Automated ash disposal system at Australian power station sets new standards

The fully automated system at Bayswater uses state-of-the-art technology to transport ash more than 11 km to a mine disposal site

H. Tod Kennedy

Asia/Pacific Editor

Pacific Power, the state electricity generating corporation of New South Wales, recently commissioned a fully automatic dense ash collecting, pumping and disposal system at its Bayswater Power Station north of Sydney. It is said to be one of the world`s highest volume systems for transporting both dry and slurried boiler ash over a long distance.

Critical to the process is the efficient pumping of highly concentrated fly ash slurry by pipeline over a distance of 11 km to fill voids at an old mining site. This method eliminates the need for a large unsightly dam and other problems related to dry disposal by truck or lean-phase slurry pumping and open sluicing. The system is proving cleaner both in collection and dumping than earlier methods and is very cost-effective according to the project engineers.

The project was initiated by Pacific Power to meet the demand for fly ash disposal in addition to its existing Pikes Gully Ash Dam. This would enable the generating plant to continue operation as a baseload station for the remainder of its 30-year life and satisfy a commitment to fill the old Ravensworth No. 2 open cut coal mine voids while helping to rehabilitate the disposal area in an environmentally friendly manner.

The overall project, estimated cost (A)$30 million, was contracted to John Holland Construction and Engineering Pty Ltd. under a turnkey agreement covering design, procurement, construction and operation. This followed a basic concept specified by Pacific Power and further developed and prepared by consulting engineers CMPS&F for the downstream slurry/water system, including silo design, pipework and installation procedures. Kockums Bulk Supply Systems Pty Ltd. designed and manufactured the dry fly ash automatic collection equipment. Both CMPS&F and Kockums were engaged as sub-contract consultants to the main contractor.

Fly ash system flow

The fly ash from the station`s four boilers collects on 40 filter bags. The bags are automatically agitated sending the fly ash into collection hoppers–one for every five filters. The dry ash is then passed on to enclosed, sloped pneumatic conveyors which “fluidize” the material, allowing it to float by gravity to a junction where it is automatically distributed to 32 collection vessels or diverted to the old sluice system. Kockums supplied the dry ash handling system and equipment.

The collection vessels, which receive up to 300 t/hour of dry fly ash, are automatically pressurized using Drexelbrook level measuring equipment, and the material is pumped pneumatically in sequence, with a maximum air pressure of 500 kPa from the general compressed air supply. The dry ash travels through 16 steel pipes of 150 mm (six inch) diameter and further through 16 pipes of 130 mm (five inch) diameter to the intermediate silo. This silo has a 500 m3 capacity.

Conveying vessels receive the dry ash for transport on a sequential basis, and at this location a compressor station with three model LG 315 Compair units provides the additional pneumatic energy to pump the dry ash to two main storage silos, each with a capacity of 2,000 m3. At this stage the material passes from the silo discharge hoppers for conversion to slurry. However, the dry ash can also be loaded into trucks at the rear of the silos through special dust-free filler nozzles.

Recycled water from the mine area is mixed with the ash in a two-stage process to form a high-concentrate slurry which is stored in two large reservoirs. From these reservoirs, the slurry is pumped at demand to the suction ends of two high-capacity slurry pumps.

John Holland engineers have found that by keeping the slurry mix around a density of 74 percent, the pumped paste-like material does not hang up or cause blockages in the pipelines, and the process is relatively quick to stop and start up at any time.

Main pumps

Each of the two slurry pumps is a GEHO model TZPM 1600 piston diaphragm, three-cylinder (triplex) unit driven by a 700 kW dc electric motor. The advanced pump design provides a reliable and economic solution for the high-pressure, long-distance transportation of solids, whether abrasive or of high specific gravity. According to the supplier, Weir/EnviroTech, efficiencies up to 95 percent can be expected along with low energy consumption.

The pump`s single-acting, high-pressure pistons are driven by a crankshaft via reduction gearbox and pinions to provide the horizontal motions. Each piston pushes oil against a pre-formed rubber diaphragm which pumps the slurry and also acts as a protective barrier between the mechanical parts and the slurry. Since the three pistons are out of phase 120 degrees, pumping progresses with little fluctuation to produce a continuous stream of slurry flowing from suction to delivery. All piston and valve pumping cycles are recorded graphically in real time on a computer at the control station.

Each pump has an uptime valve for durability and a diaphragm stroke control system for high pressures. The only wearing parts are the suction and discharge NR valves. A pulsation dampener is mounted near each pump outlet.

The pumps are capable of transporting up to 250 m3/hr of slurry and can operate at a minimum of 20 m3/hr at 15 MPa. Slurry is pumped a distance of 11 km through two 200 mm (eight inch) diameter welded steel pipes at the rate of 300 t/hour to the disposal site. The journey takes about 80 minutes.

A control station at the old mine area allows for automatic or manual distribution of slurry through movable discharge pipes to fill various voids. Once exposed to atmosphere the already near dry slurry quickly forms a hard dust-free surface which usually can be walked on within 24 hours.

A pond in the area collects any groundwater and run-off from disposal. This clean water is recycled back to the slurry plant by a pumping station mounted on a floating pontoon. All controls are automatic and integrate by telemetry with a main control station.

PLC controls

Paklog has installed a graphical and user-friendly programmable logic controller (PLC) automated control system at the main control station. Modicon 984 PLCs cover the dry fly ash collection phase and the wet mixing and slurry pumping phases. The system utilizes 2,600 input/output points. Paklog also completed cubicle construction and commissioning of the control and associated radio telemetry systems.

The pros and cons

The advantages of the high-density slurry disposal system include:

– no need for ash retaining dams;

– low water consumption;

– very little water release at the disposal area with rapid drying;

– no appreciable leachate from the disposal site;

– can use some waste liquids to mix the slurry;

– pumping has quick stop and start without line flushing;

– ash disposal outlets can be moved around to fill voids or stack the product;

– allows for progressive rehabilitation of the disposal site phased with ash dumping; and

– less personnel to monitor and maintain the plant.

There are also some disadvantages to the disposal system. For example, some operators distrust the computer controls and enclosed handling system. However, proper training should overcome this reluctance to change. In addition, the capital cost of a modern automated plant is initially high. Nonetheless, the Bayswater ash disposal system is expected to benefit from long-term savings.

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Intermediate storage silo. The adjacent compressor station provides air to pump dry fly ash by pipe to main storage.

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Above, at this point the dry fly ash is mixed into slurry, passing through holding vessels (below) to feed the GEHO slurry pumps (foreground). Below, fly ash from hoppers at Bayswater power station is pneumatically conveyed to collection vessels (two in picture) then pumped, sequentially, through overhead pipes to an intermediate storage silo.

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