Non-premixed fluidized bed combustion of C 1 –C 4 n-alkanes Jean-Philippe Laviolette, Gregory S. Patience, Jamal Chaouki ⇑ Department of Chemical Engineering, École Polytechnique de Montréal, P.O. Box 6079, Station Centre-ville, Montréal, Canada H3C 3A7 article info Article history: Received 31 May 2010 Received in revised form 5 April 2011 Accepted 18 April 2011 Available online 30 April 2011 Keywords: Combustion Methane Propane Ethane n-Butane abstract The non-premixed combustion of C 1 –C 4 n-alkanes with air was investigated inside a bubbling fluidized bed of inert sand particles at intermediate temperatures: 923 K 6 T B 6 1123 K. For ethane, propane and n-butane, combustion occurred mainly in the freeboard region at bed temperatures below T 1 = 923 K. On the other hand, complete conversion occurred within 0.2 m of the injector at: T 2 = 1073 K. For meth- ane, the measured values of T 1 and T 2 were significantly higher at 1023 K and above 1123 K, respectively. The fluidized bed combustion was accurately modeled with first-order global kinetics and one PFR model to represent the main fluidized bed body. The measured global reaction rates for C 2 –C 4 n-alkanes were characterized by a uniform Arrhenius expression, while the global reaction rate for methane was signif- icantly slower. Reactions in the injector region either led to significant conversion in that zone or an autoignition delay inside the main fluidized bed body. The conversion in the injector region increased with rising fluidized bed temperature and decreased with increasing jet velocity. To account for the pro- moting and inhibiting effects, an analogy was made with the concept of induction time: the PFR length (b i ) of the injector region was correlated to the fluidized bed temperature and jet velocity using an Arrhenius expression. These results show that the conversion of C 2 –C 4 n-alkanes can be estimated with one set of critical bed temperatures and modeled with one Arrhenius kinetics expression. Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved. 1. Introduction In the context of increasingly more severe environmental regulations due to global warming, new fluidized bed processes are currently under development to produce energy and chemical products from biomass and natural gas feedstocks. Combustion is inherent to many of these processes, which include selective oxi- dation of alkanes, biomass gasification, combustion of non- conventional feedstocks (biomass and co-firing) and many more. Combustion may involve many hydrocarbons present in the feed- stock or produced from thermal cracking during the reaction. A good knowledge of the combustion process and the operating con- ditions necessary for complete combustion in the fluidized bed re- gion is essential for process design and safety during operation, startup and shutdown. The combustion of methane, liquefied petroleum gas (LPG) and propane in fluidized beds of inert particles has been the subject of several studies in premixed [1–10] and non-premixed modes [11–16]. Fluidized bed temperature has been reported as a key parameter that determines in-bed fuel conversion: the conversion front moves towards the distributor as the fluidized bed tempera- ture is increased. At low fluidized bed temperature, the combus- tion is first initiated in the freeboard region. Increasing the temperature moves the conversion front to the bubbles bursting at the upper surface of the bed, then to bubbles igniting in the bed and finally to small bubbles where ignition occurs practically at the level of the distributor and the process appears flameless [7,9]. While some studies have suggested that combustion occurs predominantly inside the bubble phase as opposed to the emulsion phase [9], methane and propane combustion have been reported in fixed beds of sand particles at temperatures above 1023 K and 973 K, respectively [1,17]. To characterize the location of the conversion front, two critical transition temperatures have been defined for fluidized bed com- bustion in inert particles. Below the lower critical temperature (T 1 ), combustion only occurs above the bed surface. Between T 1 and the upper critical temperature (T 2 ), the conversion front begins to move inside the bubbles within the bed. Finally, above T 2 , com- bustion takes place entirely within the bed and close to the distrib- utor. For methane combustion in inert sand particles, the values of T 1 and T 2 reported in the literature range between 823–1018 K and 1133–1250 K, respectively [18]. For propane, T 2 was measured in a shallow bed (H B  0.1 m) of sand particles as 1123 K [2,9]. Investi- gations on the fluidized bed combustion of other alkanes in inert particles are scarce. To accurately predict reactant conversion in fluidized beds, a reaction model that combines the gas/solids hydrodynamics and the reaction kinetics can be used. Several gas-phase hydrodynam- ics models of varying complexity have been proposed in the literature [18]. Hydrocarbon conversion in fluidized beds has been shown to vary from plug flow to back-mixed flow depending on 0016-2361/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.04.023 ⇑ Corresponding author. Tel.: +1 514 340 4711x4469; fax: +1 514 340 4159. E-mail address: jamal.chaouki@polymtl.ca (J. Chaouki). Fuel 90 (2011) 2850–2857 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel