Pulveriser in Thermal Power Plant | Bowl Mill

Introduction

The main Function of the Pulveriser in thermal power plant is to crush/grinding the raw coal coming from coal handling system through coal feeder into a pre-determined size in order to increase the surface area of the coal.

If the coal is not pulverised, the coal might not burn completely, thus resulting in wastage of fuel. Also, pulverization of coal helps the boiler to respond to load variations more promptly.

Types Of Coal Pulveriser

Slow speed mills like ball mill
Medium speed mills like bowl mill
High speed impact mill

The slow speed and medium speed mills are selected for coals ranging from sub-bituminous to anthracite. The high-speed mills are used mainly for lignite.

Operation Of Bowl Mill Pulveriser

In bowl mill pulveriser raw coal coming from feeder gets ground between the grinding rolls and bull ring segments installed on the revolving Bowl.

Bowl is made to rotate at medium speed for proper pulverization of coal. Springs exert necessary pressure on rolls for grinding. Hot air through the mill besides removing coal moisture, picks up the lighter particles and takes them through the classifier and drop down the higher size particles for further grinding.

Fine coal air mixture leaves the mill and enters the fuel piping system. Tramp iron pieces which are not required to grind, leave the Bowl due to centrifugal force and are removed through the mill reject handling system.

Operation of Ball Mill Pulveriser

The Ball mill pulveriser is basically horizontal cylindrical tube rotating at low speed on its axis, whose length is slightly more to its diameter. The inside of the Cylinder shell is fitted with heavy cast liners and is filled with cast or forged balls for grinding, to approximately 1/3 of the diameter.

Raw coal to be ground is fed from the feed end and the product is discharged through to the discharge end. As the shell rotates the balls are lifted up on the rising side of the shell and they cascade down from near the top of the shell.

Larger pieces of coal are broken by impact and the fine grinding is done by attrition and crushing as the balls roll and slide within the charge. Hot air flow is induced through the mill in order to dry the coal and remove the fines from the pulverizing zone.

If the rate of feed is increased, coarser product will be obtained and if the speed of rotation is increased the fineness for given capacity increases.

During grinding, balls themselves wear and are continuously replaced by new ones.

Common Coal Pulveriser Problems, Causes and Solutions

Coal pulveriser problems can reduce grinding efficiency, affect coal fineness, increase power consumption, and disturb boiler combustion. Early identification of abnormal operating conditions helps improve mill availability and reliability.

Low pulveriser output: Check raw coal feed rate, coal moisture, grinding element wear, and hot air availability.
Coarse coal fineness: Inspect the classifier settings, grinding rolls, bull ring segments, and mill operating conditions.
High mill differential pressure: Check for excessive coal accumulation, inadequate primary air flow, or blockage in the mill and classifier.
High mill outlet temperature: Check hot air flow, temperature control dampers, coal feed rate, and mill operating conditions.
Low mill outlet temperature: Inspect the hot air supply, dampers, temperature control system, and coal moisture.
High mill vibration: Check grinding element wear, uneven coal distribution, foreign material, and mechanical looseness.
High mill power consumption: Inspect grinding element condition, coal grindability, feed rate, and mill loading.
Frequent mill rejects: Check tramp iron accumulation, reject chute blockage, pyrite hopper condition, and reject handling system operation.
Uneven burner coal distribution: Inspect classifier performance, coal pipe condition, and the balance of primary air and coal flow.

Preventive Maintenance of Coal Pulveriser

Regularly inspect grinding rolls, bull ring segments, classifier components, bearings, lubrication systems, seals, and mill reject arrangements. Monitor mill differential pressure, outlet temperature, primary air flow, motor current, vibration, and coal fineness to identify developing problems.

Always follow the manufacturer’s operating limits and approved plant procedures, particularly when investigating abnormal temperatures, vibration, or possible coal accumulation.