Types of Coal Pulverizers in Power Plants

Written by: johnzactruba • Edited by: Lamar Stonecypher
Published Jan 29, 2010

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

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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Prince Sambo Apr 6, 2011 10:48 AM

Causes of trips in Babcock EL 44 coal pulverisers

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 preventative measure for coal mill trip?

johnzactruba Sep 15, 2010 7:24 PM

reply amit kumar

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.

amit kumar Sep 15, 2010 3:56 PM

coal-air balancing

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.

johnzactruba Jul 28, 2010 6:31 PM

reply Balaji D

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.

Balaji D Jul 28, 2010 10:35 AM

pulverizer for boiler application

how to protect the pulverizer from fire or explotion

dhinesh Jul 20, 2010 2:58 AM

energy efficient

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

dhinesh Jul 19, 2010 4:40 AM

beater mill

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.

johnzactruba Jul 18, 2010 6:33 PM

reply dinesh t

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.

dhinesh Jul 17, 2010 1:56 AM

pulverizer

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!!

Types of Coal Pulverizers in Power Plants

1. Attrition Type

2. Impact and Attrition Type

3. Impact or Hammer Type

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