CN109519960B - Pulverized coal furnace combustion regulation and control method based on-line monitoring of oxygen content and carbon content in fly ash - Google Patents

Pulverized coal furnace combustion regulation and control method based on-line monitoring of oxygen content and carbon content in fly ash Download PDF

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CN109519960B
CN109519960B CN201811252231.6A CN201811252231A CN109519960B CN 109519960 B CN109519960 B CN 109519960B CN 201811252231 A CN201811252231 A CN 201811252231A CN 109519960 B CN109519960 B CN 109519960B
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CN109519960A (en
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王金铭
解海龙
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Northeast Electric Power University
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Northeast Dianli University
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23N—REGULATING OR CONTROLLING COMBUSTION
    • F23N3/00—Regulating air supply or draught
    • F23N3/002—Regulating air supply or draught using electronic means
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23N—REGULATING OR CONTROLLING COMBUSTION
    • F23N5/00—Systems for controlling combustion
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23N—REGULATING OR CONTROLLING COMBUSTION
    • F23N2223/00—Signal processing; Details thereof
    • F23N2223/10—Correlation
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23N—REGULATING OR CONTROLLING COMBUSTION
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Abstract

The invention relates to a regulation and control method for quasi-optimal combustion of large-scale pulverized coal furnaces, which is characterized in that heat loss is divided into operation adjustable quantity and non-adjustable quantity, an optimization target which can be regulated and controlled and reached is defined more reasonably, judgment on combustion working conditions can be realized through easily realized measurement of oxygen content and temperature and online monitoring of fly ash carbon content, combustion is regulated and organized according to the judgment, so that a boiler can quickly reach and maintain a quasi-optimal operation state when coal types and loads change, the method is simple, convenient and easy to implement, and the method has important significance for energy conservation and emission reduction work of the large-scale pulverized coal furnaces and improvement of flexibility of a thermal power generating unit for adapting to large-scale load fluctuation.

Description

Pulverized coal furnace combustion regulation and control method based on-line monitoring of oxygen content and carbon content in fly ash
Technical Field
The invention relates to a combustion regulation and control method for a large-scale pulverized coal furnace, in particular to a pulverized coal furnace combustion regulation and control method based on-line monitoring of oxygen content and fly ash carbon content.
Background
The large-scale pulverized coal furnace is a key device for providing heat energy in the thermal power generation process, and the combustion regulation and control mainly aims at fully combusting pulverized coal and minimizing heat loss, so that the utilization rate of coal is improved, and the aim of saving energy is fulfilled; in addition, the pulverized coal is ensured to be fully combusted, and meanwhile, the combustion-supporting air quantity is reduced as much as possible through regulation and control, so that the emission of nitrogen oxides (NOx) can be effectively reduced, and the method has important significance for environmental protection.
Particularly, in recent years, due to the influx of a large amount of clean energy in the energy field, thermal power generation is required to frequently and rapidly adjust the load to a large extent so as to adapt to the natural property of the clean energy. Therefore, under the condition of frequent load change, how to enable the boiler to be always in the optimal working condition through timely and effective combustion adjustment is a major engineering subject directly related to energy conservation and emission reduction.
In the boiler principle teaching material of southeast university of south-east university of south-of-the-province vibration published in 1986 by Beijing water conservancy and electric power publishing house, an expression for judging the index of the optimal working condition of the boiler, namely the thermal efficiency, is defined as follows:
Figure GDA0002301716770000011
＝100-(q2+q3+q4+q5+q6) (expression of counter balance method) (2)
The method is more specific to various heat losses by an inverse balance method, so that the method is generally adopted in engineering .
Figure GDA0002301716770000012
Wherein:
Qr: the input heat brought into the boiler by each kilogram of fuel can be similar to the low-level heating value of the coal application base
Figure GDA0002301716770000013
kJ/kg；
Q1: each kilogram of fuel boiler effectively utilizes heat, and kJ/kg can be calculated through the enthalpy values of an inlet and an outlet of a boiler working medium, steam flow and coal burning quantity;
q1＝(Q1/Qr) X 100: the boiler effectively utilizes the percentage of heat in the input heat;
q2＝(Q2/Qr) X 100: percentage of boiler exhaust heat loss to input heat, Q2The heat loss (kJ/kg) of the exhaust smoke of each kilogram of fuel can be calculated according to theoretical air quantity, combustion products and specific heat thereof, coal combustion quantity, exhaust smoke temperature and excess air coefficient;
q3＝(Q3/Qr) X 100: percentage of heat loss from chemical incomplete combustion of boiler to input heat, Q3Calculating the chemical incomplete combustion heat loss (kJ/kg) of each kilogram of fuel according to the content of combustible gas and the coal combustion amount in the measured smoke;
q4＝(Q4/Qr) X 100: percentage of heat loss due to incomplete combustion of boiler machinery in input heat, Q4For the incomplete combustion heat loss (kJ/kg) of each kilogram of fuel machinery, calculating according to the measured solid combustible content (fly ash and ash carbon content) in combustion products (smoke and ash) and coal ash;
q5＝(Q5/Qr) X 100: the percentage of the heat loss of the boiler to the input heat is related to the structure of the boiler, the heat preservation condition and the coal burning quantity;
q6＝(Q6/Qr) X 100: other heat losses from the boiler are a percentage of the heat input,such as physical heat loss from ash entrainment, etc.
It can be seen from the above heat efficiency calculation formula that if the boiler is subjected to combustion optimization regulation and control in real time by means of the index, accurate, reliable and continuous online monitoring on the weight of the coal as fired, the application base calorific value, the components of the coal as fired, the content of gas and solid combustible substances in the flue gas and the like is required, and points are required, which are difficult to achieve in engineering .
Disclosure of Invention
The invention provides a coal powder furnace combustion quasi-optimal working condition judgment index and a quasi-optimal regulation and control method based on-line monitoring of the oxygen content of smoke and the carbon content of fly ash aiming at the problems in the prior art, the heat loss is divided into operation adjustable quantity and non-adjustable quantity, the optimization target which can be regulated and controlled and reached is more reasonably defined, the judgment of the combustion working condition can be realized through the easily realized measurement of the oxygen content and the temperature and the on-line monitoring of the carbon content of the fly ash, and the combustion is regulated and organized according to the oxygen content and the temperature, so that the boiler can quickly reach and maintain the quasi-optimal operation state when the coal type and the load change.
The technical scheme adopted for realizing the invention is as follows: the invention discloses a pulverized coal furnace combustion regulation and control method based on-line monitoring of oxygen content and fly ash carbon content, and a technical scheme of the invention is derived from various heat losses related in a formula (2).
1) Simplification of heat losses in terms of thermal efficiency expression:
firstly, dividing various heat losses into online adjustable heat losses, online unadjustable heat losses and combined similar heat losses, simplifying the judgment of the optimal working condition of the boiler, and obtaining the following results:
q5、q6or Q5、Q6The heat loss proportion is small and can not be obviously improved through combustion adjustment, belongs to online unadjustable heat loss, and is used as a combustion quasi-optimal working condition judgment index to be omitted from consideration;
q3、q4or Q3、Q4The heat loss is the incomplete combustion loss, which occurs for the same reason, i.e., oxygen deficiency, insufficient temperature, and insufficient burn-up time, so if q is equal to q4Loss to the optimum state, q3This should generally also be achieved; and the gas combustible is easier to burn out than the solid combustible, the loss ratio is small, therefore, the available q is4Losses simultaneously representing q3Loss is avoided, so that the loss does not appear in the quasi-optimal combustion condition judgment index, and the difficulty brought by on-line flue gas analysis is avoided;
q2or Q2The heat loss can be actually divided into two parts, wherein part is the heat quantity taken away by the theoretical combustion products along with the smoke gas determined by the combustion reaction equation, the calculation relates to the components of the combustion products, the components are complex and are necessary products of the combustion reaction and can not be adjusted, part is the heat quantity taken away by the smoke gas due to the excess air supply, the part needing to be adjusted and reduced in combustion is which is the target of combustion optimization and is defined as adjustable smoke heat loss, and the heat loss can be used
Figure GDA0002301716770000032
The unit is kJ/kg, namely the part which can be adjusted and utilized in the heat loss of the exhaust smoke of each kilogram of fuel; in addition, the air leakage heat loss in the exhaust heat loss does not belong to the combustion adjustable part, and is not considered here;
after the above simplification is made, the formula (3) becomes the following form:
Figure GDA0002301716770000031
2) solving an optimization problem of minimizing the sum of the adjustable smoke exhaust heat loss and the mechanical incomplete combustion heat loss, and considering the influence of chemical unburned heat loss:
according to boiler theory, both heat losses are related, within the limits that are adjustable in real time, to the amount of air fed to the boiler combustion, generally indicated by the "excess air ratio" α, defined as:
α＝V/V0(5)
wherein:
v amount of air actually supplied to the boiler per kg of fuel, Nm3/kg；
V0Theoretical amount of air per kg fuel, Nm3The/kg can be converted by a combustion reaction equation according to the components of coal and air entering the furnace,
in order to provide sufficient oxygen to promote the most complete combustion of the coal, V > V0α is more than 1, α volume content O of oxygen in the smoke gas measured on line by an oxygen meter2(%) was calculated by the following formula:
Figure GDA0002301716770000041
then equation (4) is written as:
according to the optimization theory, the minimum value
Figure GDA0002301716770000046
This may be achieved by solving the following equation to obtain an optimum excess air factor α for minimal heat loss2，4：
Figure GDA0002301716770000043
The goal of combustion optimization control can therefore be summarized as finding
Figure GDA0002301716770000044
Q4(α) and hold(7) The formula holds: according to the definition of the adjustable smoke exhaust heat loss, the calculation formula is expressed as follows:
wherein:
Figure GDA0002301716770000051
average constant pressure-volume specific heat of air, kJ/Nm3The temperature can be detected according to physical parameters of air;
t2: exhaust gas temperature, DEG C;
t1: the temperature of the air when entering the furnace is the ambient temperature;
order:
Figure GDA0002301716770000052
then (8) has the following form:
Figure GDA0002301716770000053
when the coal type, the exhaust gas temperature and the ambient temperature are , the design values are obtained for simplifying the calculation, A can be regarded as a constant, and the physical meaning of the constant is the heat absorbed by the theoretical air quantity in the furnace as shown in the formula (9),
Figure GDA0002301716770000056
the Q- α coordinate is a straight line 1 with increasing, namely the adjustable smoke exhaust heat loss "
Figure GDA0002301716770000057
In direct proportion to the excess air ratio α, there are:
Figure GDA0002301716770000054
heat loss Q due to incomplete combustion of machinery4Can be expressed as:
Figure GDA0002301716770000055
wherein:
afh、alz: the proportion of fly ash and slag in the total ash content, afh、alz＝1；
AyCoal application base ash, coal quality is constant at regular time;
Cfh、Clz: carbon content of fly ash and slag, Cfhα, it can be measured by the fly ash carbon content on-line monitoring device in real time;
in the above formula, in order to avoid the trouble of on-line monitoring the carbon content of the ash, the carbon content of the ash is approximately replaced by the carbon content of the fly ash, namely Cfh＝ClzBecause the ash and slag share of the large-capacity pulverized coal furnace is very small, great errors cannot be brought;
from the formula (12), fly ash carbon content CfhIs a function of α, i.e. Q4The relationship is also α function, but the relationship is relatively complex, even if the relationship of boilers has large change under different coal types and different loads, the relationship cannot be expressed by simple formulas, but the change rule can be qualitatively expressed from the following discussion;
according to the combustion theory, when the excess air ratio is small, the opportunity for contact of oxygen with fuel is reduced as the combustion condition, so that more unburned particles are produced, and Q is obtained4Larger, with increasing α, Q4Gradually decreased and maintained in a smaller range, and when α is large enough, it will destroy another conditions of combustion, i.e. at higher excess air temperature, combustion will be delayed and unburned particles will be increased, causing Q4Again increasing with α, and thus Q4Curve 2, which is a concave, single valley, small curve according to the variation law of α, will change with the changes of boiler load, coal type, etc., but the shape or variation law will not change, and is expressed by the formula (12), and the derivative of the curve is:
Figure GDA0002301716770000061
because of the fact that
Figure GDA0002301716770000062
Is a line of , then
Figure GDA0002301716770000068
Curve 3, also , having concave, single valley, small curvature features;
substituting equations (11) and (13) into equation (7), the conditions for obtaining the quasi-optimal combustion condition are as follows:
Figure GDA0002301716770000063
or
Figure GDA0002301716770000064
Defining:
Figure GDA0002301716770000065
Figure GDA0002301716770000066
Ektthe method is defined as a new 'quasi-optimal working condition judgment index' of the large-scale pulverized coal furnace, and when:
Figure GDA0002301716770000067
when the formula (7) is satisfied, the boiler combustion is in a quasi-optimal working condition;
due to CfhThe relationship with α is uncertain, and the actual calculation will substitute for the limited real-time monitoring values, and the expression (16) can be approximately expressed as:
Figure GDA0002301716770000071
definition of
Figure GDA0002301716770000076
Is an average quasi-optimal working condition judgment index
Figure GDA0002301716770000077
Meanwhile, the boiler combustion is in an approximate quasi-optimal working condition;
(16) or (19) when load and coal type are timed, the denominator B is a fixed number greater than 0, or can be calculated by approximate design value, and during operation, the air supply amount is adjusted, i.e. α is changed, and for CfhOn-line monitoring, taking α, CfhThe monitoring value is substituted into the formula (19) to obtainAccording to
Figure GDA0002301716770000078
Judging whether the combustion condition is good or bad according to the deviation degree of 1;
if it is
Figure GDA0002301716770000073
The optimum direction is the direction to decrease α ifThe optimization direction is the direction of increasing α, the optimization algorithm can be selected to optimize, and equation (18) holds, so that the boiler is in the quasi-optimal combustion condition, i.e. the condition represented by point 4 in curve 3 of FIG. 1, and α is in this case2，4For "optimum air excess factor", in particular whenWhen, namely:
Figure GDA0002301716770000075
description of Q4Reaching the lowest value, the condition shown by point 5 in curve 2 of figure 1, the boiler combustion condition is close to the quasi-optimal condition, which is only based on the carbon content in fly ash to monitor on-line during combustion regulationThe result of the measurement can be distinguished as an important mark.
Considering that oxygen-lean combustion will be advantageous for reducing Nitrogen Oxides (NO)x) Can reduce the denitration cost, and E should be controlledktAnd the variation is within small range which is larger than 1 (determined by economic and technical comparison).
The invention provides a quasi-optimal combustion condition judgment index and a control method of a large-scale pulverized coal furnace based on-line monitoring of the oxygen content of flue gas and the carbon content of fly ash. Although the specific amount of energy saving cannot be exactly specified, the maintenance of the quasi-optimal combustion working condition indicates that the boiler always works in the most energy-saving state, and the effect is necessarily reflected in cost accounting.
The pulverized coal furnace combustion regulation and control method based on the on-line monitoring of the oxygen content and the carbon content of the fly ash has the beneficial effects that:
1. meanwhile, the difficulty that series operation parameters such as coal burning quantity, steam flow, fuel heating quantity, smoke components and the like need to be monitored in real time and all heat losses are calculated when the combustion optimization control is guided by a thermal efficiency method is avoided, the judgment on the combustion working condition can be realized only by easily realizing the measurement of the oxygen content and the temperature of the smoke in the current engineering and the online monitoring of the carbon content of the fly ash, and the combustion is regulated and organized according to the operation parameters, so that the boiler can quickly reach and maintain the quasi-optimal operation state when the coal type and the load change, and the method is simple, convenient and easy to implement;
2. the pulverized coal furnace combustion regulation and control method based on the on-line monitoring of the oxygen content and the carbon content of the fly ash has important significance for energy conservation and emission reduction of large pulverized coal furnaces and improvement of the flexibility of the thermal power generating unit to adapt to large-scale fluctuation of load.
Drawings
FIG. 1 is a qualitative schematic diagram of the relationship between boiler heat loss Q and excess air factor α as a function of a pulverized coal furnace combustion regulation method based on-line monitoring of oxygen content and fly ash carbon content;
in the figure: adjustable smoke exhaust heat loss "
Figure GDA0002301716770000086
Curve, 2. mechanical unburned heat loss Q4Curve, 3.
Figure GDA0002301716770000081
Curve, 4.
Figure GDA0002301716770000082
Minimum point of heat loss, 5.Q4And heat loss is a minimum value point.
Detailed Description
The present invention is further illustrated in conjunction with FIG. 1 and the embodiments described herein, which are meant to be exemplary only and not limiting.
For example, the coal type is flat-topped mountain bituminous coal and the pulverized coal furnace is under any load (other coal types can be used, such as 'mixed combustion', but the coal-fired component is relatively stable in periods of time), the online monitoring of the oxygen content of the flue gas adopts a 'GD 36-800 zirconia oxygen analyzer' produced by Shanghai precision instruments and meters Limited, the online monitoring of the carbon content of the fly ash adopts a 'BAC-A type burning fly ash carbon measuring system' produced by Kaiyuan scientific and technology Limited of northeast China institute of Electrical service in Jilin, the temperature measurement can be thermocouples optionally, the cold air and the flue gas temperature can be obtained by actual measurement, the cold air temperature t is set in the embodiment1At 25 deg.C and exhaust gas temperature t2The average constant pressure-volume specific heat is obtained by checking the air property at 140 DEG C
Figure GDA0002301716770000083
kJ/Nm3.℃。
The coal application base composition is shown in the following table:
Figure GDA0002301716770000084
solving the following steps: ekt1-hour boiler combustion quasi-optimal operating condition and optimal excess air ratio α2，4(attached withFIG. 4 dot display)
The on-line monitoring device ensures that a certain load of the boiler and the coal type for burning are relatively stable, the central position of flame and the fineness of coal powder are adjusted to the optimal values, the on-line monitoring device for the oxygen content at the outlet of a hearth, the exhaust gas temperature, the environmental temperature and the carbon content in fly ash of the boiler works normally, and the mechanism for adjusting the air supply quantity is sensitive.
1. Solving the denominator B in the formula (19)
The theoretical air quantity V can be calculated from the coal-fired components according to the boiler theory0＝6.1Nm3Kg, mixing V0、
Figure GDA0002301716770000085
Ay、t1And t2Substituting into formula (17) to obtain:
2. determining approximate quasi-optimal working condition of boiler combustion under current working condition
1) Judging the current combustion condition of the boiler
Measuring the oxygen content O of the current smoke2Substituting into equation (6) to obtain the excess air coefficient α(0)Is an initial point; measuring the carbon content C of the current fly ashfh(α(0)) Let Δ α > 0 (a sufficiently small empirical value), precision L > 0, adjust blower mechanism to α(1)＝α(0)+ Δ α, determination Cfh(α(1)) Δ α, Cfh＝Cfh(α(0))，ΔCfh＝Cfh(α(1))-Cfh(α(0)) Calculated by substituting equation (19)
Figure GDA0002301716770000096
If:
Figure GDA0002301716770000092
2) at α(i)Setting the step length h- Δ α (or h-h, i ≠ 0) as the starting point, adjusting the blowing mechanism to α(i+1)＝α(i)+ h, determination of Cfh(α(i+1)) Changing Δ α to α(i+1)-α(i)，Cfh＝Cfh(α(i))，ΔCfh＝Cfh(α(i+1))-Cfh(α(i)) Where i is 0, 1, 2, and 3 … …, and the calculation is performed by sequentially substituting the formula (19)
Figure GDA0002301716770000093
When:
Figure GDA0002301716770000094
3) at α(i)Setting the step length h as Δ α (or h as h, i not equal to 0) as the starting point, adjusting the blower mechanism to α(i+1)＝α(i)+ h, determination of Cfh(α(i+1)) Changing Δ α to α(i+1)-α(i)，Cfh＝Cfh(α(i))，ΔCfh＝Cfh(α(i+1))-Cfh(α(i)) Where i is 0, 1, 2, and 3 … …, and the calculation is performed by sequentially substituting the formula (19)
Figure GDA0002301716770000095
When:
Figure GDA0002301716770000101
obtained by solution
Figure GDA0002301716770000104
To "quasi-optimal excess air ratio" (see the figure), at which time
Figure GDA0002301716770000102
Namely:
Figure GDA0002301716770000103
at this time, the boiler combustion is in an approximate 'quasi' optimal condition.
3. When in operationWhen the state changes (e.g. load, coal type … …), EktAnd (4) repeating the steps until the value is not equal to 1, and determining and keeping the combustion approximate to the quasi-optimal working condition of the new boiler.
Through the embodiment, the judgment and regulation of the optimal combustion condition of the pulverized coal furnace based on the on-line monitoring of the oxygen content of the flue gas and the carbon content of the fly ash are realized.
The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and improvements can be made without departing from the principle of the present invention, and these should be considered as the protection scope of the present invention.

Claims (1)

1, method for regulating and controlling combustion of pulverized coal furnace based on-line monitoring of oxygen content and carbon content in fly ash, comprising the steps of:
1) classic thermal efficiency method for judging optimal working condition of boiler
The thermal efficiency is defined as:
Figure FDA0002276240940000011
because the counter-balance method is more targeted to various heat losses, the problem to be solved by combustion optimization is that the thermal efficiency of the boiler is always maximized under different working conditions by combustion adjustment, namely:
Figure FDA0002276240940000012
wherein:
Qr: the input heat brought into the boiler by each kilogram of fuel can be similar to the low-level heating value of the coal application basekJ/kg；
Q1: each kilogram of fuel boiler effectively utilizes heat, and kJ/kg can be calculated through the enthalpy values of an inlet and an outlet of a boiler working medium, steam flow and coal burning quantity;
q1＝(Q1/Qr) X 100: the boiler effectively utilizes the percentage of heat in the input heat;
q2＝(Q2/Qr) X 100: percentage of boiler exhaust heat loss to input heat, Q2The unit is kJ/kg for heat loss of exhaust gas of each kilogram of fuel, and the heat loss is calculated according to theoretical air quantity, combustion products and specific heat thereof, coal combustion quantity, exhaust gas temperature and excess air coefficient;
q3＝(Q3/Qr) X 100: percentage of heat loss from chemical incomplete combustion of boiler to input heat, Q3The chemical incomplete combustion heat loss of each kilogram of fuel is kJ/kg, and the unit is calculated according to the content of combustible gas and the coal combustion amount in the measured smoke;
q4＝(Q4/Qr) X 100: percentage of heat loss due to incomplete combustion of boiler machinery in input heat, Q4The unit is kJ/kg for incomplete combustion heat loss of each kilogram of fuel machinery, and the carbon content of fly ash and ash slag and the ash content of coal are calculated according to the measured solid combustible content in combustion product smoke and ash slag;
q5＝(Q5/Qr) X 100: the percentage of the heat loss of the boiler to the input heat is related to the structure of the boiler, the heat preservation condition and the coal burning quantity;
q6＝(Q6/Qr) X 100: other heat losses of the boiler account for the percentage of input heat, such as physical heat loss carried away by ash;
the invention is characterized in that the classic thermal efficiency method is simplified, thereby leading out pulverized coal furnace combustion regulation and control methods based on-line monitoring of oxygen content and fly ash carbon content;
2) simplification of heat losses in terms of thermal efficiency expression:
firstly, dividing various heat losses into online adjustable heat losses, online unadjustable heat losses and combined similar heat losses, simplifying the judgment of the optimal working condition of the boiler, and obtaining the following results:
q5、q6or Q5、Q6Heat lossThe occupation ratio is small and can not be obviously improved through combustion adjustment, and the method belongs to online unadjustable heat loss and is used as a combustion quasi-optimal working condition judgment index to omit the combustion quasi-optimal working condition judgment index from consideration;
q3、q4or Q3、Q4The heat loss is the incomplete combustion loss, which occurs for the same reason, i.e., oxygen deficiency, insufficient temperature, and insufficient burn-up time, so if q is equal to q4Loss to the optimum state, q3This should generally also be achieved; and the gas combustible is easier to burn out than the solid combustible, the loss ratio is small, therefore, the available q is4Losses simultaneously representing q3Loss is avoided, so that the loss does not appear in the quasi-optimal combustion condition judgment index, and the difficulty brought by on-line flue gas analysis is avoided;
q2or Q2The heat loss can be actually divided into two parts, wherein part is the heat quantity taken away by the theoretical combustion products along with the smoke gas determined by the combustion reaction equation, the calculation relates to the components of the combustion products, the components are complex and are necessary products of the combustion reaction and can not be adjusted, part is the heat quantity taken away by the smoke gas due to the excess air supply, the part needing to be adjusted and reduced in combustion is which is the target of combustion optimization and is defined as adjustable smoke heat loss, and the heat loss can be used
Figure FDA0002276240940000021
The unit is kJ/kg, namely the part which can be adjusted and utilized in the heat loss of the exhaust smoke of each kilogram of fuel; in addition, the air leakage heat loss in the exhaust heat loss does not belong to the combustion adjustable part, and is not considered here;
after the above simplification is made, the formula (3) becomes the following form:
3) solving an optimization problem of minimizing the sum of the adjustable smoke exhaust heat loss and the mechanical incomplete combustion heat loss, and considering the influence of chemical unburned heat loss:
according to boiler theory, both heat losses are related, within the limits that are adjustable in real time, to the amount of air fed to the boiler combustion, generally indicated by the "excess air ratio" α, defined as:
α＝V/V0(5)
wherein:
v amount of air actually supplied to the boiler per kg of fuel, Nm3/kg；
V0Theoretical amount of air per kg fuel, Nm3The/kg can be converted by a combustion reaction equation according to the components of coal and air entering the furnace,
in order to provide sufficient oxygen to promote the most complete combustion of the coal, V > V0α & gt 1, α volume content o of oxygen in smoke measured on line by an oxygen meter2(%) was calculated by the following formula:
Figure FDA0002276240940000032
equation (4) is written as:
Figure FDA0002276240940000033
according to the optimization theory, the minimum value
Figure FDA0002276240940000034
This may be achieved by solving the following equation to obtain an optimum excess air factor α for minimal heat loss2，4：
The goal of combustion optimization control can therefore be summarized as findingQ4(α) and maintaining equation (7) true, which is expressed as the formula according to the definition of "tunable exhaust heat loss":
Figure FDA0002276240940000037
wherein:
Figure FDA0002276240940000041
average constant pressure-volume specific heat of air, kJ/Nm3The temperature can be detected according to physical parameters of air;
t2: exhaust gas temperature, DEG C;
t1: the temperature of the air when entering the furnace is the ambient temperature;
order:
Figure FDA0002276240940000042
then equation (8) has the following form:
Figure FDA0002276240940000043
when the coal type, the exhaust gas temperature and the ambient temperature are , the design values are obtained for simplifying the calculation, A can be regarded as a constant, and the physical meaning of the constant is the heat absorbed by the theoretical air quantity in the furnace as can be seen from the formula (9),the Q- α coordinate is a straight line 1 with increasing, namely the adjustable smoke exhaust heat loss "
Figure FDA0002276240940000045
In direct proportion to the excess air ratio α, there are:
Figure FDA0002276240940000046
heat loss Q due to incomplete combustion of machinery4Can be expressed as:
Figure FDA0002276240940000047
wherein:
afh、alz: the proportion of fly ash and slag in the total ash content, afh+alz＝1；
AyCoal application base ash, coal quality is constant at regular time;
Cfh、Clz: carbon content of fly ash and slag, Cfhα, it can be measured by the fly ash carbon content on-line monitoring device in real time;
in the above formula, in order to avoid the trouble of on-line monitoring the carbon content of the ash, the carbon content of the ash is approximately replaced by the carbon content of the fly ash, namely Cfh＝C1zBecause the ash and slag share of the large-capacity pulverized coal furnace is very small, great errors cannot be brought;
from the formula (12), carbon content in fly ash CfhIs a function of α, i.e. Q4The relationship is also α function, but the relationship is relatively complex, even if the relationship of boilers has large change under different coal types and different loads, the relationship cannot be expressed by simple formulas, but the change rule can be qualitatively expressed from the following discussion;
according to the combustion theory, when the excess air ratio is small, the opportunity for contact of oxygen with fuel is reduced as the combustion condition, so that more unburned particles are produced, and Q is obtained4Larger, with increasing α, Q4Gradually decreased and maintained in a smaller range, and when α is large enough, it will destroy another conditions of combustion, i.e. at higher excess air temperature, combustion will be delayed and unburned particles will be increased, causing Q4Again increasing with α, and thus Q4Curve 2, which is a curve with dips, single valleys and small curves according to the variation rule of α, the curve 2 will change according to the changes of boiler load, coal type and the like, but the shape or variation rule will not change, and the derivative of the curve 2 is represented by equation (12):
Figure FDA0002276240940000051
because of the fact that
Figure FDA0002276240940000052
Is a straight line 1, then
Figure FDA0002276240940000053
Curve 3, also , having concave, single valley, small curvature features;
taking equations (11) and (13) into equation (7), the conditions for obtaining the quasi-optimal combustion condition are:
Figure FDA0002276240940000054
or
Figure FDA0002276240940000055
Defining:
Figure FDA0002276240940000057
Ektthe method is defined as a new 'quasi-optimal working condition judgment index' of the large-scale pulverized coal furnace, and when:
when the formula (7) is satisfied, the boiler combustion is in a quasi-optimal working condition;
due to CfhThe relationship with α is uncertain, and the actual calculation will substitute for the limited real-time monitoring values, and equation (16) can be expressed approximately as:
Figure FDA0002276240940000061
definition of
Figure FDA0002276240940000063
Is an average quasi-optimal working condition judgment index
Figure FDA0002276240940000064
Meanwhile, the boiler combustion is in a quasi-optimal working condition;
in the equation (16) or (19), when the load and the coal type are timed, the denominator B is a fixed number which is and is greater than 0, and the design value can be approximately calculated, the air supply quantity is adjusted during the operation, namely α is changed, and C is treatedfhOn-line monitoring, taking α, CfhThe monitoring value is calculated by substituting formula (19)
Figure FDA0002276240940000065
According toJudging whether the combustion condition is good or bad according to the deviation degree of 1;
if it is
Figure FDA0002276240940000067
The optimum direction is the direction to decrease α if
Figure FDA0002276240940000068
The optimization direction is the direction leading to increase of α, and the optimization algorithm can be selected to make the formula (18) hold, so that the boiler is in the quasi-optimal combustion condition, i.e. the condition represented by the lowest point in the curve 3, at this time, α2，4To "optimum air excess factor"; when in use
Figure FDA0002276240940000069
When, namely:
Figure FDA0002276240940000062
description of Q4The working condition represented by the lowest point in the curve 2 is reached, and the combustion working condition of the boiler is connectedThe method is close to the quasi-optimal working condition, and is an important mark which can be distinguished only by the online monitoring result of the carbon content of the fly ash during combustion regulation.
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