What is a Fluidised Bed Boiler?

[norske]

Hi all,

What is a Fluidised Bed Boiler? What general advantages does this have?

Many thanks in advance.

 

[davefitz]

There are 2 generic types, and many small commerical differences between different vendors.

The FBB can be designed as a bubbling bed boiler or a circulating fluid bed boiler. The bubbling bed boiler uses a relatively low gas velocity relative to the bed particle size's terminal velcoity ( saltation velocity) such that most particles are not entrained and carried over past the furnace enclosure. In a circulating fluid bed boiler, the gas velocity is higher than the velocity needed to entrain most particles ( saltation velocity), so most particles leave the furnace and must be recaptured in a cyclone and reinjected to the bed for complete comustion.

The temperature used for bed combustion is relatively low, about 1600 F ( compared to 2600F + in a pulverized fired furnace). This helps lower the rate of formation of NOx and wet slag. The "bed material" is usually either a non reactive material ( such as rounded riverbed sand) or a reactive material ( limestone) to absorb SO2. The bed matieral has a huge thermal inertia, and this aids in sustaining combustion with hard to ignite fuels. Some fuels which cannot be burned in a radiant furnace can be easily burned in a fluid bed boiler. Many such fuels can be obtained at low cost, or no cost, thus the operating economics are good in some special cases.

The original purposes was to burn low cost, hard to burn fuels in an environmentlaly benign manner. The improved environmental performance of modern wet scrubbbers and growing trend to reduce greenhouse gases ( CO2 and N2O) has reduced the competitiveness of the fluid bed boiler compared to a large supercritical pulverized coal unit that is burning normal coals .

At low loads the fluid bed boiler emits large amounts of N2O ( nitrous oxides, laughing gas). It has a high aux power requirement to drive the PA fans, equivalent tot he aux power needed to operate the coal mills on a pulverized coal unit.

 

[qalab]

hi,
fbc boiler are the ones which use the property similiar to boiling water. when a jet of air is passed through a layer of sand, the sand starts bubbling like boiling water. this technic is used for the combustion of soild fuels like coal,wood, husk etc.