Kinetics of softwood kraft lignin inert and oxidative thermolysis

Abstract

Oxidative and inert lignin thermolysis generate methane, CO, CO2, and char but also intermediate valuable compounds. These thermos-sensitive compounds may either repolymerize or react further to light gases and char. Here, we characterized the lignin degradation kinetics versus temperature in a micro-fluidized bed reactor operating up to 870 K in nitrogen and air and measured product selectivity. The liquid yield during pyrolysis and oxidative thermolysis reached ∼20% (predominantly aromatics). In the latter tests, oxygen reacted with lignin and mainly formed CO, CO2, and CH4 while in an inert atmosphere char was the principal product. We also monitored the mass loss of lignin in a thermogravimetric analyzer at heating rates of 5 K min−1 to 30 K min−1 (0.08 Ks−1 - 0.33 Ks−1). The heating rate dramatically affected the degradation of lignin in the oxidative thermolysis process. A two-step model characterized the lignin degradation behavior in both oxidative and inert lignin thermolysis. During lignin thermolysis; first, the weak bonds broke and char and gas formed. The remaining lignin depolymerized and reacted to aromatics and char. Lignin conversion, liquid yield and selectivity depended on the temperature. The activation energy between 600 K to 850 K was 47 kJ mol−1 and it was 95 kJ mol−1 at ∼560 K. The pre-exponential factor varied with heating rate that indicated lignin degradation is limited by heat and mass transfer. The activation energy of lignin oxidative thermolysis at first step was lower compared to pyrolysis (40 kJ mol−1) but considerably higher (200 kJ mol−1) in the second step.