temperatures within design tolerances
•
Prevention of water-wall wastage and tube metal overheating
•
Maintaining design or better unit heat rate
Prevention of fuel line stoppages, pulsation or slugging Obtaining acceptable levels on unburned carbon in Flyash
furnace
slagging
and
fouling
Numerous variables influence distribution of air and fuel between a pulverizer’s separate fuel lines. These variables are as follows:
•
Size of pulverizer coal particles leaving the pulverizer (Fineness)
• •
System resistance of each individual fuel conduit
•
Velocity of air/coal mixture ing through each burner line
•
Maintenance of critical components such as the classifier, classifier cone, grinding rings, balls, cabbage cutter, orifices and burner components
Total airflow through the pulverizer which is controlled on a ratio of weight of air to fuel
Coal Fineness and Fuel Balance Utilization of Pre-NSPS burners required 70% ing 200 mesh and <1% remaining on 50 mesh. Many operators today are forced to utilize Low NOx burners. Low NOx burners require a higher degree of precision in delivery of fuel and air to the furnace. To compensate for fuel and air imbalances and lower exposure time of fuel carbon to free oxygen at temperatures above ignition points, coal fineness standards required reevaluation. Low NOx burner firing configurations require >75% ing 200 mesh with >99.9% ing 50 mesh. Coal fineness is not always required for acceptable NOx emissions. However, optimum combustion and flyash with <6% unburned Carbon (L.O.I.), requires improved fineness. Fineness levels of 75% ing 200 mesh and 99.9% ing 50 mesh improves fuel distribution as well as flyash unburned carbon. Higher coal fineness level will also promote flame stability at lower loads, increasing boiler turn down without the of oil flames.
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Optimum Operation and Maintenance of El Pulverizers Coal Fineness and Fuel Balance As fineness increases, fuel balance improves. The finer the coal, the coal and air mixture characteristics are closer to a fluid than solids in suspension. The more fluid coal/air mixture makes achieving complete homogenization of the mixture leaving the pulverizer much easier. A more homogenous mixture of air and coal results is more even distribution between burner lines. EL Pulverizers utilize a classifier employing centrifugal separation to return coarse coal particles to the grinding zone for regrinding. The “Swirl” imparted by centrifugal classification separates coarse and fine coal particles. Homogenization of the stratified mixture leaving the pulverizer can be achieved with intense swirl. More massive coal particles (lower fineness) have more momentum when entrained in air at a certain velocity and are more easily stratified than finer coal particles that have less mass thus lower momentum. After separation of coarse and fine coal particles, fuel and air balance is further aggravated by airflow imbalance. Typically, burner lines that receive the largest quantity of coarse coal particles have the lowest air velocities. The graph below illustrates data collected on a unit with (5) separate pulverizers with differing maintenance intervals and classifier setting. This graph indicates improving fuel balance with higher fineness level despite other parametric differences. Relationship Between Fuel Line Distribution and Coal Fineness
Max. Fuel Imbalance %Dev. from the Mean
40.00% 35.00%
1. 2.
3.
Increase the raw coal feed to the pulverizer to the maximum pulverizer capacity. Make the transition from hot to cold air operation. Avoid making sudden changes in air flow through the pulverizer. Monitor pulverizer outlet temperature. After outlet temperature has returned to normal, continue operation until temperature stabilizes. Coal flow then may be reduced according to pulverizer demand. Ensure reductions in fuel flow are accompanied by proportional decreases in air flow to maintain a fuel rich mixture.
Pulverizer Skidding Pulverizer skidding is the sliding or “skidding” of pulverizer balls rather than rotation with upper and lower grinding rings. Skidding happens immediately without any type of warning. Skidding is usually the result of insufficient spring compression, also causing poor fineness, fuel balance and capacity. Skidding pulverizers are indicated by extreme noise in the immediate area of the pulverizer, sudden and large increase in pulverizer motor current (amps) and increased mill differential indicating a “plugged mill”. Suspension of pulverizer skidding warrants immediate pulverizer tripping. Heat generated by skidding can crack or break balls or the top grinding ring. Besides insufficient spring compression, flat spotted or banded balls and/or large coal chunks or tramp metal (from raw coal feed) jammed under the balls can cause skidding.
30.00% 25.00% 20.00% 15.00% 10.00% 5.00% 0.00% 62.00% 64.00% 66.00% 68.00% 70.00% 72.00% 74.00% 76.00% 78.00% 80.00% 82.00% Weighted Average Pulverizer Coal Fineness - %ing 200 Mesh Maximum Fuel Imbalance - %Dev. from the Mean
Linear (Maximum Fuel Imbalance - %Dev. from the Mean)
Coal Size Maintaining optimum sizing of raw coal to the pulverizer is mandatory in achieving capacity and fineness. Optimum raw coal feed size for EL pulverizers is ¾” to 1”. Raw coal feed should never exceed 1¼”, raw coal this size can plug throats and cause vibration. Raw coal that is too fine can plug the pulverizer or delivery components upstream of the pulverizer. Regular yard crusher inspection and maintenance is required to maintain optimum raw coal sizing. Mill Fires Sudden and large increase in pulverizer outlet temperature usually indicate a mill fire. External inspection during mill fires will reveal a hot mill housing that may peel or char off housing paint. Mill fires require immediate action to prevent further combustion. To extinguish a mill fire, air available for combustion must be reduced. During a mill fire, pulverizer air flow should not be increased. Increased air flow could result in explosion. The procedure applied to extinguish a fire on an in-service pulverizer is as follows:
2
Figure 2 - Grinding Ball Banding and Flat Spotting
Figure 3 - Normal Ball rotation and during skidding
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Optimum Operation and Maintenance of El Pulverizers Spring Compression Springs provide compressive force required to grind coal and prevent rotation of the top ring. Insufficient spring tension decreases capacity, decreases fineness and could cause a grinding ring failure. Too much spring tension can cause unnecessary increase in mill power consumption. Spring compression (vertical length under load) decreases as grinding elements wear, this decreases force exerted on the top ring and requires periodical adjustment of spring bolts. Adjustment is facilitated by tightening spring adjusting nuts that move spring bolts up or down. During inspection, spring settings are determined by direct measurement from the pulverizer interior. Before and after each spring adjustment, spring setting and external spring bolt dimension shall be recorded. External measurement of the spring bolt is an indication of ring and ball total wear. This also allows determination and spring adjustment with the pulverizer on-line. Figure 4 documents required spring compressions.
Pulverizer Size
Total Spring Force (lbs)
Spring Setting (inches)
Wire Diameter (inches)
Spring Free Length (inches)
Number of Springs
EL-50
20,400
4 23/32
1½
8¼
4
EL-53
22,100
4 3/8
1½
8¼
4
EL-56
22,100
4 3/8
1½
8¼
4
EL-64
25,500
5 5/16
1½
8¼
6
EL-70
28,900
4 29/32
1½
8¼
6
EL-76
30,500
4 29/32
1½
8¼
6
Spring Compression Accurate maintenance and inspection records may facilitate prediction of spring compression change based on hours of operation. When adjusting spring setting by spring bolt external measurement, counter-clockwise turning of the spring bolt adjustment nut increases spring compression. One counter-clockwise turn of the adjusting nut will compress the spring bolts approximately 1/8”. Figure 6 illustrates a theoretical situation where records indicate an increase in total wear measurement every 1000 hours of operation.
Figure 6 - Spring Adjustment by External Measurement
Figure 4 - Desired Spring Compression for El Mills Compressed Length of Spring in Inches Pulverizer Size EL-50 EL-53 EL-56 EL-64 EL-70 EL-76
~Pounds of Force per Ball 1100 1200 1300 5 31/32 5 3/4 5 9/16 5 3/4 5 17/32 5 5/16 5 3/4 5 17/32 5 5/16 6 5/16 6 5/32 6 6 3/32 5 7/8 5 11/16 5 31/32 5 3/4 5 9/16
1400 5 11/32 5 1/16 5 1/16 5 13/16 5 1/2 5 11/32
1500 5 1/8 4 27/32 4 27/32 5 5/8 5 5/16 5 1/8
1600 4 15/16 4 5/8 4 5/8 5 15/32 5 1/8 4 15/16
1700 4 23/32 4 3/8 4 3/8 5 5/16 4 29/32 4 23/32
1800 4 1/2 – – 5 1/8 4 23/32 4 1/2
Not Recommended←
Number of Balls 12 13 13 15 17 18
→ Recommended
Figure 5 - Spring Dimensions on El Pulverizers
3
Ball Diameter 12 1/4 12 1/4 12 1/4 12 1/4 12 1/4 12 1/4
As a result of wear, balls become smaller and wear deeper pockets into the grinding rings. As balls wear and become smaller, available grinding surface decreases and space between the balls increases. Average gap between grinding balls should be approximately 5/8”. Fill-in balls are added when additional space between balls becomes large enough to accommodate extra balls to increase total grinding surface. EL-50 through EL-76 pulverizer grinding balls are 12¼”Ø when new. Worn balls, or sets of balls, of various size are usually classified and kept on hand. When balls are added, balls inserted must be within ¼” of the average diameter of other balls in the pulverizer. When average ball size is smaller than 9¼” diameter, the pulverizer should be overhauled and all balls replaced with 12¼” diameter balls.
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Optimum Operation and Maintenance of El Pulverizers Common Pulverizer Modifications Less than desired pulverizer performance after mechanical and maintenance variables have been addressed may be due to marginal pulverizer capacity or coal quality different from the pulverizer design. Pulverizer modification is often required to achieve desired fineness and fuel balance.
•
or more will complicate pulverizer disassembly during routine maintenance. 16” FEED PIPE
OUTLET TO EXHAUSTER
Extended classifier blades. Increased blade length increases swirl rejecting more coarse particles and improving fuel distribution. Extended classifier blades can improve fineness between 5% and 15% ing 200 mesh. Figure 7 illustrates typical classifier blade extensions for an EL pulverizer and improvement in fuel distribution.
1” EXTENSION OF DEFLECTOR RING
COAL-AIR MIXTURE
INCREASED DOWNWARD MOMENTUM RETURNS HIGHER PERCENTAGE OF COARSE PARTICLES TO GRINDING ZONES OF PULVERIZER
NEW VANES 12.75" IN LENGTH 4" LONGER THAN ORIGINAL
ORIGINAL VANES 8.75" IN LENGTH
Figure 9 - Inverted Top Hat Extension
• PRE-MODIFICATION TESTING DIRTY AIR VELOCITIES
POST-MODIFICATION TESTING
RECOVERED SAMPLE (g)
+8.27% +13.2% -6.07% -18.3%
DIRTY AIR VELOCITIES MAXIMUM DEVIATION
MINIMUM DEVIATION
RECOVERED SAMPLE (g)
+1.55% +5.45% -2.33% -6.05%
DEVIATIONS ARE %DEVIATION FROM MEAN VELOCITY OR RECOVERY WEIGHT
Figure 7 - Improvement in Fuel Balance with Extended Classifier Blades
•
Installation of angled classifier blades will improve coal fineness by changing the diameter of the swirl. Blades are angled further towards the outside of the mill and the back of other classifier blades. Larger coal particles, which are accelerated more than fines due to higher momentum collide with classifier blades and are rejected.
Installation of raw coal deflectors. If raw coal deflectors are absent on EL pulverizers, raw coal is dumped on the outside of the grinding ring. This prevents coal from ing from the inside to the outside diameter of the grinding ring as required for efficient pulverization. The lower pulverizer ring rotates at approximately 90 Rpm on an EL-64,70 or 76 and 100 Rpm on an EL-50 at the shaft. Coal dumped on the outside of the grinding ring will not be able to overcome the centrifugal acceleration to through the balls. When this occurs, coal must be transported to the classifier by air through the throat and then circulated to the grinding zone through the tailing discharge. Improper circulation through the mill results in reduced capacity, high mill differential and reduced fineness. Figure 10 illustrates a typical raw coal deflector installed in an EL Pulverizer.
CLASSIFIER OUTLET RAW COAL FEED
Figure 8 - Angled Classifier Blades on an EL Pulverizer
PULVERIZER HOUSING
CLASSIFIER BLADES
SEAL WELD ACROSS TOP BAFFLE TWO 3/8” THICK GUSSETS
•
Extension of classifier outlet skirts or “inverted top hat”. This is performed to change the direction of the coal particles in a downward direction towards the classifier reject area. The increased downward momentum and 180° turn of the particles with higher mass allows less of these large particles to be carried to the fuel lines. A 1” extension of the inverted top hat is usually sufficient. Extension of 2”
4
TOTAL OF THREE 3/4” ROUND ROD BRACES AT ENDS AND CENTER OF DEFLECTOR
Figure 10 - Raw Coal Deflector for EL Pulverizer
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Optimum Operation and Maintenance of El Pulverizers Pulverizer Vibration Non-optimum pulverizer air flow can sometimes cause EL pulverizers to vibrate. The mill level also plays a factor in vibration. Pulverizers that are operating fuel lean will vibrate much more than a pulverizer operating at correct air/fuel ratio. Excessive vibration after verification of mill coal level and air flow warrants an internal inspection. Mechanical factors that can cause excessive vibration are as follows:
• • • • •
resistance’s are not caused by dissimilar positions. Proper diff position is also required to maximize diff effectiveness and to promote efficient mixing of fuel and air at the burner. Optimum diff position is achieved when the diff is centered in the coal nozzle with ±1/8” tolerance and the diffs trailing edge is 1” beyond the coal nozzle tip. (Figure 11)
Ball flat spotting or banding Foreign objects in the pulverizer Broken grinding rings Excessive ring run-out Large raw coal sizing
Clean Air Balancing Balancing system resistance of fuel lines on clean air is the first step of an empirically derived approach to balancing pulverizer fuel and air. Clean air balance is defined as balance of airflow between a pulverizer’s fuel lines in the absence of fuel. This is achieved by forcing air through the pulverizer at normal operating mill outlet temperatures with primary air fans while the feeder remains off-line. Clean air balance is determined by measuring the velocity of air flowing through each individual fuel line with a standard Pitot tube. Primary airflow optimization will be discussed later in this report. In most cases, primary airflow is higher than desired and will be reduced to optimize performance. Prior to optimization of primary airflow it is prudent to perform clean air balancing. This will insure minimum fuel line velocities are maintained after optimization of primary airflow. Air velocity in all fuel lines must exceed 3,300 feet/minute (Fpm). Fuel line velocities above 3,300 Fpm will insure coal entrainment in primary air. Velocities below 3,000 Fpm allow coal to fall out, accumulate or “dune” in horizontal runs of fuel lines. The 3,300 Fpm minimum velocity includes a 10% safety margin above the absolute minimum line velocity of 3,000 Fpm allowing for some imbalance in air flow between pipes. Coal line accumulations may cause burner pulsation’s, flame instabilities and possible stoppages. Fuel lines are balanced by an iterative process utilizing 10 Gauge carbon steel trial orifices. Clean air balance with a maximum of ±2% deviation from the mean between all fuel lines on a pulverizer must be achieved. After optimum orifice configuration is determined, permanent 400 Series stainless orifices are installed. Typical coal line orifices are illustrated by Figure 12. Computer modeling is sometimes performed to determine orifice sizing. Following installation of orifices, clean air balance should always been verified by Pitot traverse of fuel lines. Clean air tests by Pitot traverse is also required to ascertain if any fuel line resistance’s not shown by drawings are present. Boilers with EL pulverizers utilizing pre-NSPS burners typically utilize a concentric ring coal diff. If so equipped, diffs on all burners must be at uniform position to ensure differences in fuel line system
5
Figure 11 - Proper Position of B&W Con Ring Diff
FLANGE
ORIFICE PLATE Trial Orifice - 10 Ga. CS Perm Orifice - 3/8" 400 Series S.S.
xx
STAMPED W/ORIFICE SIZE
FLANGE
ORIFICE
NOTE: ORIFICES MUST BE AS FAR UPSTREAM OR DOWNSTREAM OF TEST TAPS AS POSSIBLE. >10 PIPE DIAMETERS PREFERRED
Figure 12 - Typical Coal Lines Orifices
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Optimum Operation and Maintenance of El Pulverizers Pulverizer Airflow High primary airflow can cause poor coal fineness as well as poor fuel balance. Primary airflow should be ramped against fuel flow. Optimum pulverizer airflow for a EL pulverizer is 1.75 to 1.8 pounds of air per pound of coal. The graph below illustrates a typical air to fuel relationship for EL pulverizers. Characterization of P.A. flow requires accurate control and measurement of primary air and fuel.
Primary Airflow - lbs./hour
Optimum Primary Air Flow Ramp 75,000 70,000 65,000 60,000 55,000 50,000 45,000 40,000 35,000 30,000 25,000 20,000 15,000 10,000 5,000 0
Minimum Primary Airflow to Maintain Minimum Fuel Line Velocity at 3,250 Fpm
0
5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
Coal Flow – Pounds per Hour Recommended -- 1.8 Air to Fuel Ratio
Figure 13 - Typical Primary Airflow Ramp
Pulverizer throat clearance must be verified prior to primary air flow optimization. Accumulation of pulverizer operating hours will result in throat gap enlargement as throat plates and lower grinding ring wear. As the gap becomes wider, velocity decreases, allowing smaller coal particles to fall through the throat. If throat gap is not closely monitored, serious coal dribble can result. If throat gaps are too small, mill differential will be high, possibly causing reduced capacity and trapping of pyrites such as rocks or tramp metal in the throat. Excessive coal dribble can be temporarily avoided with fineness and performance penalties by increasing primary air flow. Operation with increased primary air flow should only be used until proper maintenance can be performed. Figure 14 documents desired throat opening on EL pulverizers, sizes EL-50 through EL-76.
Figure 15 - EL Throat and Seal ring clearance
Flow nozzles or venturis usually provide the most accurate and reliable means to measure and control primary airflow. In many cases, the averaging Pitot tubes provide adequate accuracy for measurement of primary air flow. The much endorsed procedure of calibration by cold clean air flow should be abandoned. Calibration should be performed by measuring air flow at the P.A. fan discharge upstream of the averaging Pitot tube. At this location, Pitot calibration will be achieved at operating temperature of ~325°F rather than clean air at ~150°F.
Nominal Throat Size Pulverizer Size
Throat Opening 32nd of Inch
Open Area Ft2
EL-50
20
1.03
EL-53
20
1.14
EL-56
20
1.25
EL-64
22
1.48
EL-70
24
1.70
EL-76
30
2.00
Figure 14 - EL Pulverizer Throat Openings
Seal ring clearance is critical too prevent air from bying the throat to the pyrite section. Design seal ring clearances are as follows: EL-21 to EL-56: EL-64 to EL-76:
6
3/32” Maximum 1/8” Maximum
1/16” Minimum 1/16” Minimum
Figure 16 - Typical EL Pulverizer Averaging Pitot Tubes
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Optimum Operation and Maintenance of El Pulverizers Pulverizer Airflow High primary airflow contributes to poor coal fineness by increasing classifier air velocities and reducing overall coal retention time in the grinding zone. Increased primary air flow increases velocities at the classifier outlet. Higher classifier outlet velocities will have sufficient energy to entrap larger coal particles. Lower velocities allow larger particles to be returned to the grinding zones. At higher velocity, coal particles have higher kinetic energy or momentum, increasing stratification of coal and air when centrifugal forces are imparted on coal particles by the classifier. The combined effect of poor fineness and separation of coarse and fine coal particles causes poor fuel balance. High primary air will also result in the following:
•
•
•
Poor flame stability at low load and flame ignition points unattached to burner nozzles. Primary airflow of 1.8 Lbs. air/Lb. fuel will maintain coal nozzle tip velocities within the best range of flame propagation speed for flame stability and improved combustion. Increased air heater exit gas temperatures. Higher dry gas losses are the result of increased tempering air usage. Higher air to fuel ratio requires lower pulverizer inlet temperature to maintain outlet temperature, decreasing hot air from the air preheaters. Additional dry gas losses are incurred by heat loss to heat additional ambient (tempering) air after injection into the boiler “boundary." Increased furnace exit gas temperatures. High primary air increases the differential in velocity between the primary air/fuel mixture and the secondary (combustion) air. This stages or delays combustion allowing increased heat release above the burner belt zone. Lower heat release in the burner belt zone results in lower waterwall heat absorption and a subsequent increase in furnace exit gas temperature.
•
All mating surfaces between the classifier and classifier cone sections must seal with no gaps. Gaps in mating surfaces will allow coal and air to by the classifier louvers thereby reducing classifier efficiency.
•
seal ring clearance. Air escaping through excessive clearance open will reduce air flow across the throat and could result in coal spillage or dribble.
•
Insure all segments of the seal ring are securely fastened to the housing unit. The loss of a segment will allow air to by the throat and cause nonuniform distribution of air around the periphery of grinding zone. The resulting friction from a loose segment in with a grinding ring will generate extreme heat possibly causing the ring to be damaged or a mill fire.
• •
Ensure the reject gate functions properly.
•
Inspect the grinding rings and balls for wear. Document the number and size of balls.
•
Document the vertical height between the grinding rings.
•
Insure all spring bolts are properly adjusted and uniform. Dissimilar compression’s of different springs will cause uneven loading of the grinding elements and poor grinding performance. Poor grinding performance could result in fineness and capacity reduction.
•
no retaining bolts are missing from the spring bolt shoes. These bolts hold springs and spring bolts in their sockets. Missing bolts will allow extra motion between the spring bolts, springs and shoes, contributing to excessive wear of these components.
•
Inspect spring bolt shoes. Excessively worn shoes may allow springs to push through the shoe eliminating proper spring setting.
•
Insure spring bolts are vertical. Out of line spring bolts may result in bent or broken spring bolts.
tailings discharge (cabbage cutter) seals with tailing section. Clearance between cabbage cutter and tailings discharge should be less then 1/8”.
• • • •
Higher mill fire or explosion propensity
Increased auxiliary power, due to increased P.A. Fan horsepower demand to convey additional primary airflow
•
•
Higher Nitrous Oxide emissions. Increased Nitrous Oxide emissions are facilitated by injecting more of the total air, for a given excess air, as primary air in the high temperature root of the burner flame rather than secondary (combustion) air.
Inspect the condition of the housing liners. Excessive erosion of the consumable housing liners may result in erosion of the mill housing.
•
Closely inspect the top and bottom rings for cracks. Cracks may worsen during operation that can ultimately lead to a broken grinding ring that can lead to breaking of the pulverizer’s main shaft.
•
Inspect the classifier blades. Make sure all the adjustable portion (thin gauge) of each blade or louver is at uniform insertion into the classifier. Ensure all nuts that secure the adjustable portion of the classifier blade to the stationary blade are tight. Nuts that loosen with vibration may result in dislodging of blades. Dislodged blades can damage the pulverizer, cause excessive vibration and decrease coal fineness.
Higher slagging or fouling propensity Increased coal line and burner nozzle wear
EL Pulverizer - Critical Inspection Items Pulverizers should be inspected on a monthly basis. During pulverizer inspection, the following items:
•
Pulverizer throat gap. Throat gap variation around the circumference should be less than 1/16” to ensure equal air distribution around the grinding zone perimeter. Throat gap should be measured with a taper gage for accurate measurement.
7
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Optimum Operation and Maintenance of El Pulverizers • •
•
•
Inspect the inverted top hat and ensure it is not excessively worn. Ensure all swing valves open fully and seal securely in the closed position. Swing valves that do not open fully may bias dirty air flow and fuel flow between pipes. Air flow through a single pipe with a partially opened swing valve may result in low pipe velocities insufficient to maintain suspension of coal, causing a coal line stoppage. Inspect the primary air fan blades and inlet bells. the inlet bell to fan wheel clearances. Inlet bell to fan wheel clearance should be 1/8” at bottom and 1/4” at top. Inspect the primary air averaging Pitots tubes. Make sure the impact and static holes are perpendicular to the flow direction. Ensure the Pitots are not distorted and no holes are plugged.
and low outlet temperature is still experienced, available hot air temperature may be insufficient for the moisture content of a particular fuel. Required temperature can be calculated using the method shown in Figure 21. If the required temperature is not available, corrective action would require coal flow to be reduced or primary air flow increased. Higher air flow may cause decreased coal fineness, however, these cases are the exception and not the norm and should be implemented to prevent low outlet temperature.
Input Parameters Coal Flow : Air to Fuel Ratio : Primary Air Flow : Mill Oultet Temperature :
35,000 Lbs./hr. 1.80 # air/ # Coal 63,000Lbs./hr. 160 °F
h (water) @ ambient = h (vapor) @ mill outlet = Enthalpy Rise =
Ambient Temp. of Coal : Coal Moisture : of air : of Coal :
100°F 7.5% 0.24 0.30
68 Btu/Lb. 1130 Btu/Lb. 1062 Btu/Lb.
Heat Required to Dry Coal (Btu's)
Pulverizer Outlet Temperature Maintaining optimum mill outlet temperature is required to insure evaporation of fuel moisture and for proper ignition of coal at the burners. Pulverizer outlet temperature should be >160°F on EL pulverizers. Temperatures less than 160°F will not promote complete evaporation of fuel moisture. Low outlet temperature can lead to coal accumulation on fuel line pipe walls, poor combustion and poor fineness. Pulverizer ability to grind coal to fines is debilitated by fuel moisture resulting in undesirable fineness. Low outlet temperature results in a higher percentage of total primary airflow as tempering airflow rather than hot air. Tempering air is at ambient temperature and introduced through atmospheric damper or ducting from the forced draft fans. Tempering air that has not been heated by the air heaters, is injected into the boiler boundary resulting in higher dry gas losses. This also facilitates less air ing across the air heater which in turn results in a slight increase in air heater exit gas temperature. In some cases, low outlet temperature can result in lower than minimum fuel line velocities due to differential in velocity at varying temperature for a given weighted flow. When low outlet temperature is observed the following action should be taken: 1. 2. 3.
Enthalpy Rise of Coal = C oal Flow × [ 0.30 (Tin - Tout) + {Coal Moisture × (Enthalpy Rise)}] = 3,417,750Btu/hr. Heat Content of Primary Air (Btu's) Heat Req'd. to Dry Coal = (air) (air)××Mass MassPA PAFlow Flow××(Tin (Tin- -Tout) Tout) = 386.04°F Steam Properties Table - Data Taken From 1967 ASME Steam Tables Enthalpy of Vapor @ 1 Atmosphere Enthalpy (Btu/Lb.) Temp. (°F) 1124 145 1126 150 1128 155 1130 160 1132 165 1134 170 1136 175 1138 180 1140 185 1142 190
Enthalpy of Water (Liquid) @ 1 Atmosphere (14.7 PSIA) Temp. (°F) Enthalpy (Btu/Lb.) 40 8 50 18 60 28 70 38 80 48 90 58 100 68 110 78 120 88 130 98
Figure 21 - Pulverizer Heat Balance
calibration and functional checks of the temperature sensing devices. stroke and functional checks of the hot and tempering air dampers. Collect a raw coal sample and perform a proximate fuel analysis. Calculate the required temperature at the pulverizer inlet to maintain desired outlet temperature. Compare the required outlet temperature to actual pulverizer inlet temperature. If there are gross differences pulverizer air flow.
In some cases low pulverizer outlet temperature may be caused by insufficient air flow or extremely wet coal. This situation would be indicated by functional dampers that result in a fully open hot air damper and fully closed tempering air damper. If air flow is verified to be optimum
8
Innovative Combustion Technologies, Inc. y 2367 Lakeside Dr. Ste A-1, Birmingham, AL 35244 www.innovativecombustion.com
Optimum Operation and Maintenance of El Pulverizers
P.A. Temperature Required with Varying Coal Moisture 700°F 600°F 500°F 400°F
Typical correction for EL pulverizer capacity for coal grindability is illustrated by Figure 22.
EL Pulverizer Capacity Correction for Coal H.G.I. at a Specified Fineness 1.60 Capacity Factor (× base capacity at 50 H.G.I.)
Pulv. Inlet Temperature for 160° Outlet
The graph below illustrates increases in required pulverizer inlet temperature to maintain 160°F outlet temperature as coal moisture increases. This graph also illustrates increased pulverizer inlet temperature with lower pulverizer air to fuel ratio. In most cases 100% hot air will result in a maximum temperature of ~550°F. Examination of the graph will indicate that low air flow (1.5 air to fuel ratio, as an example) will result in low outlet temperature due to insufficient hot air temperature. At optimum air flow (1.8 air to fuel ratio), outlet temperature will likely not be achieved at coal moisture contents greater than 13%.
1.40 1.20 1.00 0.80 0.60 0.40 20 30 40 50 60 70 80 90
300°F 6%
8%
10%
12%
14%
Coal Hardgrove Grindability Index
% Moisture Content of Coal 1.8 Air-Fuel Ratio
1.5 Air-Fuel Ratio
Figure 22 - Effect of Coal H.G.I. on Pulverizer Capacity
Effect of Coal Grindability (H.G.I.) Design capacity of EL pulverizers is based on coal grindability index of 50 H.G.I. Pulverizer capacity for a specified coal fineness will vary directly with coal grindability below 50 H.G.I. As an example, an EL-76 rated at 40,000 Lbs./Hr. coal flow capacity with 50 H.G.I. coal will have a 32,000 Lbs./Hr. coal capacity with 40 H.G.I. coal. The converse is also true, that is, increased capacity can be achieved with higher coal grindability. Capacity correction for grindability above 50 H.G.I. is slightly less than proportional. The graph provided as Figure 22 illustrates the relationship of pulverizer capacity and coal grindability. As an example a capacity of 46,800 Lbs./Hr. coal flow could be achieved with 60 H.G.I. coal on an EL-76 with than design coal H.G.I. will require reduction in pulverizer capacity unless the pulverizer is modified to compensate for lower coal grindability. In some cases, design capacity can be achieved with lower grindability coal by increasing primary air to fuel ratio. This will achieve pulverizer capacity and unit electrical generation but with reduced fineness and boiler efficiency. Increasing air flow on coals that have both low H.G.I. and low volatile coal (20 to 29% V.M.) will cause significant flame instability requiring alternative actions.
9
Innovative Combustion Technologies, Inc. y 2367 Lakeside Dr. Ste A-1, Birmingham, AL 35244 www.innovativecombustion.com
Optimum Operation and Maintenance of El Pulverizers Pulverizer Airflow Calibration of averaging Pitot tubes is performed to establish a Pitot tube “K” factor. “K” factors are obtained by measuring air flow while documenting differential pressure across the averaging Pitot tubes. Determination of “K” factors allow calculation of primary air flow for a given differential pressure across the averaging Pitot tubes. This should be performed at several pulverizer air flows to generate a curve. Typical test points include:
•
100% full pulverizer load P.A. flow (based on a 1.8 air to fuel ratio 75% of full pulverizer load P.A. flow Minimum pulverizer air flow calculated to yield fuel line velocities of 3,300 Fpm After collection of data, Calculate “K” factors utilizing the following formula:
• • •
K=
The figures below illustrate typical measures to improve the accuracy of primary air flow measurement elements. Flow nozzles, as shown by Figure 17, are the preferred and most accurate means to measure primary flow. Typical modifications to primary air ducting intended to increase velocity and smooth flow profiles by use of dams or angle iron are shown by Figures 19 and 20. If flow nozzles are not installed, installation of venturi section, as shown by Figure 18, is the preferred method to increase accuracy of existing averaging Pitot tubes.
W ΔP × δ
δ
Air Density at Pitot (Lbs./Ft³)
W ΔP
Mass Flow (Lbs. per Hour) Pitot Differential (“w.c.)
Figure 17 - Flow Nozzle installed at PA fan inlet
Calculated “K” factor at each of the three different air flows are averaged. No single “K” factor should deviate more than ±3% from the average. If “K” factors shift more than ±3% deviation from the mean, P.A. ducting must be modified to increase accuracy or Pitots must be replaced with an alternate flow measurement element. After determination of “K” factor, primary air flow can be calculated for given Pitot differential by rearranging the previous equation.
W = K × ΔP × δ
Figure 18 - Venturi to Improve Avg Pitot Accuracy
A primary air flow ramp can then be generated by rearranging the equation once again to calculate required Pitot differential for desired primary air flow.
⎛ W ⎞ ⎟⎟ ΔP = ⎜⎜ ⎝ (K ) • δ ⎠
2
Desired primary air flow is calculated by multiplying coal flow by 1.8 above minimum fuel line velocity. Figure 19 - Typical Air Dam in EL P.A. Ducting
P.A. Differential – "w.c.
Coal Flow Vs. Primary Air Differential – Utilizing P.A. “K” Factor of 2,746 3.50 3.25 3.00 2.75 2.50 2.25 2.00 1.75 Min. Air Flow to Yield 1.50 3300 Fpm in Pipes 1.25 1.00 0.75 0.50 12,000 17,000 22,000
Basis: P.A. Fan Discharge Conditions: 400°F 10" w .c. Static Prs. Density = 0.04736 Lbs./ft³
27,000
32,000
37,000
Coal Flow - Pounds per Hour Recommended – 1.8 Pounds of Air per Pound of Coal
Figure 20 - Angle Iron to Increase Velocity and Smooth Flow Profile Across Avg. Pitot
10
Innovative Combustion Technologies, Inc. y 2367 Lakeside Dr. Ste A-1, Birmingham, AL 35244 www.innovativecombustion.com
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