boilers and classification of boilers

🔎What is a boiler? 

🔥A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, central heating, boiler-based power generation, cooking, and sanitation.

🔥CLASSIFICATION OF BOILERS🔥

♨️Boilers are classified in many different ways. some important methods are discussed below.

♨️ Boilers are commonly classified as fire tube or water tube boiler based on the way the water and gases pass through them.

Based on the type of fuel used, boilers are classified as coal-fired, oil fired or gas-fired boilers.

♨️Boilers are also classified based on their method of manufacture such as packaged boilers or field-erected boilers.

Based on end application, boilers are classified as utility, industrial, or marine boilers.

♨️A further of classification includes pressure level(low pressure, subcritical pressure, supercritical pressure, etc.) and the type of circulation(natural, pumped, once through, combined).

1️⃣ FIRE TUBE BOILER

♨️A typical externally fired, fire-tube boiler schematic as shown in figure.

The main advantages of fire-tube boilers are that they are simpler in design and generally lower in initial cost compared to water of equal capacity.

♨️They require less draught.

♨️Limitations of fire-tube boilers are limited pressure and capacity. So it is not used in a power sector.

♨️Compared to water-tube boilers, they are slower to respond to demand for steam. Also, stresses are greater in boilers because of their rigid design and subsequent inability to expand and contract easily.

TYPES OF FIRE TUBE BOILER 

♨️According to the location of furnace there are two types of fire tube boiler and these are external furnace and internal furnace type.
♨️There are mainly three types of external furnace fire tube boiler.

1. Horizontal return tubular fire tube boiler.
2. Short fire box fire tube boiler.
3. Compact fire tube boiler.

♨️There are also two types of internal furnace fire tube boiler

1. Horizontal tubular.
2. Vertical tubular fire tube boiler.

🔥WORKING OF HORIZONTAL RETURN FIRE TUBE BOILER🔥

♨️Horizontal return fire tube boiler is most suitable for low capacity thermal power plant . The main constructional features of this boiler are one big size steam drum which lies horizontally upon supporting structures. There are numbers of fire tubes come from furnace and also aligned horizontally inside the drum. When the drum is filled with water these tubes are submerged in water.

♨️The fuels (normally coal) burnt in the furnace and combustible gasses move into the fire tubes, travel through these tubes from rear to front of the boiler drum and finally the gases come into the smoke box. The hot gasses in the tubes under water transfer heat to the water via the tube walls. Due to this heat energy steam bubbles are created and come upon the water surface. As the amount of steam is increased in that closed drum, steam pressure inside the drum increases which increase significantly the boiling temperature of the water and hence rate of production of steam is reduced. In this way a fire tube boiler controls its own pressure. In other words this is a self pressure controlled boiler.

🔥Advantages of Fire Tube Boiler🔥

1. Compact in construction.
2. Fluctuation of steam demand can be met easily.
3. Cheaper than water tube boiler.

🔥Disadvantages of Fire Tube Boiler🔥

1. Due to large water the required steam pressure rising time quite high.
2. Output steam pressure cannot be very high since the water and steam are kept in same vessel.
3. The steam received from fire tube boiler is not very dry.
4. In a fire tube boiler, the steam drum is always under pressure, so there may be a chance of huge explosion which resulting to severe accident.

2️⃣ PACKAGED BOILER

♨️Packaged boilers are company-made plug-and –play types of units.

♨️They have inherent advantages like compactness, short, easy operation, and hence, they are preferred   in process industries.

♨️Packaged boilers are generally of shell type with a large number of small- diameter tubes resulting in faster evaporation due to rate of convective heat transfer.Steam parameters vary from 10 bar saturated to 100 bar, 550 deg C.

♨️Further classification of packaged boilers includes number of passes of flue gas.

♨️Package boilers are fired from fuel oil in the form of liquid or gas. Fuel is ignited in the burners which creates an explosion within the boiler. Such package boiler do not require purifiers (filters) because they burn consistently removing all contaminants within the fuel. 

♨️To create constant combustion within the boiler, the forced draft fan forces air into the burner, causing a tornado effect creating turbulence to keep the flame ignited and the furnace pressurized. 

♨️Other essentials such as Burner electronics provide auto-ignition and on-demand lighting feature that monitors the flame and pressure within the boiler. tube boiler Package boilers are commonly called water or fire tube Boilers. 

♨️Water tube boilers use convection heating, which draws the heat from the fire source, and passes against the generating tubes of the boiler, causing water inside those tubes to boil off into steam.

 ♨️The fire tube boiler arrangement utilizes conduction heating which transfers heat from physical contact. Fire tube boilers are not commonly used due to their method of conduction heating because pipes in direct contact with fire and cold water could damage the pipes. ♨️The package boiler is usually a two or three-pass fire-tube boiler with an internal furnace tube. This is similar to the much earlier Scotch boiler.

3️⃣ WATER TUBE BOILER

♨️The tubes extend between an upper header, called a steam drum, and one or more lower headers or drums.

♨️ Water tube boilers are designed to permit higher pressure and capacity. Because the pressure is confined inside the tubes, water tube boilers are fabricated in larger sizes compare to fire tube boilers.

♨️The prime advantage of a water-tube boiler is it can burn almost any solid, liquid, or gaseous fuel.

♨️Apart from common fuels, other fluids like biomass, Municipal Solid Waste (MSW), Tire Derived Fuel (TDF), and Refuse-Derived Fuel(RDF) can be used.

♨️ Another advantage of water-tube boilers withstand very high pressure and temperature, respond quickly to change in demand, expand and contract more easily than fire tube boilers, and due to this, generally have a longer service life.

♨️A high pressure watertube boiler (also spelled water-tube and water tube) is a type of boiler in which water circulates in tubes heated externally by the fire. 

♨️Fuel is burned inside the furnace, creating hot gas which heats water in the steam-generating tubes. In smaller boilers, additional generating tubes are separate in the furnace, while larger utility boilers rely on the water-filled tubes that make up the walls of the furnace to generate steam.

♨️The heated water then rises into the steam drum. Here, saturated steam is drawn off the top of the drum. In some services, the steam will reenter the furnace through a superheater to become superheated.

 ♨️Superheated steam is defined as steam that is heated above the boiling point at a given pressure. Superheated steam is a dry gas and therefore used to drive turbines, since water droplets can severely damage turbine blades.

♨️Cool water at the bottom of the steam drum returns to the feedwater drum via large-bore 'downcomer tubes', where it pre-heats the feedwater supply. (In large utility boilers, the feedwater is supplied to the steam drum and the downcomers supply water to the bottom of the waterwalls). 

♨️To increase economy of the boiler, exhaust gases are also used to pre-heat the air blown into the furnace and warm the feedwater supply. Such watertube boilers in thermal power stations are also called steam generating units.

♨️The older fire tube boilers design, in which the water surrounds the heat source and gases from combustion pass through tubes within the water space, is a much weaker structure and is rarely used for pressures above 2.4 MPa (350 psi). 

♨️A significant advantage of the watertube boiler is that there is less chance of a catastrophic failure: there is not a large volume of water in the boiler nor are there large mechanical elements subject to failure.

♨️A water tube boiler was patented by Blakey of England in 1766 and was made by Dallery of France in 1780.

♨️The main disadvantage is initial cost due to more sophisticated construction.

The three major configuration of water tube boilers are

👉Straight-tube longitudinal drum

👉Straight-tube cross drum 

👉Stiriling boiler

☝️STIRLING BOILER DIAGRAM☝️

5️⃣ STOKER FIRED BOILER

♨️Stroke fired boiler have a mechanical system designed to feed solid fuel into the boiler.

♨️These stokers are designed to support combustion process and to remove ash as it accumulates.

♨️Modern mechanical strokes includes

♀️A fuel admission system

♀️A stationary or moving grate assembly that supports the burning of fuel and provides a pathway for the primary combustion air

♀️A secondary air in system that supplies additional air for complete combustion and minimize atmospheric emissions and

an ash-discharge system. 

♨️Stoker Fired Boiler are classified according to the method of feeding fuel to the furnace and by the type of grate. The main types of stokers are:

1. Chain-grate or travelling-grate stoker

♨️In Travelling Grate  boiler coal is fed at one end of a moving steel chain grate. As grate moves along the length of the boiler furnace, the coal burns before dropping off at the end as ash. 

♨️Some degree of skill is required, particularly when setting up the grate, air dampers and baffles, to ensure clean combustion leaving minimum of unburnt carbon in the ash.

♨️The coal-feed hopper runs along the entire coal-feed end of the furnace. A coal grate is used to control the rate at which coal is fed into the furnace, and to control the thickness of the coal bed and speed of the grate. 

♨️Coal must be uniform in size, as large lumps will not burn out completely by the time they reach the end of the grate. 

♨️As the bed thickness decreases from coal-feed end to rear end, different amounts of air are required more quantity at coal-feed end and less at rear end. 

2.Spreader stoker boiler

♨️Spreader stokers utilize a combination of suspension burning and grate burning. The coal is continually fed into the furnace above a burning bed of coal. 

♨️The coal fines are burned in suspension; the larger particles fall to the grate, where they are burned in a thin, fastburning coal bed. 

♨️This method of firing provides good flexibility to meet load fluctuations, since ignition is almost instantaneous when firing rate is increased. Hence, the spreader stoker is favored over other types of stokers in many industrial applications.♨️Spreader feeders have the capability to uniformly feed coal into a device that can propel it along the depth of a grate in an evenly distributed pattern. Many designs have been used successfully over the years. 

♨️The coal feed mechanisms include gravity reciprocating plates and metering chain conveyors. The mechanisms that propel the coal into the furnace include steam and air injection as well as underthrow and overthrow rotors. Steam or air assist can be used with the rotor systems.

♀️It can be further classified as either underfeed or overfeed stokers.

👉Underfeed stokers. 

👉Overfeed stokers.

👉Mass-feed stokers

👉Spreader stokers

7️⃣ PULVARIZED COAL BOILER

♨️The pulverized coal boiler is the most favourite method of burning coal.

In this, coal mechanically pulverized into a fine powder, which enables it to burn like gas and allow more efficient combustion. Many water tube boilers  use pulverized fuel in which coal is crushed to a powder form so that less than 2% is +300µm size and 75% is -75µm size.

♨️Pulverized coal is supplied with primary air to the burners.

♨️The main advantage is of quick response to load  variation  and ability to use high preheat air Temperature. 

The concept of burning coal that has been pulvarized into a fine powder stems from the belief that if the coal is made fine enough, it will burn almost as easily and efficiently as a gas. The feeding rate of coal according to the boiler demand and the amount of air available for drying and transporting the pulverized coal fuel is controlled by computers. Pieces of coal are crushed between balls or cylindrical rollers that move between two tracks or "races." 

♨️The raw coal is then fed into the pulvarizer along with air heated to about 650°F / 340°C from the boiler. As the coal gets crushed by the rolling action, the hot air dries it and blows the usable fine coal powder out to be used as fuel. The powdered coal from the pulverizer is directly blown to a burner in the boiler. 

♨️The burner mixes the powdered coal in the air suspension with additional pre-heated combustion air and forces it out of a nozzle similar in action to fuel being atomized by a fuel injector in modern cars. ♨️Under operating conditions, there is enough heat in the combustion zone to ignite all the incoming fuel. 

🔥ASH REMOVAL🔥

♨️There are two methods of ash removal at furnace bottom:

• Dry bottom boiler
• Wet bottom boiler, also called Slag tap

♨️The fly ash is carried away with the flue gas and is separated in various hoppers in the path and finally in an ESP or a bag filter.

8️⃣ FLUIDIZED BED BOILER

♨️Fluidized- bed combustion boilers are capable of burning a variety of solid fuels including biomass fuel.

♨️ As shown in figure, the fuel burns in a bed of hot incombustible particles suspended by an upward flow of air. The advantage of FBC boiler is that fuels containing high concentration of ash, sulphur and nitrogen burn efficiently with lower emission levels. 

♨️To start FBC boilers, natural gas or fuel oil is used.

♨️Advantage of FBC boilers are combustion takes place at relatively lower temperature which results in lower NO2 emission and reduced slag information; another advantage is limestone added to the fluidized bed helps to remove sulphur. 

♨️In its most basic form, fuel particles are suspended in a hot, bubbling fluidity bed of ash and other particulate materials (sand, limestone etc.) through which jets of air are blown to provide the oxygen required for combustion or gasification. ♨️The resultant fast and intimate mixing of gas and solids promotes rapid heat transfer and chemical reactions within the bed. 

♨️FBC plants are capable of burning a variety of low-grade solid fuels, including most types of coal and woody biomass, at high efficiency and without the necessity for expensive fuel preparation (e.g., pulvarizing). In addition, for any given thermal duty, FBCs are smaller than the equivalent conventional furnace, so may offer significant advantages over the latter in terms of cost and flexibility.

♨️FBC reduces the amount of sulphur emitted in the form of sulphur dioxide emissions. Limestone is used to precipitate out sulfate during combustion, which also allows more efficient heat transfer from the boiler to the apparatus used to capture the heat energy (usually water tubes).

 ♨️The heated precipitate coming in direct contact with the tubes (heating by conduction) increases the efficiency. Since this allows coal plants to burn at cooler temperatures, less nitrogen dioxide is also emitted. 

♨️However, burning at low temperatures also causes polycyclic aromatic hydrocarbon emissions. FBC boilers can burn fuels other than coal, and the lower temperatures of combustion (800 °C / 1500 °F) have other added benefits as well.

🔥BENEFITS OF FLUIDIZED BED BOILER

♨️There are two reasons for the rapid increase of FBC in combustors. First, the liberty of choice in respect of fuels in general, not only the possibility of using fuels which are difficult to burn using other technologies, is an important advantage of fluidized bed combustion. ♨️The second reason, which has become increasingly important, is the possibility of achieving, during combustion, a low emission of nitric oxides and the possibility of removing sulfur in a simple manner by using limestone as bed material.

♨️Fluidized-bed combustion evolved from efforts to find a combustion process able to control pollutant emissions without external emission controls (such as scrubbers-flue gas desulfurization).

♨️ The technology burns fuel at temperatures of 1,400 to 1,700 °F (750-900 °C), well below the threshold where nitrogen oxides form (at approximately 2,500 °F / 1400 °C, the nitrogen and oxygen atoms in the combustion air combine to form nitrogen oxide pollutants); 

♨️it also avoids the ash melting problems related to high combustion temperature. The mixing action of the fluidized bed brings the flue gases into contact with a sulphur-absorbing chemical, such as limestone or dolomite. More than 95% of the sulfur pollutants in coal can be captured inside the boiler by the sorbent. ♨️The reductions may be less substantial than they seem, however, as they coincide with dramatic increases in polycyclic aromatic hydrocarbons, and possibly other carbon compound emissions.

♨️Commercial FBC units operate at competitive efficiencies, cost less than today's conventional boiler units, and have SO₂ and NO₂ emissions below levels mandated by Federal standards. 

♨️However, they have some disadvantages such as erosion on the tubes inside the boiler, uneven temperature distribution caused by clogs on the air inlet of the bed, long starting times reaching up to 48 hours in some cases.

1. FBC has a lower combustion temperature of 750 °C whereas an ordinary boiler operates at 850 °C.
2. FBC has low sintering process (melting of Ash).
3. Lower production of NO_x due to lower temperature.
4. Lower production of SO_x due to capture by limestone.
5. Higher combustion efficiency due to 10 times more heat transfer than other combustion processes because of burning particle.
6. Less area is required for FBC due to high coefficient of convective heat transfer.
7. Iso-thermal bed combustion as temperature in free belt and active belt remain constant.

🔥TYPES OF FLUIDIZED BED BOILERS 🔥

♨️FBC systems fit into essentially two major groups, atmospheric systems (FBC) and pressurized systems (PFBC), and two minor subgroups, bubbling (BFB) and circulating fluidized bed (CFB).

🔥Fluidized Bed Combustible🔥

♨️Atmospheric fluidized beds use limestone or dolomite to capture sulfur released by the combustion of coal.

 ♨️Jets of air suspend the mixture of sorbent and burning coal during combustion, converting the mixture into a suspension of red-hot particles that flow like a fluid. These boilers operate at atmospheric pressure.

🔥Pressurized Fluidized Bed Combustion🔥

♨️The first-generation PFBC system also uses a sorbent and jets of air to suspend the mixture of sorbent and burning coal during combustion. 

♨️However, these systems operate at elevated pressures and produce a high-pressure gas stream at temperatures that can drive a gas turbine . 

♨️Steam generated from the heat in the fluidized bed is sent to a steam turbine , creating a highly efficient combined cycle system.

🔥Advanced PFBC🔥

• A 1½ generation PFBC system increases the gas turbine firing temperature by using natural gas in addition to the vitiated air from the PFB combustor. This mixture is burned in a topping combustor to provide higher inlet temperatures for greater combined cycle efficiency. However, this uses natural gas, usually a higher priced fuel than coal.
• APFBC. In more advanced second-generation PFBC systems, a pressurized carbonizer is incorporated to process the feed coal into fuel gas and char. The PFBC burns the char to produce steam and to heat combustion air for the gas turbine. The fuel gas from the carbonizer burns in a topping combustor linked to a gas turbine, heating the gases to the combustion turbine's rated firing temperature. Heat is recovered from the gas turbine exhaust in order to produce steam, which is used to drive a conventional steam turbine, resulting in a higher overall efficiency for the combined cycle power output. These systems are also called APFBC, or advanced circulating pressurized fluidized-bed combustion combined cycle systems. An APFBC system is entirely coal-fueled.
• GFBCC. Gasification fluidized-bed combustion combined cycle systems, GFBCC, have a pressurized circulating fluidized-bed (PCFB) partial gasifier feeding fuel synhas to the gas turbine topping combustor. The gas turbine exhaust supplies combustion air for the atmospheric circulating fluidized-bed combustor that burns the char from the PCFB partial gasifier.
• CHIPPS:A CHIPPS system is similar, but uses a furnace instead of an atmospheric fluidized-bed combustor. It also has gas turbine air preheater tubes to increase gas turbine cycle efficiency. CHIPPS stands for combustion-based high performance power system.