The Order with Respect to Oxygen and Activation Energy of the Burning of an Anthracitic Char in O 2 in a Fluidised Bed, as measured using a Rapid Analyser for CO/ CO 2 . P.S. Fennell * , J.S. Dennis and A.N. Hayhurst Department of Chemical Engineering, University of Cambridge. Pembroke St, Cambridge, CB2 3RA, UK. Abstract The use of a novel, extremely rapid analyzer for measuring [CO] and [CO 2 ] with a time constant as low as 12 ms has been investigated, together with the effects of minimizing the total mixing times, when measuring the rates of oxidation for the char from an anthracite inside a small fluidized bed. It was found that the order of reaction in O 2 was 1.2 ± 0.4, with an associated activation energy of 145 ± 20 kJ / mol for the first-order rate constant. This contrasts with previous measurements using larger fluidized beds and a slower analyser, where the true kinetics were probably obscured. Although the major product of oxidation was found to be CO 2 when the bed was hotter than 800 o C, but CO below 800 o C, the indications are that CO is the major product of combustion. Nomenclature Symbol Description Units C m Measured concentration of gas mol / m 3 C t ‘True’ concentration of gas entering the particulate phase of a fluidised bed mol / m 3 d p Diameter of a particle m D Gas phase diffusivity m 2 / s D eff Effective diffusivity in the pores of a particle m 2 / s E a Activation Energy kJ / mol k First order rate constant, corrected for the effects of mass transfer m 3 / s g k’ Measured first order rate constant m 3 / s g k’’ Measured first order rate constant per unit geometric surface area m / s k g External mass transfer coefficient m / s m Mass of a batch of char g q Molar flowrate of fluidising gas mol / s Q tot Cold flowrate of fluidising gas ml / s T Temperature of the bed o C U Superficial velocity through the bed (hot) m / s U mf Minimum fluidising velocity m / s x x Mole fraction of species x Z Conversion of a particle Sh Sherwood number = k g d p / D ε Porosity of a particle ε o Initial porosity of a particle η Effectiveness factor τ o Time constant of analyser s τ bed Time constant of the bed s τ m Measured time constant s ∗ Corresponding author: psf20@cam.ac.uk Proceedings of the European Combustion Meeting 2007 Introduction Many studies [1,2] have investigated the oxidation of a coal char or graphite in O 2 ; some [1-3] have utilized the fact that in a fluidised bed the rates of heat and mass transfer to and from a reacting particle are large and characterised well. Here, we explore some inaccuracies highlighted previously [2] of measurements of the rates of burning coal chars. Previously, Fennell et al. [2] found that the oxidation of Cynheidre (an anthracite) coal-char in O 2 , when examined in a laboratory-scale fluidised bed (i.d. 300 mm, depth ~ 790 mm), was first order at 700 o C, but zeroth at 900 o C. It was suggested [2] that problems arose from a combination of the slow response-time of the analysers (~ 4 s) and the overall mixing of gases when filling such a large fluidised bed. The resulting time-constant for the entire system was ~ 7.9 s. The purpose of this work is to investigate the effects on the measured kinetics of oxidation of reducing both the mixing time and the response time of the analysers. Experimental A small bed (i.d. 30 mm; unfluidised depth 30 mm) of quartz sand (355 – 425 μm), supported on a sintered quartz distributor was held in a quartz tube and fluidised by a mixture of O 2 and N 2 (flowrate = 83 ml / s N.T.P. to give U / U mf between 7.2 and 9.7, i.e. a vigorously bubbling bed ) containing 5 - 100 vol. % O 2 . The flowrates of O 2 and N 2 were measured by calibrated rotameters. The bed was surrounded by a tubular furnace. The temperature of the sand was measured using a type K thermocouple, which controlled the bed’s temperature to a steady set-point between 650 and 900 o C. Small batches of coal-char (sieved to 75 – 106, 150 – 212 or 300 – 355 μm), between 2 and 50 ± 0.1 mg were weighed, mixed with a small quantity (~ 0.1 g) of sand and dropped on to the hot bed from above. The exact quantity of char depended on the rate of reaction: more char was used when the rate was low to enable appreciable [CO] or [CO 2 ] to be measured in the off-gases. A tightly fitting steel lid was used to restrict any ingress of laboratory air into the freeboard. The char was THIRD EUROPEAN COMBUSTION MEETING ECM 2007