ENERGY EFFICIENT DRIVE FOR
COAL GRINDING MILLS
Abstarct:
Since the implementation of vertical roller mill for coal grinding the design concept of the
gearboxes to drive the mill changed totally. Based on the experience for large vertical mill
applications the gearbox supplier introduced the bevel-planetary gear arrangement also for coal
grinding. But the focus lies not in increasing drive power but in improving efficiency, reliability and
simplification of maintenance.
The following paper describes the design concept of the initial and the state-of-the-art gearbox
design. It point out the latest design improvements based on recent redesign work of individual
western gearbox suppliers. Benefits of the actual design are stated together with a comparison of
both gear units.
A case study describing the replacement project located in Poland (Europe) closes the theoretical
discussion about the different gear designs and shows customer benefit and satisfaction.
Authors
Martin Baechler, Product Manager, FLSmidth MAAG Gear Switzerland,
[email protected]
Tomasz Wroblewski, Engineering Office Manager , FLSmidth MAAG Gear
Poland, [email protected]
Lukasz Orlowski, Designer Engineering, FLSmidth MAAG Gear Poland,
[email protected]
Rethinakumar Ganesan, Head of MAAG Gear India Operation, FLSmidth
MAAG Gear India, [email protected]
1 INTRODUCTION
The needs and demands of coal mill differ from the vertical mills in other
industrial applications. The coal grinding system must take into account
the available heat sources for drying the raw coal as well as the fire and
explosion hazards of coal dust. The entire grinding plant must therefore be
shock-resistant and equipped with a number of explosion relief valves.
On the other hand the overall layout of the gearbox for vertical coal mill
doesn’t distinguish very much from the other vertical mills gearboxes. In
contrary to the operating conditions, they are depending on the firing
system. In direct firing systems the load and operation time is direct linked
to the burner process. While an indirect firing process, where the grinded
coal is stored in a silo before burning, the load applied to the gearbox is
more constant. In general the design power of gearboxes for coal mills is
considerably smaller than in other application. Because of these facts it
makes sense to introduce a special design focused on coal mill application.
2 DESIGN CONCEPTS
The gearbox for vertical roller mills has to fulfil three main requirements:
1. Transmitting the power from the horizontal drive motor to the vertical
mill table
2. Reducing motor speed to mill speed
3. Absorbing the grinding forces and supporting the mill table
Requirement 1 and 2 are fulfilled by the specific gear arrangement, where
in the bevel stage a first reduction of motor speed and the change of load
direction are realized. With addition gear stages the specific mill speed is
realized.
The absorption of the grinding forces is ensured by a hydraulic tilting
thrust bearing directly located underneath the output flange of the gearbox.
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The reaction forces from the grinding process flows from the grinding
table throw the gearbox casing directly into the foundation.
The main driver for gear design changes in general is the increasing size
and therefor also the increasing drive power of the vertical roller mills.
Today drive power above 6000kW are common requirements in large
vertical mill applications. In coal grinding the main focus is not on
increasing power but on better efficiency and higher reliability.
2.1 INITIAL GEARBOX DESIGN
The first generation of gearboxes for vertical coal mills were designed on
the basis of a bevel-spur-gear arrangement. The challenge of this design is
to balance the ratio of the two gear stages equally. This allows keeping the
dimensions of the bevel wheel and the spur gear in a manageable range.
Fig. 1: First generation of vertical coal mill gear box.
2.2 STATE-OF-THE-ART GEARBOX DESIGN
Approximately 40 years ago the power limits for large vertical mills
driven with bevel-spur-gear arrangements were reached. Gear suppliers
have chosen a new gear arrangement to fulfil the demands from the
industry. The spur gear was replaced by a planetary stage and
consequently it was possible to reduce the overall dimension and weight
compared to the bevel-helical gears. The same design change was applied
for coal mill gears to improve the efficiency and availability. But the
optimal coal mill gear box is not simply a scaled version of the larger
vertical mill gear.
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The reduced load and less severe operating conditions compared to other
vertical mill application allow the consequent use of antifriction bearings
for all geared rotation parts. The Bevel gear stage is supported by roller
bearings located on both sides of the gearing. This special bearing
arrangement is possible thanks to a unique design of the gear casing. It
ensures perfect alignment of the bevel gear stage, equalized load
distribution and optimized operating condition for variable loads. The
lifetime of the antifriction bearing is calculated for 100’000 operating
hours.
Fig. 2: State of the art coal mill gear box
The torque is transmitted from the main motor trough the bevel gear to the
planetary gear stage via a toothed coupling. Three planet wheels are
located in the planet carrier. With this arrangement a perfect load shearing
between the planets is realized and each planet is designed for one third of
the total torque. They are considerable smaller and easier to handle
compared to one large spur gear. Spherical roller bearings, mounted on
planet shafts, ensure best possible position of planet wheel during
operation in each load condition. The rotating planet carrier is bolted and
pinned to the output disc and drives the grinding table.
The grinding table itself is supported by a tilting thrust bearing in vertical
direction and by a radial slide bearing in radial direction. The pads of the
high-quality tilting-pad thrust bearing are lined with babbitt metal. The
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purpose of these bearings is to absorb the static and dynamic grinding
forces. They are of hydrodynamic type lubricated by low-pressure pumps
or hydrostatic type on demand. Their temperature is continuously
monitored.
The gear casing consists of a robust welded steel construction or casted
well ribbed structure with a very small interruption in the circumference
caused by the bevel stage. The grinding load including grinding table
weight is transmitted straight and uniformly to the foundation via the
tilting-pad thrust bearing and the casing wall.
Fig. 3: Flow of reaction forces of grinding process
2.3 COMPARISON INITIAL VS. ACTUAL DESIGN
The table below shows very clearly the advantages of the new state-of-theart gear unit. The huge difference in weight point out the compactness of
modern coal mill drives. Exchanges or even power upgrades of the coal
mill drives therefore can be realised without complex modification of the
foundation or the mill pedestal.
But a smart designed bevel-planetary gearbox also simplifies the
maintenance: Large circular openings allow easy access for inspection of
the gear parts. Shims in the bevel gear arrangement allow dismounting of
bevel pinion for bearing changes without opening the entire gear box. The
bevel gear is designed in two parts, to simplify the exchange and reduce
cost of spare parts.
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Presented at GETS 2014 http://www.NTPCGETS.com
Old design
Actual design
Gear
size
Power
factor
[kW/rpm]
Ratio
Weight
[kg]
Efficiency
[%]
Weight
[kg]
Efficiency
[%]
1
11
25
17’000
~96%
11’000
~98%
2
16
29
20’000
~96%
14’000
~98%
3
24
29
34’000
~96%
21’700
~98%
4
34
31
47’800
~96%
37’000
~98%
Table 1: Comparison of ancient and modern design
3 CASE STUDY
One of the biggest and oldest companies in Poland’s power industry
knows about the importance of having the drive system up-to-date. In their
power plant, located in the south of Poland, they produce electrical and
thermal energy, powered by coal with an own heat distribution centre. The
power plant is one of the oldest in Poland. It has been opened in 1917 in
order to supply power to neighbouring coal mines. The installed power of
11.6MW was gradually increased in the consecutive years. Today installed
units were built in two stages between 1963 and 1972. In 1975, after
accomplishment of the second plant extension covered 7% of the domestic
power production. In the late 20th century the six existing power units
were consequently upgraded and reinforced. In general, thanks to the
upgrades, the total increase of capacity amounted to 115MW, without
building new power generating units.
With continuous investments and improvements this power plant is today
a state-of-the-art power plant. Nevertheless further investments related to
environmental protection are of the paramount importance. In January
2011 a contract to implement four catalytic flue gas denitrification
installations was signed. Retrofitting of the power unit equipment and
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joined installations were carried out simultaneously. Therefore it comes,
that the old gearboxes were exchanged by new state-of-the-art gear units
specially designed for coal mill applications. In spring 2012 the new gear
units were commissioned and went into full load operation. The
installation and commissioning of the gearbox were carried out to the
customer’s complete satisfaction.
4 CONCLUSION
Thanks to the elaborated design concept of bevel-planetary gear with a
very special bearing arrangement, customers benefit from various
advantages. The state-of-the-art gear units are quickly to install and are
very maintenance-friendly. The planetary gear stage and the thrust bearing
for the grinding table have a very long service life. Not to mention the
available customer-specific design of the gear unit, especially in case of
retrofit and replacement projects. The high operational reliability and the
very high efficiency are proven in the all the state-of-the-art gear units that
are already running all over the world.
Distributed with permission of author(s) by GETS 2014
Presented at GETS 2014 http://www.NTPCGETS.com