Wear Protection
Grinding elements maximise pulveriser operating
Wear characteristics and design optimisation of grinding elements for vertical mills. Ken Birchett reports.
C
oal pulverisation is essential to the overall system process of a coal-fired power station, not just in terms of furnace performance and heat rate but in terms of the mechanical reliability and integrity
of the furnace. Pulveriser performance plays a key role in back- end emissions in terms of particulate loading, LOI, NOx and required excess air. Poor pulveriser grinding efficiency can be a contributing factor to poor combustion efficiency at the furnace and a myriad of downstream effects. Coal pulverisation is viewed as a high wear process and generally considered the ‘key’ to combustion performance; thus the primary consideration of many when it comes to optimising system performance.
Vertical Mills Vertical Mills (VMs) are the predominant design for coal pulverisation and are designed to achieve a specific performance (discharge fineness and capacity) based on known coal parameters. Within the VM there are two (2) primary zones: the grinding zone in the lower mill body and the classification zone in the upper mill body. Te grinding zone contains the grinding elements and the primary air entry for primary coal classification. Te grinding elements consist of the table (or
bowl depending upon VM design) and the tire (or roll). Te grinding elements are crucial to pulveriser performance. When we consider the grinding elements it is critical that they are in good condition, properly aligned and preloaded (pressure) to achieve optimum grinding performance.
Maintaining performance Due to ever increasing pressure to stretch operating campaigns while maintaining pulveriser performance, it is critical that the grinding elements maintain an efficient profile for as long as possible. As the grinding elements wear, the grinding efficiency of the mill declines. Adjustments can be made to the grinding elements and other mill components through the lifecycle but a point will be reached in which either the discharge fineness or the discharge capacity will be affected thus impacting boiler efficiency, heat rate and back-end emissions. As environmental pressures mount, an increasing
number of sites are switching coal sources or grinding coals from multiple sources of varying qualities, adding to the challenge of pulveriser performance, decreasing spare mill capacity and pushing the mills and the furnace beyond their original design constraints. To meet this demand the power industry has
evolved by suppliers providing grinding elements with increased wear resistance and improved life cycles. Understanding the various mechanisms of wear on the grinding elements permits the potential to improve product supply and performance. Te type and rate of wear experienced within the mill is a function of the coal quality, the primary air volume and the pulverised coal and air velocity vectors above the bowl.
Fig 1. Left: Typical Roll and Bowl VM Design. Right: Typical Tyre and Table VM Design.
Te primary mechanisms of wear in a VM are associated with Compression, Abrasion and Impact with abrasion and compression being the primary constituents. Compression wear is an attrition process that acts normal to the surface. Abrasion is a velocity driven component that acts at a sharp angle shearing the surface. Abrasion is a function of the coal abrasivity, the pulverised coal/air velocity and velocity vector and the volume.
Fig. 2. Grinding Roll Test Piece HiCr vs Xwin. Result: Xwin Concentrated Design four times better.
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