WELCOME TO SESSION ON BOILERS FUNDAMENTALS/COMBUSTION
AJAY SHUKLA
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
EFFECT OF VARIOUS PARAMETERS IN RANKINE CYCLE
SUPERHEATING – improves efficiency INCREASING PRESSURE – marginal improvement in efficiency LOWERING EXHAUST PRESSURE- improves efficiency REHEATING – improves efficiency REGENERATIVE FEEDWATER HEATING – improves efficiency
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.
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 0 0 200 MW: 157 bar MS Pressure, 540 C/540 C 0 0 500 MW: 179 bar MS Pressure, 540 C/540 C 0 0 660 MW: 246 bar MS Pressure, 545 C/563 C Advanced Supercritical Parameter 0 0 310 bar MS Pressure, 610 C/610 C
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.
CLASSIFICATION OF BOILER
Based on Steam Parameters
Sub Critical Operates below the critical pressure of water (221.2 bar)
Super Critical Operates above the critical pressure of water (221.2 bar).
Once Through No Thermodynamic fixed point i.e. evaporation point keeps shifting in the water tubes depending on firing rate.
Universal Pressure Operate at constant pressure
Drum type Provides a thermodynamic fixed point at drum, which remains at constant temp.
Sliding Pressure Operate at sub-critical pressure at reduced loads.
Natural Circulation Boilers use the difference in water and steam density to drive the water/steam mixture through the water tubes.
Assisted Circulation Boilers have Circulating Water Pump which assists the natural convective flow through the water tubes.
CLASSIFICATION OF BOILER
Based on Flue Gas Arrangement
Two Pass Most of the SH, RH and Eco heat transfer surfaces are placed in the horizontal and second passes. Some pendant SH and RH surfaces placed above the furnace. Pendant section tubes cannot be drained.
Tower Type All heat exchangers are arranged horizontally above the furnace. Provides easy draining of the SH and RH tubes and headers.
CLASSIFICATION OF BOILER
Based on Firing Arrangement
Tangential Fired Burners are arranged over many elevation to fire around an imaginary circle. One mill normally feeds one coal elevation. individual Sec. Air control is not provided.
Wall Fired Burners are arranged in rows over many elevation on front and rear walls. Mill to burner distribution optimized for stable combustion at low loads. Each burner flame independent with individual Sec. Air control.
Downshot Fired Burners are arranged to fire downwards in rows over many elevation on front and rear walls. Better suited to low volatile coals as it gives a high furnace residence time.
CLASSIFICATION OF BOILER
Based on Bottom Ash
Wet Bottom Bottom Ash collected in slag form. Mostly used for low ash coals with low fusion temperatures.
Dry Bottom Bottom ash is cooled in water in the hopper before removal in the clinker form. Suited for Indian coals with high ash content.
Boiler Classification (contd…)
Once Through Boiler Continuous Path of water through economizer to water walls and to superheaters. These are necessarily forced circulation boilers. They are suited for fast startup and load cycling, as the thick drum is eliminated. Once Through Boiler
Boiler Classification (contd…)
Drum Type Boiler (Natural Circulation)
Drum Type Boiler (Assisted Circulation)
Boiler Classification (contd…)
Supercritical Boiler (Universal Pressure)
Supercritical Boiler (Sliding Pressure)
OUTLINE
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Boiler fundamentals
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Boiler components (water side)
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Boiler combustion (air side)
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Boiler classification
Basic Knowledge of Boiler
Basic boiler :
Steam
Water
Steam / water system Blow down
Mixing of fuel and air
Furnace
Heat transfer Surface
Flue gas
AIR
FUEL
Ash
Phenomenological Model Hot Flue Gas
Thermal Structure SH Convection & Radiation HT
Steam
Convection HT
Rise in Enthalpy of Steam
Drop in Enthalpy of Flue Gas Mechanism of Heat Transfer Source/Supply
Thermal Structure
Sink /Demand
STEAM GENERATOR COMPONENTS
• • • • • • • • • •
FURNACE DRUM BOILER CIRCULATING PUMPS CONVECTION PASS ¾ SUPERHEATER ¾ REHEATER ¾ 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
BOILER LAYOUT AND PA FAN
DPNL SHTR
Platen SHTR
Drum
Reheater S C R E E n Gooseneck
LTSH
Chimney
Downcomer waterwall
Fireball
Economiser
ID fan
APH Bottom Ash
ESP
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.
Type of Circulation ¾
¾
¾
Natural circulation (upto 165 ksc)
Density difference & height of water column Assisted by external circulating pump (CC/ BCW pump)
Forced/ assisted circulation (185-200 ksc) Once thru boiler Below 221.5 bar 1. Sub critical 240-360 bar 2. Supercritical
Circulation ratio It may be defined as the ratio of feed water flow thru down comers to the steam generated in water wall.
CR CR CR CR CR
= = = = =
30-35 Industrial boilers 6-8 Natrual cir. Boilers 2-3 Forced cir. Boilers 1 Once thru boilers (Sub critical) 1 Supercritical boilers
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
More Details of Pulverized Fuel fired SG
Boiler Auxiliaries
TO
Session on Boiler
Combustion
-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
Boiler fundamentals FACTORS AFFECTING COMBUSTIONTIME,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)
Combustion •Burning of fuel (chemical reaction) •Rapid combination of o2 with fuel, resulting in the release of heat •For fuel to burn ,the following conditions must be present • The fuel must be gasified •The oxygen and fuel mixture should be proper. •Temp should be above ignition
FUELS Combustible substances which, when combined with oxygen in air & ignited, burn giving heat.
CLASSIFICATION OF FUELS Solids
Liquids
Gaseous
Coal Lignite Peat Bagasse Gas Husk
Kerosene Petrol HSD LDO
Natural gas Methane LPG Producer
FO LSHS
MAIN CONSTITUENTS OF FUEL Carbon Hydrogen Sulphur Nitrogen Oxygen Water Vapour Ash
PROXIMATE ANALYSIS OF TYPICAL INDIAN COAL
DESIGN W ORST BEST
TOTAL MOISTURE ASH VOLATILE MATTER FIXED CARBON
% % % %
15 42 21 22
16.5 44 19.5 20
14 38 23 25
TOTAL
%
100
100
100
PROPERTIES OF FUEL (Typical Analysis of F.O.) Carbon Hydrogen Sulphur Calorific value Sp. Gravity at 30oC Flash point Viscosity at 40oC Water Percentage Sediment Percentage
83.52% 11.68% 3.27% 10,000 Kcal/kg 0.95 65oC 1500 RW Sec No 1 0.15 0.3 (Variable)
COMBUSTION • Combustion is rapid oxidation of fuel resulting in constituents getting converted into respective oxides, liberating heat. Fuel +Air Oxides + Heat (Prs of combustion) CO2 + Heat 43,968 Kcal C +O2 : 2H2 +O2 : 2H2O + Heat 61,979 Kcal S +O2 : SO2 + Heat 3175 Kcal Incomplete Combustion 2C + O2 : 2CO + Heat 26,429 Kcal 1 Kg of liquid fuel + 15 Kg of Air Oxides +
COMBUSTION PROCESS LIQUID FUEL
PRESSURISED + PREHEATED ATOMISED
HEATED BY FURNACE HEAT VAPORISED
IGNITED BY FLAME COMBUSTION
COMBUSTION REACTIONS
C O
C O
C O O
2C + O2
2CO + LESS HEAT
C
COMBUSTION INCOMPLETE
COMBUSTION REACTIONS C O
O C+O 2
C
O
CO2 + HEAT H O H
H H O O
O
2H2 + O2 H H
H O H
2H20 + HEAT COMBUSTION COMPLETE
COMBUSTION
FLAME & FLAME FRONT *
*
FLAME : IT IS AN ENVELOPE OR ZONE WITHIN WHICH COMBUSTION REACTION IS OCCURRING AT SUCH A RATE AS TO PRODUCE VISIBLE RADIATION.
FLAME FRONT : IT IS THE 3 D CONTOUR ALONG WHICH COMBUSTION STARTS IT IS THE DIVIDING LINE BETWEEN FUEL-AIR MIXTURE AND COMBUSTION PRODUCTS.
REF. : NORTH AMERICAN COMBUSTION HANDBOOK
EXCESS AIR Fuel + Theoretical air required + 15% to 40% T.A.
Combustion
FOR COMPLETE COMBUSTION... : Fuel has to be atomised. : Raise the temperature to ignition temperature. : Electrical spark of ignition. : Proper mixing of fuel and air. : Distribution of Primary and Secondary air.
GOOD COMBUSTION REQUIRES ....... Ñ
3 T’s - TIME, TEMPERATURE & TURBULENCE
Ñ
PROPER PROPORTIONING OF FUEL & AIR
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CORRECT CONTROL OF FUEL & AIR
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THOROUGH MIXING OF FUEL & AIR
Ñ
INITIAL & SUSTAINED IGNITION
MEASUREMENT OF COMBUSTION
CO2
:
12 - 13%
SMOKE INDEX
:
2-3
STACK TEMPERATURE
:
As per design.
O2
:
3%
Arrangement of fuel input in furnace
Coal is pulverized in mills at a fineness of 70% thru 200 mesh. Dried powdered coal is conveyed to furnace (at a temperature < 95-100oC) Total coal flow is distributed among running mills and fed thru coal burners at 20-25 m/sec. Coal flow is arranged in tiers. Maximum heat release rate must not exceed plain area heat loading. It generates excessive NOx and making ash fused.
Combustion air arrangement in furnace
Fuel air is supplied around coal nozzles (at velocity of 30-35 m/sec). Secondary air is supplied in adjacent tiers of sec. air dampers from wind box (Hot air from Secondary APH) Overfire/ Tempering air is supplied at the top of the burnaer zone for NOx control. Gas recirculation is adopted for steam temperature control in oil/ gas fired units. Furnace draft is maintained at -5 mmwcl with Forced and Induced draft fans (balanced draft)
Pulverized Fuel Boiler (Contd..) Advantages
Its ability to burn all ranks of coal from anthracitic to lignitic, and it permits combination firing (i.e., can use coal, oil and gas in same burner). Because of these advantages, there is widespread use of pulverized coal furnaces.
Disadvantages
High power demand for pulverizing Requires more maintenance, flyash erosion and pollution complicate unit operation
SAFETIES ý
Unauthorised flame presence during pre-purge and after controlled shut down.
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Pilot flame safety
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Main flame safety
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High gas pressure safety
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Low gas pressure safety (optional)
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Double Block & Bleed valves in main gas line
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Combustion air failure safety
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Interlock with boiler safeties
THANK YOU
