Mechanical Size Reduction. Size reduction is the term applied to the transformation process to reduce the size of the waste materials. The objective of size reduction is to obtain a final p that is reasonably uniform and considerably reduced in size in comparison with its original for that size reduction does not necessarily imply volume reduction. In some situations, the total Table 14-4 Transformation processes used for the management of solid waste Transformatlon Transforma TransformatIon or principat means or method conversi。 n product(s Copar ents found in chanical separation Volume reduction energy The original waste reduced in volume Size reduction nergy The original waste components altered in n the form of shredding. form and reduced in siz ch Thernal oxidation Carbon dioxide (cO2), sulfur dioxide (SO2), other oxidation products, ash Pyrolysis A gas stream containing a variety of gas ar and/or oil. and a char Gasification Starved air comb A low-Btu gas, a char containing carbon and the inerts originally in the fuel, an erobic biological Compost (humus-like material used as a composting Anaerobic digestion Anaerobic biological ethane(CH4),carbon dioxide(cO2) aerobic biological Methane (CH4), carbon dioxide(CO2) composting conversion digested-waste Anaerobic composting occurs in landfills (see Chapter 11) of office paper). In practice, the terms shredding, grinding and milling are used to describe mechanical size-reduction operations. Chemical Transformations Chemical transformations of solid waste typically involve a change of phase(eg, solid to liquid, solid to gas, etc. ) To reduce the volume and/or to recover conversion products, the principal chemical processes used to transform MSw include(1)combustion(chemical oxidation),(2) pyrolysis, and (3) gasification. All three of these processes are often classified as thermal processes Combustion( Chemical Oxidation). Combustion is defined as the chemical reaction of oxygen with organic materials, to produce oxidized compounds accompanied by the emission of light and rapid generation of heat. In the presence of excess air and under ideal conditions, the combustion of the organic fraction of Msw can be represented by the following equation Organic matter excess air- N2+CO+H20+O+ash heat (14-14) Excess air is used to lete combustion. The end products derived from the combustion of MSW, Eq (14-14), include hot combustion gases--composed primarily of nitrogen(N2), carbon dioxide(COz), water(H2O, flue gas), and oxygen(O2-and noncombustible residue In practice small amounts of ammonia(NH3), sulfur dioxide(soz), nitrogen oxides(NOx), and other trace gases will also be present, depending on the nature of the waste materials Pyrolysis. Because most organic substances are thermally unstable, they can be split, through a combination of thermal cracking and condensation reactions in an oxygen-free atmosphere, into gaseous, liquid, and solid fractions. Pyrolysis is the term used to describe the process. In contrast with the combustion process, which is highly exothermic, the pyrolytic process is highly endothermic. For this reason, destructive distillation is often used as an alternative term for pyrolysis The characteristics of the three major component fractions resulting from the pyrolysis of the portion of MSW are(1)a gas stream containing primarily hydrogen(H2). methane( CH4), monoxide (CO), carbon dioxide (CO2), and various other gases, depending on the characteristics of the waste material being pyrolyzed; (2)a tar and/or oil stream that is liquid at room 14-714-7 Mechanical Size Reduction. Size reduction is the term applied to the transformation processes used to reduce the size of the waste materials. The objective of size reduction is to obtain a final product that is reasonably uniform and considerably reduced in size in comparison with its original form. Note that size reduction does not necessarily imply volume reduction. In some situations, the total volume of the material after size reduction may be greater than that of the original volume (e.g., the shredding of office paper). In practice, the terms shredding, grinding, and milling are used to describe mechanical size-reduction operations. Chemical Transformations Chemical transformations of solid waste typically involve a change of phase (e.g., solid to liquid, solid to gas, etc.). To reduce the volume and/or to recover conversion products, the principal chemical processes used to transform MSW include (1) combustion (chemical oxidation), (2) pyrolysis , and (3) gasification. All three of these processes are often classified as thermal processes. Combustion (Chemical Oxidation). Combustion is defined as the chemical reaction of oxygen with organic materials, to produce oxidized compounds accompanied by the emission of light and rapid generation of heat. In the presence of excess air and under ideal conditions, the combustion of the organic fraction of MSW can be represented by the following equation: Organic matter + excess air → N2 + CO2 + H2O + O2 + ash + heat (14- 14) Excess air is used to ensure complete combustion. The end products derived from the combustion of MSW, Eq. (14- 14), include hot combustion gases—composed primarily of nitrogen (N2), carbon dioxide (CO2), water (H2O, flue gas), and oxygen (O2)— and noncombustible residue. In practice, small amounts of ammonia (NH3), sulfur dioxide (SO2), nitrogen oxides (NOx), and other trace gases will also be present, depending on the nature of the waste materials. Pyrolysis. Because most organic substances are thermally unstable, they can be split, through a combination of thermal cracking and condensation reactions in an oxygen-free atmosphere, into gaseous, liquid, and solid fractions. Pyrolysis is the term used to describe the process. In contrast with the combustion process, which is highly exothermic, the pyrolytic process is highly endothermic. For this reason, destructive distillation is often used as an alternative term for pyrolysis. The characteristics of the three major component fractions resulting from the pyrolysis of the organic portion of MSW are (1) a gas stream containing primarily hydrogen (H2). methane (CH4), carbon monoxide (CO), carbon dioxide (CO2), and various other gases, depending on the organic characteristics of the waste material being pyrolyzed; (2) a tar and/or oil stream that is liquid at room Table.14- 4 Transformation processes used for the management of solid waste