coal mill

coal mill - shanghai zenith company

A coal mill is the important auxiliary equipment for coal-powder furnace. It has three methods to crush the coal lumps and to grind them into powders, that is, crushing, impacting and grinding.Coal mill is used to grind, pulverize and dry coal before the coal is transmitted the boiler. The coal is fed into the coal mill via an inlet pipe so that the coal mill can pulverize the coal into particles.

As the mill rotates, steel balls work to grind the coal. The mill rotates approximately once every couple of seconds.The grinding system uses either 'open circuit' or 'closed circuit'. In an open circuit system, the feed rate of coal is adjusted to achieve the desired fineness of the product. In a closed circuit system, coarse particles are separated from the finer ones and sent back for further grinding. When final products reach the fineness standard, they are discharged out of the mill.

Location:Saham, Oman Material:Limestone Input Size:Below 720mm Output Size:0-5mm, 5-10mm, 10-20mm, (Oman standard) Capacity:300t/h

Location:Russia Material:Plagiogranite Input Size:Below 700mm Output Size:0-5mm, 40-70mm (0-5mm, 5-10mm, 10-20mm) Capacity:300-350t/h

Location:Mecca Material:Granite Input Size:Below 1000mm Output Size:0-10mm, 10-13mm, 13-20mm, 20-25mm Capacity:400-500TPH (12 hours per day)

long-lasting coal mill for efficient grinding | flsmidth

Our ATOX Coal Mill is a compact vertical roller mill that can grind almost any type of raw coal. The coal mill utilises compression and shear force generated between the large rollers and the rotating table to crush and grind raw coal, removing the need for a separate piece of equipment for crushing. The coal mill can also grind petroleum coke and anthracite down to a fineness below 5% +90 micromillimeters (mm) when coupled with a variable speed mill motor.

As the feed material is added to the mill, the moisture evaporates almost immediately. A continuous gas stream from the nozzle carries the finer particles to the separator, where they will be assessed for the specified fineness. For safety reasons, the coal mill is designed with no external recirculation of materials. Material is prevented from spilling over the edge of the dam ring as both the dam ring height and grinding pressure are adjustable.

The ATOX Coal Mills efficient separator allows material that has reached the required fineness to leave the grinding mill and sends oversized material back for further grinding. The fineness is controlled by adjusting the rotor speed. The high efficiency of the separator is due to the rotor being equipped with an outer ring of louvre plates that are hardfaced for longevity.

The durable grinding mill can handle virtually any type of raw coal and has been designed to grind feed materials with varying moisture levels. The ATOX Coal Mill handles feed materials with less than one percent moisture and up to 25 percent moisture, where abrasiveness and stickiness is not an issue for grinding.

The materials used to produce the ATOX Coal Mills segmented wear surfaces are durable and therefore, last longer. The reversal of liner segment option maximises use of these hard surfaces, making sure every surface is exploited while helping to address uneven wear at the same time.

The separator, with the highest mill capacity, ensures high separation efficiency through having a low bypass to reject. An efficient separator leads to a number of benefits including low specific power consumption for the mill motor, low vibrations, energy savings due to minimal pressure loss, and optimised airflow.

Long-lasting durability is extended to the separator where the inside of the reject cone and outlet top section are faced with Densit. The wear plates for the louvres are also hardfaced to maximise wear life.

The ATOX Coal Mills equipment is located at or above floor level for easy maintenance and cleaning. There are easy-access doors for inspection and maintenance of all of its parts, where roller replacement can occur while still inside the mill. For comprehensive maintenance, the large door is removable for crane access.

Our coal mill offers the flexibility of non-inert and inert operation, depending on the exploding potential of your coal type. The tailored layout considers your coal grinding needs, ensuring simple and optimised operation. For example, the availability of heat sources to dry raw coal helps determine the final layout.

Liner segments can be reversed so that the wear, which normally occurs on the outer edge, can be spread evenly, making use of the entire segment surface. The coal mills liner segments can also be repeatedly hardfaced for maximal longevity.

The heated rotary feed sluice ensures uniform flow of feed into the ATOX Coal Mill for optimum mill operation including minimising power consumption. It is the right solution for a sticky feed material as hot gas from the mill inlet is guided through the rotor, preventing the sticky

FLSmidth provides sustainable productivity to the global mining and cement industries. We deliver market-leading engineering, equipment and service solutions that enable our customers to improve performance, drive down costs and reduce environmental impact. Our operations span the globe and we are close to 10,200 employees, present in more than 60 countries. In 2020, FLSmidth generated revenue of DKK 16.4 billion. MissionZero is our sustainability ambition towards zero emissions in mining and cement by 2030.

coal mill safety

Global Cement (GC): Please could you introduce your experience in the cement sector to date? Vincent Grosskopf (VG): I started working in the bulk handling arena in the early 1970s until 1992, when I joined Thorwesten. At that point Thorwesten Vent had begun to work with explosion vents, predominantly for its own coal silos. We were then approached by others, such as Polysius and FL Smidth, which wanted to licence vents for their own designs. Learning about the use of explosion vents in collaboration with such firms led to applications in coal grinding equipment, including bag filters. On the surface this appears to be quite simple but the situations are quite different. The inside of a silo is big and wide, which means the explosion front propogates relatively slowly. By contrast, inside the narrow ducts of a coal grinding and de-dusting system, explosions move much more rapidly and with greater pressure. At Thorwesten Vent we found solutions around these differences and other issues, gaining unrivalled experience in explosion venting.

GC: What led you to establish Coal Mill Safety? VG: I established Coal Mill Safety (CMS) as a consultancy after I retired in 2011. If a cement producer wants to install a new coal grinding system, they can commission CMS to look at the suppliers design and probe it from a safety angle. If it has an existing system, it can ask CMS how it can improve it.

GC: How would you characterise the state of coal mill system safety in the cement sector in 2019? VG: At best, coal mill safety is not well understood and, at worst, it is ignored. When it comes to coal mill systems, most cement plant operators just presume that the supplier of the equipment knows all of the standards and rules and is 100% capable of making a system that conforms to these and is therefore safe. However, this is not the case.

GC: Why is this not the case? VG: The suppliers designs have undergone relatively little development over time from a safety standpoint. They contain legacy solutions. Even some of the best European suppliers lack the necessary expertise to really maximise coal mill safety in-house. There is no reason why an old design should be re-used just because it is convenient for the supplier and may be the most cost-effective solution for the user. There is too much at stake and the hidden costs, for example in excessive use of steel and concrete and poor maintenance access, quickly eat into the perceived economic advantages. Suppliers also say that a tailored solution will take longer. This is a major reason that old designs are repeated over and over again.

On top of this, suppliers are generally large companies that are not particularly dynamic, the designs take a lot of time and money to update and, frankly, there are more interesting projects to work on. Coal mill systems and their safety have taken a back seat for decades.

Of course, where there is not a focus on explosion protection, the supplier can create systems that are really dangerous. There are numerous suppliers all over the world, but particuarly in Asia, that do not understand the safety principles and, some might say, dont particularly want to.

Despite the use of oil and gas in many regions and the rapid rise of alternative fuels, coal remains the major cement production fuel. It must be safely handled when ground and fed into the kiln and, even though it is a difficult topic to broach, the onus to have the conversation is on the cement producer. (Picture: GlobalCementMagazin)

GC: What are some of the common faults? VG: These include: Explosion pressure shock resistance and explosion isolation issues on the inlet side of mills; Incorrectly protected vertical roller mill reject discharges; Incorrectly designed mill-to-bag house riser duct configurations; Incorrectly protected main bag houses with their downstream equipment for conveying pulverised fuel; Incorrectly designed and protected pulverised fuel silos; Incorrectly designed/installed gas analyser configurations and; Incorrectly configured emergency inerting systems. The methods and means of protection of raw coal stockpiles against fire are rarely organised and the designs of filters for the de-dusting of raw coal conveyor belt transition points are almost always wrong, from both fire and explosion protection points of view.

GC: Why have users not demanded improvement? VG: Producers do not have the expertise and very often dont have the time to ask the right questions or put their finger on the design flaws. The fact that the designs are so old lulls users into thinking that they must be safe, creating the perception that theres no need to act.

GC: What are some of the common ways that people get injured? VG: Many people are killed and maimed as a result of a coal dust explosions but often you wont hear about it. Even if you do hear about it, you wont get any details, which makes analysis of wider trends really difficult. A very common incident is when people open the system in, for example, a baghouse, with the expectation of fighting a fire thats inside. Oxygen enters the relatively oxygen-poor environment inside the system, there is a backdraft and anyone in the way is killed, or at least very badly burned.

A coal grinding system with a mill-to-bag house riser duct (marked with red line) that is very long. Through it, unmitigated flame front propagation could reach a velocity too high for the installed protection to effectively protect the bag house. This is a typical situation and it needs to be corrected.

GC: What data exists on the number of injuries and/or deaths caused by these systems? VG: There is pretty much no centralised data on this subject, which means we dont really know how bad things actually are. What we do know is that in many places around the world there are fatalities and maimings with alarming regularity. Some might reach the local news but there are many more that dont.

Even in developed markets, there are injuries and deaths as the result of explosions. They will be reported to local safety authorities but its very hard to get a picture of the scale of the situation beyond that.

GC: Its not possible to say how much improving safety would reduce harm then, is it? VG: Even if there was a good set of data, I still think it would be hard to act on, especially in regions where safety is not a major concern. Perhaps a major association could collate the data, but there are many other jobs, monitoring environmental performance for example, that demand their attention. I am pessimistic that this situation will change soon.

GC: Do those with first-hand experience of coal explosions take them more seriously afterwards? VG: When things go wrong with coal grinding systems, consultants like CMS will get called. On day one after the explosion, the plant staff will be very concerned and ask, What happened?, How do we stop it happening again? and so on By day three, the plant managers downtime clock is ticking louder and louder and the onus returns to production. The plant then carries on, with many of the same flaws in place and a possible repeat of the incident on the cards.

GC: Where is coal mill safety the best in the world? VG: This is not a question that can be answered geographically. There is no best or worst country at the moment, even when you look at litigious markets like Europe and the US. Id even go so far as to say that there isnt one completely safe cement sector coal mill system, anywhere in the world.

There may be some marginal improvements coming in Germany, where some inspections are now finally taking place, after ATEX Directives were transposed into national law. In Egypt, the ATEX Directives will have to be complied with by all coal-using industries very soon. This will be a very interesting process to observe.

A cyclone of an indirect firing coal mill system that hardly could have been laid out worse. The cyclone has been installed inside a building, which disallows protection by means of explosion venting. Equipping it with explosion vents has been aborted, as evidenced by the blind covers that have been installed in place of explosion vents. The explosion pressure shock resistance will be very low, if present at all. Flame front propagation would run into the cyclone completely unmitigated, since no explosion de-coupling upstream of the cyclones dirty inlet is installed. The configuration shown here is disastrous, since disintegration of the cyclone could cause a dust cloud inside the building, which, if ignited, could blow up the building itself.

GC: Can a coal mill system actually ever be safe? VG: Absolutely! If you combine all of the knowledge available to properly design and engineer your system, operate it correctly and maintain it, there is no reason why the system cannot be completely safe. This is why it is such a shame that the reality is so far from the situation we could have. If an explosion were to happen in such a system, there would be no loss of life, no injury and no major system damage.

GC: What can be done to improve the situation? VG: It starts with the cement plant operator asking the right questions during the design phase. To do that they may need the help of a consultant like CMS. Whoever is asking the questions, they need to have the power to actually demand changes to the design. Otherwise there is no point. Once, a major European cement multinational asked me to help negotiate the purchase of a coal system from a Chinese supplier. However, I was not given authority by the purchaser in that situation and the result of my efforts were negligable. The cement producer needs to understand that being the customer means they should be knowledgeable enough to not accidentally get the somewhat flawed 40 year old design the supplier wants to sell! You also need to operate the system safely and know how it needs to be used. It needs to be maintained properly too. Otherwise the system will become unsafe within three or four years. Even if the plant staff are really on the ball there will still be a place for experts. I was once at a plant in the Philippines where an explosion had occurred during the night before I visited. The plant staff were poring over their computers and control systems to try and find out about the incident. They could, for example, work out where the temperature rose too far and where there was too much oxygen in the system, but, looking at the damage quickly proved that their efforts to understand the effects of the explosion and why their protection had failed went nowhere. 45 years experience allows you to understand that part, without computers.

GC: Does it surprise you that after 45 years, an expert such as yourself is still needed at the plant? VG: No, Im not surprised. Plant staff in the cement industry need to focus on producing cement. Fire and explosion protection for coal grinding is a highly specialised field. You cannot expect that plant staff recognise flaws in the system that has been put in front of them, normally with no or very little input from their end.

GC: Will there be a brain drain in this area as consultants like yourself leave the field? VG: Thats a risk, yes. I just have to pass on as much information as I can in the remaining time that I can have in the sector.

GC: Are attitudes gradually changing? VG: Overall, no. Nothing is really changing at this point. Some producers are making sporadic efforts to understand this area and improve, but such large companies move so slowly. Many suppliers are listening to Thorwesten Vent, which is good. However, Thorwesten Vent can only influence certain aspects of fire and explosion protection of coal grinding systems, not everything.

GC: Could the standards be improved? VG: The standards and codes are very complicated and difficult to follow. They are always referred to but not understood. In some cases the standards leave a lot of room for interpretation. So, you see, you cannot even blame the engineers for misinterpreting the situation they are doing their best! The information from ATEX or the EN codes tell you all kinds of interesting information but they are not, and cannot, be exhaustive in terms of engineering solutions. You wont find answers to all the workarounds you need, most probably because it hasnt been needed before. There may be warnings at best. Ive already mentioned flame front propagation through a duct. That is something that the standards speak of, but they dont say how to deal with it.

GC: Is that because those writing the standards also dont know? VG: Its not that this is unknowable, but there are no standards with a focus on indirect firing coal grinding systems, which typically have some special conditions. NFPA 85, in spite of its pro-forma applicability to the indirect firing coal grinding systems of the cement industry, in reality only covers direct firing for the power generation industry, almost completely neglecting the elements that would form the basis of correct fire and explosion protection of indirect firing grinding systems.

GC: Would you advocate that a cement group standardise its coal mill safety solutions? VG: Yes. It would be good to issue a group guideline covering both design specifications/requirements and best practices. Compliance needs to be part of each plants quality management, with strong monitoring by the groups management. However, such an approach has become more difficult in the past decade or so, with the closures and downsizings of some groups technical centres. Lafarge, Holcim, HeidelbergCement and others used to have several of their own technical centres that would have some degree of in-house know-how and responsibilities, which certainly improved situations in the group. They would look at selected new and existing situations, but were not able to support, let alone control the safety of all the systems of their large groups. Now the big groups have closed or downsized several of their technical centres and delegated responsibilities to their plants management, where the necessary know-how will definitely be insufficient.

An awkwardly designed and installed explosion vent on a pulverised coal silo. The silo has been installed in a building, which disallows explosion venting without special measures that control the blasts exit from the building. These are not present. The explosion vent is of a design that will not reclose, due to design faults that will cause its hinged lid to deform and not to fall back after the explosion, leaving it open to ingress of O2 and uncontrolled losses of inerting medium that will make firefighting impossible. Explosion pressure shock resistance of the silo and the explosion vent are lacking. The explosion effects will affect the silos in-feed drop chutes. When venting, the blast will hit the concrete ceiling, which is far too close to prevent the flame bodys dangerous deflection, spread and expansion into the building.

GC: What kinds of producers are most proactive in coal mill safety? VG: The multinationals are starting to move in the right direction on paper, but its really slow. They dont help themselves with constant personnel changes. I have been in a situation where Ive been training say 8-10 individuals across a group. Everything goes well and then, six months later, I try to reconnect with them to see their progress. The problem is, theyre almost certainly in a different role by then! Theyve most likely forgotten everything they ever knew about coal mill safety and probably didnt transfer knowledge to the next person in their old role.

GC: What is the one easiest thing to do to improve an existing system with poor safety? VG: Sometimes the best solution is to rip it out and start again. That way you have a clean slate and can avoid so many of the common mistakes. When thats not possible, there is no easy win. Its all hard work! All situations are different in any case.

GC: It seems that your final answer sums up the whole issue VG: Indeed. Improving coal mill safety in the cement sector is a continuous and varied challenge. I hope that by highlighting some of the most common problems and failings in these pages in terms of systems, attitudes and regulations I can make others aware of how they can influence this area for the better. This will help the suppliers, cement producers and, most importantly, the men and women that risk their lives working with these unsafe systems.

coal pulverizer maintenance improves boiler combustion

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:

coal grinding - cement plant optimization

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

coal mill in cement plant, air-swept coal mill | coal mill

Air-swept coal mill is the main equipment of cement plant, also called coal mill in cement plant, used for grinding and drying various hardness of coal, crushing the coal briquette into pulverized coal. It is the important auxiliary equipment of pulverized coal furnace. The air-swept coal mill runs reliably, can work continuously, also has a large production capacity and less energy consumption. Air-swept coal mill is mainly used in cement, industrial boiler, ceramic products, thermal power generation, industrial boiler, chemical fertilizer, and other industries.

It mainly consists of a feeding device, main bearing, rotating part, driving device, feeding and discharging device, high-pressure starting device, and lubrication system. The feeding device includes the feeding pipe, the air inlet pipe, and the support. The feeding device of the coal mill adopts the oblique inlet wind and the oblique inlet louver structure to make the feeding more smooth, while the discharging device adopts the curved tube structure.

Raw coal is fed into feeding device of coal mill in cement plant, the hot air with temperature about 300 through the duct into the feeding device, there is a special lifting board in drying storehouse board will raise the raw coal, and helps the raw coal exchange heat and be dried, dried coal enter into the grinding storehouse.

There is a grinding body (steel ball) in the silo where the coal is crushed and ground into pulverized coal. At the same time when the coal is crushed, the pulverized coal and gas are taken out of the coal mill by the special induced fan through the discharging device of the coal mill, and the coarser particles will be returned to the pulverized silo through the return screw of the discharging device for further grinding.

coal mills for all requirements | gebr. pfeiffer

Coals and similar materials are combustible and may also be explosive depending on the amount of volatiles they contain. Thanks to our experience gained by the supply of more than 2200 coal mills to the most different industrial sectors, we know everything about safety issues: when is inert operation imperative, when is it dispensable provided specific safety concepts are provided, and how do these safety concepts have to be integrated into the grinding process? With petcoke and sponge coke, but particularly with the sphere-shaped petcoke, quite a different challenge needs to be met: despite the fineness of these feed materials, mill operation must be stable and smooth. And this is ensured by the geometry of the grinding elements of our coal mills which is particularly advantageous in this respect.

Hard coal is a sedimentary rock which was formed by deposition and carbonization of plant remains. It is hard and resistant. More than 50% of its weight is made up by carbon. Therefore it is classified as high-rank coal. Other fossil fuels like sub-bituminous coals with high moisture contents as well as brown coal and lignite are lower ranked. Lignite, which is the lowest ranking fossil fuel, is defined between peat and hard coal.

Petcoke is not a natural product. It is a byproduct derived from the oil refining process. Mainly consisting of carbon, it is above all used as fuel. The production of petcoke is similar to that of coke produced from coal.

Coal, lignite or petcoke are ground in the Pfeiffer vertical mill by using available hot process gases. The grinding plant can be inerted. The raw coal is dried while being ground in the mill even if feed moisture exceeds 25%. The feed size that can be handled is up to 100 mm, making two-step pre-crushing unnecessary.

Coal, lignite or petcoke are ground in the Pfeiffer vertical mill by using hot gases from a hot gas generator. The grinding plant can be inerted. The raw coal is dried while being ground in the mill even if feed moisture exceeds 25%. The feed size that can be handled is up to 100 mm, making two-step pre-crushing unnecessary.