Pretreatment of biomass feed for gasification

A process for preparing a biomass feedstream for gasification comprising:

• a) comminuting a moisture-containing lignocellulosic biomass feedstream to an effective particle size;
• b) passing said comminuted lignocellulosic biomass feedstream to a first stage torrefaction containing a first continuous torrefaction reactor having a first outer jacket through which an oil-based heat transfer fluid is conducted to provide heat to the first reactor and wherein the biomass is torrefied at a temperature from about 200° C. to about 350° C. in a substantially non-oxidizing environment and for an effective residence time to result in at least about 90 wt. % of the moisture, based on the total amount of moisture, of said biomass being removed and wherein a torrefied biomass feedstream and a first vapor phase product stream are produced;
• e) passing said torrefied biomass stream from said first torrefaction reactor to a second stage torrefaction containing a second continous torrefaction reactor having a second outer jacket through which an oil-based heat transfer fluid is conducted to provide heat to the second reactor and where the torrefied biomass is subjected to temperatures from about 200° C. to about 350° C., in a substantially non-oxidizing environment and at an effective residence time, to result in a second vapor phase product stream and a particulate biomass stream having an average particle size from about 1 to 500 microns;
• d) passing at least a portion of said particulate biomass stream from said second torrefaction reactor to a gasification zone;
• e) conducting said first and second vapor phase streams to a condensing zone wherein a fraction of both of said vapor phase streams condenses to form a condensate liquid stream comprised of a water component and a hydrocarbon component and a remaining uncondensed vapor phase stream;
• f) conducting an effective amount of said uncondensed vapor stream to both first and second torrefaction reactors to sweep vapor phase products from each torrefaction reactor;
• g) conducting any remaining portion of said vapor phase stream to a flare;
• h) vaporizing said condensate liquid stream in a burner thereby resulting in a hot flue gas;
• i) passing said 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;
• 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 exchange thereby resulting in a heated heat transfer medium;
• j) venting said flue gas exiting said heat exchanger;
• k) passing said heat transfer medium to the outer jackets of the first and second torrefaction reactors thereby providing at least a fraction of the heat of reaction for each torrefaction reactor, wherein said heat transfer medium is not in direct contact with said biomass in either torrefaction reactors.