FLUIDIZED BED COMBUSTION

Fluidized bed combustion (FBC) is a combustion technology used in power plants. FBC plants are more flexible than conventional plants in that they can be fired on coal and biomass, among other fuels. Fluidized beds suspend solid fuels on upward-blowing jets of air during the combustion process. The result is a turbulent mixing of gas and solids. The tumbling action, much like a bubbling fluid, provides more effective chemical reactions and heat transfer.

FBC reduces the amount of sulfur emitted in the form of SOx emissions. Limestone is used to precipitate out sulfate during combustion, which also allows more efficient heat transfer from the boiler to the apparatus used to capture the heat energy (usually water tubes). The heated precipitate coming in direct contact with the tubes (heating by conduction) increases the efficiency. Since this allows coal plants to burn at cooler temperatures, less NOx is also emitted. However, burning at low temperatures also causes increased polycyclic aromatic hydrocarbon emissions. FBC boilers can burn fuels other than coal, and the lower temperatures of combustion (800 °C / 1500 °F ) have other added benefits as well.

Benefits

There are two reasons for the rapid increase of fluidized bed combustion (FBC) in combustors. First, the liberty of choice in respect of fuels in general, not only the possibility of using fuels which are difficult to burn using other technologies, is an important advantage of fluidized bed combustion. The second reason, which has become increasingly important, is the possibility of achieving, during combustion, a low emission of nitric oxides and the possibility of removing sulphur in a simple manner by using limestone as bed material.

Fluidized-bed combustion evolved from efforts to find a combustion process able to control pollutant emissions without external emission controls (such as scrubbers). The technology burns fuel at temperatures of 1,400 to 1,700 °F (750-900 °C), well below the threshold where nitrogen oxides form (at approximately 2,500 °F / 1400 °C, the nitrogen and oxygen atoms in the combustion air combine to form nitrogen oxide pollutants). The mixing action of the fluidized bed results brings the flue gases into contact with a sulfur-absorbing chemical, such as limestone or dolomite. More than 95% of the sulfur pollutants in coal can be captured inside the boiler by the sorbent. The reductions may be less substantial than they seem, however, as they coincide with increases in carbon dioxide and polycyclic aromatic hydrocarbons emissions.

Commercial FBC units operate at competitive efficiencies, cost less than today's units, and have NOx and SO2 emissions below levels mandated by Federal standards.

Types

FBC systems fit into essentially two major groups, atmospheric systems (FBC) and pressurized systems (PFBC), and two minor subgroups, bubbling or circulating fluidized bed (BFB or CFB).

PFBC

The first-generation PFBC system also uses a sorbent and jets of air to suspend the mixture of sorbent and burning coal during combustion. However, these systems operate at elevated pressures and produce a high-pressure gas stream at temperatures that can drive a gas turbine. Steam generated from the heat in the fluidized bed is sent to a steam turbine, creating a highly efficient combined cycle system.

Advanced PFBC

• A 1-1/2 generation PFBC system increases the gas turbine firing temperature by using natural gas in addition to the vitiated air from the PFB combustor. This mixture is burned in a topping combustor to provide higher inlet temperatures for greater combined cycle efficiency. However, this uses natural gas, usually a higher priced fuel than coal.

• APFBC. In more advanced second-generation PFBC systems, a pressurized carbonizer is incorporated to process the feed coal into fuel gas and char. The PFBC burns the char to produce steam and to heat combustion air for the gas

 

 

turbine. The fuel gas from the carbonizer burns in a topping combustor linked to a gas turbine, heating the gases to the combustion turbine's rated firing temperature. Heat is recovered from the gas turbine exhaust in order to produce steam, which is used to drive a conventional steam turbine, resulting in a higher overall efficiency for the combined cycle power output. These systems are also called APFBC, or advanced circulating pressurized fluidized-bed combustion combined cycle systems. An APFBC system is entirely coal-fueled.

• GFBCC. Gasification fluidized-bed combustion combined cycle systems, GFBCC, have a pressurized circulating fluidized-bed (PCFB) partial gasifier feeding fuel syngas to the gas turbine topping combustor. The gas turbine exhaust supplies combustion air for the atmospheric circulating fluidized-bed combustor that burns the char from the PCFB partial gasifier.

• CHIPPS. A CHIPPS system is similar, but uses a furnace instead of an atmospheric fluidized-bed combustor. It also has gas turbine air preheater tubes to increase gas turbine cycle efficiency. CHIPPS stands for combustion-based high performance power system.

Fluidized bed

A fluidized bed is formed when a quantity of a solid particulate substance (usually present in a holding vessel) is placed under appropriate conditions to cause the solid/fluid mixture to behave as a fluid. This is usually achieved by the introduction of pressurized fluid through the particulate medium. This results in the medium then having many properties and characteristics of normal fluids; such as the ability to free-flow under gravity, or to be pumped using fluid type technologies.

The resulting phenomenon is called fluidization. Fluidized beds are used for several purposes, such as fluidized bed reactors (types of chemical reactors), fluid catalytic cracking, fluidized bed combustion, heat or mass transfer or interface modification, such as applying a coating onto solid items.

Properties of fluidized beds

A fluidized bed consists of fluid-solid mixture that exhibits fluid-like properties. As such, the upper surface of the bed is relatively horizontal, which is analogous to hydrostatic behavior. The bed can be considered to be an inhomogeneous mixture of fluid and solid that can be represented by a single bulk density.

Furthermore, an object with a higher density than the bed will sink, whereas an object with a lower density than the bed will float, thus the bed can be considered to exhibit the fluid behavior expected of Archimedes' principle. As the "density" of the (actually the solid volume fraction of the suspension) of the bed can be altered by changing the fluid fraction, objects with different densities comparative to the bed can, by altering either the fluid or solid fraction, be caused to sink or float.

In fluidized beds, the contact of the solid particles with the fluidization medium (a gas or a liquid) is greatly enhanced when compared to packed beds. This behavior in fluidized combustion beds enables good thermal transport inside the system and good heat transfer between the bed and its container Similarly to the good heat transfer, which enables thermal uniformity analogous to that of a well mixed gas, the bed can have a significant heat-capacity whilst maintaining a homogeneous temperature field.

Application

Fluidized beds are used as a technical process which has the ability to promote high levels of contact between gasses and solids. In a fluidized bed a characteristic set of basic properties can be utilized, indispensable to modern process and chemical engineering, these properties include:

• Extremely high surface area contact between fluid and solid per unit bed volume • High relative velocities between the fluid and the dispersed solid phase. • High levels of intermixing of the particulate phase. • Frequent particle-particle and particle-wall collisions.

 

 

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When the packed bed has a fluid passed over it, the pressure drop of the fluid is approximately proportional to the fluid's superficial velocity. In order to transition from a packed bed to a fluidized condition, the gas velocity is continually raised. For a free-standing bed there will exists point, known as the minimum or incipient fluidization point, whereby the bed's mass is suspended directly by the flow of the fluid stream. The corresponding fluid velocity, known as the "minimum fluidization velocity", umf.

Beyond the minimum fluidization velocity ( ), the bed material will be suspended by the gas-stream and further increases in the velocity will have a reduced effect on the pressure, owing to sufficient percolation of the gas flow. Thus the pressure drop from for u > umf is relatively constant.

At the base of the vessel the apparent pressure drop multiplied by the cross-section area of the bed can be equated to the force of the weight of the solid particles (less the buoyancy of the solid in the fluid).

∆pw = Hw(1 − εw)(ρs − ρf)g

Geldart Groupings

Dr. Geldart Schüttgüter proposed the grouping of powders in to four so-called "Geldart Groups". The groups are defined by their locations on a diagram of solid-fluid density difference and particle size. Design methods for fluidized beds can be tailored based upon the particle's Geldart grouping.

Group A For this group the particle size is between 20 and 100 um, and the particle density is typically 1400kg/m3. Prior to the initiation of a bubbling bed phase, beds from these particles will expand by a factor of 2 to 3 at incipient fluidization, due to a decreased bulk density. Most powder-catalyzed beds utilize this group.

Group B The particle size lies between 40 and 500 um and the particle density between 1400 and 4500 kg/m3. Bubbling typically forms directly at incipient fluidization.

Group C This group contains extremely fine and subsequently the most cohesive particles. With a size of 20 to 30 um, these particles fluidize under very difficult to achieve conditions, and may require the application of an external force, such as mechanical agitation.

Group D The particles in this region are above 600 um and typically have high particle densities. Fluidization of this group requires very high fluid energies and is typically associated with high levels of abrasion.

FLUIDIZED BED REACTOR

A fluidized bed reactor (FBR) is a type of reactor device that can be used to carry out a variety of multiphase chemical reactions. In this type of reactor, a fluid (gas or liquid) is passed through a granular solid material (usually a catalyst possibly shaped as tiny spheres) at high enough velocities to suspend the solid and cause it to behave as though it were a fluid. This process, known as fluidization, imparts many important advantages to the FBR. As a result, the fluidized bed reactor is now used in many industrial applications.

 

 

Basic principles

The solid substrate (the catalytic material upon which chemical species react) material in the fluidized bed reactor is typically supported by a porous plate, known as a distributor. The fluid is then forced through the distributor up through the solid material. At lower fluid velocities, the solids remain in place as the fluid passes through the voids in the material. This is known as a packed bed reactor. As the fluid velocity is increased, the reactor will reach a stage where the force of the fluid on the solids is enough to balance the weight of the solid material. This stage is known as incipient fluidization and occurs at this minimum fluidization velocity. Once this minimum velocity is surpassed, the contents of the reactor bed begin to expand and swirl around much like an agitated tank or boiling pot of water. The reactor is now a fluidized bed. Depending on the operating conditions and properties of solid phase various flow regimes can be observed in this reactor.

History and current uses

Fluidized bed reactors are a relatively new tool in the industrial engineering field. The first fluidized bed gas generator was developed by Fritz Winkler in Germany in the 1920s. One of the first United States fluidized bed reactors used in the petroleum industry was the Catalytic Cracking Unit, created in Baton Rouge, LA in 1942 by the Standard Oil Company of New Jersey (now ExxonMobil). This FBR and the many to follow were developed for the oil and petrochemical industries. Here catalysts were used to reduce petroleum to simpler compounds through a process known as cracking. The invention of this technology made it possible to significantly increase the production of various fuels in the United States.

Today fluidized bed reactors are still used to produce gasoline and other fuels, along with many other chemicals. Many industrially produced polymers are made using FBR technology, such as rubber, vinyl chloride, polyethylene, and styrenes. Various utilities also use FBR’s for coal gasification, nuclear power plants, and water and waste treatment settings. Used in these applications, fluidized bed reactors allow for a cleaner, more efficient process than previous standard reactor technologies.

Advantages

The increase in fluidized bed reactor use in today’s industrial world is largely due to the inherent advantages of the technology.

• Uniform Particle Mixing: Due to the intrinsic fluid-like behavior of the solid material, fluidized beds do not experience poor mixing as in packed beds. This complete mixing allows for a uniform product that can often be hard to achieve in other reactor designs. The elimination of radial and axial concentration gradients also allows for better fluid-solid contact, which is essential for reaction efficiency and quality.

 

 

• Uniform Temperature Gradients: Many chemical reactions produce or require the addition of heat. Local hot or cold spots within the reaction bed, often a problem in packed beds, are avoided in a fluidized situation such as an FBR. In other reactor types, these local temperature differences, especially hotspots, can result in product degradation. Thus FBRs are well suited to exothermic reactions. Researchers have also learned that the bed-to-surface heat transfer coefficients for FBRs are high.

• Ability to Operate Reactor in Continuous State: The fluidized bed nature of these reactors allows for the ability to continuously withdraw product and introduce new reactants into the reaction vessel. Operating at a continuous process state allows manufacturers to produce their various products more efficiently due to the removal of startup conditions in batch processes.

Disadvantages

As in any design, the fluidized bed reactor does have it draw-backs, which any reactor designer must take into consideration.

• Increased Reactor Vessel Size: Because of the expansion of the bed materials in the reactor, a larger vessel is often required than that for a packed bed reactor. This larger vessel means that more must be spent on initial startup costs.

• Pumping Requirements and Pressure Drop: The requirement for the fluid to suspend the solid material necessitates that a higher fluid velocity is attained in the reactor. In order to achieve this, more pumping power and thus higher energy costs are needed. In addition, the pressure drop associated with deep beds also requires additional pumping power.

• Particle Entrainment: The high gas velocities present in this style of reactor often result in fine particles becoming entrained in the fluid. These captured particles are then carried out of the reactor with the fluid, where they must be separated. This can be a very difficult and expensive problem to address depending on the design and function of the reactor. This may often continue to be a problem even with other entrainment reducing technologies.

• Lack of Current Understanding: Current understanding of the actual behavior of the materials in a fluidized bed is rather limited. It is very difficult to predict and calculate the complex mass and heat flows within the bed. Due to this lack of understanding, a pilot plant for new processes is required. Even with pilot plants, the scale-up can be very difficult and may not reflect what was experienced in the pilot trial.

• Erosion of Internal Components: The fluid-like behavior of the fine solid particles within the bed eventually results in the wear of the reactor vessel. This can require expensive maintenance and upkeep for the reaction vessel and pipes.

Current research and trends

Due to the advantages of fluidized bed reactors, a large amount of research is devoted to this technology. Most current research aims to quantify and explain the behavior of the phase interactions in the bed. Specific research topics include particle size distributions, various transfer coefficients, phase interactions, velocity and pressure effects, and computer modeling. The aim of this research is to produce more accurate models of the inner movements and phenomena of the bed. This will enable scientists and engineers to design better, more efficient reactors that may effectively deal with the current disadvantages of the technology and expand the range of FBR use

Sources: Wikipedia

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