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Study on the actual operation characteristics of fluidized bed heat exchanger in a supercritical CFB boiler

On the basis of measurements on the fluidized bed heat exchanger (FBHE) in a 600MW supercritical CFB boiler fueled by lean coal under different boiler load, the actual operation characteristics of the FBHEs, such as post-combustion characteristics and heat absorption shares in the FBHE were obtained in this paper, as well as ash flow rate through the FBHE. The measured and calculated results indicate that the post-combustion phenomenon exists in the FBHEs of the boiler fueled by lean coal. As steam parameter raises from subcritical to supercritical parameter, heat absorption share of heating surface in the FBHEs are quite different from each other regardless of boiler load. Moreover, as the boiler load decreases, ash flow rate through FBHE arranging reheater is almost the same, while ash flow rate through FBHE arranging superheater changes significantly. These results can provide referential materials for the optimized design and operation of the FBHE in large-scale CFB boilers.


Introduction
Circulating fluidized bed (CFB) boilers are usually consisted by furnace, cyclone, loop seal and fluidized bed heat exchanger (FBHE). As a key component of the CFB boiler, FBHE is favorable for adjusting furnace temperature and steam temperature [1]. To an extent, heat absorption share and postcombustion phenomenon in the FBHE represents the regulation ability of furnace temperature, which are important issues in the design of supercritical CFB boiler at this stage and ultra-supercritical in the future [2][3].
Research works on characteristics of the FBHEs were mainly conducted on laboratory-scale test rigs [4][5][6][7][8][9]. However, because of limit of experiment condition, the practical value of the experimental data was rather limited. Lu, J.Y. et al. measured gas concentration at outlet of the FBHE in a 300MW CFB boiler burning lean coal, and post-combustion characteristics of the external circulation loop were studied [10]. Zhang Consequently, studies on operation characteristics of the FBHE in a 600MW supercritical CFB boiler fueled by lean coal were carried out in the present work. In this study, operation parameters, such as steam gauge pressure, steam temperature and flow rate of heating surfaces in the FBHEs, flue gas and ash temperatures at the inlet and outlet of each chamber in the FBHE were measured. After analyzing the operation parameters of FBHE, some suggestions are given in an attempt to provide reference for the optimized design and operation of FBHE in large-scale CFB boilers.

Experimental objects
It involved a 600MW supercritical CFB boiler, shown in Figure 1(a). Six FBHEs were placed on the two sides of the boilers' furnace symmetrically, which arranged in the front wall, the middle wall and the back wall of the furnace, respectively. Every FBHE was divided into two chambers along ash flow direction, one was in the absence of heating surfaces, not covered in Figure 1

Experiment methodology
A measuring point was installed specifically in the ceiling at the exit of each FBHE for gas sampling. A custom-made probe was used to take gas samples from the FBHE [10]. A MGA5 infrared gas analyzer (O 2 precision accuracy of ±0.2%, CO precision accuracy of ±5% measured value) was employed to monitor the online gas concentrations. To prevent condensation inside the probe, the gas temperature was kept at 120°C by an electrical heating system. Other ash-side and steam-side operation parameters, such as ash temperatures at the inlet and out of the FBHE, steam gauge pressure, steam temperature and flow rate of heating surfaces, volumetric flow rate and the fluidizing air temperature of each chamber in the FBHE, were all obtained from the Distributed Control System (DCS) database.
Heat absorptions of the heating surfaces were products of the corresponding steam mass flow rates and enthalpy increments from inlet to outlet. Heat released by residual char combustion in each FBHE, directly according to combustion reaction equation of carbon in every FBHE, then ash flow rate through every FBHE soon obtained according to mass and heat balances in the chamber [16].

Test process
The tests were conducted under 60% and 99% boiler load. Proximate and ultimate analyses for the feeding coal were shown in Table 1.

Field measurements
The steam-side and ash-side thermal parameters of the FBHEs at different boiler loads are given in Tables 2-4.   These Figures also show there are differences of combustion characteristic among different FBHEs, regardless of boiler load. For example, actual operation parameters obtained from the DCS database show the openings of the incoming ash flow control valves of the FBHEs at 99%BMCR are 28% (front FBHE), 25% (middle FBHE) and 21% (rear FBHE), respectively. Meanwhile, the inlet air flow rates of the three FBHEs are nearly the equivalent, so it is easy to understand that there are gas concentration differences at the outlet of the three FBHEs. Actual operation parameters also show that the inlet ash temperature of three FBHEs are all about 880 °C, but the outlet ash temperatures are 695 °C (front wall side), 674 °C (middle) and 594 °C (rear wall side), which indicated there is a great difference of the inside ash temperature distribution between the FBHEs. It may be another reason for gas concentration differences at the FBHEs outlet.

Heat absorption shares of heating surface in the FBHE at different boiler load
Heat absorption shares of reheater and superheater in the FBHE at different load are shown in Figures  4 and 5. It is seen that there exists a remarkable difference in the heat absorption shares of reheater and superheater in different FBHEs. Heat absorption shares of reheater and superheater in the FBHE are 7.61% and 8.40% at 60% boiler load, while they are 6.73% and 11.35% at 99% boiler load, indicating that when steam parameter from subcritical to supercritical parameter, heat absorption shares of the FBHE would be quite different from each other.   Figure 6 gives ash flow rate through the arranging reheater and superheater FBHEs at different load. It is shown that with the boiler load decreasing total ash flow rate through the FBHEs decreases. Moreover, to adjust the furnace temperature, ash flow rate through the arranging superheater FBHE changed obviously, while ash flow rate through the arranging reheater FBHE a little raised. It is mainly influenced by the heat transfer characteristics inside the FBHE. The heat transfer characteristics inside the FBHE are affected by ash flow rate through the FBHE and solid temperature at the entrance of the FBHE. In the tests, inlet solid temperatures at the front wall FBHE obtained from the DCS database were 880 °C (left side) and 912 °C (right side) at 99% boiler load, while they were 787 °C (left side) and 850 °C (right side) at 60% boiler load. To ensure outlet steam temperature of reheater and keep sufficient heat absorption, more hot particles are needed to pass through the FBHE to improve the ash-side heat-transfer intensity. Additionally, DCS database also showed that the openings of the incoming ash flow control valves of the front wall side FBHEs were 35% (left side) and 28% (right side) at 99% boiler load, but they were 42% (left side) and 34% (right side) at 60% boiler load. To a certain extent, it indicates that the ash flow rate through the arranging reheater FBHE indeed doesn't decrease as the boiler load decreases.

Conclusions
Actual operation characteristics of the FBHEs in a supercritical CFB boiler were studied in this paper. The main findings of this paper include: Measured results indicate O 2 concentrations at the outlet of the FBHEs in the boiler fueled by lean coal are about 15% at different boiler load, which shows that there exists post-combustion phenomenon in the FBHEs. In addition, there are also differences on combustion characteristic among different FBHEs, regardless of boiler load. With steam parameter raises from subcritical to supercritical steam parameter, heat absorption shares of heating surface in the FBHEs would be quite different significantly. Heat absorption shares of reheater and superheater in the FBHEs are 7.61% and 8.40% at 60% boiler load, while they are 6.73% and 11.35% at 99% boiler load. As the boiler load decreases, ash flow rate through front FBHE adjusting reheater steam temperature changs from 370.17kg/s to 394.54kg/s, while ash flow rate through middle and rear FBHEs adjusting furnace temperature changes from 600kg/s to 316.95kg/s.