FLUIDIZED BED COMBUSTION

FLUIDIZATION is a method of mixing fuel and air in a specific

proportion, for obtaining combustion.

A FLUIDIZED BED may be defined as the bed of solid particles behaving

as a fluid.

It operates on the principal that when an

evenly distributed air is passed upward

through a finely divided bed of solid

particles at low velocity, the particles

remain undisturbed, but if the velocity of air

flow is steadily increased, a stage is reached

when the individual particles are

suspended in the air stream.

If the air velocity is further increased, the bed becomes highly turbulent

and rapid mixing of particles occur which appear like formation of

bubbles in a boiling liquid and the process of combustion as a result is

known as FLUIDIZED BED COMBUSTION.

The velocity of air, causing fluidization depends on a number of

parameters-

1. Size of fuel particles. 2. Density of air fuel mixture.

Hence, these parameters are given due consideration, while

manipulating with air flow velocity for desired rate of combustion.

A fluidized furnace has an enclosed space with a base having openings

to admit air.

Crushed coal, ash and crushed dolomite or limestone is mixed in the

bed furnace and high velocity combustion air is then passed through

the bed, entering from the furnace bottom.

With the steady increase in the velocity of air, a stage will be reached

when the pressure drop across the bed becomes equal to the weight

per unit cross-section of the bed, and this particular critical velocity is

called the minimum fluidizing velocity.

With further increase in velocity of air, the bed will begin to expand and

allow passage of additional air, in the form of bubbles. When the air

velocity becomes 3 to 5 times the critical velocity, the bed resembles to

that of a violently boiling liquid.

A pictorial representation of fluidized bed combustion is given in the

figure below:

The evaporator tubes of boiler are directly immersed in the fluidized

bed and the tubes, being in direct contact with the burning coal

particles, produce very high heat transfer rates. Because of this, the unit

size is reduced to a great extent, and also produces combustion with

very high efficiency.

TYPES Fluidized Bed combustion can be in 2 variants

1. VERTICAL TYPE FBC These are generally used in smaller plant, and has the capacity to produce steam of up to 6 tons per hour only.

Their vertical shape reduces the overall dimension of the steam boiler, and is extremely efficient in plants, where space provision is limited.

2. HORIZONTAL TYPE FBC There are almost 10 times in capacity when compared to vertical type fluidized bed combustion. They can produce as much as 60 tons of steam per hour, and are placed horizontally with respect to

the boiler tubes. The high capacity of the horizontal type Fluidized boilers coupled with their high efficiency, makes them an extremely desirable choice for the coal fired thermal power generating station.

ADVANTAGES FBC is being used exhaustively these days in all major power stations all over the globe, owing to numerous advantages that it offers over the other pre-dominant methods of combustion. Few of those are:

1. High thermal efficiency. 2. Easy ash removal system, to be transferred for made cement. 3. Short commissioning and erection period. 4. Fully automated and thus ensures safe operation, even at extreme

temperatures. 5. Efficient operation at temperatures down to 150° C (i.e. well below the

ash fusion temperature). 6. Reduced coal crushing etc. (pulverized coal is not a necessity here). 7. The system can respond rapidly to changes in load demand, due to

quick establishment of thermal equilibrium between air and fuel particles in the bed.

8. The operation of fluidized bed furnace at lower temperature helps in reducing air pollution. The low temperature operation also reduces the formation of nitrogen oxides. By adding either dolomite (a calcium-magnesium carbonate) or lime stone (calcium carbonate) to the furnace the discharge of Sulphur oxides to the atmosphere can also be reduced if desired.

PULVERIZED COAL FIRING SYSTEM [ADVANTAGES AND DISADVANTAGES]

In the pulverized fuel firing system, the coal is reduced to a fine powder with the help of grinding mills and projected into combustion chamber with the help of hot air current. The amount of air required to complete the combustion is supplied separately to the combustion chamber. The resulting turbulence in the combustion chamber helps in the uniform mixing of fuel (coal) and air and through combustion. The amount of air which is used to carry the coal and to dry it before entering into combustion chamber is known as “primary air” and the amount of air which is supplied separately for complete the combustion is known as “secondary air”. The amount of primary air may vary from 10% to almost the entire combustion air requirement as per the type of pulverize and load on it.

The efficiency of the pulverized fuel firing system mostly depends on the size of the powder. The fineness of the coal should be such as 70% of it should pass through 200 mesh sieve and 98% through 50 mesh sieve..

ADVANTAGES The main advantage of pulverized firing system lies in the fact that

by breaking a given mass of coal into smaller pieces exposes more surface area for combustion.

Greater surface area of coal per unit mass of the coal allows faster combustion as more coal surface is exposed to heat and oxygen. This reduces the excess air required to ensure complete combustion and the required fan power also

Wide variety and low grade coal can be burnt more easily when the coal is pulverized

Pulverized coal gives faster response to load changes as the rate of combustion can be controlled easily and immediately. Automatic control applied to pulverized coal fired boilers is effective in maintaining an almost constant steam pressure under wide load variations

This system is free from clinker and slagging troubles This system works successfully with or in combination with the gas

and oil It is possible to use highly pre-heated secondary air (350oC) which

helps in rapid flame propagation The pulverized system can be repaired easily without cooling the

system as the pulverizing equipment is located outside the furnace Large amount of heat release is possible in this system compared

to stoke firing system The banking losses are low compared to stoke firing system The boiler can be started from cold very rapidly and efficiently.

This is highly important when grid stability is of the important concern

The external heating surface is free from corrosion and fouling as smokeless combustion is possible

There are no moving parts in the furnace or boiler subjected to high temperature. Therefore, the life of the pulverized fuel firing system is more and operation is trouble-less

Practically no ash handling problem in this type of firing system The furnace volume required is considerably less as the use of the

burners which produce turbulence in the furnace makes it possible to complete combustion with minimum travel.

DISADVANTAGES The capital cost of the pulverized coal firing system is considerably

high as it requires many additional auxiliary equipment. Its operation cost is also high compared to stoke firing system

The system produces fly ash (fine dust) which requires special and costly fly-ash removal equipment’s as electrostatic precipitators

The flame temperatures are high and the conventional types of refractory lined furnaces are not inadequate. It is always necessary to provide water cooled walls for the safety of the furnace. The maintenance cost is also high as working temperature is high which causes rapid deterioration of the refractory surface of the furnace

The possibility of the explosion is more as coal burn like gas The storage of powdered coal requires special attention and high

protection from the fire hazards

COMBUSTION EQUIPMENT FOR BURNING COAL

PULVERISED COAL BURNERS

Burners are devices that allow uniform mixing of fuel with air hence lead to

efficient and complete combustion. The burner receives the fuel along with

the primary air in a central passage, while the secondary air is supplied

around the passage. A good design of the burner is essential to achieve

complete combustion of the fuel. Thus, a good burner should meet a number

of design requirements.

LONG-FLAME OR U - FLAME OR STREAM LINED BURNER

The arrangement of a long flame, U-shaped burner is schematically shown

Fig. The burner is placed such that it produces a long, u-shaped flame. The

burner injects a mixture of primary air and fuel vertically downwards in thin

streams practically with no turbulence and produces a long flame.

Secondary hot air is sup plied at right angles to the flam e which provides

necessary turbulence e and mixing for proper and rapid combustion. A

tertiary air is supplied around the burner for better mixing of the fuel with

air. In this burner, due to long flame travel, high volatile coals can be burnt

easily. Velocity of the air fuel mixture at the burner tip is around of 25 m/sec.

SHORT FLAME OR TURBULENT BURNER

The schematic arrangement of a short-flame or turbulent burner is

illustrated in fig. These burners are generally built into the furnace walls, so

that the flame is projected horizontally into the furnace. Primary air and the

fuel mixture is combined with secondary air at the burner periphery, before

the entry into the furnace as shown in figure. This burner gives out a

turbulent mixture which burns rapidly and combustion is completed within a

short distance. Therefore, the combustion rate is high. The velocity of mixture

at the burner tip is about 50 m/sec. In such burners, the bituminous coal can

be burnt easily. Modern high capacity power plants use such burners.

TANGENTIAL BURNERS

These burners are built into the furnace walls at the corners. They inject the

air-fuel mixture tangentially to an imaginary circle in the center of furnace.

As the flames intercept, it leads to a swirling action. This produces sufficient

turbulence in the furnace for complete combustion. Hence in such burners,

there is no need to produce high turbulence within the burners. Tangential

burners give fast and high heat release rates.

CYCLONE BURNER

This burner burns the coal particles in suspension, thus avoiding fly-ash

problems, which is common in other types of burners. This burner uses

crushed coal (about 5 to 6 mm size) instead of pulverized coal. This burner

can easily burn low grade coal with high ash and moisture content. Also, this

can burn biofuels such as rice husk. The cyclone burner consists of a

horizontal cylinder of about 3 m diameter and about 4 m length. The cylinder

wall is water cooled, while the inside surface is lined, with chrome ore. The

horizontal axis of the burner is slightly inclined towards the boiler. The coal

used in cyclone burner is crushed to about 6 mm size. Coal and primary air

(about 25% of the combustion or secondary air) are admitted tangentially

into the cylinder so as to produce a strong centrifugal motion and turbulence

to the coal particles. The primary air and fuel mixture flows centrifugally

along the cylinder walls towards the furnace. From the top of the burner, the

secondary air is also admitted tangentially, at a high velocity (about 100 m/s).

The high velocity secondary air causes further increase in the centrifugal

motion, leading to a highly turbulent whirling motion of the coal air mixture.

Tertiary air (about 5 to 10% of the secondary air) is admitted, axially at the

center as shown in fig, so as to move the turbulent coal-air mixture towards

the furnace.

PRINCIPLES

NATURAL CIRCULATION

Boilers are designed with Economizer,

Evaporator and superheated

depending on the Design parameters.

HEAT INPUT STEAM DRUM

DOWNCOMER TO FURNACE TUBES

FIG 1. FLOW DUE TO DENSITY

DIFFERENCE Economizers add sensible

heat to water. The economizer water

outlet temperature will be closer to

saturation temperature. The water is

forced through the economizer by the

boiler feed pumps. Super heaters add heat to steam. That is the heat is added

to steam leaving the Boiler steam drum / Boiler shell. The steam passes

through the superheated tubes by virtue of the boiler operating pressure.

Evaporators may be multi tubular shell, Water wall tubes, Boiler bank tubes

or Bed coils as in FBC boiler. In evaporators, the latent heat is added. The

addition of heat is done at boiling temperature. The Flow of water through

the evaporator is not by the pump but by the fact called thermos siphon. The

density of the water, saturated or subcooled is higher as compared the water

steam mixture in the heated evaporator tubes. The circulation is absent once

the boiler firing is stopped.

FORCED CIRCULATION

A forced circulation boiler is a

boiler where a pump is used to

circulate water inside the boiler.

This differs from a natural

circulation boiler which relies on

current density to circulate water

inside the boiler. In some forced

circulation boilers, the water is

circulated twenty times the rate of

evaporation. In water tube boilers,

the way the water is recirculated

inside the boiler before becoming

steam can be described as either natural circulation or forced circulation. The

forced circulation boiler begins the same as a natural circulation boiler, at

the feed water pump. The secondary pump takes the feed water going to

the boiler and raises the pressure of the water going in. In a natural

circulation boilers, the circulation of water is dependent on the differential

pressures caused by the change of density in the water as it is heated. That

is to say that as the water is heated and starts turning to steam, the density

decreases sending the hottest water and steam to the top of the furnace

tubes. In contrast to the natural circulation boiler, the forced circulation

boiler uses a water circulation pump to force that flow instead of waiting for

the differential to form. Because of this, the generation tubes of a forced

circulation boiler are able to be oriented in whatever way is required by space

constraints.

NATURAL DRAFT

Natural draft uses the stack or chimney affect. Flue gases are hotter and less

dense than surrounding air around the stack opening. As flue gases rise

through the stack, a natural convection current is formed creating a pressure

gradient through the stack, ducting and furnace. This causes combustion

gases to be sucked into the stack and combustion air to be drawn into the

furnace. Natural draft boilers are not as efficient as mechanized draft boilers.

Natural draft furnaces operate below atmospheric pressure.

FORCED DRAFT

A forced draft furnace uses a fan or blower to force combustion air through

the system. Control is accomplished by regulating the fan speed or damper

operation. This type of furnace is operated slightly above atmospheric

pressure. Forced draft furnace must be airtight to prevent leakage of flue

gases into surrounding area.

INDUCED DRAFT

An induced draft fan draws the gases through the flue ducting and the

combustion air into the furnace making high stacks unnecessary. Control is

accomplished by regulating the fan speed or damper operation. An induced

draft furnace is operated slightly below atmospheric pressure. Induced draft

is used with solid fuel burning or stokers, because the furnace is not airtight.

BALANCED DRAFT

Furnaces equipped with both an FD (Forced Draft) and ID (Induced Draft)

fans are called balanced draft systems; see Figure 1. In balanced draft

systems, the forced and induced draft fans work together to move

combustion air and flue gases through the furnace. The FD fan is used to

regulate the combustion airflow and the ID fan is used to regulate furnace

pressure. Balanced draft furnaces are typically operated slightly below

atmospheric pressure.

INTRODUCTION AND CLASSIFICATION OF STEAM TURBINES

INTRODUCTION

De Laval, Parsons and Curtis developed the concept for the steam turbine in the 1880s. Modern steam turbines use essentially the same concept but many detailed improvements have been made in the intervening years mainly to improve turbine efficiency.

Steam turbines are used in all of our major coal fired power stations to drive the generators or alternators, which produce electricity. The turbines themselves are driven by steam generated in ‘Boilers ‘or ‘Steam Generators ‘as they are sometimes called.

Energy in the steam after it leaves the boiler is converted into rotational energy as it passes through the turbine. The turbine normally consists of several stages with each stage consisting of a stationary blade (or nozzle) and a rotating blade. Stationary blades convert the potential energy of the steam (temperature and pressure) into kinetic energy (velocity) and direct the flow onto the rotating blades.

CLASSIFICATION

EXTRACTION STEAM TURBINE

The extraction turbine contains two outlets as shown in figure 1. The first outlet extracts the steam with intermediate pressure for the feeding of the heating process while the second outlet extracts the remaining steam with low-pressure steam for the condensation. The extraction of heat from the first outlet can be stopped to generate more output. Steam control valves at this outlet make this steam very flexible and allow adjusting the output as per demand. The

steam from the second outlet goes to the condensation chamber where cooling water brings the temperature of the steam down. The condensed water then goes back to the boiler for the regeneration of the electricity of power, therefore, it is also known as the regenerative steam turbine. The scheme of extraction turbine with cogeneration system is shown in figure 1. This turbine has following benefits and disadvantages.

Advantages:

This type of steam turbine can be used to generate a high amount of electricity.

It is a flexible turbine with the ability to regulate output as per changing need.

Disadvantages:

It is costly turbine with lots of auxiliary components Heat rejection in the condensation process reduces the overall

efficiency of the system. It is usually used on industrial level and requires complex configuration

BACK PROCESS STEAM TURBINE

The non-condensing steam turbine uses high-pressure steam for the rotation of blades. This steam then leaves the turbine at the atmospheric pressure or lower pressure. The pressure of outlet steam depends on in the load, therefore, this turbine is also known as the back-pressure steam turbine. This low-pressure steam uses for processing and no steam is used for condensation. The schematic diagram of the back-process steam turbine with cogeneration system is

shown in figure 2. There are lots of benefits of this steam turbine but at the same time it has few disadvantages which are listed below.

Advantages:

The configuration of this steam turbine is very simple It is relatively inexpensive as compared to extraction steam turbine It requires very less or no cooling water Its efficiency is higher as it does not reject heat in the condensation

process

Disadvantages:

The biggest disadvantage of this type of steam turbine is that it is highly inflexible. The output of this turbine can’t be regulated as it does not allow changing the pressure and temperature of steam in the turbine, therefore, it works best with the constant load.

The thermal load of this turbine defines the flow of steam mass which makes it difficult to change the output value. Other methods to regulate output reduce the efficiency of the overall system.