Coal-air synchronous dynamic coordinated control method for coal-fired unit

Abstract

A coal-air synchronous dynamic coordinated control method for a coal-fired unit is provided, comprising: determining functional relationship between unit loads and designed coal feed rates and functional relationship between unit loads and flue gas operation wet-basis oxygen contents, respectively; obtaining a theoretical wet flue gas volume and a combustion-supporting dry air volume per unit mass of burning coal, and calculating an actual combustion-supporting dry air volume per unit mass of burning coal; calculating an actual low calorific value of feed coal; calculating a combustion-supporting dry air volume and an outlet wet flue gas volume; according to the target value of load instruction at a future time point, calculating a coal feed rate variation and a combustion-supporting dry air volume variation; obtaining an operation wet-basis oxygen content variation; and obtaining target values of the coal feed rate and the operation wet-basis oxygen content to be adjusted.

Description

FIELD OF TECHNOLOGY

The following belongs to the technical field of coal-fired units, and in particular relates to a coal-air synchronous dynamic coordinated control method for a coal-fired unit.

BACKGROUND

AGC (Automatic Generation Control) is an important function in the EMS (Energy Management System) of the unit. The common traditional series coal-air sequence control logic is shown in FIG. 1: after receiving the unit load up or down instruction, turning up/down the high pressure-intermediate pressure governing valve of the steam turbine; after the steam pressure reduces/rises, issuing an instruction to increase/decrease the coal feed, the burning coal enters the furnace for combustion through the coal mill, pulverized coal pipeline and burner; if the oxygen content of the flue gas at the outlet of the economizer decreases/increases, issuing an instruction to turn up/down the damper of the forced draft fan, then the combustion-supporting air enters the furnace for combustion through the coal mill and the air box, to achieve the desired change in the operation wet-basis oxygen content.

In the process of flexible peak load regulation and rapid load up and down of coal-fired units at 2% MCR/min and above, this traditional serial sequence control logic mode is likely to cause the combustion-supporting air volume entering the furnace to lag behind the increase or decrease of coal feed for a long time, breaking the air-coal balanced combustion atmosphere, resulting in violent fluctuations in the concentration of nitrogen oxides in the flue gas at the outlet of the boiler economizer and the operation wet-basis oxygen content. When the load increases, the operation wet-basis oxygen content decreases with the increase of the coal feed, and the concentration of nitrogen oxides decreases greatly; when the load decreases, the operation wet-basis oxygen content increases with the decrease of the coal feed, and the concentration of nitrogen oxides increases greatly.

The concentration of nitrogen oxides in the flue gas at the outlet of a typical boiler economizer varies from 180 to 350 mg/m³ in the load up and down process, while the lag time of online CEMS measurement of nitrogen oxides at the inlet of the downstream SCR flue gas denitration system is about 1 to 3 minutes, which further causes the adjustment of denitration ammonia injection to lag behind the change of unit operating conditions, so it is easy to cause excessive injection of ammonia in a local period of time, the emission concentration of nitrogen oxides is very low, and the escape concentration of ammonia is very high, which aggravates the blockage of ammonium bisulfate in the air preheater, or causes insufficient ammonia injection in a local period of time, and the emission concentration of nitrogen oxides to exceed the standard.

At present, some technologies are trying to solve the control strategy problem of asynchronous coal and air:

Referring to CN110658721A, an adaptive pre-coal feeding method and a system applied to an AGC-R mode of a thermal power unit are disclosed. This method shortens the lag time of increasing and decreasing fuel relative to the load instruction and improves the coordinated load-change ability of the unit. However, this method only enables the unit to predict in advance the increase or decrease of the coal feed during the process of increasing or decreasing the load and cannot solve the advance prediction of the combustion-supporting air volume.

Referring to CN103513640A, an integral optimization method and a system for an automatic power generation control system of a coal-fired unit are disclosed. Although this method improves the coordination of air-coal combustion to a certain extent, it still does not shorten the lag time of the large delay between the instruction to increase and decrease the coal feed and the arrival of the increase and decrease of the air, so it cannot solve the unbalanced combustion situation during dynamic and rapid load changes.

SUMMARY

An aspect relates to a coal-air synchronous dynamic coordinated control method for a coal-fired unit, which is used to solve the problem that the combustion-supporting air volume lags behind the adjustment of the coal feed in the process of rapid peak load regulation of the unit.

To achieve the above purpose, a technical solution provided by the present disclosure is:

A coal-air synchronous dynamic coordinated control method for a coal-fired unit, comprises the following steps in sequence:

• • S1: in a steady-state operation mode of the unit, acquiring designed coal feed rates and designed flue gas operation wet-basis oxygen contents under different loads, and then determining a corresponding functional relationship between the unit loads L and the designed coal feed rates, and a corresponding functional relationship between the unit loads and the designed flue gas operation wet-basis oxygen contents;
  • S2: using the regression analysis method, obtaining a regression function of a theoretical wet flue gas volume per unit mass of burning coal and a theoretical combustion-supporting dry air volume per unit mass of burning coal based on a low calorific value by fitting, judging the accuracy of the regression function, and calculating an actual combustion-supporting dry air volume per unit mass of burning coal, and according to the air temperature, relative humidity and atmospheric pressure, calculating the air saturated vapor pressure and a water vapor proportion per unit volume of dry air, and then calculating an actual combustion-supporting wet air volume per unit mass of burning coal and a wet flue gas volume actually produced per unit mass of burning coal;
  • S3: calculating predicted values of the required combustion-supporting dry air volume and predicted values of the produced wet flue gas volume for the total burning coal per hour under different loads, and by comparing the predicted values and design values of the required combustion-supporting dry air volume, and the predicted values and design values of the produced wet flue gas volume for the total burning coal per hour under different loads, verifying whether relative deviations are within an acceptable range;
  • S4: acquiring the current load and the actual coal feed rate of the unit, calculating a designed coal feed rate under the current unit load through the corresponding functional relationship between the unit loads and the designed coal feed rates, and calculating an actual low calorific value of feed coal;
  • S5: according to the actual coal feed rate, the actual operation wet-basis oxygen content and the actual low calorific value of feed coal of the unit, calculating a combustion-supporting dry air volume and a combustion-supporting wet air volume entering the boiler, and the wet flue gas volume at the outlet of a boiler economizer;
  • S6: according to a load instruction curve of the unit, determining the target value of a load instruction at a future time point, and calculating a unit load change rate within the time, calculating the coal feed rate at the future time point, and calculating a coal feed rate variation within the time, and then calculating a combustion-supporting dry air volume variation and a combustion-supporting wet air volume variation within the time;
  • S7: calculating an operation wet-basis oxygen content variation caused by the change of the combustion-supporting dry air volume, and according to the corresponding functional relationship between the unit loads and the designed flue gas operation wet-basis oxygen contents, calculating the flue gas operation wet-basis oxygen content at the future time point, and calculating a set wet-basis oxygen content variation within the time, and obtaining an operation wet-basis oxygen content variation within the time;
  • S8: on the basis of the coal feed rate instruction and the operation wet-basis oxygen content instruction of the original sequence control logic of the unit, respectively superimposing the coal feed rate variation and the operation wet-basis oxygen content variation simultaneously in advance to obtain a target value of the coal feed rate to be adjusted and a target value of the operation wet-basis oxygen content to be adjusted.

Due to the use of the above technical solutions, the present disclosure has the following advantages over the conventional art:

By accurately calculating the combustion-supporting dry air volume and the wet flue gas volume online in real time, and setting the adjustments of the coal feed rate and the operation wet-basis oxygen content synchronously in the same proportion according to the instruction change of the unit load, the present disclosure proposes an air-coal synchronous dynamic coordinated control method that parallel presets coal feed rate and operation wet-basis oxygen content, which maintains the balanced combustion situation of the air-powder in the boiler, effectively improves the following responsiveness of the combustion-supporting dry air, greatly reduces the response lag time of the operation wet-basis oxygen content at the outlet of the boiler economizer, improves the synchronization of the adjustment between the load, burning coal and combustion-supporting dry air, improves the stability of nitrogen oxide concentration at the boiler outlet, effectively reduces fluctuations of steam pressure and temperature caused by excessive regulation, and creates important conditions for the stable operation of the SCR denitration system.

BRIEF DESCRIPTION

Some of the embodiments will be described in detail, with references to the following Figures, wherein like designations denote like members, wherein:

FIG. 1 is a logic block diagram of a traditional series air-coal sequence coordinated control; and

FIG. 2 is a logic block diagram of a coal-air synchronous dynamic coordinated control in an embodiment.

DETAILED DESCRIPTION

The technical solutions of the present disclosure are explained clearly and completely below in conjunction with the accompanying drawings, and apparently, the described embodiments are merely a part of the embodiments of the present disclosure, not all the embodiments. Based on the embodiments of the present disclosure, all other embodiments obtained by one of ordinary skill in the art without creative work fall within the protective scope of the present disclosure.

As shown in FIG. 2, a coal-air synchronous dynamic coordinated control method for a coal-fired unit can calculate the combustion-supporting dry air volume and the wet flue gas volume online in real time, obtain the target value of the coal feed rate to be adjusted and the target value of the operation wet-basis oxygen content to be adjusted according to the instruction change of the unit load, and simultaneously set the adjustments of the coal feed rate and the operation wet-basis oxygen content in the same proportion. It comprises the following steps in sequence:

S1: in a steady-state operation mode of the unit, acquiring designed coal feed rates and designed flue gas operation wet-basis oxygen contents under different loads, and then determining: a polyline function of the designed coal feed rates W_coal,b with the unit load L as an independent variable, and a polyline function of the designed flue gas operation wet-basis oxygen contents O_2,b, with the unit load L as an independent variable:W_coal,b=f(L,Q_net,b),O_2,b=g(L),where,

• • L is unit load, in MW,
  • W_coal,b is designed coal feed rate, in t/h,
  • Q_net,b is designed low calorific value of coal, in MJ/kg,
  • O_2,b, is designed flue gas operation wet-basis oxygen content, in %.

S2: using the regression analysis method, obtaining a regression function of a theoretical wet flue gas volume W_flue,th,wet per unit mass of burning coal and a theoretical combustion-supporting dry air volume W_air,th,dry per unit mass of burning coal based on the low calorific value by fitting the elemental analysis data and industrial analysis data of multiple existing sets of coal samples from utility boilers, judging the accuracy of the regression function by variance, if the variance is greater than 0.9, it indicates that the accuracy of the fitting is high, and then calculating an actual combustion-supporting dry air volume W_air,act per unit mass of burning coal, and according to air temperature, relative humidity and atmospheric pressure measured in real-time, calculating the air saturated vapor pressure P_H2O and a water vapor volume proportion φ per unit volume of dry air, and then calculating an actual combustion-supporting wet air volume W_air,act,wet per unit mass of burning coal and an actually produced wet flue gas volume W_flue,act,wet per unit mass of burning coal:

$W_{\mathrm{flue},\mathrm{th}}={\alpha }_1\times Q_{\mathrm{net}}+{\beta }_1,$ $W_{\mathrm{air},\mathrm{th}}={\alpha }_2\times Q_{\mathrm{net}}+{\beta }_2,$ $W_{\mathrm{air},\mathrm{act},\mathrm{dry}}=W_{\mathrm{flue},\mathrm{th},\mathrm{wet}}\times \frac{O_2}{21-O_2\times \left(1+\phi \right)}+W_{\mathrm{air},\mathrm{th},\mathrm{dry}}\times \frac{21-O_2}{21-O_2\times \left(1+\phi \right)},$ $P_{H_{2}O}=611.7927+42.7809\times T_{\mathrm{air}}+1.6883\times T_{\mathrm{air}}^2+0.012079\times T_{\mathrm{air}}^3+0.00061637\times T_{\mathrm{air}}^4,$ $\phi =0.3866\times \left(\varphi \times P_{\mathrm{air}}/100\right)/\left(P_{\mathrm{atm}}-\varphi \times P_{\mathrm{air}}/100\right),$ $W_{\mathrm{air},\mathrm{act},\mathrm{wet}}=W_{\mathrm{air},\mathrm{act},\mathrm{dry}}\times \left(1+\phi \right),$ $W_{\mathrm{flue},\mathrm{act},\mathrm{wet}}=W_{\mathrm{flue},\mathrm{th},\mathrm{wet}}\times \frac{21}{21-O_2\times \left(1+\phi \right)}+W_{\mathrm{air},\mathrm{th},\mathrm{dry}}\times \frac{21\times \phi }{21-O_2\times \left(1+\phi \right)},$ where,

• • W_flue,th,wet is theoretical wet flue gas volume per unit mass of burning coal, in m³/kg,
  • W_air,th,dry is theoretical combustion-supporting dry air volume per unit mass of burning coal, in m³/kg,
  • W_air,act,dry is actual combustion-supporting dry air volume per unit mass of burning coal, in m³/kg,
  • W_flue,act,wet is wet flue gas volume produced by the wet basis of per unit mass of burning coal, in m³/kg,
  • Q_net is low calorific value of coal, in MJ/kg,
  • O₂ is flue gas operation wet-basis oxygen content, in %,
  • α₁, α₂, β₁, β₂ are all constants, and are 0.2467, 0.2496, 0.718, 0.3125, respectively,
  • P_H2O, is air saturated vapor pressure, in Pa,
  • T_air is air temperature, in ° C.,
  • Φ is air relative humidity, in %,
  • P_atm is atmospheric pressure, in Pa,
  • φ is water vapor volume proportion per unit volume of dry air.

S3: according to the results of industrial analysis (received base ashes, received base water, low calorific values) and elemental analysis (carbon, hydrogen, oxygen, nitrogen, sulfur), obtaining design values of the required combustion-supporting dry air volume W_air,b and design values of the produced wet flue gas volume W_flue,b for the total burning coal per hour under different loads;

calculating predicted values of the required combustion-supporting dry air volume W_air,h and predicted values of the produced wet flue gas volume W_flue,h for the total burning coal per hour under the designed wet-basis oxygen contents of different loads:W_air,h=W_air,act,dry×W_coal,b×(1−γ)×1000,W_flue,h=W_flue,act,wet×W_coal,b×(1−γ)×1000,where,

• • W_air,h is predicted value of the required combustion-supporting dry air volume for the total burning coal per hour, in m³/h,
  • W_flue,h is predicted value of the produced wet flue gas volume for the total burning coal per hour, in m³/h,
  • W_air,act,dry is actual combustion-supporting dry air volume per unit mass of burning coal, in m³/kg,
  • W_flue,act,wet is predicted value of the theoretical wet flue gas volume per unit mass of burning coal, in m³/kg,
  • W_coal,b is designed coal feed rate, in t/h,
  • γ is incomplete combustion heat loss ratio of feed coal, and its value is 0.42%.

Calculating the relative deviation δ₁ between the predicted values W_air,h and design values W_air,b of the required combustion-supporting dry air volume for the total burning coal per hour under different loads, and the relative deviation δ₂ between the predicted values W_flue,h and design values W_flue,b of the produced wet flue gas volume for the total burning coal per hour under different loads, if the relative deviations δ₁ and δ₂ are both within −5%˜+5%, the fitting deviations are within the acceptable range:

${\delta }_1=\frac{W_{\mathrm{air},h}-W_{\mathrm{air},b}}{W_{\mathrm{air},b}}\times 100\%,$ ${\delta }_2=\frac{W_{\mathrm{flue},h}-W_{\mathrm{flue},b}}{W_{\mathrm{flue},b}}\times 100\%,$ where,

• • W_air,h is predicted value of the required combustion-supporting dry air volume for the total burning coal per hour, in m³/h,
  • W_flue,h is predicted value of the produced wet flue gas volume for the total burning coal per hour, in m³/h,
  • W_air,b is design value of the required combustion-supporting dry air volume for the total burning coal per hour, in m³/h,
  • W_flue,b is Design Value of the Produced Wet Flue Gas Volume for the Total Burning Coal Per Hour, in m³/h.

S4: acquiring the current load L_i and the actual coal feed rate W_coal,act displayed by the DCS system of the unit, calculating a designed coal feed rate W_coal,b,τ under the current unit load L_τ through the corresponding functional relationship between the unit loads L and the designed coal feed rates W_coal,b, and calculating an actual low calorific value Q_net,act of feed coal;

$Q_{\mathrm{net},\mathrm{act}}=\frac{W_{\mathrm{coal},b,\tau }\times Q_{\mathrm{net},b}}{W_{\mathrm{coal},\mathrm{act}}},$ where,

• • Q_net,act is actual low calorific value of feed coal, in MJ/kg,
  • Q_net,b is designed low calorific value of feed coal, in MJ/kg,
  • W_coal,act is actual coal feed rate under the current unit load L_τ, in t/h,
  • W_coal,b,τ is designed coal feed rate under the current unit load L_τ, in t/h.

S5: according to the actual coal feed rate W_coal,act, the actual operation wet-basis oxygen content O_2,act and the actual low calorific value of feed coal Q_net,act displayed by the DCS system of the unit, calculating a combustion-supporting dry air volume BW air,act,dry and a combustion-supporting wet air volume BW_air,act,wet entering the boiler, and the wet flue gas volume BW_flue,act,wet at the outlet of a boiler economizer under the actual unit load L_τ:

${\mathrm{BW}}_{\mathrm{air},\mathrm{act},\mathrm{dry}}=\left(W_{\mathrm{flue},\mathrm{th},\mathrm{wet}}\times \frac{O_{2,\mathrm{act}}}{21-O_{2,\mathrm{act}}\times \left(1+\phi \right)}+W_{\mathrm{air},\mathrm{th},\mathrm{dry}}\times \frac{21-O_{2,\mathrm{act}}}{21-O_{2,\mathrm{act}}\times \left(1+\phi \right)}\right)\times W_{\mathrm{coal},\mathrm{act}}\times 1000,$ ${\mathrm{BW}}_{\mathrm{air},\mathrm{act},\mathrm{wet}}={\mathrm{BW}}_{\mathrm{air},\mathrm{act},\mathrm{dry}}\times \left(1+\phi \right),$ ${\mathrm{BW}}_{\mathrm{flue},\mathrm{act},\mathrm{wet}}=\left(W_{\mathrm{flue},\mathrm{th},\mathrm{wet}}\times \frac{21}{21-O_{2,\mathrm{act}}\times \left(1+\phi \right)}+W_{\mathrm{air},\mathrm{th},\mathrm{dry}}\times \frac{21\times \phi }{21-O_{2,\mathrm{act}}\times \left(1+\phi \right)}\right)\times W_{\mathrm{coal},\mathrm{act}}\times 1000,$ where,

• • BW_air,act,dry is combustion-supporting dry air volume, in m³/h,
  • BW_air,act,wet is combustion-supporting wet air volume, in m³/h,
  • BW_flue,act,wet is outlet wet flue gas volume, in m³/h,
  • W_air,th,dry is theoretical combustion-supporting dry air volume per unit mass of burning coal, in m³/kg,
  • W_flue,th,wet is theoretical wet flue gas volume per unit mass of burning coal, in m³/kg,
  • O_2,act is actual flue gas operation wet-basis oxygen content, in %,
  • φ is water vapor volume proportion per unit volume of dry air,
  • W_coal,act is actual coal feed rate under the current unit load L_τ, in t/h,

S6: according to a load instruction curve of the unit set in advance by the power grid dispatch center, determining the target value of a load instruction at a future time point Δτ, and calculating a unit load change rate E_L within the time Δτ, calculating the coal feed rate W_coal,τ+Δτ at the future time point Δτ and calculating a coal feed rate variation ΔW_coal within the time Δτ, and then calculating a combustion-supporting dry air volume variation ΔBW_air,act and a combustion-supporting wet air volume variation ΔBW_air,act,wet within the time Δτ:

$E_L=\frac{L_{\tau +\Delta \tau }-L_{\tau }}{L_{\tau }}\times 100,$ $W_{\mathrm{coal},\tau +\Delta \tau }=W_{\mathrm{coal},\mathrm{act}}\times \left(1+\frac{E_L}{100}\right),$ $\Delta W_{\mathrm{coal}}=W_{\mathrm{coal},\tau +\Delta \tau }-W_{\mathrm{coal},\mathrm{act}},$ $\Delta {\mathrm{BW}}_{\mathrm{air},\mathrm{act},\mathrm{dry}}=W_{\mathrm{air},\mathrm{act},\mathrm{dry}}\times \Delta W_{\mathrm{coal}}\times 1000,$ $\Delta {\mathrm{BW}}_{\mathrm{air},\mathrm{act},\mathrm{wet}}=\Delta {\mathrm{BW}}_{\mathrm{air},\mathrm{act},\mathrm{dry}}\times \left(1+\phi \right),$ where,

• • E_L is unit load change rate within the time Δτ, in %,
  • L_τ is current unit load, in MW,
  • L_τ+Δτ is unit load after the time Δτ, in MW,
  • W_coal,τ+Δτ is coal feed rate at the future time point Δτ, in t/h,
  • W_coal,act is actual coal feed rate under the current unit load L_τ, in t/h,
  • ΔW_coal is coal feed rate variation within the time Δτ, in t/h,
  • ΔBW_air,act,dry is combustion-supporting dry air volume variation within the time Δτ, in m³/h,
  • ΔBW_air,act,wet is combustion-supporting wet air volume variation within the time Δτ, in m³/h,
  • W_air,act,dry is actual combustion-supporting dry air volume per unit mass of coal, in m³/kg,
  • φ is water vapor volume proportion per unit volume of dry air.

S7: according to the combustion-supporting dry air volume variation ΔBW_air,act,dry converting an operation wet-basis oxygen content variation ΔO_2,1 caused by the change of the combustion-supporting dry air volume, and according to the corresponding functional relationship between the unit loads L and the designed flue gas operation wet-basis oxygen contents O_2,b, obtaining the flue gas operation wet-basis oxygen content O_2,b,τ at the future time point Δτ, and calculating a set wet-basis oxygen content variation ΔO_2,2 within the time Δτ, and then calculating an operation wet-basis oxygen content variation ΔO₂ within the time Δτ;

$\Delta O_{2,1}=\frac{\Delta {\mathrm{BW}}_{\mathrm{air},\mathrm{act},\mathrm{dry}}\times 21}{{\mathrm{BW}}_{\mathrm{flue},\mathrm{act},\mathrm{wet}}+\Delta {\mathrm{BW}}_{\mathrm{air},\mathrm{act},\mathrm{wet}}},$ $\Delta O_{2,2}=O_{2,b,\tau }-O_{2,\mathrm{act}},$ $\Delta O_2=\Delta O_{2,1}+\Delta O_{2,2},$ where,

• • ΔO_2,1 is operation wet-basis oxygen content variation caused by the change of the combustion-supporting dry air volume, in %,
  • ΔBW_air,act,dry is combustion-supporting dry air volume variation within the time Δτ, in m³/h,
  • ΔBW_air,act,wet is combustion-supporting wet air volume variation within the time Δτ, in m³/h,
  • BW_flue,act is outlet wet flue gas volume, in m³/h,
  • ΔO_2,2 is set wet-basis oxygen content variation within the time Δτ, in %,
  • O_2,b,τ is flue gas operation wet-basis oxygen content at the future time point Δτ, in %,
  • O_2,act is actual operation wet-basis oxygen content, in %,
  • ΔO₂ is operation wet-basis oxygen content variation within the time Δτ, in %.

S8: on the basis of the coal feed rate instruction and the operation wet-basis oxygen content instruction of the original DCS sequence control logic of the unit, respectively superimposing the coal feed rate variation ΔW_coal and the operation wet-basis oxygen content variation ΔO₂ simultaneously in advance to obtain a target value W_coal,new of the coal feed rate to be adjusted and a target value O_2,new of the operation wet-basis oxygen content to be adjusted:W_coal,new=W_coal,act+ΔW_coal,O_2,new=O_2,act+ΔO₂,where,

• • W_coal,new is target value of the coal feed rate to be adjusted, in t/h,
  • W_coal,act is actual coal feed rate under the current unit load L_τ, in t/h,
  • ΔW_coal is coal feed rate variation within the time Δτ, in t/h,
  • O_2,new is target value of the operation wet-basis oxygen content to be adjusted, in %,
  • O_2,act is actual operation wet-basis oxygen content, in %,
  • ΔO₂ is operation wet-basis oxygen content variation within the time Δτ, in %.

Specific embodiments are given below for detailed explanation:

A unit of 350 MW was selected, and the calculation of the target value of the coal feed rate to be adjusted and the target value of the wet-basis oxygen content to be adjusted within 30 seconds when the current unit load was 300 MW, comprised the following steps:

S1: In a steady-state operation mode of the unit, the designed coal feed rates and flue gas operation wet-basis oxygen contents under the loads of 367.5 MW, 350 MW, 367.5 MW, 175 MW, and 87.5 MW were acquired, and the designed low calorific value of coal Q_net,b was 21.652 MJ/kg, as shown in Table 1. Then the specific functional relationship between the unit loads L and the designed coal feed rates W_coal,b was:W_coal,bk×L+b, where,

• • L is unit load, in MW;
  • W_coal,b is designed coal feed rate, in t/h;
  • k and b are both constants, and are 0.3545 and 9.7697, respectively,

TABLE 1
Coal feed rates and flue gas operation wet-
basis oxygen contents under different loads
Items	Units	Numeral values
Load rate	%	105	100	75	50	25
Unit load L	MW	367.5	350	267.5	175	87.5
Designed flue gas	%	3.6	3.6	3.6	4.5	6.8
operation wet-basis
oxygen content O2, b
Designed coal feed	t/h	139.23	135.38	103.39	72.36	40.75
rate Wcoal, b

S2: It was known that the wet-basis oxygen content O_2,b was 3.6%, the designed low calorific value of coal feed Q_net,b was 21.652 MJ/kg, then it can be calculated that W_flue,th,wet=6.059548 m³/kg and W_air,th,dry=5.716839 m³/kg, and by fitting the elemental analysis data and industrial analysis data of more than 500 sets of coal samples from utility boilers, the variances of the theoretical wet flue gas volume W_flue,th,wet per unit mass of burning coal and the theoretical combustion-supporting dry air volume W_air,th,dry per unit mass of burning coal were respectively 0.9763 and 0.9858, both of which were greater than 0.9, therefore the accuracy of the fitting was high, and the actual combustion-supporting dry air volume per unit mass of burning coal were calculated to be W_air,act,wet=6.97741 m³/kg. The designed air temperature was 20° C., the relative humidity was 55%, the atmospheric pressure was 101000 Pa, and the water vapor proportion in unit volume of dry air was calculated to be φ=0.00499.

S3: When the unit load was 367.5 MW, according to the results of industrial analysis and elemental analysis, the design value W_air,b of the required combustion-supporting dry air volume for the total burning coal per hour under the unit load of 367.5 MW was 952746 m³/h, and the design value W_flue,b of the produced wet flue gas volume was 1030231 m³/h; it was known that the designed coal feed rate W_coal,b was 139.23 t/h, then the predicted value of the required combustion-supporting dry air volume for the total burning coal per hour was W_air,h=6.97741×139.23×(1-0.42%)×1000=967385 m³/h, and the predicted value of the produced wet flue gas volume for the total burning coal per hour was W_flue,h=7.3552×139.23×(1-0.42%)×1000=1019768 m³/h; the relative deviation of the combustion-supporting dry air volume was

${\delta }_1=\frac{\left(967385-952746\right)}{952746}\times 100\%=1.54\%,$ and the relative deviation of the produced wet flue gas volume was

${\delta }_2=\frac{\left(1019768-1030231\right)}{91030231}\times 100\%=-1.02\%.$

The calculation process under other loads was the same as that under the load of 367.5 MW, and will not be repeated here. The specific calculation results are shown in Table 2.

According to the calculation results in Table 2, it can be seen that the relative deviations between the predicted values W_air,h and design values W_air,b of the required combustion-supporting dry air volume for the total burning coal per hour under different loads were 1.54%˜2.61%, the relative deviations between the predicted values W_flue,h and design values W_flue,b of the produced wet flue gas volume were −0.1.02%˜+0.63%, and the relative deviations were all between −5% and 5%, so it can be judged that the fitting deviation was within the acceptable range.

TABLE 2
Predicted values and design values of the required combustion-supporting dry air
volume and the produced wet flue gas volume for the total burning coal per hour
Items	Units	Numeral values
Unit load L	MW	367.5	350	267.5	175	87.5
Design value of the required	m3/h	952746	926373	707502	524053	341609
combustion-supporting dry air
volume for the total burning coal
per hour Wair, b
Predicted value of the required	m3/h	967385	940635	718365	531706	350539
combustion-supporting dry air
volume for the total burning coal
per hour Wair, h
Relative deviation δ1	%	1.54	1.54	1.54	1.46	2.61
Design value of the produced	m3/h	1030231	1001769	765077	564154	363923
flue gas volume for the total
burning coal per hour Wflue, b
Predicted value of the produced	m3/h	1019768	991569	757264	559082	366224
flue gas volume for the total
burning coal per hour Wflue, h
Relative deviation δ2	%	−1.02	−1.02	−1.02	−0.90	0.63

S4: When the current unit load L_τ was 300 MW, the actual coal feed rate W_coal,act displayed by the DCS system of the unit was 121.5 t/h, and through the corresponding functional relationship between the unit loads L and the designed coal feed rates W_coal,b, the designed coal feed rate under the current unit load of 300 MW was calculated to be W_coal,b,τ=0.3545×300+9.769=116.12 t/h, then the actual low calorific value of feed coal was calculated to be

$Q_{\mathrm{net},\mathrm{act}}=\frac{116.12\times 21.652}{121.5}=20.693\mathrm{MJ}/\mathrm{Kg}.$

S5: When the current unit load L_τ was 300 MW, the actual operation wet-basis oxygen content O_2,act was 3.2%,

• • then the combustion-supporting dry air volume entering the boiler can be calculated to be:

$B{W}_{air,a\mathrm{ct},\mathrm{dry}}=\left(\left(0.2496\times 20.693+0.3125\right)\times \frac{21-3.2}{21-3.2\times \left(1+0.00499\right)}+\left(0.2467\times 20.693+0.718\right)\times \frac{3.2}{21-3.2\times \left(1+0.00499\right)}\right)\times 121.5\times 1000=793414m^3/h,$

• • the wet flue gas volume at the outlet of the boiler economizer was calculated to be:

$B{W}_{\mathrm{flue},\mathrm{act},\mathrm{wet}}=\left(\left(0.2467\times 20.693+0.718\right)\times \frac{21}{21-3.2\times \left(1+0.00499\right)}+\left(0.2496\times 20.693+0.3125\right)\times \frac{21\times 0.00499}{21-3.2\times \left(1+0.00499\right)}\right)\times 121.5\times 1000=839350m^3/h.$

S6: The current time was 13:15:00, the current unit load displayed by the DCS system of the unit L_τ was 300 MW, and according to the load instruction curve of the unit set in advance by the power grid dispatch center, the unit load L_τ+Δτ after 30 seconds was 303.85 MW or 295.765 MW,

• • therefore, the unit load change rate within 30 seconds was calculated to be

$E_L=\frac{303.85-300}{300}\times 100=1.283\%\mathrm{or}$ $E_L=\frac{295.765-300}{300}\times 100=-1.412\%,$

• • the coal feed rate after 30 seconds was calculated to be

$W_{\mathrm{coal},\tau +\Delta \tau }=121.5\times \left(1+\frac{1.283}{100}\right)=123.059t/h\mathrm{or}$ $W_{\mathrm{coal},\tau +\Delta \tau }=121.5\times \left(1+\frac{-1.412}{100}\right)=119.784t/h,$

• • and the coal feed rate variation within 30 seconds was calculated to be ΔW_coal=123.059−121.5=1.559 t/h or ΔW_coal=119.784-121.5=−1.716 t/h,
  • then the combustion-supporting dry air volume variation within 30 seconds was calculated to be

$\Delta B{W}_{air,a\mathrm{ct},\mathrm{dry}}=\left(\left(0.2496\times 20.693+0.3125\right)\times \frac{21-3.2}{21-3.2\times \left(1+0.00499\right)}+\left(0.2467\times 20.693+0.718\right)\times \frac{3.2}{21-3.2\times \left(1+0.00499\right)}\right)\times 1.559\times 1000=10180m^3/h\mathrm{or}\Delta {\mathrm{BW}}_{air,a\mathrm{ct},\mathrm{dry}}=\left(\left(0.2496\times 20.693+0.3125\right)\times \frac{21-3.2}{21-3.2\times \left(1+0.00499\right)}+\left(0.2467\times 20.693+0.718\right)\times \frac{3.2}{21-3.2\times \left(1+0.00499\right)}\right)\times \left(-1.716\right)\times 1000=-11205m^3/h.$

S7: According to the combustion-supporting dry air volume variation ΔBW_air,act,dry, air,act,dry, the operation wet-basis oxygen content variation caused by the change of the combustion-supporting dry air volume was converted to be

$\Delta O_{2,1}=\frac{10180\times 21}{839350+10180\times \left(1+0.00499\right)}=0.252\%\mathrm{or}$ $\Delta O_{2,1}=\frac{-11205\times 21}{834350-11205\times \left(1+0.00499\right)}=-0.286\%,$

• • according to the corresponding functional relationship between the unit loads L and the designed flue gas operation wet-basis oxygen contents O_2,b, the design value O_2,b,τ of the flue gas operation wet-basis oxygen content after 30 seconds was 3.6%, and the set wet-basis oxygen content variation within 30 seconds was calculated to be ΔO_2,2=3.6−3.2=0.40%,
  • then the operation wet-basis oxygen content variation within 30 seconds was calculated to be ΔO₂=0.252+0.40=0.652% or ΔO₂=−0.286+0.40=0.114%.

S8: On the basis of the coal feed rate instruction and the operation wet-basis oxygen content instruction of the original sequence control logic of the unit, the coal feed rate variation and the operation wet-basis oxygen content variation were respectively superimposed simultaneously in advance to obtain the target value of the coal feed rate to be adjusted:W_coal,new=121.5+1.559=123.059 t/h or W_coal,new=121.5−1.716=119.784 t/h

and the target value of the operation wet-basis oxygen content to be adjusted:O_2,new=3.2+0.652=3.852% or O_2,new=3.2+0.114=3.314%.

According to the obtained the target value W_coal,new=119.784 t/h of the coal feed rate to be adjusted and the target value O_2,new=3.314% of the operation wet-basis oxygen content to be adjusted, the adjustments of the coal feed rate and the operation wet-basis oxygen content were simultaneously set in the same proportion.

Although the present invention has been disclosed in the form of embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

For the sake of clarity, it is to be understood that the use of ‘a’ or ‘an’ throughout this application does not exclude a plurality, and ‘comprising’ does not exclude other steps or elements.

Claims

1. A coal-air synchronous dynamic coordinated control method for a coal-fired unit, comprising the following: S1: in a steady-state operation mode of the coal-fired unit, acquiring configured coal feed rates and configured flue gas operation wet-basis oxygen contents under different loads, and then determining a corresponding functional relationship between unit loads L and the configured coal feed rates Wcoal,b, and a corresponding functional relationship between the unit loads L and the configured flue gas operation wet-basis oxygen contents O2,b;S2: using a regression analysis method, obtaining a regression function of a theoretical wet flue gas volume Wflue,th,wet per unit mass of burning coal and a theoretical combustion-supporting dry air volume Wair,th,dry per unit mass of burning coal based on a low calorific value by fitting, judging accuracy of the regression function, and calculating an actual combustion-supporting dry air volume Wair,act,dry per unit mass of burning coal, and according to air temperature, relative humidity and atmospheric pressure, calculating air saturated vapor pressure PH2O and a water vapor proportion φ per unit volume of dry air, and then calculating an actual combustion-supporting wet air volume Wair,act,wet per unit mass of burning coal and a wet flue gas volume Wflue,act,wet actually produced per unit mass of burning coal;S3: calculating predicted values Wair,h of a required combustion-supporting dry air volume and predicted values Wflue,h of a produced wet flue gas volume for the total burning coal per hour under different loads, and by comparing the predicted values Wair,h and design values Wair,b of the required combustion-supporting dry air volume, and the predicted values Wflue,h and design values Wflue,b of the produced wet flue gas volume for the total burning coal per hour under different loads, verifying whether relative deviations δ are within an acceptable range;S4: acquiring current load Lτ and actual coal feed rate Wcoal,act of the coal-fired unit, calculating a configured coal feed rate Wcoal,b,τ under the current unit load Lτ through the corresponding functional relationship between the unit loads L and the configured coal feed rates Wcoal,b, and calculating an actual low calorific value Qnet,act of feed coal;S5: according to the actual coal feed rate Wcoal,act, actual operation wet-basis oxygen content O2,act and the actual low calorific value Qnet,act of feed coal, calculating a combustion-supporting dry air volume BWair,act,dry and a combustion-supporting wet air volume BWair,act,wet entering a boiler in real time, and the wet flue gas volume BWflue,act,wet at an outlet of a boiler economizer;S6: according to a load instruction curve of the coal-fired unit, determining a target value of a load instruction at a future time point Δτ, and calculating a unit load change rate EL within the time Δτ, calculating a coal feed rate Wcoal,τ+Δτ at the future time point Δτ, and calculating a coal feed rate variation ΔWcoal within the time Δτ, and then calculating a combustion-supporting dry air volume variation ΔBWair,act,dry and a combustion-supporting wet air volume variation ΔBWair,act,wet within the time Δτ;S7: calculating an operation wet-basis oxygen content variation ΔO2,1 caused by a change of the combustion-supporting dry air volume, and according to the corresponding functional relationship between the unit loads L and the configured flue gas operation wet-basis oxygen contents O2,b, obtaining a flue gas operation wet-basis oxygen content O2,b,τ at the future time point Δτ, and calculating a set wet-basis oxygen content variation ΔO2,2 within the time Δτ, and then obtaining an operation wet-basis oxygen content variation ΔO2 within the time Δτ;S8: on the basis of a coal feed rate instruction and an operation wet-basis oxygen content instruction of an original DCS sequence control logic of the unit, respectively superimposing the coal feed rate variation ΔWcoal and the operation wet-basis oxygen content variation ΔO2 simultaneously in advance to obtain a target value Wcoal,new of the coal feed rate to be adjusted and a target value O2,new of the operation wet-basis oxygen content to be adjusted; andadjusting the coal feed rate and the operation wet-basis oxygen content of the coal-fired unit based on the target value Wcoal,new of the coal feed rate and the target value O2,new of the operation wet-basis oxygen content, obtained in S8.

2. The coal-air synchronous dynamic coordinated control method for the coal-fired unit according to claim 1, wherein the corresponding functional relationship between the unit loads L and the configured coal feed rates Wcoal,b, and the corresponding functional relationship between the unit loads L and the configured flue gas operation wet-basis oxygen contents O2,b are respectively: Wcoal,b=f(L,Qnet,b),O2,b=g(L),where,L is unit load, in MW,Wcoal,b is configured coal feed rate, in t/h,Qnet,b is configured low calorific value of coal, in MJ/kg,O2,b is configured flue gas operation wet-basis oxygen content, in %.

3. The coal-air synchronous dynamic coordinated control method for the coal-fired unit according to claim 1, wherein the theoretical wet flue gas volume Wflue,th,wet per unit mass of burning coal and the theoretical combustion-supporting dry air volume Wair,th,dry per unit mass of burning coal are obtained by fitting elemental analysis data and industrial analysis data of multiple existing sets of coal samples from utility boilers.

4. The coal-air synchronous dynamic coordinated control method for the coal-fired unit according to claim 3, wherein the industrial analysis data comprises received base ashes, received base water and low calorific values, and the elemental analysis data comprises contents of carbon, hydrogen, oxygen, nitrogen, and sulfur.

5. The coal-air synchronous dynamic coordinated control method for the coal-fired unit according to claim 1, wherein the theoretical wet flue gas volume per unit mass of burning coal Wflue,th,wet, the theoretical combustion-supporting dry air volume per unit mass of burning coal Wair,th,dry, the air saturated vapor pressure PH2O, water vapor proportion per unit volume of dry air φ, the actual combustion-supporting wet air volume Wair,act,wet per unit mass of burning coal, the wet flue gas volume Wflue,act,wet actually produced per unit mass of burning coal, and the actual combustion-supporting dry air volume Wair,act,dry per unit mass of burning coal are respectively:

6. The coal-air synchronous dynamic coordinated control method for the coal-fired unit according to claim 1, wherein the accuracy of the regression function is judged by the variances, and the variances are greater than 0.9.

7. The coal-air synchronous dynamic coordinated control method for the coal-fired unit according to claim 1, wherein the predicted value Wair,h of the required combustion-supporting dry air volume and the predicted value Wflue,h of the produced flue gas volume under a configured operation wet-basis oxygen content for the total burning coal per hour are respectively: Wair,h=Wair,act,dry×Wcoal,b×(1−γ)×1000,Wflue,h=Wflue,act,wet×Wcoal,b×(1−γ)×1000,where,Wair,h is predicted value of the required combustion-supporting dry air volume for the total burning coal per hour, in m3/h,Wflue,h is predicted value of the produced wet flue gas volume for the total burning coal per hour, in m3/h,Wair,act,dry is actual combustion-supporting dry air volume per unit mass of burning coal, in m3/kg,Wflue,th,wet is theoretical wet flue gas volume per unit mass of burning coal, in m3/kg,Wcoal,b is configured coal feed rate, in t/h,γ is incomplete combustion heat loss ratio of feed coal.

8. The coal-air synchronous dynamic coordinated control method for the coal-fired unit according to claim 1, wherein the acceptable range of the relative deviations δ is −5%˜+5%, and the relative deviations δ comprises a relative deviation δ1 between the predicted values Wair,h and design values Wair,b of the required combustion-supporting dry air volume for the total burning coal per hour, and a relative deviation δ2 between the predicted values Wflue,h and design values Wflue,b of the produced wet flue gas volume for the total burning coal per hour, and the relative deviation δ1 and the relative deviation δ2 are respectively:

9. The coal-air synchronous dynamic coordinated control method for the coal-fired unit according to claim 1, wherein the actual low calorific value Qnet,act of feed coal is:

10. The coal-air synchronous dynamic coordinated control method for the coal-fired unit according to claim 1, wherein the current unit load Lτ, the actual coal feed rate Wcoal,act, the actual operation wet-basis oxygen content O2,act, and the actual low calorific value Qnet,act of feed coal are all obtained through a DCS system of the unit.

11. The coal-air synchronous dynamic coordinated control method for the coal-fired unit according to claim 1, under the actual coal feed rate, the combustion-supporting dry air volume BWair,act,dry, the combustion-supporting wet air volume BWair,act,wet, and the wet flue gas volume BWflue,act,wet at outlet are respectively:

12. The coal-air synchronous dynamic coordinated control method for the coal-fired unit according to claim 1, wherein the load instruction curve is set in advance by a power grid dispatch center.

13. The coal-air synchronous dynamic coordinated control method for the coal-fired unit according to claim 1, wherein the unit load change rate EL within the time Δτ, the coal feed rate Wcoal,τ+Δτ at the future time point Δτ, the coal feed rate variation ΔWcoal within the time Δτ, and the combustion-supporting dry air volume variation ΔBWair,act,dry and the combustion-supporting wet air volume variation ΔBWair,act,wet within the time Δτ are respectively:

14. The coal-air synchronous dynamic coordinated control method for the coal-fired unit according to claim 1, wherein the operation wet-basis oxygen content variation ΔO2,1 caused by the change of the combustion-supporting dry air volume, the set wet-basis oxygen content variation ΔO2,2 within the time Δτ, and the operation wet-basis oxygen content variation ΔO2 within the time Δτ are respectively:

15. The coal-air synchronous dynamic coordinated control method for the coal-fired unit according to claim 1, wherein the target value Wcoal,new of the coal feed rate to be adjusted and the target value O2,new of the operation wet-basis oxygen content to be adjusted are respectively: Wcoal,new=Wcoal,act+ΔWcoal,O2,new=O2,act+ΔO2,where,Wcoal,new is target value of the coal feed rate to be adjusted, in t/h,Wcoal,act is actual coal feed rate under the current unit load Lτ, in t/h,ΔWcoal is coal feed rate variation within the time Δτ, in t/h,O2,new is target value of the operation wet-basis oxygen content to be adjusted, in %,O2,act is actual operation wet-basis oxygen content, in %,ΔO2 is operation wet-basis oxygen content variation within the time Δτ, in %.