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Medupi, Kusile boilers a large-scale milestone

MILESTONE POWER STATION Mitsubishi Hitachi Power Systems Africa predicts future power station build prospects in Africa being smaller than Eskom’s Medupi and Kusile power stations

LOW NOX BURNER Mitsubishi Hitachi Power Systems can retrofit low-nitrogen oxide burners to existing boilers

BURNER RENEWAL Mitsubishi Hitachi Power Systems has retrofitted hundreds of burners for international utilities in the US, Europe and Japan

BURNER RETROFIT Eskom will soon be required to modify the existing burners of its coal-fired fleet to meet stricter emissions limits

5th June 2015

By: Donna Slater

Features Deputy Editor and Chief Photographer

  

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The sheer size of the boilers being installed at State-owned power utility Eskom’s Medupi and Kusile power stations might not be repeated in Africa again, owing to future build prospects indicating a requirement for smaller boilers, says power systems company Mitsubishi Hitachi Power Systems (MHPS) Africa.

Medupi power station, in Limpopo, and Kusile power station, in Mpumalanga, both comprise six 800 MW units that use super- critical steam supplied at about 250 bar.

MHPS Africa sales and business development head David Milner points out that most of Eskom’s existing coal-fired fleet – even the newest and fully operational Majuba power station, in Mpumalanga, which was also built using MHPS technology – use subcritical steam supplied at about 170 bar.

Milner explains that supercritical steam enables a power station to produce electricity at a higher efficiency than a power station that uses subcritical steam technology.

Meanwhile, Unit 6 at Medupi is currently operational and producing power, with full installed capacity expected in the next few months.

Medupi and Kusile are the fourth-largest coal-fired power stations in the world and serve as benchmarks for what MHPS is capable of building.

However, Milner adds that, in future, MHPS envisages power stations, particularly in Africa, that will require much smaller units of 250 MW or 500 MW. He says that 800 MW units, such as those installed at Medupi and Kusile, are unsuitable for smaller grids found in countries outside South Africa.

Therefore, MHPS is focusing on the development of two power unit packages: a 250 MW unit and a 500 MW unit, which use a technology that turns coal into a gas, a process called integrated gasification combined cycle (IGCC).

“The 500 MW unit is more efficient, owing to economies of scale,” explains Milner, adding that MHPS’s range of boilers is largely based on utility-sized power plants of about 300 MW and larger.

Further, MHPS has access to biomass- fuelled boiler technology from its South America division. However, Milner says, to a large extent, MHPS’s focus in Southern Africa has been on coal-fired utility-sized projects, such as Medupi and Kusile, owing to the country’s large coal base.


Milner says MHPS is continuing with the development of its IGCC technology because the electricity industry currently requires higher operational efficiencies with lowered emissions, including carbon dioxide (CO2).

IGCC technology uses coal in a gasification process to develop a syngas, which, in turn, is used as a combustion fuel to a high- efficiency gas turbine, instead of directly feeding the coal into the boiler and burning it, producing large quantities of CO2 and ash as a result.

An added benefit of IGCC technology is that the gasification process produces a glassy slag by-product, which can be used in the manufacture of asphalt or concrete.

Before the syngas enters the turbine and is combusted, the impurities are removed.

Milner points out that one of the major advantages of IGCC technology is its significantly lower CO2 emissions, compared with other technologies, particularly circulating fluidised bed (CFB), a technology that is primarily used in applications that use poor- quality coal. “The problem with CFB is its low efficiency and high CO2 emissions.”

IGCC technology can also be used to combust low-quality coal when high-quality coal is unavailable.

Further, other harmful emissions, including mono-nitrogen oxides and sulphur dioxides, are much lower when IGCC technology is implemented. Particulate matter is also significantly reduced.

Milner notes that, with some CFB technologies, limestone has to be used in the boiler to neutralise the sulphur generated during combustion, resulting in the slag from that process being contaminated. Such slag requires specialised and costly disposal procedures.

He adds that IGCC technology is up to 30% more efficient than the combustion technology currently being used by Eskom’s older coal-fired units. There is also a corresponding near-30% reduction in CO2 emissions.

MHPS has a 250 MW IGCC unit operating in Japan and has received orders for the engineering of an additional two 500 MW units from another customer in Japan.

Edited by Samantha Herbst
Creamer Media Deputy Editor

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