Size reduction, biomass

Particle Size Reduction. Changes in the physical characteristics of a biomass feedstock often are requited before it can be used as a fuel. Particle size reduction (qv) is performed to prepare the material for direct fuel use, for fabrication into fuel pellets, or for a conversion process. Particle size of the biomass also is reduced to reduce its storage volume, to transport the material as a slurry or pneumatically, or to faciHtate separation of the components. 

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. 

Reduction in physical size is often required before biomass is used as a fuel or feedstock. Size-reduction techniques are employed to prepare biomass for direct fuel use, fabrication into fuel pellets, cubes, and briquettes, or conversion. Smaller particles and pieces of biomass reduce its storage volume, facilitate handling of the material in the solid state and transport of the material as a slurry or pneumatically, and sometimes permit ready separation of components such as bark and whitewood. The size of the pieces or particles can be critical when drying is used because the exposed surface area, which is a function of physical size, can determine drying time and the methods and conditions needed to remove moisture. There are a few exceptions where size reduction is not needed, such as in whole-tree burning. 

If the particle size of the biomass fuel or feedstock is not predetermined by its history, as is the case for sawdusts, nutshells, and a few other waste biomass materials, size reduction is usually carried out with one or more units that make up the front end of the total processing system. Many different kinds of machines are employed. Generally, the size of the feed is reduced by grinding, cutting, or impact mechanisms. Not all of the designs are suitable for biomass energy applications because the equipment is customized for certain uses or the cost of size reduction is excessive. Agricultural crops and woody biomass are also usually processed by different types of machines. A brief review of the basic types of machines that are or have been used for biomass follows to illustrate the variety of size-reduction equipment and their biomass applications. 

The particle size reduction tests for various selected biomass types have been executed in the pilot installation of CIEMAT. A flow diagram of the installation is shown in Figure 1. Basically, the equipment consists of two hammer mills connected in series, the first one has a nominal power of 11 kW and is used for pre-grinding (emshing). The second hammer mill has a nominal power of 15 kW and is used for refining the pre-ground biomass. 

Processes for the bioproduction of ethanol from cellulosic materials have been studied extensively. Some of the process steps are specialized and beyond the scope of this chapter. However, there are many recent review articles dealing with some specific subjects. Basically, the processes consist of a number of steps. They are availability and collection of raw feedstock [20], size reduction, pretreatment, fractionation of biomass components, enzyme production [21, 22], saccharification, enzyme recycle [23, 24], pentose fermentation, improvement of pentose-fermenting biocatalyst, overcoming of product inhibition, overcoming inhibition by substrate-derived inhibitors, ethanol recovery [25], steam generation and recycling [26], waste treatment, and by-product utilization. 

De Mooij T, Janssen M, Cerezo-Chinarro O, et al Antenna size reduction as a strategy to increase biomass productivity a great potential not yet realized, J Appl Phycol 27(3) 1063-1077, 2014. 

Mechanical pretreatment is usually required for high solid content feedstocks before any other kind of pretreatment, and actually refers to milling, through which particle size reduction of feedstocks is accomplished. Mechanical pretreatment disintegrates the solids and/or decreases the particle size of the biomass and so increases the surface area, resulting in increased substrate availabiUty for the microorganisms. Consequently, microbial growth and AD process are improved (Ariunbaatar et al., 2014). 

The general steps anticipated in a modem hydrothermal liquefaction facUity are summarized in Fig. 17.6(a) for hgnoceUulosic biomass. The processing and pumping of feed slurries of low particle size may require pretreatment for certain feedstock. For woody biomass this typically includes size reduction and alkaline treatment to obtain a pumpable and stable slurry. 

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