[Objective] Deep peak regulation has become an essential operational requirement for thermal power units to adapt to the large-scale grid integration of new energy sources. Lean coal-fired subcritical boilers frequently encounter unstable combustion and require auxiliary oil ignition during ultra-low load operation, which restricts the flexible peak-shaving capacity of power plants. Taking a successful deep peak regulation renovation project of a lean coal-fired boiler as the research basis, this paper conducts a comprehensive study on the adjustable recirculation zone staged ignition combustion technology. It systematically elaborates the core content of the technology from three key dimensions: technical working principle, computational numerical simulation, and practical engineering application effects. This research aims to validate the technical feasibility of applying the proposed technology to realize full oil-free stable combustion for subcritical lean coal-fired boilers under ultra-low load conditions, and to provide reliable technical support and practical guidance for the promotion and implementation of similar boiler renovation projects. 

[Methods] In this study, a high-precision refined Computational Fluid Dynamics (CFD) model was established to carry out full-furnace combustion numerical simulation under the typical ultra-low load condition of 30% Economic Continuous Rating (30% ECR). Based on the simulation calculation data, the dynamic variation characteristics of furnace internal flow field distribution, combustion temperature field uniformity, and water wall heat load were comprehensively analyzed. In addition, the structural composition and stable combustion mechanism of the adjustable recirculation zone staged ignition burner were expounded in detail, and the field operation data and test results of the engineering renovation were summarized and analyzed to verify the practical performance of the technology. 

[Results] The numerical simulation results demonstrate that excellent combustion performance is achieved at the 30% ECR ultra-low load working condition. Specifically, the pulverized coal burnout rate reaches 99.86%, and the flue gas carbon monoxide (CO) concentration is as low as 4.38 mg·m-3, indicating thorough fuel combustion. The field steady combustion tests conducted at 30% ECR load after technical renovation further verify the remarkable practical effect. When the boiler operates with lean coal, the minimum oil-free stable combustion electric load is successfully reduced to 90 MW. During the whole ultra-low load operation period, the furnace negative pressure fluctuates steadily within the normal safe range. The denitrification system can maintain continuous and stable operation, and all flue gas emission indicators fully meet the national ultra-low emission standards. 

[Conclusion] The adjustable recirculation zone staged ignition burner adopts a slotted bluff body structure to construct an active staged ignition mechanism, combined with a flexible adjustable perimeter air design. The innovative structural design effectively solves two key operational problems of lean coal-fired boilers, realizing stable combustion reinforcement under low load conditions and preventing furnace slagging under high load conditions. After the renovation, the corrected thermal efficiency of the boiler reaches 91.47%, which realizes high-efficiency and stable operation of the boiler in a wide load range. All operational indicators of the renovated boiler fully meet the design expectations. This study provides a valuable and practical technical reference for the deep peak regulation transformation of the same type of subcritical lean coal-fired boilers, and has good engineering application and promotion value.