CA1229327A

CA1229327A - Safety system for coal pulverizers

Info

Publication number: CA1229327A
Application number: CA000457368A
Authority: CA
Inventors: Marshall H. Cooper; Robert E. Pocock
Original assignee: Babcock and Wilcox Co
Current assignee: Babcock and Wilcox Co
Priority date: 1983-08-01
Filing date: 1984-06-25
Publication date: 1987-11-17
Also published as: BR8403542A; KR850001561A; EP0132974A3; SG63889G; MX160409A; EP0132974B1; EP0132974B2; IN162793B; JPS60143844A; HK96689A; AU3131384A; DE3476851D1; JPH0128613B2; ES534210A0; EP0132974A2; ES8504487A1; KR900002655B1

Abstract

SAFETY SYSTEM FOR COAL PULVERIZERS

ABSTRACT OF THE DISCLOSURE

A safety control system is disclosed for a coal pulverizer utilizing the measurements of net oxygen and carbon monoxide measurements of the pulverizer atmosphere. First levels of net oxygen and rate of carbon monoxide change are utilized in a control log-ic system to actuate alarms and automatic inerting of the pulverizer is accomplished utilizing a second level of net oxygen measurement and absolute carbon monoxide level in the pulverizer.

Description

~2;~327 - 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 ~22 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 ~22~ 7 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 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.

~Z2~32~

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.

~.~2~3; :7 -pa-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 ...

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 ~2;2~13~7-- 7 --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 2~3~7 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

3~2~g3~ :

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 332~

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 ~22~

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 3~7 .
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.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A safety system for a coal 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 setpoint signal indicative of a potentially hazardous net oxygen level and establishing a first control signal therefrom;

means for determining the actual rate of carbon monoxide level change in the coal pulverizer and estab-lishing a signal indicative thereof;

means for comparing the signal from said determining means with a predetermined setpoint signal indicative of a potentially hazardous rate of carbon monoxide level change in the coal pulverizer and establishing a second control signal therefrom; and alarm means responsive to said first control signal and also to said second control signal, to indicate an alarm condition indicative of a potentially hazardous condition in the coal pulverizer.

2. A safety system as set forth in claim 1 including:

second means for comparing the signal from said net oxygen measuring means with a second predetermined setpoint signal lower than said first predetermined set-point signal and establishing a second control signal therefrom; and inerting means responsive to said second control signal for inerting the coal pulverizer.

3. A safety system as set forth in claim 2 wherein said inerting means includes:
a source of carbon dioxide for providing an inert-ing atmosphere to the coal pulverizer;
valve means for controlling said source of carbon dioxide; and controller means responsive to the signal from said measuring means for controlling said valve means.

4. A safety system as set forth in claim 3 including switching means mounted between said control-ler means and said valve means being, responsive to said second control signal to allow control of said valve means by said controller means.

5. A safety system for a coal pulverizer comprising:
means for determining the actual rate of carbon monoxide level change in the coal pulverizer and establishing a signal indicative thereof;
means for comparing the signal from said determin-ing means with a predetermined setpoint signal indica-tive of a potentially hazardous rate of carbon monoxide level change in the coal pulverizer and establishing a control signal therefrom; and alarm means responsive to said control signal for indicating a potentially hazardous condition in the coal pulverizer,

6. A safety system as set forth in claim 5 wherein said determining means includes.
means for measuring the actual carbon monoxide level in the coal pulverizer;
a derivative action controller connected to said measuring means for providing an output signal indica-tive of the rate of actual carbon monoxide level change in the coal pulverizer.

7. A safety system as set forth in claim 6 further including:
second means for comparing the signal from said measuring means with a predetermined setpoint signal indicative of a hazardous carbon monoxide level in the coal pulverizer and establishing a control signal therefrom; and inerting means responsive to said control signal for inerting the coal pulverizer.

8. A safety system as set forth in claim 7 further including:
means for measuring the actual net oxygen level in the coal pulverizer and establishing a signal indica-tive thereof; and second means for comparing the signal from said net oxygen measuring means with a first predetermined setpoint signal indicative of a potentially hazardous net oxygen level and establishing a control signal for actuating said alarm means.

9. A safety system as set forth in claim 8 further including:
means for comparing the signal from said net oxygen measuring means with a second predetermined setpoint signal lower than said first predetermined setpoint signal and establishing a second control signal therefrom for actuating said inerting means.

10. A safety system as set forth in claim 9 wherein said inerting means includes:
a source of inerting atmosphere for inerting the coal pulverizer;
valve means for controlling said source of inert-ing atmosphere; and controller means responsive to the signal from said means for measuring the net oxygen level in the coal pulverizer for controlling said valve means.

11. A safety system as set forth in claim 10 including switching means mounted between said controller means and said valve means and being responsive to con-trol signals from either the comparing means comparing the absolute carbon monoxide level in the coal pulverizer with a predetermined setpoint or the control signal from the comparing means comparing the net oxygen level in the coal pulverizer with a second predetermined setpoint for allowing control of said valve means by said controller means.

12. A safety system for a coal pulverizer comprising:
means for measuring the actual net oxygen level in the coal pulverizer and establishing a 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 measuring means with predetermined setpoints for establishing respectively independent control signals whenever the predetermined setpoints are exceeded;
and alarm means responsive to either of said control signals for indicating a potentially hazardous condition in the coal pulverizer.

13. A safety system as set forth in claim 12 further including:
second comparing means for comparing the signal indicative of the actually measured net oxygen level in the coal pulverizer with a second setpoint indicative of a hazardous condition in the coal pulverizer and establishing an inerting signal whenever the actual net oxygen level in the coal pulverizer exceeds this second setpoint; and automatic inerting means responsive to said inerting control signal for providing an inerting atmosphere to the coal pulverizer.