Schade Lagertechnik lends its expertise
Schade Lagertechnik (Aumund Group) is generally associated
with large stacker and reclaimer equipment used for coal and
iron ore stockpiling, recovery and blending at the mine site and
power plant, handling often up to 4,000tph (tonnes per hour) on
a single machine. However, the by-products of these industries,
flue gas de-sulphurized (FGD) gypsum and granulated blast
furnace slag are now important commodities in their own right.
However, from Schade’s viewpoint, this is the end of the chain
for FGD gypsum handling and the story begins at the power
plant where the wet scrubber process demands are processed.
A large (4GW) power plant may require some 10,000 tonnes
of limestone weekly and produce some 15,000 tonnes of FDG
gypsum weekly which represents a considerable logistical
challenge.
The limestone is generally delivered by hopper bottom
railcars and discharged to under rail hoppers and feeders before
transfer to the primary crushers and eventually ground and
processed to be injected into the fuel gas to combine with the
sulphur dioxide present, generated by the burning of coal
forming, almost pure gypsum.
For smaller power plants, the limestone may also be received
by truck and delivered to process plant but in either case a
suitable hopper and feeder arrangement will be required to
meter the material to the following belt conveyors.
Whilst there are many alternative designs such a feeder could
be based on the PKF design from the Aumund Group of which
Schade has been part since 2001.
The PKF is particularly suitable since, with a width of 3
metres, the hopper depth may be minimized and the overall
construction depth deduced.
As illustrated above the PKF is a totally enclosed design with
no risk of spillage or dust escape which is particularly important
with underground installations to reduce or even virtually
eliminate housekeeping costs in areas difficult to access.
To ensure a constant and reliable feed to the grinding station
significant on-site storage is required. Also, generally the raw
material is delivered only in batches either by rail or road often
within restricted operating hours meaning that material must
discharge rapidly to storage allowing the transport to be
released quickly avoiding any surcharges.
At the top end of the scale is this large portal reclaimer (see
below), delivered to Iberdrola for the Andorra Teruel thermal
plant in Spain, to store and reclaim limestone at stacking and
reclaim rates of 500tph.
The travelling boom stacker, also by Schade, may be seen in
the background with the parallel yard conveyors going to the
stacker and from the reclaimer to the crusher and mill.
Generally for most power plants the semi-portal reclaimer is
the preferred solution offering the benefit of enclosed operation
within a compact envelope.
Such a machine is shown below and comprises approximately
half of the full portal using the raised outer building wall to
support the portal bogie at high level.
Material is recovered from the chain scraper reclaim
conveyor over a concrete wharf formed into the floor structure
and to the recovery conveyor running parallel to the stockpile.
The illustration below shows the general concept with the
outer building wall not only supporting the reclaimer rail at high
level but also retaining the material.
Such a system was recently supplied for the Turceni power
plant in Romania including semi-portal reclaimers by Schade plus
SamsonTM surface-mounted intake feeders from Aumund supplied
as a package. Two reclaimers handle the limestone at a rate of
200tph plus a third handles the FGD gypsum at a rate of 360tph.
The semi-portal reclaimer building design generally provides
for a reversing and travelling overhead shuttle conveyor to
distribute the material within the storage allocated areas.
In many applications two reclaimers operate within the same
building often recovering dissimilar materials from separate
dedicated working zones.
Using the reversing shuttle system material may be delivered
to one zone whilst being recovered from another and the two
operations may run simultaneously.
The building illustrated above is at the power plant of VKR
KW Knepper near Dortmund and provides storage for FDG
gypsum. This is a typical design format for smaller plants of
around 500mW.
From the storage building the FGD gypsum is recovered by
the semi-portal reclaimer and conveyed to short term silo
storage generally by belt conveyor. This material, with moisture
content around 10%, handles well on a conventional troughed
belt. That is providing suitable precautions are taken at the
transfer points to limit chute angles to around 70° and provide
effective belt cleaning. FGD gypsum is prone to accumulation in
chutes and generally lining or construction in stainless steel is
the preferred solution.
Whilst FDG gypsum coming from the de-watering equipment
is definitely not dusty, quite the reverse, after storage the surface
layer will dry and become dusty for subsequent handling and as
a safeguard small insertable dust filters are a sensible precaution
to contain any fugitive dust.
Also, at troughed conveyor feed sections the Aumund design
Kleen-Line feed boot provides a clean transfer and effectively
eliminates spillage even with the high material fall often dictated
by the required steep chute angle particularly to pick up fines
from the belt scrapers.
The Kleen-Line concept replaces the conventional wing
rollers of the standard three roll troughing set with a continuous
slider plate. In this manner belt deflection between the impact
rolls is eliminated and therefore effective contact with the skirt
rubber is ensured.
Outside of the major plant items such as the reclaimer it is
these details that contribute to the smooth and reliable running
of the plant as a whole.
FGD gypsum is notoriously difficult to reclaim from silo
storage and for this operation the Aumund BEW-K rotating
rotary discharge machine is a proven and effective solution for
silos up to 12 metres diameter. Based on the CentrexTM concept
the BEW –K comprises a rotating arm that recovers the
material from the edge of the silos using a parallel outer wall
thus avoiding any tapering of the silo. The rotating arm is itself
mounted to a rotating carrier such that the arm moves
continuously around the periphery of the silo recovering
material from the whole circumference. An undercut allowing
the arm to pass below the silo wall promotes reliable flow
avoiding bridging.
In this manner lamina flow is ensured and the bridging and
blockage associated with conventional tapered hopper and
feeder systems is totally eliminated.
However, most importantly the operation of the extractor
ensures a first in first out regime eliminating compaction at the
silo base and eliminating the need to re- circulate material to
maintain fluidity.
Illustrated on p77 is a power plant in Indiana where the FGD
gypsum is stored in two silos each of 1,000m3 capacity, height 20
metres and 8m diameter.
A BEW-K extractor is fitted in each silo with a reclaim rate
of 500tph discharging directly to tipping trucks for subsequent
distribution to board plant.
Where convenient or possible shipment by barge or small
bulk carrier is always preferred being both less expensive per
tonne mile and much less polluting compared to road or rail
transportation. This is particularly true where the power plant is
located close to a suitable berth, as shown above.
In this picture a mobile shiploader from B&W (Aumund
Group) is used in a power plant close to Copenhagen where
FGD gypsum is exported from the same berth as the coal is
imported and distributed by ship to Gyproc Board plants in the
Baltic, mostly operated by BPB (part of St. Gobain).
So now we go full circle returning to the subject of FGD
gypsum intake to the board plant and the storage and reclaim
facilities. Illustrated on the opening page we see a Schade portal
reclaimer installed at the new plaster board facility of BPB at its
Sherburn site in the UK, the new storage hall specifically is
illustrated below.
The demand for plasterboard products in the UK was
stimulated (until the current challenging economic climate) by
continuing expansion of domestic and commercial building, plus
changes in building practice with dry lining replacing plaster, plus
timber framing and increased fire resistance demands, have all
increased the area of board and therefore volume of gypsum per
unit of construction.
As in most established board plants the plant location is
determined both by the availability of suitable natural gypsum
and access to local markets. The gypsum board market is
extremely competitive with the cost of distribution making a
significant proportion of the delivered price.
Additionally to provide high quality of service and high
product availability at short notice board plants tend to be
located primarily close to economic gypsum deposits but also as
close as practical to the point of sale or usage.
This new plant, which increased British Gypsum’s (BPB in the
UK) total board capacity by around 20%, is crucial to meet the
growing demand for higher performance, higher thickness
plasterboards and is central to the company’s plans to meet the
demands of the 2012 Olympic building programme.
Drawing on best practice from more than 40 plasterboard
plants worldwide, the Sherburn plant is one of the first to
successfully achieve a ‘vertical’ start-up, a short–form
commissioning programme, which has resulted in the plant
operating at almost maximum planned capacity within a short
period of time... exceeding all expectations, with high quality
board produced from the outset.
The new plant has been designed to meet the very highest
environmental and safety standards in common with the
company’s other plants at East Leake, in Leicestershire, Kirkby
Thore, in Cumbria, and Robertsbridge, in East Sussex.
The FGD Gypsum is sourced from the nearby Drax power
plant and replaces the natural gypsum mined previously at
Sherburn which ceased operations in 1987.
After the mine closure natural gypsum was sourced from
another UK operation plus a proportion imported from Spain
adding significantly to freight costs and additionally increasing the
plant global carbon footprint.
The change from natural gypsum to 100% FGD Gypsum
represents a significant reduction in operating costs and by
eliminating mining, haulage, crushing and the transportation of
raw natural gypsum (particularly when imported) the plants
overall carbon footprint is significantly reduced. This is further
improved by the inclusion of reclaimed or scrap board; facilities
for which were included in this complete project.
Whilst the Sherburn site is extensive the available space for
the new intake and storage facility was limited requiring a
compact and innovative design.
This is further complicated when handling FGD gypsum since
there is no practical solution for the vertical conveying of this
material in large volumes and generally it is accepted that
conventional troughed belt conveyors are the only practical and
clean solution for the material transport.
The maximum angle of inclination on the troughed belts is
limited to around 18 degrees which in itself determines the
position of the storage hall relative to the mill building and the
intake facility.
The FGD gypsum is received from tipping trucks into twin
SamsonTM feeders and the discharge from each Samson is
delivered to a common cross mounted transfer conveyor to
minimize chute angles and avoid any risk of bridging or blockage.
In this project the SamsonTM, complete with an integral
enclosure, is included within an outer building envelope
integrated to the ongoing conveyor galleries.
In most installations the SamsonTM is installed at ground level
with a slightly inclined truck access ramp as illustrated below in
this project in the USA.
For the reclaim of FGD gypsum the expertise of Schade
(Aumund Group) is legendary, being involved in the application
of chain scraper reclaim technology from the very beginning in
Germany back in 1952; Schade has great experience in this
industry.
As previously illustrated at Sherburn a twin boom reclaimer
was supplied with the primary boom discharging to a collecting
conveyor running parallel to the stockpile at low level. The
secondary boom is jointed to the primary boom using a special
hinged mechanism such that the material is transferred between
the booms to clear the whole stockpile width.
In this manner the two booms always remain within the
portal envelope to minimize the height of the building.
The reclaimer system comprises a chain conveyor with the
primary boom hinged through the drive axle at the lower end
and arranged to scrape the FGD gypsum over a concrete wharf
directly onto the reclaim belt.
The conveyor chain is fitted with close pitch shovels, as
illustrated below, designed to scrape off incremental layers from
the stockpile.
The result is a clean and controlled recovery of material from
the stockpile and with fully automated control requires the
minimum of operator intervention.
In operation the boom moves along the stockpile at a
predetermined speed scraping incremental layers from the
stockpile face, the speed of travel primarily controls the
discharge rate to the following conveyor.
Clearly it is imperative the portal remains always at 90
degrees to the rails and accurate alignment is critical to the
operation and reliability of the system. Schade does not use
flanged wheels to control the alignment preferring instead lateral
guide wheels as illustrated below, a superior solution…
In this case single supporting wheels are employed, that is a
total of four wheels altogether, with one axle per side fitted with
a shaft mounted integral motor and reduction gear unit.
Illustrated above the primary scraper boom mounted to a
through pivot shaft and discharging directly into a feed boot
made integral to the reclaimer but aligned to the troughed belt
recovery conveyor.
Since the recovery conveyor must receive material at any
point along the length of the reclaimer working area a close
pitch idler design is necessary to minimize belt deflection at the
feed point with consequent spillage.
Whilst the reclaimer rate is cyclic with a peak as each shovel
passes the wharf the peaks are relatively small and therefore the
impact to the following belt is insignificant in comparison to the
Samson for example and therefore the Kleen-Line design, as
previously described, is not required in this location.
For this project a special conveyor chain with outboard
rollers is employed, as shown on p81 and as illustrated top right.
Each roller is fitted with twin ball bearings and a multipath
labyrinth seal to prevent the ingress of foreign matter causing
premature bearing failure. Normally this design is reserved for
highly aggressive environments handling abrasive materials (such
as granulated blast furnace slag) but in this application the
outboard bearing chain was selected for its quiet running
characteristics in order to remain within the maximum sound
pressure levels specified for the project as a whole.
This contract scope included the complete electrical control
package including motor control centres, PLC equipment and
the related software for the control of the tripper and reclaimer
specifically.
With travelling systems such as the tripper and reclaimer one
of the core problems concerns the telemetry and transferring
signals from the moving part which generally implies the use of a
cable winder and slip ring system. For instrumentation signals
multiple segment slip rings are expensive and often over time
will degrade the signal causing spurious results.
For this project it was decided to use a cable winder for the
main power supply but that each travelling unit should have its
own motor control centre (MCC) including dedicated PLC
which would then communicate with the main PLC and plant
control system using a wireless data network similar in principle
to that used for mobile computing.
In this manner there is no effective limitation to the number
of inputs or outputs and the system can be easily configured
without changes to physical wiring.
All of the instrumentation on both the tripper and the
reclaimer is hard wired to its dedicated motor control centre
(MCC) and the wireless data system is then the only
communication method to the fixed central MCC and PLC in
the storage hall which then communicates with the main plant
control room using a SCADA system.
Both the tripper car and the reclaimer are fitted with rotary
encoders on one non-driven axle and the output from these
encoders computed to determine the unit position along the
length of its operating zone.
The operating zones for both tripper car and reclaimer are
interdependent and interlocked to prevent the tripper
discharging above the reclaim zone or indeed above the
reclaimer itself.
For the tripper the actual position is given both from the
encoder output and from proximity type sensors which pickup a
reference signal at known positions along the track. In this
manner the tripper may be hyper accurately positioned and for
example is pre-set to avoid discharging over the lateral building
support beams.
Within its operating zone the tripper moves automatically on
a high level signal using a Radar type level detector
(Endress+Hauser Micropilot) to determine the stockpile height.
When the predetermined stockpile height is attained the tripper
moves on a timed distance plus whatever extra travel is
necessary to avoid the building structure.
The reclaimer may be set to operate within its working zone
and the unit then travels automatically within this zone to
recover the material.
A local control cabin is supplied on the reclaimer portal, as
illustrated below, containing the control equipment plus an
operators control unit allowing the system to be manually
driven.
In addition to the control equipment within the storage hall
an additional dedicated control unit is provided for the pair of
SamsonTM feeders again interlocked to the main control
equipment with traffic lamps to signal to the truck driver the
installation is running and ready to receive material. A start
initiation press button is provided for the driver to enable a
controlled start-up routine through to the tripper car.
Of course board production is not the only use for FGD
gypsum and in the cement industry significant volumes are
employed at the cement grinding stage replacing natural gypsum
with compatible benefits.
If we now consider the major co-product of the steel
industry we have similar issues where a material otherwise sent
to landfill may now be utilized effectively and economically as an
alternative raw material in the cement industry. The cement
industry is a major green-house-gas polluter and whilst not on
the scale of coal-fired power plants, the volumes are nonetheless
significant.
Even in the best cement plants around 700 kilograms of CO2
are produced for every tonne of cement released both from the
chemical reaction burning limestone to make clinker and from
the kiln fuels.
Ground granulated blast furnace slag can replace Portland
cement directly in a blend of up to 85% thus significantly
reducing the green-house-gas release associated with the kiln
operation.
Granulated blast furnace slag is generated at the steel plant
by quenching the hot raw slag into cold water causing the
material to shatter into a coarse sand like material that is easy
to handle but incredibly abrasive.
Of the total blast furnace charge, depending on the process,
around 20% will be removed as slag which represents a huge
volume of material. Granulation or pelletization offers both an
effective means of disposal, without landfill, and turn this waste
material into an independent profit centre.
As with FGD gypsum there are a variety of ways the
granulated slag may be transported, by rail, barge and by sea but
inevitably the final stage of the delivery process is by road.
Generally the steel plant is required to store a large volume
of material which must remain easily available for shipment and
for this purpose Schade has supplied stacker and reclaimer
equipment similar in principle to that used for FGD gypsum.
However, illustrated above is a circular storage system with
cantilevered stacker and reclaimer booms handling granulated
slag at Slagment in South Africa, you can see the steel plant in
the background.
This is an early example of this style of storage with a
commissioning year of 1984 and a handling rate of only 100tph
makes this a modest sized installation by modern standards.
The illustration below is a more typical size machine at CCB
Italcementi Group in Belgium with a handling rate of 600tph
handling imported slag from the local steel plants.
These examples of both FGD gypsum and granulated slag
handling systems demonstrate the developing use of these
alternative materials benefiting from energy already invested in
their creation as a means of reducing the global carbon footprint
of the building materials supply industries. Not only is this good
for the environment by reducing green-house-gas generation and
reducing landfill it is a mechanism for the creating of value from
by-product that would otherwise cost money to dispose of.
Schade within the Aumund Group remains at the forefront of
these technologies by incrementally developing existing designs
to satisfy the specific requirements of these demanding
applications.