28.05.2009 | Editor: Marcel Dröttboom
Coal is powering the developing economies of the Far East with huge new investments in coal burning power plants for India and China demanding corresponding developments in coal production and shipping plus storage and reclaim at the utilities.
Coal is still a favoured fuel for many power utilities second only to oil on a world wide scale, with oil representing about a third of total capacity. This ratio changes more in favour of coal with many of the new power plants built in recent years and presently under construction based on coal for mainstream base load generation. With accelerating demand for power in India and China one could see a real expansion in coal trades even surpassing recent volumes. This was clearly witnessed in the dry bulk shipping industry where the Chinese demand for iron ore was driving the market with supplies from Brazil and Australia mainly; and in these areas loading facilities were over stretched resulting in up to 665 ship days per week for coal and 140 ship days for iron ore being lost simply waiting to enter the port. Australia attempted to correct this with the addition of new capacity, for example at Gladstone being fed from Dawson Creek where Schade is involved with its very high capacity stockyard systems.
Combining the demands of vessel loading and discharge operations to minimise port delays and the storage, handling and blending requirements at the end user creates a significant opportunity for equipment suppliers. Responding to the challenge Schade have developed the enclosed circular storage and reclaim system as illustrated in the following editorial with now 50 units operating for China alone. In most circumstances circular storages represent the most economical and environmentally friendly solution compared to traditional longitudinal design.
Clearly the power industry leads the field and absorbs by far the largest share of coal. Nevertheless the cement industry is a substantial and expanding consumer. With the accelerating cement demand in India particularly and with expansion in China continuing unabated the eastern cement industry now leads the field by a substantial margin. A great deal of this new cement capacity is also coal fired creating new logistical demands for fuel transportation, handling and storage facilities, but also for raw materials such as limestone plus aggregates such as gypsum and granulated blast furnace slag.
During the last 50 years Schade Lagertechnik Company (Aumund Group) specialised in chain scraper reclaimer systems the first of which was ordered in 1952 for a German power plant. This was followed by a multitude of export orders for many different industries and countries. The early reclaimer machines were based on the cantilevered boom principle which even today remains a viable option and is an economical solution for smaller stockpiles (Fig. 1).
The bulk of the chain scraper reclaimer business centred on power stations of the German Ruhr area. A typical plant design includes a high capacity stacking system to two parallel longitudinal strategic stockpiles each with a portal type reclaimer and two additional blending beds supplied by a central radial boom stacker with two longitudinal stockpiles and two bridge type reclaimers to deliver a homogenised blend of coals. In parallel with the development of integrated storage and blending systems for power plant Schade is also active in major coal terminals where typical installations are designed for handling several millions of tons annually. Such a project with two portal frame reclaimers and a central radial boom travelling stacker servicing two longitudinal stockpiles is in Fig. 2.
Some of the largest ever automated stacker and reclaimer equipment thus far produced were delivered by Schade for the Dawson project managed on an alliance basis by Thiess and Sedgeman. At $ 350 million this was the largest contract ever awarded in the Australian coal mining industry and will increase the output of the Dawson mine from 7 mtpa. to 12 mtpa. Located 140 km to the west of Gladstone, Dawson, one of Queensland‘s leading export coal operations, is owned by the Moura Joint Venture, comprising Anglo Coal Australia Pty Ltd (51 %) and Mitsui Coal Holdings Pty Ltd (49 %). The mine is operated by Anglo Coal Australia.
Coal is delivered from three separate mining resources by a system of overland belt conveyors to the raw coal stacker and reclaiming system, designed by Schade, with a design handling rate of some 4,000 t.p.h. per unit and a longitudinal stockpile capacity of some 200,000 tons. After processing the coal is stocked out by another Schade boom system to stockpiles of capacity 270,000 tons using travelling stackers operating in windrow mode. Reclaimers similar to the raw coal section automatically recover the processed coal from the product stockpiles which is then transferred to automated train load out stations able to load the 7,300 ton coal trains in two hours.
With a rail span of 52 metres these are not the widest machines Schade have ever produced (65 m) but the combination of handling rate and span makes this equipment the largest and most important contract Schade have thus far undertaken. Blending and homogenisation is an important subject both for the power industry to ensure a controlled mix of various coal grades and similarly in the cement industry to mix and blend various grades of limestone and other materials.
There are various solutions for blending depending upon the stockpile strategy which is effectively controlled by the type of stacker. There are fixed boom, luffing boom and luffing plus radial boom options; the latter offering the maximum flexibility. A radial and luffing boom stacker enables material to be laid down in beds either conically (cone shell) or longitudinally in strata or Chevrons. For the best blending effect the Chevron stacking pattern is generally preferred; built using the travelling and luffing function to generate the stockpile height incrementally with the boom always discharging to the centre of the blending bed (Fig. 4). 4
In this manner the stacker is in almost continuous motion using a combination of level sensors and other detectors linked to a central on-board PLC for fully automated operation. This system has a further advantage in that as the coarse material always falls to the outside of the stockpile these larger lumps are then effectively spread across the whole stockpile base and not concentrated on the outside. Since the material is spread evenly from each source along the length of the stockpile no one source is concentrated in one section. Clearly the smaller the height increment for each pass the greater the final blending effect.
For longitudinal stockpiles the horizontal chain scraper conveyor is deflected upwards at the discharge to transfer the recovered material onto a conventional belt conveyor running alongside the stockpile. This arrangement saves on civil works costs since the receiving belt conveyor, installed parallel to the stockpile, maybe at the same level as the stockpile base and a simple retaining wall employed to contain the material.
For circular storages the chain scraper generally discharges horizontally through a central outlet to a belt conveyor running in an underground gallery.
Material is delivered to the stacker by belt conveyor arranged to span over the bridge reclaimer and discharge to the central axis of the stacker/reclaimer supporting column which can support the head of the incoming belt conveyor also, thus saving on the cost of the bridge structure and eliminating any load transfer to the light weight fabricated dome cover (Fig. 5)
The bridge reclaimer with oscillating harrow provides effective blending of all of the various layers of the stockpile where homogenisation is required. Circular storage offers an economical and compact solution compared to a longitudinal design with simplified enclosure requirements; ideal in today‘s environmentally sensitive market. This is well demonstrated by the recent projects Schade have undertaken for the power plant as illustrated in Fig. 6. In this project coal is imported from Handymax size ships using a continuous ship unloading system discharging to belt conveyors which deliver the material to the Schade radial stacking boom conveyor, as illustrated below, with central column supporting both the stacker and reclaimer booms plus counterweight.
A few years ago a total of nine units at 120 metres diameter for Taiwan plus a further 9 units of similar design for China with a stacking capacity 4,000 t.p.h. and reclaim capacity 2,000 t.p.h. were delivered. These were at the time the largest designs of that type ever produced. Blending and homogenisation of the stockpile is an essential element in the stockyard management for most power plant operators. For this purpose Schade have developed various stacking routines in order that the reclaimer may combine elements of each sample within the total stockpile into a homogenous output. For the greatest level of homogenisation the Chevron or Chevcon solutions are preferred, as previously described herein, where material from all sources is spread over the entire stockpile length.
The situation is further complicated by the need generally to stock and reclaim at the same time and often combine strategic stockpiles with minimum blending but high handling rates with smaller blending beds often located away from the main stockpile but nevertheless integrated to operate together as a complete stockpiling and blending operation.
All machines are supplied with comprehensive instrumentation to signal the machine status to a central control system using generally a PLC mounted to the reclaimer plus sometimes the stacker which communicates with the main plant control system often some distance away. Clearly the level of control integration must be tailored to the demands of the plant and operators and Schade engineers are always pleased to advise on such systems.
In 2001 Schade joined the world renowned Aumund Group benefiting from Aumund’s international sales and service organisation with strategically located offices in major business centres worldwide.
Schade Lagertechnik GmbH
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