Wear Protection
Extending the life of grinding elements
The grinding elements within the mill are an essential component of reliable and efficient coal pulverisation and performance. Ken Birchett reports.
Los elementos de esmerilado dentro de la planta constituyen un componente esencial para la pulverización y el rendimiento confiable y eficiente del carbón, afirma Ken Birchett.
Die Mahlteile in der Mühle wirken sich entscheidend auf das zuverlässige und effiziente Mahlen der Kohle und die Leistung aus. Ken Birchett berichtet.
T
he design of coal pulverisers (mills) is based on a known coal of specific parameters pulverised to a specific fineness and achieving a rated
design capacity. Typical coal mill design capacity is based on coal of specific parameters, usually:
l Moisture: 6-8%; l Hardgrove Index: 55 HGI; l Discharge fineness: 70% pasing 200 mesh (74µ).
Also, for optimum combustion performance it is generally expected that a minimum of 99% of the pulverised coal leaving the mill passes a 50 mesh sieve. Additionally, coal mills supplying a given furnace and grinding a specific coal or range of coals
are grinding coals that challenge the rated capacity for which they were originally designed. Additionally, many operators are tuning the coal mills to achieve greater performance and at the same time expecting longer operating campaigns – placing an even greater demand on the mill and the mill internals. Vertical roller mills (VRMs) are
considered attrition mills. Pulverisation is achieved by crushing the material against itself, under pressure, between the grinding elements. Te two most common VRM coal mill types are the roll and bowl type mill and the tire and table type mill (Fig 1).
Fig. 1. grinding elements in roll & bowl type mill (left) and tyre & table type mill (right).
should be selected based on achieving the required furnace heat input with not more than 90% utilisation of the total pulveriser milling capacity.
Fig. 2. worn out grinding element.
Te discharge fineness and the discharge capacity are the critical parameters for coal mill performance. Coals of differing quality, particularly HGI and moisture content, will directly impact the design capacity. Efficient pulverisation of coal is a critical function to the overall operation of the coal mill and the mill itself is a primary consideration and influence to downstream combustion performance and back-end emissions. Today, due to cost considerations, competition and other factors such as environmental compliance, many sites
Both VRM mill types utilise grinding elements for the pulverisation of coal. Te grinding elements are critical to the operation of the mill and to its ability to achieve required performance parameters (Fig. 2).
Te grinding elements are also subject to wear and degradation over time. As they wear, the grinding profile is compromised, reducing the grinding efficiency and compromising the pulveriser performance. At a certain point the grinding elements are no longer effective in maintaining the required performance and must be replaced. Grinding element lifetime is a function of the wear resistance of the grinding element alloy and the (quality), specifically abrasivity, of the coal
www.engineerlive.com 49
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52