Devolatilization of coals of northeastern India in inert atmosphere and in air under fluidized bed conditions R.C. Borah a,b , P.G. Rao a , P. Ghosh b, ⁎ a Chemical Engineering Division, North-East Institute of Science and Technology, Jorhat – 785006, India b Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati – 781039, India abstract article info Article history: Received 9 February 2009 Received in revised form 26 July 2009 Accepted 17 August 2009 Keywords: Air Argon Coal Devolatilization Fluidized bed Pyrolysis Devolatilization of five coals having volatile matter in the range of 31 to 41% was studied in argon and in air under fluidized bed conditions. The diameter of the coal particles varied between 4 and 9.5 mm. The variation of devolatilization time with particle diameter was expressed by the correlation, t v =Ad v n . The superficial gas velocity was found to have a significant effect on the rate of devolatilization. The devolatilization rate increased with the increase in the oxygen concentration in the fluidizing gas. The correlations developed in this study fitted the mass versus time profiles of the coal particles satisfactorily. The same correlations were found to be appropriate for predicting devolatilization of a batch of coal particles. The correlations developed in the present study will be useful for the design of fluidized bed combustors. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Devolatilization is the first step in most coal conversion processes such as combustion, gasification and liquefaction. It has a significant influence on the subsequent stages of processing. The volatile material released can account for up to half of the coal's heating value [1]. The yield of volatile matter can be determined by pyrolysis, which gives the proportion of coal volatilized during the devolatilization step. Over the past few decades, the fluidized bed combustion and gasification technologies have been well-recognized to utilize the low-rank coals for power generation. The combustion of volatile matter in a fluidized bed has several consequences related to its design and operation because it affects the distribution of oxygen across the bed, the feed- point spacing of the coal particles, the split in heat release (and hence the heat transfer surface between the bed and freeboard), and the release of nitrous oxide from the bed. Therefore, it is important to understand the release of volatiles and their subsequent mixing with oxygen for combustion. The rate of devolatilization of coal in a fluidized bed combustor depends on several parameters such as particle size, atmosphere, fluidization velocity, bed temperature, coal type, and the amount of volatile matter. The small particles are likely to release volatile matter quickly, but the large particles can release volatile matter above the bed because they are heated at a slower rate than the smaller particles [2]. This causes variations in the composition in the gas. For a particular design of the bed, this variation can retard the conversion of the gaseous components and shift the conversion to the downstream region of the furnace, which can influence burn-up and cause undesirable emissions. In some cases, additional air has to be introduced in the freeboard to reduce the emission levels [3]. Hence, it is important to ascertain the amount of volatile matter released by a single coal particle over time after its injection into the bed. For practical purposes, it would be better to investigate a batch of coal particles, but it has been reported in the literature [4] that the coal particles fuse together during pyrolysis and form a single large particle. There are several reports in the literature [2,5–7] on devolatiliza- tion time of coal particles in presence of air. However, these works have not reported the variation of mass of a single particle with time. Thunman and Lackner [8] studied the devolatilization of a batch of coal particles in air and developed an empirical equation for the variation of mass with time. The extent of devolatilization of large particles has been found to be less in comparison with the small particles in a fluidized bed [9,10]. There are differences among the results reported in the literature for devolatilization time, which can be attributed to the experimental conditions and the definitions used to measure it [11]. To simplify the procedure, a thermogravimetric analyzer (TGA) may be used to measure devolatilization time. However, the rate of heating in a fluidized bed is much greater than that in a TGA, and the devolatilization kinetics varies considerably with such large variation in the heating rate. Fuel Processing Technology 91 (2010) 9–16 ⁎ Corresponding author. Tel.: +91 361 2582253; fax: +91 361 2690762. E-mail address: pallabg@iitg.ernet.in (P. Ghosh). 0378-3820/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2009.08.014 Contents lists available at ScienceDirect Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc