CFD study of nonuniformity of gas-solid flow through a chemical looping combustion system with symmetrical series loops

Highlights

• Nonuniformity in a symmetrical CLC unit is studied using Eulerian multifluid model.

• Nonuniformity of gas-solid distribution is observed in a symmetrical CLC model.

• Enlarging total bed inventory and gas aeration rate can enhance nonuniformity.

• Specularity coefficient has an insignificant influence on the degree of nonuniformity.

Abstract

Multiple identical paths are usually connected to form circulating loops in many industrial reactors including chemical looping combustion (CLC), however the flow pattern may be highly nonuniform in these identical paths, compromising the stable operation. In this study, by using the Eulerian multifluid model, the nonuniformity in a CLC apparatus is studied. First, the symmetry of computational domain is verified by simulation and the model setting is validated by experimental data. Then the typical nonuniformity phenomenon is discussed and evaluated in terms of bed inventory evolution and gas-solid distribution in symmetrical parts. The effects of key operating parameters including total bed inventory, gas aeration rate, and wall roughness on nonuniformity degree are then studied. The simulating results show that the nonuniformity degree between the two symmetrical parts increases with the increase of total bed inventory and gas aeration rate and this trend will be intensified in the predictable short-term future. And through the analysis of pressure fluctuation, the instability of the system is aggravated. However, wall roughness shows an indefinite influence and tendency. This work provides an insightful understanding of nonuniformity and will help further optimise symmetrical CLC systems.

Introduction

Multiple paths are usually connected to form circulating loops in many industrial applications, where the paths configuration can be identical or nonidentical, and they may be connected in parallel or in series. The chemical looping combustion (CLC) system is regarded as one of the most promising clean combustion technologies because of the inherent CO₂ separation [[1], [2], [3], [4]]. A typical CLC system consists of two fluidised bed reactors: an air reactor (AR) and a fuel reactor (FR). They are structurally interconnected but atmosphere isolated, enabling the clean combustion and inherent CO₂ separation. Oxygen carriers continuously circulate between the two reactors to transport oxygen in the air from the AR to the fuel in the FR. Usually, the AR can be a fast fluidised bed reactor [[5], [6], [7]] due to the fast oxidation of reduced oxygen carriers, as the role to transfer the oxygen carriers back to the FR; the FR can generally be a bubbling fluidised bed reactor [5,6,8] or a moving bed reactor [8,9]. Generally, the geometry of AR and FR is nonidentical. In recent years, many CLC units are established based on the symmetrical dual circulating fluidised bed (DCFB) where the AR and FR have identical configurations [[10], [11], [12], [13]]. This structure effectively reduces the operating cost, and the circulation rate can be flexibly adjusted by altering superficial gas velocity and bed inventory. Therefore, it is important to understand the hydrodynamics of gas-solid flows in the symmetrical CLC units, where the identical paths are connected in series.

Several experimental efforts have been made to explore the hydrodynamics of gas-solid flows in the symmetrical CLC units [10,[12], [13], [14]]. For example, IFP Energies Nouvelles [11,13] developed and studied the pilot-scale CLC consisting of two identical reactor configurations. The Cranfield Pilot-Scale Advanced CO₂ Capture Technology (PACT) chemical looping reactor comprising two identical interconnected CFB components was also developed and studied [12]. A dual circulating fluidised bed (DCFB) with symmetrical CFB components was experimentally studied by Geng et al. [10] for investigating particle behaviours between AR and FR. In these works, the boundary and operating conditions in the symmetrical CLC configuration are generally nonidentical because of the different reactions taking place in the AR and FR. On the other hand, it is widely accepted that when multiphase flows travel through identical paths, a uniform distribution is commonly expected. However, the observations in the previous works showed that the flow distribution can be significantly nonuniform among the multiple paths even the boundary conditions are identical [7,[15], [16], [17], [18]]. Such nonuniformity of gas-solid distribution is expected to deteriorate process units and cause various issues such as suboptimal reactor performance, the formation of differential erosion and fouling through different paths, more frequent shutdowns, and unbalanced heat transfer [[19], [20], [21], [22]]. It is proposed that the minor or slight asymmetry in geometry and operation conditions will possibly magnify the deviation caused by the nonuniformity compared with the completely symmetrical configuration [23]. Therefore, the asymmetry of this operation condition in the CLC unit will worsen the nonuniformity and further break the originally stable CLC operation, leading to great difficulties in controlling the flow and reaction of the system. Thus, it is necessary to study the gas-solid flow nonuniform phenomenon in detail.

Although the nonuniformity of mass distribution in CLC units can be studied by setting identical initial and boundary conditions for symmetrical paths, the experimental method shows some limits. This method is time-consuming and expensive; the reliability of experimental data may be affected by many factors and anomalies, including minor differences and fluctuation in two blower equipment. It is extremely hard to maintain precisely the same for each path in real experiments. A more high-fidelity method should be helpful for investigating the nonuniformity phenomenon in CLC units.

Computational fluid dynamics (CFD), as an alternative to the experimental method, has been increasingly used to unveil the intrinsic mechanism of gas-solid flows in the DCFB CLC units and other coal-fired processes [[41], [42], [43]]. Specifically, the CFD method can eliminate the influence of structure and external conditions on flow fields by ensuring the absolute and perfect symmetry of geometry, boundary and operating conditions in symmetrical paths.

To sum up, the fundamental mechanism of the nonuniform distribution in the CFB system has been investigated through many theoretical and experimental studies [[24], [25], [26]]. However, to date, nearly all of these studies about the nonuniformity of bed inventory in DCFB system are based on parallel-connected paths, such as pant-leg structure [27,28] or multi-cyclone arrangements [21,29] because of their extensive applications in industries. The comprehensive study about the nonuniformity of bed inventory in the symmetrical CLC unit, which is a series circulation system, has not been reported, although they were developed and practised widely [3,4,30,31]. Moreover, the simulation studies about the nonuniformity of gas-solid flow through the symmetrical CLC system which consists of two series paths have not been reported yet; and the relationship between nonuniform gas-solid distribution and the performance of the symmetrical CLC are yet being understood.

To overcome this insufficiency, in the present work, the nonuniformity phenomenon of gas-solid distribution in a DCFB CLC unit with two identical paths connected in series is numerically studied, where the two paths are perfect symmetrical in terms of geometries, grids, and operating conditions. An index for assessing the degree of solid nonuniform distribution is proposed, and the influences of several key parameters are explored. The paper is organised as follows: In section 2, governing equations of the multifluid model are formulated including drag model. In section 3, geometry configuration and computational grids are given for studying the nonuniformity phenomenon. In section 4, the proposed model is validated against the experimental data, and the symmetry setting in terms of geometry, grid and operation conditions is verified. Then, the nonuniformity phenomenon is discussed, and the effects of several key operating parameters (i.e., gas inlet velocity, total solid inventory, and wall roughness) on the degree of nonuniformity are thoroughly investigated.