Power Plant Engg. High Pressure Boilers & FBC Assi. Professor Mechanical Engineering Department
HIGH PRESSURE BOILERS High Pressure Boiler, P > 60 bar Critical Pressure Boiler, P = 221.2 bar Super Critical Pressure Boiler, P > 221.2 bar Sub Critical Boiler P < 221.2 bar generally in between 130 to 180 bar Low Capacity Boiler Ms = 4000 to 6000 kg/hr Heavy Duty boiler Ms > 100000 kg/hr
DEPENDING ON TYPE OF FIRING ADOPTED IN BOILERS THEY CAN BE CLASSIFIED AS Stoker fired Pulverized coal fired Down shot fired Fluidized bed boilers Cyclone fired Chemical recovery boilers
VARIOUS TYPES OF ARRANGEMENT ARE USED BY DESIGNERS IN DESIGNING THE BOILER FOR MEETING THE END REQUIREMENT. HENCE BOILERS ARE CLASSIFIED BASED ON THE ARRANGEMENT AS Top supported boilers Bottom supported Package boilers Field erected boilers Drum type boilers Single drum Bi drum Three drums, but these are presently out of use Tower type or single pass Close coupled Two pass boilers
ACCORDING THE "ASME BOILER AND PRESSURE VESSEL CODE" BOILERS MAY BE CLASSIFIED AS Section I Power Boilers - process boilers, power boilers and high pressure boilers boilers in which steam or other vapor is generated at a pressures exceeding 15 psig high temperature water boilers intended for operation at pressures exceeding 160 psig and or temperatures exceeding 250 degrees F Section IV Heating Boilers - commercial boilers, industrial boilers, heating boilers, low pressure boilers boilers in which steam or other vapor is generated at a pressures not exceeding 15 psig high temperature water boilers intended for operation at pressures not exceeding 160 psig and or temperatures exceeding 250 degrees F
CHARACTERISTICS OF HIGH PRESSURE BOILER Necessity of forced circulation for water Pressurized combustion Increased heat transfer area water tubes Improved medium heating
ADVANTAGES OF HIGH PRESSURE BOILERS The different advantages of high pressure boilers are listed below : 1. The tendency of scale formation is eliminated due to high velocity of water through the tubes. 2. Light weight tubes with better heating surface arrangement can be used. The space required is also less. The cost of foundation, the time of erection and cost are reduced due to less weight of the tubes used. 3. Due to use of forced circulation, there is more freedom in the arrangement of furnace, tubes and boiler components. 4. All the parts are uniformly heated, therefore the danger of overheating is reduced and thermal stress problem is simplified. 5. The differential expansion is reduced due to uniform temperature and this reduces the possibility of gas and air leakages.
6. The components can be arranged horizontally as high head required for natural circulation is eliminated using forced circulation. There is a greater flexibility in the components arrangement. 7. The steam can be raised quickly to meet the variable load requirements without the use of complicated control devices. 8. The efficiency of plant is increased up to 40 to 42% by using high pressure and high temperature steam. 9. A very rapid start from cold is possible if an external supply of power is available. Hence the boiler can be used for carrying peak loads or standby purposes with hydraulic station.
La Mont Boiler
A forced circulation boiler was first introduced in 1925 by La Mont. Working: The feed water from hot well is supplied to a storage and separating drum (boiler) through the economises. The most of the sensible heat is supplied to the feed water passing through the economiser. A centrifugal pump circulates the water equal to 8 to 10 times the weight of steam evaporated. This water is circulated through the evaporator tubes and the part of the water evaporated is separated in the separator drum. The steam separated in the boiler is further passed through the superheater as shown in Fig. and finally supplied to the prime mover. Capacity: These boilers have been built to generate 45 to 50 tons of superheated steam at a pressure of 120 bar. and at a temperature of 500 C.
Limitation The main difficulty experienced in the La Mont boiler is the formation and attachment of bubbles' on the inner surfaces of the heating tubes. The attached bubbles to the tube surfaces reduced the heat flow and steam generation as it offers high thermal resistance than water film. Benson Boiler Benson in 1922 argued that if the boiler pressure was raised to critical pressure (225 bar), the steam and water have the same density and therefore the danger of bubble formation can be easily eliminated. The technical development at that time did not allow to build turbines for such high pressures. The first high pressure Benson boiler was put into operation in 1927
During starting, the water is passed through the economiser, evaporator, superheater and back to the feed line via starting valve A. During starting the valve B is closed. As the steam generation starts and it becomes superheated, the valve A is closed and the valve B is opened. During starting, first During starting, first circulating pumps are started and then the burners are started to avoid the overheating of evaporator and superheater tubes.
Capacity The maximum working pressure obtained so far from commercial Benson boiler is 500 bar. The Benson boilers of 150 tones/ hr. generating capacity are in use. Boiler having as high as 650 C temperature of steam had been put in service. Advantages. 1. As there are no drums, the total weight of Benson boiler is 20% less than other boilers. This also reduces the cost of boiler. 2. The transfer of Benson's parts is easy as there is no drums and majority of the parts are carried to the site without pre-assembly. 3. The erection of Benson boiler is easier and quicker as all the parts are welded at sites and workshop job of tube expansion is altogether avoided. 4. The Benson boiler can be erected in a comparatively smaller floor area. The space problem does not control the size of Benson boiler used. 5. The furnace walls of the boiler can be more efficiently protected by using smaller diameter and closed pitched tubes. 6. The super heater in the Benson boiler is an integral part of forced circulation system, therefore no special starting arrangement for superheated is required. 7. The Benson boiler can be started very quickly because of welded joints.
Primary separator Primary Evaporator Primary Cricket Schmidt Hartmann Boiler
The arrangement of the boiler components is shown in Fig. The operation of the boiler is similar to an electric transformer. Two pressures are used to effect an intercharge of energy.
In the primary circuit, the steam at 100 bar is produced from distilled water. The generated steam is passed through a submerged heating coil which is located in an evaporater drum as shown in figure. The high pressure steam in this coil possesses sufficient thermal potential and steam at 60 bar with a heat transfer rate of 10,000 kj/m 2 -hr C is generated in the evaporator drum. Natural circulation is used in the primary circuit and this is sufficient to effect the desired rate of heat transfer and to overcome the thermo-siphon head of about 2 m to 10 m. In normal circumstances, the replenishment of distilled water in the primary circuit is not required as every care is taken in design and construction to prevent the leakage. But as a safeguard against leakage, a pressure gauge and safety valve are fitted in the circuit.
Advantages. 1. There is a rare chance of overheating or burning the highly heated components of the primary circuit as there is no chance of interruption to the circulation either by rust or any other material. The highly heated parts run very safely throughout the life of the boiler. 2. The salt deposited in the evaporator drum due to the circulation of impure water can be easily brushed off just by removing the submerged coil from the drum or by blowing off the water. 3. The wide fluctuations of load are easily taken by this boiler without undue priming or abnormal increase in the primary pressure due to high thermal and water capacity of the boiler. 4. The absence of water risers in the drum, and moderate temperature difference across the heating coil allows evaporation to proceed without priming.
The major difficulty experienced in La Mont boiler is the deposition of salt and sediment on the inner surfaces of the water tubes. The deposition reduced the heat transfer and ultimately the generating capacity. This further increased the danger of overheating the tubes due to salt deposition as it has high thermal resistance. This difficulty was solved in Loeffler Boiler by preventing the flow of water into the boiler tubes. Most of the steam is generated outside from the feed water by using part of the superheated steam coming out from the boiler. Thermal PP, Ukai
LOEFFLER BOILER
The pressure feed pump draws the water through the economiser and delivers it into the evaporator dram. About 65 % of the steam coming out of superheater is passed through the evaporator dram in order to evaporate the feed water. The steam circulating pump draws the saturated steam from the evaporator drum and is passed through the radiant superheater and then convective superheater. About 35% of the steam coming out from the superheater is supplied to the HP steam turbine. The steam coming out from HP. turbine is passed through reheater before supplying to LP turbine. This boiler can carry higher salt concentration than any other type and is more compact than indirectly heated boilers having natural circulation. These qualities fit it for land or sea transport power generation. Capacity: Loeffler boilers with generating capacity of 100 tonnes/hr operating at 140 bar are already commissioned.
VELOX BOILER
The velocity of flue gases exceeds the velocity of sound, therefore the heat transfer from flue gases at a much greater rate than the achieved at low velocity. Air is compressed by air compressor which is run by gas turbine. The fuel and air are injected downwards into a vertical combustion chamber The combustion chamber consists of annulus water tubes. The product of combustion (flue gages) are deflected upwards with supersonic velocity. As a result, the heat is transferred from flue gases to water at a very high rate. Capacity: Steam generating capacity: 100 tones/hours Pressure: 84 bar Limitation: The size of the Velox boiler is limited (100 tones/hour) because more power is required for running the air compressor. Power produced by gas turbine is not sufficient to run the air compressor and hence balance power from external source must be supplied to the compressor.
SUPERCRITICAL BOILER As pressure of water or steam is raised, the enthalpy of evaporation is reduced. At critical pressure (221.05 bar) the enthalpy of evaporation becomes zero. When water is heated at constant supercritical pressure suddenly it is converted into steam. the high pressure (above critical point) water enters the tube inlets and leaves at the outlet as the superheated steam. There is no drum, but there should be a transition section where the water is likely to flash in order to accommodate the large increase in volume. Guru Nanak Dev Thermal Plant in n Bathinda City
Supercritical Boiler
Advantages: Heat transfer rates are considerably large compared to subcritical boilers. There is no drum, less heat capacity of the generator and hence more stable and gives better response. There is no two phase mixture and hence the problem of erosion and corrosion are minimized. There is great ease of operation and their comparative simplicity and flexibility made them adaptable to load fluctuations. Higher thermal efficiency (about 40 to 42%) of power station can be achieved with use of supercritical boiler.
Limitations The high pressure and temperature of supercritical boiler have limits for use due to availability of material and difficulties experienced in the turbine and condenser operation because of large volumes. The additional problem is created due to the separation of solid impurities as phase changes. These solids remain in the tubes and block the passage for the flow of feed water. Therefore it is necessary to treat water thoroughly before supply to the boiler.
When the combustion is carried out under high pressure by supplying the compressed air then rate of heat transfer is increased and heating surfaces required is reduced. This theory is used in supercharged boiler. The high pressure compressed air is supplied to combustion chamber. Part of heat of hot gases in the furnace is absorbed by boiler tubes in which water evaporated and then it is superheated in superheater. The high pressure and temperature exhaust gases from combustion chamber are used to run gas turbine. The work produces by gas turbine is used to run air compressor. The exhaust gases coming from gas turbine passes over the economizer tubes. Then escape to the atmosphere through chimney. In economizer usually pressure of gas side is 5 bar and pressure to the steam side of 200 bar are preferred.
Supercharged boiler
Advantages Supercharged boiler requires 30 to 25% of heat transfer surface of conventional boiler due to very high overall heat transfer co-efficient. Due to small heat capacity of the boiler, boiler plant gives better response to control. Rapid start of the boiler is possible due to less heating surfaces and compactness. The part of the gas turbine output can be used to drive other auxiliaries. Comparatively less number of operators are required. Limitation: It requires tight passage for high pressure gas. Jaitapur Nuclear PP
Fluidized Bed Combustion It is a system in which fluidized bed which is composed of fuel and inert material is mixed with air/gas in an atmospheric or pressurized vessel and combustion take places in suspended condition of particles in gas stream. When air or other gas flows upward through bed, the bed solid particles are disturbed. If velocity increased further a stage is reached and the composed (packed) bed becomes turbulent and rapid mixing of particles occurs. The behavior of this mixture of solid particles and air or gas is like a fluid. Burning of a fuel in such a stage is known as fluidized bed combustion.
The mixture of fuel (crushed coal) and inert material (crushed dolomite* or limestone) are fed on a distribution plate and air is supplied from the bottom of distribution plate. The air is supplied at high velocity so that solid particles of feed material remains in suspension condition during burning. The evaporator tubes are directly immersed in the fluidized bed and direct contact between the burning coal particles and tubes produce very high heat transfer rates. * A kind of sedimentary rock resembling marble or limestone but rich in magnesium carbonate
The world power industry is trying to shift from oil / gas to old faithful fuel coal and this is possible by Fluidized bed combustion. There are two reasons for the rapid increase of fluidized bed combustion (FBC) in combustors. First, the liberty of choice in respect of fuels. Not only coal, there is possibility of using fuels which are difficult to burn using other technologies. This is an important advantage of fluidized bed combustion. The second reason, a low emission of nitric oxides and the possibility of removing sulfur in a simple manner by using limestone as bed material.
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). Conventional FBC 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. PFBC 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.
Conventional FBC The pressure inside the bed is atmospheric. The bed consisting about 97% limestone or inert material and 3% burning fuel, is suspended by hot primary air entering the bottom of the combustion chamber. There are two types of system based on the fuel feeding arrangement as underfeed and overfeed.
In case of underfeed fuel and limestone are introduced from bottom of the fluidized bed. The overfeed system is simple in operation and economical in running but results in smaller output per m 2 area and gives poor desulphurization performance. Under feed system provides positive load and a compact design but costly in operation. Limitation The main disadvantage using the conventional FBC is that incombustible particles (ash and metals) of fuel are came downwards and block the distributor when light materials like wood dust, agricultural waste etc. are used as a fuel. There for modifications are required for high combustion efficiency.
Bubbling fluidized bed (BFB) Bubbling caps For plants with a nominal boiler capacity of over 20 MW is suitable. In BFB furnaces, a bed material is located in the bottom part.
The primary air is supplied over a nozzle distributor plate and fluidises the bed. The bed material is usually silica sand of about 1.0 mm in diameter; the fluidisation velocity of the air varies between 1.0 and 2.5 m/s. The secondary air is introduced through several inlets in the form of groups of horizontally arranged nozzles at the beginning of the upper part of the furnace (called freeboard) to ensure a staged-air supply to reduce NOx emissions. The fuel amounts only to 1 to 2% of the bed material and the bed has to be heated (internally or externally) before the fuel is introduced. The advantage of BFB furnaces is their flexibility concerning particle size and moisture content of the biomass fuels. Furthermore, it is also possible to use mixtures of different kinds of biomass or to co-fire them with other fuels. One big disadvantage of BFB furnaces, the difficulties they have at partial load operation, is solved in modern furnaces by splitting or staging the bed.
Circulation FBC System.
To increases the efficiency of combustion - provide non uniform fluidizing velocities over the bed, - provide slope to one of the FBC walls - provide sloping distributor plate to give an air slide particles. Due to above there is significant improvement and allowed to use light materials as fuels most successfully. Light materials were burned within the bed and heavy incombustibles (ash and metals) gathered at the bottom of the sloping distributor. The solid fuel enters the furnace from the side of walls. The low velocity (LV), medium velocity (MV) and high velocity (HV) air supplied at different points along the sloping surface of the distributor plate. The secondary air is supplied over the bed. The ash port is provided at lower end of the distributor plate.
In this system pressurized air (10 bar approximately) is used for fluidization and combustion. Boiler exit gas contain enough energy about temperature 850 to 900 C to drive a gas turbine. The power output of gas turbine is utilized to run the air compressor and the electric generator. Here, the product of combustion have to be sufficient clean for gas turbine to prevent excessive erosion, corrosion or fouling of the turbine. Hence, the flue gases from the combustion chamber are passed through a cyclone separator. Advantages: In PFBC system, high rate of coal loading and burning is achieved. Comparatively less volume of furnace is required, hence plant size is reduced. It has improved desulphurization and low NOx emission. Disadvantages : (1) The controlling is difficult to control. With compare to conventional plant, life is low.