* Corresponding author. Fax: #81-296-58-8209. E-mail address: hatano@nire.go.jp (H. Hatano) Chemical Engineering Science 55 (2000) 43}50 Pressure e!ect on char combustion in di!erent rate-control zones: initial rate expression Shi-Ying Lin, Yoshizo Suzuki, Hiroyuki Hatano*, Katsumi Tsuchiya Thermal Energy and Combustion Engineering Department, National Institute for Resources and Environment, 16-3 Onogawa, Tsukuba 305-8569, Japan Department of Chemical Science and Technology, The University of Tokushima, Tokushima 770-8506, Japan Received 25 August 1998; accepted 31 December 1998 Abstract E!ect of pressure on the initial rate of coal char combustion was studied in a "xed bed and analyzed in terms of theoretical rate equations. Char particles of 0.25}0.5 mm diameter were combusted over temperature and pressure ranges of 559}1273 K and 0.1}1.6 MPa, respectively. Char combustion proceeded in "rst order with respect to oxygen. The combustion rate increased proportionally with pressure in the chemical-kinetics control zone (Zone I), exhibited a nonlinear increase with pressure in the internal-di!usion control zone (Zone II), and was invariant with pressure in the external-di!usion control zone (Zone III). Theoretical rate equations involving explicit pressure terms were developed for predicting the combustion rate under elevated pressure. For given properties of char (d  ,   ,  and k  ) and operating conditions (C  , u  , ¹ and P), the combustion rate estimated from the developed rate equations agreed well with the experimental results.  1999 Elsevier Science Ltd. All rights reserved. Keywords: PFBC; Combustion; Pressure e!ect; Kinetics; Char 1. Introduction Pressurized #uidized-bed combustion (PFBC) is one of the e$cient and environmentally acceptable technologies for coal utilization. It has recently entered the commer- cial stage (Cuenca & Anthony, 1995). Several phenomena in PFBC boilers, however, are still not well understood. One of them is how the operating pressure in#uences the char combustion rate. Previous studies on the e!ect of pressure on char combustion focused mainly on pulverized coal combus- tion which can be realized at temperatures above 1473 K (Hamor, Smith & Tyler, 1973; Tidona, 1980; Monson, Germane, Blackham & Smoot, 1995; Essenhigh & Mes- cher, 1996; Saastamoinen, Aho, Hamalainen, Hernberg & Joutsenoja, 1996; Banin, Commissaris, Moors & Veef- kind, 1997; Bayless, Schroeder, Peters & Buckius, 1997). Only a few were concerned with the #uidized-bed combus- tion which uses lower temperatures (Wallman & Carls- son, 1991; Croiset, Mallet, Rouan & Richard, 1996). Findings on the pressure e!ect reported in these studies, however, are not systematic and often fragmental. Some studies (Tidona, 1980; Wallman & Carlsson, 1993; Essen- high & Mescher, 1996) showed that the combustion rate of char/coal was not signi"cantly a!ected by elevated pressure. Other studies (Shiao, Warchol & Botros, 1989; Sellakumar, Isaksson & Tiensuu, 1993; Monson et al., 1995; Bateman, Germane, Smoot, Blackham & Eatough, 1995) reported that the combustion rate increased with pressure. Saastamoinen et al. (1996) found that while the combustion rate increased with pressure in the chem- ical-reaction control regime for a "xed oxygen fraction, the rate was less a!ected by the pressure in the di!usion control regime. It is thus inferred from these previous studies that the pressure can play di!erent roles in di!er- ent rate-control regimes which prevail under di!erent experimental conditions. Gas}solid reaction involving porous solids goes through external and internal di!usion and chemical kinetic processes (Hashimoto & Silveston, 1973; Lauren- deau, 1978; Bhatia & Perlmutter, 1981; Sotirchos & Amundson, 1984; Delikouras & Perlmutter, 1993). For char combustion, three ideal temperature zones were identi"ed and categorized by Walker, Rusinko and Austin (1959) on the basis of rate-governing process. 0009-2509/00/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 9 - 2 5 0 9 ( 9 9 ) 0 0 3 0 2 - 4