CA1197011A

CA1197011A - Loss minizimation combustion control system

Info

Publication number: CA1197011A
Application number: CA000440048A
Authority: CA
Inventors: Marion A. Keyes, Iv; Michael P. Lukas; Robert E. Pocock
Original assignee: Babcock and Wilcox Co
Current assignee: Babcock and Wilcox Co
Priority date: 1982-11-01
Filing date: 1983-10-31
Publication date: 1985-11-19
Also published as: KR880001507B1; BR8306128A; HK98988A; JPH0356040U; EP0108586A1; JPS59131824A; EP0108586B1; KR840007168A; IN160113B; DE3376871D1; ES526800A0; MX168154B; ES8501511A1

Abstract

LOSS MINIMIZATION COMBUSTION CONTROL SYSTEM
ABSTRACT OF THE DISCLOSURE
A system for minimizing combustion operation losses includes measuring a load index for the combustion operation which is proportional to the fuel demand or the output thereof, measuring an amount proportional to the air heating losses of the combustion operation and measuring an amount which is propor-tional to the fuel loss of the operation. The air heating loss is measured by multiplying a flue temperature by an amount of unburned oxygen in the flue gas. This quantity is multiplied by a cost factor for such air heating and the load index. The fuel loss is obtained by measuring an amount of by-product in the flue gas as well as the opacity of the flue gas. These are multiplied by appropriate cost factors which in the case of opacity is pro-portional to a fine that would be due for violating certain limits for the opacity. Minimum values are found for the fuel loss and air heating loss quantities, as air demand to the com-bustion operation is changed. A minimum for the sum of the fuel and air heating losses is also obtained with the air demand of the combustion operation being set so that all of the losses are as low as possible. In this way the costs of undesired air heat-ing, unburned by-products as well as potential violation of flue gas characteristic limits are utilized in determining the most economical air demand for the combustion operation.

Description

Tl.V~. _ _ _ OL SYSTEM CB e 451 7 FIELD AND BACKÇ:ROI3ND OF T~E INVENT10N
The present inven~c~on rel~tes eo the control of ~ com-bu~tion proceas in a boiler, heater, or other device in w~ch 5 fuel and air are' comblned ~nd ~urned ~o produce he~t,.
Techr!lques are known ln the are~ of combuseion cor.trol wh~ch involve the measurement of various products of combus~ion ln the flue ~a;es and the use ~f these measurements to ad~u~t the amount of excess air (or air/I'uel ratio) ~uppl~ ed beyond ehe 10 ~3to~chiome~cric level reqllired for ideal combus~on., The pr~or art r~cognl2es that there i8 a ~radeoff be~cween a hi~h level of e:~ce~s air, in which ~r heating los~6 predominate, and ~oo low a level of ea~cess alr, ~n which unburned fuel 108CIe6 predolDinate.
Priol- ~ppro~ches to op~imizlng the oombustion proce~
~all into one- of three categories, depending on what product or product~ of eombuJt~on ~re.bein~ measured in ~he flue ~ases:
s~xy~en only~ eombu~tibles only, or a eo~bination of che two.
The~e sre discussed ~epar~tely ln the fo1lou~ng.
The oxygen only appr~ae~ is u6ed in the Ba~ley Meter

2() Company Patent No~ 3,049,3009 "Cs)mbustion Con~rol for a Furnace -Flred ~ith Fuel~ Havin~ Differen~c Oxygerl-Exeess Air Characteris-tics," dated All~ust 14, 1962.. An analyæer ~ u ed to ~easure ~che oxygen in the flue ga3, a~d ehe excess air 1 reduced until the .
measured oxygen reaches a p~eselected et point.: . .
The combus~cibles . only (Carbon mvnoxlde-CO9 hydro-. carbons, ~ndlor opacity) ~pproach i~ used in S~andard O~l Co~npany (Indiana) Patene No. 4,260,363~ "Furnace Fuel Op~imizer," da*ced April 7 , t 981, a~nd the copendir~,~ application to ~che Econics J

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Corporation, as described ln a ~echnical paper by Keith Swanson, "An Advanced Combustion Control System Using Distributed Micro-computer Techniques," ISA Publication ISBN 0 87664-521-X, 1981.
An analyzer or analyzers are used to measure one or more of these 5 parameters, and excess air is adjusted until they reach a preselected set point. If more than one variable is measured and controlled, some switching between controlled variables is done to attain the most "conservative" value of excess air.
The combination of oxygen and combustibles approach is used in the Measurex Corporation Patent No. 4,162,889, "Method and Apparatus for Control of Efficiency of Combustion in a Furnace," dated July 31, 1979, and Westinghouse Electric Corporation Patent No. 4,231,733, "Combined 02/Combustibles Solid Electrolyte Gas Monitoring Device," dated November 4, 1980. In the Measurex patent, the deviation of CO from its preselected set point is used to adjust the set point of an oxygen controller in a cascade fashion. In the Westinghouse patent, excess air is adjusted to control, to a preselected combustibles set point, until the oxygen moves outside pre-selected limits. Then the control mode is switched to bring the oxygen back within limits, at which point combustibles control is resumed.
The shortcomings of the current approaches to combustion control are as follows:
All of the approaches attempt to control to arbitrary selected set points one or more of the products of combustion.
There is no guarantee that combustion conditions are such that these set points can be reached or that these set points are the best ones from an economic point of view however.

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In appro~che~ thae ~ttemp~ to ~wltch ~Tnon~ multlplevariables to be controlled, ~ likely that llmi~ cycling will occur a~ the vari~u~ switch poin~ are re~ched and the modes s~
control change. Thi~ leads to unde~irable cyelic ~tre ~es ori ehe 5 proces~ equipment, None ole the approaches attempt6 ~co dire~tlg minimize ~n~r expllcit measure of economic 10~8, ~uch a~ the co~t o unburned fuel up the ~tack, ~he co~t of hea~in~ ~he exces~ alr, or th2 C08t of viLolaeing ~overrlmen~c emi~s~on regulations 10 SU~RY OF THE INVEN~ION
The pre~en~ invent~on differs from and ~mproves upon the prior ar~ in the following re~pectso (1 ) The combu~tion eon~rol approach ~ 8 ba~ed expliciL~ly on minimizing a penal~y function ~hat repre~;en~s ~he ~uan of 15 economlc lo~sses in runnlng ehe combustion process~
~ ) The contrQl approach doe~ not rely on ~electing a ~et polnt for any one product of c:ombus~ion parameter ~e,g., CO, oxygen, or opacity) ~hat ~ay or ~ay noe be ~he bes~c one under current op erat ing cond i t i on s 20, ~3~ The control approach takes into aceolmt the eco~
nomic penalty of not meetin~ governmental emisslon regulations., The basi c concep~c behind ~che present fnvel1~lon iLnvolves measurements of exce~s alr and of each of ~he combustible~
elements. These are mul~iplied by a boiler/he~ter load index to 25 produce a "ra~ce of loæs" e~timate for ea~h element. ~ese rate~
are multl~lied by appropriate economic factor to convert them ~nto the "dollar~ lost~' per unlt ~Cilll2 of operatlon, t~hen added to~ether to produce a combined lo~ lndex. The ~lr~fuel ratio then i8 ad~usted durin~ on-line operatiorl to search for the ~ini-30 m~n value ~f thls loss index., The economlc i~pact of v~s)la~ng .
r-. .

E:nvironmental Protect~on Agency (EPA3 regulations on smoke emi~lons is taken lnto ~ccount by sign~f~cantly increasing ~h~
ra~e of penalizin~ ehe opaciey coMponent a~ ~t ~pproP~cheg the lEPA
l~mi~ . , Accordingly, an obJect of the present ~nvent~on ~ ~co provide a method of reducing lo~es in ~ colobus~lon operatiorl for burning ~uel with air at a load level with ~he combu~tion opera-t~on produeing flue gas having unburned by-product and oxygen ~nd being at ~ ~tack temperature, compr~sing, mea~uring a load ~ndex for the combu~tion operation which i8 propor~lonal ~ the load level thereof, measurin~ an air heating 108~ for the combustion operation ~l~ch i~ propor~on~l to ehe stack temperature, an amount of exces~ oxygen ln the flue ga6, a lo~d nde~, and a cost fac~or for air heatiTlg, measuring an unburned by product lo~ for the combustion operation which i9 proportlonal ~o an amount of unburned by-product iLn the flue gas, the ïo~d index and a co~t actor for the unburned by-product, measurlng ~ cilaracteristic lo~ for the combust~on operatlon whlch i~ proportional to a s:hara~teristic o the flue gas (e.g., opaclty), the load index and ~ co~t factor for that characterlstic ~eOgr j a firle exacted for exceedln~ set limits for that eharacteristic) 7 adding the unburned by-produce 1088 to the characteriL~tie lo~s to obtain a total fuel los~ for the operation, varyin~ ir demand to the cc~mbustion operation to obtain different value~ oiE the air heat-ln~ 10~8, the fuel 1089, and a summation of the air heating and fuel losses ~ and selecting an a~r demand point for the combustion operae~on at wh$ch the ~nmation of air heating and fuel los~efi i~ as low as pos~ible for a ~elec~ed load level. The air demand signal i8 ehen sent to and operates in con~unctlon with ~Lhe fuel portlon of the combustlon control ~ystem.
Another ob.~ ect of the invention ~3 to provide sn appar ~tu~ for reducing loP~e~ in a combu tion operaeion.

A still further object of the invention is to provide such an appara-tus which is simple in design, rugged in construc-tion, and economical to manufacture.
According to the present invention, a method of reduc-ing losses in a combustion operation for burning fuel with air at a load level with the combustion operation producing flue gas having unburned by-product, oxygen, and at a stack temper-ature, comprises: .
measuring a load index of the combustion operation which is proportional to the load level thereof:
measuring an air heating loss for the operation which is proportional to the stack temperature, an amount of excess oxygen in the flue gas, the load index, and a cost factor for air heating;
measuring an unburned by-produc-t loss for the combus-tion operation which is proportional to an amount of unburned by-product in the flue gas, the load index, and a cost factor for the unburned by-product;
measuring a characteristic loss for the combustion operation which is proportional to a characteristic of the flue gas, the load index, and a cost factor for that characteristic;
adding the unburned by-product loss to the character-istic loss to generate a total fuel loss for the combustion operation;
varying air demand to the combustion operation to obtain different values of air heating loss, fuel loss, and a summation of air heating loss plus fuel loss; and selecting an air demand point for the combustion operation at which the air heating, fuel, and summation losses are as low as possible for a selected load index.

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- 5a -The various features of novelty which charac-terize the invention are pointed out with particularity in the clairns annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advan-tages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.
BRIEF DESCRIPTION OF THE DRAWINGS
In the Drawings:
Fig. 1 is a block diagram of an apparatus for minimiz-ing loss in a combustion operation in accordance with the invention;
Fig. 2 is a graph plotting the best previous air demand against a load index for the combustion operation, Fig. 3 is a graph plotting the cost in dollars against the air demand which reflects the various losses in the combus-tion operation.
DESCRIPTION OF THE PREFERRED EMBODIMENT
One embodiment of the present invention is illustrated in Figs. 1 through 3. In this embodiment, the cost of heating the excess air is estimated by using measurements of the stack temperature from transmitter 30 and oxygen from transmitter 32 in -the flue gas. A function generator 34 and multiplier 36 converts these measurements into an effective heat value of the excess air. This value is multiplied by the boiler/heater load index provided in line 38. In this case this value is fuel demand as measured in fuel demand transmitter 40. It could also be steam flow in a boiler or product flow in a process heater.

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~ultipller 42, thus, ~ener~te~ ~ hea~ 1088 r~e, whlch is then multiplled by a R$ ~actor to convert,the 108~ rste ln~o ~he alr heatin~ lo~ per un~e elme ~n dollars, ~n a multiplier 44.
On ehe combuseible~ ~de, mea~urements ~re made ln j transmitter~ 46, 48, ~nd 50 of carbon monoxlde (CO)~ hydrocarbon3 (~C), and opacity~ The CO and HC measurement~ are multiplied by ~he load lndex and the R$ factors in mul~ipliers 52, 54, 56, ~nd S8, to generate a fuel lo~ rate per uni~ time. The opaclt7 measurement is handled in the ~ame way, excep~ that a func~ion generator 60 i8 used inste~d of a ~imple K$ multipl~ca~ion $actvr. The function generaeor sharply increase~ the effective R$ factor when the opa~i~y appr~sc~es the allowed EPA limi~ L, then ~ettles vut at ~he m~nleude of the fine when ehe limit 1~
re~ched or exceeded. All of the combu~tibles 108~ rates then ~re added together in a ~ummin~ unlt 6? and ~moothed (flltered in time) to generate A total fuel loss rate ~n dollars per uni~
~i~e. Summing uni~ 62, thu~ generaees a ~o~al of the unburned by-product lo~s and lo~s due ~o a characteri~ic of he flue ga~
(opacity) which may cause a fineO
- ~he air and fuel 1~85 ra~es are fed into the ~Lo8s In~ex Minimization Algorithm" block 64 ~hown in Fig. 1~ A ~'high opae~ty alarm" i8 8enerated when the opacity exceeds ~he EPA
l~mit by a lim~e and ~larm unit 66~ Thi~ alarm and the load lnde~ are also fed into the minimization algorithm block 64~ Air deman~ ifi 8et by ~n optlmum alr demand value provlded on llne 70 from block 64.
The operatlon of the "Los~ Index M~nlmizatlon Algorithm" block 64 is illuætrated in F1g~. 2 and 3~ The block keep~ track of ehe "best previo~s" value3 Qf ~ir demand that have been found for each value of load ~ndex (Fi~. 23. Al~o, ehe csrreæpondin~ dollar value~ of alr heating loæs, fuel 1~, and --7~

~ot~l loss ~the ~sm of ~he o~her two losses) ~re 8tored for each -- load ~ndex value (Flg. 3)v Under n4rmal operating condi~on~
~defined a~ occurring when ehe hlgh opacity alarm 1~ not ace~ve ~nd the boller~heater lo~d i~ not charlg~n~ he mlnimlzatlon 5 al~orithm then ~earche~ for the minimum value of ehe total 10~8 pn~ameter by ~djusting the ~ir demand output from ~che bl~ck. The ~lgorithm lncreases or decrea~e~ ehe alr dlemand, depending on ~he deYiaeion of the ~urrene v~lue6 of air and fuel los~es fro~ ehe corre~pondin~ ~'best previou8" vallle8 ~tored- Th~e i~, lf the 10 fuel 1088 parame~er is ne~r it~ previ ou~ "best v~lue" but tlle ~ir 108B i8 signiicantly higher~ ~che ~lgorithm will reduce the air demand. ~n the other hand ~ if ~he deviation in fuel lo~ e~ pre dominate~ compared to the previous besc value~, the algorithm . ~ill increase the air demand~ After-waiting for a period of ti~e 15 . e~ual ~o ~he time la~ of the process, the algorithm ehen measure~
the new value of the total 10~8 parameter~ If 1~ is less than the ~tored ".be~ previou~" value for the curren~ load lnde~, the new a{r demand replaces the old one as the "best previou~" value.
Al80, ~he correspondinR new loss para~eters then replace t~e old . 20 ~nes and the ~earch con~inu~ incremen~ally ln the same direc.tion ~ntll a mLnimum is found a~ ~hvwn ~t M in FlgJ 3.
m e optim~zat~on algor~thm operate~ a~ de~cr~bed only un~er "normal" ope.rating conditlons as d~fined ~bove. I~ the ..
. loaa inde~ i8 changlng, the optimization operation is ~uspended and the alr demand Oueput i ad~usted to match the "best previ-OU8~ value ~tored for the current load index. If the load inde~
i~ stable but the "high opacity~' ~larm 1~ active, ~he 108~ mlni-mizaeion oper~tion ~till coneinues, but the "bes~ previou~" alr demand and loss value~ found under these ~la~m condition~ are discarded after the alarm becot~es inactiveD Thi8 i8 done beoau~e .. _ _ _ _.... .. . . . . .............. . . _ .. . _ . .. . . .. .. . . . .. _ . .. . . ~

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the fuel lo~ parameter i8 made artlficially high during ~he~e ~larm cond~ tion6; therefore, lt~ value i~ not relevant unter nor-~al ope~ ating cond~ tions 1.
_. While a specif~ c embodimen~ o the invent:Lon has been 5 sho~n and de~cribed in detall to illllstra~e ' ~he applicatioll of the principle~ of the inven~ion~ ie will be lunderstood ~ha~ the ~nvention may be embodied otherwise without departing from ~uch principles .

Claims

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

1. A method of reducing losses in a combustion opera-tion for burning fuel with air at a load level with the combustion operation producing flue gas having unburned by-product, oxygen, and at a stack temperature, comprising:
measuring a load index of the combustion operation which is proportional to the load level thereof:
measuring an air heating loss for the operation which is proportional to the stack temperature, an amount of excess oxygen in the flue gas, the load index, and a cost factor for air heating;
measuring an unburned by-product loss for the com-bustion operation which is proportional to an amount of unburned by-product in the flue gas, the load index, and a cost factor for the unburned by-product;
measuring a characteristic loss for the combustion operation which is proportional to a characteristic of the flue gas, the load index, and a cost factor for that characteristic;
adding the unburned by-product loss to the char-acteristic loss to generate a total fuel loss for the combustion operation;
varying air demand to the combustion operation to obtain different values of air heating loss, fuel loss, and a summation of air heating loss plus fuel loss; and selecting an air demand point for the combustion operation at which the air heating, fuel, and summation losses are as low as possible for a selected load index.

2. A method according to claim 1, including storing air demand points for various load indexes and utilizing said stored air demand points as best previous air demand values.

3. A method according to claim 2, including supplying a best previous air demand point to the combustion operation, varying the air demand away from the best previous air demand point, and if reduced values for fuel loss, air heating loss, and the summation of fuel loss plus air heating losses is reduced, storing anew best previous air demand point.

4. A method according to claim 1, wherein the char-acteristic of the flue gas is opacity, the cost factor of the characteristic being a fine for exceeding a selected limit for opacity.

5. A method according to claim 1, including measuring a fuel demand for the combustion operation, the load index being proportional to the fuel demand.

6. A method according to claim 1, including generat-ing an alarm when a limit for the characteristic loss is reached,

7. A method according to claim 1, including measur-ing the stack temperature, calculating a heating value which is proportional to the stack temperature, measuring the amount of unburned oxygen in the flue gas, multiplying the amount of unburned oxygen by the heating value, and multiplying the outcome of this multiplication by the load index to obtain a load index weighted air heating measurement and multiplying the air heating measurement by the cost factor for air heating to obtain the air heating loss.

8. A method according to claim 7, wherein the unburned by-product is one of carbon monoxide and hydrocarbons, measuring the amount of unburned by-product in the flue gas, multiplying the measured amount of unburned by-product by the load index and multiplying the outcome by the cost factor for unburned by-product.

9. A method according to claim 8, including measuring the opacity of the flue gas which opacity comprises the char-acteristic, the cost factor of the characteristic increasing to a fine for reaching a selected value of opacity as the selected value for opacity is approached.

10. A method according to claim 9, including smoothing the fuel loss using a smoothing filter to reduce irregularities in a change for the fuel cost over time.

11. An apparatus for reducing losses in a combustion operation for burning fuel with air at a low level with the com-bustion operation producing flue gas having unburned by-product, oxygen and at a selected stack temperature, comprising:
a temperature transmitter for measuring the stack temperature;
an oxygen sensor for sensing unburned oxygen in the flue gas;
at least one unburned by-product sensor for sens-ing an amount of unburned by-product in the flue gas;
an opacity sensor for sensing the opacity of the flue gas;
means for establishing a load level for the com-bustion operation which is proportional to a load index thereof;
a first multiplier connected to the temperature transmitter and oxygen sensor for multiplying the values genera-ted thereby together;
a second multiplier connected between said means and an output of said first multiplier;
a first cost factor unit connected to an output of said second multiplier for generating an air heating loss value;

a third multiplier connected between said means and said at least one unburned by-product sensor;
a second cost factor unit connected to an output of said third multiplier for generating a quantity proportional to an unburned by-product loss for the combustion operation;
a function generator connected to said opacity sensor for multiplying an amount of opacity sensed by said opacity sensor by an amount which increases to a fine that is exacted for reaching a limit in opacity; a fourth multiplier con-nected to an output of said function generator and to said means for generating an opacity loss quantity;
a summing unit connected to an output of said second cost factor unit and said fourth multiplier for generating a total fuel loss for the combustion operation; and a loss index minimizing unit connected to an out-put of said summing unit, an output of said first cost factor unit and to said means for generating an air demand signal at which the fuel loss, the air heating loss, and a summation of the fuel loss plus air heating loss are minimized.