BOILERS FUNDAMENTALS/COMBUSTION AJAY SHUKLA DGM NTPC PMI 23rd March ,10
In early 19th Century boiler were low pressure • Invention of water tube removed the pr barrier and boiler pr rise to super critical • Between 70- 90 utility operated conservatively and used low steam pr in boiler . • Now renewed interest in high efficiency supercritical boiler .The interest arose from the environmental need to attain higher efficiency and dividend of higher eff is reduce CO2
Rankine Cycle • 1 to 2: Isentropic expansion (Steam turbine) • 2 to 3: Isobaric heat rejection (Condenser) • 3 to 4: Isentropic compression (Pump) • 4 to 1: Isobaric heat supply (Boiler) Rankine cycle is a heat engine with vapor power cycle. The common working fluid is water. The cycle consists of four processes:
Boiler/ steam generator • Steam generating device for a specific purpose. • Capable to meet variation in load demand • Capable of generating steam in a range of operating pressure and temperature • For utility purpose, it should generate steam uninterruptedly at operating pressure and temperature for running steam turbines.
OUTLINE • Boiler fundamentals • Boiler components (water side) • Boiler combustion (air side) • Boiler classification
Basic boiler : Steam Water Blow down Flue gas AIR Ash FUEL
Hot Flue Gas Thermal Structure SH Steam Rise in Enthalpy of Steam Convection HT Drop in Enthalpy of Flue Gas Convection & Radiation HT Mechanism of Heat Transfer Thermal Structure Sink /Demand Source/Supply Phenomenological Model
STEAM GENERATOR COMPONENTS • FURNACE • DRUM • BOILER CIRCULATING PUMPS • CONVECTION PASS • SUPERHEATER • REHEATER • ECONOMISER • AIR HEATER • STEAM COILED AIR PREHEATER • SOOT BLOWERS • COAL FEEDERS • PULVERIZERS • COAL PIPING • BURNERS • IGNITOR AND WARM UP BURNERS • DUCTWORK AND • INSULATION AND LAGGING
Reheater DPNL SHTR S C R E E n Drum Platen SHTR LTSH Gooseneck Chimney Downcomer waterwall Fireball Economiser ID fan ESP APH Bottom Ash
Boiler fundamentals -BOILER=CONTROLLED COMB.+HEAT TRANSFER -CHEMICAL =THERMAL -COMBUSTION-FUEL,TEMP,O2 -FUEL - BITUMINOUS COAL
Boiler fundamentals Combustion in furnace :- • Pulverized fuel by coal burners • Ignition temp. By oil firing • O2 by means of fans. Reactions:- • C+O2 = CO2, • 2H2+O2 = 2H2O • S+O2 = SO2 • Theoretical air = O2/.233
FACTORS AFFECTING COMBUSTION- TIME,TEMP., INTER MIXING OF AIR WITH FUEL(TTT), COAL FINENESS, I. Excess Air:- - (20%)-bituminous coal -(15%)-lignite A. Lower excess air:- -High unburnt loss B. Higher excess air:- -Higher heat loss (ma*cpa*dt) Boiler fundamentals
Water and Steam Circulation System • Economiser • Boiler drum • Down Comers • Water walls • Primary super heater • Platen super heater • Final super heater • Reheater
Drum • The boiler drum forms a part of the circulation system of the boiler. The drum serves two functions, the first and primary one being that of separating steam from the mixture of water and steam discharged into it. Secondly, the drum houses all equipments used for purification of steam after being separated from water. This purification equipment is commonly referred to as the Drum Internals.
Natural circulation (upto 165 ksc) Forced/ assisted circulation (185-200 ksc) Once thru boiler 1. Sub critical 2. Supercritical Density difference & height of water column Assisted by external circulating pump (CC/ BCW pump) Below 221.5 bar 240-360 bar Type of Circulation
It may be defined as the ratio of feed water flow thru down comers to the steam generated in water wall. CR = 30-35 Industrial boilers CR = 6-8 Natrual cir. Boilers CR = 2-3 Forced cir. Boilers CR = 1 Once thru boilers (Sub critical) CR = 1 Supercritical boilers Circulation ratio
Waterwall construction • Made of carbon steel (Grade-C) hollow circular tubes and DM water flows inside • Waterwalls are stiffened by the vertical stays and buck stays to safeguard from furnace pressure pulsation & explosion/ implosion • The boiler as a whole is hanging type, supported at the top in large structural columns. • Vertical expansion is allowed downwards and provision is made at bottom trough seal near ring header.
Superheater & Reheater • Heat associated with the flue gas is used in superheaters & Reheater, LTSH, economiser. • Maximum steam temperature is decided by the operating drum pressure and metallurgical constraints of the turbine blade material. • Reheating is recommened at pressure above 100 ksc operating pressure. Reheating is done at 20-25% of the operating pressure. • Carbon steel, alloy steel & SS used for tubing of SH & RH.
Superheaters • Convection Superheaters • Radiant Superheaters
Important Components of Boiler Economizer Boiler drum Water wall Superheater Reheater Boiler Pressure Part Design Code – IBR/ASME. Selection of Material based on: Creep and Fatigue strength at design temperature. Fire side oxidation resistance. Design Temperature and thickness: as per IBR. Allowable stress for chosen material – as per ASME. TWO PASS BOILER ARRANGEMENT
Additional allowance on tube design thickness to take care of erosion. Selection of Material Upto 4000C: Carbon Steel for boiler tubes and plates. Upto 5500C: Low Alloy Steels like T11/P11, T22/P22, T23 etc. Upto 5900C: Medium Alloy Steel like T91/P91. Above 5900C: Austenitic Stainless Steel like TP347H, Super 304H. Drum internals designed for removal of maximum moisture and provide required purity. TDS in Feed Water restricted to 15 to 20 ppm Dissolved solids carryover not to exceed Silica carry over - <10 ppb Sodium carry over - <3 ppb Chloride carry over - <2 ppb Copper carry over - <1 ppb Iron carry over - not detectable Erosion shield/Cassette baffles on erosion prone areas.
Steam Theory • Within the boiler, fuel and air are forced into the furnace by the burner. • There, it burns to produce heat. • From there, the heat (flue gases) travel throughout the boiler. • The water absorbs the heat, and eventually absorb enough to change into a gaseous state - steam. • To the left is the basic theoretical design of a modern boiler. • Boiler makers have developed various designs to squeeze the most energy out of fuel and to maximized its transfer to the water.
Why Steam is so popular as heat conveying media in industry? • Highest specific heat and latent heat • Highest heat transfer coefficient • Easy to control and distribute • Cheap and inert
Properties of Steam • Liquid Enthalpy • Liquid enthalpy is the "Enthalpy" (heat energy) in the water when it has been raised to its boiling point is measured in kcal/kg, its symbol is hf • Also known as "Sensible Heat” • Enthalpy of Evaporation • It is the heat energy to be added to the water in order to change it into steam. • There is no change in temperature, the steam produced is at the same temperature as the water from which it is produced. • Also known as latent heat and its symbol is hfg
The temperature at which water boils, also called as boiling point or saturation temperature (It increases as the pressure increases. ) • As the steam pressure increases, the usable heat energy in the steam (enthalpy of evaporation), which is given up when the steam condenses, actually decreases. • The total heat of dry saturated steam or enthalpy of saturated steam is given by sum of the two enthalpies hf +hfg • When the steam contains moisture the total heat of steam will be hg = hf +q hfg where q is the dryness fraction.
Superheated Steam • Superheat is the addition of heat to dry saturated steam without increase in pressure. • Degree of Superheat • The temperature of superheated steam, expressed as degrees above saturation corresponding to that pressure.
Steam generation principle • Steam power plants operate on Rankine Cycle, DM water as working fluid. • Sensible heat is added in economiser +furnace • Steam generation takes place in waterwall. • Heat transfer in furnace and enclosed superheater takes place thru radiation. SH RH w/w HPT HPH+Eco IPT BFP LPT LPH CEP condenser
Basic Knowledge of Boiler • Purpose • To produce steam (Main Steam and Reheat Steam) at rated pressure and temperature • To Convert the heat of combustion of coal/oil/gas to thermal energy of steam • Steam Parameters are decided by Turbine Cycle Requirements • Steam Parameters adopted by NTPC • 200 MW: 157 bar MS Pressure, 5400C/5400C • 500 MW: 179 bar MS Pressure, 5400C/5400C • 660 MW: 246 bar MS Pressure, 5450C/5630C • Advanced Supercritical Parameter • 310 bar MS Pressure, 6100C/6100C
Engineering Function • Selection of Unit Size • Based on load demand, coal and water availability. • Input from Feasibility Report • Selection of Steam Parameters • Choice of steam parameters is governed by overall cost of the plant. • Sub-critical boilers are more suited in places where fuel cost is low. • Both drum type and once through boilers are acceptable based on manufacturer’s experience. • Super-critical boilers are costly because of greater use of high temperature material in boiler pressure parts. • Selection of Firing System • Firing systems are generally left to manufacturer’s discretion as each manufacturer prefers his standard design.
OT Boiler Tower type Typical Layout