Torrefaction, a mild roasting process in inert atmosphere, is an emerging thermo-chemical pretreatment process that can eliminate many of the shortcomings of raw biomass, but the supply of an inert gas like nitrogen in large industrial units may not be cost-effective. This paper examines the use of air as a potential substitute for the expensive nitrogen gas through a simple innovative means. It proposes to use a mildly pressurized batch reactor instead of an open continuous reactor continuously fed by nitrogen. Torrefaction of poplar wood was conducted in a 25.4 mm diameter x 304.8 mm long batch reactor under different operating parameters (Gauge Pressures, 0, 200, 400, and 600 kPa, temperatures, 220, 260, and 300 degrees C, and residence times, 15, 25, and 35 min) in air and nitrogen. Results show that torrefaction in pressurized air has higher energy density, higher fuel ratio, and similar energy yield but reduced mass yield compared to those in pressurized nitrogen. While reactor pressure was increased from 200 to 600 kPa, fuel ratio, energy density enhancement factor, fixed carbon increased but mass yield decreased in both air and nitrogen medium. Data obtained further showed that torrefaction temperature is the most important operating parameter influencing the process. Using Response Surface Methodology, this work also developed correlations to predict mass loss for given values of temperature, pressure, and time in air and nitrogen media. Correlations to estimate torrefied product properties like energy density enhancement and fuel ratio for known mass loss during torrefaction were then established. This also offers a quantitative characterization of different modes of torrefaction that could be used for design selection.