Handling by-products from the power and steel industries

Transcription

1 Handling by-products from the power and steel industries SCHADE portal reclaimer at a board-plant of St. Gobain. 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. Intake from railcar. Road truck intake with multiple lanes. 19

2 Type PKF armoured chain feeder. 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 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 Spillage- and dust-free. half of the full portal using the raised outer building wall to support the portal bogie at high level. Semi-portal reclaimer for FGD gypsum. 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. Portal reclaimer for limestone. 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 Such a system was recently supplied for the Turceni power plant in Romania including semi-portal reclaimers by Schade plus Samson TM 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. 20

3 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. Storage hall for semi-portal reclaimer. 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. Continuous side support. 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. Aumund BEW-K silo discharger. Belt deflection carrying and impact idlers. 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 Centrex TM 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 Thermal power plant in Indiana, USA. 21

4 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 pxx is a power plant in Indiana where the FGD gypsum is stored in two silos each of 1,000m 3 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. Shiploading FGD gypsum in Denmark. Where convenient or possible shipment by barge or small bulk carrier is always preferred being both less expensive per ton mile and much less polluting compared to road or rail transportation. The 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. 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 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. FGD gypsum intake at Sherburn. British Gypsum Sherburn board plant. 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 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 22

5 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 Samson TM feeders and the discharge from each Samson is delivered to a common cross mounted transfer conveyor to minimise chute angles and avoid any risk of bridging or blockage. In this project the Samson TM, 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 on pxx in this project in the USA. Samson surface feeder intake facility. 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. Shovels on outboard roller chain. 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. The result is a clean and controlled recovery of material from the stockpile and with fully automated control requires the minimum of operator intervention. Schade twin boom portal reclaimer. Reclaimer discharge over wharf. FGD gypsum storage hall. Concrete wharf and reclaim conveyor. 23

6 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 Travel drive and lateral alignment rollers. Outboard roller scraper chain. Outboard roller scraper chain. 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. Reclaim conveyor and feed boot. 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 pxx 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 24

7 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. Portal reclaimer control cabin. In addition to the control equipment within the storage hall an additional dedicated control unit is provided for the pair of Samson TM 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 utilised 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 CO 2 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 Granulated slag circular storage. 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. Granulated slag portal reclaimer. 25