Boilers & Turbines

Cheatsheet Content

### Fluidized Bed Combustion (FBC) Principle Fluidized Bed Combustion (FBC) is a combustion technology used in boilers to burn solid fuels. The key principle is to suspend solid fuel particles in an upward-flowing stream of air. - **Fluidization:** Air is blown through a bed of inert material (like sand or limestone) and fuel particles. At a certain velocity, the solid particles become suspended and behave like a fluid, creating a "fluidized bed." - **Combustion:** Fuel is continuously fed into this fluidized bed. The intimate mixing of fuel, air, and inert material ensures efficient and uniform combustion at lower temperatures ($800-900^\circ C$) compared to conventional boilers. - **Pollution Control:** Limestone in the bed reacts with sulfur dioxide ($SO_2$) released from burning coal, capturing it *in-situ* and reducing sulfur emissions. Lower combustion temperatures also reduce nitrogen oxide ($NO_x$) formation. - **Fuel Flexibility:** FBC boilers can efficiently burn a wide variety of fuels, including low-grade coals, biomass, and industrial wastes, due to the excellent mixing and heat transfer characteristics. ### Circulating Fluidized Bed (CFB) Boiler A Circulating Fluidized Bed (CFB) boiler is an advanced type of FBC boiler characterized by a higher fluidization velocity and continuous circulation of bed material. - **Operation:** Unlike bubbling FBC (BFBC) where particles remain largely within the bed, in CFB, the fluidization velocity is high enough to entrain a significant portion of the bed material and unburnt fuel particles out of the combustion chamber. - **Cyclone Separator:** These entrained solids are separated from the flue gas by a cyclone separator and then returned to the combustion chamber. This continuous circulation improves combustion efficiency, allows for longer residence times for fuel particles, and enhances pollutant capture. - **Advantages:** - Higher combustion efficiency due to longer residence time of fuel. - Better sulfur capture efficiency due to increased contact time between limestone and $SO_2$. - Even greater fuel flexibility. - Reduced $NO_x$ emissions due to staged combustion and lower temperatures. - Compact design for larger capacities. ### Compounding of Steam Turbine Compounding in steam turbines refers to the method of reducing the rotational speed of the turbine shaft to a practical limit. High-speed steam exiting the nozzles can reach speeds of over 1000 m/s, which would lead to extremely high turbine rotor speeds (e.g., 30,000 rpm). Such high speeds cause: 1. High stresses in turbine blading. 2. High vibrational losses. 3. High reduction gear ratio requirements for power generation (typically 3000 rpm). Compounding is achieved by absorbing the kinetic energy of the steam in stages, allowing the rotor to rotate at a lower, more manageable speed. The main types are: - **Velocity Compounding:** Steam expands in a single nozzle, gaining high velocity, which is then absorbed in multiple rows of moving and fixed blades. - **Pressure Compounding:** Steam is expanded in multiple stages, with each stage consisting of a nozzle and a moving blade row. - **Pressure-Velocity Compounding:** A combination of both methods. #### Pressure Compound Method In the pressure compound method (also known as Rateau turbine), the total pressure drop of the steam is divided into a number of stages. Each stage consists of a set of fixed nozzles followed by a row of moving blades. - **Working:** 1. Steam expands in the first set of nozzles, causing a drop in pressure and an increase in velocity. 2. This high-velocity steam then impinges on the first row of moving blades, imparting kinetic energy and causing the rotor to turn. The pressure remains constant across the moving blades, but the velocity decreases. 3. The steam then enters the second set of fixed nozzles, where it again expands, experiencing another pressure drop and velocity increase. 4. This process repeats across several stages until the steam reaches the desired exhaust pressure. - **Advantages:** - Lower blade speeds and stresses compared to velocity compounding. - Higher efficiency for a given number of stages. - **Disadvantages:** Requires more stages and a longer turbine casing. ### Draught Classification & Artificial Draught **Draught** (or draft) is the difference in pressure that causes a flow of air and combustion gases through a boiler furnace, chimney, and associated ducts. It is essential for: 1. Supplying fresh air for combustion. 2. Removing flue gases from the furnace. Draught is broadly classified into: - **Natural Draught:** Created by the difference in density between the hot flue gases inside the chimney and the colder ambient air outside. - **Artificial Draught:** Created by mechanical means, such as fans or steam jets. #### Artificial Draught over Natural Draught Artificial draught systems overcome the limitations of natural draught and offer several advantages: 1. **Independent of Atmospheric Conditions:** Natural draught is highly dependent on ambient air temperature and chimney height. Artificial draught provides a consistent flow regardless of weather. 2. **Greater Control:** Fans allow precise control over the air supply and flue gas removal, enabling better combustion efficiency and easier load changes. 3. **Higher Combustion Rates:** Can supply more air for more intense combustion, leading to higher heat release and steam generation rates. 4. **Reduced Chimney Height:** Since the draught is mechanically generated, the chimney does not need to be as tall, reducing construction costs. 5. **Accommodation of Resistance:** Can overcome the resistance offered by various boiler components like economizers, superheaters, air preheaters, and dust collectors, which would otherwise reduce natural draught. 6. **Better Fuel Economy:** Improved control over air-fuel ratio leads to more complete combustion and reduced heat losses. 7. **Ability to Use Low-Grade Fuels:** Can handle the higher air requirements and flue gas volumes associated with burning low-grade fuels. ### Natural Draught Cooling Tower A natural draught cooling tower uses the principle of natural convection to cool water. It typically has a large, hyperbolic shape. - **Sketch:** *Note: This is a general schematic. Actual design may vary.* - **Working:** 1. **Hot Water Inlet:** Hot water from the condenser (e.g., in a power plant) is pumped to the top of the tower and distributed through spray nozzles. 2. **Water Distribution:** The nozzles spray the hot water downwards over a "fill" material (packing). This fill material increases the surface area of the water and slows its descent, promoting better contact with air. 3. **Air Flow (Natural Draught):** As the hot water passes through the fill, it heats the surrounding air. This warm, moist air becomes less dense than the cooler, drier ambient air outside the tower. 4. **Chimney Effect:** The difference in density creates a pressure differential, drawing cooler ambient air in through the air inlet at the base of the tower. This cooler air rises through the fill, causing a natural upward flow (chimney effect). 5. **Evaporation & Convection:** As the air flows upwards, some of the water evaporates, carrying away latent heat and cooling the remaining water. Convective heat transfer also occurs. 6. **Cooled Water Collection:** The cooled water collects in a cold water basin at the bottom of the tower and is then pumped back to the condenser. 7. **Drift Eliminators:** Before exiting the tower, the moist air passes through drift eliminators, which capture any entrained water droplets to minimize water loss. ### Evaporative Condenser An evaporative condenser combines the functions of a cooling tower and a water-cooled condenser. It uses both air and water evaporation to reject heat from the refrigerant. - **Sketch:** *Note: This is a general schematic. Actual design may vary.* - **Working:** 1. **Hot Refrigerant Inlet:** Hot, high-pressure refrigerant vapor from the compressor enters the condensing coils located inside the evaporative condenser. 2. **Water Spray:** Water is continuously sprayed over the outside surface of these coils from a distribution system. 3. **Air Flow:** A fan (either forced or induced draught) draws or pushes ambient air upwards through the unit, passing over the wetted coils. 4. **Heat Transfer & Evaporation:** As the air passes over the water-wetted coils, a small portion of the spray water evaporates. This evaporation provides significant cooling to the remaining spray water and the refrigerant coils. 5. **Refrigerant Condensation:** The refrigerant inside the coils transfers its latent heat to the evaporating water and the moving air. As it loses heat, the refrigerant vapor condenses into a liquid and flows out. 6. **Cooled Water Collection:** The non-evaporated spray water falls into a basin at the bottom, where it is collected and recirculated by a pump back to the spray nozzles. 7. **Drift Eliminators:** Similar to cooling towers, drift eliminators prevent water droplets from being carried out with the exhaust air. - **Advantages:** More efficient than air-cooled condensers, uses less water than conventional water-cooled condensers with separate cooling towers. ### High Pressure Boilers High-pressure boilers operate at pressures significantly above atmospheric pressure, typically above 100 bar. They are commonly used in large thermal power plants for efficient electricity generation. **Examples of High-Pressure Boilers:** - **Benson Boiler** - **La-Mont Boiler** - **Loeffler Boiler** - **Velox Boiler** - **Sulzer Boiler** #### Construction and Working of Benson Boiler The Benson boiler is a once-through, super-critical boiler. It operates above the critical pressure of water (221.2 bar or 22.12 MPa), where there is no distinct phase change from water to steam. Water directly transforms into steam without boiling. - **Construction:** - **Economizer Section:** Water from the feed pump first enters the economizer, where it is heated by the flue gases. - **Evaporator Section (Radiant & Convective):** The preheated water then flows through a series of tubes lining the furnace walls (radiant section) and then through convective sections. Since the pressure is above critical, there are no bubbles formed; the water simply increases in specific volume and enthalpy. - **Superheater Section:** The superheated steam then passes through superheater elements, where its temperature is further increased to the desired level. - **Reheater Section (Optional):** In some power plants, steam is reheated after partial expansion in the turbine to improve efficiency. - **Furnace:** The combustion chamber where fuel is burnt. - **No Drum:** A key feature is the absence of a steam-water drum, simplifying construction and operation. - **Working:** 1. **Feed Water Pumping:** Feedwater is pumped at a pressure higher than the critical pressure (e.g., 250 bar) into the economizer. 2. **Economizer:** Water is heated by the outgoing flue gases. 3. **Evaporator (Subcritical):** If operating subcritically, water flows through radiant and convective tubes where it boils to produce saturated steam. 4. **Evaporator (Supercritical):** If operating supercritically, water directly transforms into steam without boiling. As it absorbs heat, its temperature and specific volume increase continuously. 5. **Superheater:** The saturated or supercritical steam then enters the superheater, where its temperature is raised significantly (e.g., 550°C), making it superheated steam. 6. **Turbine:** This high-pressure, high-temperature superheated steam is then sent to the steam turbine to generate electricity. 7. **Flue Gas Path:** Flue gases from the furnace pass over the evaporator, superheater, and economizer sections before being exhausted. - **Advantages:** - No steam-water drum, reducing weight and cost. - Can operate at supercritical pressures, leading to higher thermal efficiency. - Quick startup and shutdown due to low water content. - Flexible in operation and can handle fluctuating loads well. - Reduced risk of explosion due to smaller water inventory. - **Sketch:** *Note: This is a general schematic. Actual design may vary.* ### Importance of Boiler Mountings and Accessories Boiler mountings and accessories are crucial components that ensure the safe, efficient, and reliable operation of a boiler. **Boiler Mountings:** These are fittings directly attached to the boiler shell for its safe operation. They are essential and cannot be removed without shutting down the boiler. - **Safety Valve:** Prevents over-pressurization by automatically releasing steam when the pressure exceeds a safe limit, preventing explosions. - **Water Level Indicator (Gauge Glass):** Shows the water level inside the boiler, allowing operators to maintain it within safe limits. Low water can cause overheating and explosion, while high water can lead to wet steam. - **Pressure Gauge:** Indicates the steam pressure inside the boiler, vital for monitoring and control. - **Fusible Plug:** A small plug with a low melting point alloy, located in the furnace. If the water level drops dangerously low, exposing the plug, it melts, releasing steam into the furnace and extinguishing the fire, preventing overheating and explosion. - **Stop Valve (Main Steam Valve):** Controls the flow of steam from the boiler to the main steam line or turbine. - **Feed Check Valve:** Allows feedwater to enter the boiler but prevents its backflow when the feed pump is off or fails. - **Blow-off Cock:** Used to drain water from the boiler for maintenance, to remove sediments, and to control the concentration of dissolved solids. **Boiler Accessories:** These are devices installed external to the boiler to improve its efficiency, performance, and overall operation. They can often be isolated or removed without shutting down the boiler. - **Economizer:** Preheats the feedwater using the waste heat from the flue gases before it enters the boiler, reducing fuel consumption and improving efficiency. - **Superheater:** Increases the temperature of the saturated steam to superheated steam, which improves turbine efficiency and prevents condensation in the turbine. - **Air Preheater:** Heats the combustion air using waste heat from the flue gases, leading to better combustion efficiency and higher furnace temperatures. - **Feed Pump:** Supplies high-pressure feedwater to the boiler. - **Injectors:** Alternative to feed pumps, uses steam to force water into the boiler. - **Steam Separator/Drier:** Removes moisture from the saturated steam before it enters the superheater or turbine, preventing damage and improving efficiency. **Importance:** - **Safety:** Mountings like safety valves, fusible plugs, and water level indicators directly prevent catastrophic failures (explosions) due to overpressure or low water. - **Efficiency:** Accessories like economizers, superheaters, and air preheaters recover waste heat, leading to significant fuel savings and higher overall plant efficiency. - **Control:** Pressure gauges, water level indicators, and stop valves allow operators to monitor and control boiler parameters effectively. - **Reliability & Longevity:** Proper control and maintenance, facilitated by these components, ensure the boiler operates within design limits, extending its lifespan and reducing downtime. - **Environmental Compliance:** Efficient combustion and sometimes specific accessories (e.g., flue gas treatment) contribute to reduced emissions. ### Fire Tube Boiler vs. Water Tube Boiler | Feature | Fire Tube Boiler | Water Tube Boiler | | :-------------------- | :--------------------------------------------- | :------------------------------------------------- | | **Basic Principle** | Hot flue gases pass *through* tubes, water surrounds the tubes. | Water passes *through* tubes, hot flue gases surround the tubes. | | **Pressure Range** | Generally low to moderate (up to 20 bar) | High to very high (up to supercritical pressures) | | **Steam Generation Rate** | Low to moderate | High to very high | | **Heat Transfer Rate** | Relatively lower | Higher, due to better circulation and smaller tubes | | **Working Fluid Volume** | Large water volume | Comparatively smaller water volume | | **Explosion Risk** | Higher risk of explosion (due to large water volume at pressure) | Lower risk (smaller water volume, safer if a tube ruptures) | | **Operating Safety** | Less safe at high pressures | More safe at high pressures | | **Fuel Type** | Generally limited to cleaner fuels like oil/gas | Can handle a wider range, including solid fuels | | **Maintenance** | Easier cleaning of fire tubes | More complex, but easier to replace individual tubes | | **Startup Time** | Longer (due to large water volume) | Shorter (due to smaller water volume) | | **Size & Footprint** | Bulky for high capacities | More compact for high capacities | | **Applications** | Small industrial steam, heating, locomotives | Large power plants, industrial processes | | **Example** | Lancashire boiler, Scotch marine boiler | Babcock & Wilcox, Benson, La-Mont |