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function of classifier in coal mill

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Classification performance of model coal mill classifiers . nbsp 0183 32 The first generation of the coal mill classifier used static guide vanes to produce a swirling flow to classify the particles by centrifugal force 10 The modern coal mill classifier however often has an impeller mounted close to the exit of the mill chamber The impeller is a cage with a set of blades installed around its ...

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Nov 17, 2012 4.4.Classification The circulating air is also used to classify the pulverized coal product prior to carrying it to the burners. The classifier, located on the top of a mill returns the over-size material back to the pulverizer but allows the proper-sized material to pass out of the mill to the burners. classifiers are critical in providing the ...

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Classification performance of model coal mill classifiers . nbsp 0183 32 The first generation of the coal mill classifier used static guide vanes to produce a swirling flow to classify the particles by centrifugal force 10 The modern coal mill classifier however often has an impeller mounted close to the exit of the mill chamber The impeller is a cage with a set of blades installed around its ...

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The ball mill is a slow speed mill and has two primary areas where coal pockets can exist and create fires: the inlet/ outlet ductwork and the classifier. In a ball mill the raw fuel and the primary air are mixed prior to entering the mill and initial drying of the surface moisture is .

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Mar 01, 2020 1. Introduction. The classification performance of the air classifier in a vertical coal mill determines the size of pulverized coal particles. Thus, the classification performance will influence the ignition, burning, and burnout of the coal particle, and NO x formation in the furnace as well [,,, ].The first generation of the coal mill classifier used static guide vanes to produce a ...

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The ball mill is a slow speed mill and has two primary areas where coal pockets can exist and create fires: the inlet/ outlet ductwork and the classifier. In a ball mill the raw fuel and the primary air are mixed prior to entering the mill and initial drying of the surface moisture is .

Jul 14, 2007 Most existing pulverizersboth the vertical-shaft and ball-mill typescome with a static classifier. Its blades reject coarse particles to produce a stream of coal particles that are mostly ...

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dynamic classifiers improve pulverizer performance and more

No one in this industry underestimates the difficulty of transforming an unwieldy and distinctly nonuniform substance like coal into a fuel whose physical and chemical characteristics are consistent enough to supply a workhorse power plant boiler. Designers of fuel-handling and pulverizer systems have wrestled with this problem since the first traveling-grate furnace was invented. The more predictable a fuel is, the easier it is for engineers to tune a boiler burning it for maximum performance and minimum emissions.

From the grizzly to the burner tip, the pulverizer plays an essential role in maximizing the consistency of fuel delivered to the steam generator. Most existing pulverizersboth the vertical-shaft and ball-mill typescome with a static classifier. Its blades reject coarse particles to produce a stream of coal particles that are mostly about 0.0029 inches (74 microns) across.

Obviously, not all of the particles leaving the pulverizer are that size. The fineness of coal is measured using a U.S. standard sieve with a sleeve opening of 0.0030 inches. Particles smaller than 74 microns will pass through a 200-mesh screen, and those larger will not. Passing coal through several screens in series yields a profile of the particle size distribution (PSD) of a pulverizer/classifier. The industry standard for PSD has been 70% passing through a 200-mesh sieve. Other common U.S. standard sieves are 325 and 50 mesh, which stop particles larger than 0.0017 inches (45 microns) and 0.0117 inches (300 microns), respectively.

Coarse particles of coal dont burn as quickly, easily, or cleanly as finer particles. Because they take longer to burn, coarse coal particles raise a boilers average NOx emissions. They also foster agglomeration and deposition of slag, making boilers and heat-recovery boilers more vulnerable to fouling. Coarse coal can even poison the catalyst of a selective catalytic reduction (SCR) system. If enough coarse coal passes through a boiler without being burned completely, its flyash may have too much unburned carbon (UBC) for commercial use.

Upgrading a pulverizers classifier from static to dynamic can improve two key pulverizer performance measures: its throughput and its coal fineness. As their name implies, dynamic classifiers (see box) use static as well as rotating vanes to sort coal particles more precisely by size. Retrofitting a pulverizer in this way:

A classifier separates coarse from fine coal by allowing the fine coal to pass and rejecting the coarse particles for regrinding. A dynamic classifier has an inner rotating cage and outer stationary vanes. Acting in concert, they provide what is called centrifugal or impinging classification.

A Loesche LSKS dynamic classifier (Figure 1) was retrofitted to each of four Babcock Wilcox (BW) Model 10E10 ring and ball pulverizers at E.ONs Ratcliffe-on-Soar Power Station in the UK. Ratcliffe has four 500-MW natural-circulation, single-furnace, dry-bottom BW boilers. Each pulverizer has two coal outlet pipes, which then split into three smaller pipes that each feeds a BW Mark 3 low-NOx burner (LNB). Each boilers 48 burners are arranged as four rows of 12.

1. Top of the heap. An LSKS dynamic classifier like this one was installed atop an existing Babcock Wilcox ball mill at E.ONs Ratcliffe-on-Soar Power Station in the UK. Source: Loesche Energy Systems

E.ON installed the dynamic classifiers as part of its plan to comply with the new European Large Combustion Plant Directive, which calls for reductions in emissions of NOx, SO2, and particulates by 2008 and beyond. Making the typical 35% reduction in a plants NOx levels (to 0.42 lb/mmBtu) that the directive requires would normally necessitate adding an SCR system and/or upgrading to LNBsat considerable cost.

E.ON, however, chose to reduce the NOx output of the Ratcliffe plant by adding an overfire air (OFA) system and dynamic classifiers. The utility took this approach mainly to ensure an acceptable level of UBC in the plants flyash, which is sold as filler to cement makers. But another reason was to reap the classifiers biggest benefit: more consistent coal particle sizing.

The first dynamic classifier was installed at Ratcliffe in July 2003, and six more followed in the summer of 2004. All of them had already passed their tests when the overfire air system was installed.

Pre-retrofit data indicated that with static classification, 75% of the pulverizers output could pass through a 74-micron screen, and 97.5% of particles through a 300-micron screen (Figure 2). After the dynamic classifier was installed, about the same percentage (78%) passed through the 74-micron screen, but far more (virtually all) particles could pass through the 300-micron mesh. In other words, all coarse (>300 micron) coal particles were removed from the stream of coal fed to the boiler. The slopes of the pre- and post-retrofit lines on the Rosin-Rammler graph are 31.7 degrees and 52.7 degrees, respectively.

As dynamic classifiers were added in turn to the other pulverizers at Ratcliffe, it became possible to compare their effect on the individual performance of each of the plants four identical 500-MW boilers. All had been upgraded with BW UKs LNBs in the mid-1990s. A 62% average reduction in UBC levels at normal excess-air levels (Figure 3) was noted during post-retrofit testing.

3. Reduced LOI. Plotted here are flyash loss-on-ignition levels (a measure of unburned carbon content) vs. stack O2 levels at three different dynamic classifier speeds. The data were taken following the retrofit at Ratcliffe-on-Soar Power Station. Source: Loesche Energy Systems

The retrofits bigger payoff, however, was a 13% average reduction in the plants NOx emissions (Figure 4). Testing also produced a plant CO emissions profile (Figure 5) that engineers could correlate with the NOx and UBC profiles to fine-tune operations. For example, knowing that the plant could tolerate a slight increase in CO and UBC levels, the engineers could feel safe lowering the stack O2 level by 1% or more, in order to further reduce NOx emissions.

4. Par for the coarse. Eliminating large coal particles from the fuel mix also reduced NOx emissions of the Ratcliffe plant. Shown are post-retrofit measured NOx levels vs. stack O2 levels at three different dynamic classifier speeds. Source: Loesche Energy Systems

These results were from tests conducted after adding the dynamic classifiers but before installing the overfire air system. Now that the OFA system is in place, the Ratcliffe plant can meet its 2008 NOx emissions standard using only it and the LNBs. At press time, dynamic classifiers had been retrofitted to three of the plants four 500-MW boilers; the fourth is slated to be upgraded later this year.

Tilbury Power Station in the UK. In 2006, dynamic classifiers were retrofitted to the five MPS Model 180 pulverizers feeding the 300-MW front-wall-fired Foster Wheeler boiler of this RWE plant. Two similar units are scheduled to be similarly retrofitted this year. Results for the projects first phase are just in: a 50% reduction in UBC (enabling the units flyash to be sold, rather than landfilled) and a distinct improvement in particle fineness. Now, >99.8% of particles can pass through a 300-micron screen (vs. 99% before), while 85% can pass through a 75-micron screen (vs. 70% before).

J.B. Sims Power Plant in Grand Haven, Mich. Dynamic classifiers were retrofitted to three BW Model EL56 ball mill pulverizers feeding an 83-MW BW front-wall-fired boiler. As at Tilbury Power Station, the upgrade improved particle fineness. Now, 98.8% of particles can pass through a 150-micron screen (vs. 95% before), while 92.1% can pass through a 75-micron screen (vs. just 72% before).

Dynamic classifiers can increase the throughput of a pulverizer while maintaining the fineness of its output. Such retrofits make the most sense for units burning Powder River Basin coals, whose lower heat content and higher ash and moisture content require an increase in fuel flow to maintain unit rating. They also are applicable to units that blend high-heating-value and PRB coals to maintain their output. Experience with vertical-shaft pulverizers used in the coal mining industry suggests that replacing a units static classifier with a dynamic classifier would increase its throughput by more than 30% and simultaneously increase coal fineness by 10%.

Dynamic classifiers can increase both fineness and capacity, but to a lesser extent than a system optimized to increase one or the other. Again, experience with vertical-shaft pulverizers at coal mines suggests that capacity increases of 10% to 35% and increases in fineness of 25% to 50% are achievable.

A dynamic classifier is retrofitted to a vertical-shaft pulverizer by installing a duplicate upper pulverizer casing that houses the classifiers fixed and rotating vanes, motor, and drive connections. A typical dynamic classifier assembly has a center coal feed pipe and several coal outlet pipes (Figure 6). The rotor drive is mounted on the side of the casing and driven by a belt. If the coal feed is off-center, the drive can be directly coupled, as was the case at the J.B. Sims Power Plant in Michigan (Figure 7).

7. Nice installation. At the J.B. Sims Power Plant in Grand Haven, Mich., retrofitting each of three LSKS dynamic classifiers required mounting a direct drive for it in the center of the existing Model EL 56 pulverizer. Courtesy: Loesche Energy Systems

In most cases, dynamic classifiers can be installed as "slide-in" retrofits with little need to move existing equipment. Figure 8 shows another type of dynamic classifier, with one inlet and outlet, favored by the cement industry.

It should go without saying that gains in fineness, capacity, or both, depend on many factors, such as the type of coal, the brand of existing pulverizers, and the design of the overall fuel-handling system. Likewise, the costs of a retrofit are site- and unit-specific, not so much for "as found" equipment but as a result of fuel system or other unit modifications.

For this job, the break-even point for fuel savings alone is estimated at 2.4 years. The break-even on fuel savings plus NOx credits is 1.6 years (which falls to 1.3 years if revenues from sales of flyash with less UBC are factored in).

Robert E. Sommerlad is strategic accounts manager for Loesche Energy Systems; he can be reached at [email protected] Kevin L. Dugdale is managing director of Loesche Energy Systems Ltd; he can be reached at [email protected]

flow visualisation and velocity measurements in a vertical spindle coal mill static classifier - sciencedirect

The aerodynamics within a reduced scale model of a vertical spindle coal mill static classifier are investigated to provide data for improving classifier particle separation predictions and the validation of computational simulations. Quantitative data for these purposes was obtained by measuring velocities using three-dimensional Laser Doppler Anemometry. Flow visualisation was also used to provide qualitative understanding. The results demonstrate that the flow in the main classifier volume closely resembles that reported in the literature for cyclones. However, the flow in the upper section of the classifier is highly three-dimensional. The effect of varying the inlet vane angle, within the range of industrially useful vane angles, is shown to principally only affect the tangential velocity magnitude.