Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Case /~767
~P~ATU~ ~ND METI~OD FOR
MON~TORING LOW-LEVEL COM~USTIBL~S
TECHNICAL FIELD
The present invention generally relates to control
systems for pulverizers and more particularly to an im-
proved safety control system for detecting and controllin
impending hazardous conditions in a coal pulverizing mill
BACKGRO~ND ~RT
Coal usaqe has increasedin the ~nited States for a
variety of reasons, particularly those of an economic nature.
The utility industry is burning far more coal today than it
did ten years aqo. ~ith the increased demand for coal, the
use of younger, more volatile coals like subbituminous and
lignite has increased. Consequently, the potential for spon-
taneous combustioll causing serious fires and explosions duringthe handling, qrinding and pulverizing steps has increased.
Several methods which have been given considerable atten-
tion for detecting impending pulverizing mill fires are base~
on measurinq temperature, gas flow velocity and carbon monoxide.
Single and multiple point temperature monitoring tecllniques
have been used for a number of years to warn of an over-tem-
perature condition in the mill. This approach, however,
provides information too late to stop a fire from spreading.
The qas flow velocity monitor approach has potential, but the
relationship hetween gas flow, temperature and pressure are
9~S()
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not sufficiently understood to he effective as a warninq
system~ T~e increase in the carhon monoxide level in the
pulverizlng mill has been recently given the most atten-
tion in research and practice and is a way o detectingpulverizing mill fires.
~ numher o commercial devices utilizing infrared
ahsorption techniques are available for monitoring carl-on
monoxide levels in the pulverizing mi]l. ~his method is
based upon the principle that when coal starts to oxidize,
i.e. the early stages of combustion, carbon monoxide is
produced. 8eing ahle to detect this carhon monoxide at very
low levels e.q. 25 to 50 ppm permits the mill operator to
take precautionary measures to prevent a major fire or an
explosion in the mill.
~ small pocket of oxidizing coal can become a ma~or fire
through escalation or ignition. If escalation occurs the
oxidation process intensifies as the quantity of coal involved
and temperature increase. Larqer quantities of carhon monoxi~e
are produced as the process escalates until a runaway con~
tion is reached which results in a fire. This small quantity
of oxidizinq coal also represents an iqnition source wllicl)
combined with the other elements within the mill can resu]t
in a ma~or fire or explosion. In this case, the quantity of
carbon monoxide does not need to escalate prior to the fire
or explosion since the small pocket of oxidizing coal is only
an ignition source. From the foreqoing it is apparellt that
detection ~ethods based upon carbon monoxide alone are use-
ful only after oxidation has started and do not give tlle
operator a good indication of potentially explosive conditions
within the pulverizing mill. Other factors such as the
level of oxygen and combustible gases in the pulverizing mill,
must he considered when evaluating the possibility of a fire
or an explosion within the pulverizing mill.
1~9150
Because oE the Eoregoing it has become desirable to develop an
improved safety control system for a pulverizing mill wherein a measurement
of oxygen an and aggregate measurement of not only carbon monoxide but
all combustible gases in the mill are made and utilized Eor controlling the
operation of the mill and warning the operator of a potentially dangerous
mill condition.
The invention provides an improved safety system for a coal pulverizing
mill comprising means for determining the rate of change of the level of
carbon monoxide and other combustible gases in the coal pulverizing mill and
establishing a signal indicative thereoE; means for comparing said signal
from said determlning means with a predetermined setpoint signal indicative
of a potentially hazardous rate of change of the level of carbon monoxide and
other combustible gases in the coal pulverizing mill and establishing a
first control signal therefrom; and alarm means responsive to said first
control signal for indicating a potentially hazardous condition in the coal
pulverizing mill.
The invention also provides an improved safety system for a coal
pulverizing mill comprising means for measuring the level of carbon
monoxide and other gases in the coal pulverizing mill and establishing a
signal indicative thereof; means for comparing said signal from said
measuring means with a predetermined setpoint signal indicative of a
hazardous level of carbon monoxide and other combustible gases in the coal
pulverizing mill and establishing a first control signal therefrom; and
inerting means responsive to said first control signal for inerting the
coal pulverizing mill. In the preferred form, a single point analyzer is
used which is mounted directly to the pulverizing mill to provide continuous
measurements of both the oxygen content and the carbon monoxide equivalent
(COe) level of the pulverizing mill atmosphere. The measurement of the
carbon monoxide equivalent (COe) level includes not only the level of
carbon monoxide in the pulverizing mill but also the other combustible
gases, such as hydrogen, methane, ethane, etc., in the mill. The oxygen
portion of the analyzer uses a sensor operating at a temperature where any
combustible volatile material will combine with the oxygen in the sample
that is extracted from the pulverizing mill. The sensor will then respond
to the free or uncombined oxygen remaining. The resulting measurement,
referred to as the net oxygen (2) level, is then compared with various
predetermined setpoints and correlated with the carbon monoxide (CO ) level,
which is similarly compared with various predetermined setpoints, to determine
if a potentially dangerous condition exists within the pulverizing mill. Thus,
measurements of both the net oxygen (2) level and the carbon monoxide
equivalent (CO ) level in the pulverizing mill atmosphere are used
to determine the onset of conditions witlli1l the mill w11ieh
might lea~ to a fire or explosion in same.
Bl~lEE DESCRlP'rION OF 5`11E DRA~INGS
Figure l is a schematic drawinq of the safety control
systern of the present invention.
Figure 2 is a schematic drawinq of the monitorinq a11c1
control logic assernhly of the safety control system illus-
trated in Figure l.
Figure 3 is a graph of the relationship of the carbon
monoxide equivalent (COe) level and the net oxygen (2)
level in a coal pulverizing mill to the various combustible
components which comprise same versus coal temperature.
Figure 4 is a graph of the relationship of the carboTl
monoxide equivalent (COe) level and the net oxygen (2) level
in a pulverizing mill versus time and illustrates the ch~nqcs
in these levels when a fire occurs in the mill.
Figure 5 is a graph of the relationship of the carbon
monoxide equivalent (COe) level and the net oxygen (2) level
2~ in a coal pulverizing mill versus time and illustrates the
changes in these levels when a smoldering fire exists in the
mill but ignition doe~s not occur.
Figure 6 is a qraph of the net oxygen (2~ level versus
carhon monoxide equivalent (COe) level in a pulverizinq
mill and illustrates the manner in which mill operating con-
ditiorls depend upon the foregoing levels.
DE~CRIPTION OF TIIE PREFERRED EMBODI~ENT
Referring now to the drawings where the illustrations
are for the purpose of describing the preferred emhodiment of
the present invention and are not intended to limit the inven-
tion hereto, Figure l is a schematic drawing of the safety
control system ln of the present invention. As such, the
l~ 15~
control s~stem 10 can be integrated in a facility s control
systern desi~ned to monitor the performance of and detect
impending fire or explosions in industrial coal pulveri7illq
mills ~y monitoring the net oxygen (2) level and tile call~on
monoxide equivalent (COe~ level of the combustible colm~ollents
in the pulverizing mil] atmospllere. The measurement of the
carbon monoxide equivalent (COe) level of the combustible com-
ponents includes not only carbon monoxide but also other
combustibles components such as hydrogen methane ethane
and other iligher hvdrocarbon components. The combined measure-
ment of the Ce and net 2 levels in the pulverizing mill
atmosphere is used to indicate the oxidation rate of the
coal to prevent spontaneous combustion within the mill. Jn
addition the measurement of the net 2 level, when comhine(l
with other measurements can provide the basis for overall
mill performance calculations and the quality of the pul-
verized c~al.
As shown in Figure 1, the COe/o2 sample probe 12 is
usually placed in a coal pulverizing mill 14 outlet zone.
A sample qas is drawn throuqh the probe 12 which is provid~N
with a high temperature filter 16. The filter 16 is requ;re-l
to maintain trouble-free operation of the control system 1~
by minimizing the amount of particulate matter drawn into the
analyzer. A filter 16 which can be used for this application
is of a type described in U.S. Patent No. 4,286,472.
Tlle air sample drawn from the coal pulverizer is the
anal~zed for percent by volume of oxvqen (2) content an~i
the carbon monoxide equivalent (COe) concentration of combus-
tible components in ppm (parts per million) via a known o~yqenand Ce gas analyzer 18 designed to operate in a harsh power
plant environment and havinq autocalibration capabilities.
Elect.rical signals corresponding to the net oxygen (2) level
150
i.e., the level of the free or uncomhined oxyqen withill
the sample remaininq after the combustible volatile mater-
ials therein have combined with the oxyqen in tile sampl~,
and the c~rhon monoxide equivalent (COe) level are trans-
rnitted respecti~ely to a monitoring and control logic a.ssenl-
bly 20 located in a central control room via lines 22 and 24.
The net O~ and Ce levels are displayed and/or recorded on
a strip-chart: recorder 26. If the net 2 level falls helow
a predetermined rise level, the system 10 actuates audible
and visihle alarms 28, 30, respectively, to alertthe operator
who, in turn, may manually take corrective action to inert
the pulverizinq mill 14 or permit the system 10 to continue
until it initiates an automatic inert mode of operation to
bring the pulverizing mill 14 operating parameters back under
control.
Referring now to Figure 2, the monitoring and contro]
logic assemhly 20 utilizes both the net oxygen (2) measure-
ment provided by the analyzer i8 along line 22 as well as
the carbon monoxide equivalent (COe) measurement provided
along line 24 from the analyzer 18 to actuate the alarms
28, 30, respectively at a predetermined net oxygen (2) level
and at a predetermined carhon monoxide equivalent (COe) rise
level. In addition, when the net oxygen (2) level and/or
the absolute carbon monoxide equivalent (COe) level exceed
certain critical limits, automatic inerting of the pulveriz-
ing mill 14 is undertaken by controlling the opening of a
valve 32 whic)l permits some inerting media, such as carbon
dioxide or steam, to flow along a line 34 into the pulverizinq
mill 14.
~ s for the alarm functions, the net oxygen (2~ measure-
ment from line 22 is transmitted along a line 36 to a differ-
ence station 38 having a setpoint set at a predetermined net
oxygen control point provided along a line 4G. The difference
915(t
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- station 38 conpares the actual net oxyqen (2) measurerlent
provile~ by the analyzer 1~ with the setpoint net oxy~l-n
Ievel and provides an error siqnal alonq a line 42 whith is
one input to an ~ND gate 44 rhe other input to tle ~NI) qate
44 is provi(le~l by a constant neqative signal fron a ~re(leter-
mined source along a line 46 Thus as long as tle net oxyqe
(2) level proviled to the difference station 38 is qre.t-r
than setpoint net oxyqen level a positive error signal wi~l
be transmitted along line 42 to the ~ND gate 44 which will
then fail to provide any control signal alone a line 48 thus
failing to actuate the alarm 28 As soon as the net oxy~en
(2) level lrops helow the setpoint net oxygen level the error
signal transmitted along line 42 will become negative an~ in
combination with the constant negative signal proviletl on line
46 resilts in the conduction of the AND gate 44 causinq a con-
trol siqnal to he transmitted along line 4~ to the alarm 28
actuating SAme and providing an indication of potential rro-
blems with respect to the atmosphere in the pulverizinq Ini.ll
14
Alternatively the signal representAtive of the meisllre(l
carbon monoxide equivalent (COe) level which is transmit~d
along line 24 may also provide an actuation of the alternate
alarm 30 The measure-3 carbon monoxide equivalent (CO2) leve]
signal is transmitted to a derivative action controller 5()
which is sensitive to any variations in the carbon monoxide
equivalent (COe) level and provides an output signal alon~ a
line 52 indicative of the slope or rate of change of the car-
bon monoxide equivalent (COe) level in the pulverizing mill 1
The output of the derivative action controller 50 is trans-
mitted along line 52 to a difference station 54 having a pre-
determined setpoint providel along 2 line 56 r-presentative of
a rate of change of the carbon monoxide equivalent (COe) level
which wouli indicate coal ignition in the pulverizinq mill l~i
1;~691~
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The output of the difference station 54 is transmitted alonq
a line 58 to an ~ gate 60 having a second input ofa cOn-
stant positive value provided alonq a line 62. In oc?eration,
the rate of chanqe of the carbon monoxide equivalent (COr)
level normAl.Ly stays below the setpoint applied to the ~lir-
ference s~ation 54 resultinq in a negative output siqnal
from this station 54 alonq line 58. ~henever the actual r~te
of change of the carbon monoxicle equivalent (COe) level in
the pulverizinq mill exceeds the setpoint provided alonq
line 56 to this difference station 54, the signal transmitted
along line 58 becomes positive, causing the AND gate 60 to
concluct resulting in the transmission of a control siqnal
alonq a line 64 to the alarm 8n actuating same to indicate
the existence of a potentially dangerous condition in the
pulveri%ing mill 14.
The foregoing alarms 28 and 30, when actuated, warn the
operator of a potentially dangerous condition in the pulveriz-
ing mill 14. These alarms should indicate to the operator
that close monitoring of the pulverizing mill 14 is required
and generally one al~rm will be actuated, possihly followecl
by a second alarm. Since the inerting of the pulverizinq
mill 14 may shock the pulverizer, such inerting is left to
the discretion of the operator. There are, however, certnin
conditions beyond which inerting of the pulverizinq mill
14 is mandatory and must be automatically initiated. To
provide for such automatic inerting of the pulverizing mi.ll
14, the control system 10 again utilizes both the net oxygen
(2~ measurements and the carbon monoxide equivalent (COe)
measurements provided via lines 22 and 24, respectively.
~ utomatic inerting of the pulverizinq mill 14 is actuated
by a difference station 66 which has a net oxygen level set-
point provided to it along a line 68. The net oxygen level
5~
_9_
setpoint provided to the difference station 66 is signifi-
cantly lower thall the setpoint level provided to the differ--
ence station 38 Tllus ~luring normal operation of the p~
verizinq mill l4 the net o yqen (2~ level measured and
transmittel to the difference station 66 will exceed thc
setpoint applie(l thereto and the error signal pro~uced hy tlle
differel~(e statioll 66 will be a positive signal which is
transmitte~l aloncl a line 70 to an ~ND gate 72. The other in-
put of tne ~N~ gate 72 is provided by a constant negativesignal along a line 74. Thus during normal operation of the
pulverizinq mill 14 the inouts to the ~ qate 72 will he
positive and negative resulting in no control signal be;nq
transmitted froln the ~ND gate 72 alonq a line 76. Whenever
the net oxyqen (2) level within the pulverizing mill 14 falls
below the setpoint level applied to the ~ifference station 66
the output of this station 66 becomes negative providinq two
negative inpu.s to the ~ND gate 72 resulting in the transllis-
sion of a control signal along line 76 to a switching cir-
cuit 78 The switclling circuit 78 is a normally open cir-
cuit preventing the signal from a controller 80 from reach-
ing the control valve 32. When a control siqnal is present
along line 76 the switching circuit 78 changes to a closed
circuit condition which results in the controller 80 beinq
responsib]e for tle operation of the valve 32.
One inpl]t to the controller 80 is the actual net oxyg~n
~2) level in the pulverizing mill 14 and is provided by a line
82 which is connected to line 22. The setpoint for the con-
troller 8n is provided along a line 84 from some setpoint sta-
tion and tlle level of this setpoint is typically between t)esetpoint levels for difference stations 66 and 38. Thus
when the switching circuit 78 is actuate~ by a control signal
from the ~ND gate 72 in~iicating that the net oxygen (2)
level within the pulverizing mill 14 has fallen below the set-
point level to the ~ifference station 66 the controller 8n
~69~ jO
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will open valve 32 causinq an inertinq atmosphere, such as
carbon dioxide, to be delivered to the pulverizing mill 14
until a somewhat normal net oxYqen level is reached close to
the setpoir-t ~evel for the controller 8n. Typica]ly, 1 he
setpoint level for the controller 8n is kept some~hat lo-rer
than normal atmosphere to minimize the shock to the pul~;er-
izer 14 due to the inertinq process. When the net oxyqen
(2) level in the pulverizing mill lq reaches the setpoint
level for the controller 80, the .switching circuit 78 can
then be reset to its normally open condition by a reset sigr)al
along a line 86 from either a manual source or an automatic
source tied to some parameter indicative of the establishment
of normal operating conditions within the pulverizing mill 14.
The actuation of the automatic inertinq means is also
alternatively done upon the sensing of a predetermined ahso-
lute carbon monoxide equivalent (COe) level in the pulveriz-
ing mill 14. The carbon monoxide equivalent (CO~) siqna~
normally provided on line 24 is tapped hy a line 88 to pro-
vide one input to a difference station 90. The setpoint
of the difference station 90 is provided alonq a line 92
from a setpoint station and the level of this setpoint ic
typically set at the maximum carbon monoxide equivalent
(COe) level which can he tolerated in the pulverizinq mill
14. Thus, as long as the carbon monoxide equivalent (COe)
level stays below the setpoint for the difference station
90, a positive error signal will be transmitte~ by the dif-
ference station sn along a line 9q to an ~ND gate 96. The
other input to the ~ND gate 9fi is a constant negative siq-
nal provided along a line 98. Thus, during normal operation
3() of the pulverizinq mill 14, opposite polarity signals are
applied to the inputs to the ~ND gate 96, preventinq the
transmission of any control signal alonq a line 100 from the
AND gate 96. Whenever the absolute carbon monoxide equivalent
9~5()
(COe) level exceeds the setpoint level applied to the Airfer-
ence station 90 the error sigra] tr~nsmitted to the ~ qate
96 becomes negative causinq the conductioll of the ~1) 9a~e
96 and the establishment of a control signal alorlg line 1()()
to the switching circuit 78. ~s was previously desc~ el
Witll respect to the net oxygen 102) level control the fore-
going causes the switching circuit 7B to be conAuctive turrl-
inq control of the valve 32 over to the controller 80. ~n
this manner automatic iner~.inq of the pulverizer 14 will
occur until a reset signal is established along line 86.
causinq the switchinq circuit 78 to again become non-conductive
and causing the valve to switch back to its normally closed
position.
Oxygen fuel and an ignition source must be present in
the pulverizing mill in order for A fire or explosion to occur.
The grinding of the coal in the pulverizinq mill releases
hydrogen methane ethane and other combustible hydrocar!~or~.
Carbon monoxiAe is present only in very low levels durillq tlle
2Q grinding process unless the oxidation process has commence(~.
Once the oxidation process has commenced and the coal tempera-
ture rises all of the foregoinq combustible gases will
evolve and can be utilized as an indicator of a potential~y
dangerous condition. Figure 3 shows the general relationship
of the resultinq carbon monoxide equivalent ICOe~ level in the
pulverizing mill to the various combustible qaseous components
which comprise same versu.s increasinq coal temperature. As
shown in this Figure measuring the agqregate of all these
gaseous compone-lts produces a response that is significantly
more pronounced than that based only upon carbon monoxide
and eliminates the limitations resulting from relyinq on only
one gas viz. carhon monoxide.
It has been found that most pulverizing mill fires are
preceded by a significant increase in the carbon monoxide
1~i91~0
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equivalellt (COe) level in the rnill. This increase appears
to he caused by the oxidation of a srnall pocket of coal
within the bowl or underbowl area Investiqatitns have shown
that s ch pockets of oxitlizinq coal can exist for a Io~n
perio(1 of tine within the mil] anl have the potential oF
igniting a rulawav fire at any time. Such pockets can o~: be
detecte-l by using previous methods OT' detection hut can he
detectel througil the use of the present invention as shown
in Figure ~. This Pigure illustrates a fire that was pre-
ceded by elevated carbon monoxide equivalent (COe) levels
indicating tlt? presence of smoldering coal in the pulverizing
mill. Approximately ten minutes after the start-up of the
pulverizing mill the carbon monoxide equivalent (COe) level
lS increased to 250 ppm; thirty minutes after the increase in
the carbon monoxide equivalent (COe) level the oxygen (O~)
level spiked down to 5~ and the mill temperature went out
of control indicatinq the presence of a fire within the mi1~
The fire was quickly extinguished by increasinq the coa] feed
however observation of sparks from the underbowl sectio
verified that a fire had occurred and that coal was stil]
smoldering in the mill. The carbon monoxide equiva1en~-
(COe) level then gradurlly decreased to approximately 35 I-pm
over the next seven hours. This indicated that the smolder-
ing coal qradually burned itself out however the potential
for a second fire luring this period was indicated hy the hi~l
carbon monoxide equivalent (COe) level
~ n example of a smoldering fire which did not ignite the
pulverizinq mill is shown in Figure 5. ~s shown in this Fiq-
ure approximately one-half hour after start-up of the mill
the carbon monoxide equivalent (COe) level increased from 35
ppm to 225 ppm. The carbon monoxide equivalent (COe) level
remained at this hiqh level and the net oxygen (2) level fell
slightly from 17.75~ to 1~.75~. The carbon monoxide equivalent
(COe) and net oxygen (2) levels then returned to their normal
i9~
levels. ~nvestigation of the pu~verizing mill reveale-l a
small quantity of coal smoldering in the mill for tllirty
minutes. I`he quantity of smolderinq coa] was not larqe
enough to ignite the mill.
From t:he fore~oinq it is apparent that monitoring the
carhon monoxide eq~i~alen~ (COel level in the pulverizinq
mill provides a significant~ ~rn~r~ved 1~ethod fo~ tlle ear~
detection of a potentially danqerous condition in the mill
so that the necessary corrective measures can be taken to
avert a fire or explosion in same. Such early detection is
not possible with the detection methods previously avai]a~le.
In summary, Fiqure 6 illust~a~es the general relation-
ship c~f the c~rhon m~noxide equivalent (COe) level, net oxvclen
tO2) level, and pulverizer mill condition. The normal o~era-
tinq band 5hows a general relationship between carhon monoxide
equivalent (COe) level, net oxygen (2) level, and the type
of coal used. As the percent volatile material iTl tT-e coa I
increases, so does the expected carbon monoxide (COe) equiva-
lent level. As the percent moistureincreases, the net oxyqen(2) level will decrease due to resulting higher moisture
levels in the pulverizing mill gases. Rises in the carhon
monoxide equivalent (COe) level combined with a constant or
dropping net oxygen (2~ level indicates a smoldering condi-
tion with a potential for a pulverizer mill fire. Conversely,increasing carbon monoxide equivalent (COe) level indicates
that the pulveri%ing mill is in a potentially explosive con-
dition. From the foregoing, the value of measuring and deter-
mininq the carbon monoxide equivalent (COe) level, in conjunc-
tion with the net oxyqen (2) level, is apparent in determin-
inq the onset of a potentially dangerous condition in the
pulverizinq mill.
Certain modif~atiol)s and improvements will occur to
1~i91~
-14-
those skille(l in the art upon reading the foreqoinq. It
sho~ e un(lerstoo~ that all such modificati~ns an~
provements have ~een deleted herein for the sake of con-
cisenes~s and readal~iIity but are properly within the scopeof the followinq claims.
