Biomass produced through photosynthesis using the sun's energy, carbon dioxide from the ambient air and water produce an abundance of potential energy that can be a useful fuel to replace fossil fuels. Biomass that is merely left to decay will decompose and put all the collected carbon dioxide back into the environment along with methane and other green house gases. This wastes the stored suns energy. This invention can result in putting this biomass to use as a clean carbon neutral fuel and reduce fossil fuel use.
Biomass has been defined as plant material, vegetation, or agricultural waste used as a fuel or energy source. Thus, Biomass includes trees, grasses, algae, forest thinning and other plant debris, agricultural waste, lumber, wood and paper pulp, cow, pig, and horse manure and other solid waste. For convenience, the term “wood” is used herein to describe various embodiments of the invention. It will be understood that the term “wood” as used herein is merely exemplary of one possible kind of biomass and that virtually any kind of biomass may be used in practicing the present invention.
This apparatus and method of the present invention provides improvements over the previous patent applications of John A. Paoluccio, an inventor of the present invention. One such application is entitled “Method and Apparatus for Biomass Torrefaction Using Conduction Heating” and is identified as U.S. application Ser. No. 12/050,902 filed Mar. 18, 2008 and now published application US 20080223269.
Another is entitled “Method and Apparatus for Biomass Torrefaction, Manufacturing A Storable Fuel From Biomass And Producing Offsets for the Combustion Products of Fossil Fuels and a Combustible Article of Manufacture” and is identified as U.S. Pat. No. 7,942,942. This patent is incorporated by reference herein.
The U.S. application Ser. No. 11/618,868 as well as the description herein utilizes the term “conduction” with respect to heat transfer between the biomass and a liquid heat transfer fluid. This is intended to differentiate the use of a liquid in contact with the biomass instead of heat transfer between the biomass and a gas. Heat is transferred in many cases by a combination of conduction, convection and radiation and the process can be very complex at the surface boundary layer between the heat transfer surfaces. A liquid is approximately 1,000 times denser than an inert gas, thus considerably more molecules are in contact with the wood surface with a liquid than a gas.
The use of the term conduction is supported by the ASHRAE HANDBOOK FUNDAMENTALS, 2005 edition, Chapter 3 which states:                “Thermal conduction. This heat transfer mechanism transports energy between parts of a continuum by transfer of kinetic energy between particles or groups of particles at the atomic level. In gases, conduction is caused by elastic collisions of molecules; in liquids and electrically non-conducting solids, it is believed to be caused by longitudinal oscillations of the lattice structure. Thermal conductions in metals occur, like electrical conduction, through the motion of free electrons.” (emphasis added)        
Other authorities may describe the immersion of a biomass feedstock into a hot heat transfer fluid for direct heating contact between a hot liquid and the biomass surface differently. Such authorities may assert that the heat transfer rate is characterized by the unit surface heat transfer, or film, coefficient, h, in units of W/m2·° C. (or BTU/hr·ft2·° F.) and that the film coefficient for convection from a moving fluid to a solid surface is much greater for a liquid than for a gas, assuming equivalent flow. It will be understood by all that (1) there is the potential to create processing equipment of smaller size and less cost than an equivalent gas-phase system and (2) the higher film coefficient for torrefication using a liquid heat transfer fluid also offers several advantages compared to a gas-phase heat transfer media; these include shorter heating times and operating costs, lower fluid velocities and therefore more ease of control, smaller volumes and therefore more compact sizing, and relatively low pressures as compared to a gas phase process.
Although the invention has application to virtually any form of biomass, the term “wood” is used herein to describe various embodiments of the invention. It will be understood that the term “wood” as used herein is merely exemplary of one possible kind of biomass and that virtually any kind of biomass may be used in practicing the present invention. Biomass when collected typically will have a bulk density of 10 to 15 pounds per cubic foot and it is common to chip and shred it and then run it through a pellet mill to reduce handling and transportation costs. Typically this concentrates the biomass into convenient pellet form that may be ¼″ diameter by 1″ long with a bulk density of 50 pounds per cubic foot as explained in John A. Paoluccio's prior published U.S. patent application 20070266623. Embodiments of the present invention will often utilize wood or other biomass that is compressed into pellets prior to being processed in accordance with other aspects of this invention.
The apparatus described includes retorts. It will be understood that the term is used in the chemical industry sense where a “retort” is an airtight vessel in which substances are heated for a chemical reaction producing gaseous products to be collected in a collection vessel or for further processing. Such industrial-scale retorts are used, for example, in oil shale extraction processes that include the step of heating oil shale to produce shale oil, shale gas, and spent shale. Typically, industrial processes using such retorts use steam or hot gases for heat treatment.