A method for measuring bed temperature of circulating fluidized bed boiler

Abstract

The present invention discloses a method for measuring the bed temperature of a circulating fluidized bed boiler, which belongs to the technical field of wide-load peak regulation. The method comprises step S1: introducing a coal quality coefficient to correct the carbon mass fraction, constructing a mass conservation equation for carbon, an oxygen mass conservation equation, and a volatile matter mass conservation equation, and obtaining the combustion rate of carbon and the combustion rate of volatile matter in the furnace; step S2: calculating the combustion calorific value of the combustibles in the furnace; and step S3: calculating the bed temperature in the furnace based on the calorific value calculation result of step S2, in combination with the boiler thermal efficiency and the energy conservation equation, according to the functional relationship between the feed water flow rate, the main steam temperature, and the heat absorption of the water-cooled wall on the steam-water side of the CFB unit. The method of the present invention is relatively simple to calculate the bed temperature of a CFB boiler, has certain applicability, and can predict the bed temperature of a CFB boiler.

Description

Technical Field

[0001] The present invention relates to the technical field of wide load peak regulation, and in particular to a method for measuring the bed temperature of a circulating fluidized bed boiler. Background Art

[0002] Compared with pulverized coal boilers, the bed material of CFB boilers can be fully mixed with the fuel, and the fuel can be burned repeatedly during the circulation process, so the fuel burns more completely and the combustion efficiency in the boiler is higher; at the same time, the bed material in the CFB boiler can effectively capture gases such as SO2 and NOx, so the CFB boiler has less pollutant gas emissions and better environmental performance. In addition, the CFB boiler stores more unburned fuel, the energy storage in the boiler is larger, and it can adapt to the combustion of a variety of low calorific value fuels. The combustion stability in the boiler is better. Therefore, compared with traditional pulverized coal boilers, CFB boilers have many advantages. Today, with higher requirements for the cleanliness and flexibility of coal-fired power generation, CFB boilers are occupying an increasingly important position.

[0003] However, the combustion process in a CFB boiler involves a wide variety of fuels with complex compositions. Furthermore, the controlled variables within the boiler are highly coupled, and the controlled objects exhibit strong time-varying and nonlinear characteristics. Therefore, parameter control within a CFB boiler is challenging. Bed temperature, a key control parameter within a CFB boiler, significantly impacts combustion stability and flue gas pollutant emissions. Excessively high bed temperatures can lead to coking, while excessively low temperatures can result in incomplete combustion and even fire failure. Regarding pollutants, bed temperature influences the carbon reaction rate within the boiler, thereby affecting CO concentration and furnace reducibility. Since NOx denitrification requires a reducing atmosphere, bed temperature significantly impacts NOx emissions. Furthermore, bed temperature influences the thermal decomposition of limestone and the desulfurization of sulfur dioxide. Excessively high bed temperatures decompose calcium sulfate, reducing desulfurization efficiency. Excessively low bed temperatures slow the reaction rate between calcium oxide and SO2, resulting in a low desulfurization rate within the furnace. Therefore, establishing a CFB boiler bed temperature model to ensure that the bed temperature is stable near the expected value is of great significance to the safe and stable operation of the boiler and to reducing pollutant emissions from the CFB boiler. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for measuring the bed temperature of a circulating fluidized bed boiler, comprising:

[0005] Step S1: Introduce the coal quality coefficient to correct the carbon mass fraction, construct the carbon mass conservation equation, oxygen mass conservation equation and volatile mass conservation equation, and obtain the combustion rate of the carbon and the combustion rate of the volatile matter in the furnace;

[0006] Step S2: Calculate the combustion calorific value of the combustibles in the furnace;

[0007] Step S3: Calculate the bed temperature in the furnace based on the functional relationship between the feed water flow rate, main steam temperature, and water wall heat absorption on the steam-water side of the CFB unit, the calorific value calculation result of step S2, and the boiler thermal efficiency and energy conservation equation.

[0008] The mass conservation equation of the burned carbon in step S1 is as follows:

[0009]

[0010]

[0011] Where B(t) is the amount of carbon stored in the furnace, kg; t is time, s; F(t) is the amount of fuel entering the furnace at a certain moment, kg / s; η B is the proportion of fixed carbon in fuel, %; R c is the burning rate of the burning carbon, kg / s; D fw is the water flow rate, kg / s; u B is the fuel quantity instruction, kg / s; k B is the coal quality coefficient.

[0012] The oxygen mass conservation equation in step S1 is as follows:

[0013]

[0014] Where, is the oxygen concentration, kmol / m 3 ; t is time, s; V is furnace volume, m 3 ;k q is the molar conversion factor, mol / m 3 ; A ir is the total air volume, m 3 / s; R CO 、 are the combustion rates of H2, CO, and CH4, kg / s; R c is the burning rate of the burning carbon, kg / s; k f is the ratio coefficient related to the flue gas flow rate.

[0015] That is, the burning rate of carbon R c The calculation formula is as follows:

[0016]

[0017] Among them, R c is the burning rate of the burning carbon, kg / s; k c is the combustion rate constant; is the oxygen concentration, kmol / m3 ; B(t) is the amount of carbon stored in the furnace, kg; d c is the diameter of the carbon particles, m; ρ c is the density of carbon particles, kg / m 3 .

[0018] The volatile matter mass conservation equation in step S1 is as follows:

[0019]

[0020]

[0021]

[0022] Among them, C CO 、 The concentrations of CO, H2, and CH4, kmol / m 3 ; t is time, s; k q-CO 、 are the mass fraction coefficients of CO, H2, and CH4 respectively; F(t) is the amount of fuel entering the furnace at a certain moment, kg / s; k b-CO 、 are the combustion consumption coefficients of CO, H2 and CH4 respectively; R CO 、 The combustion rates of CO, H2, and CH4, kg / s; k f-CO 、 are the ratio coefficients of CO, H2 and CH4 carried away by the flue gas; A ir is the total air volume, m 3 / s.

[0023] The burning rate of the volatile matter is:

[0024]

[0025]

[0026]

[0027] Where R CO 、 The combustion rates of CO, H2, and CH4, kg / s; k CO 、 are the chemical reaction kinetic constants of CO, H2, and CH4 respectively; is the oxygen concentration, kmol / m 3 ; is the water vapor concentration, kmol / m 3 .

[0028] The calculation formula for the combustion calorific value of the combustibles in the furnace in step S2 is as follows:

[0029] Q s =Q B +Q2

[0030] Q B =R c ·H c

[0031]

[0032] Where Q s is the total heat released by the combustion of fuel in the boiler at a certain moment, MJ / s; Q B is the heat released by the burning carbon in the furnace at a certain moment, MJ / s; R c is the burning rate of the burning carbon, kg / s; H c is the calorific value of carbon, MJ / kg; Q2 is the calorific value of the volatile matter in the furnace at a certain moment, MJ / s; R CO 、 The combustion rates of CO, H2, and CH4 are kg / s; H CO 、 They are the calorific values ​​of CO, H2, and CH4, MJ / kg respectively.

[0033] The calculation formula for the bed temperature in the furnace in step S3 is as follows:

[0034]

[0035]

[0036] Where, T is the boiler bed temperature, K; C s is the specific heat capacity of the bed material, MJ / (kg·K); M s is the mass of the bed material in the furnace, kg; t is the time, s; η is the thermal efficiency of the boiler, %; Q s is the total heat released by the combustion of fuel in the boiler at a certain moment, MJ / s; Q t is the heat absorption of the water-cooled wall per unit time, MJ / s; k4 is the average heat transfer coefficient; T st is the main steam temperature, K; D fw is the water flow rate, kg / s; n is the model coefficient.

[0037] The beneficial effects of the present invention are:

[0038] (1) The method of the present invention is relatively simple to calculate the bed temperature of a CFB boiler and has certain applicability, and can predict the bed temperature of a CFB boiler;

[0039] (2) The method of the present invention can calculate the oxygen content in the CFB boiler, which is beneficial to the dynamic process analysis of combustion in the CFB boiler and the subsequent control of the oxygen content of the CFB boiler;

[0040] (3) The present invention can obtain the amount of residual carbon in the furnace and the calorific value of the boiler at the current moment through soft measurement, which is helpful to provide guidance for the control of coal feeding amount and improve the safety and economy of the CFB unit. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 The figure is a flow chart of the method for measuring the bed temperature of a circulating fluidized bed boiler according to the present invention. DETAILED DESCRIPTION

[0042] The present invention provides a method for measuring the bed temperature of a circulating fluidized bed boiler, which will be further described below with reference to the accompanying drawings and specific embodiments.

[0043] Figure 1 This is a flow chart of the method for measuring the bed temperature of a circulating fluidized bed boiler according to the present invention. Specifically, it includes:

[0044] S1 Calculate the heat generated in the furnace: The heat generated in the furnace is calculated based on the conservation of mass. The conservation of mass includes the conservation of the mass of the burning carbon and the mass of the volatile matter. The conservation of the mass of the burning carbon can be expressed as the carbon fed into the furnace at a certain moment minus the carbon consumed by combustion in the furnace, as shown in formula (1):

[0045]

[0046] Where B(t) is the amount of carbon stored in the furnace, F(t) is the amount of fuel entering the furnace at a certain moment, kg / s; η B is the proportion of fixed carbon in fuel, %; R c is the combustion rate of the carbon, kg / s. In CFB boilers, the influence of coal quality changes on the combustion calculation cannot be ignored. Therefore, the ratio of feed water flow rate to feed instruction is introduced to correct the mass fraction of carbon in the fuel. The correction formula is written as:

[0047]

[0048] Where η B is the mass fraction of carbon after correction, %; D fw is the water flow rate, kg / s; u B is the fuel quantity instruction, kg / s; k B is the coal quality coefficient. The combustion rate formula of the burning carbon in the furnace can be written as:

[0049]

[0050] Where k cThe combustion rate constant reflects the rate of change of the burning rate of the burning carbon in the furnace, and thus determines the rate of change of the burning carbon inventory in the furnace. is the oxygen concentration, kmol / m 3 ;d c is the diameter of the carbon particles, m; ρ c is the density of carbon particles, kg / m 3 Part of the volatile matter released from the fuel is consumed by combustion, and part is carried away by the flue gas. Based on the law of conservation of mass, the molar concentration of each component of the volatile matter can be written as:

[0051]

[0052]

[0053]

[0054] Where k q-x is the mass fraction coefficient of CO, H2, and CH4, which is related to the mass fraction of each component in the fuel; F(t) is the amount of fuel entering the furnace at a certain moment, kg / s; k b-x k is the combustion consumption coefficient of each component; f-x A is the ratio coefficient of each component carried away by the flue gas; ir is the total air volume, m 3 / s. The main products of volatile matter include CO, H2, and CH4. The calculation method of their respective combustion rates is calculated using the following formula:

[0055]

[0056]

[0057]

[0058] Where R x is the combustion rate of each component, kg / s; k x is the chemical reaction kinetic constant of each component; C x is the molar concentration of each component, kmol / m 3 Considering that the fuel will be dehydrated before entering the furnace, the moisture concentration is calculated as a constant. The reaction kinetic constant is related to temperature. The reaction kinetic constant of each volatile component is written as:

[0059]

[0060]

[0061]

[0062] Where k1, k2, and k3 are the kinetic coefficients of CO, H2, and CH4, respectively; T is the bed temperature, K. Part of the oxygen blown into the furnace is consumed by fuel combustion, and part is blown out of the furnace by wind. The average oxygen concentration in the furnace is written as:

[0063]

[0064] In the formula, V represents the furnace volume, m 3 ;k q is the molar conversion factor, mol / m 3 ;k f It is a ratio coefficient related to the flue gas flow rate, which represents the inertia of the change in oxygen concentration.

[0065] S2: Calculate the calorific value of the combustibles in the furnace; the heat released by the burning carbon in the furnace at a certain moment is:

[0066] Q B =R c ·H c (14)

[0067] Where: Q B is the heat released by the burning carbon in the furnace at a certain moment, MJ / s; H c is the calorific value of carbon, MJ / kg. The heat released by the combustion of volatile matter at a certain moment is:

[0068]

[0069] Where, Q2 is the calorific value of the volatile matter in the furnace at a certain moment, MJ / s; H x is the calorific value of each component of the volatile matter, H CO Take it as 10.08MJ / kg, is 124.24MJ / kg, The total heat generated in the furnace is:

[0070] Q s =Q B +Q2 (16)

[0071] Where, Q s It is the total heat released by fuel combustion in the boiler at a certain moment, MJ / s.

[0072] S3 predicts bed temperature: The boiler bed temperature is calculated by energy conservation. The heat released in the furnace comes from the combustion of carbon and volatile matter. Part of the released heat will be absorbed by the flue gas and carried away, which is the lost heat. Part of it will be absorbed by the boiler to heat the working fluid to do work, and part of it will be absorbed by the bed material in the furnace and stored in the furnace in the form of bed temperature. In practice, the main steam temperature is used to represent the lumped parameter of the metal wall temperature and the working fluid temperature. Therefore, the heat absorption of the water-cooled wall is expressed as:

[0073]

[0074] Where Q t is the heat absorption of the water-cooled wall per unit time, MJ / s; k4 is the average heat transfer coefficient; T st is the main steam temperature, K; D fw is the feed water flow rate, kg / s; n is the model coefficient. The effective heat of the boiler is represented by multiplying the total heat release by the boiler thermal efficiency. Based on the law of energy conservation, the calculation formula for the furnace bed temperature is:

[0075]

[0076] Where C s is the specific heat capacity of the bed material, MJ / (kg·K); M s is the mass of the bed material in the furnace, kg; η is the thermal efficiency of the boiler, %; Q s ·η represents the remaining effective heat per unit time after removing the heat carried away by the flue gas.

[0077] The method of the present invention is relatively simple to calculate the bed temperature of the CFB boiler and has certain applicability. It can predict the bed temperature of the CFB boiler, which is beneficial to provide guidance for controlling the coal feed rate and improving the safety and economy of the CFB unit.

Claims

1. A method for measuring the bed temperature of a circulating fluidized bed boiler, characterized in that: include: Step S1: Introduce the coal quality coefficient to correct the carbon mass fraction, construct the carbon mass conservation equation, oxygen mass conservation equation and volatile mass conservation equation, and obtain the combustion rate of the carbon and the combustion rate of the volatile matter in the furnace; The mass conservation equation of the burned carbon in step S1 is as follows: Where B(t) is the amount of carbon stored in the furnace, kg; t is time, s; F(t) is the amount of fuel entering the furnace at a certain moment, kg / s; η B is the proportion of fixed carbon in fuel, %; R c is the burning rate of the burning carbon, kg / s; D fw is the water flow rate, kg / s; u B is the fuel quantity instruction, kg / s; k B is the coal quality coefficient; The oxygen mass conservation equation in step S1 is as follows: Where, is the oxygen concentration, kmol / m 3 ; t is time, s; V is furnace volume, m 3 ;k q is the molar conversion factor, mol / m 3 ; A ir is the total air volume, m 3 / s; R CO 、 are the combustion rates of H2, CO, and CH4, kg / s; R c is the burning rate of the burning carbon, kg / s; k f is the ratio coefficient related to the flue gas flow rate; That is, the burning rate of carbon R c The calculation formula is as follows: Among them, R c is the burning rate of the burning carbon, kg / s; k c is the combustion rate constant; is the oxygen concentration, kmol / m 3 ; B(t) is the amount of carbon stored in the furnace, kg; d c is the diameter of the carbon particles, m; ρ c is the density of carbon particles, kg / m 3 ; The volatile matter mass conservation equation in step S1 is as follows: Among them, C CO 、 The concentrations of CO, H2, and CH4, kmol / m 3 ; t is time, s; k q-CO 、 are the mass fraction coefficients of CO, H2, and CH4 respectively; F(t) is the amount of fuel entering the furnace at a certain moment, kg / s; k b-CO 、 are the combustion consumption coefficients of CO, H2 and CH4 respectively; R CO 、 The combustion rates of CO, H2, and CH4, kg / s; k f-CO 、 are the ratio coefficients of CO, H2 and CH4 carried away by the flue gas; A ir is the total air volume, m 3 / s; The burning rate of the volatile matter is: Where R CO 、 The combustion rates of CO, H2, and CH4, kg / s; k CO 、 are the chemical reaction kinetic constants of CO, H2, and CH4 respectively; is the oxygen concentration, kmol / m 3 ; is the water vapor concentration, kmol / m 3 ; Step S2: Calculate the combustion calorific value of the combustibles in the furnace; The calculation formula for the combustion calorific value of the combustibles in the furnace in step S2 is as follows: Q s =Q B +Q2 Q B =R c ·H c Where Q s is the total heat released by the combustion of fuel in the boiler at a certain moment, MJ / s; Q B is the heat released by the burning carbon in the furnace at a certain moment, MJ / s; R c is the burning rate of the burning carbon, kg / s; H c is the calorific value of carbon, MJ / kg; Q2 is the calorific value of the volatile matter in the furnace at a certain moment, MJ / s; R CO 、 The combustion rates of CO, H2, and CH4 are kg / s; H CO 、 are the calorific values ​​of CO, H2, and CH4, MJ / kg; Step S3: Calculate the bed temperature in the furnace based on the functional relationship between the feed water flow rate, main steam temperature, and water wall heat absorption on the steam-water side of the CFB unit, the calorific value calculated in step S2, and the boiler thermal efficiency and energy conservation equation; The calculation formula of the bed temperature in the furnace in step S3 is as follows: Where, T is the boiler bed temperature, K; C s is the specific heat capacity of the bed material, MJ / (kg·K); M s is the quality of the bed material in the furnace, kg; t is time, s; η is boiler thermal efficiency, %; Q s is the total heat released by the combustion of fuel in the boiler at a certain moment. MJ / s; Q t is the heat absorption of the water-cooled wall per unit time, MJ / s; k4 is the average heat transfer coefficient; T st is the main steam temperature, K; D fw is the water flow rate, kg / s; n is the model coefficient.

Images