Optimum Ratios

Impact of coal blending on boiler performance

Coal blending is the uniform mixing of different coals in predetermined proportions. It is not confined to the mixing of imported coal with domestic coal, but also includes mixing of two domestic coals.

In recent years, coal blending has become a necessity to make up for shortfalls in the supply of domestic coal. For instance, NTPC Limited’s coal-fired units aggregating 35,000 MW require about 200 million tonnes of coal per year. Since 2005, NTPC has been importing coal from Indonesia, Australia and South Africa for blending owing to the inadequate supply of domestic coal.

Blending also improves the performance and reliability of power generation plants and helps maintain stable cost of production. In the future, blending will be required to conform to environmental norms as it helps maintain the ash content at stipulated levels. Hence, understanding the characteristics of coal, methods of blending and coal combustion behaviour becomes critical for optimising the burning of blended coal in boilers.

Moreover, as imported coal comes from different origins, its characteristics vary from Indian coal. However, most boilers are designed for domestic coal. Also, proper coal blending facilities are not available in the country. To address these issues, the Central Electricity Authority, in 2011, had issued an advisory that all new boilers should be designed for a blend ratio of 70:30, in which 70 per cent should be low grade domestic coal and 30 per cent should be high gross calorific value (GCV) imported coal.

Coal compatibility

It is important to note that only certain types of coals can be blended in predetermined proportions. The properties of coals being blended may or may not be additive. Hence, the behaviour of coal blends cannot be predicted from the properties of individual coal types.

Additive properties include volatile matter, ash, moisture and GCV. Non-additive properties include combustion reactivity of coal that is the rate at which devolatilisation and char combustion take place.

Another non-additive property is ash fusion temperature. The ash composition of two coals of different origins may be different and therefore have different fusion characteristics. This property is non-additive due to the interaction between the mineral constituents within the blend and may cause slagging and clinkering. Meanwhile, swelling characteristics of coal can cause high unburnts. The Hardgrove Grindability Index (HGI) is another non-additive property that must be taken into consideration. Coal with a high HGI value, is softer or easier to pulverise. A wide variation in HGI values of two coals results in the segregation of blended coal in the mill. Therefore, coals with wide HGI variations are not suitable for blending.

In order to check the compatibility of different coals with respect to their burning behaviour, it is important to carry out laboratory testing, on the basis of their characteristics, before actual mixing. Some of the techniques used for testing samples are differential thermogravimetric analysis to check the relative burning behaviour of coal, ash fusion temperature test (initial deformation, hemispherical and free flowing temperatures), and ash composition test.

Key considerations

Imported coal has high volatile matter of 25-45 per cent. Therefore, proper care needs to be taken to avoid spontaneous combustion in the stackyard and the mill inlet air temperature needs to be reduced to maintain safe mill outlet temperature.

Also, since imported coal has low ash fusion temperature, it is important to have adequate air in the furnace. In addition, pulverised fuel (PF) fineness, optimum air distribution and PF balance are needed to avoid clinkering and slagging.

Further, the compatibility of imported coal with respect to secondary combustion characteristics, sulphur content, SOx emissions, unburnt losses, etc. should be considered.

While it is important to attain the optimum coal blend ratio in a boiler, it is equally important to maintain the desired ratio throughout operations. With an increase in blend proportions, changes should be made in the operating regime as well for safe and efficient operations.

One of the ways to ascertain the impact of blending on boilers and optimise the blend proportions is to use simulation software. This software creates an engineering model of the unit, which is then calibrated to the existing baseline performance. Alternative fuel scenarios are run through the model and the differential impact of coal quality changes is quantified.

Such software can quantify changes in performance, operations and maintenance, emissions, and generation cost; make predictions of operating parameters, power consumption, efficiency, net heat rate, etc. prior to the actual burning of blends; and decide alternative fuel strategies for the unit.

Considering the wide variation in the GCV of “as billed” and “as received” coal, stand-alone online GCV analysers are also being used in Indian power plants. Other technologies for GCV measurement such as online ash meters have limitations. For example, these cannot accurately assess the GCV if the coal source is unknown. Also, online ash meters are not able to measure sulphur and other elements. Thus, online analysers are useful for measuring GCV beforehand to achieve better boiler and blending management. Also, in view of the recent guidelines of the Ministry of Environment, Forest and Climate Change, which have stipulated the maximum coal ash content at 34 per cent, Indian power plants will need online ash management systems such as GCV analysers.

Blending methods

Based on where it takes place, there are four methods used for coal blending.

The first one is blending in coal beds. In this method, blending takes place during reclamation with homogenisation at transfer points. The advantage of this methodology is that blending takes place at coal handling plant (CHP) transfer points. However, this process has disadvantages as the entire coal needs to be stacked first and CHPs are required to run for longer hours.

The second method is blending by silos. In this case, blending is done on the conveyor below the silo with homogenisation at transfer points. This method helps achieves accurate as well as varied blending ratios but has a high capital cost. The third method is blending using ground hoppers, wherein the blending is done at the merging transfer points with homogenisation at transfer points. It is an alternative reclaiming method, but an accurate blending ratio cannot be achieved through this.

The fourth method is blending at CHP transfer points. The blending is done on conveyors at an intermediate transfer point of the CHP to ensure proper mixing of coals. This method does not require any modification to the system, or any additional facilities and continuous mechanised blending is achieved. The only concern with this methodology is that the stockpile length is reduced by 50 metres, which is 8 per cent of the total coal storage capacity of the stockyard. Of the four above-mentioned methods, only the latter two are used at NTPC power plants.

Conclusion

In sum, the key to safe and efficient operations of boilers is correct blend ratio and appropriate blending methodology. In addition, the assessment of the compatibility of coals to be blended is important. Further, the method for optimising blend ratios should be specific to plant design and operations. In this regard, online coal analysers can help provide real-time feedback for blending. Moreover, optimum blend ratios need to be achieved and maintained. Software models can help optimise blend ratios, and predict the impact of coal quality on boiler performance and auxiliary power consumption.

Optimised coal blending can not only provide substantial gains in auxiliary power consumption but also contribute to reducing overall coal consumption and improving efficiency.

Based on a recent presentation by A.K. Arora and Basuraj Goswami, NTPC, at a Power Line conference

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