NOVELTY - Preparing and purifying a product gas comprises thermochemical conversion of biomass in a reactor (20), conducting the produced product gas loaded with 100% pollutant load into a main gas treatment step (22), cooling the product gas to condense the unwanted tars as aerosols resulting during the preparation of the product gas, and conducting the purified product gas having 10% of pollutant load, into a filter (24), which is also operated wet and is formed as fibrous deep filters with large depth, i.e. at least 1.5 kg fibers per square of inflow area.    USE - The method is useful for preparing and purifying a product gas produced by thermochemical conversion of biomass.    ADVANTAGE - The cleaning is done only at long intervals (maintenance interval), preferably at an interval of one month, so an uninterrupted operation until the next service or the next maintenance interval is possible.    DETAILED DESCRIPTION - Preparing and purifying a product gas comprises thermochemical conversion of biomass in a reactor (20), conducting the produced product gas loaded with 100% pollutant load (dust particles and tars as aerosols) from the thermochemical production and supplying this product gas into a main gas treatment step (22), which works at a wet condition and at least includes an electrostatic precipitator with pre-quench or high performance scrubbers, which can separate at least 95%, preferably at least 98%, of the pollutant load with a diameter of 1 mu m, cooling the product gas up to at least its cooling limit temperature of less than 90 degrees C, preferably below 50 degrees C, to condense the unwanted tars as aerosols resulting during the preparation of the product gas; and conducting the purified product gas having 10% of pollutant load, into a filter (24), which is also operated wet and is formed as fibrous deep filters with large depth, i.e. at least 1.5 kg fibers per square of inflow area, and providing in such a way cleaned product gas to a consumer. The product gas is produced by thermochemical conversion of biomass directly or by other methods processed biomass and provides a calorific value sufficient for the further energy conversion. The high performance scrubbers include single or multistage rotation scrubbers, Venturi scrubbers with fixed or adjustable Venturi grooves and/or adjustable groove cross-section, multi Venturi scrubber, disintegrator scrubber and Bayer rider scrubber and further wet scrubbers. An INDEPENDENT CLAIM is also included for the filtering device useful in the above method, where the filter depth with respect to flow velocity is 0.2-20 meters per minute, preferably 2-8 meters per minute, when the filter is wet operated at an operating differential pressure controlled by an additional external fluid or steam, where the filter is operatable with long maintenance intervals.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic representation of the process for the production and purification of a high calorific value product gas, in which the filter is a main wet cleaning step.    Reactor (20)    Main gas purifying unit (22)    Filter (24)