Coal unloading to the max

Cargotec’s new Siwertell screw-type unloader – the Siwertell VST 1090 – can discharge coal at 3000 tph – a significant increase on previous models.

“For decades we have been accustomed to talking about figures of hundreds or thousands of tonnes per hour when considering dry bulk discharge rates,” said technical director for Siwertell, Jonas Andersson. “Given this familiarity, we can sometimes overlook the nature of the challenge facing designers and engineers. To re-establish a proper perspective, it can be helpful to consider what is happening in shorter periods of time. So 3000 tph can be expressed as 50 tpmin. But it is even more impressive when you realise that this is very nearly a tonne of coal each second!”

Andersson explained developing the new unloader was technically demanding, involving far more than simply scaling up the existing product. “The whole conveyor chain is new and the re-use of our existing designs has been minor. A detailed design of a new unloader is not possible because our unloaders are designed on a tailored basis to exactly meet a customer’s specific needs. Our development work therefore focused on essential parts for the screw-type unloader and detailed calculations for the supporting structure,” he said.

“No-one is better placed to extend the boundaries of unloading performance in this way. For years, our coal handling systems have been in widespread use in import, export and transfer terminals, as well as in reclaiming systems for flat storage areas. It is this extensive experience, engineering expertise and thorough understanding of all the issues involved that ensures the success,” Andersson continued.

While it was possible to scale up some of the parts in the conveyor chain, the existing steel supporting structure design could not be adapted. Siwertell designers therefore developed a completely new solution. Another major challenge was dealing with the huge amount of material that needs to enter the inlet head in order to achieve the rated capacity.

The 3000 tph unloader has been developed primarily for coal-fired power plants. However, it can also be used for other materials and applications if there is a demand for high capacity operations: for example, it can be used for dedicated coal intakes in the steel industry.

In 2012, Cargotec secured a breakthrough contract from Formosa Petrochemical Corp. for two Siwertell coal unloaders, each with a rated capacity of 2400 tph. The new unloaders will feed coal to the company’s newly-built Ha Tinh steel plant in Vietnam. When compared to the grab crane systems traditionally employed in this role, the Siwertell unloaders offer savings in the region of US$ 2.5 – 3 million/year for the operator. To date, they are the biggest Siwertell unloaders on offer. This groundbreaking application could start a trend that sees other steel plants using Siwertell machines for dedicated coal unloading.

“Naturally, a high capacity unloader can only deliver its full potential when it is linked to adequate downstream infrastructure,” said Andersson. “Some existing terminal conveying systems may find it difficult or impossible to cope with the increased capacity, but this has to be considered on a case-by-case basis. If an unloader is discharging to an existing quay conveyor, then we need to follow the maximum capacity of the quay belt. However, if it is a new investment, then the quay conveyor will be designed to have a capacity to suit the specification of the intended unloader. In light of this, the new unloader is more likely to find applications in new terminals, particularly for coal intakes feeding new power plants in the Far East.

“After careful consideration of all relevant factors, we decided that 3000 tph for coal was the right target capacity for a new Siwertell machine, both from a technical point of view as well as from the operators’ perspective,” Andersson explained. “Larger capacity screw-type unloaders might be called for if ships become even bigger and these can theoretically be built. The main challenge is to be able to feed the bulk material into the inlet head and we would also need to see correspondingly enhanced downstream conveyor and storage systems.”

No let-up in demand for coal

Andersson said that the continuing global reliance on coal as a key resource and the need to reduce operators’ costs were the major drivers in Cargotec’s decision to undertake the development programme for the new 3000 tph screw type coal unloader.

According to the World Coal Association (WCA), some countries rely on coal for over 90% of their electricity generation and over the next 30 years it is estimated that global energy demands will increase by almost 60%. “As the most important fuel for electricity generation and a vital element in steel production, coal has a major role to play in meeting future energy needs,” Andersson said.

The WCA also notes that coal has been the world’s fastest growing energy source in recent years – faster than gas, oil, nuclear, hydro and renewables.

“The 3000 tph unloader is a natural extension of our capabilities and will prove to be well-placed for large-scale bulk handling operations,” Andersson continued. It is designed to discharge vessels of up to 230,000 DWT or a maximum beam of 50 m. In general, it is aimed at ships between 100,000 DWT and 200,000 DWT. It can be used to unload smaller vessels; however, the efficiency level will not be as high as experienced with larger vessels.

Hard figures demonstrate economies of scale

Managing director for Siwertell, Per Karlsson, said the bulk handling industry appreciates economies of scale and provided some impressive figures to explain exactly why this is the case.

“Bigger ships transport their bulk cargoes more cost effectively across the oceans, but they can lose time in port when discharge rates are not optimised for large vessels.

“Cost savings can be achieved both in terms of initial investment and through-life operational costs, depending on the overall unloading performance required. Specifying two 3000 tph unloaders instead of three 2000 tph machines – to achieve a 6000 tph capacity – results in an investment saving of up to 25% in favour of the 3000 tph ship unloaders.

“On the other hand, if the maximum possible capacity is required from the outset, then three 3000 tph unloaders will provide an unloading capacity of 9000 tph, as opposed to 6000 tph from three 2000 tph units.

“Considering a capesize vessel of 200,000 DWT and a through-the-ship capacity of 65%, the unloading time using three 2000 tph unloaders would be 51 hours. This would be reduced to 34 hours using three of the new 3000 tph unloaders.

“With an intake of 6 million tpa, a vessel cost of US$ 60,000/day and a working time of 22 hours/day, the annual cost saving should be in the region of US$ 1.4 million,” explained Karlsson. “Over the lifetime of the investment, say 25 years, the total saving should be around US$ 35 million, which more than justifies the higher investment costs for the unloading equipment, the conveyor belt and stacker. Additional cost savings will also be achieved in operation as there will be fewer operating hours for each unloader.”

“The new unloader will be heavier than its predecessors and this will naturally be reflected in the price. However, based on these figures, payback time should be less than three years and that is without considering the additional savings in operation and maintenance costs,” Karlsson continued.

“It is also worth noting that despite the increase in weight, the 3000 tph unloader will have more or less the same overall dimensions as the current largest Siwertell unloaders. [It will] still weigh less than a 2000 tph bucket chain unloader. This means that some customers will be able to take advantage of the increased unloading capacity without having to invest in civil engineering work associated with upgrading their jetty specification,” added Karlsson. “Those investing in new terminals will be able to save considerable sums in jetty construction costs in view of the relatively light weight of the new machines.”

Written by Cargotec. Edited by Jonathan Rowland.

Read the article online at: https://www.worldcoal.com/handling/04112014/world-coal-cargotec-siwertell-coal-unloading-to-the-max-coal1532/