Particuology 18 (2015) 58–65 Contents lists available at ScienceDirect Particuology jou rn al hom epage: www.elsevier.com/locate/partic Effect of fluidizing velocity on gas bypass and solid fraction in a dual fluidized bed gasifier and a cold model Mook Tzeng Lim a , Woei-Lean Saw b , Shusheng Pang a,∗ a Department of Chemical and Process Engineering, University of Canterbury, Private Bag 4800, Christchurch, New Zealand b Centre for Energy Technology, School of Chemical Engineering, University of Adelaide, Adelarde, Australia a r t i c l e i n f o Article history: Received 20 October 2013 Received in revised form 30 April 2014 Accepted 21 May 2014 Keywords: Cold model Hydrodynamics Steam gasification Dual fluidized bed a b s t r a c t Dual fluidized bed gasifiers (DFBG) are effective in producing nitrogen-free syngas from biomass. How- ever, to improve the gasifier performance, pressure drops and solid fractions within the DFBG system need to be controlled. In this study, the effects of varying the fluidizing velocity in the fast fluidized bed (FFB) on the pressure drops and the solid fractions in the system were investigated in a 100 kW DFBG and in a dual fluidized bed cold model (DFCM). Based on the experimental results, empirical correlations were developed to predict the height-averaged solid fraction in the bottom section of the FFB. Accuracy and advantages of the correlations were analyzed. The correlation is useful for design and modeling of the DFBG systems where the height-averaged solid fraction is required to be determined. © 2014 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved. Introduction Depletion of fossil fuel resources and generation of greenhouse gases from extensive use of these fuels have led to great inter- est in alternative energy sources (European Climate Foundation, Sveaskog, Södra, & Vattenfall, 2010). Biomass, in the form of wood chips, sawdust, demolition wood, or agricultural residues, has been recognized as a promising renewable energy source. Biomass gasi- fication is an effective technology to covert the biomass to a gas mixture consisting of H 2 , CO, and CH 4 (Göransson, Söderlind, He, & Zhang, 2011; Koppatz et al., 2009; Miccio, Kalisz, Baxter, & Svoboda, 2008). This gas mixture, known as producer gas, has a lower heat- ing value (LHV) ranging from 4 to 20 MJ/Nm 3 , depending on the type of gasifier and gasification agent used. The producer gas can be combusted in an internal combustion engine, in a gas turbine, or in a boiler to produce heat and power (Yap & Wang, 2007). Producer gas can also be converted into liquid fuels through Fischer-Tropsch synthesis (Takeshita & Yamaji, 2008). A dual fluidized bed steam gasifier (DFBG) is an advanced gasi- fication technology that can produce hydrogen-rich producer gas with high LHV of 10–14 MJ/Nm 3 (Pfeifer, Rauch, & Hofbauer, 2004; Saw & Pang, 2012a, 2012b; Saw, McKinnon, Gilmour, & Pang, 2012). ∗ Corresponding author. Tel.: +64 3 364 2538. E-mail address: shusheng.pang@canterbury.ac.nz (S. Pang). The DFBG consists of a fast fluidized bed (FFB) combustor and a bub- bling fluidized bed (BFB) gasification reactor as shown in Fig. 1(a). This configuration of DFBG allows the combustion and gasification processes to be separated and, under optimized operating condi- tions, the system is able to produce N 2 free producer gas (Hofbauer, Rauch, Bosch, Koch, & Aichemig, 2003; Koppatz et al., 2009; Pfeifer et al., 2004; Pröll, Siefert, Friedl, & Hofbauer, 2005; Saw & Pang, 2012a, 2012b; Saw et al., 2012). High N 2 content in the producer is undesirable as it dilutes the producer gas, thereby decreasing the LHV. In the DFBG, biomass is fed to the BFB at the bed position and steam is fed to the BFB at the bottom. The char generated from the initial stage of the gasification flows together with the bed mate- rial (normally sand) through a chute from the BFB bottom to the FFB. In the FFB, the char is combusted to heat up the bed mate- rial which is then circulated by fluidizing agents and separated in the FFB cyclone. After this, the hot bed material returns to the BFB through a siphon to provide heat for the biomass steam gasification. DFBG performance is closely related to the fluidizing velocity in the FFB as it affects the char combustion rate, the bed mate- rial circulation rate, the solids fraction, and the pressure drop. An excessive FFB fluidizing velocity can cause a bypass flow of flue gas and air from the FFB to the BFB through the interconnect- ing chute (Fig. 1(a)). Once this occurs, N 2 content in the producer gas is increased, causing the producer gas LHV to decrease (Kaiser, Löffler, Bosch, & Hofbauer, 2003; Löffler, Kaiser, Bosch, & Hofbauer, 2003; Pröll, Rauch, Hofbauer, & Aichernig, 2007; Xu, Murakami, Suda, Matsuzawa, & Tani, 2006). However, quantification of the http://dx.doi.org/10.1016/j.partic.2014.05.007 1674-2001/© 2014 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.