13.2.1 Supercritical Fluid Processing

Samples of Sitka spruce (Picea sitchensis) and Alaska birch (Betula neoalaskana) were dried to a moisture content of 4 wt% and processed in a Willey mill to pass through a 2 mm sieve. Approximately 4.5 g of each biomass stream were intro­duced into a 75 ml high-pressure reactor (Parr Instruments model 4740), where methanol (35 ml) was introduced to a volume ratio of 0.46 [18]. The reactor was placed inside a temperature-controlled heating block, where the internal reaction temperature was raised to 375°C. Both temperature and pressure were monitored with reaction time by means of a thermocouple and a pressure gauge mounted directly to the reactor vessel. Given the batch nature of the reactor, the pressure increased to above the supercritical pressure point of methanol (8.1 MPa) and increased to a maximum of 42 MPa. At this maximum pressure, the reactor was removed from the heater block and quenched in an ice-water bath until cold. The solid material and liquid were separated by means of vacuum filtration through a glass fiber mat, creating a bio-oil fraction and an insoluble biochar component and yields determined.

13.2.2 Chemical Characterization

Fourier transform infrared spectra were obtained on wood and biochar samples in the attenuated total reflectance (ATR) mode (ZnSe, single bounce) on an Avatar 370 spectrometer (ThermoNicolet). The collected spectra were ATR and baseline corrected using Omnic v7 software. Lignin condensation indices (CIs) were cal­culated according to Faix [29] with a slight modification to the method. The sum of all spectra minima intensity between 1,500 and 1,050 cm-1 was divided by the sum of all spectra maxima intensity between 1,600 and 1,030 cm-1 and was used to calculate the CI.

The volatile components in the bio-oil (methanol soluble fraction) were determined by GC-MS analysis on a Polaris Q instrument (ThermoQuest). An aliquot of the methanol soluble fraction (200 il) was transferred to a GC vial to which dichloromethane (1 ml containing 0.1 mg/ml anthracene as an internal standard) was added. Separation was achieved using a ZB-1 capillary column (Phenomenex, 30 m, 0.25 mm 0) with helium as a carrier gas and a temperature program of 40 (2 min) to 300°C (10 min) at the ramp rate of 5°C/min. Data analysis was performed using the Xcalibur software v2 (Thermoscientific). The compounds were identified by comparison with standards, their corresponding mass spectra and NIST 2008 library.