International Journal for Research in Engineering Application & Management (IJREAM)

ISSN : 2454-9150 Vol-04, Issue-03, June 2018

Pulveriser Mill Performance Analysis & Optimisation

in Super Critical Thermal Power Plant using
imported coal
Ashish Kumar Rai, Ph.D. (Scholar) Department of Energy Technology RGPV Bhopal MP, India,
ashishrai1972@yahoo.co.in
Dr. Mukesh Pandey, Professor Department of Energy Technology RGPV Bhopal MP, India,
mukeshrgtu@yahoo.co.in
Dr. Prashant Baredar, Associate Professor Department of Energy Technology MANIT Bhopal MP,
India, prashant_baredar@yahoo.com
Abstract - Most efficient way of utilizing coal for steam generation is to burn it in pulverised form. The coal is
pulverised to fineness such that 70-80% passes through a 200 Mesh sieve. Normally low speed ball mills or medium
speed vertical spindle mills are employed in thermal power plant for this purpose. The milling system is an important
part of the auxiliary equipment of a coal-fired power plant. Pulveriser mill are one of the critical equipment of coal
fired thermal power plant. They are required to feed pulverized coal in required quality and quantity to meet boiler
steam output and eventually power generation.

This paper tries to give the pulveriser mill specifications, functional, operational details and performance analysis at
different operating loads. Paper also mentions the measures for optimization of the pulveriser mill to improve the
availability & reliability of super critical thermal power plant using imported coal.

Keywords - Pulveriser, BMCR, PA fan, Availability, Reliability.

I. INTRODUCTION
Coal Pulveriser Mill - Function and Operation details.
Pulverizer as shown in fig.1 is used in power plant to
reduce the size of the incoming raw coal to a fineness that
will efficiently burn in a furnace [1].

Fig.2. HP Pulveriser Coal & Air Flow

The rate of coal feed is normally determined automatically

by a combustion control system, taking into account the
MW demand on the unit, coal quality, and the number of
Fig 1. H P Pulveriser actual view and parts pulverizer in service. Centrifugal force causes the coal in
the bowl to move radially outward, building up a bed of
As in fig.2 the raw coal (normally 25 mm) is fed into a
coal on the grinding ring. The bed of coal passes under
rotating bowl through a centre feed pipe

156 | IJREAMV04I033980 DOI : 10.18231/2454-9150.2018.0316 © 2018, IJREAM All Rights Reserved.

 International Journal for Research in Engineering Application & Management (IJREAM)
ISSN : 2454-9150 Vol-04, Issue-03, June 2018

pivoted grinding assemblies called journals. Here, spring An inverted cone is positioned inside of the inner cone to
produced loads are imparted to the coal by the rotating reduce the velocity of the transport medium entering the
rolls. Size reduction takes place in the localized area inner cone through .the cone spout and ensure that the
between the grinding ring (called the bullring) and journal heavier coal particles are not prevented from returning to
roll through a process known as attrition or friction the bowl. The exiting coal air mixture passes through a
grinding. The radial and circumferential movement of the venturi where the mixture is first concentrated, and then
coal carries the crushed coal up over the edge of the bowl expanded to obtain an even coal/air distribution in each fuel
into the path of a transport medium. The transport medium, pipe. The fuel piping carries the coal/air mixture to the
typically hot air, has three primary functions [2] furnace, where the combustion process takes place as
1. It supplies the dynamics necessary within the pulverizer shown in fig. 3. The difficult to grind foreign material
to classify the coal (control coal fineness). which falls through the air stream into the mil side is
2. The heated air aids the grinding process by partially moved by scraper assemblies, attached to the bowl hub
drying the coal as it is reduced in size. skirt to an opening in the mill side floor. The rejected
3. It is responsible for transporting the pulverized coal from foreign material enters the pyrite system. The reject
the pulverizer to the furnace. material usually consists of rock found as over or under
burden in the coal seam and/or parts of the machinery used
As the coal is continuously reduced in size, the
in the mining or coal processing operations.
smaller/lighter particles are swept from the bowl by the
gaseous transport medium, hot air transport air is normally
supplied by primary air (P.A.) fans located up stream of the
pulverizer. However, in some cases it is provided by
individual exhausters located downstream of the pulverizer.
In both systems, the air is admitted below the pulverizer
bowl into an area known as the mill side. The mill side is at
a positive gauge pressure as air is supplied by a primary air
(PA) fan. From the mill side, the air flows upward around
the outside diameter of the rotating bowl. Vanes attached to
the bowl (called the vane wheel) change the air flow to a
vertical direction. The smaller lighter particles of coal at the
edge of the bowl are carried upwards in the air steam,
dense, difficult to grind foreign material falls through the
air stream into the mill side. Above the bowl, the lighter
airborne coal particles undergo a three-stage classification
process (refer to figure 2). Fig.3 Mill fuel piping connections to furnace.

1. The first stage of classification takes place just above II. COAL MILL SPECIFICATIONS & PERFORMANCE
bowl level. Stationary air deflectors mounted on the ANALYSIS
separator body causes the heaviest coal particles to abruptly
Typical Coal Pulveriser Mill Details in 660 MW Super-
change direction, they lose momentum and are returned
Critical [3] Unit is as given in table
directly to the bowl for further size reduction.
Table No. 1.
2. The lighter particles are carried by the transport medium Table.1
to the separator top where the second stage of classification Coal mill
occurs. Here curved adjustable vanes (deflector blades) Bowl Type Medium-speed
impart a cyclonic motion to the coal/air mixture again Type
Coal mill
causing the heavier particles to lose momentum and fall out Number of Mills 6set/ boiler
of the air stream. Spring-biased/spring-opposed
Type of loading
3. After this, the mixture is passed around a vertical “dip- loading
tube” called the venturi collar. Further classification takes Item Unit Designed coal
place, and the desired fineness level is achieved. Maximum output t/h 99.6
Calculated output t/h 75.26
The heavier coal particles separated by the deflector blades
and venturi collar return to the mill bowl grinding zone by Firm output t/h 89.64
sliding down the inside of the inner cone. The cone Minimum output t/h 24.9
separates the pulverizer's “turbulent” flow area from the coal mill inlet drying medium
°C 274
classified particles. The absence of turbulence in this area temperature
allows gravity to return these particles to the bowl. Coal mill outlet temperature °C 65 - 70

157 | IJREAMV04I033980 DOI : 10.18231/2454-9150.2018.0316 © 2018, IJREAM All Rights Reserved.

 International Journal for Research in Engineering Application & Management (IJREAM)
ISSN : 2454-9150 Vol-04, Issue-03, June 2018

Coal mill rotational speed r/min 27.7 7. Minimum fuel line velocity of 1,005 meter/min. or 16.75
Unit power consumption of meter/second (or 3,300 ft./min.)
kwh/t 9.2
coal mill 8. During start up along with steam cold air can be given in
mill just before starting mill to avoid coal spillage to
Table 2 with graph represents the Pulveriser mill under bowl.
performance parameters [4] at different loads.
IV. CONCLUSION
Table 2. Mill Performance parameters at different loads. Pulverizer mill performance optimization is the first step to
Sr. Item Unit BMCR 90% 60% 30%
a successful combustion optimization program and the
No BMCR BMCR BMCR
inter-relationships of the pulverizers must be considered
1. Coal Fineness(% % 20 20 20 20
when attempting to optimize combustion, overall unit
through 200mesh)
2. Mill Operating Nos. 5 4 3 2 performance, operability, reliability, and capacity.
3. Total Mill outlet air t/h 710.4 684.3 444.3 226.2 Pulverizer capacity seems to be an challenge for power
flow (including seal plant while many units today are undergoing drastic coal
air) quality (calorific value) changes. Considering there seems
4. Mill outlet temp. °C 65 65 65 65 to be a huge disconnect when correlating mill performance
5. Mill outlet coal % 18.75 18.75 18.75 18.75 with such issues as fuel line distribution, heat rate, NOX and
moisture. environmental control equipment performance, from this
6. Mill inlet Primary t/h 676 657 421 257 technical paper tries to provide better understanding of
air flow performance, how mechanical optimization & tuning of the
7. Mill inlet Primary °C 324 313 321 301
pulverizers can yield overall improved plant performance.
air temp.
* BMCR – Boiler Maximum Continuous Rating. REFERENCES
Pulveriser Mill Performance Parameters at [1] British Electricity International 1991. “Modern Power
Differen Loads Station Practice: Vol.B: Boilers and Ancillary Plant”
Pergamon.
Mill Operating
Nos. [2] diagnosis of milling systems performance based on
Total Mill outlet air Operating window and mill consumption correlations
flow including seal air(tons/hr)
710.4 676 684.3 657 Inmaculada arauzo and cristóbal cortés Centro de
investigación del rendimiento de centrales eléctricas
444.3 421 (circe) University of Zaragoza María de luna, 50015
324 313 321 301 zaragoza Spain.
226.2 257
[3] Analysis for optimization of a milling system
65 65 65 65
5 18.75 4 18.75 3 18.75 2 18.75 I.arauzo, e. Domingo, i.iranzo, centro de investigación
100% BMCR 90% Load 60% Load 30% Load
del rendimiento de centrales eléctricas (circe) c/maria
(660MW) (594MW) (396MW) (198MW) de luna, 3 50015, zaragoza, spain.
[4] Best Practices Manual for Indian Supercritical Plants
Feb.2014 Partnership to Advance Clean Energy
III. MEASURES FOR OPTIMIZATION OF PULVERISER
Deployment (PACE-D) PACE-D Technical
MILL
Assistance Program.
1. Pipe-to-pipe fuel balance within ±10 percent of the mean
fuel flow [5]. [5] S Mahajan “Adaption of super critical technology an
2. Pipe-to-pipe dirty airflow balance within ±5 percent of affirmative step by NTPC ltd”.
the mean airflow. [6] Partnership to Advanced Clean Energy-Deployment
3. Optimized mill air to fuel ratio. (PACE-D) Technical Assistance Program
4. Minimum fineness level: greater than or equal 75 percent Combustion and Boiler Performance Optimization
passing 200 mesh and less than or equal 0.1 percent Best Practices Presented by: Stephen K. Storm,
remaining on 50 mesh. C.E.M. November 20, 2013
5. Mill-to-mill mass air and fuel balance within ±5% of the
mean.
6. Mill outlet temperatures optimal (65°C) and based on
volatility, inlet temperature limitations and considering
the mills heat balance for a respective fuel [6].

158 | IJREAMV04I033980 DOI : 10.18231/2454-9150.2018.0316 © 2018, IJREAM All Rights Reserved.