It is an enclosed pressure vessel in which water is converted into steam by gaining heat from any source (coal, oil, gas etc).
Boiler in thermal power plant accumulates the steam and build up a pressure to expend it in turbine and convert thermal energy to mechanical energy. The generator which is connected to turbine converts the mechanical energy into electric energy.
Types of boiler
1. Based on Tube Content
Fire Tube
Water Tube
2. Base on Operating Pressure
Ultra-supercritical boiler: Pressure ≥ 27.0MPa or rated outlet temperature ≥ 590 ℃ boiler
There are two types of boilers are used in sub critical, super critical and ultra-super critical coal fired thermal power plant:
Pulverized coal-fired Boiler
Fluidized Bed combustion (FBC) boiler
Note: Both types of boiler are water tube boiler
Pulverized coal-fired Boiler or Pulverized fuel Boiler
A pulverized coal-fired boiler is an industrial boiler that generates thermal energy by burning pulverized coal.
How does a steam boiler work?
In pulverized coal fired boiler,The coal is pulverized to a fine powder, so that less than 2 % is +300 micro meter and 70 – 75 % is below 75 microns. The pulverized coal is blown with part of the combustion air into boiler plant through a series of burner nozzles. Combustion takes place at temperature from 1300 – 1700 °C, depending largely on coal grade.
The flue gas formed in the furnace passes over the superheater, economizer, air-preheater, ESP and finally is evacuated by the induced draft fan into the chimney.
The ash from the combustion chamber, which falls down is collected in the bottom ash hoppers. The ash which is fine & flies with the flue gas is separated by ESP and collected in ESP hoppers.
The Feed water enters the boiler through the economizer tubes provided in the path of the flue gas. The feed water is heated in the economizer and then enters the boiler drum situated outside the furnace at the top of the Boiler.
The water is circulated in the tubes and converted into steam by gaining heat inside the furnace.The dry and saturated steam from the boiler drum then passes through the superheater section and finally available at the boiler outlet header.
Advantage of Pulverized Coal-Fired Boiler
Ability to burn all ranks of coal.
It permits combination firing (i.e. can use coal, oil and gas in same burner)
Disadvantage of Pulverized Coal-Fired Boiler
High power demand for pulverizing.
Require more maintenance.
Fly ash erosion
Pollution complicate unit operation
Fluidized Bed combustion boiler (FBC)
Pulverized coal fire boiler has been the preferred method for solid fuel firing but in the last few years, fluidized bed combustion boilers have begun to disturb this equilibrium by offering reliable solutions in the areas not served well by pulverized fuel boiler.
Working Principle of fluidized bed combustion boiler
When gas or air is passed through an inert bed of solid particles such as sand or limestone supported on a fine grid or mesh, the particle are undisturbed at low velocity. As air velocity is gradually increased, a stage is reached when the individual particle are suspended in the air stream. With further increase in the velocity of the air, the particles attain a state of high turbulence. Under such conditions, the bed assumes the appearance of a fluid and exhibits the properties associated with a fluid and hence the name “Fluidized Bed combustion”.
If the sand or limestone in a fluidized state is heated to the ignition temperature of the coal and the coal is injected continuously into the bed, the fuel will burn rapidly and the bed attains a uniform temperature due to effective mixing.
The fluidized bed combustion takes place at a temperature of 840°C to 950 °C. Since the temperature is below the ash fusion temperature, melting of ash and associated problems are avoided.
There are three types of fluidized bed combustion boilers:
1. Atmospheric fluidized bed combustion system (AFBC) or Bubbling fluidized bed combustion (BFBC)
AFBC is atmospheric fluidized bed combustion, where the furnace pressure is atmospheric pressure and velocity of fluidized air is in the range of 1.2 to 3.7 m/sec. The in-bed tubes carrying water generally act as the evaporator. The flue gas from the furnace pass over the superheater sections of the boiler flow past the economizer, the dust collector and the air preheater before being exhausted to atmosphere.
2. Circulating fluidized bed combustion (CFBC)
In this system, the fluidized velocity in circulating beds ranges from 3.7 to 9 m/sec and the flue gas is recirculated with a cyclone to capture the unburnt carbon. There are no steam generation tubes immersed in the bad. Generation and super heating of steam takes place in the convection section, water wall and at the exit of the riser.
3. Pressurised fluidized bed combustion system (PFBC)
In PFBC, a compressor supplies the forced draft (FD) air and the combustor is a pressure vessel. The heat release rate in the bad is proportional to the bed pressure and hence a deep bed is used to extract large amount of heat. This will improve the combustion efficiency. Steam generated from the heat in the fluidized bed is sent to a steam turbine and hot flue gases drive a power generating gas turbine.
The PFBC system can be used for combined cycle operation (gas turbine and steam turbine). This improves the overall conversion efficiency by 5 to 8 %.
Advantages of fluidized bed combustion (FBC) boilers
Reduce emission of NOx due to lower temperature.
Reduce emission of SOx due to capture by limestone.
Low corrosion and erosion.
Easier ash removal i.e. no clinker formation
Simple operation and quick start up.
Reduce maintenance
High efficiency of power generation
Heat Transfer In Steam Boiler
A steam boiler is designed to absorb the maximum amount of heat released from the process of combustion. There are three way (Radiation, convection & Conduction) that heat is transfer in the boiler and relative percentage of each heat transfer within system boiler is dependent on the type of steam boiler, fuels and the designed transfer surface.
1. Radiation
Radiation is the process of continuous interchange of energy by means of electromagnetic waves without a change in the temperature of the medium between two bodies involved.
The heating surface in the furnace are receives heat primarily by radiation. The steam boiler tubes absorb the radiant heat from flame and radiate a small portion of the heat back to the furnace.
2. Convection
Convection is the process of transferring heat by movement energy through a material.
Example of heat transfer by convection in steam boiler:
Heating surface in the steam boiler receives heat by convection from the hot flue gases.
water or steam in the steam boiler receive heat by convection from the heating surface.
When any fluid in a steam boiler is heated, its density is decreases and fluid become lighter. Then the heavier or cooler fluid will flow and replace the heated portion of the fluid. In turn the cooler portion become heated are example of convection.
3. Conduction
Conduction is the transfer of the heat from one part of a body to other or from one body to another in physical contact.
Heat received by the heating surface travels through the metal by conduction.
Steam Boiler Mountings
Boiler Mountings are the safety devices which are fitted on the boiler for its safe & efficient working. The following mountings are usually installed on the boiler:
Safety Valve, Water Level Indicator, Pressure Gauge, Steam Stop Valve, Feed check Valve, Blow of Cock, Man and Mud Hole.
Steam Boiler Accessories
Boiler accessories are the components which are installed to increase the efficiency of the steam power plant. The following accessories are given below:
Air Preheater, Economiser, Superheater, Feed Pump, Injector
