Flexing Conveyor Muscles: Influences of Fly Ash Grades on Power Consumption

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From Figure 5, it will be seen that power requirements for conveying the coarse grade of fly ash over the range of flow rates considered varied from about 4 to 30 kW. For the fine grade of fly ash in Figure 6, these range from 4 to only 15 kW and the material flow rates for the fine grade of fly ash are significantly greater.

Specific Energy Demand for Fly Ash Handling

Although the data on power requirements clearly shows the effects of air flow rate, it does not present a clear picture if a comparison is to be made with respect to the system air supply pressure. To do this, it is necessary to superimpose a family of curves in terms of specific energy onto the conveying characteristics. By this means, a fully comprehensive comparison will be possible. The lines of constant specific energy can be plotted quite simply by dividing the power requirements data on Figures 5 and 6 by the corresponding material flow rates. The results, in terms of specific energy in kJ/kg, are presented with Figures 7 and 8.

The specific energy curves on both Figures 7 and 8 follow a similar pattern to those of constant conveying line inlet air velocity and show that conveying should be at the lowest value of velocity possible or practicable.

More particularly, however, they clearly show that low velocity dense phase conveying is more efficient than dilute phase conveying. For the coarse grade of fly ash, the lowest value of specific energy was about 12 kJ/kg but for the fine grade of fly ash this was just 1 kJ/kg.

Off Spec Ash and Other Problems

A modern power station is likely to be generating between one and two million ton per year of fly ash. Ash handling plant failure, therefore, is not an option, but there is much that can be done to ensure that the plant can meet its operating targets. A particular problem is that of ‘off-spec’ ash. After a period of time, operational problems can occur with coal processing and combustion, which is likely to result in a change in specification of the fly ash produced.

If the

ash handling plant has been ‘fine-tuned’ to handle ‘perfect ash’ it should not be surprising, from the vast differences in conveying capability of different grades of fly ash reported here, that operational problems with the conveying system will result.

A particular problem, however, relates to the fact that there may be 150 or so hoppers in which the ash is collected on a boiler plant. Ideally each one requires equipment to ensure the reliable discharge of ash when demanded, and that this ash will be fed into the pipeline at a controlled rate. It is not surprising, therefore, that operational problems can result, particularly if the ash is ‘off-spec’, due to combustion or coal crushing plant problems.

* Dr. Mills is Pneumatic Conveying Consultant; Dr Agarwal is Professor at IIT, New Delhi, India

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