Fluidized Bed Combustion and Coal Gasification
Fluidized bed power plants offer an alternative to the pulverized coal-fired plant. The fluidized bed design is especially useful for smaller plant sizes and for burning low quality fuels. The earliest fluidized bed was the bubbling bed, developed by Fritz Winkler. A later design, the circulating fluidized bed, superseded this as the most popular type for power generation. A pressurized version of the bubbling bed reactor has also been developed. This produces high pressure, hot flue gases that can be used to drive a gas turbine as well as using the reactor to raise steam for a steam turbine. Fluidized beds have the advantage of being able to capture sulphur within the bed, with the addition of limestone, thereby reducing sulphur dioxide emissions from the boiler. Another way of using coal to generate electrical power is to gasify it, turning it into a gas that can be burnt to provide energy. The most popular way of carrying this out is in an integrated gasification combined cycle power plant that uses both gas and steam turbines to extract energy from the gasified coal.
Keywords
fluidized bed; bubbling bed reactor; circulating fluidized bed; CFB; pressurized fluidized bed; sulphur capture; limestone; coal gasification; fixed bed gasifier; fluidized bed gasifier; water shift reaction; synthesis gas; integrated gasification combined cycle power plant; IGCC
The pulverized coal power plant is the most efficient large-scale coal-based generating plant but it is most effective at a very large scale with units of up to 700 MW in size and single plant capacities of 2000 MW and more. There are alternatives which can be more cost-effective at smaller scale. One of the main alternatives is the fluidized bed combustion plant.
This type of plant can burn a wider range of fuels than a pulverized coal plant, fuels including poor-quality coals, waste materials, and biomass, as well as high-quality coals. It operates at a lower temperature than a pulverized coal plant, with a typical combustion temperature of 1000°C, so the generation of nitrogen oxides is much less of a problem. In addition, it is possible to incorporate sulfur capture within the fluidized bed itself rather than in a separate chemical separation plant that scrubs the exhaust gases, a significant simplification of the overall plant. Further, fluidized bed combustion provides a more uniform temperature in the combustion zone, making heat capture and transfer possible with smaller capture surfaces and this can, in principle, reduce the overall size and cost for a given heat output.
The principle of the fluidized bed is simple and elegant. The fluidized bed is a bed or layer of small solid particles, normally with diameters of less than 6 mm in the case of a power plant – although they can be larger in other applications – held initially at the bottom of a combustion chamber. In operation, air is blown into the chamber through ports in its floor. This air has sufficient velocity that as it passes up through the particles it entrains them so that they become suspended above the bottom of the combustion chamber floor. How far they are suspended above the floor depends on the air velocity. This suspension of solid particles now displays many of the properties of a fluid, so that the particles move and jostle one another as would molecules in a liquid or gas. The small particles also have a large surface area. These two factors allow solid state reactions between particles of different reactants to take place quickly as well as reactions between the solid particles and the gas phase in which they are suspended. Both aspects are important for the operation of fluidized beds.
The person normally credited with the construction of the first fluidized bed is the German chemist Fritz Winkler. While working for the company BASF, Winkler was trying to improve on the production of synthesis gas (a mixture of mainly hydrogen and carbon dioxide produced from carbon-containing fuels such as coal) for the production of ammonia. In 1924 he developed the principle of the fluidized bed technique and applied it to the coking of fine-grain lignite, generating an excellent combustible gas according to reports from the time.1
The technique was not initially applied to energy production but it was used in a range of industrial processes. As well as synthesis gas production, BASF developed it as a means of roasting pyrites in order to make sulfuric acid and it was used for other chemical and metallurgical processes. Today the fluidized bed technique is used in a wide range of industrial and commercial applications, including fast freezing in the food industry for small items such as peas.
The fluidized bed developed by Winkler is what is now commonly known as a bubbling fluidized bed (BFB). Work on this technology was taken up in the USSR in 1940 and in China in the 1960s, and by UK and US organizations during the 1970s when it was applied to power generation. Meanwhile at the end of the 1930s two scientists at the Massachusetts Institute of Technology, Warren Lewis and Edwin Gilliland, began to develop what has now become known as the circulating fluidized bed (CFB). Initially intended for mineral oil cracking, this design was later adapted for power generation applications too and has since become the main type of fluidized bed in use for coal combustion. Meanwhile, during the 1980s a refinement of the BFB called a pressurized fluidized bed combustion (PFBC) reactor was developed as a more efficient version of the standard bubbling bed design. Pressurized versions of a CFB have also been proposed though none has yet been built. Today all three designs are exploited in different parts of the power industry.
The BFB Reactor
The BFB is the simplest of the fluidized bed reactors that has been developed for power generation applications. The basis of the operation is similar to that of a pulverized coal boiler with a combustion chamber constructed using water walls and further heat capture surfaces in the hot gas path before the flue gases exit the boiler. However, the combustion process takes place in a bed at the bottom of the combustion chamber instead of in a fire ball suspended within it and further heat-capture water-tubes are often placed within the BFB itself, leading to very efficient energy capture.
The bed is composed primarily of a refractory and incombustible material, often sand, to which coal particles are added via feeders located above or to the side of the bed. In a typical BFB, only 5% or less of the bed is coal while the remainder is the inert bed material. In order to keep the bed in a liquidized state, high-pressure air (the primary combustion air) is introduced through the bottom of the bed and there may also be further air inlets in the sides of the chamber providing secondary combustion air. Air feeds are normally preheated using residual heat from the plant.
In coal combustion, the reaction taking place within the bed is a solid phase–gas phase reaction between coal and air. This will take place rapidly under the conditions within the bed, although reaction time is generally longer than in a pulverized coal furnace because of the lower temperature. In addition, the addition of limestone to the refractory bed material enables a similar gas–solid phase reaction to take place between any sulfur dioxide generated during combustion from sulfur in the coal and the limestone particles, capturing and removing the sulfur dioxide from the flue gases.
Since the temperature in the fluidized bed it typically around 950°C instead of up to 1700°C in a pulverized coal combustion chamber, the rate at which nitrogen oxides are formed from air is much reduced. This may remove the need for additional nitrogen oxide removal, depending upon the environmental restrictions in force at the power plant site. Even where further capture is necessary, the quantities and concentrations involved will be relatively lower than for a high-temperature combustion plant.
The BFB was developed as a coal-burning technology up until the 1980s. Units were built in the USA and extensively in China, where by 1980 there were around 200 in use.2 However, the technology fell out of use because the overall combustion efficiency and the efficiency of sulfur capture was judged relatively unfavorable compared to the alternative CFB technology. In addition, erosion of the water tubes within the BFB itself became an issue, as did the ability to scale up the bubbling bed to larger sizes to compete with the alternative pulverized coal technology. However, the use of BFB technology has continued with both biomass and waste material where the small scale and overall efficiency are less of an issue. In these cases there are often no heat transfer tubes within the BFB and there may be an additional pulverized coal burner above the fluidized bed combustion chamber to improve efficiency when burning waste or biomass.
The Circulating Fluidized Bed
Whereas the air blown into a BFB creates an easily defined suspended layer of solids, the air blown into a CFB is at much higher velocity and when it picks up and entrains the particles, it spreads them much further vertically, in some cases carrying them up to the top of the combustion chamber. Since these entrained particles must not escape the combustion plant, in a modern CFB plant the flue gases and entrained particles pass from the top of the combustion chamber into a cyclone filter which captures the solid particles and returns them to the bottom of the combustion chamber while allowing the flue gases to exit the boiler. Initial particle size is less than 10 mm for coal, under 50 mm for biomass, and smaller than 1 mm for limestone used to capture sulfur. The coal particle size is much larger than in a pulverized coal plant and can be achieved with a coal crusher instead of high-performance coal mills. A schematic of a CFB power plant is shown in Figure 5.1.
The advantage of the high-velocity fluidization in the CFB is that it accelerates the reaction between the solid and gaseous phases, a feature that was first recognized by Lewis and Galliland in the 1940s. However, it was not until the 1970s that a dedicated coal combustion plant based on this design was first constructed. Since then a number of designs have been proposed, each with slightly different features. One of the key variables is the velocity of the air that enters from the bottom of the combustion chamber. The fluid nature of the fluidized bed leads to a spread of particles depending upon their density, with more dense particles remaining lower in the chamber while less dense particles are carried higher. In some CFB designs the air velocity is relatively low and there is a pronounced bed at the bottom of the combustion chamber and a much lower particle density towards the top. This is sometimes called a turbulent bed to distinguish it from the BFB and the CFB. In fast fluidized beds, on the other hand, the bed is spread vertically to the top of the combustion chamber and there is no identifiable bed towards the bottom.3
The CFB can remove around 90–95% of the sulfur contained in the coal it burns, compared to the BFB which can only achieve 70–90% removal efficiency. Energy conversion efficiency of around 43% is achievable, close to that of a pulverized coal plant. However, this level of efficiency can normally only be reached with large CFB plants that employ supercritical steam conditions and large, efficient steam turbines.
The earliest CFB power plants were under 100 MW in generating capacity but since the middle of the 1990s there has been a major effort to build larger plants. Since then plants of 200 MW and 300 MW have been built. The largest CFB boiler so far constructed is a 460 MW unit at Lagisza in Poland which began operating in 2009. This plant uses a supercritical boiler to achieve a claimed efficiency of over 43%. Steam conditions are 275 bar/560°C/580°C.
The aim of scaling up the CFB unit size is to enable it to compete with pulverized coal technology. If high efficiency can be achieved, combined with the ability to burn a range of both good- and poor-quality fuels including lignite and biomass, this technology could offer an alternative to the more conventional plant type in the future. In addition, some companies are working on CFB designs in which the combustion air is replaced by oxygen, a strategy that can be used to reduce carbon dioxide production. Again this offers a potential alternative to a pulverized coal combustion plant with carbon capture.
Pressurized Fluidized Bed Combustion
PFBC was developed during the late 1980s and the first demonstration plants using the technology began operating during the 1990s. These plants have all been based on BFB technology. Operating the BFB at elevated pressure leads to an efficiency increase and to a more compact design. All the PFBC plants that have been built are of the bubbling bed design. A pressurized version of the CFB has also been proposed, although no units have yet been built.
The PFBC differs in one important aspect from the atmospheric BFB; it is a combined cycle plant that uses both gas and steam turbines. Operation at high pressure means that the hot flue gases exiting the combustion chamber, once cleaned, can be used to drive a gas turbine at the same time as steam produced in the pressurized boiler is exploited in a steam turbine. Heat remaining in the exhaust gases once they have exited the gas turbine is also captured to raise further steam. It is the combination of the two types of turbine which provides the route to higher efficiency. A diagram of a PFBC is shown in Figure 5.2.
PFBC plants generally operate at 1 MPa to 1.5 MPa (10–15 atmospheres) although pressures in the range 0.5 MPa to 2 MPa have been used. The units are particularly useful when burning high ash coals since additional refractory material simply remains in the bed. The combustion temperature within the bed is between 800°C and 900°C so nitrogen oxide production is low. Both the combustion chamber and the initial gas cleanup cyclones needed to prepare the gases for the gas turbine are contained within the pressure vessel so fuel and any sorbent added to remove sulfur must be pumped across the pressure boundary. Ash must be removed across the pressure barrier too. However, a simplification when burning good-quality hard coal is to turn the coal and sorbent into a paste, with 25% water, and then feed this mixture into the combustion chamber.
The power production from the generating units in a PFBC is broadly 20% from the gas turbine and 80% from the steam turbine. The gas turbine has to be specially designed to operate at a much lower gas inlet temperature than would be normal when burning natural gas and great care must be taken to ensure that the flue gases driving the gas turbine are free from particles and corrosive vapors that might damage the blades.
Most of the PFBC units so far built have had capacities of less than 100 MW but two larger units, one of 250 MW and a second of 360 MW, have been built in Japan. The latter also has a supercritical boiler to improve overall efficiency. The best efficiency of current designs is around 40%. More advanced designs will aim to exceed 45% efficiency but none has yet demonstrated an efficiency approaching this level.
The role of fluidized bed combustion as a power generation technology appears to be focused primarily on the combustion of low quality coals and biomass. New developments and advanced designs may lead to them being able to compete with pulverized coal plants but for the moment these latter offer the highest energy conversion efficiency for the combustion of good quality coals.
Coal Gasification
As an alternative to burning coal in air to generate heat and raise steam to power a steam turbine, it is possible to convert coal into a combustible gas. This can then be burned in a gas turbine power plant or, depending upon the composition of the gas, used to provide fuel for a fuel cell.
The production of gas from coal has a long history, and town gas, a potent mixture of hydrogen, methane and carbon monoxide, was commonly used as a domestic fuel until natural gas became widely available. One form of town gas is made simply by heating coal in the absence of air, driving off its volatile components and leaving an almost pure form of carbon called coke. The coke is used in metallurgical processes such as iron production and was originally a substitute for charcoal made from wood.
Modern gasification processes usually involve a more complex reaction than the simple heating of coal. These modern processes generally require partial combustion of the coal in a mixture of steam with air or oxygen, often followed by a further reaction with water vapor to produce a gas rich in hydrogen. The partial combustion uses some of the calorific value of the coal in order to drive the overall gasification process, so a certain amount of energy is lost in this way.
Modern interest in gasification technology is based on the potential to design an efficient coal-burning power plant around a gasifier. The coal is first turned into a combustible gas, then cleaned and the gas is burned in a gas turbine. Waste heat from both the gasifier and from the gas turbine exhaust is used to raise steam which drives a steam turbine. A plant of this type, called an integrated gasification combined cycle (IGCC) plant can achieve an efficiency of around 40%. It is also possible to design an IGCC plant in which all the carbon dioxide produced is captured instead of being released into the atmosphere. This configuration is discussed below.
There are a number of different gasifier designs that have been developed for modern gasification plants. The simplest is a fixed bed gasifier, a gasification vessel which is fed with fuel from the top while ash is removed from the bottom. Air is also injected from the bottom of the vessel – this type of gasifier is called an up-draft, or counter-current, gasifier – while the combustible gas is removed towards the top of the vessel. An alternative, down-draft gasifier employs steam–air/oxygen injection from the sides of the vessel while the product gas is removed from the bottom.
A more complex alternative to the fixed bed gasifier is a fluidized bed gasifier in which the gasification process takes place within a BFB reactor. As already noted above, this is an efficient way of promoting reactions between solid particles and gases. These gasifiers are particularly effective when the residues from the gasification process are corrosive since the fluidized bed keeps the ash and residues away from the walls of the reactor vessel.
Two other gasifiers have also been developed. An entrained flow gasifier is similar in some respects to a pulverized coal burner. A pulverized fuel, mixed with steam and oxygen, is burned rapidly in a combustion chamber. Combustion takes place at a higher temperature than in either a fixed bed or fluidized bed gasifier. Finally, a plasma gasifier uses an extremely high-temperature electric arc to gasify solid materials. While applicable to coal, this type of gasifier is being developed mainly for solid waste disposal.
While there are a range of complex processes that take place during coal gasification, the overall process can be considered to be the combustion of coal under reducing (limited oxygen) conditions in the presence of water vapor. The main reactions taking place are carbon combustion:
the partial combustion of carbon:
and the water gas reaction:
The reaction is carried out with around 20% of the oxygen that would be needed for the carbon to be reacted completely. Some carbon dioxide is formed but the amount is limited. The combustion and the partial combustion reactions of carbon are exothermic and the energy they release provides the driving force for the water gas reaction. Other reactions that can take place include the methanation reaction:
and the Boudouard reaction:
The result of all these reactions is to produce a gas that is primarily a mixture of carbon monoxide and hydrogen with some carbon dioxide and a small amount of methane. This gas is called synthesis gas or syngas and has been used as a feedstock for a variety of industrial processes. However, since the main components are combustible it can also be burned in a gas turbine. The gas will be produced in a mixture with nitrogen if the gasifier uses air rather than oxygen. Air gasifiers tend to produce a gas of lower calorific value than oxygen gasifiers and for modern IGCC plants, oxygen gasification is normally preferred.
The conditions in the gasification reactor are severely reducing so any sulfur present in the coal is converted into hydrogen sulfide or carbonyl sulfide. Both are readily removed from the gas mixture and can be converted either to pure sulfur or into sulfuric acid for industrial use. The reducing conditions mean that nitrogen oxide production is minimized too.
The gasification process can be taken a stage further by reacting the syngas over a catalyst with more steam. This process, called the water shift reaction, converts the carbon monoxide in the syngas into a mixture of hydrogen and carbon dioxide through the reaction:
Any methane present will also be converted into hydrogen and carbon dioxide. The final gas is now a mixture of carbon dioxide and hydrogen with some impurities. It is relatively simple to separate the two main components, hydrogen and carbon dioxide, allowing the latter to be stored away from the atmosphere and leaving hydrogen as the main product. Hydrogen can be burned in a gas turbine or in a fuel cell to generate electricity.
If gasification is to supply fuel to an IGCC power plant, the gas must first be scrupulously cleaned to remove any particulate material and impurities that might damage the gas turbine. In order for the gas cleaning to be energy-efficient it needs to be carried out at the high temperature at which the gas exits the gasifier. Otherwise energy is lost from the process.
Once cleaned, the gas is used to fuel a gas turbine generator which provides an electrical output. The hot exhaust gases from the exit of the gas turbine are then taken though a heat recovery steam generator which extracts the remaining usable heat to raise steam with which to drive a steam turbine. A plant of this type is more complex than most other types of coal combustion plant since it requires an oxygen plant in addition to the gasifier, gas cleanup, and gas and steam turbine generation elements. A cross-section of an IGCC plant is shown in Figure 5.3.
There are a limited number of major IGCC plants burning coal in existence in Europe, the USA, China, and Japan. Nominal power outputs range from 250 MW to 620 MW. One plant, in the Czech Republic, burns lignite, others burn a mixture of bituminous coal and petroleum coke, pulverized coal, and asphalt refinery residue. Typical efficiency is around 39–41% based on the lower heating value (calorific value) of the fuel although efficiencies of up to 46% are thought possible. However, all the plants currently in operation are essentially demonstration plants and there has been no full-scale commercial roll-out of the technology. Cost remains on obstacle. Another is plant availability which appears to be lower than for more conventional plant configurations.
An IGCC plant offers one power-generating configuration based on the gasification of coal. However, if full carbon capture is being considered, leaving a gas that is rich in hydrogen, then another important option is to use the gas in a fuel cell. There are a number of fuel cell technologies commercially available and all offer relatively high efficiencies when burning hydrogen.