The scope for generating bio-oils with relatively low oxygen contents via hydropyrolysis

The primary oils obtained in high yields from fast (fluidised-bed) pyrolysi s of biomass generally have high oxygen contents (ca. 40% w/w). The scope f or using pyrolysis under hydrogen pressure (hydropyrolysis), to give oils w ith much lower oxygen contents compared to normal pyrolysis has been invest igated. Fixed-bed hydropyrolysis tests have been conducted on cellulose, su gar cane bagasse and eucalyptus wood using hydrogen pressures up to 10 MPa, with heating rates of 5 and 300 degrees C min(-1). A colloidal FeS catalys t was used in some tests (Fe loading of 5%, w/w) to increase overall conver sions. Further, the attractive option of using a two-stage reactor; in; whi ch the primary oil vapors are passed though a bed of hydrotreating catalyst is also described. Raising the hydrogen pressure from atmospheric to 10 MP a reduced the oxygen content of the primary oil by over 10% to below 20% w/ w. The addition of a dispersed iron sulphide catalyst gave conversions clos e to 100% for all three biomass samples investigated at 10 MPa under condit ions in the fixed-bed reactor where significant diffusional resistances exi sted and reduced the oxygen content of the bio-oil by a further 10%. Althou gh NMR indicated that the oils became increasingly aromatic as more oxygen was removed, the increase in hydrogen pressure decreased the extent of over all aromatisation that occurs primarily due to the lower char yields obtain ed. In two-stage tests for cellulose, using a commercial sulphided Ni/Mo ga mma-Al2O3 catalyst at 400 degrees C, increasing the hydrogen pressure from 2.5 to 10 MPa decreased the oxygen content of the oil by over 20% to 10% w/ w. The H/C ratios were higher and O/C ratios smaller for the two-stage bio- oils compared to their single stage counterparts. However, the differences in the O/C ratios between the single and two-stage bio-oils increase with p ressure. (C) 1999 Elsevier Science Ltd. All rights reserved.