NOVELTY - The apparatus has a hot gas filter (60) whose inlet is in fluid communication with an outlet of a pyrolysis reactor. The hot gas filter includes a circle cross-sectional shape filter (80) with a filter insert that reduces void volume of the filter. A condenser includes an inlet that is in fluid communication with the outlet of the hot gas filter. A temperature reducing apparatus is in thermal communication with the inlet of the hot gas filter to reduce temperature of vapor entering the hot gas filter. A heat exchanger is in thermal communication with the inlet of the hot gas filter.    USE - Apparatus for producing pyrolysis oil i.e. liquid biomass-derived pyrolysis oil.    ADVANTAGE - The apparatus can purge an interior of the filter insert with a small portion of quench gas introduced through the inlet in manifold and exiting through purge vents so as to prevent damage to the filter insert from pressure differential or deposits formed inside the hot gas filter in an effective manner. The apparatus can combine gas from the condenser with the gaseous pyrolysis products from a cyclonic separator that increases a volumetric flow rate of a combined gas stream to a filtration device, thus reducing residence time of the gaseous pyrolysis products in an improved manner. The apparatus can improve hot gas filtration to reduce liquid fuel loss caused by prolonged residence time at high temperatures by reducing the residence time at elevated temperature, temperature of pyrolysis vapor and volume of the pyrolysis vapor at the elevated temperature and increasing the volumetric flow rate at constant volume of the pyrolysis vapor.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a method for producing pyrolysis oil having improved stability.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a hot gas filter including a filter insert.    Hot gas filter (60)    Hot gas (65)    Clean gas (70)    Manifold (75)    Circle cross-sectional shape filter (80)    Filter housing (105)