Optimization of waste cork powder torrefaction to improve biofuel parameters and storage properties

Waste cork torrefaction was optimized in order to improve the properties of this material for its application as biofuel. A two-level factorial design involving three factors -torrefaction temperature, torrefaction time and heating rate- was proposed with two-responses -mass yield (MY) and high heating value (HHV)-. Factorial design results revealed that torrefaction time and especially temperature were the most influential factors. Using simultaneous optimization of the two variables MY and HHV, and giving them two different weights, two different sets of optimum conditions were obtained. The first one (246 ºC, 60 min and 5 ºC/min) led to a biofuel with similar HHV as the raw material, therefore it did not give any benefit in terms of biofuel upgrading. The second one (275 ºC, 60 min and 5 ºC/min) gave a biofuel with 10% more energy density than the raw material, which allowed to classify this biofuel as lignite. Both torrefied samples showed significant increase in hydrophobic character. Finally, ATR-FTIR results revealed the occurrence of carbonization reactions during torrefaction.