Metal matrix composite technology

Paul Boughton

Ken Birchett, PE outlines a solution to improve the grinding element lifetime for pulverised coal-fired thermal power stations in India.

India possesses some of the largest coal reserves in the world. As a result coal is an important natural resource for India and an important consideration for electrical generation and generation sufficiency.

Today India's electrical generation from coal accounts for almost 60 per cent of the total generation. As a growing economic power with a large population India faces the daunting challenge of mitigating the severe deficiencies that currently exist between demand and supply and must plan rapidly for additional generation capacity to service current and future growth expectations of its national economy. As a result there is a critical need for new blocks of reliable power to be added to the Indian electrical grid. To achieve these requirements it is expected that India will increase its power output over the next several decades by more than 3-5 times its current production. Much of the planning for this new generation is based on coal-fired thermal power plants grinding coal to a specific fineness and transporting the pulverised coal to the furnace for spontaneous combustion.

In face of this daunting challenge to enhance generation capacity another complexity is added because the Indian coal reserves are typically the most abrasive coals in the world having a low calorific value and high ash content. The calorific value of Indian coal typically ranges from 2,500-4,500kCal/kg (4,497-8,095Btu/lb).

The ash content in the coal generally ranges from 35-50 per cent with the silica in the ash ranging anywhere from 45-60 per cent. The ash and the silica contained in the ash offers no heating value but, being an integral part of the coal, must be processed as part of the pulverisation process. For pulverised coal-firing this translates to the need for higher quantities of pulverised coal in order to achieve the required furnace heat input resulting in greater wear on the pulverising equipment, fuel lines and furnace components.

Of particular importance to pulveriser operation are the grinding elements for which lifetimes in India, because of high ash and silica, are measured in hours instead of months or even years in other less abrasive locations. Proper operation and condition of the grinding elements is a critical component to efficient mill operation. This coupled with proper control and distribution of primary air (PA) is critical to pulveriser performance and has a direct impact on all aspects of downstream operations and performance.

The grinding rolls or tires are particularly susceptible to abrasion wear because of their location in the mill relative to the pulverised coal/air stream; being in the direct path of the PA and entrained pulverised coal. Depending upon the distribution, the direction and the magnitude of the PA velocity vectors the greater the potential impact of abrasion wear on the roll. Likewise the higher the ash content and the higher the silica content in ash the greater the effect of abrasion wear on the roll. It is thus no wonder that the grinding element lifetimes in Indian coal-fired power plants are measured in running hours. As a result a strong need exists for grinding elements with better wear resistant properties. Mono-metal high chromium alloys and high chromium hard-surfaced products do not provide sufficient wear resistance to adequately serve the market needs in India.

A recent case in point is a power station located in the state of West Bengal in eastern India in which the coal's average heating value is 3,600Kcal/kg (6,476Btu/lb) with an average ash and silica content of 42 per cent and 62 per cent respectively. This can be considered typical of many coal-fired power stations in India. The historical lifetime of NiHard IV rolls at this site was approximately 1,000 running hours (RHs) with a maximum wear of 50mm.

In recent years large advances have been made in wear resistant products, namely ceramic composite materials also known as metal matrix composites, in which ceramic of specific properties is combined with a high chromium alloy. The result is a composite material that provides the extreme wear resistance of ceramic but maintains the mechanical properties of the high chromium base metal.

One such product is Xwin by Magotteaux. Xwin grinding rolls were installed on the mills at the power station in West Bengal beginning in 2010 with a guaranteed lifetime of 5,000 RHs. Since that date the Xwin rolls have achieved an average lifetime of 5,268 RH's with a maximum wear of 51 mm; a 4x improvement over NH4. These results correlate closely with results obtained at other Xwin locations in India. In 2011 an advanced ceramic technology called Neox, also by Magotteaux, was installed at the site in West Bengal. Performance results of the Neox rolls have projected a lifetime in excess of 10,500 RHs which is 9x better than NH4 and 2x better than Xwin. Like Xwin the performance results of Neox rolls at other locations in India are correlating closely.

The Xwin and the Neox ceramic metal matrix composite technologies have advanced the wear resistance of pulveriser grinding elements far beyond the wear resistance offered by NH4, high chromium alloys or high chrome hard-surfacing. The increased wear resistance offered by Xwin and Neox not only provides Indian coal-fired power stations longer running life of the grinding elements and increased maintenance cycles but the prolonged grinding profile consistency maintains the overall pulveriser efficiency for longer and its impact on all the downstream influences such as combustion performance and backend emissions.

As a result of the introduction of ceramic based metal matrix composite technology in the Indian pulverised coal-fired power generation market all vertical spindle roller mills in this market, with the exception of E mills, have converted the grinding elements to these products such as Xwin and Neox. Additionally other coal-fired power markets such as China, Europe, South Africa, South-East Asia and the USA are recognising the advantages that this technology offers and are switching to Xwin or Neox.

For more information at www.engineerlive.com/ipe

Ken Birchett, PE is Product Manager with Magotteaux Inc, USA. www.magotteaux.com

Recent Issues