Motors help minimising impact of sulphur dioxide on environment

Cost saving and increased efficiency motors installed at power station are helping to reduce sulphur dioxide emissions.

E.ON, one of the UK's leading power and gas companies, has recently installed two new large WEG motors at Ratcliffe-On-Soar Power Station near Nottingham. Both motors have been installed in the flue gas desulphurisation (FGD) plant which helps to reduce SO2 emissions, while also creating commercial quality gypsum. Ratcliffe Power Station takes its environmental responsibilities seriously and the site is run with efficiency as a top priority. The top criteria when sourcing the motors were energy efficiency and reduced maintenance requirements.

Chris Bennett, a senior electrical engineer at Ratcliffe, said: "It is always surprising to people who visit the site that we are so efficiency conscious, but they're often even more surprised that we have to pay for the power we use during the generation process so it's essential we don't use more energy than we actually need. Even more important is minimising the environmental impact of the power station and ensuring the reliability of the plant; outages have to be avoided where possible so critical equipment such as the motors have to be reliable."

The first motor to be installed is a 6150KW, 10 pole, 11KV WEG M-line MGF 1000 unit which is used to drive the booster fan on the FGD plant. The motor is an IEC1000 frame size with an output torque of more than 100000Nm and is the largest of the WEG motors currently installed at the power station. The booster fan is used to power the flue gas through the FGD plant.

The second motor installed is also a MGF high-voltage unit from WEG's M-Line of high voltage units, it is smaller IEC 400 frame size MGF 400E 750KW, 4 pole 3.3KV motor. This motor drives an absorber recirculation pump on one of the FGD towers. These pumps are used to transfer up to 8000 tonnes of limestone slurry an hour to the top of the 50 metre high towers to be sprayed into the flue gas.

Ratcliffe Power Station is recognised as one of the most efficient coal fired power stations in the UK and this played an important part in specifying the WEG motors, both are highly efficient, the larger unit up to 96.4 per cent at peak efficiency. The smaller motor is just 1 per cent lower at 95.4 per cent efficiency.

High efficiency has been a constant demand of smaller LV motors in recent years, but larger units from other manufacturers have lagged behind in efficiency, WEG's constant design development and refusal to compromise on the quality of materials used in its HV and MV motors has resulted in some of the highest efficiency figures on offer for large motors, and available on these standard model ranges.

The Ratcliffe site has a generation capacity of 2000 MW from four 500MW units, enough to meet the needs of approximately 1.5 million homes. The site covers 700 acres and has been producing electricity for more than 40 years.

Ratcliffe Power Station takes its environmental responsibility extremely seriously, ensuring that its impact on the surrounding area is kept as minimal as possible. Large numbers of native trees and shrubs have been planted and a biodiversity action plan exists to enhance the ecological value of the land. The FGD plant is the centrepiece of the drive to cut emissions, it was commissioned in 1993 and is used to drastically reduce SO2 emissions.

The desulphurisation process passes flue gas through a heat exchanger to cool it below 90°C. The gas is then drawn into the tower where it comes into contact with the limestone slurry. This creates a chemical reaction, removing the SO2 and converting the limestone into calcium sulphite (which is in turn processed to create commercial quality gypsum); during the process 95 per cent of the hydrogen chloride is also removed. Once the SO2 has been removed the gas is reheated to allow it to rise through the chimney.

As the FGD plant plays such a vital role in cleaning the emissions from the power station it is vital that down time is kept to a minimum. This means that all equipment in the plant must have the longest operating life possible with low maintenance requirements. It was vital that the new motors were able to survive in an outside environment for a long period, working at a continuously high load.

WEG motors provide a solution as they deliver outstanding performance, reliability and safety in the most arduous operating environments worldwide. The motors are designed to be compact and robust, using cast iron frames and corrosion resistant finishing and galvanised mounting components. The windings are sealed using a vacuum pressure impregnation (VPI) system which guarantees high quality insulation and stator coil protection through the application of a special epoxy based resin.

Operating temperature is crucial to both the efficiency and the longevity of electric motors, and so the WEG units employ a combination of an internal cooling circuit, external cooling fins and high efficiency cooling fans to guarantee an optimum temperature gradient within the motor. These, coupled with specifically dimensioned, high quality bearings ensure that the motors have a long running life.

To further increase the motors operating life WEG motors have dynamic balancing at motor operating speeds. In this case the standard balancing is on two planes, (WEG are also capable of providing three plane balancing on request). Both are computer controlled and capable of very low vibration levels, this extends both bearing and motor life.

ue to their careful design WEG motors require very little maintenance to keep them running efficiently, however, as with all motors, it is important to keep them adequately lubricated.

WEG has employed a pressure grease relief system which is an innovative grease lubrication system enabling old grease to be removed and replenished with new while the motor is still running, reducing the need for maintenance down time.

Chris Bennett: "We opted to install the WEG motors as they run smoothly and we could be confident that they would require a low level of maintenance. It was important that we install robust motors as they are running continuously on a high load in conditions that can become harsh at times.

"As well as the two large motors installed on the fan and the pump we have several smaller WEG motors throughout the power station."

Technical support is supplied by UK distributors and from WEG's dedicated LV and MV technical support team based at the company's HQ in Redditch. The team has many years of experience specifying, installing and supporting large motors in power in power stations and similar applications throughout the country. The level of support available for WEG products in the UK may not be required during the motor's life, but it does provide a high level of security for end users such as Ratcliffe power station.