Brief introduction to the conditions before and after the reform of the boiler air cap

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I. Brief Overview Before 2003, the air nozzles used in the circulating fluidized bed boilers manufactured by various boiler manufacturers were generally of the straight-blow mushroom shape, “7” shape, or segmented straight-blow type; these types of air nozzles all had certain drawbacks to varying degrees. We will take the 75t/h circulating fluidized bed boiler produced by Jinan Boiler Factory as an example to illustrate the situation before and after the renovation. The circulating fluidized bed boilers produced by this factory are generally equipped with mushroom-shaped direct-blowing air nozzles; the number of such nozzles is around 550. The air outlets are arranged in three rows in the upper part – there are many of them, with small pore sizes – and the spacing between the nozzles is also small. This configuration makes wear and damage due to opposing airflow currents very likely during operation. Since this type of air cap uses a direct-blowing mechanism, the pressure in the air chamber fluctuates constantly during boiler operation. As a result, the large amount of material inside the furnace is drawn back into the air chamber through the openings in the air cap due to these pressure fluctuations, leading to severe accumulation of slag in the air chamber. It would be fine if everything could be drawn back into the air chamber, but a considerable amount of material, as soon as it is drawn back into the air cap cavity through the air cap opening, is blown back out again by the fluctuating air pressure. Since the diameter of the air cap hole is constant, many materials with a diameter smaller than that of the air cap hole can be easily drawn into it. However, materials with irregular sizes larger than the diameter of the air cap hole may sometimes pass through it at this angle; but once the angle changes inside the air cap chamber and the fluctuating air pressure pushes them back into the furnace, they are unable to pass through the air cap hole and get stuck on the inside of it, preventing them from passing through when being sucked back – ending up stuck just outside the opening of the air cap. Over time, such issues become more and more frequent, and the number of obstructions also increases, which leads to uneven air distribution and higher air distribution resistance ； Some wind caps, when blocked, cannot be cooled by air, which often leads to overheating and damage ； As the air flow decreases when the wind cap blocks the openings, the resistance to air supply increases, which can easily lead to an increase in bed temperature, make it difficult to operate under load, and result in waste of factory electricity ； Due to the reduced resistance of the air cap caused by damage resulting from overheating, the air flow is bypassed, leading to uneven fluidization of the bed surface. Due to clogging over a long period of operation, the air distribution on the bed surface becomes uneven, which easily leads to poor fluidization or channeling and surging, resulting in coking on the bed surface; such accidents occur frequently. Excessive slag accumulation in the air chamber leads to extra labor costs; over time, failure to remove the slag in a timely manner can also cause coking in the air chamber, posing a serious threat to safe operation. Uneven air distribution and poor fluidization lead to reduced combustion efficiency, which in turn results in a decrease in the boiler’s efficiency as well. If it is replaced with a newly designed labyrinth-type bell-shaped air cap of my own, developed in 1994 based on the structural characteristics of the fifth-generation air caps used in Siemens’ boilers (patent number: ZL 94 2 36043.5), by appropriately reducing the number of air caps, increasing the spacing between them, and enlarging the diameter of the air outlets, the airflow can be directed more efficiently; this improves the stiffness of the airflow at the outlet and enhances the quality of fluidization ； Since the air outlet of this wind cap is at the bottom, the head never gets worn out. If it is changed to this type of wind cap, it should operate without problems for three years. Moreover, there is no slag accumulation in the air chamber at all, and no maintenance work is required on a regular basis; when the furnace is shut down normally, there is no need to remove the bottom material to check the air caps. It’s also very convenient to replace it when it gets damaged after a few years of use. Several years of experience, as well as the performance of boilers recently produced by various boiler manufacturers when equipped with these labyrinth-type bell-shaped air caps, have shown that the results are quite good. II. Calculation of parameters for the original mushroom-shaped air caps 1. The total number of original air caps was approximately 550; each mushroom-shaped air cap had 24 holes, with a diameter of φ7 for each hole. The exit cross-sectional area of each air cap was: F1 = 1/4×π×D²×24 = 923.16 mm² = 0.00092316 m². Total area of all air cap outlets: F_total = F1×550 = 507738 mm². = 0.507738m². 2. During normal operation of the boiler, the primary and secondary air flows are divided in a ratio of 6:4. When coal with a calorific value of 5,000 kcal/kg is used and the boiler efficiency is 86%, approximately 11.34 tons of coal are burned per hour. In this case, the total air flow Q is 81,000 m³/h, of which the primary air flow is about 48,600 m³/h of air at room temperature. It is 75,384 m³/h when it becomes hot air at 150°C. At this point, the wind speed at the outlet of the wind cap is: V = Q/3600×Total F = 7.5384/0.507738×3600 = 41.24 m/s. 3. For the calculation of the wind speed inside the wind cap cavity (still using the above air volume as an example), the diameter of the cavity is generally φ38 mm, and its total area is ； F_cavity = 1/4×π×D²×550 = 0.623447 m²
V_cavity = Q/3600×F_cavity = 75384/0.623447×3600 = 33.585 m/s
F_exit = πR²×24×550 = 3.14×0.0035²×24×550 = 0.507738 m²
V_exit = Q/3600×F_exit = 75384/0.507738×3600 = 53.25 m/s
4. Calculation of wind speed at the openings in the air distribution plate: Generally, the opening rate of such plates is around 4%, and the wind speed is typically V = 59.83 m/s. In our country as well as in various countries around the world, the sum of the resistance of the fluidized bed distribution plates and air nozzles is between 2800 and 3200 Pa. III. Parameter calculation for the conversion to a new type of labyrinthine bell-shaped air cap 1. Under the same conditions of load, parameters, coal consumption, and air supply, the wind speed at the outlet of the holes in the modified labyrinthine bell-shaped air cap is slightly higher than that of the original air cap. In order to meet the requirements, reduce resistance, save energy, and ensure that the resistance of the new air cap is roughly similar to that of the original one, we conducted numerous calculations and tests. As a result, it was determined that around 550 mushroom-shaped air caps should be replaced with approximately 280 labyrinthine bell-shaped air caps. Although the quantity has been reduced by nearly half, the service life has been **extended** thanks to a more rational design and the use of high-quality cast alloy steel materials that are resistant to wear and heat. 2. The layout after the modification remains the same as before, with a deranged arrangement. Only the spacing and intercept are slightly larger than the original dimensions. Since the wear near the three slag discharge ports was greater than in other areas, we paid sufficient attention to this issue during the renovation, thereby effectively preventing wear in those areas. 3. Calculation of the flow area, wind speed, and resistance at various parts of the modified labyrinth-type bell hood: (1) Calculation of the area, wind speed, and resistance at the openings in the air distribution plate: Area of the opening: F_opening = 1/4×π×D²×280 = 1/4×3.14×0.038²×280 = 0.318 m² Wind speed at the opening: V_opening = Q/3600×F_opening = 75384/0.318×3600 = 65.85 m/s Resistance at the opening: The resistance coefficient ξ is taken as 0.68; ω_opening = ξ_opening×V_opening²/2g = 0.68×65.85²/2×9.8 = 1474.315 Pa (2) Calculation of the area, flow velocity, and resistance of the labyrinth core tube, corners, interlayers, and outlet: The combined resistance coefficient ξ is taken as 0.80. Area of the core tube: F_core = 1/4×π×D²×280 = 1/4×3.14×0.040²×280 = 0.35168 m² Wind speed in the core tube: V_core = Q/3600×F_core = 75384/0.3518×3600 = 59.5 m/s (3) Calculation of the area and wind speed of the interlayer and the outlet of the bell hood: Area of the interlayer: F_interlayer = (πR² – πr²)×280 = 3.14×0.038² – 3.14×0.02125²×280 = 0.87255 m² Wind speed in the interlayer: V_interlayer = Q/3600×F_interlayer = 75384/0.8725×3600 = 24.143 m/s Area of the outlet: F_outlet = πR²×8×280 = 3.14×0.0075²×8×280 = 0.39564 m² Wind speed at the outlet: V_outlet = Q/3600×F_outlet = 75384/0.39564×3600 = 53.25 m/s Combined resistance coefficient: ω_combined = ξ×V_outlet²/2g = 0.80×53.25²/2×9.8 = 1134.225 Pa Total resistance: ω_total = ω_opening + ω_combined = 1474.315 + 1134.225 = 2608.54 Pa (4) Considering that the boiler might operate under overload conditions, both the air volume and resistance could increase; therefore, a coefficient of 1.3 is applied to the total resistance. ω ultra ×1.3 = 2608.54×1.3 = 3391.102 Pa. IV. Economic analysis of the use of the original air nozzles: (1) The mushroom-shaped porous air nozzles supplied by the original boiler manufacturer are made from high-quality materials, and their typical service life is around one year. If the cost per air cap is 50 yuan each, then 550×50 = 27,500 yuan. Each replacement requires 5,000 yuan for refractory materials, 8,000 yuan for labor and auxiliary materials, resulting in a total cost of 40,500 yuan per replacement. 121,500 yuan is required for three years. (2) It needs to be replaced once a year, and each replacement takes three days. Each furnace generates a value of around 130,000 per day; with a 10% profit margin, the total cost associated with starting up the furnace each time is about 15,000 yuan, amounting to roughly 84,000 yuan over three years. The total cost for the two items over three years will be 205,500 yuan more. Factors affecting the economic operation of the boiler have not been considered. V. Economic analysis of converting it into a new type of labyrinth-style bell cover air cap: (1) The new type of labyrinth-style bell cover air cap features a rational design, and is made from high-quality materials that are resistant to wear and heat; its service life is guaranteed to be at least three years, with almost no maintenance costs required each year. (2) After replacing it with a labyrinth-type bell-shaped draft hood, the air distribution and fluidization conditions inside the furnace improved significantly. With the same load, it is possible to reduce the amount of air supplied and drawn, thereby lowering the power consumption for factory operations. Each furnace can save 25 KW/h per hour; at a cost of 0.35 yuan per KW/h, this results in annual savings of 63,000 yuan assuming 300 days of operation per year. (3) The new labyrinth-type bell-shaped air cap, thanks to its use of large holes for concentrated air distribution, **improves the fluidization on the bed surface as well as the spatial material concentration inside the furnace. It reduces the amount of air required, thereby minimizing smoke emission losses, and enhances combustion efficiency while reducing the combustible content in the fly ash. Tests conducted on the boiler after this modification show that its efficiency can increase by more than 0.8 percentage points. With 300 days of operation per year, 750 tons of raw coal can be saved annually; at a cost of 380 yuan per ton, this amounts to savings of 285,000 yuan per year. More than 400,000 yuan in costs can be saved each year. (4) By replacing it with a new type of labyrinth-shaped bell-type air cap, savings of at least 400,000 yuan can be achieved each year; the environmental and social benefits resulting from reduced coal usage have not been taken into account. (5) Thanks to the adoption of the new labyrinth-type bell hood, there is no longer any need for extensive work each time the furnace is shut down to replace the hood ； The operating conditions inside the furnace have been improved; issues such as fluidization disruptions, channeling, unstable combustion, and coking on the bed surface and in the air chamber are no longer present, **which enhances the furnace’s reliability in safe operation. **It reduces the workload of maintenance staff, contributing to cost savings and increased efficiency for the enterprise. (6) By adopting the new type of labyrinth-shaped bell-type air cap, the fluidization within the furnace became more uniform; there was no longer any issue of uneven loading of the materials in the furnace. The localized wear on the water cooling walls, furnace walls, and various heating surfaces at the rear of the furnace was reduced, **which extended the operational life of the furnace. (7) The initial renovation cost for each furnace is around 120,000 yuan, and the entire investment can be recovered within less than half a year of operation. The above are my some analysis and suggestions regarding the modification of the stove’s air cap; please calculate whether the results are the same based on your own circumstances. There may be many inaccuracies in what I have written; please point them out to my company. We also carried out modifications to all the equipment in the boiler island, particularly: the central tube, anti-wear devices, operation control systems, economizers, superheaters, air preheaters, etc. About the author: Zhao Jiaen, a senior engineer and the person in charge of Yantai Panshi Anti-Wear Equipment Co., Ltd., was responsible in 1990 at the thermal power plant in Jinxi City, Liaoning Province (now Huludao City) for the installation, commissioning, and operation of the first, second, and third YG75/5.29—M type sub-high pressure, medium temperature external high-temperature cyclone-separated circulating fluidized bed boilers. These boilers were jointly designed by Jinan Boiler Factory and the Institute of Thermal Physics of the Chinese Academy of Sciences, with Jinan Boiler Factory handling their production. In 1992, I was hired by Hualu Thermal Power Co., Ltd. in Yantai Economic and Technological Development Zone, where I was responsible for the fourth, fifth, and sixth circulating fluidized bed boilers of the same type manufactured by Jinan Boiler Factory. In March 1993, I participated in the thermal tests conducted by the Shandong Electric Power Pilot Plant on this type of furnace, including tests under full load and overload conditions, as well as tests using various types of coal. In April of the same year, I attended the appraisal meeting for the new scientific and technological achievements related to this type of furnace, organized by the Science and Technology Commission of Shandong Province with the participation of **relevant ministries and departments, and delivered a presentation on its installation, operation, and debugging at the conference. Over the past decade or so, I have developed unique insights and proven solutions in areas such as the supervision and installation of circulating fluidized bed boilers, cold and hot state commissioning, operation management, maintenance and renovation, the use of refractory and wear-resistant materials, as well as addressing issues related to smoke, ash, and air leakage. Has extensive practical experience and a broad range of theoretical knowledge. In 1991 and 1992, he personally drafted the operating procedures, accident handling protocols, job responsibility systems, and various management regulations for the YG75/5.29—M type sub-high pressure, medium temperature, external high-temperature cyclone-separated circulating fluidized bed boilers produced by Jinan Boiler Factory. In 1993, he organized and participated in the development of the maintenance process standards and procedures for this type of furnace, which was quite pioneering at that time in China. It can be said that these efforts have made significant contributions to the development of circulating fluidized bed boiler technology in our country.