Identification of the running status of membrane walls in an opposed fired model boiler under varying heating loads

Highlights

• Membrane wall temperature, heat flux, strain and stress distributions were mapped.

• An area with overlapped high temperature and stress is identified on side walls.

• Wall temperature and fix constraints determined strain and stress distributions.

• Average values of strain and stress kept almost stable under varying heat loads.

Abstract

To understand the running status of membrane walls in an opposite firing boiler, a scale-down model furnace was established, and the temperature, heat flux, strain and stress distributions are investigated under four heating loads. Results show that the average membrane wall temperature and heat flux present a continuous increase from 42 °C and 16 W/m² to 96 °C and 50 W/m², respectively, with the heating load increase from 25% to full load. The average strain and stress also rise from 88.7 µm and 0.094 MPa to 152.5 µm and 0.148 MPa when the heating load increases from 25% to 50%, but then they keep stable when further increasing the heating load. General distribution patterns of each tested parameter are found relatively similar under varying heating loads. High strain and stress distributions are always detected at the middle left zone of side walls and the middle of the rear wall, where wall temperatures are measured high. External fixed constraints and high-temperature thermal strain is found jointly affecting the strain and stress distribution of the membrane wall. A simplified mechanism of how the strain and stress on boiler membrane walls evolve is proposed after comprehensive discussion of the measurement results.

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.