Introduction
Coal has many diverse properties, each one of which impacts on some aspects of power plant performance. The value of coal to the user is a maximum when the set of coal properties is matched to the particular requirements of the user’s power plant. Because there are so many diverse properties, there may be few coals that give a good match for all properties.
This shortage of matching coals is favourable to neither the coal producer nor the coal buyer, hence the need for blending - this increases the scope tremendously for presenting coals that fully satisfy a strict specification.
A normal coal specification is based on standard chemical and physical laboratory tests. The ability to satisfy a specification by blending is limited by:
· The properties of the available coals, and
· The additivity of properties, that is whether they are a weighted average of the properties of the component coals.
The next major issue with blending is how a coal blend performs. Achieving a coal specification does not infallibly predict the coal’s performance in power plant, though it is often all we have to make these predictions. Satisfying a specification does not guarantee satisfactory performance, even in the case of unblended coals, which raises the question as to whether standard laboratory tests are less reliable predictors for blends than for single coals.
Benefits of Blending
Both coal producers can benefit from blending and blends are produced anywhere between the mine and the power plant. Coal producers may blend for a combination of the following reasons:
· To enable them to offer a product that satisfies customers’ specifications,
· To enable them to sell coals that may be otherwise unsaleable,
· To increase their tonnage sales,
· To offer products that can be produced more cheaply and are more competitive on the market,
· To formulate new supplies to match an established product that may come from a mine approaching the end of its life,
· To win favour with customers by supplying more consistent quality,
The above benefits can also accrue from blending with a competitor’s coal and there may be more of this in the future. Power station operators may blend for a combination of the following reasons:
· To design acceptable feedstocks from coals that may be unsuitable in the unblended state,
· The coals used can be cheaper, thus saving fuel costs,
· To make boiler plants more easy to operate by improving the consistency of coal quality,
· To diversify the sources of the coal supply, thereby increasing competition and securing the supply,
· To enable poor quality domestic coals to be used.
Blending Logistics & Methods
Coal blends may be produced at:
· The mine, from different seams or areas,
· The outgoing seaport, where competitors may make blends for mutual advantage,
· The receival/distribution port, where customers may specify blend proportions,
· The power station stockyard, where two coals may be reclaimed simultaneously and
combined on the conveyors,
· In the power plant, where different coals may be sent to separate mills1.
Methods of blending include:
· Stacking the coal in different layers. Reclaiming is performed in a different direction, across the layers, in order to achieve a degree of mixing.
· Reclaiming simultaneously from more than one stockpile and feeding onto a common
belt.
· Feeding from separate coal bins onto a common belt.
1) Though this is not really blending, many of the impacts of plant performance are like blending. After the milling of the separate coals and their ignition at the burners, the remaining processes occur with “blended” products of combustion.
It is clear that a degree of homogeneity, that is adequate mixing, is needed after blending to avoid slugs of unblended coals entering the boiler. Nevertheless it is recognised that some mixing occurs as a result of materials handling after the point of blending.
Targeting Coal Blend Characteristics
Coal Analysis:
The initial approach in formulating a blend is to aim for a particular set of coal properties, based on the standard analysis of blends. This is logical since these results are the normal basis for coal specifications that satisfy coal supply contracts. Additionally, it is standard coal analysis that provides the first (and sometimes only) indication of how a coal will perform in a power plant.
Coal Performance:
Ultimately it is the actual performance of a coal in the power plant that matters. It is difficult enough to rely on coal analysis to predict the performance of single coals, and it may be expected that it would be more difficult for coal blends, particularly when the properties of the component coals are very different.