The kinetics and mechanism of the combustion of the carbon in biochar from oak, as studied in an electrically heated, fluidised bed of sand

• The rate of combustion of tiny particles (180–212 μm) of char from oak was measured in a fluidised bed of fine sand (150–180 μm) from 500 to 800 °C. • Burning such small particles was shown to be controlled by chemical kinetics at 700 °C or below in such a fluidised bed, where mass transfer is unusually rapid. • Hurt and Calo’s simple outline mechanism of the oxidation of a solid carbon was confirmed, for which measurements were made of the activation energies of the two rate-determining steps. • It appears that the combustion of this char was catalysed, probably by potassium, for which the mechanism is discussed. The oxidation of small particles (size 200 μm) of char from oak has been studied, by adding a small batch of them to an electrically heated bed of inert, silica sand, fluidised by mixtures of O 2 and N 2 . The concentrations of CO, CO 2 , and O 2 , in the off-gases from the bed were continuously monitored, enabling the rate of combustion and also the fraction, X , of the carbon oxidised in each particle to be measured throughout combustion at different, well-controlled temperatures. The rate of oxidation declined as (1 – X ) during burnout , b e c a u s e t h e O 2 freely contacted the carbon in these tiny particles, whose oxidation was kinetically controlled at 700 °C and below. In fact, these tiny char particles burned in the fluidised bed at 500–700 °C in Zone 1 according to S 0 (1 – X )( k 5 C O2 + k 6 ) per g of char. This matches Hurt and Calo’s three-step mechanism, whereby a porous char burns at a rate controlled by two reactions, one first-order in O 2 , the other zeroth-order in O 2 . Reactions (V) and (VI) were found to be, respectively, k 5 = 1.7 × 10 3 exp (−139 ± 18 kJ mole −1 /R T ) m s −1 and k 6 = 1.5 exp (−92 ± 20 kJ mole −1 /R T ) mole s −1 m −2 . Reaction (V), between O 2 and a surface complex containing oxygen, became faster as X increased, if the temperature exceeded 600 °C; catalysis by potassium was suspected as providing this extra reactive boost.