types of boiler coal pulverizers

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Written by: johnzactruba • Edited by: Lamar StonecypherPublished Jan 29, 2010

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Home > Engineering > Mechanical Engineering > Energy/ Power Plants

Types of Coal Pulverizers in Power Plants

Pulverizing the coal to a fine powder is an important function in the operation of a power plant. This article

explains the different methods of pulverizing.

The key input to the coal fired power plant is the coal. Coal in the form that is available from the mines is not

suitable for efficient combustion in the boiler of the power plant. The pulverizing system prepares the coal for

combustion.

There are basically three different types of pulverizing.

1. Attrition Type

Coal feeds onto a steel rotating table. Tapered, cylindrical, or spherical rollers on top of the table, under

spring pressure, exert force on the coal particles on the table. When the table rotates, the coal passes

between the rollers and the table and gets ground due to attrition. The hot air under pressure carries this

ground coal upwards. Larger particles fall back to the grinding table. These coal particles then pass through a

classifier where the large particles get separated due to centrifugal action and are returned back to the

grinding table. Different manufacturers use different type of rollers. “Bowl mills” or Raymond mills use tapered

rollers. MPS mill use tire type rollers and are called “roller mills.” The “ball and race mills” use spherical

rollers. The main characteristics are:

Lower investment costs than other types.Smaller capacity for each mill means a better degree of combustion flexibility.They require periodic roller changes as they wear out during pulverizing, increasing the maintenance

cost.Harder coals require this type of pulverizers.

2. Impact and Attrition Type

Coal is fed into a rotating cylinder containing steel balls of around 50 mm diameter. When the cylinder rotates,

the balls rotate along with the cylinder and fall down when they reach the top position. Coal is ground due to

attrition and impact. Coal feeds into the cylinder axially at one side and discharges through the other side. Hot

air or gas carries away the fine coal through the other end of the Tube. This called the “Tube mill” or “Ball

mill.” These types of mills have:

Initial costs that is higher.Maintenance that is considerably less.As the balls wear out during operation, the addition of fresh ball charges can be done while the mill

is running.Can give a very high fineness coal with a high percentage of fines.

3. Impact or Hammer Type

This type consists of a rotating shaft inside a stationary cylinder . The shaft contains arms that impact the

coal inside the cylinder. The impact powders the coal. The hot air carries the coal to the burners. This is more

suitable for softer coals with a higher Grindability index. Periodic replacement of hammers because of wear is

a requirement. Another version of this in Lignite fired power plants.This is in the form of a fan and doubles as

a fan and a mill. The impellers act as the hammers. The main charectristics are:

Lower costs.Suitable only for softer coals.Low fineness output.

Selection of the type of pulverizer depends on

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Grindability of the coal.Capital cost and maintenance cost is another criteria.Patent rights and licence agreements also decide which type of pulverizer the boiler suppliers

recommend.

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Dear Sir,

We are interested to purchase the new pulverizer 613 BOWL Mill.

Kindly send us technical literature and price at the earliest material to be ground Rock Phosphate.

Regards,

Muhammad Afzal Chaudhry

Managing Director

Lyallpur Chemicals & Fertilizer(Pvt) Ltd

Jaranwala, PAKISTAN

Ph # +92-41-4313021-22

Fax # +92-41-4312628

Hi I am Prince Sambo. I work for a copper smelter. I sometimes experience problems of a coal mill tripping. There are 4

coal mills sized EL44. These millls were manufactured by Babcock & Wilcox. Came you please help me with the following

questions:

What are the common causes of a coal mill to trip? What is the remedy once the problem has occurred? What is the

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2 of 4 11/5/2011 12:37 PM

preventative measure for coal mill trip?

Coal pulverizers require a lot of maintenanace and is usually periodically shutdown for maintenance. So if you have a

removable type of orifice you can simply remove this and put a new one.

This could be something like the optometrists testing glasses where they keep on changing the lens to fin the correct

lens size.

You could mount the orifice in between two flanges. By loosening the flanges the orifice can be slipped in and out.

Or if you want a more elaborate method you can design something like a cammera lens shutter , which by turning from

outside will be able to change the orifice size.

respected sir,

coal-air balancing in pulverized coal (PC PIPES) pipes is one of the major problem in a thermal power plant. so design of

optimum geometry of orifices is one of the solution of this problem.but in this design of orifice is changed due to erosion

after some time. so we have to stop the work to change these orifices.so balancing cannot be done online. so to remove

this problem insertion of wedge element is another method rectangular slot is made on the top of pipe so that a

rectangular wedge can be inserted in pipe. BY adjusting the amount of insertion("h/d" ration of wedge),flow resistance

across the pipeline could be adjusted. where "h" is the distace from the tip of wedge to bottom of pipe and "d" is the dia

of pipe.

sir would u suggest me some other methods to overcome this coal-air balance problem

i m presently studying in iit delhi n my guide warning that u hv to be very careful on 18th september , my presentation is

held on that day. sir i m mentally upset and continously thinking about what can be others method. but hopeless as time

is very less. Obtaining good grading is now not the criterion for me. In front of him i will unable to stand comfortably even

single minute if i dnt suggest him some another method. plz sir help me out of this problem i will be highly thankful to u in

my future life.

Inerting systems are used to ifrst kill the fire. CO2, N2 or steam is used as an inerting system.

On indication of fire air and fuel feed is cut off. The inerting medium is injected. Water is used only at the last to flush or

clean.

how to protect the pulverizer from fire or explotion

thank u sir...how can we say beater mill is energy efficient sir?

sir

how can we say beater mill saves power?for the same capacity beater mills rotate at high speed compare to bowl

mill.then how its save power?

and,did u mean to say if we are using beater mill, then no fans are required to take the coal into the furnace.

Lignites are generally softer coals requiring lesser grinding power than bituminous coals. Lignites have much higher

moisture content and require large quantities of hot air or gas. This is why beater mills are used. The mill doubles as fan

and pulveriser saving power and equipment cost.

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3 of 4 11/5/2011 12:37 PM

sir,

i'm dhinesh working in coal based thermal power plant.i have a doubt related to pulverizer.why cant we use a mill other

than beater mill for pulverizing lignite?

what's the power consumption of beater mill(210T/hr) and other mills for handling the same capacity?can we use bowl

mill for lignite fired boilers?can u help me!!

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types of boiler coal pulverizers

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