Circulating fluidized bed
The resultant mixing of gas and solids promotes rapid heat transfer and chemical reactions within the bed.
Extensive research has been conducted on this technology within the past 15 years due to increasing concerns over pollution caused by traditional methods of combusting coal and its sustainability.
[3] The Mercury and Air Toxic Standards (MATS) enacted in December 2011 in the United States by the Environmental Protection Agency have forced all the countries in Europe and America to strictly adhere to this policy.
In 1923, Winkler's coal gasifier represented the first significant large-scale industrial application of fluidized bed [5] (Kunii and Levenspiel, 1991).
The fluidization is a function of several parameters such as the particles’ shape, size and density, velocity of the gas, beds' geometries etc.
The gas-solid system shows large instabilities with bubbling and gas channelling with rise in flow rate beyond minimum fluidization.
In the present work, fast fluidized bed is used to operate the CFB where the pressure drop decreases dramatically in this regime.
At the same time, sulfur-absorbing chemical such as limestone or dolomite will be used to mix with the fuel particles in the fluidization phase, which will absorb almost 95% of the sulfur pollutants.
Alternatively, the sulfur absorbing chemical and fuel will be recycled to increase the efficiency of producing a higher quality steam as well as lower the emission of pollutants.
Therefore, it will be possible to use circulating fluidized bed technology to burn fuel in a much more environmental friendly method as compared to other conventional processes.
Circulating fluidized bed technology can be implemented in many different fields ranging from oil and gas to power stations.
One of the applications of a circulating fluidized bed scrubber is at power stations which utilize a dry sorbent usually Ca(OH)2 to reduce pollutants like HF, HCL, SO2 and SO3 in a flue gas stream.
[10] Currently, Basin Electric Power Cooperative are the only company operating the best available circulating fluidized bed scrubbing technology for a coal-fired boiler plant near Gillette, Wyoming since 2011.
This process is first used at the Gussing power plant in Austria[12] based on the steam gasification of biomass in the internally circulating fluidized bed.
In the gasification process, fuel will be gasified at 850 °C[12] in the presence of steam to produce a nitrogen-free and clean synthetic gas.
Wet FGD uses lot of water, however only marginal metals like mercury and acid gases such as HCl, HF, SO2 and SO3 can be captured.
At low operating temperature of CFBG, a longer residence time of solid can be achieved leading to a higher gasification yield.
This type of CFBS consists of three distinct feedback control loops which are for temperature, pressure drop and sulphur dioxide emission.
[20] Though the circulating fluidized beds are used widely, the CFD, which can be, describe by non-uniformity flow patterns and a thorough back mixing still possess significant radial gradients in the particle density and a lower solid holdup inside the riser interior compared to the wall of the reactor.
[22] This operational condition is chosen to avoid a distinct interface between the dilute region and the dense bed inside the riser.
However, the high gas velocities and the recirculation of solids may make the CFB system much more expensive in term of power requirement and investment compared to conventional fluidized bed reactors.
If the total carbon conversion efficiencies gets over 98% it shows good separation process that leaves simply a minor proportion of unburned char in the residues.
In designing a circulating fluidized bed, with constant temperature distribution for either endothermic or exothermic reactions, in order to determine the appropriate design for cooling or heating of the circulating fluidized bed reactors, a good approximation of heat transfer rates are necessary for better control so that the reactor can change its performance for different operating conditions.
In many industrial processes that involved small, porous or light particle which have to be fluidized with more viscous fluid in the present of gas, a gas–liquid–solid circulating fluidized bed (GLSCFB) is more preferred compared to conventional system because it can minimize dead zone and increase the contacting efficiency among gas, liquid and solid phases by improving the shear stress between those phases.
Gas–liquid–solid circulating fluidized bed also can provide higher gas holdup, produce more uniform bubble size, better interphase contact, and good heat and mass transfer capabilities.
[26] As for circulating fluidized bed scrubbers (CFBS), it is more preferred in industry due to its ability to produce higher purity product while avoiding the corrosion issue.
[27] Some modification is necessary at the inlet in order to eliminate loss of solids materials at the bottom of bed during low-load operation.
[17] Another major field that is currently being looked into is the further development of in-bed heat exchanger used with circulating fluidized bed technology.
