Boiler Design Fundamentals PMI Revision 01

STEAM-GENERATOR FUNCTIONS Evaporating water to steam at high pressure Produce steam at exceptionally high purity Superheat the steam in the unit to a specified temperature, and maintain that temperature Reheat the steam which is returned to the boiler and maintain that reheat temperature constant Reduce the gas temperature to a level that satisfies the requirement for high thermal efficiency and at the same time is suitable for processing in the emission-control equipment downstream of the boiler

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Types of Firing System HORIZONTALLY FIRED SYSTEMS Fuel is mixed with combustion air in individual burner Coal and primary air are introduced tangentially, imparting strong rotation within the nozzle. Adjustable inlet vanes impart a rotation to the preheated secondary air from the windbox Air swirl, coupled with the flow-shaping contour of the burner throat, establishes a recirculation pattern extending several throat diameters into the furnace. Once the coal is ignited, the hot products of combustion propagate back toward the nozzle to provide the ignition energy necessary for stable combustion. Major combustion process must take place within the recirculation zone so the air/fuel ratio to each burner is within close tolerances.

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Flow pattern of horizontal The burners are located in rows, either on the front wall only or on both front and rear walls. The latter is called opposed firing

TANGENTIALLY FIRED SYSTEMS Based on the concept of a single flame envelope Fuel and combustion air are projected from the corners of the furnace along a line tangent to a small circle, lying in a horizontal plane, at the center of the furnace. Turbulence and mixing that take place along its path are low compared to horizontally fired systems Significance of this factor on the production of oxides of nitrogen Possible to vary the velocities of the air streams and change the mixing rate of fuel and air, and control the distance from the nozzle at which the coal ignitesPMI

Tangentially Firing System Provides great flexibility for multiple-fuel firing Fuel and air nozzles tilt in unison to raise and lower the flame to control furnace heat absorption and S/H & R/H Temp

Tangentially-Fired Furnaces Advantages: The efficient mixing, due to vortex, rapid contact between fuel and air, and flames interaction, that would ensure a reliable combustion with uniform temperature distribution. Uniform heat flux to the furnace walls; consequently failures due to high thermal stresses have been avoided. The air and fuel streams can be admitted inclined either upward or downward from the horizontal, a feature that is used to vary the amount of heat absorbed by the furnace walls and to control the superheater temperature. Vortex motion at the furnace center prevents or minimizes slugging of the furnace walls, erosion due to impingement and local over- heating. NO, in tangentially fired unit is lower than other firing types. NO, emissions from TF boilers are about half the values from wall firing systems. PMI Revision 01

VERTICALLY FIRED SYSTEMS Used principally to fire coals with moisture and-ash-free volatile matter between 9 and 13 percent

Require less stabilizing fuel than horizontal or tangential systems Have more complex firing equipment and more complex operating characteristics. Portion of the heated combustion air is introduced around the fuel nozzles and through adjacent auxiliary ports High pressure jets are used to avoid, short-circuiting the fuel/air streams to the furnace discharge Tertiary air ports are located in a row along the front and rear walls of the lower furnace Firing system produces a long, looping flame in the lower furnace, with the hot gases discharging up the center

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Flow pattern of vertical firing

LIMITS OF STEAM TEMPERATURE AND PRESSURE Materials of superheater govern the practical limits of steam temperature and pressure. The large majority is in the 400 to 565C temperature range Problems do arise during sustained elevated temperature operation because of the adverse effects of certain fuel constituent on unit availability.

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Steam generator energy flow Tube diameter and thickness are of concern from the standpoints of circulation and metal temperature. Natural circulation boilers generally use larger diameter tubes than circulation in once-through boilers. Small-diameter tubes is an advantage in high- pressure boilers because the lesser tube thicknesses required result in lower outside tube-metal temperatures.

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Superheaters and reheaters For relatively low final outlet temperatures superheaters solely of the convection type are generally used. For higher temperatures, surface requirements are Larger. superheater elements located in very high gas- temperature zones. Metallurgy: selection of materials for strength and oxidation resistance, the use of high steam pressure requires very thick walls in all tubing subject to steam pressure.

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Furnace Design The most important design criteria in large pulverized-fuel tangentially fired furnaces are Net heat input in W /m2 of furnace plan area (NHI/PA) Vertical distance from the top fuel nozzle to the furnace arch Furnace dimensions must be adequate to establish the necessary furnace retention time to properly burn. And to cool the gaseous combustion products. Gas temperature at the furnace outlet plane has to be well below the ash-softening temperature of the lowest quality coal burned. Heat-absorption characteristics of the walls are maintained using properly placed wall blowers

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Combustion Limits on Furnace Design The lower limit of the furnace volume is dominated by the space

required for burning the fuel completely, or to an extent less than the allowable unburned fuel loss. To complete the fuel combustion within the furnace space, the fuel

injected into the furnace has to reside there for a time longer than

critical time. The fuel residence time can also be estimated by the residence time of the combustion gas produced in the furnace.

Fuel combustion time is mainly dominated by the combustion reaction velocity and the rate at which oxygen is supplied into the reaction zone. The combustion reaction velocity depends on chemical characteristics of the fuel. Main technical factors that affect the combustion time are: Combustion characteristics of the fuel. Mixing characteristics. Fluid flow characteristics of the furnace. PMI Revision 01

IMPACT OF FUEL ONBOILER DESIGN Three very important parametric influences on furnace sizing are: Fuel reactivity Gaseous-emission limitations (particularly those concerning oxides of nitrogen), and Fuel-ash properties. Ash properties particularly important for designing of coal-fired furnaces: Ash fusibility temperatures Ratio of basic to acidic ash constituents Iron/Calcium ratio Fuel-ash content in terms of kg of ash/million joules Ash friability PMI Revision 01

Effect of coal rank on sizing of a pulverized-fuel furnace 26

MI DRY BOTTOM VRS WET BOTTOM FURNACES Current pulverized-coal units are of the dry-bottom type: the ash dislodged from the furnace walls is below the ash-melting point In the wet-bottom design. the lower part of the flame has to sweep the furnace floor at all loads to maintain the fluidity of the ash.

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ADVANTAGES OF THE PENDANT PANEL DESIGN The support elements are out of the gas stream above the furnace roof. Superheater and reheater are free to expand downward. and have only simple alignment devices in the gas stream

No relative motion between the furnace tubing and the superheater or reheater tubes The above support and sealing arrangement favors shop modularization of tubes, headers, attachments and supports Field erection, major pressure-part construction can be carried out in several areas simultaneously. Widely spaced panels along with steam cooled wall sections in the upper furnace. Have high radiant-heat absorption. resulting improved steam-temperature control range.

ADVANTAGES OF THE HORIZONTAL- SURFACE DESIGN Vertical gas flow through the superheater and reheater surface minimizes the potential for localized tube erosion in a 90 turn into the rear gas pass. Horizontal tubing facilitates designing for drainability, which simplifies freeze-protection procedures, boilout, and hydrostatic testing Large fused ash deposits that are removed by sootblowers will usually drop through wider spaced tube sections below, directly to the furnace bottom. Horizontal arrangement requires that in start-up there is adequate cooling flow through the vertical hanger tubes that support and align the horizontal tube bundles. Thermocouples should be used to monitor hanger-tube temperatures on start- up, especially in tubes with downward flow.

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CONVECTION-PASS DESIGN For designing a proper balance is required to maintain a thermal head with which to transfer heat from gas to steam as the heating-surface use is optimized and undesirably high metal temperatures are avoided

To limit pressure-part erosion from flyash, the flue-gas velocity must not exceed reasonable limits. It is impractical to propose a steam generator capable of burning any kind of coal. Certain coals need wide transverse tube spacing to reduce the fouling rate and possible bridging of ash deposits. This arrangement minimizes serious fouling problems. The transverse spacing of the convection - pass tube banks is reduced as the gas temperature is reduced along its flow path

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Combustion : What is Coal The molecule consists of network of aromatic clusters that are linked and cross linked by brides. Most bridges are aliphatic in nature, but may also include other atoms such as oxygen and sulfur. Bridges that contain oxygen as ethers are relatively weak in nature. A mobile phase also exists. The mobile phase consist of smaller molecular group that are not strongly bonded to the macromolecule. The percentage of aromatic carbon usually increases with coal rank. Other important elements, a small fraction, in coal are sulfur and nitrogen. They account for almost all the pollutants formed during coal combustion.

Steps of Coal Combustion Coal is ground to size of between about 5 and 400 m in diameter, and carried by the combustion air to the furnace burners. Combustion takes place at temperatures from 1300C to 1700C, depending largely on the rank of the coal. The steps are:

Drying Devolatalization Volatile Combustion Char Burning

The drying Process Evaporation of surface moisture and, subsequently, the loss of inherent moisture, starts at temp of 100-110C, complete dehydration at about 350C. Heat is driven from the furnace environment to the particle surface by radiation and convection. Heat transfer in the process is influenced by the furnace temperature, coal-particle size, particle moisture content and particle porosity

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Volatile Matter Volatile matter is an important parameter, providing a rough indication of the reactivity or combustibility of a coal, the ease of ignition, and hence the flame stability. However, while char reactivity is one of the main factors determining combustion efficiency, there is no standard test for its determination; Standard Methods do not take into account the actual firing conditions. Volatile matter contains both combustible and non- combustible (for example, carbon dioxide and water). So the calorific value of volatiles can be significantly different for coals with the same proximate volatile- matter yield.

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Devolatilisation Generally, devolatilisation starts at the particle surface and then proceeds toward the centre.

The devolatilisation initiation temperature varies with the type of fuel, and typically ranges from 450C to 500C for coal particles. Heating rate also has a significant influence. Generally, the weaker carboxyl, hydroxyl and aliphatic bonds break up at lower temperatures, while the stronger heterocyclic components decompose at higher temperatures .After the weak bonds break up, the functional groups decompose to release gases, mainly CO2, light aliphatic gases and some CH4 and H2O. Devolatilisation is affected by coal rank, macerals presents, Coal density, heating rate and the gas environment.

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Significance of Various Parameters in Proximate Analysis

Fixed carbon is the solid fuel left in the furnace after volatile matter is distilled off. It consists mostly of carbon but also contains some hydrogen, oxygen, sulphur and nitrogen not driven off with the gases. Fixed carbon gives a rough estimate of heating value of coal Volatile Matter Volatile matters are the methane, hydrocarbons, hydrogen and carbon monoxide, and incombustible gases like carbon dioxide and nitrogen found in coal. Thus the volatile matter is an index of the gaseous fuels present. Typical range of volatile matter is 20 to 35%.

Proportionately increases flame length, and helps in easier ignition of coal. Sets minimum limit on the furnace height and volume. Influences secondary air requirement and distribution aspects. Influences secondary oil support

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Significance of Various Parameters in Proximate Analysis (contd.)

Ash Content

Ash is an impurity that will not burn. Typical range is 5 to 40% Reduces handling and burning capacity. Increases handling costs. Affects combustion efficiency and boiler efficiency Causes clinkering and slagging

Moisture Content

Moisture in coal must be transported, handled and stored. Since it replaces combustible matter, it decreases the heat content per kg of coal. Typical range is 0.5 to 10% Increases heat loss, due to evaporation and superheating of vapour Helps, to a limit, in binding fines Aids radiation heat transfer

Sulphur Content

Typical range is 0.5 to 0.8% normally Affects clinkering and slagging tendencies Corrodes chimney and other equipment such as air heaters and economisers

PMI Revision 0Limits exit flue gas temperature 1Combustion

3 Ts of Combustion TIME All combustion requires sufficient Time which depends upon type of Reaction TEMPERATURE Temperature must be more than ignition temperature TURBULENCE Proper turbulence helps in bringing the fuel and air in intimate contact and gives them enough time to complete reaction. Air for combustion

Stoichiometric Combustion

The amount of air required for complete combustion of the fuel depends on the elemental constituents of the fuel that is Carbon, Hydrogen, and Sulphur etc. This amount of air is called stoichiometric air , Say a fuel has the following composition

Constituents Carbon Hydrogen Oxygen Nitrogen Sulphur H2O` Ash

% By weight 85.9 12 0.7 0.5 0.5 0.35 0.05

Calculation for Requirement of Theoretical Amount

Element Molecular of Air Weight kg / kg mole

C12 O2 32 C+O2 CO2 H2 2H2 +1/2O2 H2O S32 N2 28 S+O2 SO2 CO2 44 SO2 64 H2O 18

C+O2 CO2 12 +32 44

12 kg of carbon requires 32 kg of oxygen to form 44 kg of carbon dioxide therefore 1 kg of carbon requires 32/12 kg i.e 2.67 kg of oxygen

(85.9) C + (85.9 x 2.67) O2 315.25 CO2 229.07 kg of oxygen Calculation for Requirement of Theoretical Amount of Air (contd.) 2H2 +O2 2H2O 4+32 36

4 kg of hydrogen requires 32 kg of oxygen to form 36 kg of water, therefore 1 kg of hydrogen requires 32/4 kg i.e 8 kg of oxygen

(12) H2 + (12 x 8) O2 (12 x 9 ) H2O 96 kg of oxygen

Calculation for Requirement of Theoretical Amount of Air (contd.) S+O2 SO2 32 +32 64

32 kg of sulphur requires 32 kg of oxygen to form 64 kg of sulphur dioxide, therefore 1 kg of sulphur requires 32/32 kg i.e 1 kg of oxygen

(0.5) S + (0.5 x 1) O2 1.0 SO2 0.5 kg of oxygen

Calculation for Requirement of Theoretical Amount of Air (contd.) Total Oxygen required Oxygen already present in 100 kg fuel (given) Additional Oxygen Required Therefore quantity of dry air reqd. (air contains 23% oxygen by wt.) Theoretical Air required

= 229.07+96+0.5 = 325.57 kg = 0.7 kg = 325.57 - 0.7 = 324.87 kg = (324.87) / 0.23 = 1412.45 kg of air = (1412.45) / 100 = 14.12 kg of air/ kg of fuel

Optimizing Excess Air and Combustion

In practice, mixing is never perfect, a certain amount of excess air is needed to complete combustion and ensure that release of the entire heat contained in fuel oil. If too much air than what is required for completing combustion were allowed to enter, additional heat would be lost in heating the surplus air to the chimney temperature. This would result in increased stack losses. Less air would lead to the incomplete combustion and smoke. Hence, there is an optimum excess air level for each type of fuel. PMI Revision 01

PULVERIZING PROPERTIES OF COAL 58

GRINDABILlTY.-measures the ease of pulverization. It should not be confused with hardness of coal. It is affected by Moisture in the Coal.

MOISTURE: comprised of equilibrium moisture and surface or free moisture. Surface moisture adversely affects both pulverizer performance and the combustion process. The surface moisture produces agglomeration of the fines in the pulverizing zone, and reduces pulverizer drying capacity because of the inability to remove the fines efficiently and as quickly as they are produced. Sufficient hot air at adequate temperature is necessary in the Milling System

COAL-ASH SLAGGING AND DEPOSITION Parameters for Coal ash Behaviour

ash-fusibility temperatures

base/acid ratio iron/calcium ratio silica/alumina ratio iron/dolomite ratio dolomite percentage ferric percentage

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