Published on February 26, 2014
Combustion Theory Lecture 12 Combustion in Boilers
Learning Outcomes 1. Overview of general boiler designs and applications – Heat balance/heat transfer/ heat availability/excess air/size and shape of the combustor – Boiler configurations: Smoke tube (Fire tube) and water tube/ Water wall designs Natural Circulation and Forced Circulation 2. Type of fuels used and different combustion configurations – Gas, liquid and solid fuels burn in boilers 3. Types of burners used and the related emission control methods in boiler designs – Gas burner designs/Liquid fuel burner designs/Solid fuel PC burners, burning beds etc. 4. Emission cleaning methods in boiler operation – Species of concerns and appropriate techniques/layouts
Overview of Boiler Operations • Boilers are being used since 18th century • Applications range from: small hot water/steam applications in domestic/industrial use large scale steam production for power generation • Improvements in the quality of steam produced for industrial applications A couple of Bars Pressure & a few 100’s °C Temperature (Saturated Steam) ~ 200-300 Bars pressure & 600 °C Temperature (Superheated Steam) [Large power generation systems]
Heat Balances – Condensing Power Flue gas heat loss ~ 15 – 20% Heat balance: Boiler Energy in fuel 100% Condensing power plant Electricity (25 - 40%) Heat distribution loss ~ 10 – 15% Heat balance: Steam plant Heat rejection by condenser ~ 35 – 45%
Available Heat Available Heat = Gross heating value of the fuel – Enthalpy lost from the process by hot exhaust gases
Optimum Excess Air Depends on • Fuel type - coal, gas, oil, and rate of combustion • Furnace/ combustor design - influences – Combustion time – Mixing – Temperature • Type and number of burners which affect fuel/air mixing • Burner turndown • Air preheat, oxidant – enhances combustion rate
Excess Air Air rate too high ⇨ Increased sensible heat loss Air rate too low ⇨ Increased potential heat loss Flue gas temp. too high ⇨ Increased sensible heat loss
Operating Excess Air Range fro Common Fuels
Estimation of the Shape & Size of a Combustion Chamber • Useful criteria: – Cross sectional surface loading QC = Qfuel/Ac [MW/m2] – Volumetric Loading QV = Qfuel/V [MW/m3] Where AC – Cross sectional area of the furnace [m2] V – volume of furnace [m3] Qfuel – fuel power [MW]; This is related to electrical power Qe by Qe = Qfuel · ηboiler · ηcycle (ηboiler -Boiler efficiency; ηcycle- efficiency of the thermodynamic cycle) • At a given air excess, – QC - associated with cross sectional average gas velocity – QV - connected to gas residence time
Volume and Surface Loadings of Suspension Fired Boilers
Burn-up Limit and Exit Temperature Limit for Tangentially Fired Pulverized Coal Boilers
Types of Fuels Used and Different Combustion Systems Size comparison: Combustion chamber of same thermal power boiler with different types of fuels, Gas, liquid and solid
Fire Tube (Smoke Tube) Boilers • • • • Maximum pressure 20 Bar No superheated steam Gas/oil fired Industrial applications Chimney Steam Water Burner Water Steam at 150°C Chimney Water 350°C Burner 200°C Water
Water Tube Boilers Water tube boiler- drum boiler • • • • • • Pressures - up to 80 - 100 Bar Steam - Superheated steam Fuel - Gas/oil/solid Water circulation: Natural or forced Steam flow through tubes Applications - Industrial and power plants
Water Wall Boilers –Large Scale Water tube boiler - once Installations through boiler • Pressure - High, up to 80 200 Bar • Steam - Superheated/ Supercritical steam • Fuel - Gas/oil/solid • Forced water /steam flow through tubes • Applications - Power plants
Gas/Liquid/Solid Powder Fuel Burning Systems • Use different types of burners as appropriate for the fuel • Gas - least complicated o lean premixed methods are employed • Liquid – Lean premixed/prevaporized o Low excess air burners and staged burning employed • Solid – premixed with air o Staged burning and reburning techniques are applied
Solid Fuel Burning Systems • Grate Furnace – Solid fuel low burning rate – No internal emission control • Pulverized Coal – High burning rate • Fluidized Bed – Solid fuel – emission control • Bubbling Circulation Bed – Solid fuel – High burning rate & emission control
Solid Fuel Burning Systems Applications 1. Bubbling Fluidized Bed (BFB)
Components of an FBC (Fluidized Bed Combustion) Boiler
Gas/Liquid Fuel Burning Systems • Oil burning – high burning rate – Emission control – Expensive • Gas – Very good fuel – Uneconomical to burn in boilers for steam power
Solid Fuel Burning Systems Applications Contd… Circulating Fluidized Bed (CFB)
Solid Fuel Burning Systems Applications Contd… (Pressurized Fluidized Bed Combustion)
Oil Burners - Stabilized by Flame Holder Blockage ratio : A disc B= A ex. air nozzle Swirl number : S0 = GΦ G x ⋅ R throat
Oil Burners Stabilized by Swirl
Solid Fuel Burning Systems Burners for Powder • Factors influencing ignition – Size distribution of the fuel – Properties of the fuel – Burner design and interaction with the combustion chamber • Two main groups of pulverized fuel burners 1. Jet burners 2. Swirl Burners
Solid Fuel Burning Systems Burners for Powder Contd… Direct injection through a pipe (Arrows ⇨ entrainment of gas) Jet burner arranged for pulverized fuel
Solid Fuel Burning Systems Burners for Powder Contd… Tangential Burners for large coal fired boilers
Solid Fuel Burning Systems (Pulverized Coal Combustion) courtesy of Hitachi Ltd. Group • 1st generation: Low-NOx burner ‘HT-NR burner’ – based on "In-flame NOx reduction” concept • 2nd generation: Low-NOx burner ‘HT-NR2’ – with further enhanced NOx decomposition capability – extremely reliable and has many applications in both the domestic and foreign industrial and power boiler markets • 3rd generation: Low-NOx burner ‘HT-NR3’ – further reduced NOx emission levels, improved combustion efficiency and ease of maintenance – a wider and shorter, highly stable flame for excellent fuel combustion characteristics with extremely Low NOx levels.
Solid Fuel Burning Systems (Pulverized Coal Combustion) Contd… courtesy of Hitachi Ltd. Group • Features of the 3rd generation HT-NR3 – Rapid ignition with Flame Stabilizing Ring – Effective separation of outer air to produce reducing conditions in the center zone – PC concentration for higher combustion efficiency in reducing condition
Emission Control Methods in Boiler Designs Nox Reduction • Nonpremixed combustion (Furnace) – Re-burning technique for NOx reduction
Solid Fuel Burning Systems (Pulverized Coal Combustion -Hitachi) History of Low NOx Burners
In-Flame Nox Redcution Mechanism (Courtesy: Hitachi Ltd. Group) Reaction in a high temperature reducing flame Characteristics of NOx Emission Flame structure Oxidizing Zone NOx Reduction zone
Premixed/Nonpremixed Combustion Exhaust Treatment for NOx Reduction in Industrial Applications • Selective Catalytic Reduction (SCR) • Selective Non Catalytic Reduction (SNCR)
Emission Control in Boiler Designs Reduction of CO2 Emissions by Improving the Efficiency of Coalfired Power Plants • Subcritical – E.g. FBC’s • Supercritical & Ultrasupercritical – (New) pulverized coal combustion systems • IGCC – Integrated Gasification & combined cycle
Emission Control in Boiler Designs Nonpremixed Combustion (Furnace) Oxy-Fuel Combustion Steam Turbine Electricity CO2 Boiler Cooling Water Mechanical Energy Cooler & Condenser Steam Condenser CO2 Compression Sulfur Removal Cooler & Compressor Particle Removal Heat Nitrogen Heat Water Sulfur Water Air Separation fuel Fly Ash • Extra cost of oxygen Oxygen Recycle (mainly CO2 & water vapor) • Leak tight operation requirement Energy Air Bottom Ash • CO2 capture
Emission Control in Boiler Designs Pre-combustion Capture (IGCC)
Emission Control in Boiler Designs Post-combustion Capture
Summary This section was focused on different aspects on combustion in boilers such as, • An overview - general boiler designs and applications – Heat balance/heat transfer/ heat availability/excess air/size and shape of the combustor – Boiler configurations: Smoke tube (Fire tube) and water tube/ Water wall designs Natural Circulation and Forced Circulation • Types of fuels used and different combustion configurations – Gas, liquid and solid fuels • Types of burners used and the related emission control – Gas burner designs/Liquid fuel burner designs/Solid fuel PC burners, burning beds etc. • Emission cleaning methods in boiler operation – Species of concerns and appropriate techniques/layouts
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