Fuel Firing

Fuel Firing, Burner arrangement, Air and Flue gas System 1

Power Management Institute, NTPC Ltd

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TANGENTIALLY FIRED SYSTEMS 3  

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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 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 ignites


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Tangentially Firing System 4 

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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


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Tangentially-Fired Furnaces 5

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 overheating.  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.  Tangential-fring technique is characterized by lower carbon losses (do not exceed 1%), and greater adaptability for the combustion of "difficult" fuels (e.g. fuels with low calorific value, high melting-point ash, or low volatile content) Power Management Institute, NTPC Ltd

2/9/2012

VERTICALLY FIRED SYSTEMS 6 

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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 7


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Purpose of the Pulverizer 8

Reduce the coal to small particles by grinding for better combustion  Dry the coal  Classify the particle size of the coal leaving the mill  Transport the coal from the classifier to the exhauster 

Power Management Institute, NTPC

2/9/2012

TYPES OF COAL PULVERIZER 9 

Tube or ball mills: 

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Vertical spindle mill: 

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Low-speed machines that grind the coal with steel balls in a rotating horizontal cylinder. If the diameter of the cylinder is greater than the length of the cylinder, the mill is called a ball mill. If the length of the cylinder is greater than the diameter of the cylinder, it is called a tube mill.

Medium-speed machines that include bowl mills, ring roll mills, and ring and ball mills. The bowl mills are further divided into deep bowl or shallow bowl mills.

Impact mill: 

High-speed impact machines that use beater wheels to crush the coal


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Tube Mill 10 

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Horizontal cylindrical drum rotates at about 20 rpm There is a charge of balls from 50 to 20 mm in diameter Coal lumps are fed to the mill and a cascade of balls from the mill rotation breaks up the lumps Excellent for highly abrasive coal as the wearing component, the balls, can be added with the coal feed as they wear. Does not handle high moisture content coal well and capacity falls about 3% for every 1% increase in moisture content

Tube Mill 

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The oldest pulverizer design still in frequent use. 25% to 30% of cylinder volume is filled with wear resistant balls of 30 – 60mm. Specific power consumption 22 kWh per Ton. Highly reliable in requires low maintenance. Bulky and heavy in construction.

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Coal Pulverizer System 12


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Fans in Power Station 13


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Power Plant Schematic

Need of Fans

Combustion Air

Boiler

Flue Gases

Air needed for combustion

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Flue are needed to be evacuated

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Losses due to flow need to be overcome

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Fan Operation 

Fans cause pressure increase by: 

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Centrifugal force created by rotation of the column of air trapped between two blades Kinetic energy is supplied to the air through the impeller Total pressure = velocity head + static pressure

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Selection Considerations Quantity of air to be moved per unit time  Estimated system resistance and expected variations  Amount of noise permitted  Space available for fan  Economic implications 

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SYSTEM RESISTANCE When a gas is forced through a duct system, a loss in pressure occurs. This loss in pressure is called system resistance.  System resistance is composed of two components:  Friction losses and  Dynamic losses. 

Variation of System Resistance with Flow

Draft Losses

Total losses

p

Furnace, SH & RH Losses Economizer Losses Ducts & dampers losses Percent Boiler Rating

Fan Classification 

Designated as dynamic pumps 

Centrifugal 

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Further classification by entry and exit of fluid/gas through impeller

4 aerodynamic classifications: Axial flow  Radial flow : referred to as “centrifugal fan”  Cross flow  Mixed flow 

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Centrifugal Fans

Components of Centrifugal Fan

Centrifugal Fan Operation 

Fans cause a pressure increase through two methods – Centrifugal force is created by the rotation of the column of air trapped between two blades. – Kinetic energy is supplied to the air through the impeller – Total pressure = velocity head + static pressure

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Blades are airfoil-type, backward-curved, forwardcurved, or radial (straight) – Airfoil-types are complex and expensive but very efficient;

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they’re used for large systems where the cost is justified.

Wheel Blade Types and Rotation

Applications where the fans will handle clean air or clean gas, the highly efficient backward inclined airfoils are the preferred design. In an application where the fan is subject to erosion due to heavy dust loading, a straight radial type fan provides erosion resistance, but at the expense of efficiency.

Backward-Curved Fan Blades 

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Have a self-limiting power characteristic, so if sized correctly the motor won’t overheat or burn out even if conditions change. High efficiency and stable operation make this blade type popular. Choose the operating point to be just to the right of the peak pressure flow rate to achieve both high efficiency and a stable flow rate. This type of fan operates stably because the pressure difference provided by the fan drops if the flow rate goes up. If the opposite were true, increased an increased flow rate would cause increase fan power, which is unstable.

Section of a Centrifugal Fan

Radial Blades 

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Similar performance to a backwardcurved except that it’s easier to overheat because as flow rate goes up, so does power. Easier to maintain, so it’s used in dirty situations (easy to clean straight blades, and they don’t collect as much) Blades are stronger than other types. Used primarily in industrial systems in a corrosive or erosive environment, such as material handling of airborne particulate or where high static pressure is required.

Airfoil-blade centrifugal fan

Forward-Curved Blades Have problems with instability because a specified pressure rise can fit three different flow rates.  Burnout can also be a problem because fan power increases with flow rate.  Quieter than other fans; used for most furnace blowers  Usually limited to clean service applications 

Axial Flow Fans 

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Common types: propeller, tubeaxial, vaneaxial Tubeaxial: impeller is inside a tube to guide airflow and improve performance Vaneaxial: like a tubeaxial except vanes either up or downstream of the impeller are used to reduce swirl and improve performance Used to deliver large flow rates but small increase in pressure Examples include fans used for ventilation without ductwork, mobile room fans, and fans used to cool computers

Two-Stage Axial Fan Assembly

Axial-flow fan for induced-draft service

System Pressure Effects 

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Fan curves are typically given in terms of total pressure vs. volumetric flow rate A typical fan running at a fixed speed can provide a greater volumetric flow rate for systems with smaller total pressure drops (if we’re to the right of the peak in the fan curve). Total pressure loss=static pressure loss+dynamic pressure loss

If exit and inlet area of a duct are about the same, the dynamic pressure loss (or gain) may be minimal.

Effect of Blade Type on Erosion Resistance and Efficiency

Fan Laws 

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Law 1 – relates to effect of changing size, speed, or density on volume flow, pressure, and power level Law 2 – relates to effect of changing size, pressure, or density on volume flow rate, speed, and power Law 3 – shows effect of changing size, volume flow, or density on speed, pressure, and power The laws only apply to aerodynamically similar fans at the same point of rating on the performance curve.

Scheme of Air and Gas Path

9 February 2012

PMI Revision 00

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Draught System Pressure Variation 

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FD Fan

Pressure drop calculation in air & gas path and its comparison with design value. Assessment of ID and FD fan power as a function of furnace pressure. Assessment of effective kinetic rate coefficient as a function of furnace pressure.

Duct APH

Duct Furnace Back pass

Duct APH

c u D t

ESP

c ID u D tFan

Chimney

Fuel Firing

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