Power generation industry studies have shown that coal pulverizers are an area where improved equipment reliability is badly needed. The Electric Research Institute (EPRI) has determined that 1% of plant availability is lost on average due to pulverizerrelated problems.1 EPRI also identified oil contamination and excessive leakage as two areas where pulverizer drive train failures account for 53% of pulverizer problems.
Pulverization is currently the favored method of preparing coal for burning. Mechanically pulverizing coal into a fine powder enables it to be burned like a gas, thus allowing more efficient combustion. Transported by an air or an air/gas mixture, pulverized coal can be introduced directly into the boiler for combustion.
This type of mill consists of a rotating tube filled with cast alloy balls. Coal is introduced through two hollow trunnions on each side of the tube. As the tube rotates, the balls tumble onto the coal, crushing and pulverizing it.
Grinding Action is carried out by a series of hinged or fixed hammers revolving in an enclosed chamber with wear resistant plates. The hammers impact on the coal, crushing it against the plates. Further pulverization is achieved as the smaller coal particles are ground through attrition against each other and the grinding face.
This mill uses hydraulically loaded vertical rollers resembling large tires to pulverize raw coal fed down onto a rotating table. As the table rotates, the raw coal is pulverized as it passes underneath the rollers. Hot air forced through the bottom of the pulverizing chamber removes unwanted moisture and transports the pulverized coal dust up through the top of the pulverizer and out the exhaust pipes directly to the burner. The more recent coal pulverizer designs are Vertical Roller Mills. Figure 2 shows a cutaway view of a Babcock and Wilcox MPS Pulverizer.
Mills A ball or roller between two races or rings provides the grinding surfaces on which pulverization occurs. One or both of the races may rotate against a ball or roll (in a Ring-Roll Mill the rolls may rotate while the ring is stationary). Ring-Roll (Bowl-Mill) and Ball-Race Mills comprise the majority of coal pulverizers currently in service at power generating facilities.
In this design the grinding rolls are stationary, while the ring (or bowl, as it is sometimes called) is rotated by a worm gear drive. Powerful springs force the grinding rolls against the ring, providing the pressure required to pulverize the coal.
Raw coal enters the top of the pulverizer through the raw coal feed pipe. The raw coal is then pulverized between the roll and rotating ring. Hot air is forced in through the bottom of the pulverizing chamber to remove unwanted moisture and transport the coal dust up through the top of the pulverizer and out the exhaust pipe directly to the burner. Coal that has not been pulverized into fine enough particles cannot be blown out of the top of the unit; it falls back to the ring and roll to be further pulverized.
The gears and bearings in the gearbox are oil lubricated. Fine coal particles and wear metals from grinding surfaces enter the lube oil through worn bearing and shaft seals, as well as being inhaled through reservoir vents. Historically, the design of pulverizers has been based on the expectation of few drive system problems under prescribed operation and maintenance. In practice, this has often been found not to be true.
Many coal pulverizer designs do not incorporate any filtration in their lube circuits. The pulverizers that do not incorporate filtration use coarse filtration such as 40-micron cleanable mesh or 200-micron cleanable, stacked disk filters. Such OEM-supplied filtration is often unable to keep up with the inherently high ingression rate. This results in contamination levels often exceeding ISO code 30/30, particularly on older designs. This high level of contamination can severely diminish bearing, gear, pump, and seal life, leading to premature need for replacement or rework. Coal pulverizer downtime can be a major factor in reducing overall plant availability and reliability.
Upgrade to Achieve Total Cleanliness Control (See diagrams on back page) The majority of pulverized coal particles are in the 4-30m range, with 70% of these particles smaller than 10m. Ingression rates vary with manufacturer, model, and age ofunit, with older units usually admitting contaminants faster than newer ones. Particulate contamination in the lube system can result in rapid damage to critical components.
In order to protect the coal pulverizer lube system components, Pall recommends maintaining a fluid cleanliness level of ISO 16/13 or better. This can be accomplished through the use of Ultipleat SRT AS grade (12(c) 1000) or finer filters. Ultipleat SRT filters, with their high particle removal efficiency and dirt-holding capacity, are ideally suited to cost-effectively control contamination in this high-ingression application.
When upgrading in-line filtration, a Pall Duplex Assembly is recommended so that elements can be changed out while the pulverizer is operating. Although putting filtration in-line is preferred, difficulty in getting system specifications from the OEM, combined with the typically low pump pressure associated with this application, may make kidney loop filtration a more viable alternative. Reservoir volumes typically vary from 15-300 gallons.
A 20% reservoir volume per minute flow through a kidney loop is generally sufficient to overcome the ingression rate of most applications. The high-viscosityof gear lube oil (2,200 SUS at operating temperature) along with the inherently rapid ingression rate usually associated with these units makes it necessary in most cases to utilize at least one UR619 housing with a UE619 element (12(c) 1000 or finer) for every 50 gpm of flow to provide superior filtration with long element life. Since the pulverizers come on- and off-line, it is important to size the system for the oil viscosity at the coldest possible ambient plant temperatures. Line diameters in the kidney loop should be large enough to facilitate flow of highly viscous lube oil.
Other applications where Pall high-performance filtration is useful include coal-carrying cars and conveyor belts. Many of these applications have both hydraulic and lube systems that are vulnerable to coal dust contamination. This equipment is required to transport the coal stored on-site to the coal pulverizers. Because this equipment is essential to theoperation of the power plant, it is critical that this equipment be free from contamination-related failure.
In March 2003, a major Canadian utility derated its Unit 4 due to a failure of the B mill. The mill was expected to be out of service for about a month. With lost production of approximately 864 MWh per day, the total estimated revenue loss was around $2,000,000. Repair costs for this outage added up to more than $400,000 due to the severity of the damage to drive train components. An analysis concluded that there were multiple causes of this problem, including poor predictive/preventive maintenance practices and poor oil cleanliness.
Pall provided a filter housing for a sixmonth trial to show that oil cleanliness could be improved to industry standards and maintained without the incurrence of substantial element costs. Oil cleanliness went from 20/19/17 to 18/16/13 in approximately 2 hours and has been maintained at this level since.
Every coal pulverizer is designed with a particular fuel grinding capacity or throughput at a certain Hardgrove grindability index (HGI), based on a defined raw coal size, moisture content, and desired fineness level. These factors are significant and must all be taken into account when discussions of pulverizer capacity become serious. A well-performing pulverizer also requires optimum airflows and good fuel fineness, among other performance characteristics, to achieve optimum combustion for your boiler. Part I of this series focuses on a discussion of pulverizer capacity and the maintenance and repair procedures used to return a pulverizer to service after a fire or after being out of service for a long period of time.
Lets use an MPS-89 pulverizer as an example. Most MPS-89 pulverizers are rated for about 125,000 pounds of coal per hour, often referred to as the mill capacity (Figure 1). We prefer to discuss coal throughput as one of the three major components of capacity; HGI and fineness are the other two. A typical correction curve for this pulverizer is shown in Figure 2 for an HGI of roughly 50 and 70% passing a 200 mesh sieve (Figure 2, green line). The design "capacity" of that same pulverizer is 135,000 pounds per hour with fuel that is 55 HGI, 3/4-inch raw coal maximum size, and 7% moisture. The pulverizer will then have the capability to produce pulverized coal that is 70% passing 200 mesh fineness (Figure 2, red line). This is commonly expected performance for this model pulverizer.
Now suppose this same pulverizer must grind sufficient coal flow to produce full load on the boiler with fuel that is 40 HGI, still 3/4-inch maximum size with 7% moisture, but with an increased fineness requirement of 80% passing a 200 mesh sieve. Check the correction curve now and you will see that the rated "capacity," which we prefer to call throughput, drops to 83,000 pounds per hour (Figure 2, blue line). In essence, the performance of a pulverizer is a delicate balancing act between the HGI, fuel fineness, and throughput.
1. A typical MPS-89 pulverizer found in many coal-fired power plants. Courtesy: Storm Technologies Inc. 2. Typical expected performance for an MPS-89 pulverizer is based on the coal HGI and fineness. Source: Storm Technologies Inc.
What is your first step after experiencing a burner fire, mill fire, mill puff, or a mill explosion? After the damage is repaired, how can the mill and burner system be safely and confidently returned to service? In our experience, the following steps have been found to be a safe and prudent course of action. Only after the mill is thoroughly checked out should it be released for operation (Figure 3). 3. Typical locations within a mill that should be inspected after a fire or other operating anomaly. These are the locations of what we consider to be the most critical measurements and tolerances in the MPS pulverizer designs. On the left is a typical Alstom Power mill, on the right an MPS-89 of similar capacity. These two mill types represent perhaps 70% of the pulverizers used in modern pulverized coal plants. Courtesy: Storm Technologies Inc.
Blueprinting a pulverizer isnt 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 1. 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 capacity with improved 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 4). Dont expect optimum performance if the grinding elements are well-worn or the tires are "flat" (Figure 5). Unusual wear patterns are often the result of uneven spring frame tolerances, alignment issues, pressure variations, geometry, and/or eccentricity issues.
Step 2. 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 fineness of greater than 75% 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 (Figure 6). Ensuring sufficient grinding pressure is absolutely essential, and it begins with setting this 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 (Figure 7). 6. The "button" clearance between the spring canister and the journal assembly is a critical tolerance. Courtesy: Storm Technologies Inc. 7. The MPS spring frame hydraulic preload must be carefully balanced and the spring frame centered for optimum mill performance. Courtesy: Storm Technologies Inc.
Step 3. 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 (Figure 8). 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. The throat area also must be properly designed to be compatible with the flow areas of the burner coal pipes and coal nozzles (Figure 9). Only with the best possible pulverizer performance, proper fuel distribution, and optimum air/fuel ratio is optimum furnace performance possible. 8. Ensure optimum arrangement of the mill throat and the coal flow path to improve mill performance. Courtesy: Storm Technologies Inc. 9. 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. Courtesy: Storm Technologies Inc.
Step 4. 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 10). To achieve this fineness, the pressurized mill classifier must perform two functions:
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 or discharge doors of the MPS original design mills are extremely important for acceptable pulverizer performance and of course, optimum furnace performance.
Coal pulverizers are the heart of a pulverized coal-fueled boiler. Often, the root causes of nonoptimized combustion lie with the pulverizers. Capacity; reliability; and environmental issues such as slagging, fouling, and higher-than-desired CO or NOx emissions; overheated superheater and reheater tube metals; and cinder fouling of selective catalytic reduction catalyst and air heaters have all, at times, been linked to poor pulverizer performance.
It is common in our experience to find pulverizers that are performing poorly, yet the degree to which unit reliability, efficiency, capacity, and environmental emissions are affected by them is often underappreciated. However, there are steps that can be taken to measure, quantify, and monitor pulverizer operation so that changes can be made to improve performance.
Obtaining representative samples of coal fineness and fuel distribution is the first step. The best method we have found to do this is by using an isokinetic coal sampler. All fuel lines must be sampled and the fineness sieved from each coal pipe separately. The fuel mass flow to each burner must also be measured.
An isokinetic sampler similar to the one shown in Figure 1 can be used with a dirty-air velocity probe to establish the proper sample extraction rate. The fuel line velocities that are measured are used to compute the primary airflow and air/fuel (A/F) ratios of each coal pipe. The velocities and A/F ratios are valuable for diagnosing combustion issues.
Tuning improvements can only be implemented after the true current performance is measured. Sampling single pipes, or sampling at a single location, is totally unacceptable in our experience. All fuel pipes must be sampled and sieved individually for best accuracy.
The fuel lines must be tested/sampled under normal operating conditions. Often during testing, we have observed that operating conditions are changed. For example, we have seen primary airflow reduced, classifiers reset for best fineness, and fuel flow brought back to mill design fuel flow rates. In other words, the assessment is not representative of normal operation. Testing under special conditions proves nothing. Only testing under normal operational conditions enables a useful diagnosis.
Acceptable standards for best low-NOx burner performance are coal fineness of 75% passing a 200-mesh sieve and less than 0.1% remaining on a 50-mesh sieve. Fuel balance should be within the range of plus or minus 10%. However, in our experience, it is common to find fuel fineness that is well below 65% passing through a 200-mesh sieve and more than 1% remaining on a 50-mesh sieve. Furthermore, it is common to see fuel imbalances that exceed plus or minus 30% to the burners.
2. Lopsided fuel distribution. This test data shows pulverizer fuel flow rates measured during an actual test. The fuel distribution is poor in this case. It should be balanced within plus or minus 10%. Source: Storm Technologies
Once the data are compiled, out-of-specification readings must be investigated. An internal inspection should be completed to check the wear of grinding elements and classifier housings, vanes, and other internal components. Also, check for foreign matter that might be blocking fuel flow paths. Any problems identified should be corrected.
Achieving best fuel balance is done by first balancing the system resistance in all of the fuel lines using orifices and then increasing fuel fineness. Figure 3 shows the typical results of this approach to fuel balancing. Of course, internal pulverizer blue printing to best mechanical tolerances and optimizing an accurate and repeatable air/fuel ration is also important.
There are various adjustments and mechanical tuning measures that can be completed to improve the performance of a modern coal pulverizer. Locations identified on Figure 4 are keyed to the following improvements (journal pressures listed are for a #943-size pulverizer):
Install synchronized straight vane coarse particle guide blades (A). The retrofit lengthens the classifier blades, improves material to 3/8-inch-thick AR400 or better, and implements critical synchronization of the classifier blades for fuel/air two-phase mixture homogenization.
Install orifice housings (E) to support future balancing efforts. The change offers two advantages: It is easier for maintenance personnel to change out orifice plates, and it speeds testing and balancing efforts.
Verify that roll-to-ring clearances (I) are absolutely no greater than 1/4-inch over the full grinding length of the rolls and that the clearance is parallel to the bowl for the full width of the rolls.
Additionally, ensure that venturi sensing lines, connections, and transmitters are all in good condition. Tempering air dampers should be stroked and corrections made to ensure that they close at least 99%. This should also be done for hot air dampers.
All internal mill surfaces must be smooth so that the swirl of the coal/air mixture may enter and leave the classifier without spoiling or turbulence caused by double layer tiles, welded pad eyes, or other internal surface discontinuities. This, combined with precise primary airflow measurement and control, is important for uniform fuel distribution at the classifier outlet. All internal dimensions should be verified and technically directed by a qualified service engineer during installation of performance parts and before closing the mill.
Overhauling Stock gravimetric feeders can also be worthwhile. The refurbishment should include calibrating load cells properly, installing modern microprocessors, adjusting belt tension appropriately, and completing accurate speed calibration.
Another pulverizer performance monitoring technique is to observe the drive motor power input in correlation with the coal feed rate. The relationship of ton/hour to kWh power input is a very helpful leading indicator (Figure 5). A reduction in the power used by a coal pulverizer does not usually result in an improved heat rate. Instead, more grinding power nearly always correlates with better coal fineness. The only exception is with a ball tube mill.
Pulverizer capacity is not simply a measure of coal throughput; capacity refers to a certain coal throughput at a given fineness, raw coal sizing, HGI (Hardgrove Grindability Index), and moisture. Often, if the desired coal throughput or load response is not achieved, the primary airflow will be elevated to higher flow rates than are best for capacity. However, increased throughput achieved in this way sacrifices fuel fineness (Figure 6).
6. A negative correlation. The three main factors that constitute pulverized capacity are Hardgrove Grindability Index (HGI), fineness, and coal throughput. Increasing throughput will adversely affect fineness. Source: Storm Technologies
This is very common. When the primary airflow is higher than optimum, it creates entrainment of larger-than-desired coal particles leaving the mills, promotes poor fuel distribution, lengthens flames, and impairs low-NOx burner performance. We have found that targeting an A/F ratio around 1.8 lb of air per lb of fuel is best. For some pulverizer types, such as ball tube mills and high-speed attrition mills, often a 1.6 A/F ratio is optimum. Never have we observed good combustion conditions or good mill performance with A/F ratios of 2.5 or greater. However, it is common to find A/F ratios of 2.2 to 2.5 during baseline testing.
Results of as-found airflow to fuel flow testing from a sample plant are shown in Figure 7. In this particular case, the A/F ratios tested were well above the desired A/F ramp. When operators bias the primary airflow up, above the optimum, it may improve wet-coal drying, load response, and reduce coal spillage from the grinding zone, but it is not good for the furnace burner belt performance.
7. Missing the mark. The air-to-fuel (A/F) flows shown in this graph are much higher than optimum. Installing properly sized rotating throats is often required to achieve targeted A/F ratios. Source: Storm Technologies
All combustion airflow inputs should be measured and controlled, if possible. We prefer to use the tried and proven venturi or flow nozzles for this purpose because they are rugged, reliable, offer repeatable results, and are less prone to impulse line plugging.
Several components can be retrofitted to improve the performance of MPS mills. The changes may cost a significant amount of money, but the work will usually pay for itself through improved heat rate. One 450-MW coal-fired unit in the Midwest spent $750,000 on testing, changes, and tuning, but calculated that it saved millions by improving heat rate and by allowing higher-slagging fuel to be used at a reduced cost, which greatly increased its market competitivness.
8. Extending component lives. Getting 8,000 hours per year performance requires condition-based maintenance utilizing periodic isokinetic coal sampling and venturi hot K testing and calibration. Source: Storm Technologies
Cold air has a different density than hot air, which can result in a variance in measured velocity at similar mass flow rates. Because the K-factor will vary, we prefer to conduct Hot-K airflow calibrations that use typical operational air or gas density when developing an average K-factor. That information is important when developing a pulverizer primary airflow curve and when measuring all combustion airflows.
Most instrumentation technicians can calibrate and check using the Hot-K method to verify calibration accuracy. As previously mentioned, high primary airflows are one of the most common root causes of poor pulverizer performance, in our experience, so obtaining accurate and representative measurements is very important.
The goal is to obtain the best possible burner belt combustion because it improves heat rate, reduces slagging/fouling, lowers emissions, and reduces cost. All of the following actions can help improve burner belt combustion:
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Through continuous innovation and dedication to quality, Williams Patent Crusher has become the industry leader in coal crushing and pulverizing equipment. Weve maintained that commitment for over 150 yearsand we strive to keep it that way.
Our engineers have designed the most advanced coal pulverizing and crushing systems in the field. Whether youre looking for a direct fired coal pulverizer, a low-capacity coal size reduction machine, or something in between, Williams has you covered. See below for the various products we manufacture to help you decide which machine is right for your application.