Towards cleaner bulk ship unloading

Apart from shipboard self-discharging systems, there are three recognised methods of unloading dry bulk cargo from a vessel to the quayside: by grab (using either a purpose-designed grab gantry unloader, a rope crane, or an articulated-boom hydraulic excavator type of machine), by continuous mechanical unloader, or by pneumatic unloader which sucks cargo from the ship’s hold in the manner of a giant vacuum cleaner. These three systems have their respective merits and drawbacks. For example, grab handling has the advantage of being straightforward and very versatile, but traditionally has the disadvantage of causing unacceptably high levels of spillage and dust pollution. Continuous mechanical unloaders cause zero spillage and significantly reduced dust nuisance, but they provide limited adaptability for handling different types of cargo. Pneumatic unloaders cause neither spillage nor dust pollution but they are only able to handle powders, pellets, grains and materials of very small lump size. They also tend to consume more energy than the other two methods.

However, latest technical advances have provided solutions to many of these drawbacks, so the distinction nowadays between these three basic modes of ship discharge is far less clear cut.

From an environmental perspective, dust pollution is the single most important factor to consider when selecting a dry bulk ship unloader but here again it is dangerous to categorise the different types of ship unloader too rigidly. For example, as described later in this article, the latest generation of grab gantry unloaders (traditionally among the most dust polluting of all ship unloading methods) have been proved to cause less dust nuisance than continuous mechanical unloaders which are supposed to be environmentally superior is this respect.

Pneumatics: coping better with the last cargo layer

 Alain de Visscher, commercial director of Vigan Engineering, which makes both pneumatic and continuous mechanical unloaders, explains that continuous mechanical unloaders operate in a more or less dust free manner while the intake head remains submerged in the cargo. However, when the hold is nearly empty and there is no more than a metre or so of cargo remaining, the rotating device normally fitted to the intake head becomes partly exposed. Under these circumstances the device will agitate the material with ambient air and this turbulence will blow dust into the nearby environment. Wheeled loaders and similar auxiliary clean-up machines lowered into the hold to help feed residual cargo towards the unloader intake will also generate airborne dust. The suction nozzle of a pneumatic unloader on the other hand does not normally require any mechanical feed device. The powerful air suction will guide all grain and dust particles into the nozzle, conveying them directly into the vertical and horizontal piping. “With its capability to suck in the lowest remaining grain on the hold bottom floor, the pneumatic unloader will act as a real vacuum cleaner. For similar reasons, the auxiliary equipment will be less heavy and will have to work for less time”, de Visscher explains.

Vigan has just delivered a pneumatic unloader to IRALCO at Iran’s Bandar Imam Khomeini port for discharging alumina and petroleum coke. The machine will be required to discharge some 30,000t of alumina a month with minimal dust pollution. One of the main factors that led to the choice of a pneumatic unloader was that previously employed grab unloading methods had given rise to unacceptable levels of dust pollution.

This unloader incorporates special features to cope with handling large volumes of alumina, a dusty and highly abrasive mineral powder. Once unloaded, the cargo is transferred into existing rubber tyred mobile hoppers via an adjustable chain conveyor system which allows for flexibility in the positioning of the hopper. A loading spout with a suction device at its extremity has been designed to reduce almost to zero the amount of alumina dust escaping into the environment. A hoist system on the boom allows wheeled or tracked loaders to be lowered into the holds for fast final clean-up operations.

Although pneumatic unloaders enjoy an excellent record in minimising airborne dust pollution they are traditionally less energy efficient than continuous mechanical unloaders, a drawback which becomes more apparent when higher discharge capacities are required. Partly for this reason Vigan in 2007 acquired AS-C and its Simporter twin-belt ship unloading technology which had earlier been developed by Simon-Carves. The Simporter, which in recent months has undergone further development by Vigan, consumes less energy than other designs of continuous mechanical unloader. A further advantage is that it causes minimal degradation to delicate cargoes since the material being unloaded is sandwiched between the two moving belts. Vigan last year won contracts to supply Simporters to Vietnam (1200t/h capacity), Korea (750t/ h) and Pakistan (1000t/h). The first of these machines is already operational, the second is currently being commissioned and the Pakistan machine is scheduled to be delivered at the end of 2010. This will work alongside a pneumatic unloader to discharge agribulks.

Dust-suppressed grab unloaders

although ship unloading by grab traditionally gives rise to more spillage and dust pollution than is the case with continuous ship unloaders (whether pneumatic or mechanical), steps have been taken in recent years to improve the environmental performance of grab unloaders. For example, two 1200t/h capacity AGD Konecranes grab unloaders recently supplied to Bristol Port in the UK have been fitted with highly efficient dust extraction and dust suppression systems. These measures have proved to be so effective that the grab unloaders generate less dust than two continuous mechanical unloaders that operate on the same wharf. Consequently the continuous unloaders are normally restricted to handling coal while more dusty cargoes are discharged by the grab unloaders, although normally one would expect the converse to apply. It was essential to restrict dust emissions at this location since there is a car import terminal in the immediate vicinity.

AGD grab unloaders employ an electrical four-drum system which was introduced by Konecranes in 1999 and is said to have almost completely replaced the conventional rope trolley type of unloader. With this latest concept only four ropes are used, which makes rope reeving very straightforward. The modern AGD control system includes special features to minimise dust pollution and to help the operator attain highest levels of performance. Feedback from several recent installations suggests that availability rates in excess of 98% can be achieved with this system.

Last year Konecranes delivered two of its latest AGD grab unloaders to Dong Energy’s new coal import terminal at Stigsnas, Denmark, which has just become operational. Each unloader has a lift capacity of 40t and can service Capesize vessels with a free digging capacity of 1900t/h. Because the machines are located close to domestic dwellings they have been fitted with special noise attenuation features. These include isolation of drive mechanisms, a fully insulated machinery house, a trolley runway with plastic-coated trolley wheels, sound insulation of hopper sidewalls and portal conveyors mounted inside a gallery. The target value for noise emission of each crane is 103dB(A), representing a 10dB(A) diminution compared with traditional grab unloaders. A Konecranes spokesman explains that this is effectively a 90% reduction when gauging the sound level in watts, which is how the human ear senses noise.

At the beginning of last year Konecranes delivered a second 1500t/h capacity AGD grab unloader as part of a turnkey contract to the Port of Koper, Slovenia. The previous unloader, which was commissioned in 2003, had been built by Sandvik (previously Voest-Alpine Materials Handling) as main contractor, with Konecranes supplying the main drive and AGD control package. The machines are employed to discharge coal and iron ore from Capesize ships.

Koper now has three grab unloaders on the same jetty, two of them employing the Konecranes AGD system. The port is today among the most efficient in the Mediterranean region and the bulk terminal is among the cleanest of its type in Europe.

Electric drive advantages for mobile harbour cranes

High energy consumption, sometimes with the added drawback of exhaust emissions, and noise pollution often go hand in hand. Gottwald Port Technology, the leading manufacturer of mobile harbour cranes, has recently made major progress in combating these problems. A key advantage in this respect is that the company’s cranes can be connected to the harbour mains allowing them to be operated by external power supply. This avoids exhaust fumes on the quayside and significantly reduces noise emissions.

Since Gottwald cranes, unlike those of its competitors, are equipped with electrical drives anyway (in most cases power is generated on board the mobile harbour crane by a state-of-the-art diesel generator), the efficiency of the drive system increases even more. Gottwald’s Peter Klein explains that in bulk handling applications, often involving the company’s portal harbour cranes normally fitted with four-rope grab forming part of a complex terminal infrastructure, these cranes are nearly always operated with power from the harbour mains. A further advantage of this arrangement is that energy, recovered from braking and lowering motions, can be fed back into the harbour’s power grid and therefore sold back to the utility. On the other hand, power recovered in this way can also be used to operate other handling equipment in the port area.

For harbour cranes which cannot be connected to the grid, because of absence of the required infrastructure, Gottwald is currently developing a future-orientated hybrid system which involves both advanced diesel engines and ultracaps for energy storage. Energy which is recovered from lowering and braking motions is stored in ultracaps and returned to the drive system for the next cycles. As part of the company’s R&D and testing programme these ultracaps have recently been installed in a prototype crane being used by a Gottwald customer who at present prefers not to be identified. Tests so far have proved quite promising, with fuel savings in the double-digit percentage range with regard to different loading and unloading cycles and cargo mixes.

Fuel savings of around 25%

The LHM series of mobile harbour cranes from Liebherr-Werk Nenzing can also be equipped with an electric main drive, allowing them to be powered directly from the harbour mains. The same applies to the company’s LPS range of rail-mounted portal cranes. All of Liebherr’s mobile harbour cranes are equipped as standard with a hydrostatic drive system, allowing the same components to be used throughout the whole range (LHM 120 through to LHM 600). A further advantage is the very fast hoist acceleration provided by the hydrostatic system, allowing a hoist speed of 120m/min to be reached in less than three seconds, which is some four to five times quicker than a comparable linear DC machine. The hydrostatic units also allow energy to be easily recaptured from the cranes’ braking and lowering motions.

All LHM diesel engines meet Europe’s latest exhaust emission requirements as well as those of EPA/CARP. Recent ECO Control innovations have helped achieve fuel consumption savings in the region of 25%, without any negative impact on operational output. A further beneficial side effect is that low engine rpm has a direct impact on reduction of noise output. Elastic suspension of winches has also helped reduce both noise and vibration.

The company has an ongoing programme to develop new ecologically sensitive technologies and its continuing efforts to reduce carbon dioxide emissions remain a high priority. Clearly these measures are proving popular with customers who in 2009 took delivery of 72 Liebherr mobile harbour cranes, 38 of which were for bulk cargo grab handling applications. Although this was a 27% drop compared with the previous year, the company did comparatively well in difficult trading conditions, especially as the global mobile harbour crane market including all manufacturers fell by about 40% during 2009.