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Success stories Published on 26-Feb-2004

CFB COMBUSTORS
Title A solid bed for efficient combustion

Circulating fluidised-bed boilers have significant environmental benefits. They can burn a range of solid fuels, including biofuels, and they could help eastern European countries to reduce emissions. The CFB-Combustors project investigated the dynamics of an operational furnace using specially designed probes. The project has provided important data and confirms the accuracy of simulated models. The results will help operators to run their plants more effectively and encourage manufacturers to design and build larger units to compete with more traditional power-generation technologies.

Isometric sketch of one of the Turow 235 MW CFB boilers (Goidich and Hyppänen, 2001).
Isometric sketch of one of the Turow 235 MW CFB boilers (Goidich and Hyppänen, 2001).
Pressure is mounting to cut Europe’s greenhouse gas emissions. But although renewable energy sources certainly limit increases in CO2 emissions, Europe’s future energy needs far surpass the potential supply of ‘green electricity’. Large, central power stations fired by fossil fuels will continue to meet the bulk of our energy needs in the coming decades. But new technology will make it cleaner and more efficient than ever before.

The CFB-Combustors project has investigated one of the least understood combustion technologies. Not all combustion needs air. In a circulating fluidised bed (CFB) system, it takes place within a gas-solid suspension – the fluidised bed. The bed consists of an inert material such as silica, sand or ash, which behaves like a fluid at high temperatures. Solid fuel ignites when it is added to the bed.

This type of technology has several attractive environmental benefits. It has low NOx emissions, and limestone can be added to the bed to capture sulphurous gases. It is also possible to mix solid fuels together, allowing for the co-combustion of coal and biofuels and thus permitting biomass combustion in the existing power plant infrastructure.

Big ambitions
Yet, despite these benefits, fluidised beds have much smaller capacities than the large pulverised coal power plants – the largest furnaces have a capacity of around 250 MW. “The problem is that we do not really know what is going on inside,” says Professor Filip Johnsson, CFB-Combustors’ coordinator. “The penetration and mixing of the fuel is critical, but no one understands how this happens. These furnaces are a bit of a black box, and we have no laws for scaling up. Manufacturers can only take incremental steps – and each involves a big risk.”

The CFB-Combustors project brought together industrial and academic partners to combine their expertise and reveal the secrets of fluidised bed technology by inserting probes into a functioning furnace and collecting data under controlled conditions.

Probing questions
A probe inserted in a measurement port.
A probe inserted in a measurement port.
The probes measured gas and solids concentrations, material momentum, pressure, and the velocities of solids over the cross-section of the boiler. Some of the probes are highly innovative. A dual-pressure probe, for example, calculates the velocity of solid particles by measuring the time lag between a particle’s contact with two probe tips. “We designed the size and shape of the probes to fit between the boiler tubes and reach deep into the furnace,” Johnsson explains. “Previously we had only used these probes on laboratory units and pilot plants with built-in measurement ports. This time we had to come up with something that could be inserted into an operational boiler. Now we have a tool that can be used on any full-scale boiler and will make future in situ testing much easier.”

The experimental part of the project was carried out on a 235 MW CFB boiler at the Turow power plant in Poland. This new boiler plays a part in Poland’s modernisation of its power production system by lowering emissions and increasing thermal efficiency.

“This was a really bold step for Elektrownia Turow,” says Johnsson. “Many industrial partners are often concerned about the extra costs of R&D. Turow was willing to take the risk of allowing us to make holes in their furnace! Their reward? First-hand knowledge of the research results, which they can apply directly to their operations. They are even considering a patent for certain operational processes.”

Of course, the ‘hole punching’ was planned in the finest detail. The group spent a year drawing up its specification, agreeing where to open up ports on the boiler, developing and building the probes, and designing the testing programme.

Fuel for thought
Initial results suggest that the boiler contains two main regions. Against the wall, a region up to 50 cm wide has a higher concentration of solids and lower oxygen compared to the boiler core. Conversely, the back of the furnace has higher oxygen levels than at the front. “Results like these all have practical implications,” notes Johnsson. “Fuel fed to the ports at the rear, for example, might need to be increased in order to establish a more even distribution in gas concentrations.”

Dynamic tests looked at the behaviour of the boiler when conditions were changed – for example, when the load was altered. The results from the dynamic measurements were then compared with results from mathematical simulation models compiled by Foster Wheeler Energia OY.

Johnsson points out that the information obtained by the project forms a unique set of data which will be used in CFB boiler research for many years to come, including the verification of mathematical modelling tools.

The agreement between the simulated and actual results will help boiler operators assess control strategies and give CFB manufacturers the confidence to build bigger boilers. “Our work shows industry that it is worth conducting advanced development work,” Johnsson comments. “Traditionally the industry bases its designs on experience, rather than designs optimised through R&D. Our data shows that you can perform investigations on large systems which reveal ways to improve them. This project should encourage manufacturers to invest, and potential customers to buy.”




  • Title
    Processes in large-scale circulating fluidised bed combustors (CFB-COMBUSTORS)
  • Reference
    ENK5-CT-1999-00005
  • Programme
    FP5: Energy, Environment and Sustainable Development
  • Contact
    Professor Filip Johnsson
    Chalmers University of Technology
    Sweden
    Fax: +46 31 772 1449
    fijo@entek.chalmers.se
  • Partners
    Chalmers University of Technology, Sweden
    Elektrownia Turow (Turow Power Plant), Poland
    Foster Wheeler Energia OY, Finland
    Technical University Hamburg-Harburg, Germany
    Technical University of Czestochova, Poland
    Vattenfall Generation Services Thermal AB, Sweden
    VSB-Technical University of Ostrava, Czech Republic