Hydroprocessing of Fischer–Tropsch biowaxes to second-generation biofuels

Abstract

Upgrading of Fischer–Tropsch (FT) biowaxes to second-generation biofuels via hydroprocessing is the final step for increasing the fuel amount of the overall biomass conversion route: gasification of lignocellulosic biomass, FT synthesis, and hydroprocessing. The typical FT product portfolio consists of high molecular weight paraffinic waxes as the main product and FT fuels in the diesel and naphtha boiling range. OMV's objective and contribution to the project focus on achieving coprocessing of FT biowaxes with fossil feedstock using existing hydrotreating plants of crude oil refineries. Various test runs have been examined with a conventional refining catalyst under mild conditions (380–390°C, 5.8 MPa; WHSV, 0.7–1.3 h−1) in a pilot plant. Pure FT biowax is converted to gases, fuels, and an oil/waxy residue in a fixed-bed reactor with a porous catalyst layer technology. The presence of hydrogen in the reaction chamber reduces the fast deactivation of the catalyst caused by the formation of a coke layer around the catalyst particle surface and saturates cracked hydrocarbon fragments. Another approach is the creation of synthetic biodiesel components with excellent fuel properties for premium fuel application. Basically, premium diesel fuel differs from standard diesel quality by cetane number and cold flow properties. Hydroprocessed synthetic biodiesel (HPFT diesel) has compared to conventional diesel advantages in many aspects. Depending on the catalyst selected, premium diesel quality can be obtained by shifting cold flow operability properties of HPFT fuels to a range capable even under extreme cold conditions. In addition, a high-quality kerosene fraction is obtained to create bio jet fuels with an extremely deep freezing point, as low as −80°C. The isomerization degree, as well as the carbon number distribution of high paraffinic profile, and the branching degree have a major impact on the cold flow properties and cetane number. FT diesel has, compared to HPFT diesel, a slightly higher derived cetane number (DCN > 83) and a cloud point of −9°C, whereas HPFT diesel reaches values as low as −60°C. Although the HPFT naphtha obtained consists of high amounts of isoparaffins, the RON/ MON values are comparable to fossil straight-run naphtha. The reason is that the branching degree of isoparaffins from the naphtha fraction is not sufficiently high enough to reach the typical octane number values of gasoline products delivered at filling stations. Assuming the goal of launching a premium biodiesel or biokerosene fuel to the market, these hydroprocessed synthetic biofuels from FT biowaxes are ideal blending components.