Fluidisedbed combustion

If a layer of sand, of finely ground coal, or of another fine solid material is placed in a container and high-pressure air is blown through it from below, the particles, provided they are small enough, become entrained in the air and form a floating, or fluidised, bed of solid particles above the bottom of the container. This bed behaves like a fluid in which the constituent particles constantly move to and fro and collide with one another. As a type of reactor, this offers some significant advantages.

The fluidised bed was used first in the process industries to enhance the efficiency of chemical reactions between solids by simulating conditions of a liquid-phase reaction. Only later was its application for power generation recognised. Its use is now widespread, and the fluidised bed can burn a variety of coals as well as other poorer fuels such as coal-cleaning waste, petroleum coke, wood and other biomass.

A fluidised bed used for power generation contains only around 5% coal or fuel. The remainder of the bed is primarily an inert material such as ash or sand. The temperature in a fluidised bed is around 950°C, significantly lower than the temperature in the heart of a pulverised-coal boiler. This low temperature helps minimise the production of NOX. A reactant such as limestone can also be added to the bed to capture sulphur, reducing the amount of sulphur dioxide released into the exhaust gas. One further advantage of the fluidised bed is that boiler pipes can be immersed in the bed itself, allowing extremely efficient heat capture (but also exposing the pipes to potentially high levels of erosion).

There are several designs for fluidised-bed power plants. The simplest is called a bubbling-bed plant. This, and a second, more complex plant called a circulating fluidised bed can both operate an atmospheric pressure. The circulating bed can remove 90-95% of the sulphur from the coal while the

Coal

Limestone

Cyclone

Combustion chamber partition

Secondary air

Cyclone

Combustion chamber partition

Secondary air

Cyclone Coal Power Plant

feedwater Generator

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Solid waste to disposal

Steam turbine

Figure 3.4 Flow diagram for a circulating fluidised-bed power plant. Source: Tri-State Generation and Transmission Association, Inc.

feedwater Generator

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Solid waste to disposal

Steam turbine

Figure 3.4 Flow diagram for a circulating fluidised-bed power plant. Source: Tri-State Generation and Transmission Association, Inc.

bubbling bed can achieve between 70% and 90% removal. Maximum energy conversion efficiency is 43%, similar to that of a traditional pul-verised-coal plant. However such high efficiencies can only be achieved with larger plants that can employ larger, and generally more efficient, steam turbines under optimum steam conditions.

A third type of fluidised-bed design, called the pressurised fluidised bed, was developed in the late 1980s and the first demonstration plants employing this technology were constructed in the mid-1990s. The pressurised bed is like a bubbling bed, but operated at a pressure of between 5 and 20 bar (1 bar is equivalent to atmospheric pressure).

Operating the plant under pressure allows some additional energy to be captured by venting the exhaust gases through a gas turbine. This provides a higher efficiency (currently up to around 45-46%) while maintaining the good emission performance of the atmospheric pressure fluidised bed. The largest pressurised fluidised-bed plant in operation is a 360 MW unit in Japan.

Atmospheric fluidised-bed power plants with boiler capacities of over 400 MW are commercially available. These can provide supercritical steam to gain the best efficiency. The technology is still under active development, with the prospect of more efficient capture of pollutants coupled with an efficiency of around 50% within the next 10-15 years.

A standard fluidised-bed power plant can meet the emission-control requirements in many regions of the world without further emission-control measures. However in regions with the most stringent regulations capture technologies are required. These are likely to include NO*, sulphur oxides (SO*) and particulate capture measures. The techniques employed to provide additional emission control are the same as those used in a conventional coal-fired power station.

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