South Africa's state-owned utility company Eskom produces 95% of the country’s electricity and almost half that of the entire continent. But power generation is stretched to capacity, leading the company to invest in a series of bold new projects – including the 4.8GW Kusile power station, which will be one of the world's biggest coal fired power stations. The facility comprises six 800MW units, scheduled for phased introduction from 2014 to 2018. Its boilers stand 115m tall and its chimneys tower 220m into the sky. During its eight year construction, the project is expected to be the largest single stimulus to South Africa’s economy, creating thousands of jobs.
Water is fundamental to the power production process, used to generate electricity, to control temperature and to treat waste. Eskom is keen to make the most efficient use of water possible in a region where it is scarce. The company has also set out a ‘zero liquid effluent’ policy to protect the natural environment. These requirements call for sophisticated wastewater treatment and purification processes that help make Kusile one of the most advanced coal fired power stations in the world. We've been leading the engineering, procurement and construction of Kusile’s array of cutting-edge water treatment facilities.
Eliminating liquid effluent
Electricity is generated when superheated steam is forced through turbine blades. Feed water is repeatedly cycled through the system and must be kept pure to prevent build up of mineral deposits when it turns to steam. Not only do deposits reduce capacity by clogging up pipes, boilers and turbines, they also form a layer of insulation, which makes heat transfer from the boilers to the water less efficient. Purifying the water also reduces the risk of the water becoming acidic, which would corrode metal components.
Purification at Kusile is a complex, multi- stage process. The steam must first be condensed back into liquid. This then passes through a combination of reverse osmosis filtration and ion exchange, which removes harmful or undesired properties at the molecular level. Carbon dioxide and oxygen are also removed, to further guard against degradation of the turbine impellers. At Kusile, this process is able to purify almost 1600 tonnes of water per hour.
Purification produces brine which is often released into estuaries to mix with seawater. But at Kusile, the brine is treated in a further process to convert it into dry salt suitable for landfill disposal.
Cutting SO2 by 90%
Water is also important in flue gas desulphurisation (FGD), a technology that reduces emissions of sulphur dioxide – SO2 – which results from burning coal and which causes air pollution and acid rain. It is being used for the first time in South Africa at the Kusile plant.
The FGD technology being put to work at Kusile removes pollutants from flue gases in devices called wet scrubbers. These spray the gases with a slurry of water and pulverised limestone. The droplets absorb sulphur dioxide molecules, preventing more than 90% of SO2 leaving the chimneys. The resulting waste material is used to make drywall lining and ceiling materials.
Although most water loss in the power station is caused by evaporation, the FGD process does produce some wastewater, which is highly contaminated and must be treated in a dedicated facility.
Intelligent safety systems
As Kusile is a fully automated plant, we had to design bespoke systems to run the water purification and treatment operations that would fit seamlessly into the overall plant design. One of these is the automated sample analysis process, which uses leading edge technology to analyse water purity. The system provides immediate analysis of samples; by continually measuring water quality, operators can detect problems before they arise.
We also used an intelligent 3D computer model to assist with scenario planning. The model plots outcomes depending on specific operational conditions to mitigate the risk of accidents. It enables accurate hazard studies, which improves efficiency and safety for operators and maintenance crews.
Benefits now for local people
As this is the first time FGD technology has been used in South Africa, the project has called on specialists from the UK, Canada, Germany, Italy and Hungary, where the process has been used for many years. But local suppliers and skills are being extensively used too: 40% of the workforce and over 50% of materials have been sourced locally, with particular attention given to developing skills in engineering, construction and quality assurance.