Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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- 1 - Case 4574
SAFETY SYSTEM FOR COAL PULVERIZERS
TECHNICAL FIELD
The resent invention generally relates to control
systems for coal pulverizers and particularly Jo safety
control systems for detecting and controlling hazardous
conditions in a coal pulverizer.
BACKGROUND APT
Known pulverized-coal systems pulverize coal,
deliver it to the fuel-burning equipment, and accomplish
complete combustion in the furnace with a minimum of
excess air. The system operates as a continuous process
and, within specified design limitations, the coal
supply or feed can be varied as rapidly and as widely as
required by the combustion process.
A small portion of the air required for combustion
~15 to 20% in current installations is used to transport
the coal to the burner. This is lcnown as primary air.
In the direct-firing system, primary air is also use to
dry the coal in the pulverizer. The remainder of the
combustion air (80 to 85~/~) is introduced at the burner
and is known as secondary air.
All coals, when exposed to air, undergo oxidation
even at room temperature. This tendency varies with coal
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type: anthracite and semi-anthracite, for example, are
little affected whereas many bituminous coals are part-
ocularly liable to absorb and combine with oxygen. The
process of oxidation continues with increasing rapidity
as the temperature ruses. Heat is generated which, if
allowed to accumulate, could result in thermal decom~o-
session and ignition of toe coal. Volatile components of
the coal, such as methane and related compounds, are no-
leased during the decomposition. Accumulation of these gaseous materials may be ignited at fairly low tempera-
lures and rapidly propagate fire or explosion.
Spontaneous combustion of coal is dependent on a
sufficient supply of oxygen to maintain the reaction and
on the surface area exposed. Coals with a Leigh surface
area, due to small particle size, as in pulverized coal
fuel, are particularly liable to self heating. This
problem is of special significance to the safe operation
and performance of industrial coal pulverizers. Spinet-
nexus combustion may result in deterioration in the
quality of the coal, in damage to the power plant, and
in certain cases, for example, where critical concentra-
lions of coal dust are involved, may provide the ignition
source for an explosion.
Present systems for fire detection in industrial
coal pulverizers use either thermocouples to measure the
rise in outlet temperature of the pulverizing mill or
infrared gas analyzers to detect the buildup of C0 pro-
duped in tile mill.
Thermocouples or Rods are normally part of the
control system for mill operation. However, they are a
relatively insensitive means for detecting pulverizer
fires. At best, they warn of impending trouble only a
few minutes before it actually occurs, and in some cases,
do not even detect a significant temperature rise before
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a fire or explosion is evident. The ineffectiveness of
thermocouples and Rods in this application is due, in
part to the shielding used to protect them from the
corrosive coal particles. Shields reduce heat conduct
lion, slowing response time.
Actual CO measurements are also used for fire de-
section in coal pulverizers since that CO buildup is no-
fated directly to the oxidation rate of coal. Infrared
gas analyzers are used to compare the CO content of the
oncoming and outgoing mill air and in effect, the amo~mt
of CO produced in the mill. Currently available infer-
red gas analyzers require extensive filtering and dewy-
duration of the gas sample extracted from the mill, to
prevent interference by water vapor and particulate
matter. Due to the high cost and maintenance requirements
of infrared absorption analyzers, it is the usual practice
to use one analyzer for several measurement points.
Continuous measurement of each mill is not provided, thus,
slowing response time. Nevertheless, this provides an
improvement over the thermocouple and ROD method described.
Additional problems occur, at some power plants, where
appreciable concentrations of CO can be found in the air
supply to the mill. Since in such plants CO in the
boiler flue gases is transferred to the combustion air
via the regenerative air heater and it thus becomes
necessary to provide an analysis of the air entering the
mill.
Thus, it is seen that an accurate and reliable
safety system was required for coal pulverizers which
would provide an early warning of impending safety probe
lets in coal pulverizers.
SAGER OF THE I~VENTI ON
The invention described herein overcomes the
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stated problems of prior art safety systems and pro-
vises an improvement over the existing art. It is not
dependent on the measurement of mill outlet temperature,
toe removal of moisture and all particulate mutter from
the sample extracted from the mill or multi-point same
poling. 'Ire invention incorporates the use of a standard
single point oxygen and CO analyzer directly mounted to
the coal pulverizing mill providing a continuous percent
by volume measurement ox oxygen content and a continuous
measurement of C0 gas concentration of the mill atoms-
phone. The 2 portion of the analyzer uses a sensor
operating at a temperature where any combustible vote-
tile material will combine with I in the sample. The
sensor will then respond to the free or uncombined 2
remaining. The resulting measurement, denoted net or
residual, 2~ can be correlated with the amount of come
bustible volatile within the mill. An additional sign
nificant indicator of a potentially hazardous condition
is, thus, provided, augmenting the C0 measurement. The
combined measurement of C0 and net 2 concentration in
the mill atmosphere is used to indicate and alarm both
the onset and progress of spontaneous combustion within
the mill.
Thus, one aspect of the present invention is to
provide an automated system capable of being integrated
into a plant's pulverizer management and combustion
control system designed to monitor the performance of
and detect impending fires and explosions in industrial
coal pulverizers and alarm such conditions.
Another aspect of the present inventive is to
provide an automated alarm system based on a net oxygen
measurement in the coal pulverizer.
Yet another aspect of the present invention is to
provide an automated alarm system based on a predator-
mined carbon monoxide rise per tire.
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Still yet another aspect of the present invention is
to provide an automated inverting control of the coal pulper-
sizer upon detection of either a predetermined net oxygen
level or an absolute carbon monoxide level.
The invention consists of a safety system for a goat
pulverizer comprising: means for measuring the actual net
oxygen level in the coal pulverizer and establishing a
signal indicative thereof; means for comparing the signal
from said net oxygen measuring means with a predetermined
10 set point signal indicative of a potentially hazardous net
oxygen level and establishing a first control signal there-
from; means for determining the actual rate of carbon monk
oxide level change in the coal pulverizer and establishing
a signal indicative thereof; means for comparing the signal
15 from said determining means with a predetermined set point
signal indicative of a potentially hazardous rate of carbon
monoxide level change in the coal pulverizer and establish-
in a control signal therefrom; and alarm means responsive
to said first control signal and also to said second control
20 signal to indicate an alarm condition indicative of a potent-
tally hazardous condition in the coal pulverizer.
The invention further consists of a safety system for
a coal pulverizer comprising means for measuring the actual
net oxygen level in the coal pulverizer and establishing a
25 signal indicative thereof; means for measuring the rate of
change of carbon monoxide level in the coal pulverizer and
establishing a signal indicative thereof; comparing means
for comparing the actual signals measured by the net oxygen
measuring means and the rate of carbon monoxide change
30 measuring means with predetermined sweatpants for establish-
in respectively independent control signals whenever the
predetermined sweatpants are exceeded; and alarm means
responsive to either of said control signals for indicating
a potentially hazardous condition in the coal pulverizer.
These and other aspects of the present invention
will be more fully understood upon a perusal of the
following description of the preferred embodiment
considered in combination with the drawings.
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BRIEF DESCRIPTION OF THY DRAWINGS
Fig. 1 is a schematic drawing of the safety control system
of the present invention.
Fig. 2 is a schematic of the monitoring and control logic of
the Fig. 1 safety control system.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings, the invention described herein
is a reliable, relatively low-cost automated safety system 8
capable of being integrated into a plant's computer control system
designed to monitor the performance of and detect impending fires
and explosions in electric-utility and industrial coal pulverizers
by monitoring the level of carbon monoxide (CO) and net oxygen
(2) concentration in a pulverized coal mill atmosphere. The
combined measurement of CO and 2 concentration in the mill atoms-
phone is used to indicate the oxidation rate of the coal to preclude
spontaneous combustion. Additionally, the measurement of net 2
concentration, when combined with other measurements may provide
the basis for overall mill performance calculations and the quality
of the pulverized coal.
As shown in Fig. 1, the CO/02 sample probe 10 is ...
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typically placed in a coal pulverizer 12 classifier
outlet zone. A sample of was is drawn Thor the probe
10 which has a porous high temperature filter 14. The
filter 14 is required to maintain trouble-free operation
by min;rnizing the amount of particulate matter drown
into the analyzer. A suitable filter 14 for this apply-
cation is of a type described in US. Patent No.
4,2~6,472.
The air sample drawn from the coal pulverizer 12
is then analyzed for percent by volume of oxygen (2~
content and CO gas concentration in Pam (parts per mill
lion) via a known oxygen and CO was analyzer 16 designed
to operate in a harsh power plant environment and having
auto calibration capabilities. A suitable analyzer for
this application is one manufactured by the Bailey
Controls Company of Babcock and Wilcox and is known as
the Type OX Oxygen and CO Analyzer. This analyzer 16
has a CO range of 0-1000 Pam and an Ox range of 0.1-25%.
Electrical signals corresponding to carbon monoxide and
oxygen concentrations are respectively transmitted to a
monitoring system control 18 located in the central con-
trot room along lines 20 and 22. CO and Ox concentra-
lions are displayed and/or recorded on a strip-chart
recorder 24. During normal pulverizer 12 operation, net
2 levels represent typically 16% 2 and normal CO
levels range between 40 and 80 Pam. If the net 2 con-
cent ration falls below a certain predetermined level,
typically 15% andtor the amount of CO produced exceeds
a predetermined rise level considered cause for concern,
typically a 50 ppm/minute sudden rise, the system 8
activates audible and visible alarms 26, I to alert the
operator who in turn may manually take corrective action
to inert the pulverizer 12 or permit the automatic monk
storing system S to continue until it initiates an
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automatic inert to bring the pulverizer 12 operating
parameters under control.
Referring now to Fig. 2, it will be seen that the
S monitoring and control logic assembly 18 utilizes both
a net oxygen measurement provided by the analyzer 16
along line 20 as well as a carbon monoxide measurement
provided along line 22 from analyzer 16. To, on the
one hand, actuate alarms 26 and 28 at predetermined
levels of net oxygen and predetermined rise times of
carbon monoxide concentration. Also when the net ox-
gun levels and the absolute carbon monoxide levels
exceed certain critical limits, automatic inverting of
the pulverizer 12 is accomplished by controllable
opening a valve 30 which allows some inheriting media
such as carbon dioxide for steam to flow along a line
32 into the pulverizer 12.
Turning first to the alarm functions, it will be
seen that the net oxygen measurement from line I is
transmitted along a line 34 to a difference station 36
having a set point set at a predetermined net oxygen
control point transmitted along line 38. The difference
station 36 corianders the actual net oxygen measurement
provided by the analyzer 16 representing the net oxygen
level in the pulverizer 12 and compares it with the set-
\ point oxygen level which, in the present situation, is
set at 15%. The present set point of 15% is based on the
assumption that the typical atmosphere in the pulverizer
representative of normal conditions is approximately 16%
and the initial alarm condition us desired to be a warn-
in indicative of potential problem areas.
The difference station 36 thus compares the Tao
signals and provides an error signal along line 40 which
is one input of an ED gate I The other input of the
AND gate 42 is provided by a constant negative signal
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from a predetermined source along line 1~4. Thus, as
long as the net oxygen level provided to the difference
station 36 along line 34 is greater than the 15~ set-
S point, a positive level error signal will be transmitted along line 40 to the AND gate 42 which then will fail to
provide any control signal along line 46, failing to
actuate the alarm 26. As soon as the net oxygen level
falls below the 15~ set point, the output along line 40
lo becomes negative and in combination with the constant
negative signal along line 44, will result in a conduct
lion of the AN gate 42, causing a control signal to be
transmitted along line 46 to the alarm I to thus actuate
it and provide an indication of potential problems in the
pulverizer 12 atmosphere.
Alternatively, the measured carbon monoxide signal
transmitted along line 22 may also provide an actuation
of the alternate alarm 28. The measured carbon monoxide
signal is transmitted to a derivative Patton controller
48 which will be sensitive to any variations in the car-
bun monoxide level and will effectively provide an out-
put signal along line 50 indicative of the slope or rate
of change of the carbon monoxide level in the pulverize
in mill 12. 'rho output of the derivative action con-
troller 48 is transmitted to a difference station avowing a predetermined set point along line .54 indicative
of a rate of carbon monoxide change which would indicate
coal ignition in the pulverizer 12. Such a rate of
change is typically taken to be a 50 pPm/minute rate of
carbon monoxide change. The output of the difference
station 52 is transmitted along the line 56 to an AND
gate 58 having a second input of a constant negative
value provided along line 60. In operation, the rate of
carbon monoxide change normally stays Boyle the 50 Pam/-
minute set point resulting in a negative output signal
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from the difference station 52. I~enever the actual
rate ox carbon monoxide change exceeds the set~oint of
line 54, the signal transmitted along lone 56 turns
positive, causing the AND Nate 58 to start conducting a
control signal along line I to the alarm 28 actuating
the alarm 28 to indicate a potentially hazardous atoms-
phone in the pulverizer 12.
m eye individual alarms, when actuated, warn the
operator of potentially hazardous conditions in the
pulverizer. This should indicate to the operator that
close monitoring of the pulverizer is required and typo
icily one alarm will be actuated, possibly followed by
the second alarm. Since the inverting of a pulverizer
may shock the pulverizer, such inverting is left to the
discretion of the operator and his supervisor. How-
ever, there are certain conditions beyond which inverting
of the pulverizer 12 is mandatory and should be automat-
icily initiated. To provide for such automatic inert-
in, the control system 8, again, utilizes both the net oxygen measurements and the carbon monoxide measurements
provided by lines 20 and 22, respectively.
Automatic inertia of the pulverizer 12 is act
tufted by a difference station 64 which has a set point
provided to it along line 66 having a net oxygen level
significantly lower than the set point level provided to
difference station 36. Typically, the difference station
64 has a net oxygen set point of 9%. Thus, during normal
pulverizer 12 operation, the net oxygen level measured
and transmitted to the difference station 64 will exceed
the I set point and the error signal produced by the dip-
furriness station 64 will be a Positive level signal trays-
milted along line 68 to an ED Nate 70. The other input
of the AND gate 70 is provided by a constant negative
level signal transmitted to the AND gate 70 along line
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72. Thus, during normal operation, the inputs to the
AND gate 70 will be positive and negative, providing no
control signal from the output of the AND gate along
line 74. envier the oxygen level of the pulverizer 12
falls below the 9% set point level, the output ox the
difference station 64 turns negative, providing two
negative inputs to the ED gate 70 and resulting in a
control signal along line 74 being transmitted to a
switching circuit 76. The switching circuit 76 is a
normally open circuit, preventing the signal transmitted
from a controller 78 from reaching the control valve 30.
Zen the control signal from line 74 is present, the
switching circuit 76 changes to a closed-circuit condo-
lion, turning over control of the valve 30 to the controller 78.
The controller 78 has an input signal indicative
of lie actual net oxygen level in the pulverizer 12
which is provided by a parallel line 80, paralleling the
net oxygen signal in line 20. The set point of the con-
troller is provided along line 82 from some predator-
mined set point station and is typically set at a 12~D
level. 'plus, when the switching circuit 76 is actuated
by a control signal from the ED gate 70, the controller
78 will open valve 30, causing an inverting atmosphere,
\ such as carbon dioxide, to be delivered to the pullover-
zero 12 until a somewhat normal ambient is reached close
to the set point level of 12%. The reason for keeping
the set point of the controller 78 at a somewhat lower
than typically normal atmosphere is to minimize the
shock to the pulverizer 12 due to the inertia process.
The switching circuit is then switched back to its no-
molly open condition by a reset signal provided along
line 84 from either a manual source or an automatic
source which can be tied to some parameter indicative
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of the reestablishment of normal ambient conditions in
the pulverizer 12.
The actuation of the automatic inertia& means is
also alternatively done upon the sensing of a predator-
mined absolute level of carbon monoxide in the pullover-
per 12. ale carbon monoxide signal normally provided
along line I is tapped by a lone I to provide one
input of a difference station 88. The set point of the
difference station 88 is provided along line 90 from a
predetermined set point station typically set at an Abe
solute carbon monoxide level of 200 Pam. Thus, as
long as the carbon monoxide level stays below a 200 Pam
value indicative of normal operation, a positive error
signal will be transmitted by the difference station 88
along line 92 to an AND gate 94. The other input to
AND gate 94 is provided by a line 96 connected to a Jon-
slant negative level source. Thus, during normal put-
varier 12 operation, opposite polarity signals are pro-
voided to the AND gate 94, preventing the establishment of any control signal along line 98 from the AND gate
94. I~enever the absolute carbon monoxide level exceeds
the predetermined set point of 200 Pam, the error signal
transmitted to the ASP gate I turns negative, causing
the conduction of the AND gate 94 and the establishment
of a control signal along line 98 to the switching air-
cult 76. As was described earlier, with reference to
the net oxygen level control; this causes the switching
circuit 76 to become conductive, turning control of the
valve 30 over to the controller 78. Again, automatic
inverting of the pulverizer 12 occurs until a reset sign
net is established along line 84, causing the switching
circuit I to again become non-conductive and causing
the valve 30 to switch its normally closed position.
It will be understood that certain modifications
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.
and improvements will occur to those skilled in the art
upon a reading of this specification. All such ~odifi-
cations and improvements have been deleted herein for
the sake of conciseness and readability but are pro-
pertly intended to fall within the scope of the
following claims.
