NOVELTY - Producing (P1) olefins and aromatic hydrocarbons, involves: (a) supplying a feed comprising a biomass pyrolysis-oil or its fraction to a steam cracking unit operating at 600-1000 degrees C and recovering at least one hydrocarbon effluent fraction.    USE - For producing olefins and aromatic hydrocarbons (claimed).    ADVANTAGE - The biomass pyrolysis-oil can be upgraded by being fed directly (without hydrotreating) to a steam cracker, either alone or jointly with a fossil hydrocarbon feedstock, where part of the biomass pyrolysis-oil is converted to olefins, mainly ethylene and propylene, and aromatics, while oxygen in the biomass pyrolysis-oil is rejected as carbon monoxide (CO), carbon dioxide (CO2), water (H2O), and a heavy fraction is also produced that can be used as a fuel for burners and furnaces. The reaction is exothermic, whereas steam cracking of fossil hydrocarbon feedstocks is endothermic. Thus, by supplying a mixed biomass pyrolysis-oil/fossil hydrocarbon feedstock to the steam cracker, the heat requirements for the operation can be reduced. The biomass pyrolysis-oil can be supplied to a higher temperature, thermal conversion reactor, such as a reverse flow reactor (RFR), where the majority hydrocarbon product is acetylene while oxygen in the biomass pyrolysis-oil is rejected as CO, CO2, and H2O. Conversion to acetylene is net endothermic and the acetylene can be converted to olefins, aromatics and other valuable chemicals.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) producing (P2) olefins and aromatic hydrocarbons, involving: (a2) pyrolyzing biomass in a reactor under conditions to convert the biomass to a vapor condensable into pyrolysis oil, non-condensable gases and solid biochar; and (b2) supplying a feed comprising at least part of the condensable vapor or the condensed pyrolysis oil to a steam cracking unit operating at 600-1000 degrees C and recovering at least one hydrocarbon effluent fraction;    (2) producing (P3) olefins and aromatic hydrocarbons, involving: (a3) supplying a feed comprising a biomass pyrolysis oil or its fraction to a reverse flow reactor operating at 900-1700 degrees C and recovering at least one hydrocarbon effluent fraction including acetylene; and (b) converting at least a portion of the acetylene to olefins and/or aromatics; and    (3) producing (P4) olefins and aromatic hydrocarbons, involving: (a4) supplying a feed comprising a biomass pyrolysis oil or its fraction along with steam to an initial heating zone at a temperature to vaporize a portion of the biomass pyrolysis oil in the presence of the steam and produce a two phase stream; (b4) feeding the two phase stream of (a3) to a vapor-liquid separator to produce a vapor stream and a liquid stream; and (c4) feeding the vapor stream of (b1) to a thermal cracking zone to produce a product stream enriched in olefins and aromatics.