**1 Overview** Compared to traditional intermediate storage pulverizing systems, direct-fired pulverizing systems are simpler, lacking equipment such as fine powder separators, powder silos, and adjustable feeders. They also offer significant advantages in ignition, stable combustion, and burnout of lean coal. In the recent large-scale construction of thermal power plants using lean coal, direct-fired pulverizing systems are widely adopted. To improve automatic adjustment levels, load response speed, combustion efficiency, burnout, and prevent localized slagging, direct-fired boilers place higher demands on the uniformity of pulverized coal concentration and fineness, air-coal distribution, pulverized coal fineness, air-coal ratio, and the uniformity and adjustability of parameters in each duct. This paper elucidates the importance and necessity of monitoring the primary air volume, velocity, and flow rate of each duct in direct-fired boilers. It also introduces a solution for measuring primary air velocity and flow rate in high-concentration gas-solid two-phase flows and its successful application in practice. [b]2 Requirements for Online Monitoring of Primary Air in Direct-Fired Boilers[/b] 2.1 Need for Combustion Adjustment For boilers equipped with direct-fired pulverizing systems, the balance of primary air and pulverized coal flow directly affects the uniformity and stability of combustion. Due to factors such as different lengths and numbers of bends in the pulverized coal conveying pipes, the overall resistance coefficients of each pulverized coal pipe are different. Parallel operation under the same pressure differential can cause uneven air-coal distribution. Resistance leveling of each primary air pipe in the boiler pulverizing system is generally achieved using the cold-state pure air resistance leveling method. That is, in a cold state, when pure air flows through the same layer of primary air pipes, throttling rings with appropriate openings are added to several primary air pipes with lower resistance to balance them with the primary air pipe with the highest resistance. In essence, this resistance adjustment method can only ensure the balance of resistance between the primary air ducts on the same floor of the boiler combustion system and the burner outlet under cold conditions. Because the pure air resistance characteristics of the pulverized coal conveying pipes differ from the resistance characteristics of the pulverized coal gas-solid two-phase flow, it cannot guarantee the resistance balance when the air-coal mixture flows through the pipes during actual operation. This results in inconsistencies in the air-coal flow velocity and concentration in the primary air ducts during actual boiler operation. Even with hot-state pulverized coal resistance balancing, uneven wear may occur in each throttling ring, leading to a deterioration in primary air uniformity. After operating for a period of time, a 600MW boiler exhibited an abnormal phenomenon where the reheat steam temperature was lower on one side, affecting the unit's economic and safe operation. This boiler's combustion system uses a double-inlet, double-outlet cold primary air fan positive pressure direct-fired system. Twenty-four swirl-type DS burners are arranged in three layers on the front and rear walls of the furnace. Pulverized coal and air are fed in from the front and rear walls and burn in opposing directions in the furnace. Each pulverized coal pipe inlet of this boiler has a throttling ring (contraction orifice). By adjusting the opening size of each contraction orifice, the resistance of each pipe and the primary air flow can be balanced. During the commissioning of this boiler unit, the resistance of the pulverized coal pipes was adjusted. However, after long-term operation, uneven wear occurred in the throttling orifice rings of each pulverized coal pipe, causing the air-coal uniformity to deteriorate again, thus affecting the combustion uniformity within the boiler and resulting in large deviations in the main steam temperature and reheat steam temperature on both sides A and B. If online monitoring of primary air is adopted, abnormalities during operation can be detected in a timely manner, and measures can be taken to adjust them. 2.2 The need for adjustment of pulverized coal fineness and air-coal ratio When the type of coal used in the boiler changes, the required economical pulverized coal fineness changes accordingly. The pulverized coal fineness should be adjusted at any time to adapt to changes in boiler operating load and fuel. Domestic power plants usually only adjust the pulverized coal fineness during equipment commissioning, and do not pay much attention to the corresponding adjustment work during operation. Therefore, the economical pulverized coal fineness cannot be guaranteed in actual operation, affecting the economic efficiency of power plant operation. For direct-fired boilers with high levels of automation, the signal of boiler load change first adjusts the coal feeder's coal rate, and then correspondingly adjusts the primary air fan's flow rate. Within the 40%–100% mill operating load range, there is a linear relationship between mill ventilation and mill load rate. However, due to the minimum ventilation requirements of the pulverizing system, the ventilation must be maintained at 70% of its rated value. Generally, when the mill operates at its rated output and corresponding rated ventilation, a suitable air-coal ratio for combustion can be obtained. However, as the mill output decreases, the air-coal ratio increases, and the pulverized coal concentration decreases. At low loads, the furnace temperature is already low; coupled with an excessively high air-coal ratio, this is even more detrimental to pulverized coal ignition and stable combustion. Especially in domestically produced units with lower levels of automation, many power plants basically do not adjust the ventilation during mill load changes, causing the air-coal ratio to deviate significantly from the suitable value at low loads. Online monitoring of primary air duct velocity, air volume, and pulverized coal concentration provides a basis for adjusting the adjustable pulverized coal distributor. Alternatively, continuous adjustment of the pulverized coal distributor during operation via a closed-loop control system can maintain optimal pulverized coal fineness and air-to-coal ratio in the air fed into the furnace. 2.3 The Need for Rapid Load Adjustment: When boiler load commands change, the opening of the primary air volume regulating damper or the primary air fan speed is adjusted simultaneously with the coal feed rate. This changes the primary air volume entering the pulverizer, resulting in a change in ventilation output. Since the rate of change in airflow is much greater than the rate of change in drying rate, the change in primary ventilation output is rapid. By using a method of proactively changing the ventilation volume, the ventilation volume is changed simultaneously with the coal feed rate. Although the grinding output may not change in time, the amount of pulverized coal carried by the ventilation has already changed. Therefore, maintaining the accuracy and sensitivity of pulverizer air volume adjustment can reduce the impact of the hysteresis characteristics of the pulverizing system on boiler load adjustment. In conclusion, online monitoring of primary air is of great significance for direct-fired pulverizing systems. [b]3 Challenges and Solutions for Online Monitoring of Primary Air in Direct-Fired Boilers[/b] Due to the complexity of gas-solid two-phase flow, the application of online monitoring of primary air in direct-fired boilers faces many challenges: (1) The measuring equipment cannot meet the anti-clogging requirements during long-term operation in a gas-solid two-phase flow environment, and continuous or periodic backflushing must be used. (2) The measuring equipment is not wear-resistant. The measuring equipment is prone to deformation and wear during long-term operation, which cannot guarantee the accuracy of the measurement and cannot provide useful operating parameters for boiler combustion. (3) The measuring equipment cannot guarantee low pressure loss, which increases the original air resistance of the pipeline. The primary air pipelines at the coal mill outlet have been adjusted according to the air resistance of each pipe. After the measuring equipment is installed, the additional pressure loss may affect the original operating conditions. (4) The pipelines in the process are limited by the spatial location and often cannot meet the requirements of the flow rate meter for the length of the straight pipe section. (5) The accuracy of the flow rate meter is not universally recognized, which restricts the development, research and application of new flow rate meters. (6) Low speed, large temperature and pressure changes and other adverse factors affect the accuracy and reliability of flow rate meters, making it difficult to improve the performance of flow rate meters and meet the needs of users. The LUSE type primary and secondary air online monitoring system of Langkun Company is composed of a speed measuring device, a transmitter and a human-machine interface based on the Bernoulli equation principle. For the detection environment of high-speed gas-solid two-phase flow, the speed measuring device adopts proprietary technology to completely solve the long-standing problems of blockage, wear prevention, large pressure loss and low accuracy in the online monitoring of primary air of direct-fired boilers. The LUSE type primary and secondary air online monitoring system has the following features: (1) The pressure tap adopts a special design to take into account high pressure tapping efficiency, prevent blockage and protect the static pressure measuring tube. (2) The specially designed anti-blocking element uses the kinetic energy of the measuring medium to clean the entire wall of the pressure tapping pipe in real time. (3) A multi-curved labyrinth pipe is used to prevent dust from entering the pressure tapping pipe. (4) High-temperature alloy ceramics with outstanding wear resistance are used. (5) The pendulum principle is used to fully absorb the kinetic energy of the detection medium. (6) Integrated wear-resistant thermal resistors provide temperature compensation. [img=272,285]http://www.luculent.net/images/huanbao_pic.jpg[/img] [b]4 Analysis of the effect of online monitoring of primary air in direct-fired boilers[/b] 4.1 Coal saving cost After using the online monitoring system for primary air, the wind speed in each pipe is displayed in real time on the DCS screen. During the start-up and shutdown process of the pulverizing system, the boiler operator can clearly see whether the coal mill outlet door and the burner door are fully open or tightly closed. This ensures that the boiler operates under a more reasonable air-coal ratio, greatly improving the boiler combustion efficiency. 4.2 Equipment cost saving If the system lacks a monitoring system, flame deviation will cause the burner nozzle to burn out, while after the monitoring system is implemented, such situations will be completely avoided. If water-cooled wall tube rupture is caused by local coking, it will cause huge economic losses to the power plant. 4.3 Reduced Maintenance and Repair Costs By eliminating issues such as eccentric coking in the furnace and duct blockage, significant savings are potentially achieved annually in maintenance and repair costs, as well as losses due to shutdowns and production stoppages, while extending boiler lifespan. There are also other social benefits, such as reduced fly ash carbon content and decreased environmental pollution. The Guizhou Anshun Power Plant, located on a plateau, burns Guizhou Jiaozishan anthracite and local small-scale coal mine coal. The first phase consists of two 300MW boilers, designed and manufactured by Dongfang Boiler Factory using FW technology, specifically the DG1025/18.2-II10 type subcritical pressure, intermediate reheat, natural circulation W-flame boiler. The pulverizing system is equipped with four FWEC D-10D type double-inlet, double-outlet ball mills, forming a positive pressure direct-fired pulverizing system. The boiler's full-load parameters are as follows: [table][tr][td=1,1,167]Parameter Name[/td][td=1,1,109]Unit[/td][td=1,1,144]Value[/td][/tr][tr][td=1,1,167]Steam Drum Pressure[/td][td=1,1,109]MPa [/td][td=1,1,144]17.8 [/td][/tr][tr][td=1,1,167]Feedwater Flow Rate[/td][td=1,1,109]t/h [/td][td=1,1,144]1045 [/td][/tr][tr][td=1,1,167]Feedwater Pressure[/td][td=1,1,109]MPa [/td][td=1,1,144]18.3 Feedwater temperature[/td][td=1,1,167]℃[/td][td=1,1,109]℃[/td][td=1,1,144]265[/td][/tr][tr][td=1,1,167]Superheated steam flow rate[/td][td=1,1,109]t/h[/td][td=1,1,144]943[/td][/tr][tr][td=1,1,167]Superheated steam outlet pressure[/td][td=1,1,109]MPa[/td][td=1,1,144]16.6[/td][/tr][tr][td=1,1,167]Superheated steam outlet temperature[/td][td=1,1,109]℃[/td][td=1,1,109]℃ [td=1,1,144]538 [/td][/tr][tr][td=1,1,167]Superheater desuperheating water flow rate[/td][td=1,1,109]t/h [/td][td=1,1,144]75 [/td][/tr][tr][td=1,1,167]Reheater inlet/outlet pressure[/td][td=1,1,109]MPa [/td][td=1,1,144]3.7/3.5 [/td][/tr][tr][td=1,1,167]Reheater inlet/outlet temperature[/td][td=1,1,109]t/h [/td][td=1,1,144]278/539 [tr][tr][td=1,1,167]Reheater desuperheating water flow rate[/td][td=1,1,109]t/h [/td][td=1,1,144]0 [/td][/tr][tr][td=1,1,167]Primary air pressure[/td][td=1,1,109]KPa [/td][td=1,1,144]11.9 [/td][/tr][tr][td=1,1,167]Secondary air pressure[/td][td=1,1,109]KPa [/td][td=1,1,144]1.0 [/td][/tr][tr][td=1,1,167]Wind box pressure[/td][td=1,1,109]KPa [td=1,1,144]0.8 [/td][/tr][tr][td=1,1,167]Primary air volume[/td][td=1,1,109]t/h [/td][td=1,1,144]134 [/td][/tr][tr][td=1,1,167]Secondary air volume[/td][td=1,1,109]t/h [/td][td=1,1,144]924 [/td][/tr][tr][td=1,1,167]Primary air temperature[/td][td=1,1,109]℃ [/td][td=1,1,144]322 [/td][/tr][tr][td=1,1,167]Secondary air temperature[/td][td=1,1,109]℃ [td=1,1,144]312 [/td][/tr][tr][td=1,1,167]Economizer outlet oxygen content[/td][td=1,1,109]% [/td][td=1,1,144]3.1 [/td][/tr][tr][td=1,1,167]Exhaust gas temperature[/td][td=1,1,109]℃ [/td][td=1,1,144]135 [/td][/tr][/table] After the unit was put into operation, it was difficult and unstable to add air during high-load operation, the combustion economy was very poor and insufficient steam temperature was easy to occur. The problem could not be solved even after multiple combustion adjustments by the manufacturer. During the major overhaul in 2001, an online monitoring system for primary air was installed. This system scientifically organized the air-coal mixture and the ratio of primary and secondary air for each burner, ensuring the rationality of the pulverized coal concentration and the primary and secondary air velocities for each burner. This resulted in rapid ignition and complete combustion. The carbon content of fly ash at full load was reduced from about 30% to about 13% (minimum 9% to 10%), and the carbon content of slag was reduced from about 30% to about 15%. The problem of insufficient steam temperature was completely solved, significantly improving the boiler's combustion safety and economy. As a result, the two units in the first phase of Anshun Power Plant can save 67,392 tons of coal annually, equivalent to RMB 7.413 million. [b]5 Conclusion[/b] The adoption of the LUSE series primary and secondary air online monitoring system provides a practical solution for combustion optimization of direct-fired boilers. Through coordinated operation with the adjustable pulverized coal distributor, the distribution of primary air is optimized, improving the safety and economy of boiler operation. It avoids a series of problems such as burner burnout, coking of water-cooled walls or other areas, tube rupture caused by uneven heat load, and local high-temperature corrosion under reducing atmosphere. It lays the foundation for combustion operation optimization and has high social and technical economic benefits.