Fluidized bed combustion (FBC) is a combustion technology used in power plants. This technology allows greater flexibility in the use of fuel: coal, biomass, garbage, etc., in addition to a better use of the fuel and a better transfer of the heat produced during combustion. A fluid bed is made up of the fuel in pieces (approximately 2-5% of the total weight of the bed) and the bed itself (ash, limestone, additional material,…). Bed melting is not allowed, so the temperature is limited to 850-900ºC. The fluidized bed supports the solid fuel while air is pumped upward during combustion. The result is the formation of eddies that favor the mixture of gas and fuel.
The concept of fluidized bed combustion arose in the early 1960s in the United States and the United Kingdom, but it did not attract the interest of electricity companies, mainly due to the high cost and the insignificance of polluting emissions at that time. In 1968, an experimental plant was built in the United Kingdom where the technology was improved and the possibilities offered by this type of plant were tested. However, the British government was not very interested in the plant, so most of the studies were financed by Swedish companies. The first plant to supply electricity to the grid was built in the early 1970s, it was a small coal-fired plant in Rivesville, West Virginia. After many economic and technical problems, due to the high maintenance cost of the plant, the plant was closed at the end of that same decade. Other pioneer plants also experienced many problems with erosion, air entrainment, and coal feeding. In the mid-70s, the construction of plants began in the United States and the United Kingdom that finally managed to prove their effectiveness and benefits, some of these plants are even in operation. At the same time, at the beginning of the 80s, a great development and construction of plants in Germany and the Scandinavian countries took place, which served to improve the existing technology as well as the creation of new concepts. Currently, fluidized bed combustion is experiencing a boom, due to the better use of coal and the lower emission of polluting gases.
Types and characteristics
The different types of fluidized beds are classified according to the parameter R that indicates the amount of material that recirculates between the material that is introduced into the bed. If: 0<R<1 Stationary fluid bed 1<R<20 Circulating fluid bed.
Stationary fluid bed
Bed velocity: 1-2 m/s Layer height: 1-1.5 m Q= 1.2-1.6 MW/m^2 Advantages: Lower combustion temperature, good heat transfer (surface of metal to favor the transmission of heat within the bed), internal desulphurization by means of limestone, the formation of other polluting gases such as halogens is avoided, it allows the burning of coals rich in ash and fuels that are difficult to ignite (for example garbage). Disadvantages: High own consumption and household losses, poor response to partial load work, cold start is complicated, technically and economically expensive, the exchange surface is susceptible to erosion, the NOx emission limit is frequently exceeded.
Circulating fluid bed
Comparison with stationary fluid bed: More technical complexity, that is, more components: recirculation cyclone, there is no heat exchanger inside the bed, gas velocity greater than the particle fall velocity (5-6 m/s), better solids mixing (better desulfurization, less lime consumption), constant temperature throughout the cycle, less losses in the home, lower NOx emissions (combustion is staggered).