When an evenly distributed air or gas is passed upward through a finely divided bed of solid particles such as sand supported on a fine mesh, the particles remain undisturbed at low velocities. As the air velocity is gradually increased, a stage is reached when the individual particles are suspended in the air stream and the bed is called “fluidized”. With further increase in air velocity, there is bubble formation, vigorous turbulence, rapid mixing and formation of dense defined bed surface. The bed of solid particles exhibits the properties of a boiling liquid and assumes the appearance of a fluid – “bubbling fluidized bed”.
Fluidization depends largely on the particle size and the air velocity. The mean solids velocity increases at a slower rate than does the gas velocity. The difference between the mean solid velocity and mean gas velocity is called as slip velocity. Maximum slip velocity between the solids and the gas is desirable for good heat transfer and intimate contact. If oil shale particles in fluidized state are heated to the ignition temperatures and oil shale particles is injected continuously into the bed, the oil shale will burn rapidly and the bed attains a uniform temperature.
The fluidized bed combustion (FBC) takes place at about 840°C to 950°C. Since this temperature is much below the ash fusion temperature, melting of ash and associated problems are avoided. The lower combustion temperature is achieved because of high coefficient of heat transfer due to rapid mixing in the fluidized bed and effective extraction of heat from the bed through in-bed heat transfer tubes and walls of the bed. The gas velocity is maintained between minimum fluidization velocity and particle entrainment velocity. This ensures a stable operation of the bed and avoids particle entrainment in the gas stream.
Any combustion process requires three “T”s – that is Time, Temperature and Turbulence. In FBC, turbulence is promoted by fluidization. Improved mixing generates evenly distributed heat at lower temperature. Residence time is many times higher than conventional grate firing. Thus an FBC system releases heat more efficiently at lower temperatures. Since oil shale contains limestone which can be used to control of SOx emissions in the combustion chamber without any additional control equipment such as de-sulfurization. This is one of the major advantages over conventional boilers.
Types of Fluidized Bed Combustion Boilers:
There are three basic types of fluidized bed combustion boilers:
1. Atmospheric Fluidized Bed Combustion System (AFBC).
2. Atmospheric circulating (fast) Fluidized Bed Combustion system (CFBC).
3. Pressurized Fluidized Bed Combustion System (PFBC).
Major performance features of the CFBC system are as follows:
CFB has emerged as a growing challenger to PC combustion in recent decades, and is well known for its flexibility in terms of fuel quality and capability of firing difficult-to-burn fuels efficiently and with very low emissions.
- Oil shale is crushed in small particles (6 –12 mm size).
- The particles are suspended in a stream of upwardly flowing air (60-70% of the total air.
- The fluidizing velocity in circulating beds ranges from 3.7 to 9 m/sec.
- The constant combustion takes place at 840-900 °C, and the fine particles (<450 microns) are elutriated out of the furnace with flue gas velocity of 4–6 m/s.
- The circulating bed is designed to move a lot more solids out of the furnace area and to achieve most of the heat transfer outside the combustion zone – convection section, water walls, and at the exit of the riser.
- CFBC requires huge mechanical cyclones to capture and recycle the large amount of bed material, which requires a tall boiler.
- The boiler is required to fire low-grade fuel.
- The low combustion temperature results in minimal NOx formation and sulphur present in the fuel is retained in the form of calcium sulphate and removed in solid form.
- The combustion air is supplied at 1.5 to 2 psig (pounds per square inch gauge)
- CFB has high combustion efficiency (over 99%) and it has a better turndown ratio.
- Erosion of the heat transfer surface in the combustion chamber is reduced, since the surface is parallel to the flow.
- The electro-static precipitator (ESP) is capable of operating under all conditions from initial start up to complete shutdown and will achieve a particulate emissions limit of 30 mg/Nm3.
- The ash handling system features a dry pneumatic system up to the ash silos.
Advantages of CFBC Boilers:
- High Efficiency: CFBC boilers can burn fuel with a combustion efficiency of over 99% irrespective of ash content.
- Reduction in Boiler Size: High heat transfer rate over a small heat transfer area immersed in the bed results in overall size reduction for the boiler.
CFBC boilers can be operated efficiently with a variety of fuels. Even fuels like flotation slimes, washer rejects, agro waste can be burnt efficiently. These can be fed either independently or in combination with coal or oil shale into the same furnace.
- Ability to Burn Low Grade Fuel:
CFBC boilers would give the rated output even with an inferior quality fuel. The boilers can fire oil shale with ash content as high as 80% and having calorific value as low as 900 kcal/kg.
Oil Shale containing fines below 6 mm can be burnt efficiently in CFBC boiler, which is very difficult to achieve in conventional firing system.
SO2 formation can be greatly minimized by limestone which is already exist in oil shale for high sulphur oil shale (3%) limestone is required for every 1% sulphur in the oil shale feed). Low combustion temperature eliminates NOx formation.
- Low Corrosion and Erosion:
The corrosion and erosion effects are less due to lower combustion temperature, softness of ash and low particle velocity (around 1 m/sec).
- Easier Ash Removal – No Clinker Formation:
Since the temperature of the furnace is in the range of 750 – 900 °C in CFBC boilers, even oil shale of low ash fusion temperature can be burnt without clinker formation. Ash removal is easier as the ash flows like liquid from the combustion chamber. Hence less manpower is required for ash handling.
The CO2 in the flue gases will be of the order of 14 – 15% at full load. Hence, the CFBC boiler can operate at low excess air – only 20 – 25%.
- Simple Operation, Quick Start-Up:
High turbulence of the bed facilitates quick start up and shut down. Full automation of start up and operation using reliable equipment is possible.
- Fast Response to Load Fluctuations:
Inherent high thermal storage characteristics can easily absorb fluctuation in fuel feed rates. Response to changing load is comparable to that of oil fired boilers.
- No Slagging in the Furnace – No Soot Blowing:
In CFBC boilers, volatilization of alkali components in ash does not take place and the ash is non sticky. This means that there is no slagging or soot blowing.
- Provisions of Automatic Oil Shale and Ash Handling System:
Automatic systems for oil shale and ash handling can be incorporated, making the plant easy to operate comparable to oil or gas fired installations.
- Provision of Automatic Ignition System:
Control systems using micro-processors and automatic ignition equipment give excellent control with minimum supervision.
The absence of moving parts in the combustion zone results in a high degree of reliability and low maintenance costs.
- Quick Responses to Changing Demand:
CFBC can respond to changing heat demands more easily than stoker fired systems. This makes it very suitable for applications such as thermal fluid heaters, which require rapid responses.
- High Efficiency of Power Generation:
By operating the fluidized bed at elevated pressures, it can be used to generate hot pressurized gases to power a gas turbine. This can be combined with a conventional steam turbine to improve the efficiency of electricity generation resulting in a potential fuel savings of at least 4%.
Technical Design Parameters Of CFB in Estonia
Estonia is the only country in the world that uses oil shale to generate electricity in commercial bases. It has started in the 1940s with middle-pressure pulverized combustion, introduced high-pressure pulverized combustion (PC) at the end of the 1950s and commissioned a circulated fluidized bed (CFB) plant in 2005. As of the end of 2005, the installed electric capacity of Estonian power plants using oil shale was 2380MW.
The original blocks at Narva PP, which were equipped with boilers designed for PC technology, were commissioned between 1959 and 1973. Because of poor economical and environmental performance of these units resulting from their age, and serious problems of corrosion and fouling of their convective heat surfaces, Narva PP decided to repower two 200-[MW] units using new environment-friendly and efficient combustion technology.
Since there were no CFB boilers in operation firing Estonian oil shale before year 2000, it was essential to carry out pilot-scale combustion tests to evaluate the performance of oil shale when subject to fluidized-bed combustion. Tests were carried out using a 1-[MWth] pilot plant. Several different types of fluidized-bed combustion were tested, and CFB combustion offered the best performance in terms of process behavior and gaseous emissions..
While pilot-scale combustion tests showed very promising results, they also highlighted the need to make a number of modifications to the principles of a standard CFB boiler design to accommodate the special characteristics involved in firing Estonian oil shale.
The early operational experience of firing oil shale in the new CFB boilers has been very positive.
All the basic problems encountered with firing the fuel in PC boilers appear to have been solved by introducing CFB combustion. The new boilers have operated reliably, with good efficiency and low gaseous emissions. The combustion process has been stable and easy to operate over a wide load range, and dynamic requirements have been met easily.
After successfully operation of the new Narva (Estonia) oil shale CFB boilers, the following technical parameters have been achieved:
New power plant will help ensure the current level of electricity production and at the same time will lower the environmental impact of electricity production from oil shale
The design of the oil shale-burning CFB boiler is presented in the figure below, boilers of this type is operated at AS Narva Power Plants.
Utilization of ash (for the production of cement, autoclaved concrete elements, neutralizing of acid soils, etc.) has decreased and now accounts for a small part of the ash output.