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Chapter 3 Physical Conversion

3-1 Dewatering and drying 3-2 Size reduction 3-3 Separation 3-4 Densification 3-5 Refuse derived fuel

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Dewatering and Drying

Dewatering is the removal of water from solid material by centrifugal force. Drying is a mass transfer process consisting of the removal of water by evaporation from a solid. A source of heat and an agent (e.g. air) to remove the vapor produced by the process are often involved. Dewatering uses mechanical energy to remove water, whereas drying uses heat to overcome the latent heat of moisture to leave from biomass.

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Before energy conversion through thermal process, moisture has to be removed first to avoid consuming energy. For gasification, moisture content in biomass is usually lower than 30% and better less than 15%. The moisture content is usually lower than 10% for biomass pyrolysis. The heating value of green wood is around 8.2 MJ/kg. After drying by air, it becomes 16.0 MJ/kg. The value is 18.7 MJ/kg after the biomass undergoes complete drying in an oven.

Dewatering and Drying

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Three Mechanisms of Drying

Conduction: the transfer of internal energy by microscopic diffusion and collisions of particles within a body due to a temperature gradient. Convection: convective heat transfer takes place both by diffusion the random Brownian motion of individual particles in the fluid and by advection, in which matter or heat is transported by the larger-scale motion of currents in the fluid. Radiation: thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation.

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Drying

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Dryer Type

Perforated Floor Dryer Band Conveyor Dryer Direct Rotary Dryer Indirect Rotary Dryer Pneumatic Dryer

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Perforated Floor Dryer

The basic system comprises a bin with a perforated floor through which hot air is made to flow (70C) and the gas is released from the roof.

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Band Conveyor Dryer The most widely used for biomass is the atmospheric band conveyor dryer (belt dryer), in which the drying medium is blown through a thin static layer of material on a horizontally moving permeable band. The band dryer is used for sawdust in pellet production.

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Rotary Dryer

The dryer consists of a hollow, rotational metal cylinder providing space for direct contact between the material to be dried and the drying medium, usually hot air.

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Indirect Tubular Steam Dryer The most usual form of such a dryer is known as a steam-tube dryer. Steam tubes running the full length of the dryer are arranged in a number of concentric rows moving inwards from the dryer shell.

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Pneumatic Dryer In the dryer, the hot gas stream transports the solid particles through a pipe or flow duct and makes direct contact with the material to be dried. This gas stream is also a drying medium to supply the heat required for drying and carries away the evaporated moisture.

The material residence time is short (510 s) for drying small particle size (500 mm) of materials using hot air or flue gas as drying medium.

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Drying Cost

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Size reduction

Operating conditions and reactor geometry, even ash delivery, are subject to biomass particle size. Size reduction can

Increasing surface area, which is conducive to subsequent treatment and chemical reactions Easier for biomass transport (e.g., slurry and pneumatic transport) Using biomass as fuels directly, such as powder, pellet, block, briquette Facilitating conversion process, such as gasification and pyrolysis

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Size reduction

Crushing: reducing size by pressing using hard objects. Shearing, cutting or shredding: reducing size by sharp knife, say, rotary knife. Grinding: similar to crushing, but using grinding. Its energy-intensive, but particle sizes are smaller. Impact: reducing size using hard objects with high velocities.

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The principal types of size-reduction machines: Crushers (coarse and fine) Grinders (intermediate and fine) Ultrafine grinders Cutting machines

Size reduction

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By particle size (screening) By density By inertia force By magnetic force

Separation

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Screening

Passing materials over a screening surface, where objects of a size smaller than the holes in the screen drop through, while objects of larger size are retained on the screen. The objects on the screen are termed plus material, whereas through the screen are termed minus material.

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Mesh Size No Mesh size Diameter (mm) No Mesh size

Diameter (mm)

1 4" 101.6 23 16 1.19 2 3 1/2" 88.9 24 18 1 3 3" 76.2 25 20 0.84 4 2 1/2" 63.5 26 25 0.71 5 2" 50.8 27 30 0.59 6 1 1/2" 38.1 28 35 0.5 7 1 1/4" 31.7 29 40 0.42 8 1" 25.4 30 45 0.35 9 3/4" 19.1 31 50 0.297 10 5/8" 15.9 32 60 0.25 11 1/2" 12.7 33 70 0.21 12 3/8" 9.52 34 80 0.177 13 1/4" 6.35 35 100 0.149 14 3 5.66 36 120 0.125 15 4 4.76 37 140 0.105 16 5 4 38 170 0.088 17 6 3.36 39 200 0.074 18 7 2.83 40 230 0.062 19 8 2.38 41 270 0.053 20 10 2 42 325 0.044 21 12 1.68 43 350 0.04 22 14 1.41 44 400 0.037

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Mesh Size

Mesh 200

200 pores per inch 1/200=0.005 in Line diameter: 0.0021 in Diameter of pore =0.005-0.0021 in=0.0029 in =0.074 mm

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Separation by Density

Liquid flotation Air classification

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Magnetic Separation

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Eddy Current Separation

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Densification

Densification means the reduction of biomass volume and the increase of biomass density, say, baling and compression. After densification, the distances between biomass particles are reduced, thereby decreasing biomass volume. This is conducive to fuel transport, storage, minimizing packing space, and subsequent treatment.

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Densification

Normally, the density of baling rice straw is between 70 and 90 kg/m3, along with the moisture of 10-15 wt%. The density of unbaled rice straw is around 5-15 % of baled one. If rice straw is compressed to pellet or briquette, its density can be promoted to 350-1200 kg/m3. The density of dry wood is 600-700 kg/m3 and bulk density is 350-450 kg/m3. After densification, the density of wood briquette is 700-800 kg/m3, even as high as 1400 kg/m3.

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Refuse Derived Fuel, RDF

Refuse derived fuel (RDF) is a fuel produced by shredding and dehydrating municipal solid waste (MSW) with a waste converter technology.

RDF consists largely of combustible components of municipal waste such as plastics and biodegradable waste. RDF processing facilities are normally located near a source of MSW and, while an optional combustion facility is normally close to the processing facility, it may also be located at a remote location.

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RDF Grade and Efficiency

The lower the pretreatment extent, the lower the cost and the efficiency are. Fuel grade and efficiency increase with increasing RDF number, so RDF-7 has the highest fuel grade. The energy efficiency of RDF-1 is lower than 15%, while it is higher than 35% for RDF-7. The production of RDF is inherently related to physical conversion, but RDF-6 (pyrolysis) and RDF-7 (gasification) are related to thermal conversion.

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