Installations for biofuel combustion. Fluidized bed boilers. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Alternative energy sources

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When fuel is burned in furnaces with high temperature, a large amount of nitrogen dioxide NO₂. It is a highly toxic brown gas with a suffocating odor. When the temperature in the furnace drops to 1000°C and below, almost no nitrogen dioxide is formed. This is achieved by burning solid fuels (hard and brown coal, oil shale, wood waste, household waste, etc.) in a fluidized (pseudo-liquefied) bed.

Fine-grained combustible material placed on the furnace grate is blown from below with air at a speed exceeding the stability limit of the dense layer. The "liquefaction rate" depends on the density of the combustible material and on the size of the particles, usually it is in the range of 0,9 ... 2,3 m / s. Possible states of the "gas - fuel particles" system are shown in fig. 5.1.

Fig.5.1. Fluidized bed furnaces: 1 - layer combustion; 2 - fluidized bed with bubbles; 3 - transition to pneumatic transport; 4 - circulating fluidized bed

When the purge rate is less than the critical one (Scheme 1), the fuel particles lie in a dense layer. This is an ordinary layer firebox. In the fluidized bed furnace (scheme 2), primary air is supplied from below, the crushed fuel is introduced through the nozzle on the left. If the liquefaction rate is exceeded, an intensive circulation of particles occurs in the layer, resembling the nucleate boiling of a liquid. Part of the air passes through such a layer in the form of bubbles, which further mix the combustible material. Particles circulate in the volume of the layer until they are completely burned out. Secondary air is introduced through the right pipe to burn the particles carried out of the bed.

With a further increase in the blowing speed (Scheme 3), an increasing number of small particles are removed from the fluidized bed, and a transition to the pneumatic transport mode occurs. In this case, it is advisable to use the separation of unburned fuel (usually in cyclone gas cleaning apparatuses) with the return of the separated particles to the fluidized bed (Scheme 4). This mode of operation of the furnace is called "circulating fluidized bed".

Pipes with a heated coolant are placed inside the layer, which removes the generated heat. Contact with moving burning fuel particles leads to a significant intensification of heat transfer. Due to the relatively low combustion temperature in fluidized bed furnaces, the ash does not melt and therefore does not stick to the heat exchange surfaces of the boiler. As a result, the cost of cleaning the heat exchange surfaces is reduced.

The fluidized bed consists of 90% or more of ash particles or specially added inert material (limestone, dolomite, fireclay chips). Therefore, materials with a very high ash content can be burned in a fluidized bed. The introduction of limestone makes it possible to drastically reduce emissions of sulfur dioxide SO₂ - this toxic gas turns into harmless gypsum CaSO₄removed from the furnace along with the ash. An increase in the fuel concentration in the layer would lead to the appearance of hydrogen H in the combustion products₂ and carbon monoxide CO.

Fluidized bed furnaces are widely used in industry for roasting various ores, pyrites in sulfuric acid production, etc. In Finland and Sweden, fluidized bed boilers are widely used for the disposal of forest industry waste (wood chips, bark, sawdust) and for the combustion of milled and sod peat. In the USA, England, France, fluidized bed boilers are increasingly being used at thermal power plants. In the USA, the capacity of fluidized bed boilers has reached 200 MW. On fig. 5.2 shows a diagram of a furnace with a circulating fluidized bed from BDC (USA).

Fig.5.2. BDC fluidized bed furnace: 1 - primary air; 2 - limestone; 3 - fuel; 4 - secondary air; 5 - furnace chamber; 6 - cyclone; 7- water supply; 8 - air

Primary air is supplied to the branch pipe 1, crushed limestone is supplied through the screw feeder 2, and fuel is supplied through the branch pipe 3. In volume 5, the combustion of suspended fuel particles continues. In cyclone 6, exhaust gases are cleaned. The trapped particles burn out in the bunker, which contains coiled heat exchangers that heat the boiler feed water. Additional air for afterburning of solid entrainment is supplied through pipe 8.

The use of fluidized bed furnaces makes it possible to utilize a huge amount of coal waste accumulated in waste heaps near coal mines and processing plants. Waste rock contains a significant amount of unused solid fuel, the spontaneous combustion of which leads to air pollution with smoke, sulfur and nitrogen oxides.

Author: Labeish V.G.

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