Coal Mills MPS

COAL PULVERIZER DESIGN UPGRADES TO MEET THE DEMANDS OF LOW NOx BURNERS by: Qingsheng Lin Staff Engineer Fuel Equipment Design Craig Penterson Manager Fuel Equipment Design Riley Power Inc. 5 Neponset Street Worcester, MA 01606 (508) 852-7100 www.babcockpower.com Riley Power Inc. 5 Neponset Street Worcester, Massachusetts 01606 www.babcockpower.comT-186 Presented at: Electric Power 2004 March 30-April 1, 2004 Baltimore, MD TECHNICAL PUBLICATION © Riley Power Inc. 2004 COAL PULVERIZER DESIGN UPGRADES TO MEET THE DEMANDS OF LOW NOx BURNERS by: Qingsheng Lin Staff Engineer Fuel Equipment Design Craig Penterson Manager Fuel Equipment Design Riley Power Inc. 5 Neponset Street Worcester, MA 01606 (508) 852-7100 www.babcockpower.com ABSTRACT Coal pulverizer design and operation is an important element integral to the long term success of Low NOx combustion systems. The increased use of Low NOx burners in the past 10 years has instigated a need for further development of coal pulverizer technology in an effort to ensure efficient operation of a power boiler for minimizing gaseous emissions (NOx, CO, HC) and unburned carbon in fly ash. Riley Power Inc. (RPI), a Babcock Power Inc. company, has been developing improved coal pulverizer technology during the past several years to meet these challenging demands. All three (3) types of coal pulverizer systems supplied by RPI originally designed for low, medium and high- speed pulverization have undergone design upgrades and improvements. These machines include Ball Tube Mills (BTM), MPS mills and Atrita® Pulverizers, respectively. The Atrita® Pulverizer has been upgraded for better coal fineness and longer service life. BTM systems have been upgraded for more reliable operation and MPS mills have been upgraded for increased capacity. This paper discusses the design details behind these upgrades, reviews the impact on Low NOx burner performance (emissions and UBC) and presents the advantages of these milling system technology upgrades for switching coal types from bituminous to sub-bituminous coal. 2 INTRODUCTION As part of the continuing effort to improve combustion performance commensurate with reduced emissions in coal-fired power plants, Riley Power Inc. (RPI), a Babcock Power Inc. (BPI) company has been actively developing mill system technology to achieve better coal fineness, increased capacity, greater reliability, and longer wear life. The effort has improved the design of low, medium, and high speed pulverizers, all three of which are supplied by RPI. Improved mill system design combined with field proven Low NOx burner technology enables a utility boiler today to operate with low emissions and minimal degradation in boiler efficiency. This paper discusses the details behind the pulverizer upgrades and the benefits to utility boiler operation under Low NOx conditions. MPS MILL The MPS mill is classified as an air-swept, pressurized, vertical spindle, table/roller mill. It contains an integral classifier, a grinding section, a windbox (plenum), and auxiliary components. (Figure 1) Frequency converted electric drive Turret Louvre Rotating cage Rotary classifier SLS Sealing air circle line Pendulum adjustment Loading frame Grinding rollers Rotating nozzle ring Grinding track Bottom housing shaft seal Bottom housing Figure1. MPS Mill with SLS Dynamic Classifier. Planetary mill gearing KPV Tensioning rods with hydr. cylinder Return hopper Pendulum joints Housing Hot air inlet duct Grinding track carrier Motor Foundation 3 Raw coal is gravity-fed through a central feed pipe to the grinding table where it flows outwardly by centrifugal action and is ground between the rollers and table. Hot primary air for drying and coal transport enters the windbox plenum underneath the grinding table and flows upward through a swirl ring having multiple sloped nozzles surrounding the grinding table. The air mixes with and dries coal in the grinding zone and carries pulverized coal particles upward into a classifier. Fine pulverized coal exits the outlet section through multiple discharge coal pipes leading to the burners, while oversized coal particles are rejected and returned to the grinding zone for further grinding. Pyrites and extraneous dense impurity material fall through the nozzle ring and are plowed, by scraper blades attached to the grinding table, into the pyrites chamber to be removed. Mechanically, the MPS mill is categorized as an applied force mill. There are three grinding roller wheel assemblies in the mill grinding section, which are mounted on a loading frame via pivot point. The fixed-axis roller in each roller wheel assembly rotates on a segmentally-lined grinding table that is supported and driven by a planetary gear reducer direct-coupled to a motor. The grinding force for coal pulverization is applied by a loading frame. This frame is connected by vertical tension rods to three hydraulic cylinders secured to the mill foundation. All forces used in the pulverizing process are transmitted to the foundation via the gear reducer and loading elements. The pendulum movement of the roller wheels provides a freedom for wheels to move in a radial direction, which results in no radial loading against the mill housing during the pulverizing process. Depending on the required coal fineness, there are two types of classifier that may be selected for an MPS mill. The SLS dynamic classifier, which consists of a stationary angled inlet vane assembly surrounding a rotating vane assembly or cage, is capable of producing micron fine pulverized coal with a narrow particle size distribution. In addition, adjusting the speed of the rotating cage can easily change the intensity of the centrifugal force field in the classification zone to achieve coal fineness control real-time to make immediate accommodation for a change in fuel or boiler load conditions. For the applications where a micron fine pulverized coal is not necessary, the SLK static classifier, which consists of a cone equipped with adjustable vanes, is an option at a lower cost since it contains no moving parts. With adequate mill grinding capacity, the MPS mill equipped with SLK static classifier is capable of producing a coal fineness up to 99.5% or higher <50 mesh and 80% or higher <200 mesh, while the SLS dynamic classifier produces coal fineness levels of 100% <100 mesh and 95% <200 mesh, or better. NEW MPS MILL DEVELOPMENT Since the first application using an MPS mill to process pulverized coal in Germany in the mid 1960s, there have been over 2,000 different MPS mill installations operating in coal-fired power plants worldwide. As one of the most popular coal pulverizers in the utility industry, the MPS mill was first introduced into the US in the early 1970s. Most of the first generation design with a mechanical spring grinding force loading system (Figure 2) are still operating today in coal-fired power plants. With the development of advanced grinding technology, modern MPS mills have improved to its third generation design utilizing a hydropneumatic grinding force loading system with enhanced grinding force. To date, MPS mills have been successfully used for grinding a wide range of coals from bituminous to high moisture sub-bituminous to lignite type coals. The standard mill capacity for twenty (20) different mill sizes ranges from 10 tph to 190 tph (Figure 3). 4 Figure 2. First Generation MPS Mill with spring grinding force loading system. Figure 3. MPS Mill standard capacity. Loading frame Loading spring Guide frame Hot air inlet duct Motor Adjustable classifier vane Static classifier SLK Sealing air system Grinding rollers Nozzle ring Segmented grinding track and carrier Grinding track support Tensioning rods with hydraulic cylinder Bevel spur gearing KV 0 20 40 60 80 100 120 140 160 180 200 100 112 125 140 150 160 170 180 190 200 212 225 235 245 255 265 280 290 300 315 Mill Size M ill C ap ac ity , t/h r RPI MPS Mill Standard Capacity 5 Today's coal-fired utility boiler operation needs pulverizer designs to supply pulverized coal with required throughput and coal fineness, and also perform with lower specific power consumption, especially at reduced mill load. The designs must be capable of proper mill operation and control with quick response to boiler load demand for a variety of coal switching or coal quality fluctuations. The designs must also have greater mill turndown capability without mill vibration. Obviously, the first generation of MPS mill designs with limited adjustability for a spring-loaded grinding force loading system, in which the grinding force is produced by pretensioned compression springs, are often inadequate to satisfy today's mill operation requirements, since this nonadjustable grinding force concept produces little flexibility of grinding pressure for different mill operating conditions. A modern MPS mill design is equipped with a hydropneumatic grinding force loading system that consists of three hydraulic cylinders with one tension rod each to pull down a rigid loading frame. The grinding rollers fixed to the loading frame are thus pressed against the coal bed between the rollers and grinding table segments (Figure 4). The applied grinding load is capable of being adjusted in real time with the mill in operation. Figure 4. Hydropneumatic grinding force loading system. The hydraulic cylinder configuration shown in Figure 5 indicates that the applied grinding force is produced by the oil pressure on the piston ring surface (grinding pressure) and reduced by the oil pressure acting on the piston bottom face (counter pressure) of the loading cylinder. The counter pressure reduces the noise generated by the mill and is adjustable depending on the coal properties and required coal fineness. During operation, both pressures are adjusted proportionately to the feeder speed by means of pressure control valves to achieve an optimized grinding force characteristic throughout the mill load range. During mill startup or shutdown, a higher pressure is applied on the piston bottom face than on the piston ring face to eliminate mill vibration or rumble by reducing the grinding force against the mill table. With sufficient high pressure on the piston bottom face, the rollers can even be lifted off of the grinding table segments, which results in a significant wear reduction between the rollers and grinding segments, as well as minimal mill torque requirement during mill startup. With the successful application of hydropneumatic loading systems, further adjustments in the design have been implemented in recent years. MPS mills have experienced their "third generation" design by increasing mill grinding force by over 60%. As a result, mill capacity increases of 20%-50%, depending on the coal application, have been experienced since the mill grinding capability is directly proportional to available grinding force. This implies, for example, that only six (6) or seven (7) mills of the same physical size are required today for a boiler that previously needed eight (8) mills to supply the same coal flow. This significantly reduces the initial capital investment for mills and associated burners and coal piping systems. 6 Figure 5. Hydraulic Cylinder Configuration. Tensioning rod Leackage Grinding pressure N2 Accumulator Leackage Lifting Lowering Counter pressure Hydraulic cylinder 7 The adjustable grinding force capability with hydropneumatic loading system designs enables MPS mills to vary the grinding load as mill load demand changes. This optimizes mill grinding force loading characteristics such that the mill grinding force increases as mill load increases. Figure 6 illustrates a typical mill grinding force loading characteristic. The grinding load increasing from 100% to 160% corresponds to mill capacity increasing from 100% to 135%, which reflects mill design upgrade from second generation to third generation by enhancing this grinding force. For comparison purposes, a grinding force loading characteristic of the mill with a spring loading system and the second generation of hydropneumatic loading system design are also shown in Figure 6. For the mill equipped with spring load system, the grinding force is overloaded at reduced mill load. This will result in additional wear on roller and table segments and will produce excessive mill vibration. While at high mill load operation, the grinding force is insufficient to meet the requirement for mill load increase. Figure 6. Mill grinding force loading characteristic. ATRITA® PULVERIZER The Atrita® Pulverizer is a horizontal type high speed coal mill (Figure 7), which consists primarily of three sections: crusher, grinding and fan section. The coal feed into the mill is first reduced in size in the crushing section for primary size reduction and drying. Screened by the grid segments under the hammers, the reduced-size coal is introduced to the grinding section for pulverization. The conveying air or primary air, developed by an integral fan in the mill's fan section, transports the pulverized coal from the grinding section to the burners. In the crusher section, there are hammers and a breaker plate to perform a crushing function. Below the breaker plate, there is a crusher block, which can be adjusted to move forward or backward against the hammers to establish a gap between hammer tips and crusher block. This gap, associated with the grid segments under the hammers, controls the size of crushed coal entering the grinding section. In the grinding section, major grinding components are stationary pegs and moving clips. The clips are attached to the wheel that rotates around the mill axis at a high rate of speed. The turbulent flow and impact momentum on coal particles developed by the high speed movement of the clips create an intensive particle-to-particle attrition or a pulverizing effect to further grind the coal particles in the grinding zone. In order to control pulverized coal fineness, there is a whizzer type classifier or rejector arm assembly between the grinding section and the fan section within the Atrita® Pulverizer. In the grinding process, the V-shaped rejector arms, rotating with the pulverizer rotor, magnified the intensity of centrifugal forces within the grinding section, which retains courser coal particles in the grinding zone for additional pulverization. The finer coal particles, subjected to less centrifugal and drag forces due to reduced mass and sectional area, pass through the rejector arms with primary air into the fan section and are delivered to the burners through coal pipes. 8 Figure 7. Duplex Atrita® Pulverizer. Fan Section Pulverizing Section Crusher-Dryer Section Pulverizing Section Fan Section Coal-Air Inlet Coal-Air Inlet Peripheral Liner Swing Hammer Grid Grinding Clip Shroud Fan Blade Rotor Disc Rejector Arm Stationary Peg Impeller Clip 9 NEW ATRITA® PULVERIZER DEVELOPMENTS With more than 50 years of operational experience and more than 1600 installations, the Atrita® Pulverizer is faced with the challenge of continually improving pulverized coal fineness and reducing mill down time with design modification and material upgrades. In the present design, the rejector arm assembly is composed of an axially adjustable hub, several V-shape arms with attached guards, and a stationary rejector ring. It is essential to set a very small clearance between the side surfaces of the rejector ring and the side end of the rejector arms to achieve acceptable coal fineness. However, this tight clearance requirement is difficult to maintain due to material wear and dimensional variation. This results in the inability to control coal fineness in the pulverizing process by failure to prevent coarse particles from "leaking" through the gap between the rejector arms and rejector ring. Therefore, a new rejector arm assembly design has been developed with a dynamic seal effect by creating a labyrinth seal gap along with additional beaters. In this new design, referred to as a DynaRing™ Classifier (patent pending), a solid continuous ring made from segments is added in between the rejector ring and rejector arms. This added ring, attached to rejector arms, is rotated by the main shaft through the rejector arms. Thus, the seal gap between the rotating and stationary parts is a continuous clearance formed between the end surfaces of the dynamic ring and the rejector ring seated on the rejector ring support, instead of the rejector arms and the reject ring. To make a more effective seal, the seal gap is made with a labyrinth shape, that is, the end surfaces of the rejector ring and dynamic ring are made with offset steps to form a labyrinth type gap. On the dynamic ring, there are a number of beaters equally spaced and attached on the outer end surface facing the rejector ring. The main shaft rotates these beaters at a high speed through the rejector arms and dynamic ring, which further controls coal fineness by reducing the size of the particles entering into the gap or by preventing the coarse particles from entering the seal gap. The beaters are either mounted to or integral with the dynamic ring. The leading face of the beaters is tiled or coated with a wear resistant alloy for long wear life. In the DynaRing™ Classifier design, the rejector arms have been redesigned with an attachable capability to tailor different coal application and coal fineness requirements. A field installation test of the DynaRing™ Classifier was conducted on an Atrita® 550D Pulverizer installed at a utility located in the Northeast (Figure 8). The DynaRing™ Classifier improved coal fineness significantly. Table 1 presents the typical test results from the mill tests. The data show that with the DynaRing™ Classifier, the coal fineness improved from the previous 67.6% to 80% passing Figure 8. DynaRing™ Classifier. through 200 mesh at approximately the same fuel flow (Test 1 vs. Test 3). After the retrofit, the mill also enhanced the top size control capability significantly; the coarse residue on 50 mesh averaged 0.5% or less! Even at a higher mill throughput (16% higher capacity) the mill retrofitted with the DynaRing™ Classifier still produced much finer coal than that of the pre-retrofitted mill (Test 2 vs. Test 3). This coal fineness improvement was also demonstrated by the comparison between Mill C and Mill D that is equipped with the standard rejector arm assembly (Test 1 vs. Test 4). The improvement of coal fineness between the DynaRing™ Classifier and original or standard rejector arm designs is also illustrated in Figure 9. 10 Table 1 Test Results of DynaRing™ Classifier Figure 9. Test Results of DynaRing™ vs. Rejector Arm Classifier. 50 mesh 100 mesh 200 mesh 60 70 80 90 100 Coal fineness Pa ss in g th ro ug h, % Coal fineness comparison DynaRing Classifier vs. standard rejector arm DynaRing Classifier / Mill C Std. rejector arm / Mill C Std. rejector arm / Mill D With the DynaRing™ Classifier, derating the mill capacity is significantly minimized when it is desired to produce a high level of coal fineness product and for the pulverizer to handle high moisture PRB coal. In addition, the DynaRing™ Classifier allows much more latitude when setting the seal gap clearance. This greatly simplifies the setup of the DynaRing™ Classifier during the initial installation and should help to prevent the coal fineness from deteriorating as the relevant components wear in service. As coal fineness increases, the DynaRing™ Classifier needs more power input into the mill for additional grinding work. Preliminary data indicated that the corresponding increase in power consumption is approximately 12%. In addition to improving coal fineness with the DynaRing™ Classifier, the Atrita® Pulverizer grinding