Circular fluidised-bed (CFB) combustion power stations enable the cleaner combustion of different fuels to generate power, which enable power to be produced from lower grades of coal and diverse fuels, such as biomass, oil shale and waste wood, says French energy major Alstom Steam Business sales director Hugh Kennedy.

A CFB power plant must be designed to use a specific fuel and there is no power plant that can burn all grades of fuels to provide power. But the significant advances Alstom has made in CFB technology enables it to build plants that can use two or more different fuels simultaneously and at varying ratios.

The main development driver of CFB technology is the lower grades of coal fuels and even waste coal that are being used to generate power. The varying quality and moisture content of low-grade coal and waste coal makes them particularly difficult to combust in a conventional pulverised coal power station, he says.

The furnaces of CFB plants contain a mixture of fuel, ash and limestone and combust the fuel to produce steam at temperatures of up to 620 ºC to drive turbines. The limestone reacts with sulphur dioxide gas during this stage to form calcium sulphite.

Coarser fuel sizes can be used, compared with pulverised coal plants, as any fuel that is not combusted is separated from the lighter gases and ash in the cyclone, after the furnace stage, and then fed back into the furnace to complete combustion.

CFB plants have lower water consumption, compared with pulverised coal plants, and low sulphur dioxide and nitrous oxide emissions.

“CFB technology is not a familiar technol- ogy in South Africa, but is becoming increasingly common around the world and particularly in China, where the State-owned power equipment suppliers and design institutes obtained a licence to use Alstom’s CFB technology within China and have commissioned several CFB plants to burn waste coal.”

Further, all modern baseload power plants must be designed for reliability and flexibility because renewable sources will provide variable power to the grid, emphasises Kennedy.

Alstom’s CFB plants can modulate power output over a wide range, with a future development target to enable operation of the plant in idle mode, where power output is zero and power used to maintain the furnace and systems is 10% of maximum output.

“Alstom has experience in designing CFB plants that use a variety of different fuels. The Baima 300 MW CFB plant in Sichuan, China, burns waste anthracite and demonstrated that it can meet fuel quality variations and signifi- cant jumps in demand, while maintaining steam temperature and emissions levels.”

The 125 MW electrical Emile Huchet CFB plant, in France, was built in the 1970s to burn waste coal and discard coal left on dumps. Cyclone technology was extensively tested at the plant to separate and send the heavier particles back to the furnace.

The Seward CFB plant, in the US, has two 260 MW electrical boilers and was designed to generate power from low-grade and discard coal while meeting stringent US emissions and air regulations.

The Narva CFB plant, currently being built in Estonia, has been designed to use oil shale and biomass as its primary fuels, as well as retort gas generated during the production of oil from oil shale. The plant will come into service this year and will meet the stringent European Union sulphur dioxide and nitrous oxide and emissions standards.

South Africa has a large grid that can accommodate large CFB units that can use the fuel available while addressing emissions concerns, with a limited impact on water constraints, compared with pulverised coal plants, he concludes.