Identification of the running status of membrane walls in an opposed fired model boiler under varying heating loads
Introduction
Despite the rapid growth of renewable energy utilization, thermal power plants driven by fossil fuels will remain the dominant energy supply system at least for the coming 20 years, according to the International Energy Agency’s World Energy Outlook [1]. From the perspectives of energy sustainability and environmental protection, increasing the working steam temperature and pressure in boiler systems is a promising way to improve system efficiency and reduce emissions simultaneously [2]. However, the application of high temperature and high pressure steam could really compromise the safety of heating surfaces in the whole boiler system [3]. For example, this could result in ultra-supercritical water vapor of high temperature and pressure inside the steam generation tube and intense flame outside the tube [4]. Under this condition, ash deposition, creep deformation, fracture and high temperature corrosion could take place with serious consequences [5]. The leakage and rupture of boiler tubes in power plants is a critical factor that can lead to unscheduled and costly outages. The predominant failure location is the circumferential cracking on water wall tubes in the fireside of a boiler [6], [7], [8].
In order to protect water walls in boilers from deformation and cracking failures, several researches have been done to investigate the working status of boiler heating surfaces. Some scholars measured and tried to improve the combustion conditions within the furnace to create a better working condition for the heating surface (mostly membrane walls) [9], [10], [11]. Since the heat flux entering steam tubes in boilers is critical to the safety of the tubes, many researchers studied the temperature and heat flux of the water walls in boilers [12], [13], [14], [15], [16], which were mostly done by simulation via the analysis of finite elements of the boiler tube [17], [18], [19]. Recently, more attention has been paid to research about the effect of stresses, which are unavoidable during boiler operation. Jan Taler’s group developed different methods to monitor the thermal stresses of different boiler components [20], [21]. Thermal stress distribution [22], [23], thermal stress analysis of critical components [24], [25] and stress corrosion cracking experiments [26], [27], [28] were also extensively conducted. However, previous research works rarely involved a systematic investigation into the running status (temperature, heat flux, strain and stress) of the whole membrane wall system in an opposed fired boiler [29], especially, the detailed strain and stress distributions under different heating loads.
Therefore, we built a scale-down opposed fired spiral boiler system in the lab and proposed an experimental method to measure and calculate the temperature, heat flux, strain and stress distributions of membrane walls. In our previous work [30], the running status of the membrane system was compared between symmetry and asymmetry combustion conditions at a fixed heating load. Symmetric and asymmetric combustion conditions here mainly refer to the arrangement of the burners. Under the symmetric combustion condition, the burners were symmetrically arranged on both the front and rear walls in pairs, while under asymmetric combustion the burners were not arranged in pairs. Results revealed that the asymmetry combustion condition did have great promotion impact, especially on the stress distributions of the membrane walls. However, actual opposed fired boilers are mostly run under symmetric combustion conditions. In addition, boiler accidents caused by the deformation and fracture of boiler membrane walls are more likely to happen when the heating load is variable [6], [18], [31]. It is therefore of great importance to identify the status of the membrane system when the boiler is operated with symmetrical burner arrangement under varying heating loads.
In this paper, the temperature, local heat flux, strain and stress distributions of furnace membrane water walls were well measured on a model boiler setup when operated in a symmetric mode under four different heating loads: 25%, 50%, 75% partial loads and the full load conditions. Combustion was controlled in a symmetric manner by firing an equal number of opposite burners on the front and rear walls. It is expected to obtain a systematic understanding of how the heating load influences the working status of the heating surfaces during the operation of the boiler, and to further explore the relationship between the strain/stress distributions and the temperature or heat flux distributions. This work will provide valuable insights about the operation conditions of the entire membrane wall system under different boiler heating loads. Results in this study could potentially contribute to the optimization of opposed-firing boilers regarding their configuration, design and operation.
Section snippets
Experimental setup
Experiments were conducted in the wall-fired model boiler in our laboratory, as depicted in the Fig. 1. As described in our previous work [30], the furnace had a dimension of 0.32 m (width, x) × 0.32 m (depth, y) × 1.35 m (height, z). The boiler chamber was assembled with four membrane walls made of 8 steel plates (thickness, 2 mm) and 7 steel tubes (φ 12 × 2 mm) by pin connection. Cooling water was pumped into the membranes from the bottom, and flowing upward to the top, then returning to the
Temperatures of the furnace and outlet flue gas
The furnace flue gas temperature and outlet flue gas temperature under different heating loads were measured to estimate the furnace combustion and water wall heat transfer status. Limited by the length of the sheathed thermocouple, the furnace flue gas temperature could not represent the highest temperature in the furnace. The reason why we did not measure the flame temperature by inserting the thermocouple through the side walls is that opening up a measuring port on the membrane walls would
Conclusion
In this study, distributions of the temperature, heat flux, strain and stress of the membrane wall system were experimentally investigated in an opposed fired model boiler under four different heating loads. Tendencies of the maximum and average values of each targeted parameter were analyzed and discussed under varying heating loads. Major conclusions for the model furnace are summarized as following:
- 1.
The increase of the heating load form 25% partial load to full load leads to continuous
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Authors are grateful to the National Key Technology R&D Program of China (Contract No. 2014BAA02B02), the Postdoctoral Science Foundation of China 2019M652381, the Program for New Century Excellent Talents in University of Chinese Education Ministry (NCET-13-0468) and the Scientific Research Foundation Project of Shandong University 2018GN047.
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2021, Proceedings of the International Conference on Power Engineering 2021, ICOPE 2021