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

Abstract

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 O2 and N2. The concentrations of CO, CO2, and O2, 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,becausethe O2 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 S0(1 – X)(k5CO2 + k6) 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 O2, the other zeroth-order in O2. Reactions (V) and (VI) were found to be, respectively, k5 = 1.7 × 103 exp (−139 ± 18 kJ mole−1/RT) m s−1 and k6 = 1.5 exp (−92 ± 20 kJ mole−1/RT) mole s−1 m−2. Reaction (V), between O2 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.