Densification, biomass

Balatinez, J. J., The Potential of Densification of Biomass Utilization. Plenum Press, London, 1983, pp. 181-189. 

Chapters 6 to 12 address specific groups of processes and methods employed for converting biomass to energy and fuels. In this chapter, the physical processes employed to prepare biomass for use as fuel or as a feedstock for a conversion process are discussed. The processes examined are dewatering and drying, size reduction, densification, and separation. The physical process, a few specific examples of the process, and its relationship to the thermochemical or microbial process that may be used for subsequent conversion are described. 

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... 

TABLE 6.4 Typical Biomass Densification Hardware, Feedstocks, and Products"... 

Densification of solid wastes and biomass to produce solid fuels with more desirable physical properties is the subject of the fourth section. 

Fritz, J. J., Gordon, J. J., Henry, J. F., Nguyen, V. T., "Status Review of Wood Biomass Gasification, Pyrolysis and Densification Technologies", MTR-8031, The MITRE Corporation, McLean, Virginia, July 1979. 

An important result of this study is the finding that the work and pressure of compression or extrusion can be reduced by a factor of about two by preheating the feedstock to 200-225 C before densification, This requires extra thermal energy for complete drying and to heat the biomass (heat capacity about 1.8 J/g-C) to the higher temperature however, these are offset by lower electrical power costs, lower equipment costs because of the lower pressure requirements, possibly reduced die wear due to improved lubricity of the biomass at increased temperatures, and increased fuel value due to complete water removal and prepyrolysis. These factors must be tested at the commercial scale before any conclusions can be drawn on the desirability of preheating feedstock. 

The overall energy efficiency of densification systems may be misleading. Only 10 to 25of input energy is lost in making a biomass fuel pellet, even if electrical energy is thermally generated from the same residue when used as feedstock. 

The answers assembled by these studies have differed because the assumptions used to define the question have been different. A recently encountered expression, used in description of an entirely different human effort, is nevertheless appropriate in the context of human effort in general "...while not perfect...is probably as near to perfection as possible." There must come a time in the development of an enterprise when an end to study is called and the proposal subjected to a test. The feasibility of the densification/refinement of fuels from residual biomass sources is currently being tested. 

The destructive effects of expanding compressed air and relaxation of elastic deformation can be also reduced if the maximum pressure is held for some time, called dwell time, before it is released. Fig. 8.2 shows, that, without special technical provisions, this is only achieved in ram extruders (Fig. 8.2b, see also Section 8.4.3). In such equipment, a number of briquettes is retained in the long pressing channel and is redensified during each stroke. After the wall friction is overcome and the entire line of briquettes moves forward, the pressing force remains almost constant. A similar, but much smaller effect is obtained in pellet presses (see also Section 8.4.2). Since, as mentioned before, a dwell time and, particularly, the application of several densification cycles also helps to convert temporary elastic deformation into permanent plastic deformation, these techniques are especially suitable for the densification of elastic materials such as, for example, biomass. 

Vanhercke, T., Petrie, J.R., Singh, S.P., 2014a. Energy densification in vegetative biomass through metabolic engineering. Biocatal. Agric. Biotechnol. 3, 75-80. 

During HTC the biomass undergoes energy densification to a hydrochar which resembles a lignite-like coal with an energy density of 28 MJ/kg (Libra et al.,... 

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