An investigation of performance of a conventional U type loop-seal for CFB reactors with side and bottom aerations

doi:10.1016/j.cherd.2020.08.013

Chemical Engineering Research and Design

Volume 163, November 2020, Pages 58-66

https://doi.org/10.1016/j.cherd.2020.08.013 Get rights and content

Highlights

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The loop-seal with side aeration has advantages over the conventional U type.
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The conventional U type requires a higher aeration flow rate for operating.
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The loop-seal with supply side wall aeration provides the highest sensitivity.
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Maximum solid circulation rate is limited by the rate through the passage.

Abstract

Experiments were carried out in a circulating fluidized bed (CFB) system. The riser has a cross sectional area of 100 × 100mm², while the supply and recycle chambers of the loop-seal have the same cross-sectional areas. Sand having a Sauter mean diameter of $310 μ m$ was used as bed material. Throughout the experiment, the superficial air velocity of the riser and the height of the solid particles in the storage column were kept constant. Four types of loop-seal categorized by their aeration scheme were investigated: aeration at the bottoms of supply and recycle chambers with side aeration at the supply chamber wall (U_SRQ_S), aeration at the recycle chamber bottom with side aeration at the supply chamber wall (U_RQ_S), aeration at the supply chamber bottom with side aeration at the recycle chamber wall (U_SQ_R), and aeration at the bottoms of supply and recycle chambers with side aeration at the recycle chamber wall (U_SRQ_R). The relation between the solid circulating rate and the side aeration flow rate was modeled as a saturated function. The results show that the U_RQ_S and the U_SQ_R types provide the same initial solid circulation rate, which is higher than for the conventional U type loop-seal. For a certain fluidization number, the loop-seals with side aeration provide higher solid circulation rate and require lower total aeration flow rate. The loop-seal with side aeration at the supply chamber wall tends to provide higher solid circulation rate and sensitivity of the circulation rate. For a certain solid particles height in the storage column, the maximum solid circulation rate was restricted by the flow through the horizontal passage.

Introduction

The circulation fluidized bed (CFB) reactor is popularly used as it has many advantages. For example, it has excellent mixing and circulation of solid particles around an endless loop of its riser and return leg, which leads to a higher reaction rate and a complete reaction, respectively.

As it has a large difference in pressure between the riser and the return pipe, a device which is able to transfer the particles from the return leg bottom to the riser bottom without back flow of gas is crucial. In addition, as a large rate of high temperature solid particles is required to be transferred, a non-mechanical valve is used for this purpose.

Various non-mechanical valves are used in CFB reactors such as loop-seal, L-valve, V-valve or J-valve (Kim et al., 2008; Cheng et al., 1998a; Li et al., 1997; Geldart and Jones, 1991; Leung et al., 1987; Terasaka et al., 2002; Lim et al., 2013; Chan et al., 2009; Yazdanpanah et al., 2012; Chovichien et al., 2013; Arena et al., 1998; Daous and Al-Zahrani, 1998; Xueyao et al., 2013) etc. U type and N type loop-seals are extensively used in CFB boilers and gasifiers. They consist of a supply chamber and a recycle chamber, which are connected to the return leg and riser respectively. Solid particles flow down from the return leg into the supply chamber, horizontally through the partition wall between both chambers into the recycle chamber and finally into the riser. Aeration at the loop-seal bottom plays an important role in the flow.

A number of factors affect the performance of the loop-seals. Velocity of aeration, type of Geldard’s particle, average particle diameter, pressure in the loop-seal, loop-seal configuration and aeration scheme have strong effects on solid particle flow behavior, solid circulation rate, pressure in the loop-seal (Basu and Cheng, 2000; Kim et al., 2002; Cheng et al., 1998b; Cheng and Basu, 1999; Kim and Kim, 2002; Peining et al., 2014; Monazam et al., 2007; Li et al., 2018; Zhao et al., 2015; Basu et al., 2009; Hong et al., 2013; Wang et al., 2014a; Bischi et al., 2013; Saayman et al., 2014), and sensitivity of solid circulation rate.

Aeration supplied at the bottom of the loop-seal plays a most important role in the particles flow. Solid circulation rate provided by the loop-seal increases as the velocity of the aeration increases (Bareschino et al., 2014; Wang et al., 2014b; Yao et al., 2011; Li et al., 2014; Han et al., 2007; Ji et al., 2012).

For the U type with horizontal passage, the circulation rate decreases and the required aeration for initiating the circulation rate increases as the ratio of the passage length to the passage height increases (Basu and Butler, 2009). Sensitivity of solid circulation rate with respect to aeration velocity increases as the height of the opening in the partition wall increases. Configuration of the loop-seal affects the sensitivity and required velocity of the aeration for initiating the circulation rate. The N type configuration is better able to provide higher sensitivity and lower aeration velocity than the U type (Wang et al., 2014c; Dong et al., 2014).

The aeration velocity ratio of the supply chamber to the recycle chamber has an influence on the sensitivity and required aeration for initiating the circulation rate. The U type with aeration supplied at the supply chamber has a higher sensitivity than that supplied at the recycle chamber. However, the U type with aeration supplied at the recycle chamber initiates the circulation rate at lower aeration velocity (Yang et al., 2009). The circulation rate of the U type can be increased by supplying aeration at the supply chamber side wall in the direction perpendicular to the side wall of the supply chamber, and parallel to the horizontal passage. The increase in the circulation rate decreases, as the height of the side aeration nozzle from the supply chamber bottom increases (Kim et al., 1999; Namkung and Cho, 2002; Seo et al., 2011). Moreover, the sensitivity of the circulation rate with respect to the side aeration flow rate can be increased by supplying aeration via multiple nozzles around the supply chamber wall (Kim et al., 2001).

As the aeration flow rate corresponds to initial solid circulation rate, sensitivity of solid circulation rate, and maximum circulation rate provided by the loop-seal is very important information. As solid circulation rate varies with its loads, this performance reflects the ability of loop-seals to provide minimum solid circulation rate, respond to its load, and to provide its maximum load. This information is very useful in operation and for modification of existing units in order to obtain the required performance. Furthermore, it can be used to reduce the operating costs due to excessive aeration supply.

In order to minimize aeration flow rate, the aeration has to be supplied in the desired direction of the particles flow. This may be achieved by investigating the performance under operation at various combinations of the bottom and side aerations. As a U type loop-seal is excellent in gas back flow prevention, it is most popularly used, especially in CFB boilers and gasifiers. In addition, information on the effect of side aeration at the recycle chamber on performance is still lacking. In this work, the performance of the conventional U type in terms of solid circulation rate, initial solid circulation rate, sensitivity of solid circulation with respective to aeration flow rate, and maximum solid circulation rate as a function of fluidization number based on the supplied bottom aeration velocity was investigated. Four aeration schemes were studied: aeration at the supply and recycle bottoms and side aeration at the supply chamber wall, aeration at the recycle chamber bottom and side aeration at the supply chamber wall, aeration at the supply chamber bottom and side aeration at the recycle chamber, and aeration at the supply and recycle bottoms and side aeration at the recycle chamber wall. In addition, the partition wall between the supply and the recycle chambers of commercial units is so thick that it may have a significant effect on the solid flow. This effect is taken into account by using the U type with a horizontal passage.

Section snippets

Experimental apparatus

The general arrangement of the circulating fluidized bed (CFB) system used in this work is shown in Fig. 1.

The riser has a cross sectional area of $100 m m \times 100 m m$ and is $4500 m m$ high from the air distributor plate. Local sand is used as solid particles or bed material. Fluidization air is supplied by a high-pressure blower. The air is delivered from the blower discharge through the supplied pipe and enters the riser via the distributor plate. The air velocity in the pipe is measured by a Pitot tube

Theoretical analysis

Data from research (Kim and Kim, 2002; Kim et al., 2001) shows that for a certain type of aeration supplied at the bottom of a loop-seal ( $U$ ), the effect of the side aeration at the supply chamber wall of the loop-seal ( $Q_{S}$ ) on solid circulation rate ( $G_{s}$ ) may be divided into two zones: a zone in which $G_{s}$ is sensitive to $Q_{S}$ and a zone in which $G_{s}$ is not sensitive to $Q_{S}$ . The effect can be presented as in Fig. 4.

For the sensitive zone, the loop-seal starts to provide $G_{s}$ at a rate of $G_{s, i}$ when $Q_{S}$ is

Results and discussion

The solid circulation rate ( $G_{s}$ ) of the conventional loop-seals as a function of aeration velocity at the bottom of the supply and recycle chambers ( $U$ ) to the minimum fluidized velocity of the solid particles ( $U_{m f}$ ) ratio or fluidization number ( $F n$ ) of the present work has the same trend as found by other researchers and is shown in Fig. 5. It increases as $F n$ increases.

In all cases, experimental data shows that for a certain bottom aeration velocity, $G_{s}$ rapidly increases as the side aeration flow

Conclusion and recommendation

In this work, the loop-seal with aeration solely at the supply chamber or the recycle chamber is unable to operate without side aeration. The U_RQ_S and the U_SQ_R types have the same initial solid circulation rate, which is about 5 times that of the conventional U type. However, the required total aeration flow rate for initiating the circulation rate for the conventional U type is higher than that of the U_RQ_S and the U_SQ_R types. The aeration flow rate of the conventional U type is about twice

Conflict of interest

None declared.

Acknowledgements

The authors acknowledge the National Research University Project of Thailand, Khon Kaen University and Mr. Ian Thomas for financial support and kindly making the manuscript more readable, respectively.

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An investigation of performance of a conventional U type loop-seal for CFB reactors with side and bottom aerations

Highlights

Abstract

Introduction

Section snippets

Experimental apparatus

Theoretical analysis

Results and discussion

Conclusion and recommendation

Conflict of interest

Acknowledgements

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Inst. Chem. Eng.

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Adv. Powder Technol.

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Int. J. Hydrogen Energy

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Particuology