PME 426 Lecture 3 – Boilers PDF

PME 426 (Power Plant with Renewable Energy) Lecture 3 – Boilers Prepared by: Engr. Rafael P. Rebutada CMO Coverage Boiler Performance Calculations Factor of Evaporation (FE) quantifies the efficiency of a boiler in converting water into steam, reflecting the relationship betw...

PME 426 (Power Plant with Renewable Energy) Lecture 3 – Boilers Prepared by: Engr. Rafael P. Rebutada CMO Coverage Boiler Performance Calculations Factor of Evaporation (FE) quantifies the efficiency of a boiler in converting water into steam, reflecting the relationship between the heat energy supplied and the latent heat required for vaporization. Boiler Performance Calculations Equivalent evaporation (EE) refers to the amount of water that a boiler can convert into dry saturated steam at a standard temperature of 100°C (212°F) and pressure, allowing for direct comparisons between different boiler systems regardless of their specific operational conditions. Boiler Performance Calculations Equivalent Specific Evaporation (ESE) is a refined measure used to evaluate the efficiency of steam generation in boilers, specifically focusing on the amount of steam produced per unit of energy input. This metric allows for a more detailed analysis of boiler performance by considering the specific conditions under which evaporation occurs. Boiler Performance Calculations ASME Evaporation Units (AEU) are designed to efficiently remove solvents from liquid mixtures through evaporation, utilizing advanced engineering principles to optimize performance and comply with stringent safety and quality standards. The ASME evaporation unit is defined as the amount of steam produced by a boiler under specific conditions. The standard measure is based on the evaporation of water at 100°C (212°F). Boiler Performance Calculations Rated Boiler Horsepower (Rated Bo Hp) is a unit of measurement that quantifies the capacity of a boiler to generate steam. It is defined specifically in terms of the amount of steam produced from water at a standard temperature and pressure, allowing for consistent comparisons across different boiler systems. Boiler Performance Calculations Developed Boiler Horsepower (Dev Bo Hp) is a measure of the actual steam-generating capacity of a boiler, reflecting the amount of steam produced under specific operating conditions. This metric is essential for understanding the performance and efficiency of boiler systems in various industrial applications. Boiler Performance Calculations Percent Rating Developed (% Rating Dev) is a metric used to express the operational efficiency of a boiler in terms of its developed horsepower (Dev Bo Hp) relative to its rated capacity. This percentage indicates how effectively a boiler is performing compared to its maximum design output. Higher Heating Value (HHV) vs. Lower Heating Value (LHV) The Higher Heating Value (HHV) and Lower Heating Value (LHV) are two important metrics used to quantify the energy content of fuels, particularly in combustion processes. Understanding the distinction between these values is crucial for applications in energy generation, heating systems, and fuel efficiency assessments. Definitions Higher Heating Value (HHV): Also known as gross calorific value, HHV measures the total energy released when a fuel is burned, including the energy contained in the water vapor produced during combustion. It assumes that all water vapor condenses back into liquid water, releasing its latent heat of vaporization Lower Heating Value (LHV): Also referred to as net calorific value, LHV accounts for the energy lost in vaporizing water during combustion. It assumes that the water remains in vapor form and does not recover the latent heat. Essentially, LHV is derived from HHV by subtracting the heat of vaporization of the water produced Boiler Performance Calculations Overall Boiler Efficiency (eo), also known as steam generator efficiency, is a critical metric that quantifies the effectiveness of a boiler in converting the energy from fuel into usable steam. It is defined as the ratio of the useful heat output from the boiler to the total energy input provided by the fuel. Boiler Performance Calculations Boiler and Furnace Efficiency (ebf) quantifies the effectiveness of a boiler or furnace in converting fuel energy into useful thermal energy, specifically steam or hot water. This efficiency is crucial for optimizing energy use and minimizing operational costs in industrial and commercial applications. Boiler Performance Calculations Net Efficiency of a Steam Generating Unit (enet) measures the effectiveness of a steam generator in converting the energy from fuel into useful work output, specifically in the form of steam. This efficiency accounts for all energy inputs and outputs, providing a comprehensive view of the unit's operational performance. Boiler Performance Calculations Gross Station Heat Rate (GSHR) is a critical measure used in the energy sector to evaluate the efficiency of a power plant, specifically in terms of the thermal energy required to generate electrical energy. It quantifies the amount of heat energy input needed to produce one kilowatt-hour (kWh) of electricity at the generator terminal. Boiler Performance Calculations Net Station Heat Rate (NSHR) is an important metric used to evaluate the efficiency of a power plant in converting fuel energy into electrical energy. It measures the amount of heat energy required to generate one kilowatt-hour (kWh) of electricity, accounting for all internal energy losses within the plant. Boiler Performance Calculations Overall (Gross) Station Efficiency (ηo) is a key performance metric used to evaluate the efficiency of power plants in converting the energy content of fuel into electrical energy. It represents the ratio of the total electrical output generated by the power plant to the total energy input derived from fuel combustion. Boiler Performance Calculations Grate Efficiency (egr) refers to the effectiveness of a boiler's grate system in converting fuel into usable energy, specifically in the context of combustion processes. It is a crucial parameter for evaluating the performance of solid fuel-fired boilers, particularly those using biomass or coal. Recommended Articles: 0. https://www.sanfoundry.com/power-plant-engg-mcqs-steam-generators/ 1. Power Plant Questions and Answers – Steam Generator Types – II 2. Power Plant Questions and Answers – Steam Generator Types – I 3. Power Systems Questions and Answers – Efficiency of Steam Power Plant 4. Power Plant Questions and Answers – Efficiencies in a Steam Power Plant – 1 5. Power Plant Questions and Answers – Efficiencies in a Steam Power Plant – 2 6. Power Plant Questions and Answers – Steam Power Plant 7. Power Plant Questions and Answers – Steam Turbines Basics – I 8. Steam and Gas Turbines Questions and Answers – Combined Power Plant 9. Power Plant Questions and Answers – Superheaters, Reheaters and Steam Generator Control 10.Power Systems Questions and Answers – Constituents of Steam Power Plant  PME 426 (Power Plant with Renewable Energy) Lecture 3 – Boilers Prepared by: Engr. Rafael P. Rebutada CMO Coverage Boilers Definition: A steam generator is a closed vessel that generates steam at a constant pressure based on process requirements. Types of Steam: The generated steam can be wet, dry saturated, or superheated. Modern power plants typically use one boiler per turbine, simplifying piping systems https://www.anelectricalengineer.com/definition-of-boiler-and-various- and making boiler and turbine control easier. types-of-steam-boiler-or-steam-generator/ Boilers Pressure Design: Boilers can operate at critical pressure (221.2 bar), above critical pressure, or below it. Supercritical Boilers: Operate above critical pressure, also called once-through boilers. Sub-critical Boilers: Operate below critical pressure, also known as drum boilers. https://en.wikipedia.org/wiki/Supercritical_steam_generator Boilers Pressure Maintenance: Constant pressure is achieved by balancing the steam generation rate with the steam consumption rate. Fuel Source: In thermal power stations, coal is the primary source of combustion. Function in Power Generation: Heat from burning coal generates steam, which drives https://www.sciencedirect.com/topics/engineering/boiler-pressure the turbo-generator to produce electricity. Boilers Classification according to the contents of the tubular heating surface 1. Fire-tube boilers Fire tube boilers are a widely used type of boiler where hot gases from combustion pass through tubes that are surrounded by water. This design allows for efficient heat transfer, making them suitable for various applications, particularly in heating and steam generation. These boilers operate at moderate pressure (16–20 bar) https://www.iqsdirectory.com/articles/boiler/water-tube-boilers.html and are suitable for generating 3–8 tons of steam per hour, which is used in process heating. Boilers Classification according to the contents of the tubular heating surface 1. Fire-tube boilers Components Cylindrical Shell: The boiler typically has a cylindrical shape, which is strong enough to withstand pressure. Firebox: Located at the bottom, where fuel is burned. Tubes: Hot gases pass through these tubes, which are surrounded by water. Chimney: Discharges exhaust gases to the atmosphere. https://www.iqsdirectory.com/articles/boiler/water-tube-boilers.html Safety Features: Includes safety valves and gauges to blowdown refers to the process of removing water from the boiler to control the concentration of dissolved and suspended solids in the water. Over time, these solids monitor pressure and prevent accidents. can accumulate and cause issues such as scaling, corrosion, or poor heat transfer Boilers Classification according to the contents of the tubular heating surface 1. Fire-tube boilers Advantages Simplicity: Fire tube boilers are relatively simple in design and operation. Cost-Effective: They are generally less expensive to install and maintain compared to water tube boilers. Efficiency: Suitable for smaller applications with lower steam requirements. Disadvantages https://www.iqsdirectory.com/articles/boiler/water-tube-boilers.html Pressure Limitations: Typically limited to lower pressures (up to blowdown refers to the process of removing water from the boiler to control the about 17.5 kg/cm²). concentration of dissolved and suspended solids in the water. Over time, these solids Size Constraints: Less efficient for larger steam generation needs can accumulate and cause issues such as scaling, corrosion, or poor heat transfer compared to water tube boilers. Boilers Classification according to the contents of the tubular heating surface 1. Fire-tube boilers Types of Fire-Tube Boilers Grate is the part of the furnace 1. Cornish Boiler where the fuel (typically coal) is placed and burned. It consists of a 2. Lancashire Boiler metal framework or series of bars designed to hold the burning fuel while allowing air to pass through 3. Locomotive Boiler for combustion and ash to fall into an ashpit below for removal. 4. Scotch Marine Boiler 5. Cochran Boiler https://www.mechstudy.com/cornish-boiler/ The Cornish boiler is a simple horizontal fire-tube boiler consisting of a cylindrical shell with a single large flue tube running through its length, where the fuel is burned to heat water surrounding the tube. Commonly used in the 19th century, it is known for its efficiency in low-pressure steam applications, such as in textile mills and small industries. Boilers Classification according to the contents of the tubular heating surface 1. Fire-tube boilers Types of Fire-Tube Boilers 1. Cornish Boiler 2. Lancashire Boiler 3. Locomotive Boiler 4. Scotch Marine Boiler https://www.mechstudy.com/lancashire-boiler/ 5. Cochran Boiler The Lancashire boiler is an improvement over the Cornish boiler, offering higher efficiency by using twin flue tubes and providing better heat transfer. It consists of a large cylindrical shell with two parallel flue tubes running through it, where fuel is burned to heat water surrounding the tubes.This boiler is It is commonly used in industries like textile, sugar, and chemical manufacturing. Boilers Classification according to the contents of the tubular heating surface 1. Fire-tube boilers The steam dome serves as a collection point for the driest steam, ensuring minimal water carryover, and houses the main steam valve, which controls the flow of steam to the engine. Types of Fire-Tube Boilers By positioning the dome at the highest 1. Cornish Boiler point of the boiler, it allows for the separation of steam from water, 2. Lancashire Boiler improving steam quality and efficiency in the locomotive's operation. 3. Locomotive Boiler 4. Scotch Marine Boiler https://www.sps700.org/contribute/2016RebuildCampaig n/ProjectOverview.shtml 5. Cochran Boiler A locomotive boiler is a horizontal, multi-tubular fire-tube boiler commonly used in steam-powered locomotives. It consists of a cylindrical shell, a firebox where fuel is burned, and a series of small fire tubes that carry hot gases through the water-filled shell, producing steam for propulsion. Boilers Classification according to the contents of the tubular heating surface 1. Fire-tube boilers Types of Fire-Tube Boilers 1. Cornish Boiler 2. Lancashire Boiler 3. Locomotive Boiler 4. Scotch Marine Boiler https://ciciboilers.com/equipment/hurst/scotch-marine/ 5. Cochran Boiler The Scotch Marine Boiler is a horizontal, cylindrical, fire-tube boiler commonly used in marine applications. It features a large shell with multiple fire tubes and a single or double furnace that burns fuel to heat the surrounding water, producing steam for ship propulsion or other purposes. Boilers Classification according to the contents of the tubular heating surface 1. Fire-tube boilers Types of Fire-Tube Boilers 1. Cornish Boiler 2. Lancashire Boiler 3. Locomotive Boiler 4. Scotch Marine Boiler https://www.youtube.com/watch?v=Ff9a9RfmR0g 5. Cochran Boiler The Cochran boiler is a vertical, multi-tubular, fire-tube boiler known for its compact design and efficient steam generation. It features a cylindrical shell, a central firebox where fuel is burned, and multiple horizontal fire tubes that transfer heat to the water surrounding them. This boiler is commonly used in small to medium-sized industrial applications and is valued for its ability to operate at relatively low pressure while providing a reliable supply of steam. Boilers Classification according to the contents of the tubular heating surface 2. Water-tube boilers Water tube boilers are a type of boiler where water circulates through tubes that are heated externally by hot gases from a combustion chamber. This design contrasts with fire tube boilers, where hot gases pass through tubes surrounded by water. These boilers can operate at significantly higher pressures than fire tube boilers, often exceeding 250 bars. This makes them suitable for applications requiring high-pressure steam, such as power generation in thermal power plants and industrial https://www.mdpi.com/1996- 1073/13/3/677?__set_cache=yes&utm_source=TrendMD&utm_mediu processes m=cpc&utm_campaign=Energies_TrendMD_0 Boilers Classification according to the contents of the tubular heating surface 2. Water-tube boilers Components 1. Steam Drum Function: Acts as a collection vessel for steam and water. It facilitates the separation of steam from water, allowing for efficient steam generation. Features: Contains steam separators, safety valves, and connections for feed water and blowdown. 2. Mud Drum Function: Located at the bottom of the boiler, it collects impurities and sediments that settle from the water. It is essential for maintaining water quality. Features: Equipped with blowdown connections to remove sludge and control total dissolved solids (TDS). 3. Water Walls Function: Composed of tubes surrounding the furnace, they absorb heat from combustion gases to generate steam. https://talklikesm.best/product_details/93148343.html Arrangement: Can be arranged in line or staggered configurations to optimize heat absorption. https://www.researchgate.net/figure/Scheme-of-the-water-wall-tubes-the-evaporator-tubes- location-in-the-combustion-chamber_fig1_353190920 Boilers Classification according to the contents of the tubular heating surface 2. Water-tube boilers Components 4. Riser Tubes Function: Carry heated water from the mud drum to the steam drum. As water is heated, it rises due to buoyancy. Characteristics: Smaller diameter compared to downcomers to maximize heat transfer surface area. 5. Downcomers Function: Transport cooler water from the steam drum back to the mud drum. They facilitate natural circulation in the boiler. Design: Larger diameter than risers to accommodate higher flow rates. https://www.sciencedirect.com/science/article/abs/pii/S2451904920300561 6. Furnace Function: The combustion chamber where fuel is burned to produce heat. Design: Enclosed space designed to optimize combustion efficiency. Boilers Classification according to the contents of the tubular heating surface 2. Water-tube boilers Components 7. Superheater Function: Increases the temperature of steam beyond its saturation point, enhancing efficiency and energy output. Placement: Located after the steam drum and before the steam exits to turbines or other applications. 8. Economizer Function: Preheats feed water before it enters the boiler, improving overall thermal efficiency by recovering waste heat from flue gases. Design: Typically consists of a series of tubes through which feed water flows. 9. Headers Function: Distribute water and steam to various parts of the boiler, connecting multiple tubes. https://www.coalhandlingplants.com/boiler-in-thermal-power-plant/ Types: Include front headers (connected to risers) and rear headers (connected to downcomers).  https://www.boilerfabrication.com/quality-13692752-asme-carbon-steel-boiler- manifold-headers-heat-energy-absorption https://cfdflowengineering.com/basics-of-boilers-and-its-components/ Boiler headers, or boiler manifolds are designed to simplify piping near the boiler, reduce installation time, labor costs and parts used. (2015) R. K. Hedge - Power Plant Engineering- Pearson Education Boilers Classification according to the contents of the tubular heating surface 2. Water-tube boilers Types of Water-Tube Boilers 1. A-Type Boiler 2. D-Type Boiler 3. O-Type Boiler 4. Yarrow Boiler 5. Stirling Boiler https://myansteam.com/type-of-water-tube-boiler/ 6. Thornycroft Boiler An A-Type Boiler is a water-tube boiler with an "A" shaped design, featuring 7. La Mont Boiler two water drums at the base and a single steam drum at the top, connected by water tubes, commonly used for high-pressure steam generation in industrial 8. Benson Boiler and power applications. Boilers Classification according to the contents of the tubular heating surface 2. Water-tube boilers Types of Water-Tube Boilers 1. A-Type Boiler 2. D-Type Boiler 3. O-Type Boiler 4. Yarrow Boiler 5. Stirling Boiler https://myansteam.com/type-of-water-tube-boiler/ 6. Thornycroft Boiler A D-Type Boiler is a water-tube boiler with a "D" shape, consisting of a steam 7. La Mont Boiler drum at the top and water drums at the bottom, connected by water tubes. It efficiently generates high-pressure steam and is commonly used in power plants 8. Benson Boiler and large industrial applications. The design allows for rapid heat transfer and steam generation at high pressures and temperatures. Boilers Classification according to the contents of the tubular heating surface 2. Water-tube boilers Types of Water-Tube Boilers 1. A-Type Boiler 2. D-Type Boiler 3. O-Type Boiler 4. Yarrow Boiler 5. Stirling Boiler https://myansteam.com/type-of-water-tube-boiler/ 6. Thornycroft Boiler An O-Type Boiler is a compact water-tube boiler with a symmetrical "O" design, 7. La Mont Boiler featuring a steam drum at the top and a water drum at the bottom, used for efficient high-pressure steam generation in industrial and marine applications. 8. Benson Boiler Boilers Classification according to the contents of the tubular heating surface 2. Water-tube boilers Types of Water-Tube Boilers 1. A-Type Boiler 2. D-Type Boiler 3. O-Type Boiler 4. Yarrow Boiler 5. Stirling Boiler https://upload.wikimedia.org/wikipedia/commons/d/dd/Yarrow_boiler_tubes_% 28Rankin_Kennedy%2C_Modern_Engines%2C_Vol_VI%29.jpg 6. Thornycroft Boiler Yarrow boilers are high-pressure water-tube boilers developed by Yarrow & Co. 7. La Mont Boiler and widely used on ships, especially warships. They feature a three-drum design with two banks of straight water tubes in a triangular arrangement, a single 8. Benson Boiler steam drum at the top, and smaller water drums at the base. Their unique design includes straight tubes and internal bidirectional circulation within the tube banks, eliminating the need for external downcomers. Boilers Classification according to the contents of the tubular heating surface 2. Water-tube boilers Types of Water-Tube Boilers 1. A-Type Boiler 2. D-Type Boiler 3. O-Type Boiler 4. Yarrow Boiler 5. Stirling Boiler https://en.wikipedia.org/wiki/Stirling_boiler#/media/File:Stirling_boiler_(Heat_En gines,_1913).jpg 6. Thornycroft Boiler The Stirling Boiler is a water-tube boiler designed for high-capacity steam 7. La Mont Boiler generation, commonly used in power plants and industrial applications. It consists of multiple drums (typically one steam drum and two or more water 8. Benson Boiler drums) connected by large banks of water tubes. The design allows for excellent heat transfer, natural circulation of water, and high efficiency, making it suitable for high-pressure and high-temperature operations.. Boilers Classification according to the contents of the tubular heating surface 2. Water-tube boilers Types of Water-Tube Boilers 1. A-Type Boiler 2. D-Type Boiler 3. O-Type Boiler 4. Yarrow Boiler 5. Stirling Boiler https://commons.wikimedia.org/wiki/File:Thornycroft_watertube_boiler_-_Cassier's_1895-96.png 6. Thornycroft Boiler The Thornycroft Boiler is a type of water-tube boiler designed primarily for marine 7. La Mont Boiler applications, particularly in fast naval vessels. It features a compact design with straight water tubes arranged in a zigzag pattern between a single steam drum at the top and 8. Benson Boiler water drums at the bottom. This arrangement allows for efficient heat transfer, quick steam generation, and operation under high pressures. Its lightweight and compact structure made it ideal for warships and other high-speed marine vessels. Boilers Classification according to the contents of the tubular heating surface 2. Water-tube boilers Types of Water-Tube Boilers 1. A-Type Boiler 2. D-Type Boiler 3. O-Type Boiler 4. Yarrow Boiler 5. Stirling Boiler (2015) R. K. Hedge - Power Plant Engineering-Pearson Education 6. Thornycroft Boiler The La Mont Boiler is a high-pressure water-tube boiler that uses forced circulation to overcome the limitations 7. La Mont Boiler of natural circulation at operating pressures between 120 and 160 bar. Feedwater is preheated in economizer tubes using flue gases before entering the steam drum. A centrifugal pump circulates water at a rate up to 10 8. Benson Boiler times the evaporation rate, forcing it through evaporator tubes in the radiant and convective sections of the furnace. The generated steam is passed through superheater tubes for further heating before being fed to the turbo generator. The La Mont Boiler is capable of supplying steam at rates between 130 and 3000 tons per hour. Boilers Classification according to the contents of the tubular heating surface 2. Water-tube boilers Types of Water-Tube Boilers 1. A-Type Boiler 2. D-Type Boiler 3. O-Type Boiler 4. Yarrow Boiler 5. Stirling Boiler (2015) R. K. Hedge - Power Plant Engineering-Pearson Education 6. Thornycroft Boiler The Benson Boiler is a supercritical water-tube boiler operating at 225–500 bar, eliminating bubble formation by 7. La Mont Boiler working above the critical pressure. Feedwater is preheated in economizers, flows through evaporator tubes, and is directly superheated without a steam drum. It generates 150+ tonnes/hour of steam but requires periodic 8. Benson Boiler cleaning to remove salt deposits in the evaporator. During startup, a circulating system ensures safe operation before full load is achieved. FURTHER READINGS (2015) R. K. Hedge - Power Plant Engineering-Pearson Education page 184 - 216 (2012) POWER PLANT ENGINEERING REVIEWER (LECTURE) Revision 0 (Llovido) page 4 - 10 FURTHER READINGS (2015) R. K. Hedge - Power Plant Engineering-Pearson Education page 184 - 216 (2012) POWER PLANT ENGINEERING REVIEWER (LECTURE) Revision 0 (Llovido) page 4 - 10  B. FUELS AND COMBUSTION - LECTURE 1. Definitions Fuel – is composed of chemical elements which, in rapid chemical union with oxygen, produce combustion. Combustion – is that rapid chemical union with oxygen of an element whose exothermic heat of reaction is sufficiently great and whose rate of reaction is sufficiently fast that useful quantities of heat are liberated at elevated temperatures. 2. Classification of Fuels 2.1 Solid – including coal, coke, peat, briquettes, wood, charcoal, and waste products 2.2 Liquid – including petroleum and its derivatives, synthetic liquid fuels manufactured from natural gas and coal, shale oil, coal by-products (including tars and light oil), and alcohols. 2.3 Gaseous – including natural gas, manufactured and industrial by-product gases, and the propane and butane or, liquefied petroleum (LP) gases that are stored and delivered as liquids under pressure but used in gaseous form. 3. Coal Classification 3.1 Classification by rank – degree of metamorphism, or progressive alteration, in the natural series from lignite to anthracite (lignite, subbituminous, semibituminous, bituminous, semianthracite, anthracite, superanthracite). Probably the most universally applicable method of classification in which coals are arranged according to fixed carbon content and calorific value, in Btu, calculated on the mineral-matter-free basis. 3.2 Classification by grade – quality determined by size designation, calorific value, ash, ash-softening temperature, and sulfur. The size designation is given first in accordance with the standard screen analysis method followed by calorific value, and symbols representing ash, ash-softening temperature, and sulfur. 3.3 Classification by type or variety – determined by nature of the original plant material and subsequent thereof. 4. Burners for Pulverized Coal 4.1 Vertical firing – with all the secondary air admitted around the burner nozzle so that it mixes quickly with coal primary air mixture from the burner nozzle. 4.2 Impact firing – a form of vertical firing, consists of burners located in an arch low in the furnace or in the side walls and directed toward the furnace door, with high velocities of both primary and secondary air. This type of firing is used exclusively in wet-bottom or slagging type. 4.3 Horizontal firing – employs a turbulent burner, which consists of a circular nozzle within a housing provided with adjustable valves, the unit being located in the front or rear wall. 4.4 Corner or tangential firing – is characterized by burners located in each corner of the furnace and directed tangent to a horizontal, imaginary circle in the middle of the furnace, thereby making the furnace the burner in effect, since turbulence and intensive mixing occur where the streams met. 5. Coke Coke – is the solid, infusible, cellular residue left after fusible bituminous coals are heated, in the absence of air, above temperatures at which active thermal decomposition of the coal occurs. Pitch coke or petroleum coke – are obtained by similar heating of coal-tar pitch and petroleum residues. High temperature coke – is made from coal at temperature ranging from 815 C to 1093 C. Low temperature coke – is formed at temperatures below 704 C. The residue, if made from a non-cooking coal, is known as char. 1  B. FUELS AND COMBUSTION - LECTURE 6. Charcoal Charcoal – is produced by partial combustion of wood at about 400 C and with limited air. 7. Liquid Fuels Fuel Oil – is defined as any liquid or liquefiable petroleum products burned for the generation of heat in a furnace of firebox, of the generation of power in an engine, exclusive of oils with a flash point below 37.7 C. Four Classes of Fuel Oils in common uses a. Residual oils – which are topped crude petroleum’s or viscous residuum obtained in refinery operations. b. Distillate fuel oils – which are distillates derived directly or indirectly from crude petroleum. c. Crude petroleum’s and weathered crude petroleum’s of relatively low commercial value. d. Blended fuels – which are mixture of two or more of the preceding classes. Commercial Fuel Oil Specifications a. Grade no. 1 – a distillate oil intended for vaporizing pot-type burners and other burners requiring this grade of fuel. b. Grade no. 2 – a distillate oil for general purpose domestic heating in burners not requiring no. 1 fuel oil. c. Grade no. 4 – an oil for burner installation not equipped with pre-heating facilities. d. Grade no. 5 – a residual type oil for burner installation equipped with pre-heating facilities. e. Grade no. 6 – an oil for burners equipped with pre-heaters permitting a high-viscosity fuel. 8. Gasoline Gasoline – is defined as a refined petroleum naphtha which by its composition is suitable for use as a carburetant in internal combustion engines. Motor Gasoline – is a mixture of hydrocarbons distilling in the range of 37.7 C to 204.4 C by the standard method of test. 9. Kerosene Kerosene – is defined as a petroleum distillate having a flash point not below 22.8 C as determined by the Abel tester and suitable as an illuminant when burned in a wick lamp. 10. Coal Tar Coal Tar – is a product of the destructive distillation of bituminous coal carried out at high temperature. 11. Liquefied Petroleum Gases (LPG) Liquefied Petroleum Gases (LPG) – are mixtures of hydrocarbons liquefied under pressure for efficient transportation, storage, and use. They are generally composed of ethylene, propane, propylene, butane, isobutene, and butylenes. Commercially, they are classed as propane, propane-butane mixtures, and butane. They are odorless, colorless, and non-toxic. 12. Diesel Fuel Oils Refiners grade fuels classified according to methods of production. a. Distillate fuels – are produced by distillation of crudes. b. Residual fuels – are those left after the distillation process. c. Blended fuels – are mixtures of straight distillate fuels with cracked fuel stocks. 2  B. FUELS AND COMBUSTION - LECTURE Cracked stocks – are residual of fuels which have been treated thermally or catalytically to obtain yields of lighter- grade fuels or gasoline. Lightest grade distillates – classed as kerosene or No. 1 fuel oil, may have an initial boiling point of 176.6 C and end point of 260 C. Heaviest grades of distillates – classed as No. 3 or 4 fuel oil, may have an initial boiling point of 232 C to 260 C and end point of 343 C to 371 C. Residual fuels, No. 4 or No. 5 – are suitable only for the slower-speed diesel. 13. Gaseous Fuels Gaseous fuels – are commonly used in industry, whether distributed by public utilities or produced in isolated plants, are composed of one or more simple gases in varying proportions. 14. Diesel Lubricating Oils Crude oils – are frequently described as “paraffinic”, “napththenic”, or “mixed based” according to the physical characteristics of the crude. Two broad types of oil a. “Straight” oils – are produced entirely from the crudes chosen through elimination of undesired constituents by suitable refining processes. b. “Additive” oils – are produced by adding to straight mineral oils certain oil-soluble compounds that enhance the lubricating oil properties for use in a diesel engine. Additives – are used principally to inhibit or slow down oxidation, to increase film strength, to keep solids in finely divided state and to hold them in suspension, to improve the viscosity index, to lower the pour point, to decrease friction and wear under extreme pressure conditions, to reduce foaming, and as rust or corrosion inhibitors. SAE Three Types of Lubricating Oils a. Regular type – suitable for moderate operating conditions. b. Premium type – having oxidation stability and bearing corrosion preventive properties making it generally suitable for more severe service than regular duty type. c. Heavy duty type – has oxidation stability, being corrosion-preventive properties, and detergent-dispersant characteristics for use under heavy-duty service conditions. SAE Numbers – are a means of coordinating and standardizing the products of oil companies and the recommendations by the oil companies. The system of SAE motor classification is a system based entirely on viscosity and is totally unrelated the other qualities of a lubricating oil. 15. Specific Gravity Specific Gravity – a dimensionless parameter, it is the ratio of the mass of a unit volume of fuel to the mass of the same volume of a standard substance at a specified temperature. density of liquid fuel SG = density of water density of gaseous fuel SG = density of air 3  B. FUELS AND COMBUSTION - LECTURE In reporting SG data the 15.6 C or 60 F standard is common, that is, the oil is at 15.6 C or 60 F and is referred to the density of water taken at 15.6 C or 60 F. Specific gravity at other temperature with correction factor, SGt = SG15.6o C [1 − 0.0007(t − 15.6 )] in SI units SGt = SG60 o F [1 − 0.0004(t − 60)] in English units American Petroleum Institute Gravity Unit, oAPI - Is the accepted standard by the petroleum and oil industry, it was drawn up to correct vales measured by incorrectly calibrated hydrometers. o 141.5 API = − 131.5 SG at 15.6o C Baume Gravity Unit, oBaume’ or oBe’ - Another standard commonly associated with brine. o 140 Baume = − 130 SG at 15.6 o C 16. Viscosity Viscosity – is measure of resistance to flow. Absolute Viscosity – is defined as that unit force required to move one layer of a fluid at unit relative velocity to another layer of the fluid which is at unit distance from the first. Kinematic Viscosity – is defined as the ratio of absolute viscosity divided by density. Units of viscosity: Absolute viscosity, µ 1 reyn = 1 kb-sec / in2 1 poise – 1 dyne-se/cm2 = 0.1 Pa-sec Kinematic Viscosity, ν 1 stoke = 1 cm2/sec = 0.0001 m2/sec Centipoises and centistokes are more commonly used. Saybolt viscosimeter – measures the time required for a given quantity of oil at standard temperature to flow through a specified tube. SSU (Saybolt Second Universal) – is obtained by timing the interval required for 60 cc of oil to flow through tube or pass through a standard orifice. For 30 to 45 SSU at 37.8 C, Centistokes = 0.308(SSU – 26) 180 Or ν = 0.22SSU − centistokes SSU SSF (Saybolt Second Furol) – unit used for very viscous liquids using a relatively large orifice. 62 SSF = 600 SSU 4  B. FUELS AND COMBUSTION - LECTURE 17. Other Properties Flash point – is the temperature at which oil gives off vapor that burns temporarily when ignited. Flash point – is the temperature to which oil must be heated to give off sufficient vapor to form an inflammable mixture with air. Flash point – is the temperature at which ignition of the fuel vapors rising above the heated oil will occur when exposed to an open flame. Fire point – is the temperature at which oil gives off vapor that burns continuously when ignited. Pour point – is the temperature at which oil will no longer pour freely or the temperature at which oil will solidify. Dropping point – is the temperature at which grease melts. Cloud point – is the temperature at which the paraffin elements separate from oil. Conradson number (carbon residue) – is the carbonaceous residue remaining after destructive distillation, expressed in percentage by weight of the original sample. Viscosity index – indicates the relative change in viscosity of an oil for a given temperature change. Octane number – the ignition quality rating of gasoline, which is the percentage by volume of iso-octane in a mixture of iso-octane and heptanes that matches the gasoline in anti-knock quality. Cetane number – the ignition quality rating of diesel, which is the percent of cetane in the standard fuel. Aniline point – is that temperature where equal parts if oil and aniline will dissolve in each other. Volatility – is the ability of a liquid fuel to change into vapor which is manifested in the temperature range at which various portions of the fuel are vaporized. 18. Composition of Fuels a. Paraffins, CnH2n+2 – saturated hydrocarbons, very stable in characters b. Olefins, CnH2n – unsaturated hydrocarbons, characterized by the presence of a double bond between carbon atoms. c. Diiolefins, CnH2n-2 – less saturated than olefins, characterized by the presence of two double bonds. 19. Analysis of Composition 19.1 Proximate analysis – is made by heating the coal until it decomposes successively into three of the four complex items of proximate analysis. The fourth is found by the difference. A typical proximate analysis of coal determines the percentage of moisture, volatile matter, fixed carbon, and ash. a. Moisture – is determined by subjecting a 1-g sample of the coal to a temperature of 220 F to 230 F for a period of exactly 1 hr. b. Volatile matter – consists of hydrogen and certain hydrogen-carbon compounds that can be removed from the coal merely by heating it. c. Ash – is performed by heating the sample of coal used in the moisture determination to a temperature of 1290 F to 1380 F in an uncovered crucible, with good air circulation, until the coal is completely burned. 5  B. FUELS AND COMBUSTION - LECTURE d. Fixed Carbon – is the difference between 100 % and the sum of the percentages of moisture, ash, and volatile matter. 19.2 Ultimate analysis – analysis of composition of fuel which gives, on mass basis, the relative amounts of carbon, hydrogen, oxygen, nitrogen, sulfur, ash, and moisture. 20. Basis of Reporting Analysis a. As received or as fired b. Dry or moisture free c. Moisture and ash free or combustible d. Moisture, ash, and sulfur free 21. Heating Values of Fuels or Calorific Value a. Higher heating value (gross calorific value), HHV – is the heating value obtained when the water in the products of combustion is in the liquid state. b. Lower heating value (net calorific value), LHV – is the heating value obtained when the water in the products of combustion is in the vapor state. 22. Methods of Determining Heating Values 22.1 Laboratory experiment 22.1.1 Bomb calorimeter for solid and liquid fuels 22.1.2 Gas calorimeter for gaseous fuels 22.2 Empirical formulas 22.2.1 Dulong’s formula for solid fuels of known ultimate analysis.  O HHV = 33,820 + 144,212 H −  + 9,304 S kJ kg  8  O HHV = 14,600 + 62,000 H −  + 4050S Btu lb  8 22.2.2 ASME Formula for petroleum products ( HHV = 41,130 + 139.6 o API kJ kg ) ( o HHV = 17,680 + 60 API Btu lb ) 22.2.3 Bureau of Standard formula HHV = 51,716 − 8 ,793.8(SG )2 kJ kg HHV = 22,230 − 3780(SG )2 Btu lb Difference between higher and lower heating values HHV – LHV = 9H2(2442) in SI units HHV - LHV = 9H2(1050) in English units Where: 9H2 = lbs or kg of water formed per lb or kg of fuel burned. 2442 kJ/kg or 1050 Btu/lb – latent heat of vaporization of water. Also H2 = 26-15(SG), percent by weight. 23. Fuel Production Process a. Fractional distillation – the primary method of crude oil refining. 6  B. FUELS AND COMBUSTION - LECTURE b. Thermal cracking – changing heavy oil into gasoline by means of high pressure, high temperature and longer exposure time. c. Catalytic cracking – subjects oil to high pressure and high temperature in the presence of a catalyst; permit accurate control of the compounds formed and produces a gasoline of higher octane number than the one produced in thermal cracking. d. Hydrogenation – process of catalytic cracking in a hydrogen atmosphere; obtained are more saturated products than those from cracking process alone. e. Isomerization – process by which the atoms of carbon and hydrogen in normal hydrocarbons are rearranged to produce a more complex structure of higher anti-knock value. f. Polymerization – makes use of high pressure, high temperature and a catalyst to combine light and volatile gases into gasoline. g. Alkylation – process of combining an isoparaffin usually iso-butane, with an olefin, usually butane or propane, to form a large isoparaffin molecule, usually iso-octane or iso-heptane, having a very high octane number. h. Reforming –used to obtain fuels with substantially higher than 100 octane number; currently used to process about forty percent of motor gasoline. i. Hydrodesulfurization – process of adding hydrogen to unsaturated hydrocarbons and reducing the sulfur content of the resulting fuel oil. 24. Combustion Combustion – a chemical reaction between fuel and oxygen (air) which is accompanied by heat and light. 25. Composition of Air and Molecular Weights a. Composition by weight 76.8 % nitrogen, 23.2 % oxygen Or 76.8 / 23.2 = 3.3 lb of nitrogen per lb of oxygen. b. Composition by volume 79.0 % nitrogen, 21.0 % oxygen Or 79.0/21.0 = 3.76 moles of nitrogen per moles of oxygen c. Molecular weights Air = 28.97 kg/kgmole C = 12 kg/kgmole H2 = 2 kg/kgmole O2 = 32 kg/kgmole N2 = 28 kg/kgmole S = 32 kg/kgmole 26. Air Fuel Ratio Theoretical air-fuel ratio, Wta – is the exact theoretical amount, as determined from the combustion reaction, of air needed to burn a unit amount of fuel, kg air per kg fuel or lb air per lb fuel.  O  Wta = 11.53C + 34.36 H2 − 2  + 4.32 S  8  where: Wta = theoretical air, lb per lb fuel C = carbon, lb per lb fuel H2 = hydrogen, lb per lb fuel O2 = oxygen, lb per lb fuel S = sulfur, lb per lb fuel 7  B. FUELS AND COMBUSTION - LECTURE Actual air-fuel ratio, Waa – is determined by the presence of excess air which is defined as the amount of air supplied over and above the theoretical air. Waa = (1+ e)Wta W − Wta e = aa Wta where e is the excess air in decimal. 27. Typical Combustion Reaction Fuel + Air = Product of Combustion C nHm + (n + 0.25m)O2 + 3.76(n + 0.25m)N2 → nCO2 + 0.5mH2O + 3.76(n + 0.25m)N2 (n + 0.25m)(32 + 3.76 × 28) 137.28(n + 0.25m ) Wta = = 12n + m 12n + m 28. Classification of combustion reaction a. Combustion reaction with chemically-correct or stoichiometric condition general chemical formula of the fuel is CnHm. b. Combustion reaction with greater amount of theoretical air, or having a fuel-lean mixture. c. Combustion reaction with lesser amount of theoretical air, or having a fuel-rich mixture. 29. Equivalence ratio for a given mass of air, φ. W φ = ta Waa Note: φ = 1, for stoichiometric mixture. φ < 1, for fuel-lean mixture. φ > 1, for fuel-rich mixture. 30. Orsat Analyzer Orsat analyzer – is a convenient portable apparatus for determining the volumetric percentage of CO2, O2, and CO in the dry flue gas. 31. Dry Flue Gases from Actual Combustion 4CO2 + O2 + 700 Wdg = C ab 3(CO2 + CO ) Boiler test code formula corrected to account for the SO2. 11CO2 + 8O2 + 7(CO + N2 )  3  5 Wdg = C ab + S  + S 3(CO2 + CO )  8  8 where: CO2, O2, CO, and N2 are volumetric Orsat analysis Cab and S are decimal fractions by weight. 32. Weight of dry refuse from the coal analysis A Wr = 1− Cr 8  B. FUELS AND COMBUSTION - LECTURE where: Wr = dry refuse per lb coal as fired, lb A = ash in coal, lb Cr = combustible In 1 lb of refuse. 33. Carbon Actually Burned Cab = C − Wr + A Or HVr C ab = C − Wr 14,600 where: Cab = carbon actually burned per lb of fuel, lb C = carbon in 1 lb of fuel, lb HVr = heating value of the dry refuse, Btu per lb. 34. Carbon burned to CO due to incomplete combustion. CO Ci = × C ab CO2 + CO where Ci is the pounds of carbon the CO per pound of fuel burned. 35. Air Actually Used During Combustion  O  Waa = Wdg + 8 H2 − 2  − C ab − S − N2  8  Values of H2, O2, S, and N2 are obtained from the ultimate analysis of the fuel and all values are expressed as decimals. 36. Boiler Heat Balance Consist of percentage energy absorbed by boiler fluid, energy loss due to dry flue gases, energy loss due to moisture in fuel, energy loss due to evaporating and superheating moisture formed by combustion of hydrogen, energy loss due to incomplete combustion of carbon to CO, energy loss due to combustible in the refuse, and energy loss due to radiation and unaccounted for totaling to higher heating value as 100%. a. Energy absorbed by boiler fluid. The useful output of the steam generator is the heat transferred to the fluid. W (h − h ) Q1 = w 2 1 Wf in which Ww = weight of fluid flowing through the boiler during the test, lb h1 and h2 = fluid enthalpies entering and leaving the boiler, respectively, Btu per lb Wf = weight of fuel burned during test, lb Q1 expressed as a percentage of the higher heating value of the fuel is the boiler efficiency. 9  B. FUELS AND COMBUSTION - LECTURE b. Energy loss due to dry flue gas. This loss is the greatest of any of the boiler losses for a properly operated unit. Q2 = 0.24Wdg (t g − t a ) in which 0.24 = specific heat of the flue gas at constant pressure, Btu per lb per deg F. tg = temperature of the gas leaving the boiler, F ta = temperature of the air entering the boiler, F c. Energy loss due to evaporating and superheating moisture in fuel. Moisture entering the boiler with the fuel leaves as a superheated vapor in the same way as does moisture from the combustion of hydrogen. Q3 = M f (1089 + 0.46t g − t f ), when t g < 575 F Q3 = M f (1066 + 0.5t g − t f ), when t g > 575 F where Mf = moisture in fuel, lb per lb of fuel tf = temperature of fuel, F d. Energy loss due to evaporating and superheating moisture formed by combustion of hydrogen. This loss is higher for gaseous fuels containing relatively large percentages of hydrogen than for the average low- hydrogen coal. Q4 = 9H 2 (h − h ff ) where: h2 = weight of hydrogen in the fuel, lb per lb fuel h = enthalpy of superheated vapor, Btu per lb hff = enthalpy of liquid at the incoming fuel temperature or Q4 = 9H 2 (1089 + 0.46t g − t f ), when t g < 575 F Q4 = 9H 2 (1066 + 0.5t g − t f ), when t g > 575 F The proper value of H2 to be used in the equation is the amount of hydrogen in the fuel that is available for combustion. To obtain the value of H2, deduct from the value of H2 in ultimate analysis one ninth of the weight of moisture from the proximate analysis. e. Energy loss due to incomplete combustion. Products formed by incomplete combustion may be mixed with oxygen and burned again with a further release of energy. CO Q5 = 10,160C i = 10,160C ab Btu lb CO2 + CO f. Energy loss due to unconsumed carbon. All combustible in the refuse may be assumed to be carbon, since the other combustible parts of coal would probably be distilled out of the fuel before live embers would drop into ash pit. 10  B. FUELS AND COMBUSTION - LECTURE Q6 = 14,600(C − C ab ) Btu lb or Q6 = Wr HVr g. Unaccounted-for and radiation loss. This loss is due to radiation, incomplete combustion resulting in hydrogen and hydrocarbons in the flue gas, and unaccounted-for losses. Q7 = HHV − Q1 − Q2 − Q3 − Q4 − Q5 − Q6 h. Boiler Heat Balance Tabulation Item Energy, Btu per lb fuel Percentage Q1 Q2 Q3 Q4 Q5 Q6 Q7 HHV 100% - End - 11  Steam Generator 5 Contents 5.1 Fossil fuel steam generators 5.5 Boiler performance calculations 5.2 Classification of boilers 5.6 Accessories for the steam generator 5.3 Circulation in water tube boilers 5.7 Boiler mountings 5.4 Modern high-pressure water tube boilers 5.8 Questions 5.1 FOSSIL FUEL STEAM GENERATORS Steam generator is a closed vessel that is used to generate steam at constant pressure as per the process requirement. The steam generated may be wet, dry saturated or superheated in state. In modern power plants, it is very common to use one boiler (single unit) per turbine, which leads to simpler piping systems and relatively easier boiler and turbine control. These boilers are usually designed to operate either at critical pressure (221.2 bar) or above or below the crit- ical pressure. If the steam generators are designed to operate above the critical pressure, then they are known as supercritical boilers or once-through boilers. Some of the steam generators are designed to operate below the critical pressure, and they are known as sub-critical or drum boilers. In a steam generator or boiler, constant pressure is maintained by balancing the rate of steam generated with the rate of steam consumed. In a thermal power station, coal is the main source of combustion. The heat generated by burning coal is utilized to generate steam, which in turn runs the turbo-generator. 5.2 CLASSIFICATION OF BOILERS 5.2.1 Fire Tube Boilers and Water Tube Boilers Based on the contents of the tubular heating surface, boilers are primarily classified as fire tube boilers and water tube boilers. 1. Fire tube boilers Fire tube boilers are those in which the products of combustion pass through the tubes and water lies around the outside of tubes as shown in Figure 5.1. Examples include Cochran, 184 Power Plant Engineering Lancashire, Cornish, locomotive and Scotch marine boilers. These boilers operate at moderate pressure (16–20 bar) and are suitable for generating 3–8 tons of steam per hour, which is used in process heating. Fire tube Water tube In Out Out C H Water Hot flue gases o o l Hot flue gases t In Out Out d Water Hot flue gases W W In Out a a Water t Out t e Hot flue gases In Out e Water r r (a) (b) Fig. 5.1 Comparison of Fire Tubes and Water Tubes: (a) Fire Tube Boiler Tubes; (b) Water Tube Boiler Tubes 2. Water tube boilers Water tube boilers are those in which the products of combustion (hot flue gases) surround the water tubes from outside. Cold water enters the tubes and leaves hot as shown in Figure 5.2. Water boilers operate at very high pressures and are used for power generation. Examples: Babcock–Wilcox boiler and Sterling boiler Drum To the steam header Steam Water Superheater Steam Water Combustion Blow-Down Combustion gases gases (a) (b) Fig. 5.2 Principle of Operation of Fire Tube and Water Tube Boilers: (a) Principle of Fire Tube Boiler; (b) Principle of Water Tube Boiler  Steam Generator 185 5.2.2 Stationary and Mobile Boilers If the boilers are used at one place only they are termed stationary boilers. These boilers are used either for process heating in industries or for power generation in steam power plants. Examples: Babcock–Wilcox boiler and fluidized bed combustion (FBC) boiler. Mobile boilers are portable and are used in locomotives and ships. Examples: Locomotive boiler and marine boiler 5.2.3 Internally Fired and Externally Fired Boilers If the furnace is placed in the region of boiling water, then the boiler is termed internally fired boiler. Example: Lancashire boiler If the furnace is placed outside the boiling water region, then the boiler is known as ­externally fired boiler. Example: Babcock–Wilcox boiler 5.2.4 Horizontal, Vertical and Inclined Tube Boilers If the heating tubes are horizontal, then the boilers are called horizontal tube boilers. Example: Lancashire boiler If the heating tubes are vertical, the boilers are known as vertical tube boilers. Example: Cochran boiler If the heating tubes are inclined to the horizontal, the boilers are known as inclined tube boilers. Example: Babcock–Wilcox boiler 5.2.5 Based on Heat Sources Boilers may be classified based on the fuel used for combustion or heat generation source. Various heat sources used may be the following: (i) Heat generated by the combustion of fuel in solid, liquid or gaseous forms (ii) Heat generated by hot waste gases as byproducts of other chemical processes (iii) Heat generated by electrical energy (iv) Heat generated by nuclear energy 5.2.6 Natural Circulation and Forced Circulation Boilers If the circulation of water is by natural convection currents produced by the application of heat, the boilers are known as natural circulation boilers. Example: Babcock–Wilcox boiler If the circulation of water is by external means using a pump, the boilers are known as forced circulation boilers. In these types, the fluid is forced once through or controlled by partial recir- culation. Such boilers are also known as positive forced circulation boilers. These boilers work at very high pressures. Examples: La-Mont boiler, Velox boiler, Benson boiler, etc 186 Power Plant Engineering 5.2.7 Comparison of Water Tube Boilers and Fire Tube Boilers Water tube boilers Fire tube boilers Advantages The rate of evaporation is more as the area of The rate of evaporation (steam generation heating surface is more due to the presence rate) is less as the area of heating surface of large number of small ­diameter tubes. is less due to the presence of small number (usually 2) of large diameter tubes. Due to smaller ratio of water to steam space Due to larger ratio of water to steam space (W/S), steam can be generated at a faster (W/S), steam generation is not so fast. rate even if the boiler is cold. As the rate of evaporation is more, it is As the rate of evaporation is less, it is pre­ferred in steam power plants for power ­preferred in process industries. generation. Working pressure of the boiler is high as Working pressure is low as the flue tubes are the shell can withstand high temperature or subject to more thermal stresses for the same ­thermal stresses. thickness of the water tube boiler tubes. Due to higher working pressure and p ­ ositive Due to lower working pressure, there circulation of water, there are fewer chances are more chances of sediments getting of sediments getting d ­ eposited inside the ­deposited. water tubes in the shell. The rate of heat transfer from the flue gases The rate of heat transfer is less as the to the water tubes is more as the water tubes ­direction of flow of the flue gases and the are positioned to obstruct the flow of flue water flow is either parallel or counter flow. gases (cross flow). Furnace can be altered to suit the type of fuel Furnace cannot be altered and is used for used for combustion, and hence the boiler firing one fuel only. can adopt flexible firing ­methods. Boiler need not be shut down immediately just Bursting of flue tube results in serious in case water tube bursts, and hence is not ­explosion and may be fatal. fatal. Cleaning, repairing and inspection are easy Cleaning, inspection and repairing are not as all parts are accessible. easy due to inaccessible parts. Water circulates inside the tubes and hot flue Hot flue gases circulate inside the tubes and gases surround them. water surrounds the boiler shell. Boiler furnace is placed outside the boiler, Boiler furnace is placed inside the boiler, furnace alteration is easy. hence furnace alteration is difficult. Water circulation is cyclic, that is, from boiler Water circulation is limited inside the boiler drum to water tubes and again to boiler drum. shell. Disadvantages Water should be treated in a systematic manner Water need not be that much pure and to remove impurities present in it. A water needs minimum treatment such as ­passing treatment plant is used to avoid silica deposits. through a pressure filter. It has high initial cost. It has low initial cost. Not used for mobile purpose. Used as mobile boiler.  Steam Generator 187 5.3 CIRCULATION IN WATER TUBE BOILERS If the circulation of water is by natural convection currents produced by the application of heat, the boilers are known as natural circulation boilers. Example: Babcock–Wilcox boiler If the circulation of water is by external means using a pump, the boilers are known as forced circulation boilers. In these types, the fluid is forced ‘once through’ or controlled by partial recirculation. These boilers are also known as positive forced circulation boilers. These boilers work at very high pressures. Examples: La-Mont boiler, Velox boiler, Benson boiler, etc. Modern-day power plants use forced circulation boilers that have the high steam-raising capacity. In the sections that follow, few such boilers are discussed. 5.4 MODERN HIGH-PRESSURE WATER TUBE BOILERS Fig. 5.3 A Typical Modern High-Pressure Boiler 188 Power Plant Engineering In modern power plants, it is very common to use one boiler (single unit) per turbine, which leads to simpler piping systems and relatively easier boiler and turbine control. These boilers are generally designed to operate at critical pressure (221.2 bar) or above or below the critical pressure. If the boilers are designed to operate above the critical pressure, then they are known as supercritical boilers or once-through boilers. If the boilers are designed to operate below the critical pressure, then they are known as sub-critical or drum boilers (Figure 5.3). 5.4.1 Generation of Steam Using Forced Circulation, High and Supercritical Pressures High-pressure boilers may further be classified into natural circulation, forced circulation and once-through boilers. 5.4.1.1 Natural Circulation Boilers A typical flow pattern of a natural circulation boiler is shown in Figure 5.4. Here, water is ­circulated purely by density difference with most of the heat from the fuel flame being radiated to the water walls directly. The steam pressure of such boilers is limited to about 180 bar, with water and steam being separated in the boiler drum. Superheated steam Boiler drum Economizer Super heater Mad Drum Heat Feed pump Fig. 5.4 A Typical Natural Circulation Boiler 5.4.1.2 Forced Circulation Boilers Figure 5.5 shows the flow pattern of a forced circulation boiler. In these boilers water is cir- culated by using an additional pump. These boilers often use orifices, which control the flow circulation. Orifices are located at the bottom of the tubes that ensure even distribution of flow through water wall tubes. These boilers can generate steam up to about 200 bar.  Steam Generator 189 Superheater Boiler drum Circulating pump Economizer Circulating Heat pump Feed pump Fig. 5.5 A Typical Forced Circulation Boiler 5.4.1.3 Once-Through Boilers Figure 5.6 shows a flow diagram of a once-through boiler. These boilers operate above critical pressure, that is, above 221.2 bar. As the density of water and steam is same above critical pres- sure, there will be no recirculation. In these boilers, water enters the bottom of the tubes and completely transforms into steam as it passes through the tubes and reaches at the top. Thus, these boilers do not need a steam drum, and are hence often referred to as drumless boilers. Superheater Steam separator Economizer Heat Feed pump Fig. 5.6 Flow Diagram of a Once-Through Boiler 190 Power Plant Engineering 5.4.2 A Brief Account of Modern Steam Generators In the section that follow, a brief explanation of few high-pressure boilers is given. 5.4.2.1 La-Mont Boiler Figure 5.7 shows a modern forced circulation type La-Mont boiler. Feed water from the hot well is passed through the economizer tubes before entering the steam and water drum. During its flow through the economizer tubes, water gains maximum sensible heat from the flue gases escaping to the exhaust. As the boiler operates at pressures ranging between 120 bar and 160 bar, natural circulation is limited due to reduced density difference between the liquid and vapour. Hence, an external centrifugal pump is used to assist the circulation. This pump delivers large amount of water up to 10 times the rate of evaporation from the steam drum. Water from the circulating pump is forced through the evaporator tubes placed in the radiant section and convective section of the furnace simultaneously. This is done by the distributing header that distributes water from the circulating pump to the evaporator tubes through nozzles. The steam generated in the steam drum is then passed through the superheater tubes before being fed to the turbo generator. This boiler is useful to supply steam between 130 and 3000 tons/h. Cold air Hot air to Blower furnace Feed water Superheated steam to turbine Steam and Superheated tubes water drum Evaporator tubes (convective section) Flue gas Furnace wall Circulating Distributor Evaporator pump header Boiler furance (Radiant section) Fig. 5.7 A Modern Forced Circulation Type La Mont Boiler The main disadvantage of La-Mont type boiler is the formation of bubbles. These bubbles come in contact with the inner surface of the heating tubes and are subsequently attached to it. The bubbles attached to the tube surface have higher thermal resistance, and consequently reduce the heat flow and steam generation.  Steam Generator 191 5.4.2.2 Benson Boiler Figure 5.8 shows a typical Benson boiler. It is a supercritical boiler with pressure ranging from 225 bar to 500 bar. The difficulty of bubble formation is completely eliminated due to the oper- ation of the boiler above the critical pressure. Exhaust gases Cold air Hot air to furnace Blower Feed water Economizer Evaporator (Convective section) To turbine 2 Superheater tubes Starting valve 1 Evaportator furnace (Radiant section) Fig. 5.8 Benson Boiler Feed water from the hot well is forced through the economizer tubes. The hot water enters the radiant section of the evaporator tubes and then passes through convective section of the evapo- rator tubes. Since the boiler has no drum, the steam generated in the evaporator tubes enters the superheater tubes where it is heated above the critical pressure. Steam-raising capacity of the boiler is around 150 tonnes/h and above. The major disadvantage of the boiler is that when water transforms into steam in the convec- tive section of the evaporator, salts get deposited in the transformation zone. Hence, periodic flashing of evaporator tubes in the convective section becomes necessary. The boiler is started from cold by circulating the feed water from the hot well by operating the starting valve. During this period, the valve (valve 2) that supplies superheated steam to the turbine is closed. Thus, water circulates through economizer, evaporative tubes, superheater and back to the feed water circuit via the starting valve. While taking the boiler on range, starting valve is closed and valve 2 is opened. This method avoids excessive heating of the tubes. The advantages of Benson boiler are the following: 192 Power Plant Engineering 1. It is compact is size. 2. It is lesser in weight due to the absence of drum. 3. Boiler erection is easy as all parts are welded at the plant site. 4. It is suitable for both partial loads and overloads. 5. Formation of bubbles is eliminated due to supercritical pressure. 6. It can withstand sudden load fluctuations; therefore, it is more suitable for power stations. 7. It has negligible blow-down losses compared with natural circulation boiler. 8. It is prone to lesser explosion hazards due to its small storage capacity. 5.4.2.3 Velox Boiler Figure 5.9 shows a line diagram of fire tube Velox boiler. This is a fire tube boiler that uses oil or gaseous fuel. The combustion gases are circulated through the tubes with supersonic velocity to increase the heat transfer between the hot gases and the feed water. Air compressed to 2.5 bar by an air compressor is supplied to the combustion chamber. The combustion gases produced by the combustion of fuel pass through the tubes transferring heat to the water. The mixture of water and steam formed enters a steam separator with a spiral flow. The centrifugal force produced by this effect causes the heavier water particles to be thrown outword to the walls. The steam thus separated is passed to superheater where it is superheated by the gases coming from the combustion chamber and finally passed to prime mover. The water is again sent to the combustion chamber through a circulating pump. The combustion gases coming out of the superheater are used to run a gas turbine that runs the air compressor. The exhaust gases coming out of the gas turbine is passed through the economizer to heat the feed water. The feed water is circulated through the tubes with the help of a pump. Feed water tank Fuel tank Exhaust to atmosphere Fuel pump Steam Feed pump separator Hot flue gases Economiser Superheater Combustion chamber Water 150°C Steam circulating pump 500°C Air compressor Gas turbine Motor Exhaust Fig. 5.9 Velox Boiler  Steam Generator 193 5.4.2.4 Schmidt–Hartmann Boiler Figure 5.10 shows a Schmidt–Hartman boiler. There are two circuits in this boiler. In the ­primary circuit, distilled water from the water passes through the primary evaporator located in the combustion chamber of the boiler. The steam produced in the evaporator is passed through a submerged heating coil located in the evaporator drum. The high-pressure condensate formed in the submerged heating coil is circulated through a low-pressure feed heater where its natural circulation is maintained in the primary circuit. In the secondary circuit, feed pump supplies impure feed water from hot well to the evaporator drum through feed preheater. The saturated water that comes from the feed preheater is heated by the high-pressure steam in the heating coil. The steam thus produced in the evaporator drum from impure water is further passed through the superheater and then supplied to prime mover. Cold air Hot Evaporator Blower drum Secondary circuit Primary circuit Super heated steam Feed preheater Primary evaporator Feed NRV pump Water drum Combustion chamber Hot water Fig. 5.10 Schmidt–Hartman Boiler 5.4.2.5 Loeffer Boiler Figure 5.11 shows a Loeffer boiler. In this boiler, the feed water is evaporated by superheated steam. The high-pressure feed pump forces the water through the economizer and deliv- ers it to the evaporator drum. The steam-circulating pump draws saturated steam from this drum and passes it through the superheater. About one-third of superheated steam passes to the turbine, and the remaining steam enters the evaporating drum where it evaporates the feed water. 194 Power Plant Engineering Exhaust gases Air Blower Feed pump Feed water Economizer Radiant evaporator Convective superheater Steam to Flue gas turbine Steam circulating C.C pump Evaporator drum Fig. 5.11 Loeffer Boiler 5.4.2.6 Supercritical Boilers The increasing fuel costs with decreasing fuel quality have constantly persuaded power engi- neers to search for more economical methods of power generation. The most recent method to produce economical thermal power is by the use of supercritical steam cycle. Between the working ranges of 125 bar and 510°C to 300 bar and 600°C, large number of steam-generating units are designed, which are basically characterized as sub-critical and supercritical. Usually, a sub-critical boiler consists of three distinct sections: preheater (economizer), evaporator and superheater. In the case of supercritical boiler, only preheater and superheater are required. The constructional layouts of both types of boilers are otherwise practically identical. With the recent experiences gained in design and construction of supercritical boilers, it has become a rule to use supercritical boilers above 300 MW capacity units. The advantages of supercritical boilers over critical type are listed below: 1. T  hey offer higher heat transfer rates as compared with sub-critical boilers. In a s­ ubcritical boiler, the steam side heat transfer coefficient is 165 MJ/m2h-°C at r­ espective steam pres- sure and temperatures of 180 bar and 538°C, whereas the steam side heat transfer, coef- ficient for supercritical boiler is 220 MJ/m2–h-°C when the steam is generated at 240°C. 2. The pressure level is more stable due to less heat capacity of the generator and ­therefore gives better response. 3. Higher thermal efficiency (40–42 per cent) of power station can be achieved with the use of supercritical steam.  Steam Generator 195 4. T he problems of erosion and corrosion are minimized in supercritical boilers as two-phase mixture does not exist. 5. The turbo generators connected to supercritical boilers can generate peak loads by changing the pressure of operation. 6. There is a great case of operation, and their comparative simplicity and flexibility make them adaptable to load fluctuations. Although thermodynamically steam temperature and pressure are always desirable, the trend is halted due to the availability of material and difficulties experienced in the turbine and ­condenser operations due to large volumes. Presently, 246 bar and 538°C are used for unit sizes above 500 MW capacity plants. Boilers operating above critical pressure are known as supercritical boilers. They are used in power plants of capacity 300 MW and above. The constructional layout of supercritical boiler is practically identical to subcritical boiler. Usually, a sub-critical boiler consists of three distinct sections: economizer, evaporators and superheater; no distinction can be made between such components in supercritical boiler. Figure 5.12 shows Ramsin’s once-through boiler. Superheated steam Exhaust gas Economizer Superheater (CS) Flue gas Evaporator (T) Feed pump Fig. 5.12 Ramsin Boiler The boiler consists of inclined evaporator coils (T) arranged in a spiral. Forty such coils are paralleled around the furnace. Steam generated in the evaporators flows into the headers and then to the convection superheater (CS). The superheated steam is utilized for power generation. 196 Power Plant Engineering Advantages 1. Heat transfer rates are considerably large. 2. Higher thermal efficiency of power plant. 3. Problems of erosion and corrosion are minimized as two-phase mixture does not exist. 4. Adaptable to load fluctuation. Disadvantages 1. I t is costly due to increased requirement of steel for heat transfer surface, pumps and feed water piping. 5.5 BOILER PERFORMANCE CALCULATIONS Overall efficiency of the plant is defined by the ratio of plant output expressed as follows: Output h0 = × 100 Input Energy generated = Heat supplied Energy generated (KW) h0 = m f × CVf where mf = mass of fuel burnt/s, kg/s, CVf = calorific value of fuel, kJ/kg. Considering the Rankine cycle, we can define both boiler and tubine efficiencies as follows: Boiler efficiency, output hb = Input Heat supplied to steam in raising its temperature to superheat condiition = Heat input to boiler Total heat of feed water = × 100 mf × CVf h1 − hw hb = × 100 m f × CV f  Steam Generator 197 where h1 = enthalpy of superheated steam, kJ/kg hw = enthalpy of feed water, kJ/kg mf = mass of fuel burnt, kg CVf = calorific value of fuel, kJ/kg 5.5.1 Turbine Efficiency Turbine efficiency is defined as the ratio of mechanical work output to isentropic heat drop in the turbine. Mechanical work output ht = ×100 Isentropic heat drop Mechanical work output = × 100 (h1 − h2′ ) where h2′ = enthalpy of exit steam, kJ/kg. Due to heat rejection (45–49 per cent of total available heat energy) in the condenser, Rankine cycle efficiency decreases. The following Figure 5.13 shows the heat balance diagram considering the losses. 100% ∗Other losses: 10−12% Condensate loss: 45−49% Heat input 50% Generator loss etc.: 2−4% Useful heat output & electricity : 34−39% 0 Fig. 5.13 Heat Balance Diagram Note: Other losses include the following: Moisture in fuel: 0.5–1.0% Combustion of hydrogen: 3% Dry chimney gasses: 4.5–5.5% Combustible in ash: 1–2% Radiation and unaccounted: 1–2% 198 Power Plant Engineering 5.5.2 Rankine Cycle Efficiency ( η c) Rankine cycle efficiency is given by the ratio of energy available for conversion in work to energy given as heat in boiler. Actual enthalpy drop ∴ hc = Isentropic enthalpy drop h1 − h2 = h1 − h2′ where h2 = actual enthalpy of steam at turbine exit. 5.5.3 Generator Efficiency ( η g) During conversion of mechanical energy into electrical energy, some losses occur due to friction and core losses (copper loss, iron loss). Generator efficiency is given by the following equation: Electrical energy sent out, kwh hg = ×100 Mechanical work (kw) × 36000 Based on the above four efficiencies, the overall efficiency of the plant may be defined as follows: η0 = ηb × ηt × ηc × ηg A part from the above two more terms namely overall turbo-alternator efficiency and heat rate are quite useful for calculation purpose. 5.5.4 Overall Turbo-Alternator Efficiency ( η ota) Overall turbo-alternator efficiency is defined as the ratio of electrical energy sent out to the heat supplied in the boiler and is expressed as follows: Electrical energy sent out hota = ×1000 Heat supplied to steam in boiler = ηt × ηc × ηg Hence, overall station efficiency is expressed as follow: η0 = ηb × ηota  Steam Generator 199 5.5.5 Heat Rate Based on heat rate, it is sometimes convenient to express overall turbo-alternator efficiency. Heat rate is expressed as follows: Heat added to steam in boiler HR = Electrical energy sent out 1 ∴ hota = × 100 HR In addition, the overall station or plant efficiency is 1 h0 = ×100 HR of plant Note: When the plant has multiple turbines fed by a common header or bus from the boilers, the overall efficiency must be multiplied by a factor known as range factor, or range efficiency factor. 5.5.6 Boiler Performance Boiler efficiency depends on the ability to burn the fuel and subsequently utilize the heat energy to transform water into steam; in general, a lager boiler has higher efficiency in comparison with a smaller one, due to its physical size and ability to operate at higher pressure and temperature. Based on the energy consumption of auxiliaries and the type of the fuel chosen, boiler effi- ciency can be expressed as follows: 1. Gross-on-gross efficiency Gross-on-gross efficiency is based on the gross calorific value or GCV (higher calorific value, HCV) of the fuel and the gross heat supplied to the working fluid. 2. Net on gross efficiency Net on gross efficiency calculation is based on the GCV of the fuel and the net supplied to the working fluid after excluding heat equivalent energy consumption from boiler auxiliaries. 3. Gross on net efficiency Gross on net efficiency calculation is based on net calorific value of the fuel (LCV) and gross heat supplied to the working fluid. 4. Net-on-net efficiency Net-on-net efficiency calculations are based on the net calorific value (LCV) of the fuel and the net heat supplied to the fuel. Two different method are used for computing the boiler efficiency. These are as follows: (i) Direct method (ii) Indirect or losses method 200 Power Plant Engineering (i) Direct method In direct method, efficiency is calculated for a non- reheat (NRH) unit as follows: (Enthalpy of steam − Enthalpy of feed water) h = ms Heat supplied  C PS (T1 − TS ) + hfg + C pw (TS − TW )  hb = m p ×  