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A 15t/h circulating fluidized bed boiler at a paper mill in Fujian suffered from issues such as insufficient boiler output, low temperature of the superheated steam, and high flue gas temperature. To address these problems, performance improvements were carried out on the boiler, with good results achieved. The details of the renovation plan and the operational status are outlined as follows: I. Project Overview The power station of a certain paper manufacturing company in Fujian is equipped with a 15t/h circulating fluidized bed boiler along with a 1.5MW backpressure steam turbine generator. The boiler is of the low-ratio type, manufactured by a boiler factory in Hunan; it operates under medium-temperature and medium-pressure conditions. It features a single boiler drum, a water-cooled wall with straight tubes for natural circulation, a high-temperature separator equipped with an L-shaped return valve. At the rear part of the boiler, there are two stages of superheaters, three stages of economizers, and one set of air preheaters. The furnace, the separator, and the rear flue ducts all utilize a heavy-duty furnace wall structure. The boiler uses Longyan anthracite, with a lower heating value of around 4800 Kcal/kg. Since it began operating, the boiler has had the following problems: 1. The boiler’s output does not reach the rated capacity, and it is unable to meet the steam requirements for paper production; the maximum output is only about 10 t/h, and the bed temperature exceeds 1000°C. 2. The temperature of the superheated steam is extremely low; without using desuperheating water, the main steam temperature is only around 380°C, which prevents the steam turbine generator set from operating, thereby severely affecting the thermal efficiency of the unit as well as the economic benefits of the enterprise. 3. The flue gas temperature reaches as high as 220°C, causing the dust collector bags to be frequently burned out. To address the aforementioned issues and improve the unit utilization rate as well as the overall thermal efficiency, the user decided to carry out a comprehensive renovation of the boiler. II. Principles and Approaches for Renovation 1. The renovation follows the principles of safety, reliability, efficiency, economy, and environmental protection; that is, while ensuring the boiler’s output and performance, efforts are made to make use of the existing system and its structural components as much as possible. 2. The boiler operates as a single unit without a backup boiler, and there are no other heat sources in the area surrounding the plant. Modifying the boiler means that the entire plant will have to shut down; therefore, the modification plan must take into account minimizing the duration of the shutdown. 3. The original boiler was of the low-ratio circulating fluidized bed type, with a heavy furnace wall and an indoor layout. Taking into account factors such as site conditions, investment, and construction time, this renovation continues to use the low-ratio boiler design; the height of the original boiler body remains unchanged. The combustion system is redesigned by making use of the original boiler top, separator, upper part of the furnace chamber, and piping system, while the rear heating surfaces are optimized to address issues such as insufficient output, low steam temperature, and high flue gas temperature. III. Specific measures for renovation: 1. The lower part of the furnace is modified to a bed with a gradient in height; the lower section of the furnace and the air distribution system are altered. The area of the main bed is 2.24㎡, while the area of the secondary bed is 3.92㎡. The area of the air distribution plates increases from 3.8㎡ to 6.16㎡. 22 buried pipes are arranged in the auxiliary bed, covering an area of about 21 square meters. The main purpose of this change is to optimize the boiler’s combustion conditions based on the existing furnace structure; while maintaining the boiler’s output, it also improves combustion efficiency. By placing the tubes in a secondary bed where the particle size is small and the fluidization speed is low, the problem of tube wear, which is common in boilers with a low expansion ratio, can be addressed. 2. The corresponding water supply pipes and steam guide pipes are adjusted to meet the requirements of the structural changes ； 3. Considering that raising the furnace height involves high costs, a long installation period, and the need to replace all the refractory materials, any increase in efficiency is minimal; thus, the overall cost-effectiveness is poor. Moreover, calculations show that the original furnace size is sufficient to meet the boiler’s 15T output requirement. Therefore, in this design, the furnace height will not be increased, nor will the separator need to be adjusted ； 4. The number of rows in the high-temperature superheater has been increased from 7 to 11, and the tube specification has been changed to Φ32*3.5 to meet the requirements regarding flow velocity and steam parameters ； 5. The number of tubes in the low-temperature superheater has been increased from 10 to 14, and the tube specification has been changed to 32*3.5 to meet the requirements regarding flow velocity and steam parameters. All the inlet and outlet headers of the superheater, as well as the desuperheating header, have been replaced; a surface-type desuperheater is used for desuperheating ； 6. The number of high-temperature precast segments has increased from 16 to 22 ； A transition header is added between the low-temperature and medium-temperature economizers to facilitate maintenance; the number of tube sections is reduced from 24 to 22 in order to lower the flue gas flow velocity ； 8. The air preheater has been redesigned with independent upper and lower tube banks; the overall height of the tube banks has increased by 35%. The tubes are arranged separately for primary and secondary air, which helps to maintain good sealing properties while increasing the heating surface area and heat exchange efficiency. Primary air accounts for 40% while secondary air accounts for 60%, with the tube specifications and arrangement remaining unchanged. 9. The L-shaped return conveyor was replaced with a U-shaped one to facilitate control of the returned material. 10. The boiler uses Longyan anthracite, which contains a high amount of fine powder and is difficult to burn completely; therefore, the coal feeding method has been changed to positive-pressure spiral feeding into the dense-phase zone of the main furnace bed, in order to increase the residence time of the coal inside the furnace and ensure efficient combustion. 11. To meet the configuration requirements of the differential bed, an additional blower must be installed in the boiler as the primary fan (with an air volume of 9000 m3/h and a total pressure of 13500 Pa); the existing blower will be used as the secondary fan, while the exhaust fan will continue to be utilized as before. IV. Operating conditions after the renovation: The renovation of this boiler began in mid-August 2018 and was completed at the beginning of October 2018. The boiler was put into operation on January 3, 2019 (the commissioning was delayed due to the user’s production plans). Overall, the boiler has operated stably; the operating conditions after the technical upgrades are as follows: 1. The boiler’s steam production capacity reaches 15 t/h, and it can operate safely and stably for long periods at this load level, with the bed temperature maintained at around 950°C. 2. The temperature of the superheated steam reaches the designed value of 450°C, and the amount of water used for temperature reduction is within the specified range; this meets the requirements for the steam inlet parameters of the turbine, **thereby improving the overall thermal efficiency of the unit. 3. The flue gas temperature at rated load is approximately 158°C, meeting the design requirements. 4. The boiler only requires ignition using the main bed; the area of the ignition bed is reduced compared to before the modification, **which shortens the ignition time and saves on fuel costs for ignition. 5. The carbon content in the slag has decreased compared to before the renovation, mainly due to the adoption of a differential velocity bed combustion system, which enhances internal circulation and prolongs the residence time inside the furnace. 6. Since the pipes are arranged in the secondary bed, the coal fed into the boiler has a high calorific value and low ash content; as a result, the boiler’s load can be increased slowly during startup. It is recommended that users load as much coal as possible into the secondary bed before ignition, and use coal with a lower calorific value during ignition. This will enable an increase in the pressure difference across the material layer once the secondary bed is operational, thereby reducing the time required to increase the load. V. Conclusion Through a comparative analysis of the operational data before and after the renovation, it was found that the boiler’s load capacity, steam parameters, and flue gas temperature all met the design requirements. With the installation of the turbine, gradient utilization of steam thermal energy was achieved, bringing significant benefits to the enterprise. Thus, the expansion and efficiency-improvement renovation achieved its intended objectives.