Microwave torrefaction of Prosopis juliflora: experimental and modeling study.
This study is focused on evaluating the effect of particle size and microwave power on torrefaction of the invasive biomass species, Prosopis juliflora (PJF), via experiments and theoretical model. Experiments were conducted using a central composite design (CCD) approach to construct response surfaces capturing the effects of these two factors on three key characteristics of the obtained char, viz. yield, calorific value and energy recovery. Highest char yield was obtained at an intermediate microwave power for a given particle size, while the calorific value increased with both the parameters. The response surface for energy recovery was convex with maxima at 481 W power for 1.9 mm particles. A microwave-based thermogravimetric study was conducted to obtain the mass loss profile during torrefaction. Assuming a global, single-step, power-law kinetic model with Arrhenius-type rate constant for torrefaction, the apparent activation energy and reaction order were estimated to be 173.93 kJ mol-1 and 1.3, respectively. To explain the trends of char yields, a new approach was proposed that integrates the temperature profile information with the inferred kinetic model. Two parameters, viz. torrefaction time and average temperature, are introduced to quantify the temperature profiles during torrefaction. The generic model can be extended to any biomass species and reactor configuration by conducting suitable experiments. Detailed composition analysis of the bio-oil revealed that, unlike conventional torrefaction, significant degradation of lignin also occurred along with hemicellulose degradation at low bulk temperatures during microwave torrefaction.