Optimization of process parameters for torrefaction of Acacia nilotica using response surface methodology and characteristics of torrefied biomass as upgraded fuel

Abstract

The process parameters (temperature, residence time and heating rate) for torrefaction of Acacia nilotica in a fixed-bed reactor were optimized using response surface methodology. Maximum higher heating value and energy yield, both at the same time, were obtained at 252 °C, 60 min residence time, and 5 °C/min heating rate. Both the parameters were highly influenced by temperature; whereas residence time and heating rate had minimal impact. The torrefied biomass obtained at optimum condition was characterized by proximate and ultimate analysis, thermogravimetric analysis, Fourier transform infrared spectroscopy and scanning electron microscopy. Moisture content, H/C ratio and O/C ratio decreased by 73.23, 52.94, and 46.22%, respectively; while fixed carbon and higher heating value increased by 75.54 and 18.62%, respectively, as compared to raw biomass. Fuel properties such as fuel ratio increased by 87.39%, while combustibility index and volatile ignitability decreased by 83.32 and 22.71%, respectively. Flow properties such as angle of repose, Hausner ratio, Carr compressibility index and cohesion coefficient decreased by 8.04, 6.20, 22.48 and 12.5%, respectively. Enhanced fuel and flow properties make torrefied biomass a suitable feedstock for pyrolysis and gasification and optimization of this process may facilitate scale-up and reduce operational cost. © 2019