Fluidized bed combustion
The fluidized bed furnace is a furnace, with the milled fuel in a fluidized bed of hot bed material can be burned (eg quartz sand).
The fuel and the bed material are combined, maintained by the addition of a fluidization medium, such as air, into the suspension and thereby fluidized. The comminuted fuel particles have a large surface so that a good burnout can occur. The strong turbulent flow has a very good momentum and heat exchange result, so that a uniform temperature prevails in the fluidized bed. The combustion temperature can be determined by the introduced fuel mass flow. The temperature is adjusted so that the formation of harmful gases (CO, NOx) is minimized. In the fluidised bed very low nitrogen oxide emissions can be met, since a relatively low combustion temperature without temperature peaks can be driven. Low combustion temperatures have the advantage that the relatively strongly bonded nitrogen molecule of the combustion air does not dissociate, whereby the formation of thermal NOx is prevented.
The bed material is a material which is chemically inert ( inert) and does not take part in the combustion. This inert material is a procedural means to meet the different needs of combustion air (oxygen ) and fluidizing gas to reconcile. The bed material usually forms the bed ash, together with, for example gravel (sand) and one to three percent by weight of fuel. For use of sand as inert material, the abrasive effect is observed, which leads to positive (cleaning of the firebox ) and negative effects ( Higher Aschmengen, grinding the slag, higher proportion of fly ash and thus to higher disposal costs ).
A distinction is made between the stationary and circulating ( atmospheric ) fluidized bed:
- At the stationary fluidized bed, the fluidized bed remains in the combustion chamber. The ash and the bed abrasion can be detected by extraction from the combustion chamber or by downstream separator. Depending on the flow velocity, a distinction between bubbling regime, Slugging Bed, turbulent regime and Fast Fluidization.
- In a circulating fluidized bed, the fluidized bed is discharged from the combustion chamber and separator ( separation from flue gas and bed material ) and siphon back out ( circulation: circulating bed material ). Circulating fluidised bed combustion (WSF ) are structurally and procedurally complex than stationary, but allow a higher power density.
In some pilot plants, the WSF is used as a pressure- charged combustion chamber in a gas turbine process. The pressure fluidized bed combustion is a special form of WSF technology, which so far industrially could not prevail due to the problem of hot gas cleaning.
Many solid fuels require treatment prior to combustion. Typical processing steps are the crushing of the fuel, the sorting of larger non-combustible impurities and drying and compacting. For this purpose are various pieces of equipment and systems for use ( for example, shredding facility with air classifier ). In the fluidized bed can be a very wide range of fuels can be used. The use of solid, liquid, sludge-like, pasty or gaseous fuels draws the fluidised bed as a multi-fuel and Mehrstofffeuerung (simultaneous combustion of different materials). Unlike a grate firing these fuels ( pseudo- liquid) can be easily mixed into the fluidized bed. A major advantage of fluidized bed combustion is thermal recovery or disposal of hazardous substances such as sewage sludge (very humid), petroleum coke ( much sulfur ), chicken manure (low density), landfill gas ( heizwertarm ) and much more.
Integrated flue gas purification and pollution prevention
In the fluidized bed combustion in coal-fired power plants, lime is added to bind the sulfur contained in the coal. This product is gypsum. The optimum of this reaction is about 850 ° C. Therefore this is the normal operating temperature of the circulating fluidized bed combustor. The resulting gypsum is present in the mixture with the fuel ash and can only be removed together. Separation of gypsum ash nor ausgenutztem lime is not possible. The resulting dry solid but can be easily disposed of, for example, in the original coal mines. Many fluidized bed power plant operators can sell the solid obtained also in the construction industry.
The emission of nitrogen oxides can be kept alone under the typical limits by the combustion control usually. Due to the low combustion temperature and a controlled formation of so-called thermal NOx is prevented from the oxidation of nitrogen contained in the fuel to NO x is controlled and minimized by the multi-stage air.
Due to the above two measures power plants can be built without complex downstream flue gas cleaning using fluidised bed typically, creating a more compact overall system is possible as compared to the conventional method with downstream power plant furnaces, flue gas cleaning for the same performance.