Hydrodynamics of cold-rig biomass gasifier using semi-dual fluidized-bed Son Ich Ngo a , Young-Il Lim a, ⁎, Byung-Ho Song b , Uen-Do Lee c, d , Chang-Won Yang c, d , Young-Tai Choi c , Jae-Hun Song e a Lab. FACS, RCCT, Department of Chemical Engineering, Hankyong National University, Gyonggi-do, Anseong-si, Jungangno 167 456‐749, Republic of Korea b Department of Chemical Engineering, Kunsan National University, Gunsan, Jeonbuk 573‐701, Republic of Korea c Energy System R&D Group, Korea Institute of Industrial Technology (KITECH), Cheonan 331-825, Republic of Korea d Department of Green Process and System Engineering, University of Science and Technology (UST), Cheonan 331‐825, Republic of Korea e 1501 SeenTec Tower 74‐6 Sangnam-dong, Seongsan-gu, Changwon City, Gyeongnam 642‐831, Republic of Korea abstract article info Article history: Received 21 August 2012 Accepted 14 September 2012 Available online 23 September 2012 Keywords: Gasification Semi-dual fluidized-bed (sDFB) Solid circulation rate Internal mixing Computational fluid dynamics (CFD) Eulerian–Eulerian model This study reports a semi-dual fluidized-bed (sDFB) biomass gasifier, which is a novel design of dual fluidized-bed (DFB) with the internal mixing of solid particles between riser and gasifier to enhance the heat and mass transfer. A cold-rig experiment of sDFB (0.8 m width × 0.2 m depth × 3.85 m height) was performed to investigate fluid hydrodynamics and solid circulations. Pressures were sampled at 43 points along the sDFB gasifier. An external circulation rate of sand was measured for 60 s after 2 min of the operating time. In order to estimate the amount of direct back-mixing particles through the gasifier–riser interconnec- tion area, an Eulerian–Eulerian two-dimensional computational fluid dynamics (CFD) model was developed for the cold-rig sDFB. This CFD model included the kinetic theory of granular flow and the k– dispersed turbu- lence model. The CFD simulation results were validated with the experiment data. About 17% back-mixing of particles through the gasifier–riser interconnection area were obtained from the CFD simulation. This indi- cates that the sDFB has a possibility of having higher heat and mass transfer than the conventional DFB. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Nowadays, interest in biomass gasification has an accelerative development, since it is renewable, sustainable, abundantly available everywhere in the world, and the increased use of biomass can reduce the petroleum dependence [1,2]. Biomass gasification is the process by which organic matter is thermally devolatilized, followed by secondary reactions of the resulting products [3]. The chemical energy of the solid fuel is converted into both the thermal and chem- ical energy of the gas. Chemical energy contained within the gas is a function of chemical composition. Thus the chemical composition of the producer gas determines its quality as a fuel [4]. Furthermore, the producer gas of biomass gasification is used as feedstock in some upgrading systems for generating energy and fuels in a much cleaner manner. This is suitable for energy demand in the future, owning to reduce the net of carbon dioxide emission while increasing environmental safety [1]. However, biomass gasification has been known as a complex process due to the complicating nature of bio- mass composition. Fluidization has been widely used industrially because of its con- tinuous handling ability of solid particles and its good heat and mass transfer characteristics [5]. In the conventional dual fluidized-bed (DFB) system, the heat required for endothermic reactions in the gasifier is provided with solid particles (sand) transported from the combustion zone (riser) to the gasification zone (gasifier). Thus, the amount of circulating solids indicates the energy demand for the gasification process [6]. Recently, researchers have addressed the relationships between the solid circulation rate and other factors in DFB gasifiers such as the heat efficiency [6,7], breakage and attrition effects [8], and the stability of the loop-seal [9]. Using a gasifier with a higher capacity requires more energy and a higher solid circulation rate. However, Shen et al. suggested that more solid circulation rate could lead to more breakage and attrition by the hot circulating particles [8]. Seo et al. reported that the solid circulation rate has to be maintained above a certain amount for the loop-seal to be stable [9]. From our experience [6,7], the solid circulation rate should be carefully selected by considering many performance criteria such as heat efficiency, lower heating value, and additional fuel ratio. Computational fluid dynamics (CFD) modeling has become a viable tool for investigation on hydrodynamics of various processes with the aid of increasing computational capacity. However, CFD is still at the verification and validation stage for modeling multiphase flow systems such as fluidized-beds. More improvements regarding the flow dynamics and computational models are required to make CFD suitable for fluidized-beds modeling and scale-up [10,11]. In CFD, the modeling of gas–solid hydrodynamics is generally divided into two main approaches. The Eulerian–Lagrangian approach is also called discrete particle modeling. The gas phase is calculated Powder Technology 234 (2013) 97–106 ⁎ Corresponding author. Tel.: +82 31 670 5207; fax: +82 31 670 5445. E-mail address: limyi@hknu.ac.kr (Y.-I. Lim). 0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2012.09.022 Contents lists available at SciVerse ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec