Methods Of Firing Steam Boiler 10473r

Methods of Firing Steam Boiler For obtaining maximum fuel combustion efficiency it is required to proper and complete combustion of fuel inside the boiler furnace. For that, proper and sufficient supply of air and proper mixing of air with fuel are primary requirements. Adequate supply of fuel particles for proper burning of particles also to be maintained.The combustion should produce designated temperature of the steam boiler and maintains it consistently. In addition to these, the method of firing steam boiler is such that, the system may be easily handled and also, operation and maintenance should be minimum. There are mainly two methods of firing steam boiler with coal as fuel. One is solid fuel firing other is pulverized fuel firing. Let us discuss one by one. There are mainly two types of solid fuel firing system 1. Hand firing 2. Mechanical stroke firing Smaller size boiler can be operated by hand firing system. This system was commonly used to drive coal engine locomotive in past. Here, coal chips are put into the furnace frequently by shovels.

Mechanical Stoker Firing When fuel i.e. coal is put into the steam boiler furnace by means of mechanical stoker, the firing of boiler method is referred as mechanical stoker firing. There are mainly two types of mechanical stoker firing systems. Under Feed Mechanical Stoker Firing Here, combustion takes place on the grate. The primary air is fed below the grate. The primary air is fed below the grate. The secondary air is allowed at the top. when the coal is burnt, it is pushed down by fresh coal. The fresh coal is pushed on the grate by means of rams as shown .

The ignition occurs downwards against the primary air flow. The volatile matter filters through the bed and is completely burnt. The combustion rate is high. The light ash contents and combustion gases fly away to the atmosphere along with primary air. Heavier ash content comes down over the grate and ultimately falls into ash pit. Travel Grate Stoker Solid Coal Firing Here, the coal is burnt on a chain grate which continuously travel forwards slowly combustion takes place during the journey of coal from first end to last end of the furnace. At the end of the combustion heavier ash content falls into ash pit by gravitational force as the grate chain moves like conveyor belt. The lighter ash particles and combustion gases fly away with primary air. Pulvarized Fuel Firing For getting most calorific value of coal, the coal is pulvarized in fine powder and then mixed with sufficient air. The mixture of coal powder and air is fired in the steam boiler furnace to achieve most efficient combustion process.This is most modern and efficient method of boiler firing. Due to pulverization, the surface area of coal becomes much larger, and in this method air required for combustion is much less. As the quantity of required air and fuel both are less, loss of heat in this method of boiler firing is much less, hence temperature can easily be reached to the designated level. As the combustion is most efficient pulverized coal firing increases the overall efficiency of steam boiler. As handling of lighter coal dust is much easier than handling of heavier coal chips, it is quite easy to control the output of the boiler by controlling supply of fuel to the furnace. Hence fluctuation of system load can smoothly be met. In addition these advantages, pulverized coal firing system has may disadvantages. Such as 1. The initial cost of installing this plant is very high. 2. Not only initial cost, running cost of this plant is quite high as separate pulverization plant to installed and run additionally. 3. High temperature causes high thermal loss through flue gas. 4. This type of method of boiler firing has always a risk of explosion. 5. This is also difficult and expensive to filter fine ash particles from fine gas. Moreover, the quantity of ash particles in the flue gas is more in pulverized system.

Pulvarization Process Process of pulvarization is discussed here in brief. 1. First the coal is crushed by preliminary crasher. The coal is crushed to 2.5 cm. or less. 2. Then this crushed coal is ed through magnetic separator to separate any iron content in the coal. Iron must be removed, otherwise during

pulverizing iron particles will cause spark which results unwanted fire hazard. 3. After that, crushed coal is dried properly before pulverization. The moisture content must be less than 2% after drying operation. 4. Then the coal is crushed again in fine particles in ball mill. This process is referred as pulverization. 5. This pulverized coal is then puffed with air and put into furnace as fluid.

Feed Water and Steam Circuit of Boiler There are different components present in feed water and steam circuit of boiler and we should know some essential components of these circuit and these are Economizer, boiler drums, water tubes, and super heater.

Economizer 

Economizer is a heat exchanger which takes heat from the flue gas, and increases the temperature of feed water coming from feed water common header to about the saturation temperature corresponding to the boiler pressure.

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Throwing away the flue gases of high temperature into the atmosphere involves a great deal of energy losses. By utilizing these gases in heating feed water, higher efficiency and better economy can be achieved, and hence the heat exchanger is called “Economizer”.

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Structurally economizer is a collection of bent hollow tubular elements through which feed water es. Outside of the tubes are heated by Exhaust flue gases. More no. of water tubes more will be the heat exchange surface. No. of tubes and tube cross section are pre-designed as per required boiler parameters.

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In the T-S curve above, the shadow portion illustrates the zone of economiser. The heat absorbed by feed water is denoted by ‘Q eco’.

Another essential component of Feed Water and Steam

Circuit is Boiler Drum.

Boiler Drums Two types of boiler drums used in all types of boilers are ‘steam drum’ and ‘mud drum’. Both the drums have specific functions. Steam Drum The functions of steam drum in feed water steam circuit are: 1. To store water and steam sufficiently to meet varying load demands. 2. To provide a head and thereby aiding the natural circulation of water through water tubes. 3. To separate vapour or steam from water- steam mixture, discharged by the risers. 4. To aid in chemical treatments to remove dissolved O2 and to maintain required pH. Separating steam from two-phase mixtures in the steam drum: 

Steam must be separated from the mixture before it leaves the drum, because: 1. Any moisture carried with steam contains dissolved salts. In the super heater, water evaporates and the salt remain deposited on the inside surface of the tubes to form a scale. This scale reduces the life of the super-heaters. 2. Some of the impurities in the moisture (like vaporized silica) may cause turbine blade deposits.

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One of the important functions of steam-drum is to separate steam from steam water mixture. At low pressure (below 20 bar; 1 bar = 1.0197 kg/cm2) simple gravity separation is used. In the method of gravity separation the water particles disengaged from steam due to higher density.

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As the pressure inside the boiler drum increases the density of steam increases, as steam is very compressible. Hence difference between the densities of steam and water decreases. Hence gravity separation becomes in efficient.

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Hence in the steam drum of the high pressure boilers, there are some mechanical arrangements (known as ‘drum internals’ or ‘anti-priming arrangements’) for separating steams from water.

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Following picture illustrates different Anti-priming arrangements used in thermal power plants :

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“Baffles” are separators which separate the hot steam-water mixture from dry steam and provide a guided path for the dry steam.

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In the “cyclone separator” steam water two-phase mixture is allowed to move in a helical path and due to centrifugal forces the water particles separate out from the two-phase mixture. The small vanes inside the cyclone separator collect the deposited water particles.

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In the “scrubber” the two phase mixture is allowed to move in a zigzag path and it provides the ultimate stage of drying the steam.

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After scrubber steam is allowed to move to super-heated through a perforated screen.

Mud Drum Mud drum is another header which is situated at the bottom of the boiler and usually helps in natural circulation of water through the steam tubes. Mud drum usually contains water at saturation temperature, and also the precipitated salts and impurities known as slurries. It is periodically washed to remove the slurry by opening the discharge valve.

Water Tubes These are also essential for Feed Water and Steam Circuit of Boiler Water tubes are bent or straight hollow tubes through which steam water mixture circulates. There are two types of water tubes, viz. down-comer and riser. This downcomer, riser assembly is also known as “Evaporator” (or “boiler proper”). In the evaporator actual state change from water to steam occurs. In the T-S diagram beside, the zone of evaporator is illustrated. ‘Q eva’ is the heat absorbed by evaporator. It is mainly the latent heat of vaporization of water.

Down-comers Water Tubes As the name suggests down-comers are the water tubes through which water comes down from steam drum to mud drum (see fig.). No vapour bubble should flow along with saturated water from the drum to the down comers. This will reduce the density difference and the pressure head for natural circulation. Risers Water Tubes Risers are the water tubes through which steam water two-phase mixture at saturation temperature goes up from mud drum to steam drum. Risers are usually close to furnaces, while the down-comers are away from the furnaces.

Super-Heaters Super heater is another important part of Feed Water and Steam Circuit of Boiler 

Super-Heater is an important element of the feed water-steam circuit. It is basically a heat exchanger in which heat is transferred to the aturated steam to increase its temperature. In high pressure boilers more than 40% of the total heat is absorbed by the super heaters. The T-S diagram beside

illustrates the heat absorbed by the super heater and is denoted by ‘Q sh’.

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In super-heater the rate of heat absorption is more. Hence, in the modern water tube boilers there are more. Hence, in the modern water tube boilers there are more super heating surfaces.

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Super-heater tubes are exposed to the highest steam pressure and temperature on the inside and the maximum gas temperature on the outside. They are made of costliest alloys.

Functions of Super-Heater 

An increase in inlet steam temperature gives a steady improvement in cycle efficiency. Hence, the function of super-heater is to raise the overall efficiency. In addition, it reduces the moisture content in the later stages of the turbine and thus increases the turbine internal efficiency.

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However, the increase in temperature is limited by the properties of the construction materials of boilers and turbines. Usually the optimum temperature of steam is maintained 450oC at the turbine inlet.

Boiler Feed Water Treatment Demineralization Reverse Osmosis Plant Deaerator « Previous Next » All natural sources of water contain impurities as well as dissolved gasses. The amount of these impurities depends on type of water source and location.Why it is necessary to treat the raw water? Raw water coming from different sources contains dissolved salts and un-dissolved or suspended impurities. It is necessary to remove harmful salts dissolved into the water before feeding it to the boiler. Because1. The deposition of dissolved salts and suspended impurities will form a scale on the inside wall of different heat-exchangers and thus there will create excessive pressure and thermal stress (due to uneven heat exchange across the wall of heat-exchanger) inside the heat-exchangers, which may lead to the explosion and serious hazards for boilers.

2. The harmful dissolved salts may react with various parts of boiler through which it flows, thereby corrode the surfaces. 3. Corrosion damage may occur to turbine blades. Hence, boiler feed water treatment is very much required to remove such dissolved and suspended impurities from water before feeding it to boiler.

Arrangements for Boiler Feed Water Treatment For continuous supply of feed water to boiler, after removing impurities, there are two types of plant generally incorporated. These are: 1. Demineralization plant (D M plant) 2. Reverse Osmosis plant (R O plant) Demineralization plant employs a chemical method to separate out the dissolved salt in raw water. But reverse osmosis plant employs a simple physical method to separate the dissolved salts. Before feeding the raw water to these plants sand filtration is done by different filters. Along with these plants there are two deaerators, which remove dissolved oxygen in the feed water, as traces of oxygen may react with boiler tubes and thereby corrode those.

Complete arrangements and inside equipment of these plants are described below.

Demineralization Plant The function of demineralization plant is to remove dissolved salt by ion exchange method (chemical method) and there by producing pure feed water for

boiler. The salts which make the water hard are generally-chloride, carbonates, bicarbonates, silicates & phosphates of sodium, potassium, iron, calcium and magnesium. In D M plant there are three types of resin used for boiler feed water treatment process 1. Cation exchange resin 2. Anion exchange resin 3. Mixed Bed resin 1. Resins are chemical substances (usually polymers of high molecular weight) used to react with salts & eliminates them by chemical process. 2. As the name suggests, the cation exchange resin, exchanges the cation & anion exchange resin, exchanges anions with the salts dissolved in hard-water. Cation Exchange Resin NaCl + RSO3H = RSO3 - Na + + HCl Thus H2SO4, H2CO3 are also produced. We have removed Na + but the water has become acidic. Anion Exchange Resin HCl + R4NOH = R4NCl + H2O This way we have eliminated Cl - and thus acidity of the water. Similar reaction for H2SO4 also.

Mixed Bed Resins These mixed bed resins are used in Demineralization plant of boiler feed water treatment, to remove the ions (especially Na + and SO3 2- ) which may further present in the water after foregoing process of purification. Degasser The function of degasser tower is to remove carbonate ions by forming carbondi-oxide. In degasser tower stream of water is poured from top & air is blown from bottom to top. In the pressure of air the carbonic acid (H 2CO3) present in the water dissociates into H2O and CO2. H2CO3 = H2O + CO2 This CO2 is free to mix with air. Benefits of using degasser are: 1. It removes the carbonic acid and other gases mixed with water by simple physical method & thereby reduce the chances of corrosion. 2. It saves the resins which are very costly chemicals and thereby improves the economy of boiler feed water treatment process. The H2CO3 free water is now collected in degasser sump and then pumped to anion exchange resin inlet.

Reverse Osmosis Plant (RO Plant) Like demineralization plant there is another stage of water treatment which is known as reverse osmosis plant. RO plant uses the process known as reverse osmosis to produce salt-free water. The theoretical aspect is described below:-

Osmosis is a process in which only the solvent molecules through a semipermeable membrane from higher solvent density to lower solvent density (i.e. from solution of lower density to the solution of higher density). Osmotic pressure:- It is the minimum pressure that should be applied on the higher

density solution so that no osmosis takes place through the semi-permeable membrane is called the osmotic pressure (π). π = iCRT Where, C is concentration of solution, R is universal gas constant, T is temperature in Kelvin scale, i is van’t Hoff’s factor, different for different solutions. i = 1 for infinitely dilute solution. Hence osmotic Pressure is a function of temperature.

Reverse Osmosis On the higher density solution (lower density solvent) if a pressure (P), greater than osmotic pressure (π) is applied then the solvent molecules through the semi-permeable membrane from higher density solution to lower density solution. This phenomenon is called reverse osmosis. This one important stage for boiler feed water treatment process.

Reverse Osmosis Plant In RO plant using reverse osmosis phenomenon salt-free water is taken out from raw water after the sand filtration. Purity of the salt-free water depends on effectiveness of the permeable membrane.

The layout of a typical Reverse Osmosis Plant is given below

Steam air pre-heater require some steam which will reduce the efficiency of the power plant. The procedure is described below: 1. Sodium hypochlorite (NaOCl) is injected to raw water to kill the algae or bacteria present in the raw water. Otherwise they may cause harm to the multi grade-filter (MGF). 2. The multi-grade filter is the primitive type of filter where sand, stonechips, stones are used in stacks to remove the large size suspended particles from the raw water. 3. The net filter again removes medium-size suspended particles, where the raw water es through the net minute vents. 4. Then by ultra-filtration very small suspended particles are removed. After long usage of ultra-filtration unit, it requires back-wash, and then it is back-washed with water & three chemicals, viz. HCL, NaOH and NaOCl (Sodium Hypochlorite). HCl Removes iron by dissolving it. It also removes the basic salts those are rejected on UFU. NaOH ← It helps to remove acidic salt. NaOCl ← To kill algae and bacteria inside the UFU. 5. After ultra-filtration the water is stored into RO feed tank & then pumped with RO feed pump of Reverse Osmosis Plant. In the channel the water is mixed with HCL (for pH controlling, as the water coming from RO plant or RO permeate water should have pH around 6.0) and SMBS (sodium meta bi-sulphate) [Na2S2O5]. Due to the presence of sodium hypochlorite the water is chlorinated. To remove excess chlorine SMBS is used. If excess chlorine is not removed then the semi-permeable membrane may get damaged. It is also mixed with anti-scaling reagent (AS), which reacts with those chemicals which form scale inside the channel.

6. Then the water is ed through micro-cartridge filter (MCF) which removes the other suspended particles & the precipitate formed by the reaction of anti-scaling reagent with the scaling chemicals. 7. In the this stage of boiler feed water treatment the water is fed to RO unit by H/P pump, where after successive filtration by 1 st and 2nd stage RO it is fed to degasser unit. 8. After degasification the water is ed through D/M plant MB (mixed bed) resin & stored into D/M water storage tank.

Deaerator

Deaerator is a type open heater in which dissolved oxygen in the feed water is removed as much as possible by mechanical means. Gases move from higher partial pressure to lower partial pressure. Partial pressure of oxygen in air is high (as air contains almost 21% oxygen by volume) than the partial pressure of dissolved oxygen in feed water. Hence, by simple mechanical means it is not possible to eliminate the dissolved oxygen from water.

Hence, in deaerator the feed water is heated by LP or VB steam (pressure: 2.5 – 3.5kg/cm2, temperature: 1400°C). Due to heating the partial pressure of dissolved oxygen in feed water increases and solubility decreases to considerable amount. Then by mechanical means dissolved oxygen is released in air. Hence Deaerator is another very important part of boiler feed water treatment plant. The mechanical means is same as that of in degasser. But instead of air, LP steam is blown from bottom to top and feed water is poured from top to bottom. Deaerator also serves as header, to provide a net positive suction head (NPSH) to the boiler feed pumps (BFP) and here by protects the BFPs from any damage due to vapor lock and cavitations. Fuel Bituminous Coal

STOICHIOMETRIC AIR mass / unit mass of fuel 11.18

Anttiasite Coal Coke Liquite Peat Residual Fuel Oil Distillate Fuel Oil(Gas Oil) Natural Gas(Methane Base)

10.7 9.8 7.5 5.7 13.85 14.48 17.3

Combustion of Coal For sufficient air,

We have already said that mass wise there is 23.2 % O 2 presents in air. Hence the amount of air required to provide 2.67 gm of O2 is

As per ideal combustion theory, after combustion of one gm carbon(C), product of combustion contains only 3.67 gm of CO2 and (11.5 - 2.67 =) 8.83 gm of N2 Coal Combustion for Insufficient Air

By weight, the requirement of air for providing this much O 2 is

After combustion of one gm carbon(C), product of combustion contains only 2.33 gm of CO and (5.75 - 1.33 =) 4.42 gm of N 2. From equation (1) and (2) it is clear that due to insufficient air combustion, the heat lose during 1 gm of coal combustion is (33.94 - 10.12) = 23.82 kj

Combustion of Sulfur

So, air required for 1 gm sulfur combustion, is

So, combustion product, after completing 1 gm of sulfur combustion, contains 2 gm of SO2 and (4.31 - 1 = ) 3.31 gm of N2

Combustion of Hydrogen

From combustion theory of C, S and H2 it is found that 2.67 gm oxygen is required for 1 gm carbon combustion, which implies 2.67 C gm oxygen is required for C gm carbon, 1 gm oxygen is required for 1 gm sulfur combustion, which implies S gm oxygen is required for S gm sulfur and 8 gm oxygen is required for 1 gm hydrogen combustion, which implies 8H gm oxygen is required for H gm hydrogen. Hence 1 gm of coal (fuel) which contains C gm carbon, S gm sulfur and H gm hydrogen, requires (2.67 C + S + 8 H) gm of oxygen for efficient combustion. Some amount of oxygen may be contained in the fuel itself in form of different compounds and it takes part in combustion also. If O is the original weight of the oxygen presents in 1 gm of fuel, net requirement of oxygen for sufficient coal combustion is (2.67 C + S + 8 H - O) gm.For that the amount of air required is

This above mentioned analysis is called coal analysis for combustion. Before efficient combustion can take place, several basic requirements must be fulfilled, most important of them are, 1. The combustion must be done with sufficient oxygen. 2. There must be sufficient turbulence to promote throughout mixing of combustible and oxygen.

Coal Content in Proximate Analysis Moisture = 8 %, volatile material = 20 to 25 %, fixed carbon = 40 %, ash = 30 %. Fixed carbon's combustion temperature = 900°C. Basic component of ash is Si, Al and others. Now fusion temperature of Si is 1200°C. If the furnace temperature rises above 1100°C then Si will be fused and deposited on the tubes, as slag, causing improper heat transfer. Now to dilute the temperature excess air and complete combustion are required. Now, the volatile material plays important role in combustion. Less the volatile material flame will be high which may be chance for flame impingement of S/H coil. For fulfilling the point some practical steps to taken. In practice it is always necessary to supply more air to the combustion system than it is theoretically required. Reason for that air and fuel mixing process in any combustion system, as it is not possible to ensure complete and intimate mixing of the fuel with the necessary oxygen at the point of injection. So some excess air is required for proper combustion to a reasonable minimum power, stack loss and unburnt carbon in ash. Generally 20% excess air is allowed.

% of Excess Air

Unburnt Carbon in C.V. Liberated in Ash Furnace

0%

10 %

75 %

15 % 100 %

2% 0.5 %

97 % 99.5 %

Unburnt Gas Loss CO2, O2, N2, H2O, CO, CH4(15 %) CO2, O2, N2, H2, CO(1 %) CO2, O2, N2

Third process is unsatisfactory for extra fan power and convey huge amount of heat. The coal particles should be at least 74 microns in 200 mesh. So pulveriser is required for 1. Better utility of coal 2. Saving of time. There are mainly three losses occurred during coal combustion, 1. Unburnt gas loss 2. Dry flue gas loss 3. Combustible in ash loss. Unburnt Gas Loss the unburnt gas loss is mainly the result of burning carbon to carbon monoxide instead of carbon dioxide. It is seen that heat release in CO reaction is

one third of that in CO2 reaction. So adequate supply of oxygen or excess air will quickly reduce this loss to zero. Dry Flue Gas Loss A further loss of heat is that due to dry flue gas. It is often referred to as the stack loss. If more excess air is itted, this loss increases. Combustible in Ash Loss This loss is very high when there is little or no excess air because mixing of combustible material and oxygen is so poor. As the air quantity is increased, the loss falls rapidly. However it does not reach to "zero" because the loss depends upon two factors firstly on air - coal mixture and secondly on fineness of pulverized coal grain. More fine grain of pulverized coal helps to complete combustion more perfectly and resulting less combustible in ash loss. In practice, though, a stage is reached where it is not worth grinding the coal any finer because it will cost more to grind than the extra heat release. Practically the loss does not reach to zero. generally a high volatile coal is crushed until 75 % of its bulk es through a 200 mesh whereas a low volatile coal is crushed until 80 % es through similar mesh. The loss gets less as excess air is added, reaches a minimum and then increases as still more excess air is added. Thus there is only one quantity of excess air which will give lower loss for the combustion of a particular fuel. For bituminous coal 15.5 % excess air is optimum requirement for Coal Combustion. Fluidized Bed Combustion | Types and Advantages of Fluidized Bed Combustion « Previous 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 ed 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 parameter, like :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. In fluidized bed combustion, rapid mixing ensures uniformity of temperature. The main advantage of fluidized bed combustion system is that municipal waste, sewage plant sludge, biomass, agricultural waste and other high moisture fuels can be used for heat generation. A fluidized furnace has an enclosed space with a base having openings to it air. Crushed coal, ash and crushed dolomite or limestone is

mixed in the bed furnace and high velocity combustion air is then ed 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 age 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 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 of Fluidized Bed Combustion (FBC) Fluidized Bed combustion can be in 2 variants, namely :1. Vertical type FBC :These are generally used in smaller plant, and has the capacity to produce steam of up to 6 tonnes 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 tonnes 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 and Dis-advantages of Fluidized Bed Combustion 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.(pulverised 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 calciummagnesium carbonate) or lime stone (calcium carbonate) to the furnace the discharge of sulphur oxides to the atmosphere can also be reduced if desired. In view of all these advantages of fluidized bed combustion above, where fluidized bed combustion emerges as the best alternative available today, still the major drawback of this system is that the fan power has to be maintained at a considerably high value, since the air has to be supplied continuously at a very high pressure for ing the bed. This in turn increases the operating cost of the auxiliary units of the plant. But it is more than compensated by the high values of efficiency that FBC provides. Steam Boiler | Working principle and Types of Boiler Next » Boiler or more specifically steam boiler is an essential part of thermal power plant.

Definition of Boiler Steam boiler or simply a boiler is basically a closed vessel into which water is heated until the water is converted into steam at required pressure. This is most basic definition of boiler.

Working Principle of Boiler The basic working principle of boiler is very very simple and easy to understand. The boiler is essentially a closed vessel inside which water is stored. Fuel (generally coal) is bunt in a furnace and hot gasses are produced. These hot gasses come in with water vessel where the heat of these hot gases transfer to the water and consequently steam is produced in the boiler. Then this steam is piped to the turbine of thermal power plant. There are many different types of boiler utilized for different purposes like running a production unit, sanitizing some area, sterilizing equipment, to warm up the surroundings etc.

Steam Boiler Efficiency The percentage of total heat exported by outlet steam in the total heat supplied by the fuel(coal) is called steam boiler efficiency.

I t includes with thermal efficiency, combustion efficiency & fuel to steam efficiency. Steam boiler efficiency depends upon the size of boiler used. A typical efficiency of steam boiler is 80% to 88%. Actually there are some losses occur like incomplete combustion, radiating loss occurs from steam boiler surrounding wall, defective combustion gas etc. Hence, efficiency of steam boiler gives this result.

Types of Boiler There are mainly two types of boiler – water tube boiler and fire tube boiler. In fire tube boiler, there are numbers of tubes through which hot gases are ed and water surrounds these tubes. Water tube boiler is reverse of the fire tube boiler. In water tube boiler the water is heated inside tubes and hot gasses surround these tubes. These are the main two types of boiler but each of the types can be sub divided into many which we will discuss later. Fire Tube Boiler As it indicated from the name, the fire tube boiler consists of numbers of tubes through which hot gasses are ed. These hot gas tubes are immersed into water, in a closed vessel. Actually in fire tube boiler one closed vessel or shell contains water, through which hot tubes are ed. These fire tubes or hot gas tubes heated up the water and convert the water into steam and the steam remains in same vessel. As the water and steam both are in same vessel a fire tube boiler cannot produce steam at very high pressure. Generally it can produce maximum 17.5 kg/cm2 and with a capacity of 9 Metric Ton of steam per hour. Types of Fire Tube Boiler There are different types of fire tube boiler likewise, external furnace and internal furnace fire tube boiler. External furnace boiler can be again categorized into three different types1. Horizontal Return Tubular Boiler. 2. Short Fire Box Boiler. 3. Compact Boiler. Again, internal furnace fire tube boiler has also two main categories such as horizontal tubular and vertical tubular fire tube boiler. Normally horizontal return fire tube boiler is used in thermal power plant of low capacity. It consists of a horizontal drum into which there are numbers of horizontal tubes. These tubes are submerged in water. The fuel (normally coal) burnt below these horizontal drum and the combustible gasses move to the rear from where they enter into fire tubes and travel towards the front into the smoke box. During this travel of gasses in tubes, they transfer their heat into the water and steam bubbles come up. As steam is produced, the pressure of the boiler developed, in that closed vessel.

Advantages of Fire Tube Boiler 1. It is quite compact in construction. 2. Fluctuation of steam demand can be met easily. 3. It is also quite cheap. Disadvantages of Fire Tube Boiler 1. As the water required for operation of the boiler is quite large, it requires long time for rising steam at desired pressure. 2. As the water and steam are in same vessel the very high pressure of steam is not possible. 3. The steam received from fire tube boiler is not very dry. Water Tube Boiler

A water tube boiler is such kind of boiler where the water is heated inside tubes and the hot gasses surround them

. This is the basic definition of water tube boiler. Actually this boiler is just opposite of fire tube boiler where hot gasses are ed through tubes which are surrounded by water. Types of Water Tube Boiler There are many types of water tube boilers, such as 1. Horizontal Straight Tube Boiler. 2. Bent Tube Boiler. 3. Cyclone Fired Boiler. Horizontal Straight Tube Boiler again can be sub - divided into two different types,

1. Longitudinal Drum Water Tube Boiler. 2. Cross Drum Water Tube Boiler. Bent Tube Boiler also can be sub divided into four different types, 1. Two Drum Bent Tube Boiler. 2. Three Drum Bent Tube Boiler. 3. Low Head Three Drum Bent Tube Boiler. 4. Four Drum Bent Tube Boiler. Advantages of Water Tube Boiler There are many advantages of water tube boiler due to which these types of boiler are essentially used in large thermal power plant. 1. Larger heating surface can be achieved by using more numbers of water tubes. 2. Due to convectional flow, movement of water is much faster than that of fire tube boiler, hence rate of heat transfer is high which results into higher efficiency. 3. Very high pressure in order of 140 kg/cm2 can be obtained smoothly. Disadvantages of Water Tube Boiler 1. The main disadvantage of water tube boiler is that it is not compact in construction. 2. Its cost is not cheap. 3. Size is a difficulty for transportation and construction.