Pellet densified

The cost of manufacturing thermoformed, polystyrene foam sheet parts is less dependent on raw material cost than other extrusion processes. This is largely due to the combined effects of additional energy costs required to operate two extruders, heat removal requirements in the secondary extruder, cost of pelletizing (densifying) regrind and the relatively low output of the process for the equipment scale and cost. Typical cost factors for the manufacture of thermoformed polystyrene foam sheet products include raw materials 35%, labor 27%, sales and administration 16%, depreciation 8%, utilities 7% and other 7%. 

RDF can be produced as fluff or densified forms. As regards fluff RDF, it is not biologically stable and difficult to store, and it must be used within 2 or 3 days. Densified or pelletized RDF has advantages over fluff RDF because it is easier to transport, handle, and store. 

The carbon black industry takes extreme efforts to confine product during all stages of manufacture (45). Highly efficient bag filters are used to collect the product. After collection the fluffy carbon black is densified and pelletized to minimize dusting problems during shipping and use. 

To remove possible impurities, the gas black is classified and then densified, pelletized (see Section 4.4.1), or submitted to an oxidative aftertreatment (see Section 4.4.7), depending on its intended use. Since gas blacks are formed in the presence of excess air, their surface is oxidized. Acidic surface oxides are predominant. 

The production of solid fuels from renewable resources (e.g. biomass) has become more important due to increasing energy demands and also because of environmental pressure. Upgraded biofuels (refined biofuel) include powdered fuels and densified fuel such as briquettes and pellets. The raw materials are often shavings... 

Pellets are a cylindrical form of densified wood particles that have several advantages compared to the raw material (mostly sawdust) ... 

Baling has long been used to densify hays, straws, and other agricultural crops such as cotton to simplify removal from the field and to reduce storage space and transportation costs. Baled straw has a density of 70 to 90 kg/m at 10 to 15 wt % moisture content, whereas the bulk density of piled straw is about 5 to 15% of this density range. When straws are compressed to form pellets, briquettes, or cubes in specially designed dies and presses, the density can be increased to 350 to 1200 kg/ml In contrast, dried wood has a density of 600-700 kg/m and a bulk density of about 350 to 450 kg/m, whereas the bulk densities and densities of wood briquettes are 700 to 800 kg/m and up to 1400 kg/m, respectively. 

The heating value depends on the moisture and ash contents of the densified material and is usually in the range of 15 to 17 MJ/kg. The use of asphaltic binders or pelletizing conditions that result in some carbonization can yield densified products that have higher heating values. Pellets, briquettes, and logs have been manufactured by densification methods from biomass for many years. Prestologs made from waste wood and sawdust were marketed before 1940 in North America, and the market for pellet fuels made from wood... 

Folk, R. L., and Govett, R. L. (1992). A Handbook for Small-Scale Densified Biomass Fuel (Pellets) Manufacturing for Local Markets. U.S. Department of Energy, Bonneville Power Administration, Portland, OR, July. 

Granules, pellets, and tablets can be filled into capsules using automatic-filling machines. Products are prepared in these forms to modify the release rates of active ingredients, separate incompatible components, or densify a product to achieve the fill weight in a specific size of capsule. 

The cold pressing is carried out in hardened steel dies. The carbide powder is first mixed with 0.5-5% of binder lubricants, such as paraffin, camphor, Carbowax, cetyl alcohol, or beeswax dissolved in nonaqueous inert solvents such as benzene, isopropyl alcohol, or methyl alcohol. The bonded carbide powder is granulated by forcing through a screen and then loaded into the dies. The density of the cold-pressed pellets may be as high as 80% of theoretical, and the sintered pellets may be densified to 90% of theoretical. The optimum sintering condition is 1800°C for 4 h. 

Power Consumption. A previous report (2) showed the power consumption of the pellet mill as a function of throughput. Subsequent work has shown there were errors in these measurements and the result is hereby withdrawn. Later work, using processed office waste with added moisture (75% of the samples contained 10 - 30 wt%, 22% of the samples contained 30 - 50 wt%, and the remainder 10 wt%) showed that the relationship between power consumption P(kW), and feedrate to the densifier F (short tons/h, dry wt basis), could be represented by P = 50.5F + 21.7, for 63 determinations, 0.3 < F < 1.7 Mg/h, with a correlation coefficient r = 0.964 and a standard error of the regression line of 5.7. The intercept should be equal to the idling power of the machine which was separately measured as 24 kW. There was no discernible relationship between power consumption and moisture content. 

Lignin sulfonate byproducts from sulfite pulping make excellent binders for densified products (JJ, 1 2). Like other adhesives they require complete blending and adequate heating to make durable products. In excess they increase pelleting horsepower. Although available in quantity in the Northwest ligninsulfonates are presently burned for lack of markets. 

Densification is accomplished by a 300 HP California Pellet Mill Model 7162-3. The pellets are formed by extrusion through nominally 3/8 inch diameter radial holes in a cylindrical die. Pressure is exerted on a "pad" of the material to be densified by rolls which have fixed shafts. The pellet die rotates about the pressure rolls. Pellet length is controlled by a "cut-off" knife positioned to clip the pellets as they exit the rotating die. The exiting material stream from the pellet mill is mechanically lifted to a pellet cooler, which discharges to a screen to remove under-size pellet pieces and fines. The fines and pieces are recycled to the pellet mill infeed system. Finished pellets are conveyed by high pressure air system to tank storage. 

Densified biomass fuel in the form of extruded pellets have been produced from a variety of raw materials and by systems that vary the sequence of process steps from that described herein. These variations in the processes should have little effect on the composition of the combustible fraction of the product pellets, A survey of fuel properties of a number of these fuel pellets is illustrated in Figure 1. When the moisture and ash are removed... 

Densified fuel pellets may be used as boiler fuel in place of coal in stoker-fired furnaces. Direct substitution of fuel pellets was demonstrated in two informally reported instances with no furnace adjustments. The particulate emission from each of these installations were monitored with the emission rate results shown in Figure 2. As a comparison, the North Carolina Administrative Code allowable particulate emission rate schedule (a function of heating rate) is also shown. The environmentally acceptable particulate emission contributes to the economic justification of the conversion of small-to-medium size steam generators to the use of pelleted biomass fuels. Two such installations are now using pelleted fuels on a routine basis, and a third has concluded the economic benefit by entering into a contract agreement to purchase pelleted fuel. 

The types of fuel that will be considered for use in vehicle mounted gasifiers include wood chips ( 30 weight percent H2O) and densified fuel pellets ( 10 weight percent H2O) produced from agricultural residues. 

If five plants were required for the production of densified fuel pellets from agricultural residues, the investment costs for a total capacity of 580 metric tons/day would probably exceed 10 million. Therefore, the investment costs associated with the option of using vehicle-mounted gasifiers may be about 15 million. 

The volume could be 50% this amount if densified fuel pellets are used. 

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