Steam treatment of four softwood species and bark to produce torrefied wood

Debarked samples from three wood species (Spruce, Douglas-fir and Pine) and Douglas-fir bark were treated with saturated steam at 220 °C for 5 min. The objective was to quantify physical and compositional properties of the treated biomass that would affect on the degree of carbonization (torrefaction) of softwood particles. The resulting data indicated that the calorific value increases as a result of steam treatment; the highest increase was 26.0% for Spruce, from 18.7 MJ/kg to 23.6 MJ/kg. The corresponding carbon content of the treated Spruce increased from 46.5% to 57.0%. The equilibrium moisture content of the treated wood, placed in a chamber at 90% relative humidity and 30 °C air for 180 min, dropped from around 0.103 (decimal dry basis) (before treatment) to about 0.045 (decimal dry basis) for Spruce treated particles. Bulk density of Spruce particles increased the most from 87 kg/m3 to 129 kg/m3 after steam treatment. The bulk density of Douglas-fir bark decreased from 310 kg/m3 to 275 kg/m3 upon steam treatment. The increased values in calorific values, hydrophobicity, carbon content and dark brownish color indicated a mild degree of torrefaction of biomass when compared to the corresponding values of treated biomass at temperatures of 280 °C.

► Steam treatment is introduced as a wet torrefaction process at lower temperatures.
► The heating value of steam treated biomass is comparable to that of dry torrefaction.
► The increased carbon content also confirms the effectiveness of the pretreatment.
► Among different softwood species, Spruce is affected by steam treatment the most.
► Bark particles is affected by steam treatment the least..