The circulating fluidized bed (CFB) is a type of fluidized bed combustion that utilizes a recirculating loop for even greater efficiency of combustion. while achieving lower emission of pollutants. Reports suggest that up to 95% of pollutants Circulating Fluidized Bed Technology, The Circulating Fluidized Bed Technology, 2010, Innovation and Information for sustainable living can be absorbed before being emitted into the atmosphere. The technology is limited in scale however, due to its extensive use of limestone, and the fact that it produces waste byproducts.
Introduction Fluidization is the phenomenon by which solid particles are transported into a fluid-like state through suspension in a gas or liquid. The resultant mixing of gas and solids promotes rapid heat transfer and chemical reactions within the bed. Power plants that use this technology are capable of burning low grade fuels at high efficiency and without the need for expensive fuel preparation. They are also smaller than the equivalent conventional furnace, so may offer significant advantages in terms of cost and flexibility.
Circulating fluidized bed is a relatively new technology with the ability to achieve lower emission of pollutants. 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. The importance of this technology has grown recently because of tightened environmental regulations for pollutant emission.
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. This means that emissions such as metals, acid gases, organic compound, flue gas acids and other pollutants from power plants or industrial facilities have to meet the requirements set by EPA Mercury and Air Toxic Standards (MATS), Mercury and Air Toxic Standards (MATS), 2013, United States Environmental Protection Agency. and upgrades have to be done for facilities that do not meet the standards. As a result, the demand for circulating fluidized bed technology is predicted to rise.
In 1923, Winkler's coal gasifier represented the first significant large-scale industrial application of fluidized bed (Kunii and Levenspiel, 1991). CFB combustion technology continues to grow strongly in large utility power plant applications as CFB boiler technology has grown from small-scale industrial applications to large ultra-supercritical power plants in less than 20 years. Prime examples, both provided by Sumitomo SHI FW are the 460 MW supercritical CFB power plant operating since 2009 in Lagisza, Poland, and 2200 MW ultrasupercritical Samcheok (Korea) Green Power Plant successfully running since 2016.
Fluidization regimes and classification Fluidization is the phenomenon by which solid particles are transported into a fluid like state through suspension in a gas or liquid. In fact, there is a simple and precise way to classify the various fluid-particle beds (Winaya et al., 2003; Souza-Santos, 2004; Basu, 2006). Most of the CFB operating and environmental characteristics are the direct results of the hydrodynamic behaviour. Numerous researchers have studied the hydrodynamics of CFB (Yang, 1998; Basu, 2006; Rhodes, 2008; Scala, 2013). The fluidization is a function of several parameters such as the particles’ shape, size and density, velocity of the gas, beds' geometries etc. Kunii and Levenspiel (1991), Oka and Dekker (2004), and Souza-Santos (2004) defined the regimes of fluidization as described below:
(a) Fixed Bed: When the fluid is passed through the bottom of the bed at a low flow rate, the fluid merely percolates through the void spaces between stationary particles.
(b) Minimum fluidization: When the gas velocity reaches (Umf) minimum fluidization velocity, and all the particles are just suspended by the upward flowing fluid.
(c) Bubbling Fluid Bed: When the flow rate increases beyond the minimum fluidization velocity, bed starts bubbling. The gas-solid system shows large instabilities with bubbling and gas channelling with rise in flow rate beyond minimum fluidization. Such a bed is called aggregative, heterogeneous, or bubbling fluidized.
(d) Turbulent Fluidized Bed: When the gas flow rate sufficiently increases, the terminal velocity (Utr) of solids is exceeded, the upper surface of the bed disappears, entrainment becomes appreciable instead of bubbling,
(e) Fast Fluidized Bed: With further increasing in gas velocity, solids are carried out of the bed with the gas making a lean phase fluidized, this regime is used for operating CFB. In the present work, fast fluidized bed is used to operate the CFB where the pressure drop decreases dramatically in this regime.
(f) Pneumatic Transport: Beyond the circulating fluidized bed operating regime, there is the pneumatic transport region, pressure drop increases in this regime.
An appreciated contribution by Geldart (1973) classified the particles based on size and density into four groups viz. C, A, B, and D. Group B (of particle size dp between 40–500 μm and density of ρs<~1400 kg/m3) is commonly used for CFB. Yang modified Geldart's classification using Archimedes number Ar, under elevated pressure, temperature, and non-dimensional density (Yang, 2007).
Pressure and Pressure Drop The flow in a CFB is multiphase. The unrecoverable pressure drop along the riser height is a basic value for design; and this results due to solid particles distribution, voidage, gas viscosity, gas velocity, gas density, and density of solid.
Basis of technology During the combustion phase, upwards jets of air will cause the solid fuels to be suspended. This is to ensure the gas and solids will mix together turbulently for better heat transfer and chemical reactions. The fuel will be burnt at a temperature of 1400 °F (760 °C) to 1700 °F(926.7 °C) to prevent nitrogen oxide from forming. While burning, flue gas such as sulfur dioxide will be released. 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.
Range of applications Circulating fluidized bed technology can be implemented in many different fields ranging from oil and gas to power stations. This technology is highly sought after due to its numerous benefits. Some of the popular applications of circulating fluidized bed are circulating fluidized bed scrubber and circulating fluidized bed gasification system.
Circulating fluidized bed scrubber 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.Fluid Bed Scrubber, Dustex, 2012, The Dustex Advantage. Fluid Bed Scrubber. Dustex Corporation, Georgia. 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.Operating Experience of Circulating Fluidized Bed Scrubbing Technology in Utility Size Power Plants and Refineries, Bonsel, 2007, Operating Experience of Circulating Fluidized Bed Scrubbing Technology in Utility Size Power Plants and Refineries, pp. 1 – 12—
The three major components of the circulating fluidized bed scrubber in power plants are: Circulating fluidized bed absorber Fabric filter Dry lime hydration system.
In the circulating fluidized bed scrubber process, flue gas will enter the reactor from the bottom of the vessel. Simultaneously, hydrated lime will be injected into the circulating fluidized bed absorber for reaction to take place to convert SO2 and SO3 from the flue gas to calcium sulfate and calcium sulfite. Water will also be injected at the same time to control the operation temperature for maximum absorption capacity. The flue gas will then send to the bag house for further filtration. In the bag house, a series of air valves across the filters, will produce compressed air bursts to ensure a more efficient solid and dust collection. Lastly, clean flue gas will then be directed to the stack with the minimum pollutants in the flue gas stream. The schematic diagram of the process is shown in Figure 1.
Circulating fluidized bed gasification system Gasification is the process of converting biodegradable waste materials into synthetic gas without combustion. This process is first used at the Gussing power plant in AustriaThe Importance of Fluid Bed Gasification Technology, The Importance of Fluid Bed Gasification Technology. Foster Wheeler North America, pp. 1–9. based on the steam gasification of biomass in the internally circulating fluidized bed.
In the gasification process, fuel will be gasified at 850 °C in the presence of steam to produce a nitrogen-free and clean synthetic gas. Charcoal will be burnt with air in the combustion chamber to provide the heating for the gasification process as it is an endothermic process. Thermal transfer will take place between the gasification and combustion chamber. The illustrated gasification process is presented in Figure 2.
The chemical reaction that takes place in the gasification as shown in equation and whereas the reaction in combustion chamber is represent in equation .
Gasification;
C + H2O = CO + H2
C + CO2 = 2CO
Combustion;
C + O2 = CO2
Dolomite lime or limestone can also be used to increase the hydrogen concentration by absorbing carbon dioxide to increase the combustion process.
Advantages and limitations Wet flue gas desulfurization (Wet FGD) has typically been used to capture the pollutants gas. However, this machinery is expensive, hard to maintain and takes a lot of space in power plant. 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.Application of Circulating Fluidized Bed Scrubbing Technology for Multi-Pollutant Removal, Hosrt Hack, Robert Giglio and Rolf Garf, 2013, Application of Circulating Fluidized Bed Scrubbing Technology for Multi-Pollutant Removal, pp. 1–11
The use of CFB's and dry scrubbers in the Virginia City Hybrid Energy Center allows it to capture up to 99.6% of the SO2 emitted.
The new technology of circulating fluidized bed scrubber (CFBS) was introduced circa 1984. The turbulator wall design will ensure a perfect mixing and the ability to capture various pollutants. The used of alloy metals had been replaced with a carbon steel design, reducing the installation cost. It also comes in a compact size thus the capital costs could be reduced. The water usage can also be reduced with the design of plug-free water spray nozzles.Circulating Fluidized Bed, M. Usman. Circulating Fluidized Bed. University of Engineering & Technology, Lahore The CFBS can undergo a self-cleaning process, reducing the cost of maintenance. The operating temperature is lower thus the production of the nitrogen oxides, a contributor to smog, is lower.Flue Gas Desulfurization Technology Evaluation Dry Lime vs Wet Limestone FGD, Sargent and Lundy, 2007, Flue Gas Desulfurization Technology Evaluation Dry Lime vs Wet Limestone FGD, pp. 1–57