Wear characteristics and design optimisation of grinding elements for vertical mills. Ken Birchett reports.
Coal 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 (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. The grinding zone contains the grinding elements and the primary air entry for primary coal classification.
The grinding elements consist of the table (or bowl depending upon VM design) and the tire (or roll). The 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.
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. The 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.
The 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.
Coal quality influences the type and magnitude of wear experienced within the grinding zone.
Lower quality coals, such as lignite coals or brown coals, tend to be more aggressive and require high throughput.
Vertical Mill design also plays a factor; namely the orientation or position of the grinding elements relative to the throat vanes and the pulverised coal and primary air (PA) velocity vectors. The table is generally positioned at or below the centreline of the PA throat vanes thus the primary wear mechanism on the table is from compression and the sliding friction of the coal across the table.
The orientation of the tyres is generally above the centreline of the throat, thus in addition to compression, the tyres are exposed to abrasion as they rotate in the mill.
More abrasive coals have a greater influence on the tyres, thus explaining why the table to tyre wear life ratio is 2:1 or even 3:1 in some applications. It is important to consider the design of the table and the tyre separately. Xwin tyres and tables allow such a consideration.
Xwin, a composite material of high-chrome alloy and granularised ceramic, has a composite hardness of 2100 Vickers, well above the ceiling that exists with mono-metal high chrome and hard-surfaced high chrome alloys.
Xwin grinding elements offer a longer lifetime and the ability to maintain a proper grinding profile for a longer period. This translates to Xwin grinding elements assisting in maintaining mill efficiency and maximising the optimum operating campaign between maintenance cycles. Depending upon the VM type, the overall lifetime improvement using Xwin versus HiCr or hardfaced grinding elements ranges from two to three times better or more.
Because Xwin is a composite of two different materials, each with different wear velocities, it is possible to identify the different wear mechanisms acting against the grinding elements.
By understanding these materials, it is possible to better match the wear resistance of the grinding element, thus creating a grinding element that not only increases the lifetime but wears more uniformly over its life.
With Magotteaux' patented Xwin in its concentrated design version, the wear resistance of the Xwin composite is adjusted in order to better match the forces and types of wear acting against the grinding element.
Field experience has demonstrated that the Xwin concentrated design further increases the resistance against the components of wear extending the operating life, permitting a more uniform wear profile and matching more consistently the lifetime of the tyre and the table.
Ken Birchett is product Development Manager, North America, Magotteaux Inc, Franklin, Tennessee, USA. www.magotteaux.com