Multi-fuel combustion gas boiler coupling biomass direct combustion power generation method

Abstract

The invention discloses a multi-fuel combustion gas boiler coupling biomass direct combustion power generation method, and belongs to the field of biomass utilization. According to the method, after basic characteristics, combustion characteristics and dynamic characteristics of fuel are analyzed, a numerical model is established; then, calculating the numerical model of the blending combustion biomass fuel, and determining the optimal biomass fuel fineness, nozzle position and blending combustion proportion; and according to a numerical simulation result, corresponding process system design optimization and combustion adjustment measure formulation are carried out. According to the method, on the basis of raw material basic physicochemical and combustion characteristic analysis, a coal-biomass-coke oven gas large-difference fuel multi-element mixed combustion model is constructed, and on the basis of numerical simulation calculation, safe and stable mixed combustion of biomass fuel in the positive-pressure direct-fired pulverized coal boiler for large-proportion mixed combustion of surplus coal / coke oven gas of an iron and steel plant is achieved.

Description

Technical Field

[0001] The invention relates to biomass utilization, and in particular to a method for coupling a mixed-fired gas boiler with biomass direct-fired power generation. Background Art

[0002] Biomass energy is a renewable energy source that is considered to have the attribute of "zero-carbon resource". Using biomass fuel to partially replace coal in thermal power generating units can significantly reduce carbon dioxide emissions and produce huge environmental benefits. In addition, recycling waste biomass raw materials and coke oven gas and coupling combustion with coal will help reduce coal consumption and reduce power generation costs.

[0003] Biomass power generation technologies include gasification power generation, direct combustion power generation, co-combustion with coal power generation, gasification followed by coupled combustion power generation, etc. At present, the most commonly used biomass coal-fired coupled power generation in the industry is mainly divided into indirect combustion power generation and direct combustion power generation.

[0004] The above-mentioned process technologies each have their own advantages and disadvantages: the direct combustion technology of pulverized coal furnace requires small investment but has problems such as unqualified steam parameters, low burnout rate, easy slagging on the heating surface and risk of corrosion on the heating surface; the gasification coupling technology has problems such as large investment, complex process and small gas output.

[0005] The prior art does not provide technical experience guidance for the biomass coupling of positive pressure direct-blowing pulverized coal boilers that co-fire a large proportion of surplus high / coke oven gas in steel plants. Based on this, the present invention provides a method for achieving safe and stable combustion of biomass-coal-gas three-phase coupling. Summary of the invention

[0006] Purpose of the invention: The purpose of the present invention is to provide a method for coupling a mixed-fired gas boiler with biomass direct-fired power generation.

[0007] Technical solution: A method for coupling biomass direct combustion power generation with a mixed-fired gas boiler, basic characteristics analysis of biomass raw materials and common coal, blast furnace gas, and coke oven gas to be mixed for long-term stable combustion, combustion characteristics and kinetic characteristics analysis of biomass, pulverized coal and their mixtures, construction of pure pulverized coal combustion model and multi-fuel gas-solid mixed combustion model; numerical simulation calculation of biomass fuel blending under multiple working conditions to determine the optimal biomass fuel fineness, nozzle position and blending ratio; according to the results of numerical simulation tests, the corresponding powder delivery injection system control process optimization and combustion adjustment measures are formulated, so as to optimize the process system design and thermal control logic, and formulate reliable combustion adjustment measures. Realize safe and stable combustion of biomass-coal-gas three-phase coupling in a positive pressure direct-blowing pulverized coal boiler with a large proportion of mixed combustion of blast furnace gas / coke oven gas.

[0008] Preferably, the biomass raw materials to be co-combusted stably over a long period of time refer to biomass raw materials of considerable quantity, mature market circulation conditions, and high cost-effectiveness that can be stably obtained.

[0009] Preferably, the basic characteristic analysis includes conducting drying characteristic, grindability (crushing) characteristic experiments and physical and chemical property analysis of the biomass raw materials to clarify the relationship between the weight loss characteristics, preparation fineness, quality, yield and energy consumption of the biomass.

[0010] Through elemental analysis and industrial analysis, the changes in the physical and chemical properties of pretreated biomass and their impact on the co-combustion process are clarified.

[0011] Preferably, the analysis of the combustion characteristics and thermodynamic characteristics of biomass, coal powder and their mixtures includes combustion characteristics and thermodynamic experiments of biomass burning alone and mixed burning with coal powder, analyzing the volatile matter and fixed carbon combustion temperature of the biomass raw material and coal powder and the melting characteristic temperature of the ash component.

[0012] Clarify the thermal weight loss characteristics TG-DTG curve, ignition temperature Ti, burnout temperature Th under different blending ratios, and evaluate its activation energy E, burnout characteristic index Cb, and comprehensive combustion characteristic index S.

[0013] The optimal blending ratio is selected based on the degree of mutual promotion and inhibition of the ignition, burnout, ash slagging and pollutant emission characteristics during the co-combustion process.

[0014] Preferably, the pure coal powder combustion model and the multi-fuel gas-solid mixed combustion model are constructed by referring to the design specifications of the selected boiler (drawings such as furnace structure dimensions, burner and heat exchange surface layout, etc.) and using Gambit for numerical modeling.

[0015] Preferably, the model selects a suitable grid system to divide the number of grids for the boiler, determines the boundary conditions, selects the design coal type for thermal calculation based on the boiler BMCR operating conditions, generates a mesh file and imports it into the Fluent software for numerical simulation calculation. When the number of grid divisions is 980,000, 1.21 million, and 1.55 million, the corresponding simulation results are compared, and it is found that when the number of grids exceeds 1.21 million, the difference in simulation calculation results is very small, and it has nothing to do with the number of grids, so the number of grids is selected as 1.21 million.

[0016] Preferably, the accuracy of the model should be verified by comparing the numerical simulation calculation results of pure coal powder combustion and designed mixed gas combustion conditions with the relevant thermal parameters in the design manual and the actual operating condition parameters to ensure the reasonable accuracy of the model.

[0017] According to the results of numerical simulation calculations, the velocity field, temperature field and flue gas component concentration field distribution in the furnace under actual operating conditions are obtained.

[0018] Preferably, the numerical simulation calculation of the blended combustion of biomass fuels includes performing numerical simulation calculations of blended combustion of biomass fuels of different finenesses and quantity ratios through burner nozzles at different heights under pure coal combustion conditions and designed mixed combustion gas volume conditions.

[0019] Preferably, the combustion adjustment is performed by performing corresponding secondary air distribution adjustment according to changes in the blending conditions during the numerical simulation calculation.

[0020] The velocity field, temperature field, flue gas components and distribution of pollutants such as NOx during the combustion process in the furnace under different co-firing conditions are analyzed.

[0021] Evaluate the influence of biomass injection position, fineness and quantity on the overall combustion performance of the furnace and pollutant emission characteristics relative to the original design conditions. Select the optimal biomass nozzle position by analyzing the simulation results.

[0022] Preferably, the process system design includes connecting the biomass raw materials to the transfer conveying system through the discharge hopper and the crushing and screening system to the existing coal conveying belt, and switching into the corresponding silo using the existing coal plow, wherein: the crushing and screening system, involving the crusher and the screening device and its intermediate storage bin, adopts a fully enclosed automatic dust prevention system setting; the transfer conveying system design includes a discharge hopper with a dust collector, a closed transfer belt and a falling dust prevention cover and a dust suppression device;

[0023] The adjustment of the powder feeding and blowing system is to utilize the existing belt feeding facilities, coal bunker and the corresponding standby pulverizing system MPS coal mill, the hot primary air blower directly into the furnace combustion and the corresponding thermal control system logic conditions and parameter optimization;

[0024] Optimization of the thermal control logic of the process system includes the use of lifting MPS coal mill grinding rollers in the hot primary air pulverizing system, adjusting the angle of the folding baffle separator at the coal mill outlet, adjusting the primary air temperature and flow parameters, matching the wood powder concentration to meet the necessary conditions for transportation, setting the coal mill outlet temperature parameters to prevent deflagration in the mill, and setting the secondary air door air distribution opening parameters.

[0025] Beneficial effects: Based on the analysis of the basic physical and chemical characteristics of raw materials and combustion characteristics, the present invention constructs a multi-component mixed combustion model of large-difference fuels of coal, biomass and coke oven gas, and based on numerical simulation calculations, realizes the safe and stable blending of biomass fuels in a positive-pressure direct-blowing pulverized coal boiler that burns a large proportion of surplus high / coke oven gas in a steel plant. This can prevent factors such as improper fuel selection and preparation quality control, unreasonable injection position, improper blending ratio, unreasonable air distribution, etc. from causing extreme problems in the operation of the boiler system, such as deterioration of combustion, overheating of the heating surface, slagging, corrosion, unqualified steam parameters, substandard pollutant emissions, and secondary combustion in the tail flue. It has the advantages of safe process, simple system, small scope of modification, and easy switching. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is the workflow diagram of data condition preparation and numerical simulation;

[0027] Figure 2 is a schematic diagram of coupled biomass fuel delivery;

[0028] Figure 3 It is a schematic diagram of the furnace structure;

[0029] Figure 4 It is a schematic diagram of furnace grid division;

[0030] Figure 5 It is the velocity field analysis diagram in the furnace;

[0031] Figure 6 It is the temperature field analysis diagram in the furnace;

[0032] Figure 7 It is an analysis diagram of flue gas components and pollutant NO in the furnace. DETAILED DESCRIPTION

[0033] In order to make the technical solution of the present invention clearer, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0034] Example 1

[0035] Construction and numerical simulation of multi-fuel gas-solid mixed combustion model. Take the DG1165.49 / 17.5-Ⅱ21 model 350MW subcritical natural circulation, intermediate reheat, natural circulation drum boiler as an example: single furnace Π-type layout, four-corner tangential combustion, tail double flue, solid slag removal, balanced ventilation, all-steel frame suspension structure, operating floor elevation of 12.6m; four-corner layout, tangential combustion, direct current swing type pulverized coal burner, imaginary tangential circle diameter is Φ1032mm and Φ548mm; pulverizing system is medium-speed mill heat primary fan positive pressure direct blowing type, the primary fan is arranged downstream of the air preheater, using 5 medium-speed coal mills, one coal mill is equipped with one primary fan; when all the designed coal types are burned, 4 are in operation and 1 is on standby. Each corner is equipped with 20 layers of nozzles, including five layers of primary air nozzles, two layers of independent top secondary air (OFA, i.e., ember air) nozzles, eight layers of secondary air nozzles (of which three layers are equipped with fuel oil spray guns), two layers of coke oven gas nozzles, and three layers of blast furnace gas nozzles; peripheral air is arranged around the primary air nozzles, and the appropriate swing of the burner nozzle can change the position of the flame center for combustion adjustment. According to the analysis of the furnace structure (such as Figure 3 ), the area from the bottom of the cold ash hopper to the furnace outlet is taken as the entire calculation area, and the model is divided into six parts from bottom to top: the cold ash hopper area, the lower area of ​​the burner, the main combustion area, the upper area of ​​the burner, the flame folding angle area and the horizontal flue.

[0036] It is particularly pointed out that various heat exchangers are arranged on the top of the furnace and in the horizontal flue for mass transfer and heat transfer. Considering the impact of specific heat transfer on the furnace, the grid division is relatively complex and requires high computer performance. In numerical simulation, grid division is the key to simulation calculation (such as Figure 4 ).

[0037] In the burner area, pulverized coal and air continuously enter the furnace for combustion. Parameters such as temperature, velocity, and pressure change dramatically. When there is a certain angle between the flow field and the grid, there is a certain pseudo-diffusion phenomenon in the vertical direction of the dependent variable. This phenomenon will lead to inaccurate calculation results. Therefore, special attention should be paid to this problem when dividing the grid to ensure the accuracy of the calculation results.

[0038] The specific settings are required according to the actual furnace type and fuel type, and are not specifically limited in this application.

[0039] The preferred boundary condition setting is that the burner nozzle adopts the velocity inlet boundary; since the temperature difference of the water-cooled wall will not have a significant impact on the temperature field after stable combustion in the furnace, the wall is set to a constant temperature condition. When the boiler operates under BMCR conditions, the wall temperature has little effect and is set to 790K for the convenience of calculation; since the boiler is in a slightly negative pressure operating state, the model outlet is set to a pressure outlet condition with a pressure of -50Pa.

[0040] Furthermore, based on the BMCR working conditions and previous experimental data results, numerical simulation research was carried out for a 10%wt biomass blending ratio. Five layers of fuel inlet were set up, so that biomass was sprayed into the furnace from the A, C, and E layer nozzles corresponding to the upper, middle, and lower three main combustion zones, and the remaining nozzles were sprayed with pulverized coal. The total excess air coefficient was set to 1.2 to explore the influence of the biomass injection position on the overall combustion performance of the furnace and the pollutant emission characteristics.

[0041] In order to further observe the furnace temperature distribution after biomass is added, the average temperature of the furnace section along the height direction is taken to observe the influence of different biomass injection positions on the furnace temperature distribution in the height direction.

[0042] The BMCR working condition was selected to mix pulverized coal with blast furnace gas, coke oven gas, and biomass for combustion. The air distribution mode after adding coal gas to the boiler was changed according to the on-site working conditions. The velocity field and temperature field (such as Figure 5 , 6 ), flue gas components and pollutants such as NO (such as Figure 7 ) and other parameters. The influence of biomass blending on the co-combustion of gas and pulverized coal is clarified.

[0043] The total heat input to the boiler under the selected model BMCR condition is calculated as: Q = B g ×Q g .

[0044] Where, Q is the total heat input into the furnace, kJ / s; B g is the coal consumption under BMCR conditions, kg / s; Q g It is the lower calorific value of the coal involved, kJ / kg.

[0045] γ is defined as the ratio of the heat generated by coal gas to the total heat input to the boiler. Therefore, when mixing coal gas, the volume flow rate calculation formula is: V g =γQ / Q v .

[0046] Where: Q v is the volume calorific value of gas, kJ / m 3 ; V g is the gas volume flow rate required for blending, m 3 / s.

[0047] The blast furnace / coke oven gas nozzle is selected as the gas nozzle, so the gas velocity is: v g =V g / 4S.

[0048] Where: v g is the gas inlet velocity, m / s; S is the total area of ​​the gas nozzle, m 2 .

[0049] The theoretical air volume for complete combustion of blast furnace gas can be calculated as follows: V G1 =(0.5H 2 +0.5CO) / 21.

[0050] Where V G1 is the theoretical air volume for complete combustion of blast furnace gas, m 3 / m 3 .

[0051] The theoretical air volume for complete combustion of coke oven gas can be calculated as follows:

[0052] V G2 =(0.5H 2 +0.5CO+2CH 4 +3C 2 H 4 +305C 2 H 6 ) / twenty one.

[0053] Where V G2 is the theoretical air volume for complete combustion of coke oven gas, m 3 / m3 .

[0054] Therefore, the total amount of air required for gas is: V gas =V Gi ×V g .

[0055] Where: V gas is the total amount of air required for gas blending, m 3 / s.

[0056] The total amount of air required for fuel under blending conditions: V = α 1 ×V coal +α 2 ×V gao +α 3 ×V jiao +α 4 ×V bio .

[0057] Where: V is the total amount of air required for blending, m 3 / s; α 1 , α 2 , α 3 , α 4 are the excess air coefficients of coal, blast furnace gas, coke oven gas and biomass respectively; V coal 、V gao 、V jiao 、V bio The total amount of air required for complete combustion when mixing coal, blast furnace gas, coke oven gas and biomass, m 3 / s.

[0058] Due to the large mixing ratio of coal gas, in order to ensure the stability of the flow field in the furnace, the primary air share is appropriately reduced to ensure that the wind speed changes of the primary and secondary air are within a reasonable range.

[0059] According to the above simulation results of the co-combustion of coal and biomass, while ensuring that the total input heat remains unchanged, it is calculated that the efficiency of pure coal powder combustion is 86.56%. When biomass is sprayed into the furnace from the lower layer (A layer), the middle layer (C layer), and the upper layer (E layer), respectively, the corresponding thermal efficiencies are 85.75%, 84.55%, and 85.12%. The thermal efficiency decreases after adding biomass, but when eucalyptus is sprayed into the furnace from the lower layer (A layer), it is close to the pure coal powder combustion condition.

[0060] The bottom layer (Layer A) was selected as the biomass inlet, and the effects of adding biomass on the furnace temperature, velocity field and pollutant NOx distribution were analyzed.

[0061] Considering the comprehensive experimental results, when biomass is sprayed into the furnace at the bottom layer (A), the temperature distribution is close to that of pure coal powder combustion, the high temperature zone is relatively smaller, and the effect of promoting coal powder combustion is better. x The volume fraction is also reduced. At the same time, the average temperature of the furnace outlet section and the outlet flue gas flow rate are similar to the actual pure coal powder combustion conditions of the boiler on site, so it is more appropriate to choose the lowest layer as the eucalyptus injection port.

[0062] Example 2

[0063] The method also provides a design scheme for a biomass blending application process system, including a feeding and conveying system and an injection process system. The feeding and conveying system is provided with a discharge hopper with a dust collector, a crusher and a screening device and an intermediate storage bin, and adopts a fully enclosed, automated, environmentally friendly dust prevention system with explosion-proof function, a closed transfer belt, a falling dust prevention cover and a dust suppression device. The biomass raw material is connected to the transfer conveying system to the existing coal conveyor belt through the discharge hopper and the crushing and screening system, and is switched into the corresponding silo using the existing coal plow. The powder feeding and injection process system uses existing facilities for local adjustments, using the existing belt feeding facilities, coal bins and the corresponding standby pulverizing system MPS pulverizer, and the hot primary fan blows directly into the furnace for combustion, and the corresponding thermal control system logic conditions and parameters are optimized.

[0064] The optimization of the thermal control logic of the process system includes the hot primary air pulverizing system adopting the lifting of the MPS coal mill roller, adjusting the angle of the coal mill outlet baffle separator, adjusting the primary air temperature and flow parameters, matching the wood powder concentration to meet the necessary conditions for transportation, setting the coal mill outlet temperature parameter to prevent deflagration in the mill, and setting the secondary air door air distribution opening parameter, among which:

[0065] The biomass powder fineness is 50-100 mesh; the moisture content of biomass is 10-15%; the outlet temperature of the coal mill is controlled at 60-70℃; the low calorific value of biomass fuel is >3100kcal / kg.

[0066] Since biomass fuel has the phenomenon of wind rushing, the secondary air distribution should make adaptive adjustments to the damper opening.

[0067] The startup logic conditions of the coal feeder, coal mill and primary fan corresponding to biomass fuel are adaptively modified.

[0068] The system parameters are reasonably designed to meet the requirements of safe, stable and economical operation of direct combustion of biomass in boilers that burn high- / coke gas in large proportions. The system transformation is simple, with low investment and operating costs.

[0069] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A method for power generation by coupling a mixed-fired gas boiler with biomass direct combustion, characterized in that: The basic characteristics of biomass raw materials and common coal, blast furnace gas, and coke oven gas that are intended for long-term stable co-combustion are analyzed, the combustion characteristics and kinetic characteristics of biomass, coal powder and their mixtures are analyzed, and a pure coal powder combustion model and a multi-fuel gas-solid mixed combustion model are built; numerical simulation calculations of biomass fuel co-combustion under multiple operating conditions are carried out to determine the optimal biomass fuel fineness, nozzle position and co-combustion ratio; based on the results of numerical simulation tests, the corresponding powder feeding and injection system control process is optimized and combustion adjustment measures are formulated, thereby optimizing the process system design and thermal control logic and formulating reliable combustion adjustment measures.

2. The method for power generation by coupling mixed-fired gas boiler with biomass direct combustion according to claim 1 is characterized in that: The biomass raw materials intended for long-term stable co-combustion refer to biomass raw materials with considerable quantity, mature market circulation conditions, and high cost-effectiveness that can be stably obtained.

3. The method for power generation by coupling mixed-fired gas boiler with biomass direct combustion according to claim 1 is characterized in that: Basic property analysis includes drying property, grindability test and physical and chemical property analysis of biomass raw materials, clarifying the relationship between biomass weight loss property, preparation fineness, quality, yield and energy consumption.

4. The method for power generation by coupling mixed-fired gas boiler with direct-fired biomass combustion according to claim 1 is characterized in that: The analysis of the combustion characteristics and thermodynamic characteristics of biomass, coal powder and their mixtures includes the combustion characteristics and thermodynamic experiments of biomass burning alone and mixed burning with coal powder, and the analysis of the volatile matter and fixed carbon combustion temperature of biomass raw materials and coal powder as well as the melting characteristic temperature of ash components.

5. The method for power generation by coupling mixed-fired gas boiler with direct-fired biomass combustion according to claim 1 is characterized in that: The pure coal powder combustion model and the multi-fuel gas-solid mixed combustion model were built by referring to the design manual of the selected boiler and using Gambit for numerical modeling.

6. The method for power generation by coupling mixed-fired coal-gas boiler with direct-fired biomass combustion according to claim 1, characterized in that: The model selects a suitable grid system to divide the number of grids for the boiler, determines the boundary conditions, selects the design coal type for thermal calculation based on the boiler BMCR operating conditions, generates a mesh file and imports it into the Fluent software for numerical simulation calculations.

7. The method for power generation by coupling mixed-fired gas boiler with direct-fired biomass combustion according to claim 1, characterized in that: The accuracy of the model should be verified by comparing the numerical simulation calculation results of pure coal powder combustion and designed mixed gas combustion conditions with the relevant thermal parameters in the design manual and the actual operating condition parameters to ensure the reasonable accuracy of the model.

8. The method for power generation by coupling mixed-fired gas boiler with direct-fired biomass combustion according to claim 1, characterized in that: The numerical simulation calculation of the blended combustion of biomass fuels includes the numerical simulation calculation of the blended combustion of biomass fuels of different fineness and quantity ratio through burner nozzles of different heights under pure coal combustion conditions and designed mixed combustion gas volume conditions.

9. The method for power generation by coupling mixed-fired coal-gas boiler with direct-fired biomass combustion according to claim 1, characterized in that: Combustion adjustment is the corresponding secondary air distribution adjustment according to the changes in the blending conditions during the numerical simulation calculation.

10. The method for power generation by coupling mixed-fired gas boiler with direct-fired biomass combustion according to claim 1, characterized in that: The process system design includes connecting the biomass raw materials to the transfer conveying system through the discharge hopper and the crushing and screening system to the existing coal conveying belt, and switching to the corresponding silo using the existing coal plow, including: the crushing and screening system, involving the crusher and screening device and its intermediate storage bin, adopts a fully enclosed automatic dust prevention system setting; the transfer conveying system design includes a discharge hopper with a dust collector, a closed transfer belt and a falling dust prevention cover and dust suppression device; The adjustment of the powder feeding and blowing system is to utilize the existing belt feeding facilities, coal bunker and the corresponding standby pulverizing system MPS coal mill, the hot primary air blower directly into the furnace combustion and the corresponding thermal control system logic conditions and parameter optimization; Optimization of the thermal control logic of the process system includes the use of lifting MPS coal mill grinding rollers in the hot primary air pulverizing system, adjusting the angle of the folding baffle separator at the coal mill outlet, adjusting the primary air temperature and flow parameters, matching the wood powder concentration to meet the necessary conditions for transportation, setting the coal mill outlet temperature parameters to prevent deflagration in the mill, and setting the secondary air door air distribution opening parameters.

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