Method and an arrangement for efficient torrefaction of biomass

Abstract

The invention relates to a method and a system for efficient torrefaction of a biomass. Said system comprises a first zone for heating of an optionally pre-dried biomass and an insulated second zone for torrefying the heated biomass. The second zone has an inlet being connected to an outlet of the first zone and means for withdrawing torrefied biomass arranged at the bottom of the second zone, such that the biomass can move downwards through the second zone by means of gravity. Means for heating are arranged at the first zone, but not at the second zone.

1 . A method for torrefaction of an optionally pre-dried biomass, comprising the steps of:
 a) heating the optionally pre-dried biomass in at least one heating zone such that the temperature of the biomass reaches a torrefaction temperature range; and   b) torrefying the heated biomass from step a) in a torrefaction zone by maintaining the temperature within the torrefaction temperature range,   wherein the heated biomass from step a) is fed to the torrefaction zone at its top and torrefied material is withdrawn from the torrefaction zone at its bottom, such that the biomass moves downwards through the torrefaction zone by means of gravity, and wherein an oxygen-containing gas is supplied to the torrefaction zone.   
     
     
         2 . Method according to  claim 1  wherein the oxygen-containing gas is supplied to the torrefaction zone such that oxygen reacts with components of torrefaction gases under the formation of heat and wherein torrefaction gases are withdrawn from the heating zone such that the torrefaction gases moves through the torrefaction zone and through the heating zone countercurrent to the biomass transport. 
     
     
         3 . A method according to  claim 1 , wherein the torrefied material is withdrawn from the bottom by means of a transport device, such as a transport screw. 
     
     
         4 . A method according to  claim 3 , wherein the speed of the transport device is controlled to control of the residence time of the material in the torrefaction zone. 
     
     
         5 . A method according to  claim 1 , wherein the torrefaction temperature range is within 220° C. to 600° C., such as 220-500° C., such as 220-450° C., such as 220-400° C., such as 230-600° C., such as 230-500° C., such as 230-450° C., such as 230-400° C., preferably 240-500° C., preferably 240-400° C., preferably 240-350° C., most preferably 270-350° C. 
     
     
         6 . A method according to  claim 1 , wherein the biomass is transported through the heating zone in a horizontal direction. 
     
     
         7 . A system for torrefaction of biomass, comprising:
 a first zone for heating of an optionally pre-dried biomass; and   an insulated second zone for torrefying the heated biomass, said second zone having an inlet being connected to an outlet of the first zone,   means for withdrawing torrefied biomass arranged at the bottom of the second zone, such that the biomass can move downwards through the second zone by means of gravity,   wherein means for heating are arranged at the first zone, but not at the second zone.   
     
     
         8 . The system according to  claim 7 , wherein the first zone is arranged in a heating chamber comprising a screw or a belt for transporting the biomass through the first zone. 
     
     
         9 . The system according to  claim 8 , wherein the heating chamber is horizontal and the screw or belt extends in a horizontal direction for horizontal transport of the biomass. 
     
     
         10 . The system according to  claim 7 , wherein the insulated second zone is arranged in a vertical reactor. 
     
     
         11 . The system according to  claim 7 , comprising at least one oxygen inlet for injection of an oxygen-containing gas to the second zone and at least one outlet for torrefaction gases and wherein the outlet for torrefaction gases is located upstream of the oxygen inlet in relation to a biomass transport direction within the second zone.