CN114880919B - Method for calculating optimal internal and external desulfurization proportion of circulating fluidized bed unit - Google Patents

Abstract

The invention discloses a method for calculating the optimal in-furnace desulfurization ratio of a circulating fluidized bed unit. First, a comprehensive cost model of in-furnace desulfurization of a CFB unit is established, and load, coal quality, coal feed amount, total air volume, bed temperature, limestone conveying fan rated power, slurry circulation pump rated power, limestone feed flow rate in the furnace, ammonia injection amount, raw flue gas _SO2 concentration, and net flue gas _SO2 concentration are selected as input variables; when establishing the comprehensive cost model, a relationship between the in-furnace desulfurization efficiency and the load, calcium-sulfur molar ratio, bed temperature, and air-coal ratio is determined; then, the SO2 generation concentration is determined by using the input of the comprehensive cost model selected in step 1, and after assuming the in-furnace desulfurization ratio, a comprehensive cost model of in-furnace desulfurization of the CFB unit under typical load conditions is established; finally, an intelligent optimization algorithm is used to solve the optimal in-furnace desulfurization ratio under typical load conditions, and the optimal in-furnace desulfurization ratio under various load conditions is obtained by fitting.

Description

A method for calculating the optimal ratio of internal and external desulfurization in circulating fluidized bed units

Technical Field

The invention belongs to the field of pollutant control optimization of thermal power units and relates to a method for calculating an optimal desulfurization ratio inside and outside the furnace of a circulating fluidized bed unit.

Background technique

Circulating fluidized bed (CFB) units have the advantages of strong combustion stability and low pollutant treatment costs. They can achieve low emission standards through in-furnace desulfurization, and the process flow is simple. As China has proposed ultra-low emission standards for pollutant emissions from coal-fired power generation units, in order to achieve ultra-low emissions, circulating fluidized bed units are equipped with off-furnace desulfurization (flue gas desulfurization) equipment, and ultra-low emissions are achieved through a combination of in-furnace and off-furnace desulfurization. The ultra-low emission standard for SO ₂ is ≤35mg/m ^3. Currently, there is little research on the optimal in-furnace desulfurization ratio for CFB units, and there is a lack of guidance for on-site operation. On-site operators often adjust the calcium-sulfur ratio of in-furnace desulfurization to ensure that the SO ₂ concentration of the original flue gas at the furnace outlet is within a certain fixed range, ensuring that after off-furnace desulfurization, the SO ₂ emission concentration is less than 35mg/m ³ . Although this operation mode is simple, it does not take into account the changing patterns of the in-furnace/out-of-furnace desulfurization efficiency and operating costs under different load conditions of the unit, which increases the desulfurization material consumption of the CFB unit, increases the desulfurization cost, and reduces the operating economy of the unit.

Purpose of the Invention

The purpose of the present invention is to solve the problems of poor distribution of the desulfurization ratio inside and outside the furnace and poor desulfurization operation economy of the CFB unit in the prior art, and provide a method for calculating the optimal desulfurization ratio inside and outside the furnace of a circulating fluidized bed unit. On the basis of the comprehensive cost model of the desulfurization inside and outside the furnace of the CFB unit, a genetic algorithm is used to solve the desulfurization ratio inside and outside the furnace, which is conducive to solving the problems such as increased desulfurization cost caused by the poor desulfurization ratio inside and outside the furnace under various load conditions, thereby reducing the operating cost of the desulfurization system of the CFB unit.

Summary of the invention

The present invention provides a method for calculating an optimal desulfurization ratio of inside and outside the furnace of a circulating fluidized bed unit, comprising the following steps:

Step 1, establish a comprehensive cost model for desulfurization inside and outside the furnace of the CFB unit, and select load, coal quality, coal feed rate, total air volume, bed temperature, limestone conveying fan rated power, slurry circulation pump rated power, limestone feed flow rate in the furnace, ammonia injection amount, raw flue gas _SO2 concentration, and net flue gas _SO2 concentration as input variables of the comprehensive cost model;

Step 2: when establishing the comprehensive cost model, determine the relationship between the desulfurization efficiency in the furnace and the load, calcium-sulfur molar ratio, bed temperature, and air-coal ratio;

Step 3: Using the input of the comprehensive cost model selected in step 1, determine the _SO2 generation concentration, assume the desulfurization ratio in the furnace, and establish a comprehensive cost model for desulfurization inside and outside the furnace under typical load conditions of the CFB unit;

Step 4: Use the intelligent optimization algorithm to solve the optimal in-furnace desulfurization ratio under typical load conditions, and fit the optimal in-furnace desulfurization ratio under each load condition.

Preferably, the process of establishing the comprehensive cost model of the internal and external desulfurization of the CFB unit in step 1 is: the total cost of the combined internal and external desulfurization is defined as including: equipment power consumption cost, furnace denitrification cost, heat loss cost, and limestone usage cost, and the comprehensive cost is the total desulfurization cost minus the income generated by gypsum; wherein the limestone usage cost includes the limestone usage cost of the furnace desulfurization and the limestone usage cost of the external desulfurization; the equipment power consumption cost includes the power consumption of the limestone conveying fan in the furnace, The power consumption of the slurry circulation pump outside the furnace; the cost of denitration in the furnace increases with the increase of the calcium-sulfur molar ratio of desulfurization in the furnace; when the desulfurization efficiency in the furnace remains unchanged, the lower the calcium-sulfur molar ratio, the lower the heat loss cost, and the lower the converted coal consumption; the amount of limestone used for desulfurization in the furnace is determined by the calcium-sulfur molar ratio of desulfurization in the furnace, the desulfurization efficiency in the furnace, and the coal quality. The desulfurization efficiency in the furnace is related to the load, calcium-sulfur molar ratio, bed temperature, and air-coal ratio; the amount of limestone used for desulfurization outside the furnace is related to the desulfurization efficiency outside the furnace;

The desulfurization efficiency in the furnace It is expressed as shown in formula (1):

In formula (1), _Wc is the coal feed rate, _Sar is the sulfur content, _Air is the total air volume, and _kf is the dimensionless flue gas conversion coefficient. is the original flue gas SO ₂ concentration under standard conditions;

The off-furnace desulfurization efficiency It is expressed as shown in formula (2):

In formula (2), under standard conditions is the net flue gas _SO2 concentration.

Preferably, in step 2, the least square method is used to fit the relationship between the desulfurization efficiency in the furnace and the load, the calcium-sulfur molar ratio, and the bed temperature under typical load conditions, and the coefficient of the relationship is corrected using the air-coal ratio, which specifically includes the following sub-steps:

Sub-step S21, selecting bed temperature, coal feed rate, total air volume, coal quality, raw flue gas _SO2 concentration, and limestone feed flow rate in the furnace under typical load conditions;

Sub-step S22, calculating the furnace desulfurization efficiency, air-coal ratio, and calcium-sulfur molar ratio under typical load conditions, wherein the calculation of the furnace desulfurization calcium-sulfur molar ratio is shown in formula (3):

In formula (3), is the purity of limestone, is the limestone feed flow rate in the furnace;

Sub-step S23, using the least squares method to fit the relationship between the furnace desulfurization efficiency, the calcium-sulfur molar ratio, and the bed temperature under typical load conditions, is expressed as shown in formula (4):

In formula (4), A is a dimensionless coefficient, B is a function of bed temperature, and m _CFB is the molar ratio of calcium to sulfur in the furnace;

Sub-step S24, based on the operating data, using the air-coal ratio under typical load conditions to correct A, and finally obtaining the relationship between the furnace desulfurization efficiency and the calcium-sulfur molar ratio and the bed temperature under typical load conditions.

Preferably, in step 3, after assuming the desulfurization ratio in the furnace, the power consumption of the limestone conveying fan, the power consumption of the slurry circulation pump, and the limestone consumption for desulfurization inside and outside the furnace are determined, wherein the power consumption of the limestone conveying fan is determined by the limestone feed flow rate in the furnace and the rated power of the limestone conveying fan, and the power consumption of the slurry circulation pump is determined by the number of circulating pumps put into operation and the rated power, specifically comprising the following sub-steps:

Sub-step S31, fitting the relationship between the desulfurization efficiency outside the furnace and the original flue gas _SO2 concentration and the total air volume by the least square method, expressed as shown in formula (6):

In formula (6), a ₁ , a ₂ , and a ₃ are model coefficients;

Sub-step S32: Assuming that the desulfurization ratio in the furnace is x under a typical load condition, the desulfurization ratio outside the furnace is 1-x, and combining equation (1) and equation (4), the calcium-sulfur molar ratio m _CFB of the desulfurization in the furnace is determined;

Sub-step S33: the limestone consumption cost _W1 in the furnace is expressed as shown in formula (7):

In formula (7), M _{CaCO 3} and M _Cao are the relative molecular masses of CaCO ₃ and CaO, respectively, in g/mol; u1 is the unit price of limestone in the furnace, in yuan/kg;

The heat loss cost _W2 is expressed as shown in formula (8):

W ₂ =W _c [η/(η-Δη)-1]u ₂ (8),

In formula (8), η is the boiler design efficiency; Δη is the boiler heat loss; u ₂ is the unit price of coal, in yuan/kg;

The denitrification cost W ₃ is expressed as shown in formula (9):

W ₃ = k ₁ m _CFB W _c u ₃ (9),

In formula (9), k ₁ is the cost coefficient; u ₃ is the unit price of urea, in yuan/kg;

The power consumption cost W ₄ of the conveying fan is expressed as shown in formula (10):

In formula (10), α is the compressed air coefficient; u ₄ is the on-grid electricity price, in yuan/kWh;

The cost of limestone consumption outside the furnace _W5 is expressed as shown in formula (11):

In formula (11), _mCFB ,wet is the molar ratio of calcium to sulfur outside the furnace; _u5 is the unit price of limestone outside the furnace, in yuan/t.

The circulating pump power consumption _W6 is expressed as shown in formula (12):

In formula (12), n is the number of slurry circulation pumps started, which is determined by the original flue gas _SO2 concentration and load; _Pw is the power of a single slurry circulation pump, in kW;

The gypsum benefit _V7 is expressed as shown in formula (13):

Among them, η(H ₂ O) is the water content of gypsum; u ₈ is the unit price of gypsum, in yuan/kg;

The comprehensive cost of combined desulfurization inside and outside the furnace f(x) is expressed as shown in formula (14):

f(x)W1+W2+W3+W4+W5+W6-V7 0≤X≤Xmax(14),

In formula (14), _W1 , _W2 , _W3 , _W4 , _W5 , _W6 , and _V7 are all determined by the furnace desulfurization ratio x, and are respectively the furnace limestone consumption cost, heat loss cost, denitrification cost, conveying fan power consumption cost, furnace limestone consumption cost, circulation pump power consumption, and gypsum revenue; _xmax is determined by the furnace desulfurization capacity of the CFB unit.

Preferably, in step 4, the optimal in-furnace desulfurization ratio under typical load conditions is the solution obtained by optimizing the minimum comprehensive cost of in-furnace and out-of-furnace desulfurization by genetic algorithm, wherein the genetic algorithm optimization process includes the following sub-steps:

Sub-step S41, encoding: selecting an unsigned binary integer to represent the individual x _i ;

Sub-step S42, generating an initial population: randomly generating N individuals as the initial population, and setting the number of iterations to n;

Sub-step S43, fitness calculation: using the comprehensive cost function value g( _xi) as the fitness of individual _xi , select the fitness function shown in formula (15):

g( _xi ) = minf( _xi ) (15);

Sub-step S44, selection, crossover, and mutation operations: pass the individuals with higher fitness in the current group to the next generation; use the single-point crossover method to perform crossover operations, and use the basic bit mutation method to perform mutation operations, so as to save the next generation of groups, and the number of iterations increases by 1;

Sub-step S45, termination condition judgment: if the number of iterations is greater than or equal to n, the calculation is terminated, and the individual with the minimum fitness is output as the optimal solution, that is, the optimal desulfurization ratio inside and outside the furnace; otherwise, return to sub-step S43.

Further preferably, the number of populations N=20, the number of termination iterations n=80, the crossover probability is 0.4, and the mutation probability is 0.001.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic flow chart of the method for calculating the optimal in-furnace and out-furnace desulfurization ratio of a circulating fluidized bed unit according to the present invention.

Detailed ways

The specific embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. Those skilled in the art should understand that the description here is only an exemplary description of the best embodiment of the present invention and should not be regarded as limiting the scope of the present invention. Any variation or modification that does not deviate from the main purpose and idea of the present invention falls within the scope of the present invention.

The present invention discloses a method for calculating the optimal in-furnace desulfurization ratio of a circulating fluidized bed unit. FIG1 is a flow chart of the method for calculating the optimal in-furnace desulfurization ratio of a circulating fluidized bed unit according to the present invention. As shown in FIG1 , the method comprises the following steps:

Step 1, establish a comprehensive cost model for desulfurization inside and outside the furnace of the CFB unit, and select load, coal quality, coal feed rate, total air volume, bed temperature, limestone conveying fan rated power, slurry circulation pump rated power, limestone feed flow rate in the furnace, ammonia injection amount, raw flue gas _SO2 concentration, and net flue gas _SO2 concentration as inputs of the comprehensive cost model;

Step 2: when establishing the comprehensive cost model, determine the relationship between the desulfurization efficiency in the furnace and the load, calcium-sulfur molar ratio, bed temperature, and air-coal ratio;

Step 3: Using the input of the comprehensive cost model selected in step 1, determine the _SO2 generation concentration, assume the desulfurization ratio in the furnace, and establish a comprehensive cost model for desulfurization inside and outside the furnace under typical load conditions of the CFB unit;

Step 4: Use the intelligent optimization algorithm to solve the optimal in-furnace desulfurization ratio under typical load conditions, and fit the optimal in-furnace desulfurization ratio under each load condition.

In this embodiment, in step 2, the least square method is used to fit the relationship between the in-furnace desulfurization efficiency and the load, calcium-sulfur molar ratio, and bed temperature under typical load conditions, and the coefficient of the relationship is corrected using the wind-coal ratio. In step 3, after assuming the in-furnace desulfurization ratio, the power consumption of the limestone conveying fan, the power consumption of the slurry circulation pump, and the limestone consumption for in-furnace and out-of-furnace desulfurization are determined, wherein the power consumption of the limestone conveying fan is determined by the limestone feed flow rate in the furnace and the rated power of the limestone conveying fan, and the power consumption of the slurry circulation pump is determined by the number of circulating pumps in operation and the rated power. In step 4, the optimal in-furnace desulfurization ratio under typical load conditions is the solution to the minimum comprehensive cost of in-furnace and out-of-furnace desulfurization after genetic algorithm optimization.

details as follows:

1. Selection of model input variables

The circulating fluidized bed unit achieves ultra-low emissions through the operation mode of combined desulfurization inside and outside the furnace. The total cost of combined desulfurization inside and outside the furnace includes: equipment power consumption cost, furnace denitrification cost, heat loss cost, limestone usage cost, and the comprehensive cost is the total desulfurization cost minus the income generated by gypsum. Among them: the limestone usage cost includes the limestone usage cost of desulfurization inside the furnace and the limestone usage cost of desulfurization outside the furnace. The power consumption cost of the equipment includes: the power consumption of the limestone conveying fan in the furnace and the power consumption of the slurry circulation pump outside the furnace; the cost of denitrification in the furnace is affected by the calcium-sulfur molar ratio of desulfurization in the furnace. As the calcium-sulfur ratio increases, the cost of denitrification in the furnace tends to increase; the heat loss cost is also affected by the calcium-sulfur molar ratio of desulfurization in the furnace. When the desulfurization efficiency in the furnace remains unchanged, the lower the calcium-sulfur molar ratio, the lower the heat loss cost, and the lower the converted coal consumption; the amount of limestone used for desulfurization in the furnace is determined by the calcium-sulfur molar ratio of desulfurization in the furnace, the desulfurization efficiency in the furnace, and the coal quality. The desulfurization efficiency in the furnace is related to the load, calcium-sulfur molar ratio, bed temperature, and air-coal ratio; the amount of limestone used for desulfurization outside the furnace is related to the desulfurization efficiency outside the furnace.

According to the CFB unit operation data, the desulfurization efficiency in the furnace It can be obtained by the following formula:

Where, W _c is the amount of coal fed; _Sar is the sulfur content, which is determined by the coal quality; _Air is the total air volume; _kf is the dimensionless flue gas conversion coefficient; is the original flue gas _SO2 concentration (under standard conditions).

External desulfurization efficiency It can be obtained by the following formula:

in, is the net flue gas _SO2 concentration (under standard conditions).

Therefore, load, coal quality, coal feed rate, total air volume, bed temperature, limestone conveying fan rated power, slurry circulation pump rated power, limestone feed flow rate in the furnace, ammonia injection amount, raw flue gas SO ₂ concentration, and net flue gas SO ₂ concentration are selected as the input variables of the model.

2. Steps to determine the relationship between furnace desulfurization efficiency and load, bed temperature, and air-coal ratio

It includes the following sub-steps:

Sub-step S21, selecting bed temperature, coal feed rate, total air volume, coal quality, raw flue gas _SO2 concentration, and limestone feed flow rate in the furnace under typical load conditions;

Sub-step S22, calculating the furnace desulfurization efficiency, air-coal ratio, and calcium-sulfur molar ratio under typical load conditions, wherein the calculation of the furnace desulfurization calcium-sulfur molar ratio is shown in formula (3):

In formula (3), is the purity of limestone, is the limestone feed flow rate in the furnace;

Sub-step S23, using the least squares method to fit the relationship between the furnace desulfurization efficiency, the calcium-sulfur molar ratio, and the bed temperature under typical load conditions, is expressed as shown in formula (4):

In formula (4), A is a dimensionless coefficient, B is a function of bed temperature, and m _CFB is the molar ratio of calcium to sulfur in the furnace;

Sub-step S24, based on the operating data, using the air-coal ratio under typical load conditions to correct A, and finally obtaining the relationship between the furnace desulfurization efficiency and the calcium-sulfur molar ratio and the bed temperature under typical load conditions.

3. Comprehensive cost model and solution

External desulfurization efficiency It is related to the original flue gas _SO2 concentration and total air volume. The relationship between the off-furnace desulfurization efficiency and the original flue gas _SO2 concentration and total air volume can be fitted by the least squares method, which specifically includes the following sub-steps:

Sub-step S31, fitting the relationship between the desulfurization efficiency outside the furnace and the original flue gas _SO2 concentration and the total air volume by the least square method, expressed as shown in formula (6):

In formula (6), a ₁ , a ₂ , and a ₃ are model coefficients;

Sub-step S32: Assuming that the desulfurization ratio in the furnace is x under a typical load condition, the desulfurization ratio outside the furnace is 1-x, and combining equation (1) and equation (4), the calcium-sulfur molar ratio m _CFB of the desulfurization in the furnace is determined;

Sub-step S33: the limestone consumption cost _W1 in the furnace is expressed as shown in formula (7):

In formula (7), M _{CaCO 3} and M _Cao are the relative molecular masses of CaCO ₃ and CaO, respectively, in g/mol; u1 is the unit price of limestone in the furnace, in yuan/kg;

The heat loss cost _W2 is expressed as shown in formula (8):

W ₂ =W _c [η/(η-Δη)-1]u ₂ (8),

In formula (8), η is the boiler design efficiency; Δη is the boiler heat loss; u ₂ is the unit price of coal, in yuan/kg;

The denitrification cost W ₃ is expressed as shown in formula (9):

W ₃ = k ₁ m _CFB Wcu ₃ (9),

In formula (9), k ₁ is the cost coefficient; u ₃ is the unit price of urea, in yuan/kg;

The power consumption cost W ₄ of the conveying fan is expressed as shown in formula (10):

In formula (10), α is the compressed air coefficient; u ₄ is the on-grid electricity price, in yuan/kWh;

The cost of limestone consumption outside the furnace _W5 is expressed as shown in formula (11):

In formula (11), _mCFB ,wet is the molar ratio of calcium to sulfur outside the furnace; _u5 is the unit price of limestone outside the furnace, in yuan/t.

The circulating pump power consumption _W6 is expressed as shown in formula (12):

W ₆ = nPwu ₄ (12),

In formula (12), n is the number of slurry circulation pumps started, which is determined by the original flue gas _SO2 concentration and load; _Pw is the power of a single slurry circulation pump, in kW;

The gypsum benefit _V7 is expressed as shown in formula (13):

Wherein, η(H ₂ O) is the water content of gypsum; u ₈ is the unit price of gypsum, yuan/kg.

The comprehensive cost of combined desulfurization inside and outside the furnace f(x) is expressed as shown in formula (14):

f(x)＝W1+W2+W3+W4+W5+W6-V7 0≤X≤Xmax(14),

In formula (14), _W1 , _W2 , _W3 , _W4 , _W5 , _W6 , and _V7 are all determined by the furnace desulfurization ratio x, and are respectively the furnace limestone consumption cost, heat loss cost, denitrification cost, conveying fan power consumption cost, furnace limestone consumption cost, circulating pump power consumption, and gypsum revenue; _xmax is determined by the furnace desulfurization capacity of the CFB unit.

The process of genetic algorithm to find the optimal desulfurization ratio in the furnace specifically includes the following sub-steps:

Sub-step S41, encoding: selecting an unsigned binary integer to represent the individual x _i ;

Sub-step S42, generating an initial population: randomly generating N individuals as the initial population, and setting the number of iterations to n;

Sub-step S43, fitness calculation: using the comprehensive cost function value g( _xi) as the fitness of individual _xi , select the fitness function shown in formula (15):

g( _xi ) = minf( _xi ) (15);

Sub-step S44, selection, crossover, and mutation operations: pass the individuals with higher fitness in the current group to the next generation; use the single-point crossover method to perform crossover operations, and use the basic bit mutation method to perform mutation operations, so as to save the next generation of groups, and the number of iterations increases by 1;

Sub-step S45, termination condition judgment: if the number of iterations is greater than or equal to n, the calculation is terminated, and the individual with the minimum fitness is output as the optimal solution, that is, the optimal desulfurization ratio inside and outside the furnace; otherwise, return to sub-step S43.

In a specific embodiment, the number of populations N=20, the number of termination iterations n=80, the crossover probability is 0.4, and the mutation probability is 0.001.

The present invention has the following beneficial effects:

(1) According to the operating characteristics of the CFB unit, the corresponding variables are selected and the relationship between the desulfurization efficiency in the furnace and the bed temperature and the air-coal ratio is determined by the least squares method.

(2) On this basis, after assuming the desulfurization ratio in the furnace, the comprehensive desulfurization cost model under typical load conditions was established.

(3) A genetic algorithm is used to optimize the optimal in-furnace and out-of-furnace desulfurization ratio under typical load conditions, with the minimum comprehensive desulfurization cost as the objective function.

Claims (3)

1. A method for calculating the optimal desulfurization ratio of the circulating fluidized bed unit inside and outside the furnace, characterized in that it comprises the following steps: Step 1, establish a comprehensive cost model for desulfurization inside and outside the furnace of the CFB unit, and select load, coal quality, coal feed rate, total air volume, bed temperature, limestone conveying fan rated power, slurry circulation pump rated power, limestone feed flow rate in the furnace, ammonia injection amount, raw flue gas _SO2 concentration, and net flue gas _SO2 concentration as input variables of the comprehensive cost model; The process of establishing the comprehensive cost model of internal and external desulfurization of CFB units is as follows: The total cost of combined desulfurization inside and outside the furnace is defined as including: equipment power consumption cost, furnace denitrification cost, heat loss cost, and limestone usage cost. The comprehensive cost is the total desulfurization cost minus the income generated by gypsum; wherein, the limestone usage cost includes the limestone usage cost of desulfurization inside the furnace and the limestone usage cost of desulfurization outside the furnace; the equipment power consumption cost includes the power consumption of the limestone conveying fan inside the furnace and the power consumption of the slurry circulation pump outside the furnace; the denitrification cost inside the furnace increases with the increase of the calcium-sulfur molar ratio of desulfurization inside the furnace; when the desulfurization efficiency inside the furnace remains unchanged, the lower the calcium-sulfur molar ratio, the lower the heat loss cost, and the lower the converted coal consumption; the desulfurization limestone usage inside the furnace is determined by the calcium-sulfur molar ratio of desulfurization inside the furnace, the desulfurization efficiency inside the furnace, and the coal quality. The desulfurization efficiency inside the furnace is related to the load, the calcium-sulfur molar ratio, the bed temperature, and the air-coal ratio; the desulfurization limestone usage outside the furnace is related to the desulfurization efficiency outside the furnace; The desulfurization efficiency in the furnace It is expressed as shown in formula (1): In formula (1), _Wc is the coal feed rate, _Sar is the sulfur content, _Air is the total air volume, and _kf is the dimensionless flue gas conversion coefficient. is the original flue gas SO ₂ concentration under standard conditions; The off-furnace desulfurization efficiency It is expressed as shown in formula (2): In formula (2), under standard conditions is the net flue gas SO ₂ concentration; Step 2: when establishing the comprehensive cost model, determine the relationship between the desulfurization efficiency in the furnace and the load, calcium-sulfur molar ratio, bed temperature, and air-coal ratio; The least square method is used to fit the relationship between the desulfurization efficiency in the furnace and the load, calcium-sulfur molar ratio, and bed temperature under typical load conditions, and the coefficient of the relationship is corrected using the air-coal ratio. Specifically, the following sub-steps are included: Sub-step S21, selecting bed temperature, coal feed rate, total air volume, coal quality, raw flue gas _SO2 concentration, and limestone feed flow rate in the furnace under typical load conditions; Sub-step S22, calculating the furnace desulfurization efficiency, air-coal ratio, and calcium-sulfur molar ratio under typical load conditions, wherein the calculation of the furnace desulfurization calcium-sulfur molar ratio is shown in formula (3): In formula (3), is the purity of limestone, is the limestone feed flow rate in the furnace; Sub-step S23, using the least squares method to fit the relationship between the furnace desulfurization efficiency, the calcium-sulfur molar ratio, and the bed temperature under typical load conditions, is expressed as shown in formula (4): In formula (4), A is a dimensionless coefficient, B is a function of bed temperature, and m _CFB is the molar ratio of calcium to sulfur in the furnace; Sub-step S24, according to the operation data, using the air-coal ratio under typical load conditions to correct A, and finally obtain the relationship between the furnace desulfurization efficiency and the calcium-sulfur molar ratio and the bed temperature under typical load conditions; Step 3: Using the input of the comprehensive cost model selected in step 1, determine the _SO2 generation concentration, assume the desulfurization ratio in the furnace, and establish a comprehensive cost model for desulfurization inside and outside the furnace under typical load conditions of the CFB unit; After assuming the desulfurization ratio in the furnace, determine the power consumption of the limestone conveying fan, the power consumption of the slurry circulation pump, and the limestone consumption for desulfurization inside and outside the furnace. The power consumption of the limestone conveying fan is determined by the limestone feed flow rate in the furnace and the rated power of the limestone conveying fan, and the power consumption of the slurry circulation pump is determined by the number of circulating pumps in operation and their rated power. The specific steps include the following: Sub-step S31, fitting the relationship between the desulfurization efficiency outside the furnace and the original flue gas _SO2 concentration and the total air volume by the least square method, expressed as shown in formula (6): In formula (6), a ₁ , a ₂ , and a ₃ are model coefficients; Sub-step S32: Assuming that the desulfurization ratio in the furnace is x under a typical load condition, the desulfurization ratio outside the furnace is 1-x, and combining equation (1) and equation (4), the calcium-sulfur molar ratio m _CFB of the desulfurization in the furnace is determined; Sub-step S33: the limestone consumption cost _W1 in the furnace is expressed as shown in formula (7): In formula (7), M _{CaCO 3} and M _Cao are the relative molecular masses of CaCO ₃ and CaO, respectively, in g/mol; u ₁ is the unit price of limestone in the furnace, in yuan/kg; The heat loss cost _W2 is expressed as shown in formula (8): W ₂ =W _c [η/(η-Δη)-1]u ₂ (8), In formula (8), η is the boiler design efficiency; Δη is the boiler heat loss; u ₂ is the unit price of coal, in yuan/kg; The denitrification cost W ₃ is expressed as shown in formula (9): W ₃ = k ₁ m _CFB W _c u ₃ (9), In formula (9), k ₁ is the cost coefficient; u ₃ is the unit price of urea, in yuan/kg; The power consumption cost W ₄ of the conveying fan is expressed as shown in formula (10): In formula (10), α is the compressed air coefficient; u ₄ is the on-grid electricity price, in yuan/kWh; The cost of limestone consumption outside the furnace _W5 is expressed as shown in formula (11): In formula (11), m _CFB ,wet is the molar ratio of calcium to sulfur outside the furnace; u ₅ is the unit price of limestone outside the furnace, in yuan/t; The circulating pump power consumption _W6 is expressed as shown in formula (12): W ₆ = nP _w u ₄ (12), In formula (12), n is the number of slurry circulation pumps started, which is determined by the original flue gas _SO2 concentration and load; _Pw is the power of a single slurry circulation pump, in kW; The gypsum benefit _V7 is expressed as shown in formula (13): Among them, η(H ₂ O) is the water content of gypsum; u ₈ is the unit price of gypsum, in yuan/kg; The comprehensive cost of combined desulfurization inside and outside the furnace f(x) is expressed as shown in formula (14): f(x)＝W ₁ +W ₂ +W ₃ +W ₄ +W ₅ +W ₆ −V ₇ 0≤X≤X _max (14), In formula (14), W ₁ , W ₂ , W ₃ , W ₄ , W ₅ , W ₆ , and V ₇ are all determined by the furnace desulfurization ratio x, and are respectively the furnace limestone consumption cost, heat loss cost, denitrification cost, conveying fan power consumption cost, furnace limestone consumption cost, circulating pump power consumption, and gypsum revenue; x _max is determined by the furnace desulfurization capacity of the CFB unit; Step 4: Use the intelligent optimization algorithm to solve the optimal in-furnace desulfurization ratio under typical load conditions, and fit the optimal in-furnace desulfurization ratio under each load condition. 2. A method for calculating the optimal in-furnace desulfurization ratio of a circulating fluidized bed unit according to claim 1, characterized in that, in step 4, the optimal in-furnace desulfurization ratio under typical load conditions is a solution obtained by optimizing the minimum comprehensive cost of in-furnace desulfurization by a genetic algorithm, wherein the process of optimizing the minimum comprehensive cost of in-furnace desulfurization by a genetic algorithm comprises the following sub-steps: Sub-step S41, encoding: selecting an unsigned binary integer to represent the individual x _i ; Sub-step S42, generating an initial population: randomly generating N individuals as the initial population, and setting the number of iterations to n; Sub-step S43, fitness calculation: using the comprehensive cost function value g( _xi ) as the fitness of individual _xi , select the fitness function shown in formula (15): g( _xi ) = min f( _xi ) (15); Sub-step S44, selection, crossover, and mutation operations: pass the individuals with higher fitness in the current group to the next generation; use the single-point crossover method to perform crossover operations, and use the basic bit mutation method to perform mutation operations, so as to save the next generation of groups, and the number of iterations increases by 1; Sub-step S45, termination condition judgment: if the number of iterations is greater than or equal to n, the calculation is terminated, and the individual with the minimum fitness is output as the optimal solution, that is, the optimal desulfurization ratio inside and outside the furnace; otherwise, return to sub-step S43. 3. A method for calculating the optimal in-furnace and out-of-furnace desulfurization ratio of a circulating fluidized bed unit according to claim 2, characterized in that, wherein, the number of groups N=20, the number of termination iterations n=80, the crossover probability is 0.4, and the mutation probability is 0.001.