U.S. patent application number 12/745965 was filed with the patent office on 2010-12-23 for system and method for full combustion optimization for pulverized coal-fired steam boilers.
This patent application is currently assigned to ABB Technology AG. Invention is credited to Harry Dohalick, Pekka Immonen, Theodore Matsko, Richard Vesel.
Application Number | 20100319592 12/745965 |
Document ID | / |
Family ID | 40445210 |
Filed Date | 2010-12-23 |
United States Patent
Application |
20100319592 |
Kind Code |
A1 |
Dohalick; Harry ; et
al. |
December 23, 2010 |
System and Method for Full Combustion Optimization For Pulverized
Coal-Fired Steam Boilers
Abstract
A method and system for controlling a pulverized coal fired
boiler wherein the flow of a coal/air mixture flowing to each
burner is monitored and transmitted to a distributed control
system. The distributed control system also monitors and controls
the position of dampers in a splitter that feeds the coal/air
mixture to the burners. The dampers are controlled in a closed loop
fashion to achieve a optimal boiler performance.
Inventors: |
Dohalick; Harry; (Concord,
OH) ; Immonen; Pekka; (Shaker Heights, OH) ;
Vesel; Richard; (Hudson, OH) ; Matsko; Theodore;
(Chesterland, OH) |
Correspondence
Address: |
ABB INC.;LEGAL DEPARTMENT-4U6
29801 EUCLID AVENUE
WICKLIFFE
OH
44092
US
|
Assignee: |
ABB Technology AG
Surich
CH
|
Family ID: |
40445210 |
Appl. No.: |
12/745965 |
Filed: |
December 5, 2008 |
PCT Filed: |
December 5, 2008 |
PCT NO: |
PCT/US2008/085671 |
371 Date: |
July 23, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61012089 |
Dec 7, 2007 |
|
|
|
Current U.S.
Class: |
110/186 |
Current CPC
Class: |
F23K 3/02 20130101; F23K
2203/006 20130101; F23N 2223/40 20200101; F23N 1/022 20130101; F23N
2239/02 20200101; F23N 2223/08 20200101 |
Class at
Publication: |
110/186 |
International
Class: |
F23N 5/18 20060101
F23N005/18 |
Claims
1. A system for controlling a pulverized coal-fired boiler having
at least one pulverizer for pulverizing coal and forming an air and
coal mixture, a plurality of burners, each said burner fed said air
and coal mixture by a burner line, said system comprising; a
combustion optimization system having a combustion model of the
pulverized coal-fired boiler; a distributed control system in
communication with said combustion optimization system and
receiving control commands from said combustion optimization
system; a coal flow sensor positioned to monitor the velocity of
said air and coal mixture fed into each said burner; an air flow
homogenizer positioned downstream of said pulverizer, said air flow
homogenizer having a splitter for separating said air and coal
mixture into said burner lines, said splitter having a plurality of
dampers to control the flow of said air and coal mixture flowing to
said burners; and wherein said distributed control system controls
the position of said dampers in a closed loop fashion using a
signal indicative of the present position of said dampers in
combination with signals from the coal flow sensors.
2. The system according to claim 1 further comprising one or more
positioners that transmit a signal indicative of the position of
each said damper to said distributed control system.
3. The system according to claim 2 wherein said one or more
positioners move a associated one of said dampers to a setpoint
transmitted from said distributed control system.
4. The system according to claim 1 wherein said combustion
optimization system further including a cost function, said cost
function including weighted customer optimization targets.
5. The system according to claim 4 wherein said combustion
optimization system utilizes said combustion model and a cost
function to provide setpoint and setpoint bias values to the
distributed control system.
6. The system according to claim 1 further comprising flame
detectors positioned proximate to said burners to detect the
presence or absence of flame and also measure the quality of the
flame to said distributed control system.
7. The system according to claim 1 wherein said coal flow sensor
further monitors coal concentration and temperature of said air and
coal mixture flowing into each burner.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a national stage entry of
PCT/US08/85671, which claims the priority to U.S. provisional
patent application Ser. No. 61/012,089 filed on Jul. 12, 2007
entitled " A System and Method for Full Combustion Optimization for
Pulverized Coal-Fired Steam Boilers," the contents of which are
relied upon and incorporated herein by reference in their
entirety.
DESCRIPTION OF THE PRIOR ART
[0002] There are a number of advanced control techniques for the
optimization of combustion within a pulverized coal-fired boiler
(PCFB). These methods typically involve the use of advanced
model-predictive control, and/or neural net-based controls, to
monitor, balance and control the admittance of fuel and air to
various stages of the boiler, including primary, secondary,
overfire, and underfire air controls. Other variables, such as
burner tilts and attemperator spray flows may be controlled as
well, in order to optimize the combustion process. As is well
known, attemperators reduce the steam temperature by bringing
superheated steam into direct contact with water. The steam is
cooled through the evaporation of the water.
[0003] The combustion process should be controlled and optimized to
obtain the "best possible" performance thereby meeting in an
economically and/or environmentally optimized fashion the competing
goals of NOx reduction, CO and unburned fuel reduction, and heat
rate improvement. However, this optimization is in large part
limited by physical process parameters that the system used to
optimize combustion often does not have the ability to control.
[0004] One example of a system 1 using such prior art control
techniques without coal flow management is shown in the air and
fuel flow diagram of FIG. 1. As is well known in the art, the
system 1 of FIG. 1 usually includes a distributed control system
(DCS) to control the process such as the DCS 14 shown in FIG. 2 and
may also include a combustion control and optimization system (COS)
such as the COS 12 shown in that figure.
[0005] As shown in FIG. 1, ambient air enters the system 1 on the
left hand side of the diagram. Most of this air becomes primary air
whose main function is to carry the pulverized fuel out of the one
or more coal pulverizers 2.
[0006] The air and pulverized fuel must be in a stoichiometric
ratio at the burners 4 and that mix is obtained by adding secondary
ambient air as shown on the right hand side of the diagram.
[0007] FIG. 1 also shows several dampers 6a, 6b, 6c and 6d that are
associated with the flow of air. Damper 6a, known as the hot air
damper, is associated with the flow of heated ambient air that is
in the primary air duct 3. Damper 6b, known as the cold air damper,
is associated with unheated ambient air in the tempering air duct 5
to temper the hot primary air. Damper 6c, known as the primary air
damper, provides the mixture of the primary air and the tempering
air to the pulverizers 2 and the burner lines 7 associated with the
pulverizers and the burners 4 and provides the tempered hot primary
air to the burners 4. Damper, 6d, known as the control damper,
provides secondary heated air in the secondary air duct 8 to the
burners 4. As is well known to those in this art, the major
adjustment to these dampers 6a, 6b, 6c and 6d are load related and
the signals to make that adjustment come from a distributed control
system such as DCS 14 of FIG. 2.
[0008] One element which is missing from the prior art is the
ability to provide a closed-loop controllable flow of a homogeneous
and balanced air-fuel mixture to the burner systems of the PCFB.
Past technologies and implementations have used methods and
apparatus such as riffle boxes to homogenize the air-fuel mixture.
Riffle boxes have been associated with high pressure drops which
may lead to rapid-wear. Manual set-and-forget balancing techniques
have also been used, which are configured at one load condition for
the PCFB, typically with fixed orifices to balance the admittance
of primary combustion air with the stream of fuel from the
pulverizer 2.
[0009] The present invention provides an improved combustion
optimization system that is designed to monitor, modify and control
the combustion process, including the load-varying air-fuel mixing
and homogenization processes.
SUMMARY OF THE INVENTION
[0010] According to one aspect of the present invention, a system
is provided for controlling a pulverized coal-fired boiler having
at least one pulverizer for pulverizing coal and forming an air and
coal mixture, a plurality of burners, each said burner fed said air
and coal mixture by a burner line. The system includes a combustion
optimization system having a combustion model of the pulverized
coal-fired boiler. A distributed control system is in communication
with the combustion optimization system and receives control
commands from the combustion optimization system. A coal flow
sensor is positioned to monitor the velocity of the air and coal
mixture fed into each burner. An air flow homogenizer is positioned
downstream of the pulverizer and includes a splitter for separating
the air and coal mixture into the burner lines. The splitter has a
plurality of dampers to control the flow of the air and coal
mixture flowing to the burners. The distributed control system
controls the position of the dampers in a closed loop fashion using
a signal indicative of the present position of the dampers in
combination with signals from the coal flow sensors.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a partially schematic view of a prior art
pulverized coal-fired boiler;
[0012] FIG. 2 is a schematic view of a COS and DCS control system
for a pulverized coal-fired boiler;
[0013] FIG. 3 is a coal flow monitoring sensor;
[0014] FIG. 4 is a partially schematic view of a pulverized
coal-fired boiler control system according to the present
invention; and
[0015] FIG. 5 is a process flow chart for the pulverized coal-fired
boiler control system.
DETAILED DESCRIPTION
[0016] Referring now to FIG. 2, there is shown a block diagram for
one embodiment of the system 10 of the present invention. System 10
includes an advanced Combustion Control and Optimization System
(COS) 12. COS 12, models the multivariable nonlinear relationships
of the combustion process. The relationships between
signals/parameters are identified by analyzing their historical
data. COS 12 is based on advanced model predictive control
techniques and uses the combustion model and a cost function that
describes the weighted customer optimization targets to provide
setpoint and setpoint bias values 18 to the distributed control
system (DCS) 14 of system 10. DCS 14 includes operator setpoints
and provides process values 20 to COS 12. COS 12 has a model of the
process and has as inputs the constraint variable limits 22, the
controlled variable targets 24 and the manipulated variable targets
and limits 26. One example of COS 12 is the Optimax Combustion
Optimizer System, available from ABB.
[0017] The DCS 14 is connected to the boiler and final control
elements 16 of system 10. The DCS 14 provides the multiple boiler
control values 28, the air damper position 30 and the coal/air gate
position 32 to the boiler and final control elements 16.
[0018] The boiler process, with instrumentation and final control
elements 16, also includes various instruments that provide the
process values 34 to the DCS 14. In turn, the DCS 14 controls the
process by sending control signals to the final control elements.
The instruments may for example include flame detectors such as
those that detect the presence or absence of flame and also measure
the quality of the flame. This flame quality measurement can be
used to ensure that the combustion process is operating
efficiently. One example of such a flame detector is the Uvisor.TM.
SF810i system available from ABB that provides in a single housing
both flame detection and a measurement of the quality of the flame.
Associated with the flame detector is a suitable solution for
monitoring the quality of the flame such as the Flame Explorer
which is also available from ABB.
[0019] The instruments may also include a system that has sensors
to measure the velocity of the pulverized coal feeds into the
boiler, the concentration of coal therein and optionally
temperature. This system uses the input from the sensors to provide
closed loop combustion optimization of boilers fired with
pulverized coal. One example of such a system is the PfMaster
system available from ABB that with one signal processing unit can
measure up to 24 pulverized fuel (pf) burner feeds. One example of
such a sensor is the ABB coal flow monitoring sensor shown in FIG.
3.
[0020] An air and fuel flow diagram for system 10 is shown in FIG.
4. As shown therein, system 10 includes everything shown in FIG. 1
and also has the following elements that are not in the prior art
diagram of FIG. 1: (a) An air-fuel flow homogenizer 40 that has a
fuel flow splitter with dampers (identified in FIG. 4 as
control-gate dampers 42) in the burner lines 7 from the pulverizer
2 to control the flow of the homogenized air-fuel mixture of
pulverized coal to two or more of the burners 4 of the boiler. (b)
A flame scanner 46 with a combustion index which may for example be
the flame scanner described above. (c) Coal flow sensors 48 which
monitor each of the burner lines.
[0021] One example of the sensors 48 and associated coal flow
monitoring system is the Pf Master system described above. Sensors
48 may measure velocity, coal concentration and temperature with a
single sensor.
[0022] As with the prior art air and fuel flow diagram of FIG. 1,
the air dampers 6a, 6b, 6c and 6d shown in FIG. 4 are controlled by
the DCS 14. In the prior art, the dampers of the splitter 42 are
manually configured at one load condition. In accordance with the
present invention, the position setting of the dampers of the
splitter 42 are controlled by the DCS 14. DCS 14 provides closed
loop control of the dampers for splitters 42 by using a signal
indicative of their present position in combination with signals
from the coal flow monitoring system. Positioner and actuator
devices such as those available from ABB provide the signal
indicative of the damper position and to move the associated damper
to the setpoint from DCS 14.
[0023] The controlled diversion of the homogenized air-fuel mixture
results in a balanced delivery of air and fuel to individual
burners 4 within the burner array with appropriate stoichiometric
ratios. Additionally, the COS 12 can modify the overall air-fuel
delivery profile to the burner array such that the best burner
input flows amongst the burners 4 in the array may be achieved for
a given load.
[0024] One example of an air-fuel flow homogenizer 40 is the
variable area rope breaker system PF diffusing system available
from Greenbank Terotech Ltd. One example of a fuel flow splitter 42
with dampers is the coal flow control gate splitter also available
from Greenbank. As is described above, the coal flow control gate
dampers in splitter 42 are controlled by COS 12 of system 10
through the DCS 14.
[0025] As can be appreciated the conversion of the fuel flow
splitter 42 to closed-loop controlled operation, provides for the
initial balancing of the air-fuel mixture to the burners 4 fed by
its piping. This achieves the capability to dynamically balance the
air-fuel flow to individual burners of the PCFB under varying load
conditions. These varying load conditions affect the incoming
two-phase distribution of air and fuel and create the need for a
dynamic response over the desired load range.
[0026] As can be further appreciated, the coupling of the local
closed-loop controls of the fuel flow splitter 42, to the COS 12
creates the following additional benefits which are beyond what any
one of the separate elements can provide alone: (a) Complete
monitoring and control of the combustion process, from the initial
mixing of fuel with air in a homogenized and ratio-balanced
fashion, through the required distribution to various burners
within the PCFB, and finally the controlled ignition and optimized
combustion of the air-fuel stream within the confines of the boiler
interior. (b) The ability to dynamically create, monitor and
control relative air-fuel flows between the multiple-burners of a
PCFB, such that load-induced effects from the pulverization,
air-induction, and flame creation processes can be manipulated and
optimized to obtain true "best possible" performance, such that the
competing goals of NOx reduction, CO and unburned fuel reduction,
and heat rate improvement, are met in an economically and/or
environmentally optimized fashion. (c) The capabilities as
described above can be achieved in an automated fashion, where the
operators of the PCFB have a substantially reduced need to manually
balance and control the multitude of individual air and fuel flows
of the typical pulverized coal fired boiler combustion process.
[0027] A flow chart of system 10 is shown in FIG. 5. As shown
therein, the COS 12 provides, in response to the external load
demand and process values, states and control modes from DCS 14
both real-time optimization and advanced process control to DCS 14.
DCS 14 controls the actuators that are used to position the dampers
shown in FIG. 4 and sensors provide process related values such as
coal flow and flame detection and quality.
[0028] As can be also appreciated, the monitoring of flame status
and quality insures that individual burners are performing as
expected, with the MPC model from COS 12 tracking the correlation
of combustion index with individual burner load and
performance.
[0029] As can be appreciated from the above description, the
present invention provides over the prior art, substantially
improved combustion efficiencies and unit heat rate, and the
reduction and control of emissions to acceptable levels. Additional
benefits may include the mitigation of costly fan-limited
operation, due to the overall lowering of resistance in the
air-fuel paths between pulverizers and burners.
[0030] The advantages provided by the system of the present
invention include, reductions in LOI (Loss on ignition--i.e.
unburned fuel and wastage), reduced or eliminated use of auxiliary
(co-firing) fuels during low loads, reduced waterwall wastage due
to CO rich "dark zones", and reduced emissions (CO2, CO and NOx).
Further PCFB operational improvements which can result from the use
of the present invention include, improved unit heat rate (thermal
efficiency), improved unit ramp rate, improved flame and fireball
stability over a much wider load range, elimination of some/all
riffle boxes for fuel distribution, with improved draft fan
efficiency results, and controllable variations in the air/fuel
ratio to adapt to boiler load conditions.
[0031] It should be appreciated that while the embodiment for the
system of the present invention shown in FIG. 2 and its associated
air and fuel flow diagram shown in FIG. 4 can as described above
include a flame scanner with a combustion index, the system of the
present invention will provide an improvement over the systems of
the prior art even if the flame scanner used in the system does not
have a combustion index.
* * * * *