System and method for preparation of solid biomass by torrefaction

Abstract

Methods and systems for preparing a torrefied biomass fuel are disclosed. Moisture is initially extracted from relatively wet biomass fuel to produce a relatively dry biomass fuel. Remaining moisture is then extracted from the relatively dry biomass fuel in a final drying stage, using steam at a temperature of about 900° F. The resulting dried biomass fuel is conveyed downward using gravity and undergoes torrefaction, which produces torrefied biomass fuel and torrefaction gases. A gaseous mixture of steam and torrefaction gases is vented to a heat exchanger, where the gaseous mixture is heated by a flue gas, and the heated gaseous mixture is used to support the extraction of the remaining moisture in the final drying stage and to support the torrefaction of the dried biomass fuel. Embodiments disclosed herein efficiently use available energy resources to the benefit of manufacturers, consumers, and the environment.

1 - 17 . (canceled) 
     
     
         18 . A torrefaction system comprising:
 a torrefaction reactor comprising and upper portion and a lower portion, wherein the upper portion includes:   an upper portion configured to dry an input of the torrefaction reactor, and   a lower portion configured to torrefy an output of the upper portion;   a combustion unit configured to combust a portion of a gaseous mixture vented by the torrefaction reactor to produce a flue gas; and   a heat exchanger configured to exchange heat from the flue gas to the gaseous mixture.   
     
     
         19 . The torrefaction system of  claim 18 , wherein the torrefaction reactor further includes a segregation barrier between the upper portion and the lower portion. 
     
     
         20 . The torrefaction system of  claim 19 , wherein the segregation barrier is configured to enable gases to flow across the segregation barrier. 
     
     
         21 . The torrefaction system of  claim 18 , wherein the combustion unit comprises:
 an upper chamber configured to combust the gaseous mixture and a supplemental fuel to produce the flue gas; and   a lower chamber configured to combust a portion of a relatively wet biomass fuel to support production of the flue gas in the upper chamber.   
     
     
         22 . A method for preparing a torrefied biomass fuel, the method comprising:
 extracting moisture from a biomass input to produce a dried biomass fuel;   torrefying the dried biomass fuel to produce a torrefied biomass fuel and a torrefaction gas;   venting a gaseous mixture including the torrefaction gas to a heat exchanger;   heating the gaseous mixture; and   using the heated gaseous mixture to support the extraction of the moisture from the biomass input and the torrefaction of the dried biomass fuel.   
     
     
         23 . The method of  claim 22 , wherein the moisture is extracted using steam. 
     
     
         24 . The method of  claim 22 , additionally including combusting at least a portion of the gaseous mixture to produce a flue gas, wherein the gaseous mixture is heated via heat exchange with the flue gas. 
     
     
         25 . The method of  claim 24 , additionally including providing the flue gas, cooled by transfer of heat to the gaseous mixture, to the dryer for moisture extraction. 
     
     
         26 . The method of  claim 24 , further including combusting a portion of a relatively wet biomass fuel to support said combustion of at least a portion of the gaseous mixture. 
     
     
         27 . The method of  claim 26 , further including maintaining the combusted biomass in a combustion unit for sufficient time to eliminate all organic compounds, particulates, and other hydrocarbons resulting from the combustion of the biomass. 
     
     
         28 . The method of  claim 26 , further including combusting a supplemental fuel to provide additional thermal energy for the combustion. 
     
     
         29 . The method of  claim 22 , further including controlling flow of the heated gaseous mixture into an upper portion of a torrefaction reactor and into a lower portion of the torrefaction reactor to maintain the lower chamber above about 250° C. 
     
     
         30 . The method of  claim 29 , further including maintaining the lower portion of the torrefaction reactor at a lower pressure than the upper portion.