Combustion is a vital and one of the initial processes in the functioning of steam boilers. Various fuels are ignited in solid fuel boilers that require proper combustion to generate steam and avoid greenhouse emissions from unburned fuels. Fluidized bed combustion is a technique that effectively burns the fuels in solid fuel boilers. In the fluidization method, the combustion is performed by mixing fuel and air in a specific proportion. A fluidized bed can be defined as a bed of solid particles, passed through a pressurized fluid (gas or liquid) that causes the solid particles to act as a fluid in certain conditions.
In India, the coal majorly available is of low quality, low calorific value, and high ash content that hinders the conventional grate fuel firing system. Fluidized bed combustion is ideal for burning fuels such as coal, rice husk, bagasse, agro waste, etc. and offers various benefits as compared to traditional firing systems such as flexibility in fuel choice, compact design of solid fuel boiler, combustion efficiency, and reduced emission of pollutants like SOx and NOx.
Working Principle of Fluidized Bed Combustion:
Fluidized Bed Combustion occurs when a gas or air is evenly distributed and passed upwards through a bed of fine solid particles such as sand. Initially, at low velocity, the particles remain undisturbed. As the velocity gradually increases, the particles reach a stage when it is suspended in the air stream, and the bed is fluidized.
Fluidization highly depends on particle size and air velocity. The mean solid velocity increases at a slower rate as compared to gas velocity. The difference between both velocities is termed slip velocity. Maximum slip velocity assures good heat transfer. When sand particles in a fluidized state are heated to the combustion temperature of coal while the coal is administered steadily into the bed, it results in rapid burning of coal and uniform temperature of the bed. The fluidized bed combustion occurs at between 8400C and 9500C. The combustion process requires Time, Turbulence, and Temperature. Particle bed in FBC uses limestone that controls the emission of sulfur dioxide and nitrogen oxide without any pollution control equipment.
Types of Fluidized Bed Combustion in Solid Fuel Boilers:
There are three major types of fluidized bed combustion boilers that are:
Atmospheric Classic Fluidized Bed Combustion (AFBC)/ Bubbling Bed:
In AFBC, the coal is crushed to 1mm to 10 mm, depending on the quality of the coal and the type of fuel fed in the combustion chamber. The atmospheric air acts as fluidization air and combustion air that gets preheated by exhaust flue gas and flows through the bed with a velocity ranging from 1.2 m/sec. to 3.7 m/sec. The amount of fuel reacted depends on the rate at which air passes through the bed.
AFBC solid fuel boilers are generally 0.9 m to 1.5 m deep and consist of in-bed evaporator tubes in the limestone, sand, and fuel bed to extract heat and control the bed temperature. It operates with a determined temperature range of 800oC and 850oC. If the temperature exceeds 950oC, there may be a risk of clinker formation, and if it lowers than 800oC, it may result in loss of combustion efficiency.
Components of AFBC in solid fuel boiler:
- Fuel Feeding System
- Air Distributor
- The in-bed heat transfer surface
- Ash Handling System
Circulating Fluidized Bed Combustion (CFBC):
The CFBC technique performs by administering crushed fuel of 6 mm to 12 mm size and limestone into the combustor. The bottom of the furnace consists of air distribution nozzles that deliver upward flowing air in which the particles are suspended. The fluidizing velocity ranges between 3.7 m/sec. to 9 m/sec. and combustion takes place between 840oC and 900 o C. The CFBC comes without steam generation tubes, but some of them consist of external heat exchangers. The circulation of particles leads to sufficient heat transfer and residence time for the utilization of carbon and limestone.
For larger process plants, CFBC provides greater area, fuel particle, and sorbent residence time for effective combustion and to control SO2. It is more cost-effective than AFBC and an ideal choice for medium to large boiler capacity firing low-grade fuel that requires control on sulfur emission.
Features of Circulating Fluidized Bed Combustion System in Solid Fuel Boilers:
- High processing capacity due to high gas velocity
- Low combustion temperature leads to minimal NOX
- Better turndown ratio
- Reduced erosion of the heat transfer surface
- High combustion efficiency
Pressurized Fluid Bed Combustion:
Pressurized Fluid Bed Combustion (PFBC) is mainly used in cogeneration plants and large-scale coal combustion. The gas turbine is driven from the off-gas in fluidized bed combustion. Waste heat from the gas turbine is utilized for preheating the condensate from the steam turbine and used as feed water for generating steam. Combining gas and steam turbines leads to the efficient generation of electricity.
Advantages of Fluidized Bed Combustion in Solid Fuel Boilers:
- Higher Efficiency
- Flexibility in Fuel Choice
- Effective Combustion of Low-Quality Fuels
- Reduced Corrosion and Erosion
- Easy Ash Removal and No Clinker Formation
- Less Requirement of Excess Air
- Quick Startup
- Avoids Soot Blowing
- Highly Reliable
- Quick Response to Load Fluctuations
- Easy Maintenance
- High Efficiency for Power Generation
Rakhoh Boilers has been one of the leading boiler manufacturers since its inception in 1983. We manufacture a wide range of efficient and reliable solid fuel boilers, oil-fired and gas-fired boilers that assure effective combustion of the fuels for optimal productivity and efficiency. We also manufacture high-quality boiler accessories and provide excellent boiler services.
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