ENERGY EFFICIENT DRIVE FOR COAL GRINDING MILLS

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1 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, martin.baechler@flsmidthmaaggear.com Tomasz Wroblewski, Engineering Office Manager, FLSmidth MAAG Gear Poland, tomasz.wroblewski@flsmidthmaaggear.com Lukasz Orlowski, Designer Engineering, FLSmidth MAAG Gear Poland, lukasz.orlowski@flsmidthmaaggear.com Rethinakumar Ganesan, Head of MAAG Gear India Operation, FLSmidth MAAG Gear India, rethinakumar.ganesan@flsmidthmaaggear.com

2 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.

3 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.

4 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 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

5 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.

6 Old design Actual design Gear size Power factor [kw/rpm] Ratio Weight [kg] Efficiency [%] Weight [kg] Efficiency [%] ~96% ~98% ~96% ~98% ~96% ~98% ~96% ~98% 3 CASE STUDY Table 1: Comparison of ancient and modern design 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 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

7 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.