NOVELTY - Biomass conversion process comprises: (a) pyrolyzing solid particulate biomass in a lower reaction zone of a riser reactor to produce an initial pyrolysis product; (b) combining a thermal modification medium with the initial pyrolysis product to form a thermally-modified product mixture, where the thermal modification medium has a substantially different temperature than the temperature of the initial pyrolysis product; and (c) cracking at least a portion of the thermally-modified product mixture in an upper reaction zone of the riser reactor to produce a cracked pyrolysis product.    USE - The process is useful for conversion of biomass (claimed) into gaseous and liquid products that are useful in transportation fuels, petrochemicals and/or specialty chemicals.    ADVANTAGE - The process is economical and it provides high yields of high quality liquid fuels, such as bio-oil.    DETAILED DESCRIPTION - Biomass conversion process comprises: either (a) pyrolyzing solid particulate biomass in a lower reaction zone (18) of a riser reactor (12) to produce an initial pyrolysis product, (b) combining a thermal modification medium with the initial pyrolysis product to form a thermally-modified product mixture, where the thermal modification medium has a substantially different temperature than the temperature of the initial pyrolysis product, and (c) cracking at least a portion of the thermally-modified product mixture in an upper reaction zone of the riser reactor to produce a cracked pyrolysis product, where the vapor component residence time in the upper reaction zone is at least 50% greater than the vapor component residence time in the lower reaction zone; or (a1) pyrolyzing solid particulate biomass in a lower reaction zone of a riser reactor to convert at least 50 wt.% of the solid particulate biomass into vapor components of an initial pyrolysis product, where the lower reaction zone is maintained at an average pyrolysis temperature of 250-750 degrees C during the pyrolyzing, (b1) combining a quenching medium with the initial pyrolysis product to form a quenched product mixture, where the temperature of the quenching medium is at least 20 degrees C less than the temperature of the initial pyrolysis product, and (c1) catalytically cracking at least a portion of the quenched product mixture in an upper reaction zone of the riser reactor to produce a cracked pyrolysis product, where the upper reaction zone is maintained at an average cracking temperature of at least 10 degrees C less than the average pyrolysis temperature, and the average horizontal cross-sectional area of the upper reaction zone is at least 50% greater than the average horizontal cross-sectional area of the lower reaction zone. An INDEPENDENT CLAIM is included for an apparatus for converting solid particulate biomass to bio-oil, comprising a riser reactor defining a lower reaction zone and an upper reaction zone, and a temperature control system for introducing a thermal modification medium into the riser reactor at a location proximate the juncture of the upper and lower reaction zones.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of biomass conversion system.    Riser reactor (12)    Biomass source (14)    Biomass feed system (16)    Lower reaction zone (18)    Lift gas source. (20)