Pretreatment of biomass feed for gasification

In accordance with the present invention there is provided a process for preparing a biomass feedstream for gasification, which process comprises:

• a) comminuting said biomass to an effective particle size;
• b) conducting said comminuted biomass to a drying zone wherein at least about 90 wt. % of water is removed thereby resulting in a dried comminuted biomass stream and a first vapor stream;
• c) passing said dried comminuted biomass stream to a torrefaction zone where it is torrefied at temperatures ranging from about 200° C. to about 350° C., in a substantially non-oxidizing environment and at an effective residence time thereby resulting in a second vapor phase stream comprised of water vapor and small amounts of organic components, and a torrefied dried comminuted biomass stream;
• d) passing at least a portion of said torrefied dried comminuted biomass stream to a gasification zone;
• e) venting at least a portion of said first vapor stream to the atmosphere and conducting at least a portion of any remaining first vapor stream to a combustion zone;
• f) conducting at least a portion of said second vapor stream to said combustion zone and recycling at least a portion of any remaining second vapor stream to said torrefaction zone to act as a sweep gas, wherein said combustion zone is operated at an effective time and temperature that will result in the conversion of at least 99 wt. % of any volatile organic components from said second vapor phase to a hot flue gas;
• g) conducting a portion of said hot flue gas to said drying zone to provide at least a portion of the heat necessary to dry the biomass to a predetermined level;
• h) passing at least a portion of the remaining hot flue gas through a first passageway of a heat exchanger having a first passageway and a second passageway contiguous to each other but not in fluid communication with each other, wherein each of said passageways having an inlet and an outlet and wherein each passageway is constructed to allow a fluid to pass from its inlet to its outlet and to allow heat to be transferred from a fluid of one passageway to a fluid in the other passageway;
• and
• i) passing a heat transfer medium through said second passageway of said heat exchanger wherein heat is transferred from said hot flue gas passing through said first passageway of said heat exchanger thereby resulting in a heated heat transfer medium and a cooled flue gas.