Technical route and methods for retrofitting using two-stage air-staged combustion technology

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The technical approach for the technological transformation using air-segregated combustion: There are no detailed calculation methods available for the combustion process in coal-fired boilers. Relevant documents and standards involve the design of burners based on the selection of certain empirical parameters, and these same principles should be followed when carrying out transformations using air-segregated combustion. The technical roadmap for air-staged combustion retrofitting was derived from the practical experience of retrofitting 5 burners; it is applicable to burner retrofits and facilitates the widespread adoption of low-NOX combustion technologies in such retrofit projects. The technical roadmap for applying low NOX combustion technology in quadrangular cut-circle coal-fired boilers is shown in Figure 4. http://www.lyxgsh.com/d/file/news/cpzs/2016-04-05/924019c0560b06db144a4e2764518d4e.jpg Methods for determining the air excess factor in the main combustion zone: When the air excess factor in the main combustion zone is 0.7, the best results are achieved in terms of NOX reduction. However, an excessively low air excess factor can affect the organization of the aerodynamic field within the furnace. Additionally, by using a date-pit-shaped secondary air distribution pattern, radial stratification can be achieved, which helps to reduce the mixing of primary and secondary air during the early stages of combustion and thus lowers the amount of air required for primary combustion. The size of the furnace also affects the selection of the air excess factor. For boilers with larger capacity and more burner layers, the aerodynamic field within the furnace is more flexible, allowing for the use of a lower air excess factor in the main combustion zone; conversely, smaller boilers require a higher air excess factor. Therefore, the air excess coefficient in the furnace is selected after considering various factors. Based on this study, it is recommended that for boilers with a capacity of 420 t/h or less, a value of 0.9–1.0 be used, while for boilers with a capacity exceeding 420 t/h, a value of 0.8–0.9 should be used. The method for calculating the smoke flow velocity in the main combustion zone is given in Equation 2: http://www.lyxgsh.com/d/file/news/cpzs/2016-04-05/4bfa31ca06278135c6ac583cfcb6f934.jpg. In this equation, Wy represents the average upward velocity of the smoke inside the furnace; Bj denotes the calculated fuel consumption rate of the boiler, in kg/s; Vy is the volume of the smoke, as calculated, in m3/kg (under standard conditions) ； l1, l2 —— depth and width of the furnace chamber, in meters ； P — Local atmospheric pressure, Pa ； ϑ′ —— furnace exit temperature,℃ ； θa——theoretical combustion temperature, °C. The selection of the SOFA height is determined by analyzing the nitrogen oxide conversion curve during coal powder combustion; the first 0.5 seconds are the critical period for controlling nitrogen oxide formation. The selected SOFAs should, in principle, ensure that the residence time of the primary air at the top layer to the flue gas at the bottom layer of the SOFA is greater than 0.5 s. To ensure sufficient combustion after injecting SOFA, while maintaining a sufficient residence time, the height of SOFA should also be below the centerline of the uppermost burnout air and the flame deflection angle. The flue gas velocity is calculated using Equation 3. τ = l3/Wy (3) Where: l3 – the distance from the center of the primary air nozzle in the upper row of the burner to the center of the burnout air. The design of the volume, flow rate, dimensions, and number of layers of the SOFA air flow is determined as follows: The volume of air for the SOFA is calculated using equation 4: QSOFA = QLL × (1.2 – a1) (4) QSOFA – the mass flow rate of the burnout air; QLL – the theoretical mass of air; a1 – the air excess coefficient in the main combustion zone. After calculating the mass flow rate, it is converted into a volumetric flow rate. To ensure that the burnout air has sufficient penetrating power, the design of the burnout air ducts should minimize the use of bends, and the distance from the air box to the burnout air nozzles should be reduced as much as possible. The area of the ducts should be more than 1.2 times the area of the SOFA nozzles. The SOFA wind speed of 50 m/s is selected (higher wind speeds are generally not achievable due to wind pressure), and the total size of SOFA is calculated based on the volumetric flow rate and the selected wind speed. Even if the boiler capacity is less than 420 t/h, it is generally designed to have two or more layers; this way, at low loads, only one layer of SOFA is activated, allowing for a high wind speed in that layer and thus strong penetration capability. The SOFA nozzle can be designed to swing up, down, left, and right, which can help adjust the deviation in main steam temperature and control the flame height; however, a fixed design is usually sufficient. Nozzles in a swiveling design should be designed as square, while those in a fixed design should be circular, as this improves the stiffness and penetration of the airflow and enhances the combustion efficiency. The design of the installation angle for SOFA involves positioning it at an angle opposite to the direction of air flow rotation. The initial size of the installation circle is set to be either the same as the original secondary air circle or slightly larger than it; adjustments are made after calculating the degree of balance. The method for calculating the balance of the aerodynamic field inside the furnace is given by Equation 5; this equation represents the ratio of all tangent and cotangent values of the aerodynamic momentum. After modifying the flow guides, the modified nozzles are divided into separate sections for calculation. http://www.lyxgsh.com/d/file/news/cpzs/2016-04-05/c565c88748d50c70b912c01bff0ed8bb.jpg Where: jiff represents the air flow rate in the same direction as that of the power field, and the value is in m3/s; A and B represent the width and depth of the furnace chamber, respectively, with values in meters ； α1, α2 —— represent the burner angles in the same direction as and opposite to the direction of the power field, respectively. The modification of the original secondary air angle area involves reducing the area of the original secondary air to such an extent that the total area of the secondary air after the modification is close to its value before the modification. This is done to compensate for the decrease in wind speed resulting from the reduction in air volume after the introduction of SOFA. However, it is not advisable to reduce the area of the top two layers, as these layers play a role in suppressing the flame and achieving initial combustion grading; reducing their area would hinder the reduction of NOX emissions as well as the complete combustion of coal powder. When reducing the area of each secondary air nozzle, it is important to ensure that the distance between the secondary air and the primary air does not increase excessively as a result of the reduced area; otherwise, the primary air may draw in hot flue gas, leading to coking at the nozzles. When modifying the original secondary air system, it is also necessary to prevent coking in the furnace caused by oxygen-deficient combustion in the main combustion zone. Guide vanes can typically be installed in the secondary air to direct a portion of it at an angle of 15° to 20°, thereby enriching the oxygen level in the water wall area and preventing coking. The design of these guide vanes is shown in Figure 5. http://www.lyxgsh.com/d/file/news/cpzs/2016-04-05/d22475462e392b2a54d90fb4d748c2a9.jpg Luoyang Xinggang Petrochemical Equipment Co., Ltd. specializes in the production of various petrochemical equipment and catalytic cracking devices; it is particularly expert in manufacturing burners, nozzles, flue dampers, and flap valves. Contact number: 0379-62329011