Secondary Air Damper Control in Boiler
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Published: Thu, 06 Jul 2017
In this proposed work, design and development of controlling Secondary air dampers in a draft system of a power plant which is presently working on DCS technique has been accomplished by using high computing software ARDUINO and results has been shown with suitable simulation in VISUAL BASICS. The goal of this work is to control the air dampers in a different way according to the unit load thereby increasing the boiler efficiency and to reduce the pollution. A set of six interlock conditions were provided for this purpose. Since DCS is applicable only for big system not less than 5000 input and output but this is costly. It consists of separate server, processor and computers where as ARDUINO does not require a separate processor, no workstation, no operator station here directly connect interfacing card with computer itself.
Keywords- Secondary air dampers, Draft system, ARDUINO, Power plant (TTPS), Software control.
In power plant boilers, the draft is obtained by means of fans namely Forced draft and Induced draft fan. The FD and ID fans develop enough draft to move the tremendous volume of air and gases through the flue gas ducts and stack to atmosphere maintaining a balanced draft in the furnace. The secondary air for combustion primarily enters into the windbox. The wind box assembly is a tall structure divided in its height into thirteen compartments which houses 6 no. of Coal nozzles and 5 auxiliary air nozzles alternatively at different elevations from top to bottom. Louvre dampers named as secondary air dampers each of which is operated by an individual damper drive cylinder located at the side of the wind box. The coal nozzle elevations are designated as A,B,C,D,E,F elevation from bottom to top, the bottom end air and top end air elevations as AA and FF respectively. The auxiliary air nozzles in between coal elevations are designated as elevations AB,BC,CD,DE,EF. The furnace corners are designated as corner 1, 2, 3 and 4 in clockwise direction. The heat released in the furnace is received by the
Water/steam/air to the maximum extent to have boiler efficiency of the order of 86%.
II. OPERATIONAL PROCEDURE OF SECONDARY AIR
The operation of the auxiliary and fuel air dampers in the windbox assemblies are affected by the FSSS.
A. Auxiliary air dampers:
During the furnace purge period and initial operations up to 30% boiler loading all elevations of auxiliary and end air dampers modulate to maintain a predetermined (approx. 40 mmWC) set point differential pressure between windbox to furnace. 30 to 40% of total air flow to have an air rich furnace will be supplied during the above period to avoid unhealthy furnace conditions.
When the unit load exceeds 30% MCR, the differential pressure set point is changed and ramps to a higher setting (approx. 100 mmWC). Simultaneously, the auxillary air dampers associated with coal or oil elevations not in service close in timed sequence starting with the upper elevation of dampers and progressing to the lowest elevation.
When the unit loading is reduced below 30% loading, the auxillary air dampers open in a timed sequence starting with the lowest elevation of dampers. Simultaneously, the differential set point changes to its lowest setting.
All the auxillary air dampers maintain the status quotient upon a boiler trip and will open fully when both the FD fans are off. The bottom end air damper is normally kept open to a fixed predetermined position to reduce unburnt coal dust fall out.
B. Fuel (oil) air dampers:
Elevation AB, CD and EF dampers modulate according to the oil header pressure when heavy oil is being fired. Otherwise they open and close with the rest of the aux air dampers. Elevation AB dampers are kept at a fixed opening when light oil is being fired.
C. Fuel (coal) air dampers:
Their operations are independent of Boiler load. All coal
air dampers are normally closed. They open fifty seconds after the associated feeder is started, and modulate as a function of feeder speed. Fifty seconds after the coal feeders are removed from service, the associated fuel air dampers close. The fuel (coal) air dampers open fully, when both FD fans are off or when the boiler trips on master fuel trip.
Importance of secondary air:
- Protect Boiler Equipments
- Control the Pollution
- Improve the Efficiency of the Boiler
This project is to automatically control the air inlets that are termed as dampers which are operated using pneumatic valves. The pneumatic valves are controlled by sensors depending on the windbox to furnace differential pressure. Thus this project controls dampers based on arduino coding
And its visually displayed in visual basics. This forms the basic methodology for this project.
A. Hardware Description:
The hardware components consist of 4potentiometers of range 10kâ„¦. These are assumed to be the coal mills/diesel input i.e. inputs for coal. Secondary air dampers are present near all coal and diesel elevations.
The percentage opening of a damper depends on the amount of coal/diesel input. Therefore variation of potentiometers indicate amount of coal/diesel input. The damper opening depends on the variable point of the potentiometers.
4 potentiometers are so connected for explanation and they are connected to the A/D convertor pins of AT MEGA 8.0 controller.
Then the freeduino board in which the AT MEGA 8.0 is fixed is connected to PC and arduino coding is uploaded in the controller. Then VB shows graphical representation of percentage of damper opening and all other factors.
B. Software Description:
The software coding was developed based on several conditions on which the secondary air damper openings depend. These form the base for developing a program. These conditions are based on some fixed values and formulas. As already mentioned the percentages of secondary air damper opening depend on the amount of fuel input to the furnace. The algorithm for visual basics coding are as below.
1) Operation Of Coal Elevation Dampers: (A, B, C, D, E, F)
The coal dampers at elevations A, B, C, D, E and F will be fed with coal depending on the load of the power plant. Feeding (30 tonnes per hour) any three of these coal elevations is sufficient enough to supply the predetermined full load (210 MW) of the unit. All dampers always have a minimum opening of 30%. In general,less than 20% of the secondary air to the fireball will maintain substoichiometric combustion which limits the flame temperature of the fireball and provides the curtain of secondary air over the walls. The curtain of secondary air militates against the formation of slag on the furnace walls. All this proportioning of the air requires tools of adjustment available to the furnace operator.
The dampers modulate according to coal input in three different ways as:
When coal is fed into a particular(own) elevation ,the corresponding air damper starts modulating based on the fuel input to let the secondary air required for the combustion of given input.
Rather when coal is fed into an (auxillary) elevation adjacent to a particular damper, the damper modulates to maintain the windbox to furnace differential pressure, thereby controlling the constant negative pressure essential to create swirling fireballs of combustion in tangentially fired boilers.
When the condition of tripping the whole unit arises, all the six coal elevation dampers are opened to100 % (maximum).
When there is no fuel input in any of the above elevations then the dampers will be in minimum opening position. (0%)
2) Operation of oil elevation dampers
(AB, BC, CD, DE, EF):
Oil is used as fuel input during startup and during scarcity of coal. Each pair of coal nozzle elevations is served by one elevation of oil burners (4 nos. /elevation) located in the auxiliary air nozzle. Thus 12 oil guns, non retractable type, are accommodated in the three elevations AB, CD and EF. Heavy fuel oil can be fired at the oil guns of all these three elevations, whereas light fuel oil (High speed diesel oil) can also be fired at AB elevation. Each oil gun is associated with a retractable high energy arc (HEA) igniter, arranged at the side of the oil gun in the auxiliary air compartment and they directly light up the oil guns.
The dampers modulate according to oil input in three different ways as:
When oil is fed into a particular(own) elevation ,the corresponding air damper starts modulating based on the fuel input to let the secondary air required for the combustion of given input.
When steam input is higher than 30% with adjacent elevation opened then windbox to furnace differential pressure is maintained.
Rather when oil is fed into an (auxillary) elevation adjacent to a particular damper, the damper modulates to maintain the windbox to furnace differential pressure, thereby controlling the constant negative pressure essential to create swirling fireballs of combustion in tangentially fired boilers.
When the condition of tripping the whole unit arises, all the oil elevation dampers are opened to 100 % (maximum).
When there is no fuel input in any of the above elevations then the dampers will be in minimum opening position. (0%)
3) Operation of AA, FF elevation dampers:
The AA elevation of air damper is always fully (100%) opened and the FF elevation is always closed to enhance combustion.
Thus the coding for the project is developed based on the conditions of the coal and oil inputs.
IV. SEQUENCE OF OPERATION
The opening of dampers depends on the load and coal input. The above table I illustrate the percentage of coal input in each mill for given percentage of load. Here 30 Tonnes/Hr of coal is the 100% input and 10 Kg/cm2 of oil is the 100% input of oil.
When unit load is 50%, then coal mills A, B are in full operation and C mill operates 10% with AB elevation having oil.
For a load of 60%, coal mills A, B, C will operate till 30% and mill D has 10% input. AB oil elevation is fed with full percentage of oil.
If the load is 70%, then there is no need for oil but coal mills A, B, C, and D are fed with maximum input.
For 90% ,there is no need for oil but the coal mills
A, B, C, D is fed with maximum input and E with 20% of fuel input.
For 100% load, all the coal elevations are fed with maximum input.
V. SIMULATION RESULTS
Visual Basic is a third-generation event-driven programming language and integrated development environment (IDE) from Microsoft for its COM programming model first released in 1991. Visual Basic was derived from BASIC and enables the rapid application development (RAD) of graphical user interface (GUI) applications, access to databases using Data Access Objects, Remote Data Objects, or ActiveX Data Objects, and creation of ActiveX controls and objects. VBA is effectively the same language as Visual Basic but VBA-code is tied to Microsoft Office and can only run within that environment. A programmer can put together an application using the components provided with Visual Basic itself.
The four fuel inputs are received via the potentiometer to the AT MEGA 8 microcontroller. The variation of potentiometers indicate amount of coal/diesel input. The damper opening depends on the variable point of the potentiometers.
The controller board is connected to PC and arduino coding is uploaded in the controller. Then VB is used for graphical representation of damper opening.
The simulation results showing the air dampers status variation according to the various input set points is as follows:
The amount of coal input given is shown in the above fig 5. This show the percentage of input given, timer, coal mill that is active, amount of load, steam input set point and steam input in percentage.
This form shows all the elevation of dampers and denotes the damper that is open and the percentage of damper opening is also shown. In addition to that reactive power produced for corresponding load is also calculated.
Based on the graphical programming language and the data stream operating mode, the software instead of apparatus, the use of virtual instrument technology has opened up our new field of control. Compared to the traditional control it has obvious advantages. Programming is simple by the graphical programming language. A form of visual basics personalizes button design and layout, with the virtual knobs and switches to struck the friendly human, intuitive control interface makes vivid, so that the operators are involved in the operation easier to control the process and results at a glance. The main advantage of controlling secondary air dampers by using arduino is lesser cost as compared to DCS technique, since the interfacing of inputs and outputs are directly given to the PC (which works as work station i.e. block panel and operator station i.e. front panel) .This paper was made in a Coal Based Thermal power plant at tuticorin, TTPS as the existing is running on DCS system.
DCS – Distributed Control System
FSSS – Furnace Safeguard Supervision System
FD fan – Forced Draft Fan
ID fan – Induced Draft Fan
ADC – Analog to Digital converter
PC – Personal Computer
VB – Visual Basic 6.0
Draft – difference of pressure causing flow of air
Elevation -position of dampers
Windbox -secondary air storage compartment between FD fan and furnace.
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