Patent Publication Number: US-2003221363-A1

Title: Process and apparatus for making a densified torrefied fuel

Description:
BACKGROUND OF THE INVENTION  
       [0001] This invention relates generally to the field of energy, and more particularly to a process for producing a superior fuel from biomass (or other energy sources such as peat) by first torrefying the biomass and then densifying it to make pellets, cubes or logs. The process emulates to some extent the processes of Nature in making coal.  
       [0002] Biomass will be a major source of energy as fossil fuels run out or are seen as environmentally destructive. It has many advantages:  
       [0003] Renewable  
       [0004] No global warming effect  
       [0005] Widely available as low cost lumber or agricultural residues  
       [0006] but unfortunately also many disadvantages . . .  
       [0007] Low density, hard to store and ship  
       [0008] Many sizes and shapes, need to have special handling for each  
       [0009] Low energy density  
       [0010] Two processes have been developed in the last three decades which improve the properties of raw biomass.  
       [0011] Densification into pellets, cubes or logs is a commercial process making useful fuels out of sawdust, paper, plastics and agricultural residues for use in power plants, gasification and home heating. Densification removes many of the disadvantages of biomass and produces a fuel that is  
       [0012] Much higher density for easy shipping and storing  
       [0013] Easy to feed in standard equipment  
       [0014] Uniform and fungible, independent of original shape and size  
       [0015] Unfortunately it is  
       [0016] Expensive, using very large machines  
       [0017] Uses high power  
       [0018] Has high die wear  
       [0019] Torrefaction is defined in the Random House Dictionary as “to subject to fire or intense heat, to parch, roast or scorch. When applied to biomass, it is a process consisting of heating biomass to 200-270° C. which  
       [0020] Increases the mass energy density by first drying, then decarboxylation, from 18 to up to 23 MJ/kg (8000 to 10,000 Btu/lb)  
       [0021] Renders the fuel waterproof for easy storage and shipment  
       [0022] Renders the biomass friable, so that particle size reduction is greatly simplified  
       [0023] Unfortunately it reduces the volume energy density since the product has the same particle size as the starting material with some energy loss.  
       [0024] Thus, in summary, densification requires high power, has high die wear and requires very heavy machinery. Torrefaction produces a product with low energy density.  
       PRIOR ART  
       [0025] Torrefaction  
       [0026] U.S. Pat. No. 4,954,620 Thermocondensed lignocellulose material, and a method and an oven for obtaining it  
       [0027] U.S. Pat. No. 4,816,572 Thermocondensed lignocellulose material, and a method and an oven for obtaining it  
       [0028] U.S. Pat. No. 4,787,917 Method for producing torrefied wood, product obtained thereby, and application to the production of energy  
       [0029] U.S. Pat. No. 4,553,978 Process for converting ligneous matter of vegetable origin by torrefaction and product obtained thereby  
       [0030] Densification  
                                                          4015951   April 1977   Gunnerman           4026678   May 1977   Livingston           4236897   December 1980   Johnston           4308033   December 1981   Gunnerman           4324561   April 1982   Dean et al.           4395265   July 1983   Reilly et al.           4398917   August 1983   Reilly           4494962   January 1985   Christie et al.           4529407   July 1985   Johnston et al.           4561860   December 1985   Gulley et al.           4810255   March 1989   Fay, III et al.           4828573   May 1989   Jelks           4834777   May 1989   Endebrock           5141526   August 1992   Chu           5342418   August 1994   Jesse           5643342   July 1997   Andrews                      
 
       [0031] It is seen that there are patents on both torrefaction and densification, but I find no combination of densification after torrefaction which is the substance of this patent and which has a number of advantages over the processes taken separately.  
       SUMMARY OF THE INVENTION  
       [0032] The primary object of the invention is to provide a superior fuel from biomass. The fuel has up to 30% more energy per kg than biomass and can be up to 3 times as dense as wood or 20% denser than conventional pellets. It can be called a “Biomass Superfuel”.  
       [0033] Another object of the invention is to provide a fuel higher in energy content than biomass and approaching coal in its fuel value but without the detriments of coal (high ash, sulfur and fossil fuel emissions).  
       [0034] Another object of the invention is to provide a fuel higher in energy density than any other form of biomass.  
       [0035] A further object of the invention is to provide a fuel of uniform properties from a variety of forms of biomass.  
       [0036] Another object of the invention is to provide a fuel that is easier to store and ship than the many forms of biomass which occur naturally.  
       [0037] Another object of the invention is to provide a fuel that is water resistant.  
       [0038] Another object of the invention is to provide a fuel that requires less energy to make than conventional densified fuels.  
       [0039] Another object of the invention is to decrease the energy required for size reduction by initial torrefaction.  
       [0040] Other objects and advantages of the present invention will become apparent from the following descriptions, taken in connection with the accompanying drawings, wherein, by way of illustration and example, an embodiment of the present invention is disclosed.  
       [0041] The drawings constitute a part of this specification and include exemplary embodiments to the invention, which may be embodied in various forms. It is to be understood that in some instances various aspects of the invention may be shown exaggerated or enlarged to facilitate an understanding of the invention. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0042]FIG. 1 shows diagrammatically steps in making Torrefied Densified Biomass.  
     [0043]FIG. 2 shows a typical embodiment of the process.  
     [0044]FIG. 3 shows an advanced concept in which combustion of a small part of the torrefaction products to provide process heat 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0045] Detailed descriptions of the preferred embodiment are provided herein. It is to be understood, however, that the present invention may be embodied in various forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but rather as a basis for the claims and as a representative basis for teaching one skilled in the art to employ the present invention in virtually any appropriately detailed system, structure or manner.  
     [0046] Turning first to FIG. 1 there is shown diagramaticaly the steps in producing densified torrefied biomass, including size reduction and drying of the initial feed if necessary, torrefaction followed by further size reduction if necessary, and densification claimed in this invention, and finally cooling the product to 100 C for bagging or storing. The dark boxes, taken in the order shown, constitute the unique part of this invention.  
     [0047]FIG. 2 is a typical embodiment of the process, showing biomass feed ( 1 ) entering a star valve ( 2 ) and being fed to for instance multi hearth heating device ( 3 ) driven by a motor ( 4 ) and heated by burner ( 5 ) with a burner-stack ( 6 ), aan exit star valve ( 7 ) feeing pellets through a closed feeder ( 8 ) to a pelletizer, cuber or logger ( 9 ) which produces densified torrefied biomass in the chute ( 10 ) and feeds a bag or other storage device ( 11 ).  
     [0048]FIG. 3 shows a modification of FIG. 2 in which the products of torrefaction are burned to produce the heat required for torrefaction. It shows a biomass feed ( 20 ) entering a star valve ( 21 ) and being fed to for instance an auger heating device ( 22 ) driven by a motor, ( 23 ) and heated by an oven ( 24 ) heated with a fuel gas burner ( 25 ) for startup that also can burn the torrefaction products in the oven with a re-burner for burning torrefaction gases ( 26 ), an exit stock for the hot combustion gases ( 27 ) with an oxygen sensor ( 28 ) to maintain the correct air/fuel ratio during heating, a star valve ( 29 ) for feeding the finished torrefied biomass to a pelletizer, cuber or logger ( 30 ) which feed the densified torrefied biomass to a bag or other storage device ( 31 ).  
     [0049] While the invention has been described in connection with a preferred embodiment in FIGS. 2 and 3, it is not intended to limit the scope of the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.  
     [0050] In particular, the means of heating for torrefaction can be the multiple hearth kiln shown in FIG. 2, the heated auger shown in FIG. 3 (also described in U.S. Pat. No. 5,110,785), a fluidized bed heater, a rotary kiln, a batch oven, or any other device capable of raising the temperature to 300° C.  
     [0051] While a pellet mill is shown in FIGS. 2 and 3 for producing pellets, typically from ¼ to ¾ inch, the process can also be practiced using a cuber to make 1-2 inch cubes or any of several log making machines currently in use to make larger cylinders 2 to 4 inch in diameter.  
     EXAMPLE  
     [0052] The following experiment was performed in order to investigate the anticipated improvement in properties and reduction in pressure and work to be expected from densifying torrefied biomass. A quantity of pine sawdust was dried at 100° C. to provide a uniform starting material. The sawdust was loaded into a 1 inch diameter die normally used for making metallurgical specimens and capable of being heated to 300° C. The piston was put in place and the heating begun.  
     [0053] When the die temperature reached the test temperature the pressure was increased in steps of 1000 psi to 10,000 psi, with recording of the die travel at each pressure. The results of these experiments are shown in Table 1.  
               TABLE 1                          Density and HHV of DTB made from pine sawdust at 10,000 PSI and T                                                     Wt           Work           T   Density   Loss   HHV   HHV   Required 1         Form   C   g/cm3   %   kJ/g   kJ/cm3   kWh/tonne                                                 Pine Sawdust   20   0.20   0   19.3   5.8   NA       Pine pellet   20   1.20   0   19.3   23.2   6.00       Pine pellet   100   1.34   0.7   19.3   25.9   6.09       Pine pellet   150   1.37   2.4   19.6   26.9   6.23       Pine pellet   175   1.42   3.5   19.6   27.9   5.34       Pine pellet   200   1.47   4.4   19.6   28.7   4.45       Pine pellet   225   1.44   8.8   21.4   30.7   3.56       Pine pellet   250   1.32   28.8   23.0   30.3   2.67                          
 
     [0054] From this it is seen that the high heating value of the pellet is increased from 19.3 to 23 kJ/g by first torrefying at 250° C., an increase of 20%. The density is increased from 1.2 to 1.32 g/cm3, an increase of 10%.