The Esbjerg coal-fired power station on the west coast of Denmark has recently become cleaner.
As a result of a Danish law about emissions, the 378 MW power plant installed a reactor to remove the nitrogen oxide pollutants that normally would be released into the atmosphere. It already had installed a plant to remove sulfur oxides.
To support the de-nitrification (DeNOx) plant that it installed with power station owner/operator Elsam, contracting engineer Fisia Babcock installed a special wire rope hoist.
The treatment plant uses natural gas and ammonia to react with nitrogen oxides in the exhaust gas that come from the boiler, and turn them into harmless nitrogen gas and water.
These chemical reactions take place about 60m (200ft) above the ground, at the height of the tower boiler’s exhaust outlet, in the presence of catalytic converters that periodically need to be replaced. This is the hoist’s job.
Unlike most power stations, the Esbjerg facility had included a dedicated space for the DeNOx plant when it was built, according to Fisia Babcock process engineering and gas cleaning engineer Gerd Beckmann.
Although there was sufficient space for the DeNOx plant, the building’s arrangement of structural steel created some tight spots – the smallest with 115mm (4 1/2 in) of clearance – at some points along the lifting path of 65.5m.
The catalytic converter cages measure about 1m x 2m (3ft by 6ft) and weigh up to 2.5t.
Fisia Babcock’s initial concept called for a simple single-fall hoist. But there was a risk the load would swing or twist, which could result in the load jamming – potentially a huge problem because the hoist is only used during occasional two-day outages. Also, the catalytic converters are fragile and would be damaged if they banged against steel, according to the eventual hoist supplier, R Stahl.
Standard wire rope kit hoists also would not be appropriate for the job, because their drums are too small for the lift height. Also, the hook tends to move horizontally as the rope unwinds from the drum, which makes positioning the load through tight spaces more difficult.
The solution
Stahl supplied a 2.5t capacity two-fall SH60 wire rope hoist on a monorail. Each of the two falls of rope reeved on the hoist are attached to a hook. The two-fall rope design prevents lateral hook movement, and reduces the risk of the load twisting. Rope guides help correctly wind the rope on the drum.
To cope with the long height of lift, Stahl specified a long rope drum shared by both 12.5mm-diameter (1 in) wire ropes.
Fisia Babcock installed some padding on the steel structure in case the catalyst cage did smack against it. “The hoist works so well that it was not necessary,” said Fisia Babcock engineer Gerd Beckmann said.
Stahl over-specced the motor to help make sure it would not overheat in ambient temperatures of 60°C (140°F). It chose a standard hoisting motor that runs at 65% of duty cycle maximum at fast speed, 20m/min. It runs at 20% of duty cycle maximum at a creep speed of 3.3m/min. A travel motor moves the crane and load at 10m/min and 2.5m/min.
The hoist’s LEI load measuring system measures the load torque in the hoist gear and cuts off power to the hoist in the event of an overload. Normally a load sensor would be integrated into an extra sheave – but this hoist specification calls for the rope to fall directly to the hook from the drum.
Since the hoist was installed in late 2004, it has swapped out 84 catalytic converter units with new ones during a three-shift, 48-hour outage. The next change-out is scheduled for 2007.
Beckmann said that this type of long-reach low-rotation hoist may well be useful for future DeNOx plants. At the end of 2006, he says he will be buying six more hoists for six reactors for a new coal-fired power station, Torre Valdaliga near Rome.
Despite Stahl’s good work so far, a hoist manufacturer’s job never ends. “Stahl is one of our suppliers, but it is not certain to get the order,” he said.
