Almond shells: Catalytic fixed-bed pyrolysis and volatilization kinetics

The recovery of energy from lignocellulosic food residues provides a double beneficial effect on the mitigation of environmental issues: waste management and carbon-neutral fuels. In this work, almond shells were characterized by thermogravimetry to evaluate their potential for conversion into pyrolytic bio-oil. The average composition assessed by deconvolution of rate of thermal degradation curves (hemicellulose 40.9 wt%, cellulose 28.2 wt%, lignin 30.9 wt%) agrees with published data obtained using laborious conventional techniques. The same set of differential thermogravimetry data was used to estimate pyrolysis kinetics parameters using linear regression and curve-fitting methods, revealing parameters in the range of previously published data. The fixed-bed reactor pyrolysis of grounded almonds shells produced bio-oil in yields lower than 52 wt% for temperatures in the range 375–500 °C. The use of catalysts in pyrolysis tests, fluid catalytic cracking catalyst, Li and Na carbonates, and cenospheres had a beneficial effect on the bio-oil yield. Catalysts had also a positive effect on the bio-oil quality decreasing the acidic species. Cenospheres were the most performant material, being able to decrease the relative fraction of organic acid functional groups from 48 wt% to 21 wt% at 500 °C. Cenospheres are waste streams from coal power plants; this favorable behavior brings them to light as promising low-cost catalysts for biomass pyrolysis processes.