Bed Blending Efficiency – Adding Value with Stockyard Systems

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“Homogenising” defines the reduction of the variation of the parameters relative to the average, while the average is usually unchanged (Fig. 5). homogenising is often necessary to make the following production process more stable and efficient. It is therefore often done at the end of a transport chain at a place next to the use of the material. Since it often requires rehandling it is costly and space consuming. So, space and equipment used for homogenising should be operated most efficient, if possible permanently round the clock.

The most economic point in the logistical chain is to be found for the investment. While for instance in the steel industry the steel grades produced change rather frequently, requiring a flexible raw material stockyard including homogenising next to the plant in other industries like power generation the raw material quality is rather stable over the life span of a production unit. For these, it might be interesting to evaluate homogenized material from third parties or centralizing of own facilities rather than investing in a number of local homogenising stockyards.

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Adding Value with Stockyards – Already a trend?

Be it in on the supplier’s side with the Indonesian Adaro coal group forming “Coaltrade Services International” to “handle the blending of Adaro Indonesia’s with other coals for value added purposes” [1] , i.e. blending the low sulphur Adaro “Envirocoal” with other coals of inferior qualities to reap the benefits of providing coal with high blending efficiency as a supplier rather than leaving them to the end users. Be it on independent stockyards – often ports, such as EMO of Rotterdam – ideas are developed on blending and/or homogenising of materials to increase product quality and add value for the end users. On the contrary, on end user’s side discussions are going on to built centralised stockyards which feed several production lines with coal made to the specific need of each line rather than investing in a number of blending facilities next to each line.

The Particular Challenge: A large Number of Small Shipments with Random Quality

South East Asia with its fast growing economies is an area where the above mentioned global challenges become particular obvious. In the Philippines, an electricity provider with plans to build a number of new power plants on various islands is investigating the option of a centralised coal blending and homogenising stockyard next to some smaller coal mines, which shall feed three plants, one next to the stockyard the other two at separate islands (Fig. 6).

Yet, another specific case in South East Asia shall be taken as an example to discuss various topics related to the best blending/homogenising strategy in more detail. Presently, the operator of a cement plant with annual consumption of about 2 million tonnes of coal receives the coal from eight mines in the vicinity by road trucks and high quality coal by ship. The trucks come in random sequence from the mines. The material characteristics even of coal coming from the same mine varies substantially, since it is directly coming from the different mining faces. There is no coal blending next to the mine. High grade coal shipped from Kalimantan is finally blended with the coal from nearby mines in an attempt to achieve a stable coal quality (Fig. 7).

After weighing and sampling at the cement plant gate all trucks are directed to individual piles as per the source of the coal. The coal is dumped there and pushed to the piles. Wheel loaders take the coal from the individual piles as per the intended quality blend and dump it at the feeding point of the blending bed. High quality coal from Kalimantan is finally used to compensate for inferior qualities of coals from nearby. At the feeding point the coal is crushed and conveyed to the roof mounted tripper which builds up a Chevron pile. The coal is reclaimed via feeders and a tunnel mounted longitudinal belt conveyor. The blending pile feeds presently four different production lines. The client is looking for a more efficient coal handling system to supply the future five production lines featuring less costs on the stockyard as well as a more stable fuel quality.

Besides the high operation costs due to the immense labour intensity and the large number of mobile equipment employed, the homogenising capability of the existing systems does not meet the requirements of the modern cement production process any more. So, the plant operator is looking for a more efficient solution.

In the following consideration each truck is considered as an individual batch with certain characteristics. As per DIN 23006 main coal quality parameters of coal are:

• calorific value,

• ash content,

• sulphur content,

• water content,

• content of volatile maters,

• grain size distribution, and

• density.

Each batch is characterised by a certain average value and a volatility of those parameters. With large batches coming in from the various sources one would normally stack the material onto separate piles. With each batch being big enough for a number of layers over the full length of the pile desirable homogenisation within each material quality might be reached.

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