Influence of elevated pressure on the torrefaction of wood

The use of pressurized reactors in industrial processes can improve efficiency and economics. Torrefaction is a partial pyrolysis of lignocellulosic biomass designed to result in a solid product with improved fuel properties for utilization in combustion and gasification. In this work, the influence of elevated pressure on the torrefaction of wood has been investigated. Wood samples were torrefied using a pressurized thermogravimetric reactor (PTGR) with pressures of 0.1 to 2.1 MPa. The results indicate that reactor pressure, particle size of feedstock, and wood species are all factors in torrefaction yield improvements. Torrefaction at 2.1 MPa pressure improved the higher heating value (calculated) of single-particle beech cylinders from 20.4 to 22.2 MJ kg−1, the increase ranging from 7.5 to 19% from the untreated heating value. Decomposition reactions were accelerated with pressure so that a given mass yield was realized in a shorter time. At 2.1 MPa pressure and 280 °C the time was reduced by over 60% for milled aspen compared to the run made at 0.1 MPa. A key finding is that the sample torrefied at a higher pressure, but shorter residence time, despite the same mass yield had a greater carbon yield and thus also a higher energy yield. For milled aspen, most of the observed pressure-dependent effect occurred within an initial pressure increment from atmosphere up to 0.5 MPa. The findings presented will have implications for the industrial production of torrefied fuels.