[b]0 Introduction[/b] The No. 3 boiler at Mudanjiang No. 2 Power Plant is an HG-410/9.8-9 type single-drum natural circulation pulverized coal boiler designed and manufactured by Harbin Boiler Factory, and was put into operation in October 1984. After the boiler was put into operation, due to large fluctuations in the power grid load, it had to undertake peak shaving tasks, resulting in high oil consumption at low loads. When operating below 70% of the rated load, oil injection was necessary for combustion assistance. To solve the problem of stable combustion at low loads and reduce power generation oil consumption, the plant collaborated with Zhejiang University to modify the lower main burner of the No. 3 boiler into a rich-lean burner, which was then put into operation. After one year of continuous operation following the modification, tests were conducted on the thermal efficiency and stable combustion at low loads of the No. 3 boiler. The test results showed that this type of burner could achieve stable combustion without oil injection when the boiler was operating at 50% of its rated load. In 1996, this technology was further applied to the No. 2 and No. 4 boilers. The original boiler burner was designed for basic load operation, resulting in poor peak-shaving capacity. When operating below 70% rated load (70MW), oil-assisted combustion was necessary to ensure stable ignition and combustion. In 1995, the 100MW unit of Mudanjiang No. 2 Power Plant consumed a total of 2663 tons of oil for boiler start-up, shutdown, and load reduction. [b]1. Working Principle of the Flame Stabilizer[/b] The added flame stabilizer consists of a dilute-phase burner and a dense-phase burner, mainly composed of a dense-dilute phase separator, a variable-load burner, and connecting pipes. The dense-dilute phase separator utilizes the difference in centrifugal force during high-speed turning of the pulverized coal airflow to achieve dense-dilute separation of the pulverized coal. The variable-load burner is divided into a dense-phase burner and a dilute-phase burner. The dense-phase burner consists of a dense-phase nozzle, a blunt body, and an annular perimeter airflow. During low-load operation, the perimeter airflow is closed, which is beneficial for the ignition and stable combustion of the pulverized coal airflow. The dilute phase burner is located at the original primary air nozzle, above the dense phase burner. With a lower pulverized coal concentration, as long as the dense phase combustion is stable, dilute phase ignition is not a problem. Previously, the secondary air of the oil gun was closed when no oil was injected. After installing the flame stabilizer, it must be in the open position to support the flame and prevent it from shifting downwards. During high-load operation, the perimeter air of the dense phase burner must be fully open to prevent excessively strong high-temperature flue gas in the recirculation zone from causing premature ignition and damaging the burner nozzle. [b]2 Operating Indicators After Applying the Dense-Dilute Flame Stabilizer[/b] The dense-dilute flame stabilizer has strong low-load stable combustion performance and adaptability to changes in coal type, while also reducing the workload of operators. Tests show that: a. When operating at 40% rated load, no auxiliary fuel oil is needed, and combustion is stable; b. When operating at full load, after primary air leveling, the tested boiler efficiency is 90.99%, which is 0.45% higher than the design value; c. It has high combustion efficiency, with fly ash combustible content of 1.98% and slag combustible content of 8.2% when operating at full load; d. When operating at 50MW electrical load, through 2 hours of operation observation, nozzle temperature measurement, and pulverizing system disturbance, it is determined that this type of burner can fully meet the requirements for stable ignition and combustion when operating at 50% rated load, with stable operating parameters and good combustion status; e. After applying the rich-lean burner, a 410t/h boiler can save 438.5t of auxiliary fuel oil per year, saving 414,700 yuan; f. Because this burner has good low-load stable combustion performance, when there are changes in coal type or other abnormal situations, it can avoid or delay boiler shutdown, avoid accidental shutdown, and achieve significant economic benefits. **3 Technical and Economic Benefit Analysis** During the major overhaul of Boiler No. 3 at Mudanjiang No. 2 Power Plant in June 1995, the lower exhaust primary air nozzles were modified and officially put into operation in July 1995. The No. 2 and No. 4 flame stabilizers were installed in May and June 1996, respectively. 3.1 Increased Annual Electricity Consumption Due to the installation of the flame stabilizer on the 410t/h boiler, the total air pressure increased from 2450Pa to 2646Pa to prevent pipe blockage. The blower current increased by approximately 4A under the same load compared to before the modification. If calculated based on 6000h operation per year, 6kV voltage, and a power factor of 0.8, the increased electricity consumption per year (for one boiler) is: A = IUcosαt = 1.732 × 4 × 6 × 0.8 × 6000 = 199,500 kWh. If calculated at 0.15 yuan per kWh, the increased electricity cost per year is 29,900 yuan. 3.2 Annual Fuel Savings a. Fuel Saving by Reducing Pulverized Coal Before the installation of the fuel stabilizer, each reduction in pulverized coal consumption required 3 tons of auxiliary fuel oil. After the modification, fuel consumption was reduced by less than 1 ton, saving 2 tons of auxiliary fuel oil per reduction. If there are 20 reductions in pulverized coal consumption per year, 40 tons of fuel oil can be saved. b. Fuel Saving by Starting and Stopping (including Peak Shaving) Fuel-fired Boilers can be stopped 2 hours earlier during startup, eliminating the need for fuel injection during shutdown and reducing fuel consumption by 2 hours. Each boiler startup and shutdown can save 8 tons of fuel oil. If there are 30 startups and shutdowns per year, 240 tons of fuel oil can be saved. c. Fuel Saving by Stabilizing Fuel In the first half of 1995, fuel saving by stabilizing fuel consumption was 317 tons more than in the same period of 1996. On average, each boiler can save 79.25 tons of fuel oil in half a year, resulting in an annual saving of 158.5 tons of fuel saving by stabilizing fuel consumption. In the first half of 1998, the number of unit startups and shutdowns was higher than the same period last year, while the fuel oil consumption for auxiliary fuel in the first half of 1996 was lower than the same period last year. Therefore, after installing the fuel stabilizer, one boiler can save a total of 40 + 240 + 158.5 = 438.5 tons of fuel oil per year. Based on the current fuel price of 1014 yuan/ton, this translates to a fuel cost saving of 444,600 yuan per year. After deducting the extra 29,900 yuan in electricity costs for the blower per year, the operating cost saving is 414,700 yuan per year. The total cost of equipment, technology, installation, and commissioning for this project is 325,000 yuan. After deducting the 120,000 yuan cost of replacing the primary air elbow and burner nozzles that would have been necessary during a major overhaul, the actual modification cost is only 205,000 yuan, with a net payback period of only six months. Currently, the plant has successfully modified three 410t/h boilers, and conservatively estimated, the annual cost savings for these three boilers are approximately 1,244,100 yuan.