Fluidized bed combustion

The fluidized bed furnace is a furnace with which crushed fuel can be burned in a fluidized bed made of hot bed material (e.g. quartz sand ).

Procedure

The fuel and bed material are mixed together by the addition of a fluidizing medium, e.g. B. air, held in suspension and thus fluidized . The crushed fuel particles have a large surface area so that good burnout can take place. The strong turbulent flow results in very good momentum and heat exchange, so that there is a uniform temperature in the fluidized bed. The combustion temperature can be determined by the introduced fuel mass flow. The temperature is set so that the formation of harmful gases ( CO , NO _x ) is as low as possible. With fluidized bed combustion, very low nitrogen oxide emissions can be maintained, since a relatively low combustion temperature can be operated without temperature peaks. Low combustion temperatures have the advantage that the relatively strongly bound nitrogen molecule in the combustion air does not dissociate , which prevents the formation of thermal NO _x .

Bed material

The bed material is a substance that chemically inert ( inert ), and does not participate in the combustion. This inert material is a procedural means to reconcile the different requirements for combustion air (oxygen) and fluidization gas. The bed material usually forms the bed ash together with, for example, gravel (sand) and one to three percent by weight of fuel. If sand is used as an inert material, the abrasive effect must be taken into account, which leads to positive (cleaning of the combustion chamber) and negative effects (larger quantities of ash, grinding of the slag, higher proportion of fly ash and the associated higher disposal costs).

Process variants

A distinction is made between the stationary and the circulating (atmospheric) fluidized bed:

with the stationary fluidized bed, the fluidized bed remains in the combustion chamber. The ash and bed debris can be collected by removing them from the combustion chamber or by downstream separators. Depending on the flow velocity, a distinction is made between Bubbling Regime, Slugging Bed, Turbulent Regime and Fast Fluidization.

with a circulating fluidized bed, the fluidized bed is discharged from the combustion chamber and returned via a separator (separation of flue gas and bed material) and siphon (circuit: circulating bed material). Circulating fluidized bed furnaces (WSF) are more complex in terms of construction and process technology than stationary ones, but allow a higher power density.

In some pilot plants, the WSF is used as a pressurized combustion chamber in a gas turbine process. The pressurized fluidized bed combustion is a special form of WSF technology that has not yet been able to establish itself industrially due to the problem of hot gas cleaning.

fuel

Many solid fuels require conditioning prior to combustion. Typical processing steps are the crushing of the fuel, the sorting out of larger non-flammable foreign substances as well as drying and compacting . Various devices and systems are used for this purpose ( e.g. shredder system with wind sifter ). A very wide range of fuels can be used with fluidized bed combustion. The use of solid, liquid, sludge-like, pasty or gaseous fuels distinguishes fluidized bed combustion as multi-fuel and multi-fuel combustion (simultaneous combustion of different substances). In contrast to grate firing, these fuels can simply be mixed into the fluidized bed (pseudo-liquid). A great advantage of the fluidized bed combustion is the thermal recovery or removal of problematic substances such. B. sewage sludge (very moist), petroleum coke (a lot of sulfur), chicken manure (low density), landfill gas (low calorific value) and much more.

Advantages disadvantages

The advantages of combustion in the fluidized bed can be summarized as follows:

High heat transfer rates on the heating surfaces

Low combustion temperature and therefore no thermal NO _x formation

Temperatures of approx. 850 ° C allow direct desulphurization of the flue gases, i.e. a reduction in SO ₂ emissions by adding limestone

Possibility of burning ash-rich and poorly igniting coal.

Are disadvantageous

the relatively high wear of the heating surfaces due to the erosive effect of the bed material

the high internal demand, especially with circulating fluidized bed combustion

poor partial load behavior

Due to its special characteristics, fluidized bed combustion has established itself particularly in the combustion of inferior fuels in small and medium power sizes and in the implementation of combined gas and steam turbine processes with pressure combustion.

Integrated flue gas cleaning and avoidance of pollutants

In fluidized bed combustion in coal-fired power plants , lime is added to bind the sulfur contained in the coal . This creates plaster of paris . The optimum for this reaction is around 850 ° C. This is therefore the normal operating temperature for circulating fluidized bed combustion. The resulting gypsum is mixed with the fuel ash and can only be removed together. A separation of plaster of paris, ashes and also unused lime is not possible. The resulting dry solid can very easily be dumped, e.g. B. in the original coal mines. Many fluid bed power plant operators can also sell the solid matter to the construction industry.

The emission of nitrogen oxides can usually be kept below the typical limit values through the combustion control alone. The low and controlled combustion temperature prevents the formation of so-called thermal NO _x ; the oxidation of the nitrogen contained in the fuel to NO _x is controlled and minimized by the multi-stage air supply.

By means of the two measures mentioned above, power plants with fluidized bed combustion can typically be built without complex downstream flue gas cleaning , which enables a more compact overall system with the same output compared to the method of conventional power plant combustion with downstream flue gas cleaning.