A method for collecting biomass, such as straw, and for producing a pyrolysis liquid, such as oil or tar, from the biomass, includes the step of collecting the biomass from a growth site, e.g. a field, by means of a mobile unit. The biomass is continuously fed into a pyrolysis apparatus accommodated by the mobile unit, as the mobile unit is moved across the growth site. While the biomass is processed in the pyrolysis apparatus, further biomass is simultaneously being collected. The pyrolysis apparatus may be a flash pyrolysis or fast pyrolysis apparatus relying on centrifugal forces for forcing biomass towards a reactive surface in a pyrolysis reactor. The mobile unit may be self-propelled.